Diagenetic Pore Fluid Evolution and Dolomitization of the Silurian and Devonian Carbonates, Huron Domain of Southwestern Ontario: Petrographic, Geochemical and Fluid Inclusion Evidence

Abstract

Core samples from two deep boreholes were analyzed for petrographic, stable and Sr isotopes, fluid inclusion microthermometry and major, minor, trace and rare-earth elements (REE) of different types of dolomite in the Silurian and Devonian carbonates of the eastern side of the Michigan Basin provided useful insights into the nature of dolomitization, and the evolution of diagenetic pore fluids in this part of the basin. Petrographic features show that both age groups are characterized by the presence of a pervasive replacive fine-crystalline (<50 µm) dolomite matrix (RD1) and pervasive and selective replacive medium crystalline (>50–100 µm) dolomite matrix (RD2 and RD3, respectively). In addition to these types, a coarse crystalline (>500 µm) saddle dolomite cement (SD) filling fractures and vugs is observed only in the Silurian rocks. Results from geochemical and fluid inclusion analyses indicate that the diagenesis of Silurian and Devonian formations show variations in terms of the evolution of the diagenetic fluid composition. These fluid systems are: (1) a diagenetic fluid system that affected Silurian carbonates and was altered by salt dissolution post-Silurian time. These carbonates show a negative shift in δ¹⁸O values (dolomite δ¹⁸O average: −6.72‰ VPDB), Sr isotopic composition slightly more radiogenic than coeval seawater (0.7078–0.7087), high temperatures (RD2 and SD dolomite T_h average: 110 °C) and hypersaline signature (RD2 and SD dolomite average salinity: 26.8 wt.% NaCl eq.); and (2) a diagenetic fluid system that affected Devonian carbonates, possibly occurred during the Alleghenian orogeny in Carboniferous time and characterized by a less pronounced negative shift in δ¹⁸O values (dolomite δ¹⁸O average: −5.74‰ VPDB), Sr isotopic composition in range with the postulated values for coeval seawater (0.7078–0.7080), lower temperatures (RD2 dolomite T_h average: 83 °C) and less saline signature (RD2 dolomite average salinity: 20.8 wt.% NaCl eq.).

1. Introduction

The Michigan Basin has been the subject of several diagenetic studies in the last three decades (e.g., [1,2,3,4,5,6,7,8]). However, very few comprehensive investigations regarding the nature and composition of diagenetic fluids, lateral extent of the diagenetic processes and paragenetic sequence of different types of dolomite within Silurian and Devonian successions in the Huron Domain have been carried out. Most previous studies focused on the Paleozoic carbonate succession in the area were limited in stratigraphic and/or spatial resolution (e.g., [1,2,3,4]). In addition, contrasting models have been proposed to explain the fluid flow evolution and migration in the Michigan Basin, such as migration of dolomitizing fluids from the center of the Michigan Basin toward the margins [5] or from the back reef downward through Guelph limestones to the center of the Michigan basin (e.g., [6,7,8]). Numerous diagenetic processes such as dolomitization and Mississippi Valley Type (MVT) mineralization, salt dissolution, precipitation of late-stage cement and oil, and gas generation and migration have influenced the Paleozoic succession in the Michigan Basin [9]. Fractures and faults in carbonate bodies represent preferential pathways for fluids which in turn play a key role in the diagenesis and evolution of host rocks and hydrocarbon reservoirs [5]. The presence of fractures and faults in the Michigan intracratonic Basin represented an important control on the modification of host limestones and mobilization of dolomitizing fluids within the basin [4,10,11,12].

Coniglio et al. [8] performed a regional study of the Silurian Guelph Formation in southwestern Ontario. In this work, the authors underline how burial diagenesis produced the recrystallization of massive dolomites with homogeneous petrographic characteristics but with different geochemistry. According to their study, initial shallow burial dolomitization occurred due to the isolation of the Michigan Basin during the late Silurian. Isotopic values of δ¹³C and ⁸⁷Sr/⁸⁶Sr suggest a Silurian seawater source. They also recorded a decrease of δ¹⁸O values in dolomite from the platform to the lower slope which occurred with increasing burial depth. These authors suggested that seawater-derived dolomitizing fluids migrated from the margin toward the basin center with a progressive decrease in terms of the capability of the fluids to dolomitize the down-slope.

Luczaj et al. [13] conducted a study of fluid inclusion analysis in dolomite and calcite of the Middle Devonian Dundee Formation. Their work underlines that the sedimentary fabrics are modified by medium to coarse crystalline dolomite. They suggested that fluids characterized as dense Na–Ca–Mg–Cl brines interacted with host rock at high temperatures (around 120–150 °C). They invoked a model involving the transport of fluids and heat from deeper parts of the basin along with major fault and fracture zones systems.

Haeri-Ardakani et al. [5] evaluated diagenesis and fluid flow evolution within Ordovician, Silurian and Devonian carbonate successions in southwestern Ontario. Data from fluid inclusions analysis showed homogenization temperatures of replacive and saddle dolomite and late-stage calcite ranging between 75–145 °C. They suggested the involvement of hydrothermal fluids in replacement and recrystallization of dolomite and calcite cementation to explain homogenization temperatures higher than the peak expected during maximum burial for each interval (60–90 °C). Evidence of decreasing values in fluid inclusion homogenization temperatures from the center to the margin of the basin suggests that hydrothermal fluids originated from the center of the Michigan Basin. The authors suggest that the potential source of heat could be related to the buried mid-continental rift (MCR), while the preferential pathways for the migration of diagenetic fluids from the center of the Michigan Basin can be related to the Cambrian sandstones that unconformably overlay the Precambrian basement which could have behaved as regional aquifers [5].

The main purpose of this study was to conduct a thorough investigation of the characteristics and the occurrence of dolomitization within the Devonian and Silurian carbonate formations on the eastern side of the Michigan Basin (Figure 1). Dolomitization processes are examined in terms of dolomite types, distribution and geochemistry using an integrated approach utilizing petrography, multi-elements analysis, stable isotopes, Sr-isotopes and fluid inclusion analyses of rock core samples. Based on geologic, geochemical and petrographic investigations, a relative timing in terms of paragenesis of dolomitization is suggested, as well as relationships between existing regional structures and Devonian and Silurian strata-bound dolomite, nature and composition of mineralizing fluid during diagenesis, and extension of diagenetic events. This study represents an important contribution to the more extensive studies carried out by the Nuclear Waste Management Organization (NWMO) for the characterization of a suitable site to develop a Deep Geologic Repository (DGR) for low- and intermediate-level nuclear waste at the Bruce power station in Kincardine, Ontario.

2. Geologic and Tectonic Setting

The study area is located on the northeastern margin of the Michigan Basin (Figure 1a,b). It represents part of the northwestern flank of the Algonquin Arch that consists of Paleozoic sedimentary rocks that cover the subsurface basement [14]. The thickness of the Paleozoic succession ranges from a maximum of 4800 m at the center of the Michigan Basin to 850 m on the flank of the Algonquin Arch where the study area is located [15].

On a regional scale, southern Ontario is located between two major Paleozoic sedimentary basins, the Appalachian Basin to the southeast and Michigan Basin to the west (Figure 1). The Algonquin Arch, which separates the two basins, extends from northeast of the Canadian shield to the southwest near the city of Chatham. Near the Windsor area, another structural high named the Findlay Arch starts extending to the southwest into Michigan and Ohio states [3].

The rifting that separated North America and Africa occurred during Precambrian to Cambrian represents the initial episode of subsidence (approximately 1100 Ma) and deposition within the Michigan Basin [10]. Thermal subsidence of the Precambrian basement followed this event approximately 580 Ma to 500 Ma because the cooled lithosphere becomes denser [19]. The subsequent and continuous deposition of sediments into the Michigan Basin caused the bending of the basement [20,21,22]. However, the development of the basin is not only the result of continuous subsidence but represents the result of a series of tectonic events that characterized the Paleozoic [23,24].

The Taconic and Acadian orogenies were a dominant control on the Paleozoic strata of the eastern flank of the Michigan Basin [25]. In fact, during the Taconic Orogeny (Ordovician) a large-scale eastward-tilting of the Laurentian margin governed the disappearance of the concentric depositional geometry of the Michigan Basin with the consequent deposition of Ordovician limestone and overlying shale successions [24,26,27]. The regional maximum principal stress at that time was northwesterly oriented the same as the main direction of the thrust motion along the Appalachian tectonic front [25,28].

Restricted marine conditions occurred during the late Silurian with the consequent deposition of evaporites of the Salina Group [15,18]. In addition, the presence of an unconformity reveals emergent conditions at the end of the Silurian [25]. The Middle Devonian Acadian Orogeny reactivated the Algonquin-Findlay Arch system with the return of the concentric geometry of the Michigan Basin [23]. The Lower and Middle Devonian units are dominated by limestones and dolostones, whereas, the Upper Devonian strata are mostly represented by shales [15].

The Caledonian (Devonian) and Alleghanian (Carboniferous) orogenies played an important role in terms of diagenetic fluid migration [29]. Late Paleozoic sediments have been removed from the entire study area. According to Wang et al. [30], the regional analysis of apatite fission track dating in the center of the Michigan Basin suggests that late to post Alleghenian erosion removed approximately 1000 m of carbonate successions from the margins and approximately 1500 m from the center of the basin.

The main characteristics of the Silurian and Devonian stratigraphy in the study area are shown in Figure 2. The Michigan Basin has been characterized by a typical tropical climate during mid to late Silurian [31] whereby shallow water reef complexes started to develop (e.g., Gasport, Lions Head, Fossil Hill and Guelph formations). Moreover, the Michigan Basin has been isolated and becoming an evaporative basin in which alternation of evaporites and carbonates (Salina Group) deposited during the rest of the Silurian [32]. Conversely, during the Middle Devonian, Michigan Basin has been characterized by an extremely arid climate with consequent deposition of shallow intertidal and supratidal lithofacies such as Amherstburg and Lucas formations [33].

3. Materials and Methods

Core samples were collected from the Bruce Nuclear site, the proposed site for a deep geological repository for low- and intermediate-level nuclear waste, from two boreholes DGR1 and DGR8 (Figure 2) [17]. The samples were chosen based on dolomitized and fractured rocks in hand specimens. A summary of sample identification with relative age and depth is reported in Figure 3 and

Table 1. Lists of samples collected from the boreholes DGR1 and DGR8.

Cored Sample	Sample Code	Formation	Age	Depth (m)
DGR1-CR1	1-1	Lucas	Devonian	23.5
DGR1-CR2-(1)	1-2	Lucas	Devonian	26.25
DGR1-CR2-(2)	1-3	Lucas	Devonian	26.95
DGR1-CR2-(3)	1-4	Lucas	Devonian	27.75
DGR1-CR3-(1)	1-5	Amherstburg	Devonian	31.35
DGR1-CR3-(2)	1-6	Amherstburg	Devonian	31.85
DGR1-CR5	1-7	Amherstburg	Devonian	35.95
DGR1-CR8	1-8	Amherstburg	Devonian	42.58
DGR1-CR9	1-9	Amherstburg	Devonian	46.4
DGR1-CR10-(1)	1-10	Amherstburg	Devonian	48.65
DGR1-CR10-(2)	1-11	Amherstburg	Devonian	49.7
DGR1-CR11	1-12	Amherstburg	Devonian	51
DGR1-CR30-(1)	1-13	Bois Blanc	Devonian	104.05
DGR1-CR30-(2)	1-14	Bois Blanc	Devonian	105.08
DGR1-CR41	1-15	Bass Island	Silurian	128.93
DGR1-CR44	1-16	Bass Island	Silurian	133.03
DGR1-CR49	1-17	Bass Island	Silurian	142.28
DGR1-CR65	1-18	Salina—G-Unit	Silurian	176.13
DGR1-CR66	1-19	Salina—G-Unit	Silurian	178
DGR1-CR84	1-20	Salina—E-Unit	Silurian	229.32
DGR1-CR104	1-21	Salina—B-Unit (Carbonate)	Silurian	289.85
DGR1-CR105	1-22	Salina—A2-Unit (Carbonate)	Silurian	294.14
DGR1-CR116-(1)	1-23	Salina—A1-Unit (Carbonate)	Silurian	326.92
DGR1-CR116-(2)	1-24	Salina—A1-Unit (Carbonate)	Silurian	328.17
DGR1-CR117	1-25	Salina—A1-Unit (Carbonate)	Silurian	329.97
DGR1-CR118	1-26	Salina—A1-Unit (Carbonate)	Silurian	333.48
DGR1-CR119	1-27	Salina—A1-Unit (Carbonate)	Silurian	337.32
DGR1-CR121	1-28	Salina—A1-Unit (Carbonate)	Silurian	342.27
DGR1-CR122	1-29	Salina—A1-Unit (Carbonate)	Silurian	344.42
DGR1-CR130	1-30	Salina—A0-Unit	Silurian	371.27
DGR1-CR133	1-31	Guelph	Silurian	378.1
DGR1-CR143	1-32	Fossil Hill	Silurian	410.1
DGR1-CR154	1-33	Manitoulin	Silurian	440.92
DGR8-CR10	8-1	Lucas	Devonian	22.71
DGR8-CR17	8-2	Lucas	Devonian	43.46
DGR8-CR18	8-3	Amherstburg	Devonian	48.61
DGR8-CR22	8-4	Amherstburg	Devonian	59.06
DGR8-CR64	8-5	Salina—G-Unit	Silurian	182.06
DGR8-CR90	8-6	Salina—E-Unit	Silurian	238.91
DGR8-CR100	8-7	Salina—C-Unit	Silurian	265.46
DGR8-CR110	8-8	Salina—B-Unit (Carbonate)	Silurian	295.91
DGR8-CR116	8-9	Salina—A2-Unit (Carbonate)	Silurian	313.31
DGR8-CR122	8-10	Salina—A1-Unit (Carbonate)	Silurian	332.26
DGR8-CR137	8-11	Salina—A0-Unit	Silurian	377.21
DGR8-CR137	8-12	Salina—A0-Unit	Silurian	378.75
DGR8-CR139	8-13	Guelph	Silurian	382.4

.

The selected core samples were photographed before making thin sections and wafers for fluid inclusions analysis. Petrographic examination of thin sections (n = 46) was performed using a standard petrographic microscope to provide lithofacies types, textural relationships and diagenetic fabric of the studied samples, to establish the paragenetic sequences. Subsequently, all thin sections were examined using cathodoluminescence microscopy (CL) to obtain data regarding the chemistry of the fluids, mineral zonation and diagenetic textures of host rock and fracture filling cement. The equipment used to carry out the cathodoluminescence analysis was a Technosyn Model 8200 MKII with a 12–15 Kv beam and a current intensity of 0.42–0.43 mA (Cambridge Image Technology Ltd., Hatfield, Hertfordshire, UK). The thin sections were also stained using a mixture of Alizarin Red S and potassium ferricyanide dissolved in a dilute solution of hydrochloric acid [35,36] to distinguish between ferroan or non-ferroan phases of calcite and dolomite.

A total of forty-three (Silurian) and thirty-three (Devonian) samples were micro-sampled using a microscope-mounted drill assembly for oxygen and carbon isotopic composition from Silurian (n = 18 and n = 25) and Devonian (n = 12 and n = 21) specimens for calcitic and dolomitic components, respectively. Each sample was reacted in vacuo with 100% pure phosphoric acid four 4 h at 50 °C and 25 °C for dolomite and calcite respectively. The CO₂ produced during the reaction by the different diagenetic mineral phases was extracted using the method proposed by Al-Aasm et al. [37] and measured employing a Delta-Plus mass spectrometer (Thermo Electron Corporation, Bremen, Germany). The delta values of oxygen and carbon isotopes have been reported in per mil (‰) relative to the PeeDee Belemnite (VPDB) standard and corrected for phosphoric acid fractionation. Precision was better than 0.05‰ for both ¹³C and ¹⁸O. Carbon dioxide extraction and isotopic composition measurements were performed at the Stable Isotopes Laboratory, University of Windsor (Windsor, ON, Canada).

Around 10–50 mg of powdered samples (n = 22) were extracted to determine the Sr isotopic composition using a microscope-drill assembly, to be subsequently reacted with 2.5 N HCl for 24 h at room temperature. To avoid Strontium contamination from pore salts produced by drying the samples, they were washed twice using Milli-Q water. To measure the strontium isotope ratio in each sample, an automated Finnigan MAT 261TM mass spectrometer was employed, performing all the analysis in the static multicollector mode with Re filaments and normalizing ⁸⁷Sr/⁸⁶Sr ratios to ⁸⁷Sr/⁸⁶Sr = 8.375209. The mean standard error of the static multicollector was 0.00003 for NBS-987. These analyses were done at the University of Bochum, Bochum, Germany.

A thorough petrographic investigation of doubly polished thin sections was performed to identify and classify fluid inclusions trapped in diagenetic minerals. Measured ice-melting temperatures (Tmice) are reported as wt.% NaCl eq. using the relationship of Hall et al. [38] and Bodnar [39]. Detailed petrography was performed to determine the type of inclusions (primary or secondary/pseudosecondary) following the criteria proposed by Roedder [40]. For this study, attention was focused on primary liquid-vapor inclusion assemblages only. The microthermometric measurements were performed at the University of Windsor (Windsor, ON, Canada) employing a Linkam TH600 heating-freezing stage, coupled with the Olympus BX60. The device calibration was achieved using synthetic pure water and CO₂ inclusions as standard. Microthermometric analyses were completed on 126 primary fluid inclusions in selected carbonate samples from the Silurian (n = 85) and Devonian (n = 41) successions including pervasive replacive medium crystalline dolomite (RD2; n = 22 and n = 17 for Silurian and Devonian samples, respectively), saddle dolomite (SD; n = 31; observed in Silurian samples only) and late fracture-filling blocky calcite cement (BKC; n = 32 and n = 24 from Silurian and Devonian samples, respectively). Homogenization temperatures (T_h) and ice-melting temperatures (Tmice) were measured with a precision of ±1 °C and ±0.1 °C, respectively. Heating measurements were performed before freezing to avoid possible stretching of the inclusions during freezing [41].

Major, minor, trace and rare earth elements (REE) analyses were performed using inductively coupled plasma spectroscopy (ICP-MS) technique. Around 50–100 mg of powdered carbonate samples from different calcite and dolomite phases were digested utilizing 5% v/v acetic acid. Measurements were carried employing an Agilent 8800 inductively coupled mass spectrometer at the University of Waterloo (Waterloo, ON, Canada). All results are reported in part per million (ppm). The REEs’ results obtained from ICP-MS analysis were normalized to PAAS (Post-Archean Australian Shales [42]). Rare-earth element anomalies such as Ce_SN [(Ce/Ce*)_SN = Ce_SN/(0.5La_SN + 0.5Pr_SN)], La_SN [(Pr/Pr*)_SN = Pr_SN/0.5Ce_SN + 0.5Nd_SN)], Eu_SN [(Eu/Eu*)_SN = Eu_SN/(0.5Sm_SN + 0.5Gd_SN)], and Gd_SN [(Gd/Gd*)_SN = Gd_SN/(0.33Sm_SN + 0.67Tb_SN)] were calculated employing the equations proposed by Bau and Dulski [43]. The proportions of LREE (La-Nd) over HREE (Ho-Lu) were calculated via (La/Yb)_SN ratios as proposed by Kucera et al. [44]. The proportions of MREE (Sm-Dy) was calculated using (Sm/La)_SN and (Sm/Yb)_SN ratios [45] over LREE and HREE, respectively.

4. Results

4.1. Petrography of Silurian Formations

The diagenetic history of the Silurian formations includes processes such as dolomitization mechanical and chemical compaction, fracturing, dissolution, silicification, and calcite and evaporite cementation. The diagenetic processes above are described in Tortola [46], on which this paper is largely based.

4.1.1. Dolomitization

Four types of dolomite occur in the Silurian formations (Figure 4) and are classified following the criteria proposed by Sibley and Gregg [47]. These include including three types of replacive matrix dolomite and one of dolomite cement. These are a pervasive replacive micro to fine crystalline (<50 μm) euhedral to subhedral dolomite matrix (RD1) (Figure 4A); a pervasive replacive medium (>50–100 μm) euhedral crystalline dolomite matrix (RD2) (Figure 4B), which represent the main constituents of the host rock; and a less abundant selective replacive medium (>50–100 μm) euhedral crystalline dolomite matrix (RD3) only observed along dissolution seams in partially dolomitized limestones, usually characterized by cloudy-dark cores and clear rims (Figure 4C). All dolomite types are non-ferroan and show dull red to non-luminescent (RD1 and RD2) and red to bright (RD3) under the CL (Figure 4D,E). In some cases, RD2 shows undulose extinction similar to saddle dolomite (e.g., [5,48]) and is characterized by cloudy non-ferroan cores followed by later clear and iron-rich rims (Figure 4B,F). In addition to these types, a coarse crystalline (>500 μm), ferroan saddle dolomite cement (SD; Figure 4G) filling fractures and vugs are observed, commonly predating late calcite cement (Figure 4E,H) and occasionally evaporite minerals (Figure 4I). Typically, SD is fluid inclusion-rich, zoned with cloudy cores and clear rims, and characterized by curved crystal boundaries.

4.1.2. Fracturing

Three types of fractures are present in the studied formations. Thin randomly oriented hairline fractures (<0.5 cm) filled with drusy calcite cement (DC) represent the first generation of fractures (Figure 5A). Late wider subhorizontal and subvertical fractures (0.5–1 cm), both commonly occluded by saddle dolomite, late blocky calcite and evaporite cement representing the second and third generations, respectively (Figure 5B,C). Cross-cutting relationship of the last two generations of fractures shows that cementation took place after the formation of both fractures.

4.1.3. Calcite Cementation

Early and late-stage calcite cement (Figure 6) have been distinguished including from early to late: isopachous, syntaxial overgrowth, drusy, and blocky calcite. Undolomitized calcite matrix (MC) from precursor limestone was also recognized (Figure 4C and Figure 5A). Isopachous calcite cement (ISC) is characterized by cement rims growing around coated grains. It exhibits microcrystalline, bladed texture (Figure 6A) with non-ferroan, dully-red luminescence (Figure 4D) and crystal size ranging from 50 to 100 μm. Syntaxial calcite overgrowth cement (SXC) forms ferroan, bright-luminescent crystals ranging in size from 100 μm up to >500 μm, usually around echinoderm fragments (Figure 6B). Void-filling and pore-lining drusy calcite cement (DC) was mainly observed in intergranular and intraskeletal pores, molds and fractures. Common textural features show equant to elongate, anhedral to subhedral crystals ranging in size between 75–250 μm with dimension increasing towards the center of the pore space (Figure 6C). In most cases, DC is ferroan and exhibits dully-red luminescence under CL (Figure 6D).

A late fractures and void-filling blocky calcite cement (BKC) consisting of coarse non-ferroan to ferroan crystal (Figure 6E) ranging in size from 200 μm to >500 μm, shows zoned red to bright luminescence under CL (Figure 6F) and commonly postdate saddle dolomite cement.

In terms of paragenesis, early calcite cement (ISC, SXC and DC) predated RD1, RD2 and RD3 and predated fracture- and pore-filling saddle dolomite which is postdated by late blocky calcite (BKC) and evaporite (GY and ANH) cement.

4.1.4. Evaporite Cementation

Pore and fracture-filling fibrous gypsum (GY)/anhydrite (ANH) cements in the Silurian formations range in size from 50–150 μm (Figure 4I and Figure 5C,D). They are the latest stage diagenetic mineral phases. Both gypsum and anhydrite are only present in samples from the Salina Group Formation, commonly postdating fracture-filling blocky calcite and saddle dolomite cement (Figure 4I).

4.2. Petrography of Devonian Formations

4.2.1. Dolomitization

Three types of dolomite occur in the Devonian formations (Figure 7), which are characterized by the presence of pervasive replacive micro to fine (<50 μm), euhedral to subhedral, crystalline dolomite matrix (RD1) (Figure 7A,B), a pervasive replacive medium (>50–100 μm) euhedral crystalline dolomite matrix (RD2) (Figure 7B–D), which represent the main constituents of the host dolostone, and a less abundant selective replacive medium (>50–100 μm) euhedral crystalline dolomite matrix (RD3 commonly associated with dissolution seams), and usually characterized by cloudy-dark core and clear rims (Figure 7E). All three types of dolomite are non-ferroan and show non-luminescent to dull red (RD1 and RD2) and red to bright red (RD3) under the CL (Figure 7B,F).

A schematic summary of the main features with relative nomenclatures of the main carbonate minerals observed and described for both age groups is presented in

Table 2. Summary of the main petrographic characteristics of the carbonate minerals observed in Silurian and Devonian samples.

Age	Mineral	Description	Texture	Size	Luminescence	Ferroan/Non-Ferroan
Silurian	RD1	Pervasive replacive micro to fine crystalline dolomite matrix	Non-planar to planar-s	<50 μm	Dully red/bright	Non-ferroan
Silurian	RD2	Pervasive replacive medium crystalline dolomite matrix	Planar-e/planar-s	>50 μm up to 150 μm	Dully red/non-luminescent	Non-ferroan
Silurian	RD3	Selective replacive medium crystalline dolomite matrix (commonly associated with dissolution seams)	Planar-e	>50 μm up to 150 μm	Dull/red	Non-ferroan
Silurian	SD	Coarse crystalline saddle dolomite cement filling fractures and vugs	Curved crystal faces and cleavage planes and undulose extinction	>500 μm	Dull/non-luminescent	Ferroan
Silurian	ISC	Isopachous calcite cement characterized by cement rims growing around coated grains	bladed	50–100 μm	Dully red	Non-ferroan
Silurian	SXC	Syntaxial calcite overgrowth cement usually around echinoderm fragments	-	100 μm to >500 μm	Bright red	Ferroan
Silurian	DC	Void-filling and pore-lining cement in intergranular and intraskeletal pores, molds and fractures	Equant to elongate, anhedral to subhedral	75–250 μm Size increase towards the center	Dull red	Ferroan
Silurian	BKC	Blocky calcite cement consisting of coarse-grained crystals without a preferred orientation mainly filling fractures and voids	-	200 μm to >500 μm	Red/bright orange zoned	Non-ferroan/ferroan
Devonian	RD1	Pervasive replacive micro to fine crystalline dolomite matrix	Non-planar to planar-s	<50 μm	Dull/red	Non-ferroan
Devonian	RD2	Pervasive replacive medium crystalline dolomite matrix	Planar-e/ planar-s	>50 μm up to 100 μm	Dull/non-luminescent	Non-ferroan
Devonian	RD3	Selective replacive medium crystalline dolomite matrix (commonly associated with dissolution seams)	Planar-e	>50 μm up to 100 μm	Red/bright	Non-ferroan
Devonian	SXC	Syntaxial calcite overgrowth cement usually around echinoderm fragments	-	100 μm to >500 μm	Non-luminescent	Non-ferroan
Devonian	DTC	Dogtooth calcite cement characterized by sharply pointed acute crystals growing normal to the substrate (mainly skeletal grains)	Elongate scalenohedral or rhombohedral	50 μm	Red/bright mostly zoned	Non-ferroan
Devonian	DC	Void-filling and pore-lining cement in intergranular and intraskeletal pores, molds and fractures	Equant to elongate, anhedral to subhedral	75–250 μm Size increase towards the center	Dull red to bright, zoned in most cases	Non-ferroan
Devonian	BKC	Blocky calcite cement consisting of coarse-grained crystals without a preferred orientation mainly filling fractures and voids	-	200 μm to >500 μm	Red/bright zoned	Non-ferroan

.

4.2.2. Fracturing

The fractures observed in the Devonian formations can be divided in two types. The first is represented by sub-horizontal fractures (0.5–1 cm) filled by blocky calcite cement (Figure 8A). The second type is represented by sub-vertical fractures (0.1–0.5 cm) commonly filled by blocky calcite (Figure 8A,B) crosscutting chemical compactional features (Figure 8C,D).

4.2.3. Calcite Cementation

Four types of calcite cement, ranging from early to late-stage, (Figure 9) have been distinguished, including syntaxial overgrowth, dogtooth, drusy and blocky calcite. Undolomitized calcite matrix (MC) from the precursor limestone was also observed in some specimens (Figure 8D).

Syntaxial calcite overgrowth cement (SXC) forms non-ferroan, non-luminescent crystals, usually around echinoderm fragments, ranging in size from 100 μm up to >500 μm (Figure 9A).

Dogtooth calcite cement (DTC), is characterized by non-ferroan, non-luminescent, sharply pointed acute crystals (50 μm) growing normal to the substrate. It is mainly observed lining intraskeletal chambers, and commonly presenting textures from elongate scalenohedral to rhombohedral (Figure 9B).

Pore-lining and void-filling drusy calcite cement (DC) is mainly observed in intergranular and intraskeletal pores, molds and fractures (Figure 9C–E). Common textural features show equant to elongate, in some cases anhedral to subhedral crystals ranging in size between 75–250 μm with dimension increasing towards the center of the pore space (Figure 9D). In most cases, DC is non-ferroan (Figure 9E) and shows zoned, dull-red to bright orange luminescence (Figure 9C).

A late fracture and void-filling blocky calcite cement (BKC) consisting of the coarse-grained, non-ferroan crystal (Figure 9E) without preferred orientation, ranges in size from 200 μm to >500 μm, shows zoned red to bright orange luminescence under CL (Figure 9F).

Paragenetically, early calcite cement (SXC, DTC and DC) predate replacive dolomite matrix RD1, RD2 and RD3, and predate the late fracture- and pore-filling blocky calcite (BKC) cement.

4.3. Geochemistry of Silurian and Devonian Formations

4.3.1. Oxygen and Carbon Stable Isotopes

The isotopic composition of RD1 from the two age groups shows more negative δ¹⁸O and δ¹³C average values in the Silurian samples than in the Devonian samples. δ¹⁸O isotopic composition of Silurian and Devonian pervasive replacive micro to fine crystalline dolomite matrix (RD1) range from −7.83 to −4.46‰ VPDB (average: −6.43‰ ± 1.1‰) and from −7.01 to −4.46‰ VPDB (average: −5.95‰ ± 0.71‰), respectively. δ¹³C isotopic values in RD1 samples range from −2.73 to 3.78‰ VPDB (average: 0.18‰ ± 1.9‰) for Silurian formations and from 2.01 to 4.28‰ VPDB (average: 3.05‰ ± 0.88‰) for Devonian formations (n = 20; Figure 10;

Table 3. Oxygen and carbon stable isotopes, Sr isotopes results from dolomite and calcite samples.

Sample ID	Age	Phase	δ13CVPDB (‰)	δ18OVPDB (‰)	87Sr/86Sr
1-15a	Silurian	RD1	−0.47	−4.46
1-16a	Silurian	RD1	−0.27	−5.35
1-17a	Silurian	RD1	1.86	−4.49	0.708705
1-18a	Silurian	RD1	−2.73	−7.65	0.708877
1-20a	Silurian	RD1	−0.59	−7.06
1-21a	Silurian	RD1	0.22	−6.44
1-31a	Silurian	RD1	3.78	−7.15
8-6a	Silurian	RD1	−1.4	−7.36	0.708919
8-5a	Silurian	RD1	−0.63	−6.99
8-8a	Silurian	RD1	−1.16	−6.16
8-9a	Silurian	RD1	−0.19	−6.22
8-13a	Silurian	RD1	3.72	−7.83
1-16b	Silurian	RD2	−0.32	−3.85
1-24a	Silurian	RD2	1.51	−6.79	0.708469
1-23b	Silurian	RD2	1.44	−7.25
1-31b	Silurian	RD2	3.58	−7.96	0.708587
8-11a	Silurian	RD2	3.18	−5.42
8-10b	Silurian	RD2	1.82	−7.23
8-13b	Silurian	RD2	3.84	−7.86
1-30a	Silurian	RD3	4.02	−7.53	0.708905
1-33b	Silurian	RD3	0.65	−5.91
8-12	Silurian	RD3	3.31	−7.41	0.708743
1-31c	Silurian	SD	3.69	−7.72	0.708648
8-13c	Silurian	SD	0.10	−9.65
1-28a	Silurian	SD	1.27	−6.13	0.708516
1-24b	Silurian	MC	1.32	−5.55
1-27b	Silurian	MC	0.14	−8.16
1-30b	Silurian	MC	3.53	−8.09
1-33c	Silurian	MC	0.21	−4.73
8-11b	Silurian	MC	2.89	−5.35
1-23a	Silurian	ISC	1.37	−6.76
8-10a	Silurian	ISC	1.27	−7.77
1-32	Silurian	SXC	0.12	−4.81
1-33a	Silurian	SXC	−0.21	−5.54
1-17b	Silurian	DC	−0.88	−8.85	0.708154
1-25	Silurian	BKC	0.02	−4.54
1-26a	Silurian	BKC	0.00	−6.78
1-27a	Silurian	BKC	0.03	−7.52	0.708383
1-28b	Silurian	BKC	0.06	−7.51
1-31d	Silurian	BKC	3.17	−6.41
8-5b	Silurian	BKC	−3.75	−9.19	0.708054
8-9c	Silurian	BKC	−1.06	−10.19
8-13d	Silurian	BKC	3.10	−8.09
8-9d	Silurian	GY	-	-	0.708486
1-29a	Silurian	GY	-	-	0.708434
1-1a	Devonian	RD1	2.65	−6.03	0.708103
1-1b	Devonian	RD1	2.17	−6.22
1-3-1a	Devonian	RD1	3.23	−5.72
1-5a	Devonian	RD1	4.02	−6.14
1-5b	Devonian	RD1	4.28	−6.11
1-7a	Devonian	RD1	3.71	−5.88
8-1a	Devonian	RD1	2.01	−4.46	0.707884
8-4a	Devonian	RD1	2.33	−7.01
1-3-2b	Devonian	RD2	3.32	−4.75	0.708089
1-4a	Devonian	RD2	3.93	−5.94
1-7b	Devonian	RD2	3.48	−5.74
1-14a	Devonian	RD2	0.99	−6.47
8-4b	Devonian	RD2	2.03	−7.66	0.708920
1-2a	Devonian	RD3	3.47	−5.67
1-3-2c	Devonian	RD3	3.62	−3.91
1-6a	Devonian	RD3	4.63	−5.97	0.708146
1-10a	Devonian	RD3	2.60	−5.31
1-13b	Devonian	RD3	1.09	−5.04
8-2a	Devonian	RD3	3.81	−5.41
8-3a	Devonian	RD3	2.99	−5.00
8-4c	Devonian	RD3	2.45	−6.20	0.708509
1-2b	Devonian	MC	2.52	−5.87
1-4b	Devonian	MC	2.81	−5.77
1-6b	Devonian	MC	4.12	−4.53
1-8b	Devonian	MC	1.92	−6.47
8-2b	Devonian	MC	2.83	−6.59
8-3b	Devonian	MC	1.99	−6.51
1-11a	Devonian	SXC	0.97	−4.44
1-12a	Devonian	SXC	1.55	−4.42
1-13a	Devonian	DC	−0.59	−6.22	0.708051
1-14b	Devonian	DC	−1.15	−6.49
1-2c	Devonian	BKC	−1.55	−8.01
8-1b	Devonian	BKC	−2.40	−8.61	0.707982

).

The isotopic compositions of RD2 from Silurian formations show more negative δ¹⁸O and δ¹³C average values than values of RD2 from Devonian samples. δ¹⁸O isotopic composition of Silurian and Devonian pervasive replacive medium crystalline dolomite matrix (RD2) range from −7.96 to −3.85‰ VPDB (average: −6.62‰ ± 1.37‰) and from −7.66 to −4.75‰ VPDB (average: −6.11‰ ± 1.07‰), respectively. δ¹³C isotopic concentrations in RD2 samples range from −0.32 to 3.84‰ VPDB (average: 2.15‰ ± 1.37‰) for Silurian formations and from 0.99 to 3.93 ‰ VPDB (average: 2.75‰ ± 1.21‰) for Devonian successions (n = 12; Figure 10; Table 3).

RD3 from Silurian samples is characterized by more negative δ¹⁸O average values but comparable δ¹³C average values Devonian samples. δ¹⁸O isotopic composition of Silurian and Devonian selective replacive medium crystalline dolomite matrix (RD3) range from −7.53 to -5.91‰ VPDB (average: −6.95‰ ± 0.9‰) and from −6.20 to −3.91‰ VPDB (average: −5.31‰ ± 0.71‰), respectively. δ¹³C isotopic concentrations in RD3 samples range from 0.65 to 4.02‰ VPDB (average: 2.66‰ ± 1.77‰) for Silurian formations and from 1.09 to 4.63‰ VPDB (average: 3.08‰ ± 1.07‰) for Devonian successions (n = 11; Figure 10; Table 3).

The δ¹⁸O and δ¹³C isotopic values of Silurian Saddle dolomite cement (SD) range from −9.65 to −6.13‰ VPDB (average: −7.84‰ ± 1.76‰) and from 0.10 to 3.69‰ VPDB (average: 1.69‰ ± 1.83‰), respectively (n = 3; Figure 10 and Table 3).

Early calcite shows more negative δ¹⁸O and more positive δ¹³C average values in Devonian formations compared with Silurian samples. Silurian and Devonian early calcite matrix and cement (MC, ISC, SXC, DTC and DC) show δ¹⁸O isotopic composition ranging from −8.85 to −4.73‰ VPDB (average: –6.56‰ ± 1.47‰) and from −6.59 to −4.42‰ VPDB (average: −5.73‰ ± 0.87‰), respectively. δ¹³C isotopic values in early calcite samples (MC, ISC, SXC, DTC and DC) range from −0.88 to 3.53‰ VPDB (average: 0.97‰ ± 1.32‰) for Silurian formations and from −1.15 to 4.12‰ VPDB (average: 1.70‰ ± 1.52‰) for Devonian successions (n = 20; Figure 11; Table 3).

Late calcite cement (BKC) shows more negative δ¹⁸O and δ¹³C average values in Devonian formations compared with the Silurian samples. The δ¹⁸O isotopic composition of Silurian and Devonian late fracture-filling calcite cement (BKC) range from −10.19 to −4.54‰ VPDB (average: −7.53‰ ± 1.62‰) and from −8.61 to −8.01‰ VPDB (average: −8.31‰ ± 0.3‰), respectively. δ¹³C isotopic concentrations in BKC samples range from −3.75 to 3.17‰ VPDB (average: 0.20‰ ± 2.08‰) for Silurian formations and from −2.40 to −1.55‰ VPDB (average: −1.98‰ ± 0.43‰) for Devonian successions (n = 10; Figure 11; Table 3).

4.3.2. Strontium Isotopes

Due to the mixed nature of the samples, only one pure sample of drusy calcite cement (DC), for each age group, were microsampled avoiding then contamination with other carbonate minerals. The Sr isotopic values of DC were 0.708154 and 0.708051 for the Silurian and Devonian samples, respectively (Figure 12). The Silurian late calcite cement (BKC) show ⁸⁷Sr/⁸⁶Sr isotopic ratio values ranging from 0.708054 to 0.708383 (n = 2) and 0.707982 for Devonian BKC (n = 1). ⁸⁷Sr/⁸⁶Sr ratios of matrix dolomite samples (RD1, RD2 and RD3) from the Silurian (n = 7) and Devonian (n = 6) carbonate successions range from 0.708469 to 0.708919 and 0.707884 to 0.708920, respectively. Saddle dolomite sampled from Silurian formations has Sr isotopic values ranging from 0.708516 to 0.708648 (n = 2; Figure 12). Gypsum samples from Silurian formations show Sr isotopic values ranging from 0.708434 to 0.708486.

⁸⁷Sr/⁸⁶Sr isotopic ratios for dolomite and calcite by age are presented in Table 3.

4.3.3. Fluid Inclusions Microthermometry

Fluid inclusions measurements were focused only on dolomite and blocky calcite cement. Due to the small size of crystal and/or fluid inclusions, microthermometric measurements in mineral phases such as RD1and RD3 were not achievable in both age groups. The melting (Tm) and homogenization temperatures (T_h) were measured from two-phase, liquid-rich primary fluid inclusions which ranged in size from 1 µm up to 20 µm in diameter with variable shapes from irregular to nearly circular (Figure 13). In some cases, measurements of the melting temperatures for fluid inclusions hosted in RD2 were not possible due to their small size in both Silurian and Devonian successions (measured n = 9 and n = 7 for Silurian and Devonian samples, respectively). Hydrocarbon fluid inclusions were not identified in the selected samples from the studied formations.

Fluid inclusions hosted in RD2 from Silurian samples show higher T_h and salinity compared to fluid inclusions hosted in RD2 from Silurian formations. Microthermometric results of pervasive, replacive medium crystalline dolomite matrix (RD2) vary from T_h: 49.7 to 134.1 °C (average: 89.1 ± 20.9), 22.03 to 25.2 wt.% NaCl eq. (average: 23.3 ± 1.1) and T_h: 69.9 to 102.3 °C (average: 82.7 ± 9.8), 18.9 to 21.8 wt.% NaCl eq. (average: 20.8 ± 0.9) for the Silurian and Devonian samples, respectively.

Saddle dolomite (SD) hosted in Silurian successions shows values ranging from T_h: 101.2 to 193.4 °C (average: 124.8 ± 20.5), 25.97 to 32.6 wt.% NaCl eq. (average: 28.5 ± 1.8).

Fluid inclusions hosted in BKC from Silurian samples show lower T_h and higher salinity compared to fluid inclusions hosted in BKC from Silurian formations. In late calcite cement (BKC), measured values range from T_h: 70.2 to 194.3 °C (average: 120.9 ± 35.4), 21.4 to 32.3 wt.% NaCl eq. (average: 27.6 ± 3.2) and T_h: 146.20 to 237.9 °C (average: 194.87 ± 26.9), 15.17 to 23.95 wt.% NaCl eq. (average: 20.21 ± 2.31) for the mentioned age groups, respectively.

The summaries of the fluid inclusion results are presented in Figure 14, Figure 15 and Figure 16 and

Table 4. Fluid inclusions results.

Sample	Age	Host Mineral	Occurrence	Size (µm)	Th (°C)	Tmice (°C)	Salinity
1-31-1	Silurian	RD2	Cluster	2	54.2
1-31-2	Silurian	RD2	Cluster	3	55.1
1-31-3	Silurian	RD2	Cluster	1	57.5
1-31-4	Silurian	RD2	Cluster	7	100.4	−20.3	22.58
1-31-5	Silurian	RD2	Cluster	1	49.7
1-31-6	Silurian	RD2	Cluster	6	118.9	−22.3	23.89
1-31-7	Silurian	RD2	Cluster	5	92.6	−24.4	25.21
1-31-8	Silurian	RD2	Cluster	5	97.5	−21.4	23.31
1-31-9	Silurian	RD2	Cluster	4	88.9
1-31-10	Silurian	RD2	Isolated	5	85.9	−19.5	22.03
1-31-11	Silurian	RD2	Isolated	2	71.4
1-31-12	Silurian	RD2	Isolated	10	134.1	−23.6	24.71
1-31-13	Silurian	RD2	Crystal growth	2	91.5
1-31-14	Silurian	RD2	Crystal growth	1	92.8
1-31-15	Silurian	RD2	Crystal growth	1	93.6
1-31-16	Silurian	RD2	Crystal growth	1	84.7
1-31-17	Silurian	RD2	Crystal growth	1	89.6
1-31-18	Silurian	RD2	Crystal growth	4	97.1	−21.2	23.18
1-31-19	Silurian	RD2	Crystal growth	2	96.4
1-31-20	Silurian	RD2	Crystal growth	1	99.8
1-31-21	Silurian	RD2	Crystal growth	6	97.3	−20.2	22.51
8-13-31	Silurian	RD2	Crystal growth	3	111.4	−19.9	22.31
1-31-22	Silurian	BKC	Isolated	6	159.9
1-31-23	Silurian	BKC	Isolated	5	184.7	−25.3	25.76
8-13-1	Silurian	BKC	Short trail	7	119.3	−32.7	30.42
8-13-2	Silurian	BKC	Short trail	8	118.1	−33.5	30.97
8-13-3	Silurian	BKC	Short trail	6	109.3	−29.4	28.28
8-13-4	Silurian	BKC	Cluster	4	106.4
8-13-5	Silurian	BKC	Cluster	4	145.4
8-13-6	Silurian	BKC	Cluster	2	79.4
8-13-7	Silurian	BKC	Cluster	2	95.7
8-13-8	Silurian	BKC	Cluster	3	130.1
8-13-9	Silurian	BKC	Cluster	2	149.7
8-13-10	Silurian	BKC	Cluster	2	172.3
8-13-11	Silurian	BKC	Isolated	10	115.9	−31.8	29.82
8-13-12	Silurian	BKC	Crystal growth	13	81.8	−28.7	27.85
8-13-13	Silurian	BKC	Crystal growth	18	96.2	−32.6	30.35
8-13-14	Silurian	BKC	Crystal growth	15	93.3	−28.2	27.54
8-13-15	Silurian	BKC	Crystal growth	6	89.4
8-13-16	Silurian	BKC	Crystal growth	5	81.6
8-13-17	Silurian	BKC	Isolated	8	73.4	−26.9	26.74
8-13-18	Silurian	BKC	Isolated	6	84.7	−27.1	26.86
8-13-19	Silurian	BKC	Isolated	2	104.7
8-13-20	Silurian	BKC	Isolated	3	103.5
8-13-21	Silurian	BKC	Isolated	2	122.6
8-13-22	Silurian	BKC	Crystal growth	10	113.8	−35.4	32.33
1-28-6	Silurian	BKC	Isolated	12	115.2	−33.1	30.69
1-28-7	Silurian	BKC	Cluster	3	123.3
1-28-8	Silurian	BKC	Cluster	4	189	−18.6	21.40
1-28-9	Silurian	BKC	Cluster	4	139.7	−19.1	21.75
1-28-10	Silurian	BKC	Isolated	6	182.8
1-28-11	Silurian	BKC	Isolated	6	194.3
1-28-12	Silurian	BKC	Isolated	4	124.7	−22.8	24.21
1-28-13	Silurian	BKC	Isolated	10	70.2	−25.9	26.13
8-13-19	Silurian	SD	Crystal growth	5	122.6
8-13-20	Silurian	SD	Crystal growth	5	123.4
8-13-21	Silurian	SD	Crystal growth	3	142.7
8-13-22	Silurian	SD	Crystal growth	4	193.4
8-13-23	Silurian	SD	Scattered	5	121.8	−35.8	32.63
8-13-24	Silurian	SD	Scattered	6	125.9	−33.2	30.76
8-13-25	Silurian	SD	Scattered	2	117.16
8-13-26	Silurian	SD	Scattered	3	145.1
8-13-27	Silurian	SD	Scattered	2	115.5
8-13-28	Silurian	SD	Scattered	2	118.1
8-13-29	Silurian	SD	Scattered	4	119.5	−29.1	28.09
8-13-30	Silurian	SD	Isolated	10	102.3	−24.5	25.27
8-13-31	Silurian	SD	Isolated	15	110.2	−28.6	27.78
8-13-32	Silurian	SD	Crystal growth	6	112.5	−27.1	26.86
8-13-33	Silurian	SD	Crystal growth	7	123.2	−30.9	29.23
8-13-34	Silurian	SD	Crystal growth	11	105.2	−28.2	27.54
8-13-35	Silurian	SD	Crystal growth	5	115.9	−31.6	29.69
8-13-36	Silurian	SD	Crystal growth	4	108.9
8-13-37	Silurian	SD	Crystal growth	10	117.3	−28.6	27.78
8-13-38	Silurian	SD	Crystal growth	9	103.4	−27.9	27.35
8-13-39	Silurian	SD	Crystal growth	6	101.2	−31.1	29.36
8-13-40	Silurian	SD	Crystal growth	13	127.5	−29.9	28.60
8-13-41	Silurian	SD	Crystal growth	3	125.9	−32.8	30.49
8-13-42	Silurian	SD	Crystal growth	4	132.7
1-28-1	Silurian	SD	Crystal growth	6	112.3	−26.8	26.68
1-28-2	Silurian	SD	Crystal growth	8	120.5	−27.1	26.86
1-28-3	Silurian	SD	Crystal growth	5	126.6	−27.8	27.29
1-28-4	Silurian	SD	Crystal growth	3	130.3
1-28-5	Silurian	SD	Crystal growth	3	136.4
1-28-6	Silurian	SD	Crystal growth	5	125.8	−28.4	27.66
1-28-7	Silurian	SD	Crystal growth	7	186.6	−33.7	31.11
8-4-1.	Devonian	RD2	Crystal growth	3	101.1
8-4-2.	Devonian	RD2	Isolated	5	87.5	−18.6	21.40
8-4-2.	Devonian	RD2	Cluster	2	85.3
8-4-2.	Devonian	RD2	Cluster	4	79.6	−17.9	20.89
8-4-2.	Devonian	RD2	Cluster	1	88.1
8-4-2.	Devonian	RD2	Cluster	6	77.8	−15.4	18.96
8-4-2.	Devonian	RD2	Cluster	4	84.3	−17.8	20.82
8-4-2.	Devonian	RD2	Cluster	2	75.4
1-3-2-1	Devonian	RD2	Isolated	5	76.9	−19.2	21.82
1-3-2-2	Devonian	RD2	Isolated	3	85.2
1-3-2-3	Devonian	RD2	Cluster	4	93.3	−17.4	20.52
1-3-2-4	Devonian	RD2	Isolated	6	79.1	−18.3	21.19
1-3-2-5	Devonian	RD2	Isolated	1	71.4
1-3-2-6	Devonian	RD2	Isolated	2	102.3
1-3-2-7	Devonian	RD2	Isolated	1	76.3
1-3-2-8	Devonian	RD2	Isolated	1	69.9
1-3-2-9	Devonian	RD2	Isolated	2	81.1
8-1-1.	Devonian	BKC	Isolated	8	173.5	−11.2	15.17
8-1-2.	Devonian	BKC	Cluster	10	205.1
8-1-3.	Devonian	BKC	Cluster	2	166.9
8-1-4.	Devonian	BKC	Crystal growth	8	175
8-1-5.	Devonian	BKC	Crystal growth	8	208.4	−15.6	19.13
8-1-6.	Devonian	BKC	Trail	2	154.1
8-1-7.	Devonian	BKC	Trail	2	157.9
8-1-8.	Devonian	BKC	Trail	2	163.2
8-1-9.	Devonian	BKC	Scattered	8	183.1	−14.8	18.47
8-1-10.	Devonian	BKC	Scattered	8	208.6	−15.2	18.80
8-1-11.	Devonian	BKC	Scattered	4	221.2
8-1-12.	Devonian	BKC	Scattered	3	182.4
8-1-13.	Devonian	BKC	Cluster	6	199.8	−19.9	22.31
8-1-14.	Devonian	BKC	Scattered	3	168.3
8-1-15.	Devonian	BKC	Cluster	5	237.8
8-1-16.	Devonian	BKC	Cluster	7	180.2	−20.1	22.44
8-1-17.	Devonian	BKC	Cluster	8	225.4	−19.4	21.96
8-1-18.	Devonian	BKC	Cluster	5	225.8
8-1-19.	Devonian	BKC	Isolated	7	196.4	−21.4	23.31
8-1-20.	Devonian	BKC	Trail	5	207.1
8-1-21.	Devonian	BKC	Isolated	8	165.9	−15.9	19.37
8-1-22.	Devonian	BKC	Trail	8	237.9	−16.2	19.60
8-1-23.	Devonian	BKC	Isolated	8	231.1	−19.1	21.75
8-1-24.	Devonian	BKC	Isolated	7	209.8	−22.4	23.95
8-1-25.	Devonian	BKC	Scattered	8	227.1	−12.9	16.80
8-1-26.	Devonian	BKC	Scattered	4	146.2
8-1-27.	Devonian	BKC	Scattered	4	188.6
8-1-28.	Devonian	BKC	Isolated	6	200.8	−16.4	19.76
8-1-29.	Devonian	BKC	Isolated	6	195.5	−16.2	19.60
8-1-30.	Devonian	BKC	Isolated	7	223.9	−17.8	20.82
8-1-31.	Devonian	BKC	Isolated	4	154.3	−14.7	18.38
8-1-32.	Devonian	BKC	Isolated	9	214.5	−19.6	22.10

. Examples of the main characteristics of fluid inclusion assemblages hosted within the different calcite and dolomite phases are illustrated in Figure 13.

4.3.4. Major, Minor and Trace Elements Analysis

Major Elements and Dolomite Stoichiometry

The concentrations of the major elements, calcium and magnesium, were determined by ICP-MS analysis for dolomite and calcite phases to establish their stoichiometry. Ideal or stoichiometric dolomite would be characterized by the equal molar percentage of MgCO₃ and CaCO₃ and would be less soluble and more stable compared to non-stoichiometric dolomite [53]. In contrast, non-stoichiometric dolomite will have more Ca than Mg and because of its heterogeneous structure and composition is unstable and dissolve more rapidly.

All the dolomite types recognized petrographically were sampled and analyzed without any contamination from calcite components for both age groups with the only exception for the selective replacive dolomite (RD3) which could not be sampled without possible contamination by calcite or other dolomite phases in the Silurian specimens. With rare exceptions, all types of dolomite analyzed in this study were non-stoichiometric in both age groups (

Table 5. Major, minor and trace elements results.

Sample	Age	Type	MgCO3	CaCO3	Mn	Fe	Sr
			(mole %)	(mole %)	(ppm)	(ppm)	(ppm)
1-15a	Silurian	RD1	43.0	57.0	42.5	678.9	89.3
1-16a	Silurian	RD1	41.6	58.4	62.1	599.1	90.1
1-17a	Silurian	RD1	42.0	58.0	34.2	606.3	70.2
1-18	Silurian	RD1	46.8	53.2	197.6	3458.5	71.9
1-20	Silurian	RD1	40.6	59.4	130.2	2564.8	72.4
1-21	Silurian	RD1	48.7	51.3	67.9	339.3	71.9
1-31a	Silurian	RD1	49.8	50.2	446.7	2984.0	97.7
8-5a	Silurian	RD1	51.7	48.3	208.9	3515.8	94.8
8-6	Silurian	RD1	41.2	58.8	157.3	3276.2	81.2
8-8	Silurian	RD1	45.6	54.4	100.1	617.7	107.9
8-13a	Silurian	RD1	41.8	58.2	552.2	4020.5	80.2
1-16b	Silurian	RD2	43.9	56.1	66.3	809.7	81.2
1-24	Silurian	RD2	37.9	62.1	103.7	6261.1	166.3
1-31b	Silurian	RD2	42.6	57.4	702.5	5183.0	70.7
8-13b	Silurian	RD2	43.5	56.5	748.4	5929.5	71.9
1-28a	Silurian	SD	33.0	67.0	214.6	20,374.0	81.8
8-13c	Silurian	SD	36.5	63.5	1436.5	18,005.3	111.2
1-1a	Devonian	RD1	48.2	51.8	30.1	81.3	46.8
1-3-1a	Devonian	RD1	39.5	60.5	29.1	80.6	58.4
1-5a	Devonian	RD1	39.3	60.7	46.1	171.2	81.9
1-5b	Devonian	RD1	43.4	56.6	40.7	148.4	66.3
1-7a	Devonian	RD1	43.7	56.3	35.7	112.2	72.6
8-1a	Devonian	RD1	39.2	60.8	25.8	90.0	62.0
8-4a	Devonian	RD1	42.7	57.3	54.2	690.5	67.3
1-3-2a	Devonian	RD2	40.4	59.6	32.9	83.0	60.3
1-4a	Devonian	RD2	40.7	59.3	39.8	89.6	93.9
1-7b	Devonian	RD2	41.6	58.4	37.4	117.4	71.7
8-4b	Devonian	RD2	38.4	61.6	59.9	756.1	79.4
1-3-2b	Devonian	RD3	47.2	52.8	40.8	360.9	65.7
1-6a	Devonian	RD3	40.0	60.0	42.5	173.0	89.1
8-4c	Devonian	RD3	52.1	47.9	59.0	724.8	78.9

) with variable proportions of MgCO₃ and CaCO₃.

In both age group RD1 in non-stoichiometric but results less stoichiometric in Silurian formation compared to Devonian samples. Pervasive replacive micro to fine crystalline dolomite (RD1) ranges from 48.34 to 59.36 mole % CaCO₃ (average: 55.19 ± 3.73) and from 51.78 to 60.77 mole % CaCO₃ (average: 57.71 ± 3.04) for Silurian and Devonian formations, respectively (n_S = 11 and n_D = 7).

RD2 shows comparable values of mole % CaCO₃, but it is slightly more calcian in Devonian formations. Pervasive replacive medium crystalline dolomite (RD2) ranges from 56.11 to 62.12 mole % CaCO₃ (average: 58.04 ± 2.4) and from 58.39 to 61.57 mole % CaCO₃ (average: 59.71 ± 1.16) for Silurian and Devonian formations, respectively (n_S = 4 and n_D = 4).

Selective replacive medium crystalline dolomite (RD3) ranges from 47.93 to 59.96 mole % CaCO₃ (average: 53.55 ± 4.94) in Devonian formations (n_D = 3).

Saddle dolomite cement (SD), only observed and sampled from Silurian formations (n_S = 2), ranges from 63.47 to 67.00 mole % CaCO₃ (average: 65.23 ± 1.77).

Minor and Trace Elements: Iron, Manganese and Strontium

The results of trace elements in dolomitic phases are presented in Table 5.

RD1 sampled from Silurian formations show higher contents of Mn, Fe and Sr to Devonian samples. Pervasive replacive fine crystalline dolomite matrix RD1 in Silurian rocks (n = 11) is characterized by Mn contents between 34.2 to 552.2 ppm (average: 181.5 ± 161.5 ppm), Fe between 339.3 to 4020.5 ppm (average: 2060.1 ± 1404.6) and Sr between 70.2 to 107.9 ppm (average: 85.6 ± 11.9), whereas in Devonian rocks (n = 7) Mn ranges between 25.8 to 54.2 ppm (average: 37.4 ± 9.5), Fe between 80.6 to 690.5 ppm (average: 196.3 ± 204.3) and Sr between 46.8 to 81.9 ppm (average: 65 ± 10.2).

Silurian RD2 samples show higher contents of Mn, Fe and Sr compared to Devonian samples. Pervasive replacive medium crystalline dolomite matrix RD2 in Silurian rocks (n = 4) is characterized by Mn contents between 66.3 to 748.4 ppm (average: 405.2 ± 320.9), Fe between 809.7 to 6261.1 ppm (average: 4545.8 ± 2192.1) and Sr between 70.7 to 166.3 ppm (average: 97.5 ± 39.9), whereas in Devonian rocks (n = 4) Mn ranges between 32.9 to 59.9 ppm (average: 42.5 ± 10.3), Fe between 83 to 756.1 ppm (average: 261.5 ± 285.8) and Sr between 60.2 to 93.8 ppm (average: 76.3 ± 12.2). Selective replacive medium crystalline dolomite matrix (RD3) sampled from Devonian formations only (n = 3) and is characterized by Mn contents between 59 to 40.8 ppm (average: 47.5 ± 8.2), Fe between 724.8 to 173 ppm (average: 419.6 ± 229), and Sr between 89.1 to 65.7 ppm (average: 77.9 ± 9.6)

Values of trace elements in saddle dolomite cement (SD) encountered exclusively in Silurian formations (n = 2) is characterized by Mn contents between 214.5 to 1436.5 ppm (average: 825.5 ± 611), Fe between 18005.3 to 20373.9 ppm (average: 19189.7 ± 1184.3), and Sr between 81.8 to 111.2 ppm (average: 96.5 ± 14.7).

Silurian dolomites have higher Mn, Fe and Sr content than Devonian dolomites and also the progressive increase in Fe and Mn in the successive generations.

In addition to the previously mentioned results, samples (n = 10 and n = 5) from early and late calcite cement were analyzed to establish their trace elements concentrations.

Isopachous calcite cement (ISC) observed only in Silurian formations (n = 2) is characterized by Mn contents between 41.9 to 48.2 ppm (average: 45.1 ± 3.2), Fe between 164.8 to 230.5 ppm (average: 197.7 ± 32.8) and Sr between 177.1 to 244.3 ppm (average: 210.7 ± 33.6).

Syntaxial overgrowth calcite cement (SXC) in Silurian rocks (n = 2) show higher contents of Mn, Fe compared with respective values obtained from Devonian samples, but lower Sr concentrations. It is characterized by Mn contents between 272.1 to 589.23 ppm (average: 430.7 ± 158.6), Fe between 617.7 to 1124.5 ppm (average: 871.1 ± 253.4) and Sr between 230.8 to 244.3 ppm (average: 237.6 ± 6.7), whereas in Devonian rocks (n = 3) Mn ranges between 15.7 to 20.3 ppm (average: 17.9 ± 1.9), Fe between 41 to 51.4 ppm (average: 47.4 ± 4.6) and Sr between 239.1 to 498.7 ppm (average: 333 ± 117.5).

A single measurement in each age group of drusy calcite cement (DC) show values of Mn: 36.5 ppm and 116.2 ppm, Fe: 401 ppm and 217.67 ppm, and Sr: 116.8 ppm and 1425 ppm for the Silurian (n = 1) and Devonian (n = 1) samples, respectively.

Late-stage blocky calcite cement (BKC) sampled from Silurian formations show significantly higher contents of Mn, Fe, and Sr compared to Devonian samples. Trace element values from Silurian samples (n_S = 5) is characterized by contents of Mn between 82.1 and 754.2 ppm (average: 225.9 ± 264.6), Fe between 79.2 to 2097.6 ppm (average: 1036.5 ± 811.4) and Sr between 164.6 to 538.6 ppm (average: 317.3 ± 144), whereas results from the single pure sample available of blocky calcite cement from Devonian formations show values of Mn: 35.9 ppm, Fe: 8.7 ppm and Sr: 643.9 ppm. BKC

4.3.5. Rare Earth Elements (REE)

Selected samples of pervasive replacive fine crystalline dolomite matrix (RD1), pervasive replacive medium crystalline dolomite matrix (RD2), saddle dolomite cement (SD) and blocky calcite cement (BKC) were analyzed for REEs concentrations from Silurian formations. In addition to the selected samples of pervasive replacive fine-crystalline dolomite matrix (RD1), pervasive replacive, medium crystalline dolomite matrix (RD2), selective replacive medium crystalline dolomite matrix (RD3) and blocky calcite cement (BKC) were analyzed for REEs concentrations from Devonian successions (

Table 6. Rare earth elements (REE) results.

Sample	Age	Type	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	ΣREE
			(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)
1-15a	Silurian	RD1	1.51	3.42	0.44	1.65	0.36	0.09	0.31	0.05	0.26	0.05	0.16	0.02	0.12	0.02	8.46
1-16a	Silurian	RD1	3.80	9.17	1.16	4.22	0.84	0.16	0.76	0.11	0.62	0.11	0.33	0.04	0.26	0.04	21.62
1-17a	Silurian	RD1	0.78	1.42	0.15	0.52	0.10	0.02	0.10	0.01	0.06	0.01	0.04	0.00	0.03	0.00	3.25
1-18	Silurian	RD1	1.76	4.02	0.51	2.07	0.43	0.08	0.41	0.05	0.28	0.05	0.12	0.01	0.07	0.01	9.88
1-20	Silurian	RD1	3.97	9.62	1.16	4.58	1.05	0.25	1.06	0.15	0.80	0.15	0.41	0.05	0.27	0.04	23.55
1-21	Silurian	RD1	0.87	1.99	0.23	0.90	0.16	0.05	0.17	0.02	0.13	0.02	0.07	0.01	0.06	0.00	4.68
1-31a	Silurian	RD1	2.06	4.23	0.54	2.04	0.41	0.10	0.42	0.06	0.32	0.06	0.19	0.02	0.14	0.02	10.62
8-5a	Silurian	RD1	2.19	5.21	0.64	2.40	0.47	0.11	0.50	0.06	0.35	0.07	0.18	0.02	0.12	0.02	12.33
8-6	Silurian	RD1	2.75	6.99	0.87	3.46	0.75	0.19	0.82	0.12	0.64	0.12	0.30	0.04	0.20	0.03	17.27
8-8	Silurian	RD1	1.17	2.82	0.35	1.34	0.27	0.06	0.30	0.04	0.22	0.04	0.11	0.01	0.12	0.01	6.86
8-13a	Silurian	RD1	2.42	4.90	0.60	2.19	0.48	0.10	0.46	0.07	0.39	0.08	0.25	0.02	0.16	0.02	12.13
1-16b	Silurian	RD2	3.64	9.45	1.23	4.86	0.88	0.18	0.86	0.12	0.65	0.12	0.36	0.05	0.23	0.03	22.67
1-23b	Silurian	RD2	0.28	0.50	0.05	0.26	0.05	0.01	0.05	0.01	0.04	0.01	0.02	0.00	0.02	0.00	1.30
1-24	Silurian	RD2	0.32	0.69	0.09	0.35	0.06	0.01	0.07	0.01	0.07	0.02	0.04	0.01	0.02	0.01	1.77
1-31b	Silurian	RD2	2.87	6.52	0.80	2.80	0.49	0.12	0.48	0.06	0.37	0.06	0.21	0.03	0.15	0.02	14.98
8-10b	Silurian	RD2	0.29	0.53	0.06	0.23	0.06	0.01	0.06	0.01	0.05	0.01	0.03	0.00	0.02	0.00	1.37
8-13b	Silurian	RD2	3.32	7.82	0.97	3.79	0.73	0.13	0.67	0.08	0.48	0.09	0.24	0.04	0.23	0.03	18.61
1-28a	Silurian	SD	0.36	0.77	0.09	0.39	0.07	0.02	0.04	0.01	0.05	0.01	0.02	0.00	0.02	0.00	1.85
1-31c	Silurian	SD	5.75	16.24	2.14	8.07	1.39	0.23	1.06	0.13	0.68	0.11	0.33	0.04	0.21	0.03	36.41
8-13c	Silurian	SD	19.39	47.98	5.92	20.35	3.42	0.44	2.30	0.27	1.34	0.23	0.69	0.08	0.45	0.06	102.93
1-23a	Silurian	ISC	0.40	0.71	0.08	0.33	0.05	0.01	0.06	0.01	0.05	0.01	0.05	0.00	0.02	0.00	1.80
8-10a	Silurian	ISC	0.20	0.39	0.05	0.20	0.04	0.01	0.04	0.01	0.03	0.01	0.02	0.00	0.02	0.00	1.01
1-32	Silurian	SXC	6.07	10.30	1.44	5.74	1.09	0.24	1.17	0.14	0.76	0.14	0.35	0.04	0.24	0.03	27.75
1-33a	Silurian	SXC	3.86	7.09	0.94	3.92	1.08	0.26	1.42	0.20	1.39	0.29	0.93	0.12	0.80	0.10	22.42
1-17b	Silurian	DC	1.22	2.08	0.23	0.82	0.15	0.04	0.12	0.02	0.10	0.01	0.05	0.01	0.03	0.01	4.88
1-26	Silurian	BKC	2.99	7.48	0.95	4.01	1.08	0.24	1.19	0.19	1.15	0.22	0.60	0.08	0.48	0.06	20.73
1-27	Silurian	BKC	2.07	5.09	0.67	2.76	0.53	0.13	0.56	0.08	0.42	0.07	0.22	0.03	0.15	0.02	12.78
1-28b	Silurian	BKC	0.70	1.24	0.13	0.49	0.10	0.02	0.13	0.02	0.13	0.03	0.09	0.01	0.07	0.01	3.17
1-31d	Silurian	BKC	9.35	26.81	3.53	13.19	2.43	0.34	1.75	0.21	1.03	0.19	0.53	0.06	0.40	0.05	59.87
8-5b	Silurian	BKC	1.72	4.60	0.62	2.28	0.40	0.10	0.42	0.06	0.32	0.08	0.24	0.03	0.20	0.03	11.09
8-13d	Silurian	BKC	14.53	32.22	3.47	10.91	1.60	0.21	1.18	0.12	0.53	0.09	0.24	0.03	0.20	0.03	65.36
1-1a	Devonian	RD1	0.13	0.25	0.03	0.14	0.03	0.01	0.03	0.00	0.02	0.00	0.01	0.00	0.01	0.00	0.69
1-3-1a	Devonian	RD1	0.23	0.42	0.06	0.18	0.05	0.01	0.04	0.01	0.04	0.01	0.02	0.00	0.03	0.00	1.10
1-5a	Devonian	RD1	0.13	0.18	0.02	0.09	0.02	0.00	BDL	0.00	0.03	0.01	0.03	0.00	0.02	0.00	0.54
1-5b	Devonian	RD1	0.11	0.17	0.02	0.07	0.01	0.00	BDL	BDL	0.02	0.00	0.02	0.00	0.02	0.00	0.44
1-7a	Devonian	RD1	0.14	0.20	0.02	0.11	0.03	0.01	BDL	0.00	0.02	0.00	0.01	0.00	0.02	0.00	0.56
8-1a	Devonian	RD1	0.31	0.53	0.07	0.24	0.04	0.01	0.04	0.01	0.05	0.01	0.02	0.00	0.02	0.00	1.35
8-4a	Devonian	RD1	0.76	0.94	0.15	0.61	0.17	0.03	0.18	0.03	0.16	0.04	0.12	0.01	0.07	0.01	3.29
1-3-2a	Devonian	RD2	0.25	0.45	0.06	0.23	0.04	0.01	0.04	0.01	0.05	0.01	0.03	0.00	0.02	0.01	1.22
1-4a	Devonian	RD2	0.33	0.40	0.05	0.15	0.03	0.01	BDL	0.00	0.03	0.00	0.01	0.00	0.02	0.00	1.05
1-7b	Devonian	RD2	0.14	0.20	0.03	0.10	0.03	0.00	BDL	0.01	0.02	0.01	0.01	0.00	0.01	0.00	0.56
8-4b	Devonian	RD2	0.89	1.09	0.18	0.72	0.16	0.03	0.22	0.03	0.19	0.04	0.14	0.02	0.09	0.01	3.80
1-2a	Devonian	RD3	0.43	0.66	0.08	0.29	0.06	0.01	0.07	0.01	0.04	0.01	0.02	0.00	0.02	0.00	1.72
1-3-2b	Devonian	RD3	0.42	0.76	0.11	0.42	0.12	0.02	0.08	0.01	0.08	0.02	0.05	0.01	0.04	0.01	2.15
1-6a	Devonian	RD3	0.20	0.23	0.03	0.13	0.03	0.00	0.03	0.00	0.03	0.01	0.02	0.00	0.01	0.00	0.73
1-10a	Devonian	RD3	1.67	1.53	0.20	0.70	0.13	0.03	0.14	0.02	0.14	0.03	0.10	0.01	0.06	0.01	4.78
1-13b	Devonian	RD3	6.74	6.77	1.32	5.40	1.07	0.25	1.25	0.16	0.96	0.20	0.56	0.06	0.38	0.05	25.17
8-4c	Devonian	RD3	0.91	1.12	0.18	0.75	0.16	0.04	0.23	0.03	0.21	0.05	0.12	0.02	0.11	0.01	3.94
1-10b	Devonian	SXC	0.82	0.74	0.11	0.38	0.04	0.01	0.06	0.01	0.07	0.02	0.06	0.01	0.04	0.00	2.38
1-11a	Devonian	SXC	1.13	1.16	0.16	0.64	0.09	0.02	0.14	0.02	0.12	0.02	0.07	0.01	0.05	0.01	3.63
1-12a	Devonian	SXC	1.20	0.90	0.15	0.61	0.12	0.03	0.21	0.03	0.22	0.05	0.16	0.02	0.11	0.02	3.83
1-13a	Devonian	DC	5.61	5.71	1.07	4.07	0.71	0.14	0.70	0.11	0.60	0.13	0.39	0.05	0.36	0.06	19.70
8-1b	Devonian	BKC	0.88	1.22	0.14	0.48	0.07	0.01	0.07	0.01	0.06	0.02	0.04	0.01	0.05	0.01	3.08

).

Replacive dolomite RD1 and RD2 sampled from Silurian formations show sub-parallel trends compared to the pattern of Silurian brachiopods shown by Azmy et al. [54] but with substantial differences from Devonian patterns, Silurian dolomite trends are characterized by a minor negative La anomaly and both cases of positive and negative Ce anomalies (Figure 17A,B). Both types of dolomite RD1 and RD2 exhibit higher average ΣREE (11.88 ± 6.25 ppm, and 10.62 ± 8.92 ppm, respectively) compared with the average of Silurian brachiopods (2.21 ppm) shown by Azmy et al. [55]. Saddle dolomite (SD) and blocky calcite cement (BKC) show similar REE shale-normalized trends (Figure 17A,B) with average ΣREE (47.06 ± 41.95 ppm and 28.83 ± 24.47 ppm, respectively) and both significantly differ from the Silurian Brachiopods REE patterns. As shown in Figure 17, SD and BKC present both negative La and Ce anomalies (Figure 17B) as well as a slight negative Eu anomaly (Figure 17A).

All the dolomite and late calcite samples from Devonian formations exhibit shale-normalized patterns comparable to those of modern warm water brachiopods (Figure 18A).

The average shale-normalized REEs concentrations in samples from Devonian formations present all the typical characteristic of seawater patterns including depletion of LREE over HREE (averages: RD1: (La/Yb)_SN = 0.66; RD2: (La/Yb)_SN = 0.70; RD3: (La/Yb)_SN = 1.03), enrichment of MREE (averages: RD1: (Sm/La)_SN = 1.25; RD2: (Sm/La)_SN = 1.13; RD3: (Sm/La)_SN = 1.0), negative Ce anomaly (Figure 18A,B), and slightly positive La and Gd anomalies (Figure 18A). Replacive dolomite RD1, RD2 and RD3 exhibit higher and progressively increasing average ΣREE (1.14 ± 0.93 ppm, 1.66 ± 1.26 ppm and 6.41 ± 8.50 ppm, respectively) compared with the average of modern warm water brachiopods (0.25 ± 0.2 ppm) proposed by Azmy et al. [55]. Slightly different patterns, which are still comparable to the trend of modern warm water brachiopods, characterize REEs of BKC with ΣREE of 3.08 ppm which falls between those of RD1 and RD3 (Figure 18A).

5. Discussion and Interpretations

The petrographic and geochemical results obtained in this study indicate that diagenesis of the Silurian and Devonian dolomitized successions have been subjected to fluids of variable origins and compositions, as described below.

5.1. Constraints from Petrography

5.1.1. Silurian

The diagenetic history of the Silurian formations includes dolomitization, mechanical and chemical compaction, fracturing, dissolution, silicification and calcite and evaporite cementation (Figure 19). The diagenetic processes above listed are described in depth in Tortola [46], on which this paper is largely based.

Dolomitization

Replacive dolomite crystal size is often influenced by the texture of the precursor limestone [55]. Hence, fine-crystalline dolomite may represent the replacement of fine-grained limestone, whereas coarser replacive dolomite may either represent replacement of coarse-grained limestone or recrystallization of precursor dolomite matrix [55].

RD1 is commonly characterized by closely packed unimodal mosaics of micro to fine-crystalline dolomite which preserved the finely laminated texture of the precursor mudstone. The characteristics showed by RD1, are comparable to fine crystalline dolomite observed in Silurian formations from different areas of southwestern Ontario [5,7,8,56]. This dolomite has been interpreted to be formed from evaporative reflux of hypersaline brines in a sabkha-like setting in a near-surface or shallow burial setting, in the condition of relatively high salinity and low temperature [47,57,58]. This interpretation is also supported by a cross-cutting relationship with chemical compactional features, such as dissolution seams and stylolites, which evidently postdated RD1. However, there is no evidence of supratidal lithofacies and abundant evaporites.

As mentioned by Zheng [7], the relatively coarser size of replacive fine crystalline dolomite (20–50 µm) compared to modern dolomites (<4 µm) can be explained by an increase of crystal size due to early recrystallization. This early recrystallization is possibly supported by the negative shift in oxygen isotopic composition discussed in the next section. Cathodoluminescence of mainly dull to dark red luminescence for RD1 suggests replacement in a reducing environment at shallow burial conditions [59].

Medium replacive dolomite (RD2) shows slightly coarser crystal size but similar texture compared to RD1. Dolomite crystal coarsening can occur in different scenarios: (1) slow precipitation rate [60]; (2) increase of temperature due to progressive increasing burial conditions [47]; (3) recrystallization of precursor dolomite or overgrowth of later stage dolomite on existent cores of earlier dolomite [47,61,62,63,64].

Many studies (e.g., [62,64,65]) suggest that non-planar crystal boundaries, coarsening of crystal size and absence of zonation under CL can represent evidence of recrystallization. According to Machel [66], the most accredited definition of recrystallization is the one proposed by the Glossary of Geology [67], which states that recrystallization of a mineral determines the formation of a new mineral commonly characterized by coarser crystals and with either same or different composition. The coarsening of crystal size coupled with the lack of zonation under CL observed in RD2 may indicate recrystallization [64,68,69]. The occurrence of strictly non-planar crystal boundaries was not observed in RD2 from both Silurian and Devonian samples, possibly because early dolomite, formed from seawater can preserve unchanged geochemistry and texture after a certain depth of burial and several million years [70].

According to Machel [66], dolomite can be defined “significantly recrystallized” if it is characterized by the variation of at least one of the following parameters: (1) increase of crystal size or in non-planar crystal boundaries; (2) progressive ordering; (3) change in composition including stable and radiogenic isotopes, stoichiometry, trace and rare earth elements, fluid inclusions and zonation; (4) change in paleomagnetic properties.

The increase of crystal size, the negative shift of δ¹⁸O from postulated values of marine dolomite formed during Silurian and changes in trace and rare earth elements (discussed in the next sections), make RD2 “significantly recrystallized”, but also “insignificantly recrystallized” with respect to the increase of non-planar crystal boundaries and progressive ordering [1,5,66]. Crystal coarsening of fabric-destructive pervasive replacive medium (50–100 µm) crystalline dolomite (RD2) may suggest recrystallization from the precursor dolomite RD1, but the presence of cloudy, non-ferroan cores and clear, ferroan rims as a common characteristic of RD2 crystals (Figure 4F) may also suggest overgrowth on earlier dolomite [47]. Under CL, medium dolomite RD2 shows slightly more red luminescence compared to RD1, which indicates that its formation took place in slightly deeper burial conditions in a more reduced environment [59]. As the hypothesis of overgrowth of RD2 on early dolomite is not quite confirmed by CL observation, no brighter luminescent zones have been observed in correspondence of the clear rims, the interpretation of recrystallization seems more likely. Selective replacive medium crystalline dolomite (RD3) occurred in association with dissolution seams during early chemical compaction. Similar textures of patchy dolomite formed in association with chemical compactional features have been previously described (e.g., [71,72]) as dolomite formed from marine or connate waters flowed through dissolution seams which are believed to have behaved as preferential transport path for the dolomitizing fluids. Alternatively, the magnesium required for the formation of RD3 may be supplied by the remobilization of the Mg released in solution from the dissolution of high magnesium calcite [72,73]. The association with dissolution seams suggests that RD3 formed in shallow to intermediate burial realm postdating RD1 and RD2. The common presence of dark cores and clear rims as distinctive characteristics of RD3 crystals (Figure 4C) may also suggest overgrowth on the previous dolomite phases [47]. This evidence, as mentioned for RD2, is not confirmed by cathodoluminescence which shows luminescence comparable with those of RD2 without any zonation or difference in terms of luminescence between cores and rims in RD3 crystals (Figure 4D).

The less abundant saddle dolomite (SD) is observed as fracture and vug-filling cement occluding second and third generation of fracture, postdating RD1, RD2 and RD3 and predating blocky calcite (BKC) and evaporite cement (GY and ANH) (Figure 4E,G–I). This petrographic evidence suggests that saddle dolomite formed during a late diagenetic event possibly in intermediate burial conditions.

Calcite Cementation

Calcite cementation represents an important diagenetic process that affected Silurian carbonate rocks in the Michigan Basin. It started in the early-stage, during marine environmental conditions, to continue during progressively deeper burial.

Isopachous calcite (ISC) was only observed in Silurian formations as cement characterized by bladed crystals surrounding coated grains (Figure 6A) formed prior to mechanical compaction and consequently prior to SXC, DC and BKC. It is interpreted to have formed from seawater immediately after deposition in a near-surface marine environment, inhibiting and hence predating mechanical compaction. Under CL this cement is dull-red (Figure 4D), occasionally non-luminescent, indicating precipitation in a slightly reducing environment where Fe²⁺ and Mn²⁺ were available to be incorporated in the crystal lattice [59].

Syntaxial overgrowth calcite (SXC) is mainly cloudy and inclusion-rich, evidence of precipitation in near-surface marine, meteoric or mixing-zone environments [74]. The presence of dissolution seams deflecting around and cross-cutting SXC indicates that its precipitation took place before chemical compaction. Under CL this cement is red to bright red indicating precipitation in a redox environment where Fe²⁺ and Mn²⁺ were available to be incorporated in the crystal lattice [59]. Isotopic values for this cement from both Silurian and Devonian rocks (Figure 11) show overlapping and/or slightly depleted values relative to the postulated ratios for their respective ages; hence evidence for their formation form seawater parentage fluids.

Drusy mosaic calcite (DC), was mainly observed as vug, mould and fracture-filling cement (Figure 6C), which can form in near-surface meteoric as well as in burial environment [75,76] but its formation is not commonly related to the marine environment [62]. Under CL it mainly shows dull-red luminescence (Figure 6D), evidence of precipitation in a reducing condition [59]. Paragenetically, DC postdate silica and predated dissolution seams and stylolites (Figure 5A) as well as late blocky calcite cement (Figure 5B). Hence, its formation represents a relatively early diagenetic event predating chemical compaction.

Late diagenetic blocky calcite cement (BKC) is characterized by coarse crystals (>500 µm) and it is mainly observed as vug and fracture filling cement which possibly formed in a burial environment (e.g., [77]). Petrographically, BKC shows dull to bright orange zoned luminescence under CL indicating formation in a progressively reducing environment. Based on petrographic evidence, BKC is interpreted to be the latest phase of calcite cementation, postdating saddle dolomite cement (SD) (Figure 4I) and predating evaporite cementation.

5.1.2. Devonian

The diagenetic history of Devonian successions includes dolomitization mechanical and chemical compaction, fracturing, dissolution, silicification, and calcite cementation (Figure 20). These sequences are based on petrographic cross-cutting relationships supplemented by geochemical evidence.

Dolomitization

Devonian replacive dolomites RD1, RD2 and RD3 (Figure 7) show similar characteristics in terms of timing and environments compared to the respective dolomite phases observed in the Silurian formations.

RD1 is commonly characterized by closely packed unimodal mosaics of micro to fine-crystalline dolomite (Figure 7A) which preserved the original texture of the precursor mudstone. Fine-crystalline dolomites distinguished in Devonian formations in previous studies (e.g., [2,5,33,56]) show similar texture if compared to RD1 and are interpreted to be formed from evaporative reflux of hypersaline brines in a sabkha-like setting. Hence, the mimetic pervasive replacive micro to fine crystalline dolomite (RD1) is possibly formed in a low temperature, near-surface or shallow burial environment. However, there is no evidence of abundant evaporites commonly associated with this model. Slightly coarser size of RD1 crystals, in comparison with recent dolomites, suggest early recrystallization [7], which is supported by oxygen isotopic composition discussed in Section 5.2. Cathodoluminescence characteristics of RD1 showing mainly dull to dark red luminescence (Figure 7B) suggest replacement in a shallow reducing environment [53]. Cross-cutting relationship with dissolution seams and stylolites suggests that RD1 formed prior to chemical compaction.

Crystal coarsening of fabric-destructive pervasive replacive medium crystalline dolomite (RD2) (Figure 7C,D) suggests possible recrystallization of the precursor dolomite RD1 [47,61,62,63,64]. The lack of zonation under CL coupled with the coarsening of crystal size observed in RD2 may represent additional evidence of recrystallization [64,68]. Devonian medium replacive dolomite shows slightly brighter red luminescence compared to RD1 (Figure 7B), possible evidence of a slightly deeper burial formation in a more reducing environment.

Selective replacive medium crystalline dolomite (RD3) has similar characteristics to RD3 observed in Silurian formations. It is commonly associated with dissolution seams and stylolites, which act as preferential paths for dolomitizing fluids [72]. Hence, it is interpreted to have formed in a shallow to intermediate burial regime postdating RD1 and RD2 (Figure 7E,F) and alongside with chemical compaction. The common presence of dark cores and clear rims as distinctive characteristics of RD3 crystals (Figure 7E) may also indicate overgrowth on the previous dolomite phases [47]. This hypothesis is not confirmed by cathodoluminescence results which shows luminescence slightly brighter with those of RD2 but without any zonation or difference in terms of luminescence between cores and rims within the crystals (Figure 7B,F).

Calcite Cementation

Calcite cementation represents an important process within the paragenesis of Devonian carbonate rocks in the Michigan Basin (Figure 9). It started in the early-stage, during marine environmental conditions, to proceed during progressively deeper burial stages.

Syntaxial overgrowth calcite cement (SXC) is commonly observed in bioclastic grainstone, usually as nucleation over echinoderm plates or crinoid ossicles that act as single crystals (Figure 9A). Syntaxial overgrowth is mainly cloudy and inclusion-rich, evidence of precipitation in near-surface marine, meteoric or mixing-zone environments [74]. The presence of dissolution seams deflecting around and crosscutting SXC indicates that its precipitation took place before chemical compaction. Under CL this cement is dull-red indicating precipitation in a reducing environment where Fe²⁺ and Mn²⁺ were available to be incorporated in the crystal lattice [59].

Dogtooth calcite cement (DTC), which is only present in bioclastic grainstones from Devonian formations (Figure 9B), can form in different environments. According to Reinhold [78] this cement can form: (1) in a marine-phreatic environment when dogtooth calcite occurs in intraskeletal voids; (2) in shallow burial environment when observed in stromatactis cavities succeeding radiaxial fibrous calcite cement. DTC is observed growing normally to the substrate (mainly in intraskeletal pores) with sharply pointed acute crystals. It formed early within the diagenetic history of the Devonian formations, in a near-surface or shallow burial environment, predating the formation of silica and consequently predating RD1. The presence of zoned dull to red luminescence under CL indicates possible formation in a progressively reducing environment [53]. As sampling of a pure specimen for geochemical analyses was not possible, the conclusion concerning time and environment of DTC’s occurrence are based exclusively on petrographic observations.

Drusy mosaic calcite (DC), mainly observed as vug, mould and fracture-filling cement (Figure 9D,E), can form in near-surface meteoric as well as in burial environment [75,76]. Its formation is not commonly related to the marine environment [71]. CL observations reveal luminescent zones ranging from non- to orange bright luminescent (Figure 9C) indicating fluctuation from well oxidizing environment, which inhibited the incorporation of Fe²⁺ and Mn²⁺, to a slightly more reducing condition, respectively [59]. DC postdated silica and predated dissolution seams, stylolites and late blocky calcite cement (Figure 9E). Hence, its formation is thought to represent a relatively early diagenetic event which possibly took place in a shallow burial environment, predating chemical compaction.

Late diagenetic blocky calcite cement (BKC) possibly formed in the burial environment. BKC shows dull to bright zoned luminescence under CL (Figure 9F) indicating formation in a progressively reducing environment. Based on petrographic evidence, BKC is interpreted to be the latest phase of calcite cementation (Figure 20) postdating fractures.

5.2. Constraints from Stable O, C and Radiogenic Sr Isotopes

The conditions of precipitation and/or recrystallization of diagenetic minerals can be predicted using their oxygen isotopic composition, which is a function of the temperature as well as the δ¹⁸O composition of the diagenetic fluids involved [50]. The recrystallization of dolomite, and associated chemical and textural changes, is considered a frequent process in carbonates associated with increasing burial depth [50,64,66,69,79]. Isotopic data show a general overlap between δ¹⁸O and δ¹³C values of replacive dolomites (RD2 and RD3) from Silurian and Devonian successions (Figure 10). In both age groups, for these dolomite samples display similar δ¹³C compared to those of carbonates formed in equilibrium with seawater of the respective age (for Silurian: 0.5 to 2.5‰ VPDB [80]; and for Middle Devonian: 0.5 to 2.5‰ VPDB [49]) (Figure 10). The δ¹³C values possibly reflect the carbon isotopic composition of the precursor limestones (e.g., [81]). However, many δ¹³C values in Silurian RD1 departed from the postulated values for Silurian carbonates. In contrast, in both age groups, the δ¹⁸O isotopic composition shows a negative shift from the postulated values for their respective age.

5.2.1. Silurian

Dolomite formed in equilibrium with seawater should display δ¹⁸O isotopic composition enriched of about 3 ± 1‰ (e.g., [82]). Calcitic matrix is characterized by δ¹⁸O values varying from −8.16 to −4.73‰ VPDB and hence with some exceptions, mostly falling into the range of the postulated values for Silurian marine calcite (−3 to −5.5‰ VPDB [80]). Assuming that the Silurian seawater with an isotopic composition of −3.5 ‰ SMOW [83] represents the source of dolomitizing fluids for the formation of RD1, its range of δ¹⁸O values should vary from −0.25 to −2.5‰ VPDB. The calculated average value (−1.4‰ VPDB) would correspond to an estimated value of 24 °C employing the fractionation equation proposed by Land [82]. However, values obtained from RD1 show a negative shift of about 2–6‰ from the δ¹⁸O average value (−1.4‰ VPDB) expected for the Silurian dolomite (Figure 10). RD2, RD3 and SD exhibit a negative shift of about 2–8‰ compared with the δ¹⁸O average value of the Silurian marine dolomite (Figure 10). An explanation to justify this negative shift of δ¹⁸O values can be the interaction with meteoric water or formation in the burial regime at relatively high temperatures (e.g., [69]). The interaction with meteoric water should be demonstrated by evidence of subaerial exposure such as karst phenomena, presence of vadose cement or other features associated with meteoric diagenesis. Although previous studies underline the presence of an erosional unconformities (between Cabot Head and the overlying Fossil Hill formations and between Bass Island and Bois Blanc formations [34]), the lack of evidence of karstification in the studied samples (e.g., [69]), make the meteoric diagenesis an improbable scenario for matrix dolomites. Hence, the hypothesis of recrystallization with burial seems to be more likely.

RD1 shows negative δ¹³C values, possibly related to the formation of this dolomite in a shallow burial environment associated with bacterial sulphate reduction (BSR) [84]. This hypothesis is supported by the presence of minute framboidal pyrite crystals (Figure 4A) embedded within RD1 [85].

Even though, there is no evidence of the corresponding variation in terms of crystal size coarsening and depletion of δ¹⁸O values (e.g., [8]) comparing data from RD1 and coarser dolomites (RD2 and RD3), the negative shift of δ¹⁸O values could be associated to recrystallization and/or to interaction with hydrothermal fluids at relatively high temperature (e.g., [5,69,77,86,87]).

Most of the dolomite samples (both, matrix and cement) from Silurian formations are characterized by ⁸⁷Sr/⁸⁶Sr ratios falling in the range of postulated values for Silurian seawater (0.7078–0.7087 [52,53]) (Figure 12). However, some samples show slightly more radiogenic values than those of coeval seawater. This variation from the postulated values can be related to the recrystallization of early dolomite and to water/rock interaction with feldspar-bearing rocks [5]. In an overall view, the ⁸⁷Sr/⁸⁶Sr ratios for dolomite matrix (RD1, RD2 and RD3), dolomite cement (SD) and late-stage calcite cement (BKC) suggest for both a seawater-dominated parental fluid.

Most of the calcitic samples show δ¹³C values, in range with postulated values for Silurian marine calcite with the exception for late blocky calcite which clearly shows a negative shift from the equilibrium values (Figure 11). This may suggest a separate input of oxidized organic carbon possibly related to interaction with hydrocarbons (e.g., [77]).

5.2.2. Devonian

Calcitic matrix is characterized by δ¹⁸O values varying from −8.16 to −4.73‰ VPDB and hence with some exceptions (Figure 11), mostly falling into the range of the postulated values for Devonian marine calcite (−4 to −6 ‰ VPDB [49]). Most of the calcitic samples show δ¹³C values, in range with postulated values for Devonian marine calcite with the exception for late BKC which clearly shows a negative shift from the equilibrium values (Figure 11). This, as already mentioned for Silurian BKC, may suggest a separate input of oxidized organic carbon possibly related to interaction with hydrocarbons (e.g., [77]).

Considering that dolomite formed in equilibrium with seawater, it should display δ¹⁸O isotopic composition enriched of about 3 ± 1‰ (e.g., [82]), and assuming that Devonian seawater, with an isotopic composition of −2‰ SMOW [51,88], represents the source of dolomitizing fluids for the formation of RD1, its range of δ¹⁸O values should vary from −1 to −3‰ VPDB. The calculated average value (−2‰ VPDB) would correspond to an estimated value of 33 °C employing the fractionation equation proposed by Land [82]. However, RD1 shows a negative shift of about 1–5‰ from the average value (−2‰ VPDB) of δ¹⁸O expected for Devonian dolomite (Figure 10). RD2 and RD3 also exhibit a similar negative shift compared with the δ¹⁸O average value of Devonian marine dolomite (Figure 10).

This negative shift of δ¹⁸O values can be also related to the interaction with meteoric water or formation in burial regime at relatively high temperatures (e.g., [69]). As already mentioned in the previous section, the interaction with meteoric water should be demonstrated by evidence of subaerial exposure such as karst features, such as the presence of vadose cement or other evidence associated with meteoric diagenesis. Although there is no evidence of karstification in the samples collected and analyzed in this study, a previous study [89] demonstrated the presence of dissolution processes mostly active in the shallow subsurface, usually <200 m depth in south-western Ontario. At the local scale, approximately 180 m of Devonian bedrock beneath the Bruce nuclear site are karstic with karstification and higher-permeability confined intervals also found within the Salina A1 dolostone and the Guelph Formation [89]. Hence, the possibility of meteoric water influence cannot be discounted completely.

However, the absence of a negative shift of δ¹³C values, commonly associated with the meteoric diagenesis (e.g., [69]), coupled with the marked δ¹⁸O isotopic negative values for matrix dolomite may favour the hypothesis of recrystallization.

The majority of the dolomite matrix, as well as the late calcite (BKC) from Devonian formations, are characterized by ⁸⁷Sr/⁸⁶Sr ratios falling in the range of postulated values for Devonian seawater (0.7078–0.7080 [51]) (Figure 12). However, one sample shows more radiogenic isotopic composition than those of coeval seawater. This variation from the postulated values, even if isolated, can be related to the recrystallization of early dolomite and to water/rock interaction with the overlying Upper Devonian sandstones and shales [56]. Hence, the results ⁸⁷Sr/⁸⁶Sr ratios for dolomite matrix (RD1, RD2 and RD3), and late-stage calcite cement (BKC) suggest for both seawater-dominated parental fluids.

5.3. Constraints from Fluid Inclusions

As shown in Figure 15 Silurian replacive dolomite (RD2) is characterized by slightly higher homogenization temperature (89.1 °C) compared to those hosted in Devonian RD2 (83.2 °C). These values are in both cases lower than T_h values given by Barnes et al. [90] (Th: 155 °C) for the central part of the Michigan Basin, but comparable T_h values and salinity with those reported by Haeri-Ardakani et al. [5] (T_h: 104 °C; 20.3 wt.% NaCl eq.) for different areas of southwestern Ontario. Fluid inclusions hosted in saddle dolomite in Silurian formation also show slightly higher values of T_h and salinity (T_h: 124 °C; 28.5 wt.% NaCl eq.) compared to matrix dolomite and to saddle dolomite fluid inclusions analyzed in the study carried out by Haeri-Ardakani et al. [5] (T_h: 108 °C; 25.5 wt.% NaCl eq.).

Late calcite cement (BKC) fluid inclusions results show significantly higher T_h values but lower salinity for the Devonian samples compared to those hosted in the Silurian calcite. Fluid inclusions hosted in BKC of the Devonian formation show T_h values significantly higher and lower salinity if compared with values obtained in the central part of the Michigan Basin by Luczaj et al. [13] (T_h: 140 °C; 29.2 wt.% NaCl eq.), whereas it shows similar salinity and higher T_h if compared with the values proposed by Haeri-Ardakani et al. [5] (T_h: 126 °C; 19.7 wt.% NaCl eq.).

These results coupled with the negative shift of δ¹⁸O values may suggest, in both age groups, the formation of RD2 was at relatively high temperatures from saline brines (e.g., [77]). Similar ranges of homogenization temperatures characterize blocky calcite cement and saddle dolomite, in the Silurian formations, and may represent evidence of similarity in terms of mineralizing fluid responsible for their formation. This assumption is also supported by evidence from REE analysis (discussed in the following section). The overall higher salinity (Figure 15B and Figure 16B) which characterizes RD2 and BKC from Silurian formations compared to those of the same mineral phases from Devonian formations, suggests that the diagenetic fluids responsible for their formation were saline basinal brines resulted from the dissolution of evaporites hosted in Silurian formations (e.g., [5,77]). The high homogenization temperatures of late calcite cement and saddle dolomite and the relatively high T_h of fluid inclusions hosted in replacive dolomite (Figure 14A and Figure 15A) are significantly higher than the maximum burial temperatures suggested for the study area (ca 60–90 °C [91]) for both age groups and hence, it may suggest the involvement of hydrothermal fluids [5,13,90].

The distinct patterns of salinity and homogenization temperature (Figure 14, Figure 15 and Figure 16) that characterize Devonian and Silurian dolomite may suggest that dolomitization/dolomite alteration and other related diagenetic processes are the result of two different diagenetic fluid systems: (1) a diagenetic fluid system that affected Silurian carbonates characterized by high temperatures and high salinity and (2) a diagenetic fluid system that affected Devonian carbonates characterized by slightly lower temperatures and salinity (e.g., [77]).

5.4. Constraints from Geochemistry

5.4.1. Stoichiometry

Most dolomite samples, with some exceptions for RD1, show Ca-rich, non-stoichiometric composition in both Silurian and Devonian formation. Several studies on dolomite [50,68,79,92,93] suggested that the recrystallization of fine crystalline dolomite forms coarser, more stable and more stoichiometric dolomite. However, coarser pervasive dolomites are overall less stoichiometric than fine crystalline dolomite. A possible explanation for such a trend is that these results reflect the average composition of the cores and rims of the crystals. Separate measurements for cores and rims, which were not performed for these dolomites, may have shown very substantial stoichiometric differences. The model proposed by Sibley [92] suggests that progressive recrystallization of non-stoichiometric dolomite should result in an increasingly stoichiometric dolomite characterized by Ca-rich cores and stoichiometric rims. Even though, this phenomenon has been observed and documented in many studies (e.g., [72,94]), stoichiometry has not been achieved in the Silurian and Devonian formations investigated. An experimental study by Malone et al. [95], demonstrated that dolomite subjected to temperatures of 50 °C–200 °C lead to its recrystallization with a rapid increase of stoichiometry in the first part of the experiment, but with a slowing down as reactions progressed. Despite complete recrystallization at 200 °C, dolomite stoichiometry was not achieved.

The interpretation that better explains this trend is that earlier-formed dolomite has been recrystallized by later, Ca-rich, and warmer basinal fluids, under conditions that did not allow these dolomites to achieve stoichiometry (e.g., [69]).

5.4.2. Trace Elements: Iron, Manganese and Strontium

The distribution of iron and manganese in solution is strictly controlled by redox conditions (Eh) [96] as well as their concentrations in precursor limestone and precipitation rate [97]. In the condition of positive Eh (oxidizing conditions), Fe²⁺ and Mn²⁺ oxidize passing from a divalent to a trivalent state inhibiting their incorporation in carbonates crystal lattice [59].

RD1, RD2 and SD in Silurian formations and RD1, RD2 and RD3 in Devonian formations show a progressive increase in Fe and Mn concentration (Table 5). Previous studies on dolomite [65,98] suggested that the relative enrichment of Fe and Mn in later dolomites can be attributed to dolomite recrystallization. Higher content of Fe and Mn in coarser dolomites could be related to either formation under relatively more reducing conditions [99] or increasing Fe and Mn distribution coefficients resulting from higher temperatures or slower precipitation rate [100]. Despite this increase of Fe and Mn there is no evidence of a corresponding variation in terms of Sr concentration between RD1 and younger dolomite phases in both Silurian and Devonian formations. This evidence may suggest that the different phases of dolomite observed within both age groups are perhaps linked to their parental mineralizing fluids (in this case seawater-dominated fluids). The low Sr concentrations (mostly in the range between 50 and 100 ppm for all dolomite samples in both age groups) are comparable to those of ancient dolomite precipitated from marine waters (e.g., 40–150 ppm [101]), but significantly lower than modern protodolomites (500–700 ppm) formed in evaporitic marine settings [79,102]. Decrease of Sr concentration with increasing crystal size is interpreted to be associated with dolomite recrystallization in presence of Sr-depleted meteoric waters (e.g., [50,79,98,103]. However, matrix dolomites from both age groups do not show evidence of a decrease of Sr concentrations with increasing crystal size.

Despite, the lack of evidence of co-variation of Sr with either Fe and Mn, their higher concentrations in coarser dolomite (RD2 and SD in Silurian samples and RD2 and RD3 in Devonian samples) compared to RD1, can be related to their formation from Fe- and Mn-enriched fluids under more reducing conditions [7].

5.4.3. Rare Earth Elements (REE)

Previous studies focused on REE compositions of carbonate rocks suggest controversial scenarios in terms of preservation [104,105] or modification [54], after diagenesis, of rare earth element patterns and concentrations compared with those of their precursor seawater. Concentrations of REE in ancient carbonate rocks can be useful tools to establish the nature of the mineralizing source [106,107,108]. Chaudhuri and Cullers [109], Webb et al. [104] and Allwood et al. [105] suggested that during the post-depositional processes such as diagenesis and metamorphism REEs patterns can remain essentially unaffected, considering that a large amount of diagenetic fluids is required to record substantial changes [106]. In other words, the REE concentrations in carbonate rocks are principally influenced by the depositional environment [110,111]. However, according to recent studies (e.g., [5,54]) diagenetic processes may sensibly alter REEs patterns and concentrations, making them an ideal tool for the understanding of REEs relocation in carbonate rocks.

The distinctive Post-Archean Australian Shales (PAAS) normalized seawater REE concentrations have patterns characterized by the following features: depletion of light REE, negative Ce anomaly and slight positive La and Gd anomalies [43,112,113,114].

Silurian

Average shale-normalized REE patterns of RD1 and RD2 show similar trends compared to those of Silurian brachiopods [54] but with significantly higher ΣREE (Figure 17A). All dolomite and calcite samples are characterized by a slight enrichment of LREE over HREE and a more significant enrichment of MREE. The comparison between the trend of Silurian brachiopods [54] and those of RD1 and RD2 (Figure 17A), possibly suggests an initial common seawater-dominated source of diagenetic fluids partially masked afterward by a different evolution of the diagenetic fluids [5]. Differently from Devonian patterns, Silurian dolomite trends are, in fact, characterized by a minor negative La anomaly and both cases of positive and negative Ce anomalies (Figure 17A,B) possibly implying their formation from evolved seawater. Both types of dolomite RD1and RD2 exhibit higher average ΣREE (11.07 ± 0.93 ppm, and 9.79 3 ± 7.8 ppm, respectively) compared with the average of Silurian brachiopods (2.21 ppm) proposed by Azmy et al. [54].

Saddle dolomite (SD) and Blocky calcite cement (BKC) show similar REE shale-normalized trends (Figure 17A) with average ΣREE (47.06 ± 41.95 ppm and 23.74 ± 23.00 ppm, respectively) and both significantly differ from the Silurian Brachiopods REE patterns. SD and BKC both show major negative La and Ce (Figure 17B) anomalies as well as a slight negative Eu anomaly (Figure 17A). The differences between replacive dolomite matrix RD1 and RD2 compared with Saddle dolomite and blocky calcite cement include progressively higher ΣREE and a more pronounced enrichment in LREE, and hence may suggest a different source of REEs (Figure 17).

Devonian

Average shale-normalized REE patterns and their comparison with those of modern warm water brachiopods [54] show that all dolomite types and late calcite cement sampled from Devonian formations have maintained the original seawater REE characteristics with a negative Ce anomaly (Figure 18A,B), and slightly positive La and Gd anomalies [43,112,113,114]. Proceeding from older (RD1) to younger (RD3) dolomite, shale-normalized REE show similar patterns coupled with a progressive enrichment in average ΣREE (Figure 18). These characteristics may indicate a common seawater-dominated diagenetic fluid slightly modified during progressive deeper burial because of the increasing water/rock interaction [5].

5.5. Evolution of Diagenetic Fluids

The evolution of diagenetic fluids involved during dolomitization in the eastern side of the Michigan Basin can be deduced by integrating petrographic, isotopic and fluid inclusions data.

The relationships between the oxygen isotopic composition of the mineral phase, of the fluid (δ¹⁸O_fluid) and temperature of formation (T_h from fluid inclusions studies) are shown in Figure 21.

Figure 21A shows the relationship between the measured δ¹⁸O values of dolomite from Silurian and Devonian formations vs. temperature. It also shows the δ¹⁸O values of formation fluids as related to the dolomite-water oxygen fractionation equation [82]. Isotopic compositions of fluid involved in the formation of RD2 in Silurian and Devonian formations show an overlap but Devonian RD2 shows slightly more enriched δ¹⁸O_SMOW likely related to the presence of more evolved basinal fluids (e.g., [77]) and a narrow range of temperature compared to the Silurian RD2. On the other hand, Saddle dolomite shows more enriched δ¹⁸O_SMOW and higher temperatures than the RD2 matrix dolomite. Considering the most negative δ¹⁸O value from Silurian RD2 (−7.96‰ VPDB) and assuming a Silurian seawater value of −3.5‰ SMOW [83], results in a precipitation temperature of 56 °C, which is significantly lower than the average temperature obtained from fluid inclusion microthermometry (89 °C). Considering the most negative δ¹⁸O value from Devonian RD2 (−7.66‰ VPDB) and assuming a Devonian seawater value of −2‰ SMOW [51], results in a precipitation temperature of 33 °C, which also results significantly lower than the average temperature obtained from fluid inclusion microthermometry (83 °C). Thermal history reconstructions of the Michigan Basin have been carried out by many authors using different methods [4,6,7,13,91,115,116]. Considering the burial curve reconstructed by Zheng [7] the maximum burial temperature expected for Silurian formations would be ca. 63 °C, assuming the atmospheric temperature of 20 °C, a thickness of 1700 m for the overlaying sedimentary cover and a geothermal gradient of 25 °C /km. Recrystallization with the involvement of hydrothermal fluids (e.g., [5]) can be assumed considering the significantly higher (SD: 125 °C; RD2: 89 °C) temperature obtained from fluid inclusions microthermometric analysis. Considering the burial reconstruction proposed by Legall et al. [91] for Devonian formation using conodont and acritarch alteration index, the maximum burial temperature is estimated to be 60 °C, and hence, considerably lower than the temperatures obtained from fluid inclusions microthermometry. It follows that in both age groups, higher values of homogenization temperatures compared to those expected for maximum burial cannot be justified by the burial dolomitization model only [77] but must involve the presence of hydrothermal fluids [5,56].

Figure 21B shows the relationship between the measured δ¹⁸O values of late calcite cement (BKC) from Silurian and Devonian formations vs. temperature. It also shows the δ¹⁸O values of formation fluids as related to the calcite-water oxygen fractionation equation [117]. Isotopic compositions of fluid involved in the formation of BKC in Silurian and Devonian formations show an overlap. Devonian BKC, however, shows slightly more enriched δ¹⁸O_SMOW values compared to the Silurian BKC. This trend reflects a slight change in fluid chemistry in the two age groups that can be attributed to more evolved basinal fluids (e.g., [77]).

The fluid flow in intracratonic basins, such as the Michigan Basin, can be influenced by topography, compaction and convection related to density gradients (e.g., [118]).

The most likely fluid flow model, considering the low topographic relief and the carbonate rocks-dominated characteristic of the Michigan Basin, is the buoyancy-driven model related to temperature and density gradient [5]. Fractures and faults in sedimentary basins characterized by alternating carbonate rocks with variable permeability can promote the migration of cross-formational fluids [119]. In the Michigan Basin, faults and fracture systems in Paleozoic successions are controlled by the reactivation of basement structures and represent preferential pathways that promote fluid migration through low permeability sedimentary rocks such as shales and evaporites [5]. Homogenization temperatures of fluid inclusions hosted in dolomite samples from both Silurian and Devonian formations are significantly higher than temperatures estimated for maximum burial but lower than temperatures of fluid inclusions hosted in dolomites from previous studies carried out in the central part of the Michigan Basin (e.g., [13,91]). This evidence suggests that dolomite formation can be related to the presence of hydrothermal fluids that originated in the central part of the Michigan Basin migrated towards the margins [5]. The potential source of heat could be related to the reactivation of the buried mid-continental rift (MCR) during late Devonian to Mississippian time [120,121], while the preferential path for the migration of diagenetic fluids from the center of the Michigan Basin can be attributed to the Cambrian sandstones unconformably overlying the Precambrian basement [5,77]. Late calcite cement in sub-horizontal fractures near the base of the Silurian sequence dated in a recent study [120] show U–Pb ages of 318 ± 10 Ma determined by LA-ICP-MS and 313 ± 1 Ma by ID-TIMS. Hence, the authors suggest that the fluid flow of hydrothermal brines was influenced by the Alleghanian orogenies [120]. According to Sutcliff et al. [122] absolute dating of Cambrian and Silurian secondary minerals shows the same age and are contemporaneous with Alleghanian magmatism and uplift which caused the migration of hydrothermal fluids westwards from high-level Alleghanian plutons through the basal Cambrian.

6. Conclusions

Examination of cores, petrographic studies, microthermometric and geochemical analyses of Silurian and Devonian carbonates of the eastern side of the Michigan Basin (Huron Domain) led to the following conclusions regarding dolomitization and other related diagenetic processes observed in these successions:

• Silurian and Devonian formations are characterized by the presence of three types of replacive dolomite matrix RD1 (formed in shallow burial conditions), RD2 and RD3 (formed in intermediate burial conditions). In addition to these types, a coarse crystalline ferroan saddle dolomite cement (formed in intermediate burial conditions) filling fractures and vugs is observed only in the Silurian rocks. Early- and late-stage calcite cement have been distinguished in both groups of formations including isopachous, syntaxial overgrowth, dogtooth, drusy and blocky calcite.
• Isotopic data show an overlap between δ¹³C and δ¹⁸O values in Silurian and Devonian dolomites. In both Devonian and Silurian, many samples δ¹³C values fall in the range of values estimated for the marine dolomite of equivalent age. The negative shift in δ¹⁸O values, however, show evidence of dolomite recrystallization during burial and increasing temperature. Several other lines of evidence for recrystallization have been observed such as increasing crystal size, zonation, etched surfaces between cores and rims of crystals, and an increase in radiogenic strontium ratios.
• Silurian and Devonian Sr isotopic ratios show the seawater composition of their respective age as the primary source of diagenetic fluids with minor rock-water interactions. However, Silurian samples show a more radiogenic signature that documents more intense interactions of water with feldspar-bearing rocks
• REE shale-normalized patterns suggest that in both age groups the diagenetic fluids were originally of coeval seawater composition subsequently modified via water-rock interaction. The different evolution of the diagenetic fluids is more prominent in REE_SN patterns from Silurian samples and it is possibly related to brines, which were modified by the dissolution of Silurian evaporites from the Salina series.
• Homogenization temperatures characterizing fluid inclusions hosted in RD2 from the Devonian samples, and RD2 and saddle dolomite in the Silurian successions exceed the temperature estimated for maximum burial. Hence, it may suggest the involvement of hydrothermal fluids during their formation.
• The potential source of heat could be related to the reactivation of the buried mid-continental rift (MCR), whereas the preferential path for the migration of diagenetic fluids from the central part of the Michigan Basin towards the margins via faults and fractures can be attributed to the Cambrian sandstones unconformably overlying the Precambrian basement that could have behaved as regional aquifers.
• Geochemical and fluid inclusion investigations denote two distinct fluid systems related to the diagenetic and tectonic history of Michigan Basin.