Dolomitization Facilitated by Clay Minerals on Mixed Siliciclastic-Carbonate Shoals of Carboniferous Age in the Tarim Basin, China: Constraints on Element Mobility and Isotope Geochemistry

Abstract

In the western Tarim Basin, Carboniferous granular dolostones deposited on a carbonate platform contain a small amount of terrigenous materials of sand-size fraction, agglomerated clay minerals, or similar phases. However, the role of terrigenous materials on dolomitization is still unclear. The aim of this study was to reveal the dolomitization mechanism. The granular dolomites have small crystal size, earthy yellow color, and fabric-retentive texture, with relatively good order. These features indicate dolomites precipitated during early diagenesis. The ratio of rare earth elements (RREs) abundance of the stable isotopes ⁸⁷Sr/⁸⁶Sr relative to Post-Archean Australian Shale (PAAS) normalized patterns was used to study the source of the dolomitizing fluids. The composition of REEs is characterized by heavy rare earth (HREE) enrichment (average Nd_SN/Yb_SN = 0.83). There is a positive (La/La*)_SN anomaly and slightly positive (Gd/Gd*)_SN and (Y/Y*)_SN anomaly; δ¹⁸O of seawater in fractionation equilibrium with granular dolostones was from −2.8‰ to 1.7‰ PDB, implying the dolomitizing fluid was contemporary, slightly evaporated seawater. The granular dolostones on the relatively thick shoals were subject to subaerial exposure before pervasive dolomitization, with evidence that the input of detrital kaolinite predated the formation of dolomites. Higher ⁸⁷Sr/⁸⁶Sr values and ∑REE in granular dolostones than the values in equivalent limestones indicate that dolomitization was related to terrigenous materials. Within the terrigenous materials, the negative-charged clay minerals may have catalyzed the dolomitization, resulting in dramatically decreased induction time for precipitation of proto-dolomites. A greater amount of terrigenous materials occurred on the shoals at the sea level fall, resulting from enhanced river entrenchment and downcutting. As a result, after subaerial exposure, the penesaline water flow through the limy allochems sediments lead to dolomitization, with the catalysis of illite on relatively thick shoals.

1. Introduction

Stratabound dolomites occur throughout the geological record, from Pre-Cambrian to the Late Pliocene [1,2,3]. The origins of these dolomites have been the subject of much research and significant debate [4,5,6,7,8,9,10,11]. In many cases, the petrographic features; δ¹⁸O, δ¹³C; and ⁸⁷Sr/⁸⁶Sr ratio of dolomites show the dolomitizing fluid could be seawater or slightly modified seawater [4,12,13], mesohaline brines [6,8,14], hypersaline brines [15,16], and hydrothermal fluids [9,13]. The mechanism of dolomitizing fluid through carbonate sediments after deposition could be the storm-driven flooding of the near-coastal supratidal flats [17,18], reflux of seawater/slightly evaporated seawater [19], evaporative pumping [15,20], and thermal convection [21,22] or geothermal convection along fault [23,24]. Accordingly, several conceptual models for dolomitization have been proposed to reveal the origin of stratabound dolostones in the last two centuries [3,6,7,8,9,14,16,22,23,24,25,26,27,28,29,30]. In particular, the dolomitization of island dolostones (e.g., Cenozoic Bahamas Bank) at considerable depth is performed by the seawater model [3,30].

The nucleation and growth of dolomite has been restricted by many kinetic inhibitors, e.g., temperatures (below about 50 °C), Ca²⁺/Mg²⁺ ratio, and salinity [31,32,33,34]. Up to now, microbes have been regarded as effective catalysts that promote non-stoichiometric, highly disordered and metastable proto-dolomite [25,35,36,37]. However, it would be difficult for abiotic dolomitization to occur under low temperature as a result of the kinetic barrier [33], unless occurring at elevated temperatures, with most temperatures being >175 °C in simulation experiments [34,38,39]. However, in some island dolostones, dolomitization occurs in the seawater mixed with meteoric water under low temperature below 50 °C without microbial catalysts, [3,30], e.g., Little Bahamas Bank, Xisha Island, Cayman Islands. Catalysts overcoming the kinetic barrier to promote abiotic dolomite formation at low temperature might exist in the above environment. Clay minerals are ubiquitous and may contribute to dolomitization as the catalyst, rather than only providing Mg²⁺. A simulation experiment has confirmed that highly negative-charged clay minerals can facilitate the precipitation of abiotic proto-dolomite under ambient conditions [10].

In this study, granular dolostones from the Late Carboniferous at Bachu-Maikit area in the Tarim Basin, western China, have been examined. Dolostone bodies have stratabound geometry. These dolostones contain a small amount of terrigenous sand grains and clay minerals deposited on carbonate shoals, alternating with sparry granular limestone and partially dolomitzed limestones. Previous studies based on isotope and petrography show that meteoric water has influenced the granular limestones in the Lower Member [40,41], and dolomitization was likely penecontemporaneous [40,42]. However, the mechanism of dolomitization and the pathway of dolomitized fluid flow during the early diagenesis are poorly understood. In this study, the timing of dolomitization, the geochemical feature and hydrology model of dolomitized fluid, and the effect of clay minerals on early dolomitization are discussed, using petrographic observations and geochemical analyses.

2. Geological Setting

Tarim Basin, western China, is located between the Kunlun and Tienshan orogenic belts, with a boundary of the Altyn Tagh Deep Fault (Figure 1). The southern basin was a passive continental margin of the Paleo-Tethys Ocean in the Carboniferous-early Permian [43]. During the Late Carboniferous, Tarim Block drifted to about 20° N [43], which was located in the tropical to northern temperate zone of the Paleo-Tethys Ocean [44,45]. During the Late Devonian-Mississippian period, continents uplifted at the northern, eastern, and southern areas of the Tarim Block (Figure 1). A southwesterly opening lagoonal environment with restricted circulation developed in the central and western areas [46]. In the Early Pennsylvanian period, the depositional environment was restricted lagoonal and tidal flat facies [47,48]. With the rapid sea level rise, a carbonate platform formed in the central and western areas of the basin [40,48]. At the end of the Moscovian, a dramatic sea level fall terminated the growth of the carbonate platform, with obvious sedimentary discontinuities at the top of the Xiaohaizi Formation [48]. Accordingly, the sedimentation from the Early Pennsylvanian period to the Late Muscovite period in the Tarim Basin was mainly controlled by sea level change and experienced little effect from local tectonism [46].

Dolostones from the Xiaohaizi Formation of the Late Carboniferous at the Bachu-Maikit (BM) area in the northwest of Tarim Basin were studied. The BM area is composed by Bachu uplift and Maikit slope. Maikit slope dips southwest, with the burial depth of the top of the Xiaohaizi Formation from 4266 m to 4543 m. Bachu uplift is located in the western part of the central paleo-uplift, formed during the Late Permian [49]. The depth of the Xiaohaizi Formation ranges from 1868 m to 1935 m. There are two gas fields developed in the BM area, named Yasongdi and Bashituo (Figure 1). The Xiaohaizi Formation is the most important gas zone in these two fields. The Xiaohaizi Formation overlies the Kalashayi Formation of the Middle Carboniferous, and is underlain by the Nanzha Formation of the Early Permian. Mudstones and calcareous mudstones of the Kalashayi Formation deposited in the BM area [50]. The geological age of the Xiaohaizi Formation was from the end of the Bashkirian to the Early Moscovian [50]. The carbonate open platform facies with various carbonate shoals developed in the study area in the period of the Xiaohaizi Formation (Figure 1). The BM area is one of the centers of Early Permian volcanic activity in Tarim Basin. There are two deep fractures cutting from Cambrian to Permian (Figure 1). The burial history of the Xiaohaizi Formation in well BT4 was reported by Wang et al. (2013) [40]. In the BM area, the Xiaohaizi Formation is experiencing the maximal burial depth at present, and the other subsidence period happened in the Late Permian (Figure 2).

Figure 1. Location of Bachu-Mikit area and sedimentary facies distribution of Upper Carboniferous Xiaohaizi Formation (modified from [43,50]). (A)The study area is located in the lithofacies paleogeography of the whole basin of Tarim Basin. The red box shows the region of Bachu-Macheti. (B) The distribution of sedimentary facies and data wells in the study area. Note that facies were mainly shoals on shallow open marine area in Bachu-Mikit area. The western part of the study area has uncertain sedimentary facies boundary. Well BT5 is located at the eastern part of the main study area.

Figure 1. Location of Bachu-Mikit area and sedimentary facies distribution of Upper Carboniferous Xiaohaizi Formation (modified from [43,50]). (A)The study area is located in the lithofacies paleogeography of the whole basin of Tarim Basin. The red box shows the region of Bachu-Macheti. (B) The distribution of sedimentary facies and data wells in the study area. Note that facies were mainly shoals on shallow open marine area in Bachu-Mikit area. The western part of the study area has uncertain sedimentary facies boundary. Well BT5 is located at the eastern part of the main study area.

The upper and lower members developed in the Xiaohaizi Formation [41]. The main lithology of the Upper Member is marly limestones or argillaceous micrites, deposited in lagoon [51,52]. However, the Lower Member mainly contains granular limestones and dolostones, which formed on shoals of the open platform [53] (Figure 2). The lithology of the Lower Member is characterized by porous granular dolostones alternating with tight cemented granular limestone. The grains include ooids, oncoids, bioclastic debris (e.g., algae, bivalve, brachiopod, echinoderm), intraclastic material and full organism (e.g., Fusulinella sp., Taizchoella sp., Climacammina sp., Cribrogenerina sp., and Globivalvulina sp.) [40,50].

3. Methods

The petrographic studies and geochemical measurements are derived from ten wells in the Xiaohaizi Formation at the BM area (

Table 1. Sampling information of the cores from Xiaohaizi Formation.

Well	Thickness of the Cored Intervals (m)	Total Thickness of Tested (m)	The Number of Thin Sections	TH	Elements Composition	SEM Observation	δ13C	δ18O	87Sr/86Sr	XRD
M4	10.8	8.25	7		√	√	√	√	√
M10	51.62	36.22	36	√	√		√	√		√
BC1	11.65	8.76	8				√	√
BT2	9.54	8.4	5		√	√	√	√	√
BT3	15.89	14.79	9		√		√	√	√	√
BT4	18.92	16.23	18	√	√	√	√	√	√	√
BT5	21.5	12	15				√	√
BT6	16.2	15.42	29				√	√
BT7	7	6.26	3				√	√
BT8	37.23	28.75	31				√	√

). The core thickness of Xiaohaizi Formation in well M10 is the largest, so more samples are selected from there. A total of 132 thin sections of carbonate rocks were prepared for petrological observations. All thin sections were stained with Alizarin-Red S and potassium ferricyanide, allowing the distinction between calcite and dolomite and the iron content of each mineral. The petrographic features, mineral compositions, elements, stable C and O isotopes, and ⁸⁷Sr/⁸⁶Sr values were measured to determine the geochemical characteristic of dolomitized environment, and the homogenization temperature of fluid inclusion was detected to obtain the burial diagenetic condition.

Thin sections were observed with a cathode-luminescent microscope (CL). The polished thin section surface was observed using CL MK5-2 series (CITL, Hertfordshire, UK). The voltage of this equipment was 11 kV, with the current of 380 μA. CL images were captured with the exposure times of 5 s.

A total of 8 samples from well BT2, BT4, and M4 were examined using scanning electron microscope (SEM). Quanta 250 FEG (FEI Company, Hillsboro, OR, USA) and an INCA x-max20 (Energy Disperse Spectroscopy, Oxford Instruments, Abingdon, UK) were used to observe the samples. Under high vacuum, in the model of secondary electron, the resolution for imaging is 2 nm. Under the model of backscattered electron, the resolution for imaging is 2.5 nm. The samples were carbon-coated and the beam diameter was 4 μm.

The content of minerals was determined by X-ray diffraction (XRD) (Rigaku DMAX-3C with Cu Kα radiation)(Rigaku Corporation, Tokyo, Japan), as well as degrees of the order of dolomite. The experimental conditions are 40 kV, 20 mA. The angle accuracy is bigger than 0.02° (2θ). The scanning speed is 0.05 s per step, and the scanning range is 3–50° (2θ). Using the PDF2 (2004) computer using Jada 5.0 software, semi-quantitative phase analysis was completed. When the mineral content is greater than 40%, the relative deviation is less than 10%. The order degree of dolomite is calculated by the ratio d(015)/d (110) of the two diffraction peaks. The molar percentage of CaO and MgO in dolomite from 3 samples (4 collection points) in wells BT3 and BT4 was determined by electron probe microanalysis (EPMA) (EPMA-1720 H Series, Shimadzu Instruments, Kyoto, Japan). The pure metals were used as standards. The resolution is less than 129 eV, and the precision for the major elements is less than 1%.

The weight percentages of insoluble residues within dolostones in well M10 were measured by dissolving the rock with 5% HCl and weighing the residues. These insoluble residues are composed of terrigenous materials (e.g., clay mineral, quartz, and feldspar).

In order to compare the element composition of different lithology on different shoals in the study area, granular dolostones, partially dolomitized limestone, and limestones from the Xiaohaizi Formation were selected in BT2, BT3, BT4, M4, and M10 wells (Figure 1). A total of 13 samples for major elements were measured using Optima 5300 V Inductively Coupled Plasma atomic emission spectroscopy (ICP-AES) (Perkin-Elmer Company, Norwalk, USA). REEs of nine samples were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (7700 Series, Aglient Technologies, CA, USA). A quantity of 100 mg of 13 bulk sample powder was evenly divided and dried at 100 °C for 2 h and dissolved in a mixed solution consisting of 4 mL hydrofluoric acid, 2 mL hydrochloric acid, 3 mL nitric acid, 1 mL perchloric acid, and triplet of sulfuric acid. The sample dissolved in the solution is then heated to about 200 °C for 4 h until white smoke appears. A quantity of 5 mL of chlorazic acid was added to the solution to extract the element. Transfer the solution to a 50 mL volumetric bottle and dilute it with deionized water. Then, the major element was determined by ICP-AES and the trace element was determined by ICP-MS.

The stable isotopes of dolostones and limestones from ten wells were all measured to make a comparison laterally in the BM area. A total of 37 samples were analyzed for δ¹⁸O and δ¹³C of whole rock, using Thermo Scientific MAT253. The isotopes of relatively pure dolostones without calcite cements may represent that of dolomites. All isotope data are calculated as per mil deviation from the Pee Dee Belemnite (PDB) standard. The samples were treated with phosphoric acid. The digestion device is a rotary disk-type constant temperature reaction device. Phosphoric acid with 99.0% purity is added to the digestion device at 70 °C for 24 h. The precise carbon and oxygen isotope values were 0.0037‰ for ¹³C, and 0.013‰ for ¹⁸O.

A total of 12 samples from wells BT2, BT3, BT4, and M4 in the Yasongdi and Bashituo fields were analyzed for ⁸⁷Sr/⁸⁶Sr ratios using Thermo Scientific TRITON Plus Thermal ionization MS. The 50 g samples were put into the flasks to react with 2.5 mL 2.5 N HCl for 12 h, and then they were dried under 120 °C. A quanitty of 1 mL 6 N HCl was added into the flask at the temperature of 100 °C for 20 min, then dried under 120 °C. After the dissolution of sample, a quantity of 1.5 mL 2.5 N HCl was added into the flask, then the solution was transferred into the tube to centrifuge, with 3000 rpm for 10 min. After that, the pre-equilibration column was equilibrated by 5 mL 2.5 N HCl. A quantity of 1 mL liquid supernatant of the centrifuge tube was added on the column. The column was flushed with 0.5 mL 2.5 N HCl four times. Then, a quantity of 12 mL 4 N HCl was added into the column, and the eluent was removed. A new flask was used to receive the solution containing Sr, and the column was flushed again by 15 mL 4 N HCl. Finally, the solution containing Sr was put into the MS, and the ⁸⁷Sr/⁸⁶Sr ratios weredetected. The strontium isotope measurement error is expressed as 2σ±.

In order to exclude the influence of late diagenesis and hydrothermal fluid charging, the diagenetic temperature of dolostones adjacent to the deep fracture is estimated. A THMS600 Cooling-Heating Stage (Linkam Scientific, Surrey, UK) was used in the study. The homogenization temperature (TH) of inclusions in calcite veins and calcite cement was measured by using samples from 5 Xiahaizi formations near the deep fault in wells BT4 and M10. It failed to find gas–liquid inclusion within fine crystalline dolomites. The temperature range of the instrument is −196 to 600 °C with an accuracy of <0.1 °C. When approaching the critical point of rapid homogenization of gas–liquid two-phase inclusions, the heating rate can be controlled within 1 °C/min.

4. Results

4.1. Lithology and Mineral Composition

The main lithologies in the Lower Member of the Xiaohaizi Formation include granular dolostones (Figure 3A,B), granular limestones (Figure 3C,D), partially dolomitized granular limestones (Figure 3E), and dolomicrites (Figure 3F). The granular dolostones are characterized as stratabound, fabric-retentive, and dark brown in color (Figure 3A). In terms of crystal size, the granular dolostones contain fine crystalline dolomites (4–63 μm). In the bottom of a set of granular dolostones, dolostones are partially cemented by poikilotopic calcite in the M10, BT3, and BT8 wells (Figure 3B). Vadose silt could be observed in the moldic pores of dolostones in granular dolostones in well BT8 (Figure 3G,H). The thickness of shoals can be calculated by combining core description with well logging. As shown in Figure 4 and

Table 2. Relevant information of stratigraphy and lithology in Xiaohaizi Formation.

Items	Bachu Uplift				South of the Maigaiti Slope			North of the Maigaiti Slope
Well	BC1	BT2	BT3	BT5	BT6	BT8	BT9	M4	M10	BT4
The interval of burial depth (m)	1935–2033	1876–1971	1868–1944	1887.5–1970	4405–4486	4458–4544	4543–4655	4343–4417	4361.5–4457	4266–4333.5
The total thickness of the Xiaohaizi Formation (m)	98	95	76	82.5	81	86	112	74	95.5	67.5
The thickness of shoal (m)	30.35	36.5	38.5	33	36.37	54.44	49	45.5	57.5	31.81
The thickness of dolomitized carbonate rock (m)	19.75	35.5	17	33	9.4	28.12	24	11.9	38.7	21.58
The proportion of the shoal to the total thickness of the Xiaohaizi Formation (%)	30.97	38.42	50.66	40	44.9	63.3	43.75	61.49	60.21	47.13
The proportion of dolomitized shoal to the total thickness of shoal (%)	65.07	97.26	44.16	100	25.85	51.65	48.98	26.15	23.82	67.84

, dolomitized carbonate rocks are prone to occur in relatively thick shoals, while the proportion of dolomitized shoal to the total thickness of shoal has a negative relationship with the thickness of shoals accounting for the total Xiaohaizi Formation.

Granular limestones have grain-supported texture and contain various grains, including oosparite limestone (Figure 3C), intrasparite limestone, biosparite limestone (Figure 3D), and oncoidsparite limestone. Granular limestones are cemented by two generations of calcite cementation (Figure 3C,D). The first generation of calcite cementation occurs mainly as bladed calcite forming isopachous fringes on grains. The second generation is of blocky spar, filling in the void space. The observation of potassium ferricyanide stained thin section shows the ferroan-calcite is very rare, present in a very small amount of late diagenetic mineral.

Partially dolomitized granular limestones occurred in wells BT3, BT4, BT5, BT9, BC1, and M4 (Figure 3E). Partially dolomitized limestones have similar texture to granular limestones, with the microcrystalline calcite within grains dolomitized.

There are variable amounts of terrigenous material in carbonate rocks of the Xiaohaizi Formation. The detrital grains in granular dolostones are dominated by well-sorted and round quartz (Figure 3A), with minor kaolinite, dickite, illite, and uncertain terrigenous materials. The weight percentages of terrigenous material in granular dolostones range from 1.47 to 4.17 wt.% (

Table 3. Contents of acid-insoluble residues within dolostones from Xiaohaizi Formation.

Well	Depth (m)	Insoluble Residues Content # (wt.%)
M10	4397.30	3.63
M10	4398.10	3.97
M10	4400.00	4.17
M10	4404.94	1.47
M10	4406.54	1.69
BT3	1918.02	3.15

# samples were measured by dealing with 5% HCl.

). The size of terrigenous quartz ranges from 0.15 to 0.2 mm. Through the counting of terrigenous grains point by point under microscope, the contents of terrigenous detrital grains in granular dolostones in well M10 were determined to range from 1% to 8%. There are more terrigenous detrital grains in dolostones than in limestones (Figure 5). The contents of terrigenous grains increase upwards vertically in dolostones in the Lower Member of Xiaohaizi Formation on stratigraphy column of well M10 (Figure 5). The diffraction peak of illite (d = 4.28) can be seen on the XRD pattern (Figure 6).

4.2. Types and Characteristics of Dolomites

In terms of dolomite textural classification [7], dolomites from granular dolostones or partially dolomitized limestones all appear to be planar-e or planar-s (Figure 7A–C). According to crystal size classification of dolomites [8], the size fractions of dolomite are determined. Based on the observation of thin sections, and analysis of XRD (

Table 4. Determination of mineral composition by XRD.

Well	Depth (m)	Lithology	Content of Minerals (wt.%)						Degree of Order of Dolomite
			Kaolinite	Quartz	Calcite	Dolomite	Illite	Pyrite
M10	4397.30	granular dolostone	T	1	3	96	T	0	0.53
M10	4398.10	calcareous granular dolostone	2	2	33	63	T	0	0.63
M10	4400.00	calcareous granular dolostone	1	2	27	70	T	0	0.52
M10	4406.54	calcareous granular dolostone	T	T	25	75	T	0	0.5
M10	4404.94	calcareous granular dolostone	T	T	21	79	T	0	0.59
BT4	4312.11	microcrystalline dolostone	6	T	9	84	T	1	0.41
BT3	1918.02	granular dolostone	T	1	2	97	T	0	0.6

Note: T—trace.

), there are two types of dolomites in the Xiaohaizi Formation, which are described below.

4.2.1. Type A Dolomite

Dolomites within granular dolostones from the Xiaohaizi Formation are characterized by small crystal sizes; cloudy, earthy yellow color; and fabric-retentive texture (Figure 8A). Determined by SEM, the fine crystalline dolomites (about 14–32 μm) occur in granular dolostones. The CL colors of type A dolomites are non-luminescent or dull red (Figure 8B). XRD analyses indicate that dolomites in granular dolostones are relatively well ordered, with ordering ratios from 0.41–0.63 (Table 4). Determined by EMPA, the mole percent of CaO (55.16–58.58%) and MgO (39.21–43.09%) show that type A dolomites are stoichiometric (

Table 5. Mole percent of oxides in type A dolomite, determined by EMPA.

Well	BT3	BT3	BT3	BT4
Depth (m)	1913.62	1913.62	1911.72	4312.11
Lithology	Granular Dolostone	Granular Dolostone	Granular Dolostone	Dolomicrite
Crystal size	15 μm	30 μm	12 μm	10 μm
Type of dolomite	A	A	A	A
F	0.25	0.09	0.00	0.00
SrO	0.04	0.00	0.00	0.01
K2O	0.00	0.00	0.01	0.09
SO3	0.28	0.03	0.07	0.16
Na2O	0.02	0.00	0.02	0.01
BaO	0.00	0.00	0.00	0.00
MnO	0.00	0.01	0.01	0.02
CaO	58.58	58.03	57.72	55.16
MgO	39.21	40.62	40.24	43.09
TiO2	0.00	0.00	0.14	0.04
FeO	0.85	0.63	0.84	1.39
SiO2	0.32	0.29	0.45	0.03
Al2O3	0.46	0.29	0.49	0.00

). There are some kaolinite and illite-like clay minerals detected in granular dolostones (Figure 9A,B). In addition, rare anhydrite-replaced calcite or anhydrite cement occur in granular dolostones with type A dolomite.

4.2.2. Type B Dolomite

Type B dolomites partially replaced microcrystalline calcite within grains in dolostones, with cloudy center and clear rim (Figure 8C). In the SEM image, type B dolomites have coarse crystal (about 80 μm), larger than type A dolomites (Figure 7A–C). The coarse crystalline type B dolomites under CL have relatively brighter mottled red color than type A dolomites in granular dolostones (Figure 8B,D).

4.3. C, O Isotope

Bulk-rock stable δ¹⁸O of granular dolostone ranges from −4.36~2.8‰ (

Table 6. The composition of C and O isotope and the δ¹⁸O_seawater in fractionation equilibrium with dolomites.

Well	Depth (m)	Lithology	δ18Odol-PDB	δ13Cdol-PDB	δ18Oseawater-PDB
BT8	4532.05	Granular dolostone	2.8	−1.4	3.41
BT5	1933.10	Granular dolostone	−0.4	3.5	0.12
BT5	1937.01	Granular dolostone	0.1	0.1	0.63
BT5	1936.56	Granular dolostone	1.1	0.1	1.66
BT8	4512.40	Granular dolostone	−1.6	1.8	−1.12
BT8	4525.20	Granular dolostone	−2.0	−2.1	−1.53
BT8	4525.66	Granular dolostone	−2.2	−2.0	−1.74
BT8	4533.82	Granular dolostone	−2.6	−2.0	−2.15
BT8	4534.30	Granular dolostone	−1.4	−1.8	−0.92
BT8	4534.65	Granular dolostone	−3.2	−2.0	−2.77
BT4	4317.55	dolomicrite	1.3	1.9	1.87
BT5	1938.30	dolomicrite	1.7	−0.1	2.28
BT5	1944.46	dolomicrite	1.0	−2.2	1.56
BT5	1933.25	dolomicrite	0.5	0.4	1.04

). The δ¹³C of granular dolostone is from −2.1‰ to 3.5‰ PDB, ranging from isotopically light to heavy (Table 6).

It’s reported that the tropical seawater temperature in the Middle Pennsylvanian ranges from 19.0 to 30.2 °C. Given average tropical seawater temperature of 25 °C, the δ¹⁸O of seawater in fractionation equilibrium with granular dolostones is from −2.8‰ to 1.7‰ PDB, and that with dolomicrites ranges from 1.0‰ to 5.2‰ PDB (Table 6) (Equation (1) from [54].

$$1000\times \ln {\mathsf{\alpha }}_{dolomite-water}=2.73\times {10}^6{T}^{-2}+0.26$$

(1)

This equation is suitable for low temperature environments and can be used to calculate the oxygen isotopes of seawater in equilibrium with dolomite.

The δ¹⁸O values of granular limestones range from −6.5‰ to −7.9‰ PDB (

Table 7. Composition of C and O isotope of granular limestones from Xiaohaizi Formation.

Well	Depth (m)	Lithology	δ18Ocal-PDB	δ13Ccal-PDB
M10	4410.75	granular limestone	−7.90	2.30
BC1	1976.76	granular limestone	−6.90	−1.20
BT6	4450.60	granular limestone	−7.60	1.70
BT8	4515.35	granular limestone	−6.80	−1.30
BT7	4434.51	granular limestone	−6.50	1.60
BT7	4432.07	granular limestone	−7.40	1.50
M10	4430.42	granular limestone	−6.50	−2.60

), which is obviously more depleted than granular dolostones. The difference in oxygen isotope fractionation between dolomite and limestone can be as high as 2.5‰, even if the main oxygen isotope of dolomite is still heavy [6]. The δ¹³C of granular dolostones ranges from −2.6‰ to 1.7‰ PDB (Table 7), lighter than that of granular dolostones. There is a positive correlation between carbon and oxygen isotope, implying that meteoric water influence existed at the top of each upward shallower cycle.

4.4. ⁸⁷Sr/⁸⁶Sr Isotope

The ⁸⁷Sr/⁸⁶Sr values of limestones range from 0.7079 to 0.7083 (

Table 8. ⁸⁷Sr/⁸⁶Sr values for dolostones and limestones from Xiaohaizi Formation.

Well	Depth (m)	Lithology	δ18O V-PDB	87Sr/86Sr	Error (2σ)
BT2	1926.30	Granular dolostone	−4.13	0.7085	4.63 × 10−5
BT2	1923.60	Granular dolostone	−4.36	0.7083	3.43 × 10−5
BT2	1928.70	Partially dolomitized limestone	−6.73	0.7082	7.05 × 10−5
BT3	1920.27	Granular limestone	none	0.7081	4.28 × 10−5
BT4	4303.37	Granular limestone	−8.62	0.7079	9.49 × 10−6
BT4	4311.52	Granular limestone	none	0.7080	2.34 × 10−5
BT4	4316.40	Granular limestone	−7.67	0.7083	7.23 × 10−5
BT4	4319.60	Granular limestone	−7.83	0.7082	2.41 × 10−5
BT4	4309.34	Granular limestone	none	0.7079	3.23 × 10−5
M4	4388.50	Granular dolostone	−3.48	0.7082	4.05 × 10−5
M4	4386.85	Calcareous dolostone	−6.56	0.7081	2.03 × 10−5
M4	4393.29	Granular limestone	none	0.7080	3.81 × 10−5

). ⁸⁷Sr/⁸⁶Sr values of granular dolostones and calcareous dolostones (0.7081–0.7085) are higher than those of limestones (Table 8). There is a positive correlation between ⁸⁷Sr/⁸⁶Sr and δ¹⁸O (Figure 10A).

4.5. Rare Earth Elements (REEs)

The REE concentrations of carbonate rocks from the Xiaohaizi Formation are shown in

Table 9. REE concentrations of 9 samples from Xiaohaizi Formation. All concentrations in parts per million (ppm).

Well	Depth (m)	Lithology	ΣLREE	ΣHREE	ΣLREE/ΣHREE	Y/Ho	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu
BT2	1926.3	Granular dolostone	8.28	4.16	1.99	37.46	2.13	3.46	0.44	1.73	0.33	0.19	0.38	0.06	0.36	0.07	0.22	0.03	0.21	0.03
BT2	1923.6	Granular dolostone	7.70	3.98	1.94	39.88	1.95	3.19	0.40	1.61	0.34	0.21	0.35	0.06	0.34	0.07	0.20	0.03	0.17	0.03
BT4	4316.4	Granular limestone	5.89	3.12	1.89	40.90	1.48	2.49	0.31	1.25	0.25	0.11	0.27	0.04	0.25	0.05	0.16	0.02	0.15	0.02
M4	4386.85	Granular dolostone	32.20	12.45	2.59	31.59	8.05	14.60	1.60	6.38	1.32	0.25	1.42	0.23	1.28	0.25	0.72	0.10	0.59	0.09
M4	4388.5	Granular dolostone	20.99	10.35	2.03	32.43	5.02	9.67	1.05	4.18	0.91	0.17	1.03	0.17	1.02	0.21	0.59	0.08	0.50	0.07
BT2	1928.7	Partially dolomitizedlimestone	17.52	6.17	2.84	34.50	4.16	8.54	0.87	3.23	0.60	0.11	0.61	0.10	0.57	0.12	0.35	0.05	0.33	0.05
M4	4387.35	Granular dolostone	4.01	1.71	2.35	33.29	0.96	1.76	0.22	0.86	0.18	0.03	0.19	0.03	0.17	0.03	0.10	0.01	0.09	0.01
BT4	4314.46	Granular limestone	11.86	4.10	2.89	34.91	2.83	5.56	0.59	2.33	0.44	0.10	0.45	0.07	0.39	0.08	0.22	0.03	0.19	0.03
BT4	4303.37	Granular limestone	9.32	2.79	3.35	26.18	2.01	4.70	0.44	1.75	0.35	0.07	0.37	0.06	0.33	0.06	0.17	0.02	0.14	0.02

. Rare earth element data of five carbonate rocks from the Xiaohaizi Formation are normalized to Post-Archean Australian Shale (PAAS) [56]. PAAS can reflect the original REE composition of marine sediments, so PAAS is chosen as a standard. The REE profiles have a flattened heavy REE (HREE) profile (Figure 10B). The abundance of the light rare earth elements (LREEs) is higher than that of the heavy rare earth elements (HREEs). As shown in Figure 10C, the ∑REE of granular dolostones is obvious higher than that of limestones. The composition of REEs is characterized by HREE enrichment (average Nd_SN/Yb_SN = 0.83). The (Ce/Ce*)_SN [2Ce_SN/(La_SN + Pr_SN)] ranges from 0.83 to 1.15, yet without negative anomaly, identified by Figure 10D. There is a positive (La/La*)_SN anomaly (1.04–1.28), slightly positive (Gd/Gd*)_SN (1.03–1.17) and (Y/Y*)_SN anomaly (0.9–1.5) (

Table 10. The indices for the REE patterns of 9 samples from the Xiaohaizi Formation.

Well	Depth (m)	Litholoy	Nb/YbSN	La/La*SN	(Ce/Ce*)SN	(Eu/Eu*)SN	(Pr/Pr*)SN	(Gd/Gd*)SN	(Y/Y*)SN	Ba (ppm)
M4	4386.85	Granular dolostone	0.91	1.27	0.94	0.85	0.97	1.15	1.11	5.44
M4	4388.5	Granular dolostone	0.70	1.19	0.97	0.81	0.97	1.17	1.16	14.23
BT2	1928.70	Partially dolomitized limestone	0.82	1.04	1.04	0.87	0.97	1.12	1.24	4.65
M4	4387.35	Partially dolomitized limestone	0.82	1.11	0.89	0.90	1.03	1.11	1.17	4.76
BT4	4314.46	Granular Limestone	1.02	1.15	0.99	1.10	0.97	1.03	1.22	175.74
BT4	4303.37	Granular Limestone	1.03	1.10	1.15	0.94	0.91	1.12	0.90	3.49
BT2	1926.30	Granular dolostone	0.69	1.22	0.83	2.55	1.04	0.62	1.36	374.08
BT2	1923.60	Granular dolostone	0.77	1.28	0.83	2.82	1.03	0.57	1.44	542.79
BT4	4316.40	Partially dolomitized limestone	0.69	1.17	0.84	2.01	1.04	0.72	1.50	191.12

). The positive (Eu/Eu*)_SN anomaly ((Eu/Eu*) > 1) of four samples in Table 10 is caused by the relatively high concentration of Ba.

4.6. Major and Trace Elements

The major element Al in dolostones is mostly associated with fine-particle clay minerals [57]. Meanwhile, Rb is prone to be absorbed by clay minerals to enrich the terrigenous materials. The concentrations of Al and Rb in the impure dolostones from the Xiaohaizi Formation could be the proxies for the terrigenous fraction. The concentrations of the main major and trace elements are shown in

Table 11. Concentration of elements in granular dolostones from Xiaohaizi Formation.

Well	Depth (m)	Lithology	Mg/Ca	Sr (μg/g)	Ba (μg/g)	Ti (μg/g)	Mn (μg/g)	Fe (μg/g)	Al (μg/g)	Rb (μg/g)
BT3	1918.02	Granular dolostone	0.534	119	11	49	76	3581	4993	1.624
BT3	1921.48	Calcareous dolostone	0.375	168	545	56	112	5923	6090	/
BT4	4306.97	Granular dolostone	0.531	207	17	99	168	14,153	6792	2.363
BT4	4316.09	Micrite	0.032	268	178	38	36	1369	3698	0.956
BT4	4317.55	Calcareous dolostone	0.086	221	6	47	34	1500	3880	/
M10	4397.30	Granular dolostone	0.555	674	12	95	119	3575	4480	3.271
M10	4398.10	Calcareous dolostone	0.526	166	20	120	160	7102	6017	1.445
M10	4400.00	Calcareous dolostone	0.459	176	9	107	152	8423	5260	2.056
M10	4404.94	Calcareous dolostone	0.455	167	3	46	61	4138	3400	1.038
M10	4406.54	Calcareous dolostone	0.432	159	29	48	71	5290	5315	1.664

. The correlation between 1/Sr and Mg is positive (Figure 10E). The cross-plot between Al, Rb, and Mg/Ca concentration ratio shows a relative weak positive correlation (Figure 10F). The relatively high concentration of Fe in these dolostones is caused by the diagenetic pyrite filled in the dissolved pores [52].

4.7. Fluid Inclusion

The result of homogenization temperature of fluid inclusions within calcite veins or cements in granular dolostones was shown in

Table 12. Homogenization temperature of fluid inclusions within calcite in granular dolostones from Xiaohaizi Formation.

Well	Depth (m)	Host Mineral	Type	Gas/Liquid Ratio	Th (°C)
BT4	4312.11	calcite cement	saline	8	107.3
BT4	4312.11	calcite cement	saline	6	106.7
M10	4407.54	calcite cement	saline	7	103.4
M10	4407.54	calcite cement	saline	6	96.3
BT4	4316.09	calcite vein	saline	6	86.1
BT4	4316.09	calcite vein	saline	7	81.8
BT4	4316.09	calcite vein	saline	6	94.2
BT4	4316.09	calcite vein	saline	5	88.1
BT4	4316.09	calcite vein	saline	8	85.7
M10	4400.00	calcite cement	saline	8	81.7
M10	4400.00	calcite cement	saline	7	76.3
M10	4400.00	calcite cement	saline	6	84.3
M10	4400.00	calcite cement	saline	6	95.2
M10	4400.00	calcite cement	saline	7	82.4
BT4	4306.97	calcite cement	saline	10	73.6
BT4	4306.97	calcite cement	saline	8	122.1
BT4	4306.97	calcite cement	saline	7	86.3
BT4	4306.97	calcite cement	saline	7	102.1

. In terms of the homogenization temperature of saline fluid inclusions, the lowest formation temperature of calcite cements most frequently occurred at 80–90 °C, with minor at 100–110 °C and 120–130 °C (Figure 11). However, the calcite vein filled with fractures has the lowest formation temperature of 80–90 °C (Figure 11).

5. Discussion

5.1. The Implication of the Textural Characteristic of Dolomites

The formation of dolomite or dolomitization of precursor limestone could be identified based on the texture of dolomites. In the study area, there are at three crystal sizes of dolomites (Figure 3), consistent with three lithologies, e.g., granular dolostones, partially dolomitized granular limestones, and dolomicrites. The small crystal sizes, cloudy earthy yellow color, and planar crystal boundaries of type A dolomites in granular dolostones indicate the dolomitization begins or proceeds at low temperature [7,58] and with multiple-site nucleation and relatively rapid crystal growth on mud-rich limestone precursors [59]. It has been suggested that two dolomite populations with different crystal size (type A and type B dolomite) are closely related to the different temperature of dolomitization and the reactive surface area [60,61]. The coarser crystalline dolomites (type B dolomite) occurs in the partially dolomitized granular limestone, with cloudy center and clear rims, implying they undergo different fluid activity at shallow burial diagenesis under higher formation temperature than type A dolomite. There is ubiquitous two-generation calcite cementation, indicating that partially dolomitized granular limestone has experienced the calcite-saturated diagenetic fluid, compared with dolostones almost without cements. Moreover, the precursor limestones for the fine crystalline type A dolomite and the coarse crystalline type B dolomite have different reactive surface area; fine grain size could lead to more finely crystalline textures. The dolomitization of the type B dolomite with coarse crystalline might have replaced coarse precursor limestones, implying the recrystallized calcite matrix in the partially dolomitized limestone in burial diagenesis might predate dolomitization. Therefore, the formation of type A dolomites predated type B dolomites, with low temperature and large reactive surface area.

5.2. Timing of Dolomite Formation and Dolomitizing Fluid

This study focuses on the formation of type A dolomites in granular dolostones, with their high visible porosity making them a high-quality reservoir. As described above, the textural characteristic of stoichiometric type A dolomite implies that the dolomites grew under low temperature in the early diagenesis. Through the observation by SEM, the growth of dolomite was restricted by the detrital kaolinite (Figure 7G), implying that dolomite formed after the input of detrital kaolinite. Kaolinite is distributed in almost all carbonate rocks with visible pores in the Xiaohaizi Formation. It is impossible for kaolinite to be converted from K-feldspar in the burial diagenesis, for there is very limited input of K-feldspar in the carbonate shoals. Kaolinite accounts for up to 6% of bulk rock (Table 4). The occurrence of kaolinite is more likely related to the chemical weathering at subaerial emergence. It is suggested that the kaolinite could occur in carbonate rocks below the uncomformity. The earthy yellow color of type A dolomites (Figure 3A) was possibly caused by infected by organic matter during emergence [62]. Together with the vadose silt in the dolostones (Figure 3G,H), the earthy yellow color of type A dolomites indicates the possibility of subaerial exposure before dolomitization.

The δ¹⁸O of the seawater in the period of Early to Late Carboniferous ranges from −6‰ to 0‰ PDB [63], based on analyses of pristine brachiopod shells and other calcareous fossils. According to the average value of 2.5‰ as the fractionation between dolomite and calcite [64], The δ¹⁸O values of dolomite precipitated from normal Early to Late Carboniferous seawater range from −3.5‰ to 2.5‰ PDB. As shown in Table 6, in terms of the calculated δ¹⁸O of seawater in fractionation equilibrium with granular dolostones, the δ¹⁸O_dolostone resembles that of the dolostone formed from seawater. However, the δ¹⁸O values of granular dolostones are slightly heavier than that in seawater. Unless fluid rock ratios are high, dolomite should inherit the REE characteristics of temporal seawater [65,66]. The rare earth elements in PAAS-normalized (SN) model of granular dolomite can be used as an index to study the source of dolomitization fluid. As shown in Table 10, there is positive (La/La*)_SN anomaly, positive (Y/Y*)_SN anomaly, and slightly positive (Gd/Gd*)_SN anomaly in granular dolostones, implying the dolomitization fluids having affinity with seawater [67]. Moreover, the non-luminescent or dull red CL color of type A dolomites shows low Mn concentration [68], resembling seawater. The homogenization CL patterns of type A dolomites also suggest one phase of dolomite precipitation, and absence of recrystallization in the late diagenesis. Some enriched δ¹³C compositions of dolomites may indicate varying degrees of evaporation, perhaps early stages of methanogenesis [18,69]. As a result, it is credible that the dolomitizing fluid of type A dolomite is mainly contemporary seawater with slight evaporation. In addition, the absence of primary anhydrite indicates the dolomitizing fluid was below gypsum saturation.

The ⁸⁷Sr/⁸⁶Sr values of seawater during the Carboniferous Age were 0.7075 to 7080 [63]. The ⁸⁷Sr/⁸⁶Sr values of limestones in this study are higher than that in seawater, indicating the burial diagenesis overprinted these rocks. However, under similar diagenetic background, the ⁸⁷Sr/⁸⁶Sr values of granular dolostones ranging from 0.7081–0.7085 are higher than that of limestones (Table 8). The ⁸⁷Sr/⁸⁶Sr values indicate the fluid responsible for dolomitization in granular dolostones is explicitly influenced by the radioactive strontium cf. [70]. It is impossible that meteoric water contributes to the radioactive strontium for dolomitization, inasmuch as the flushing of the platform top by meteoric water will decrease the Mg/Ca ratio of seawater to levels insufficient for dolomitization [8]. The terrigenous materials in the mixed shoals of siliciclastic-carbonate could provide radioactive strontium to the dolomitizing fluid. The dolomitized fluid could derive from modified seawater mixed with a small amount of terrigenous input, especially fine components, e.g., clay minerals. Of course, there is another hypothesis for the origin of radioactive strontium. Based on the homogenization temperature of fluid inclusion within calcite cements and veins (Figure 11), the most important diagenetic fluid activity occurred at 80–90 °C. In terms of the burial history [40], the timing of diagenetic fluid activity was about 272–254 Ma in the Late Permian in the study. However, the occurrence of high homogenous temperature with more than 120 °C might be caused by the hydrothermal fluid charging, matching with the fracture activation in the Late Permian [49]. The hydrothermal fluid from underlying siliciclastic rocks entered into the dolostones along two deep fractures will carry the radioactive strontium to the porous dolostones. However, the absence of hydrothermal dolomites and very few hydrothermal minerals indicate the origin of high ⁸⁷Sr/⁸⁶Sr values of granular dolostones could not be related to the hydrothermal fluid.

There are few geochemical data of type B dolomite obtained in this study for their complicated mineral assemblies. The formation of type B dolomites in the partially dolomitized limestones has a different way distinguishing from type A dolomites. The brighter CL color of type B dolomites than the type A dolomite lattice indicates the relatively higher Mn concentration, assuming Fe concentration is less than 5000 ppm [71]. Mn²⁺ has been found to favor Mg²⁺ site in dolomite [72,73], with mobility in dolomitizing fluid. Meanwhile, the cloudy center and clear rim of type B dolomite show the calcite precursor replacement by dolomite at the burial condition during the late diagenesis [59].

5.3. The Depositional Environment of Dolomite Formation

Through the observation of thin sections, the granular dolostones with very few calcite cements have relatively high visible pores. The various residual dolomitized allochems in granular dolostones indicate they deposited on a carbonate shoal with mixed bioclast and intraclast (Figure 3A). In the vertical profile of the Xiaohaizi Formation, there are some rocks bearing clay (e.g., marlstone and marly limestone), which are interbedded with dolostones. In the upper part of this formation, lagoonal limestone containing clay minerals was deposited.

Multiple periods of sea level fluctuation appeared in the Late Carboniferous in response to Gondwanan glaciation [74,75]. The sea level fluctuation is recognizable in the sequence of granular dolostones alternating with tight granular limestone in the study area [52] (Figure 12). In the cross section, granular dolostones composed of type A dolomites are prone to occur on the top of upward shallower sequence (Figure 12). As shown in Figure 4, there is much thick granular dolostone with type A dolomite deposited when the shoal has a large scale, while the partially dolomitized limestones containing type B dolomites occurr on the relatively thinner shoals. As discussed above, the dolomitization of type A dolomite on the thick shoal occurred at the early diagenesis, while the partial dolomitization of type B dolomite on the thin shoal mostly formed at the late diagenesis. This might be caused by the sufficient dolomitized fluid flow in the interior of the thick shoal and the amount of reflux related to sea level fall. However, there is not a difference of fluid flow capacity between thick and thin shoals, because the carbonate rocks on the two types of shoals have similar grain composition resulting from similar precursor limestone. There is another hypothesis that subaerial exposure might be an important factor of the dolomitization difference between the thick and thin shoals. Based on the identification of thin sections, vadose silt related to subaerial exposure only occurred in well M10, BT8 and BT9, located on the thick shoal. As shown in Table 11, the Sr concentration of dolostones in well M10 has almost no change vertically (the abnormally high Sr concentration of sample at 4397.30 m in well 10 is caused by authigenous gypsum), while Mn concentration increases upward, indicating that meteoric water has influenced the dolostones at the top of shoal. The mechanism for the dolomitization influenced by subaerial exposure will be discussed in the next section.

Meanwhile, as shown in Figure 4, there is a relatively high partial dolomitization proportion on the thin shoal. The partial dolomitization postdating the calcite cementation occurred on the thin shoal at the burial diagenesis. From the CL color and crystal features, the partial dolomitization might be precipitated from connate seawater stored in contemporaneous limestone in burial area [76]. If the connate seawater was responsible for the partial dolomitization, it implies there will be a longer induction time for proto-dolomite precipitation in the partial dolomitization at low temperature.

5.4. The Catalysis Role of Clay Minerals on Dolomitization

Many synthesis experiments have shown that abiogenic stoichiometric dolomites are very difficult to precipitate at low temperatures [33]. The dolomitization has to experience three- or four-stage reaction and produce various intermediate products (e.g., low-magnesium calcite, high-magnesium calcite, very high-magnesium calcite, and nonstoichiometric, poorly ordered dolomite) [32,77,78], beginning with the induction stage. The induction stage is crucial to the dolomitization reaction [79]. The induction stage might last for a very long time [77,79] without the catalysts promoting the reaction. It has been proven that microbes could be effective catalysts for the reaction [25,35,36,37]. However, the inorganic chemical mechanisms that catalyze stoichiometric dolomite formation remain an enigma. Recently, new research has revealed that zinc in saline water can facilitate magnesium ion dehydration, resulting in a dramatic decrease in induction time [11]. The catalysis role of zinc on dolomitization may be a good explanation for the formation of abiogenic dolomites, but it is difficult to evaluate the different zinc concentrations in various depositional environments at the present.

In this study, clay minerals have a pivotal role in dolomitization; meanwhile, control the concentrations of elements like Fe, Sr and Mn etc. The clay minerals covered on the dolomites in granular dolostones might promote the precipitation of dolomite at low temperature. The microbe relicts are absent in the shoal containing various coarse allochems; see Figure 4, Figure 7 and Figure 9. As discussed above, the salinity of the dolomitized fluid responsible for type A dolomite was normal to slightly higher than contemporary seawater. The kinetics for the dolomitization in the fluid with normal salinity at low temperature could be overcome by highly negative-charged clay minerals (i.e., montmorillonite and illite) [10] via electrostatic binding of Mg²⁺ and Ca²⁺ ions and simultaneous desolvation of these strongly hydrated cations. As evidence from SEM images shows, illite-like clay mineral on the surface of fine crystalline dolomite was detected (Figure 9B), and several microcrystalline dolomites occurred on the terrigenous material aggregation (Figure 9C). Meanwhile, the ⁸⁷Sr/⁸⁶Sr values of granular dolostones bearing type A dolomites are heavier than limestones. There are no permeable clastic aquifers underlying the Xiaohaizi Formation. The radioactive strontium could originate from the mixed terrigenous clay minerals, rather than the dolomitized fluid interaction with clastic strata. As shown in Figure 10B, the ∑REE concentrations of granular dolostones is higher than that of partially dolomitized limestones and granular limestones, also resulting from the input of mixed clay minerals. The influence of clay mineral contaminant on the REE patterns has been excluded. The obvious positive (La/La*)_SN anomaly and relatively high (Y/Ho) ratio indicate there is no obvious clay contaminant [55]. Therefore, the petrographic and geochemical analyses both support that the clay minerals have a close relationship with dolomites. Combined with the simulation experiment from Liu et al. (2019) [10], it can be inferred that the highly negative-charged clay minerals can facilitate the precipitation of dolomites at low temperature and seawater with normal or slightly high salinity.

However, a major problem of this hypothesis is whether there is a sufficient amount of clay minerals for the dolomitization catalysis. In this study, the amount of illite in granular dolostones in the Xiaohaizi Formation is less than 5 wt.% for the very weak diffraction peak in the detection of XRD (Figure 6). However, many terrigenous materials are observed on the surface of dolomites under the SEM images, compared with the clean surface of granular limestones (Figure 7). The terrigenous quartz, feldspar, and lithic fragments could be carried by many geological processes, including offshore current and wind. Meanwhile, detrital clay minerals primarily originated from the hinterland. Keping Uplift, located in the north of the Bachu-Makit area, could provide the terrigenous detrital materials (Figure 1). River entrenchment and downcutting under falling sea level will result in more clastic input to the basin [80]. As discussed above, alling sea level also resulted in the subaerial exposure of the precursor of dolostones on relatively thick shoals. The relatively high contents of terrigenous quartz occurring in granular dolostones vertically in well 10 (Figure 5) show the possibility of dolomitization induced by terrigenous materials. Within these terrigenous materials, a small amount of illite could serve as nucleation centers for dolomite. Once these proto-dolomites are formed, they might act as nuclei for later massive dolomite formation [81].

The dolomitization in the partially dolomitized limestones occurring in the late diagenesis may be caused by the absence of sufficient illite to overcome the kinetic barrier. According to relevant simulation experiments, stoichiometric dolomites could be synthesized mostly under temperatures >175 °C [34,38,39]. Therefore, the elevated temperature could provide the dynamics to overcome the kinetics for proto-dolomite precipitation, resulting in partial dolomitization.

5.5. The Density-Dependent Convection of Dolomitized Fluid and the Mechanism of Dolomitization

Under the aforementioned dolomitized fluid and catalysts, the paleohydrological conditions for the dolomitization of granular dolostones on the mixed siliciclastic-carbonate shoals have been established (Figure 13). In this study, based on the evidence from geochemistry and petrography, the timing for the dolomitization of type A dolomite was proven to be penecontemporaneous at low temperature. The CL color of type A dolomite resembles that of the dolostones deposited in the peritidal area. However, the different hydrology systems between the peritidal area and shoals leads to a simple seepage–reflux model that could not explain the mechanism of dolomitization on the shoals. Reflux dolomitization is caused by brines percolating into previously deposited limestones, from repeated flooding and reflux of marine waters of slightly increased salinity [5,6]. In this study, the δ¹⁸O_dolostone of −3.2‰ to 2.8‰ PDB and the presence of free protogenetic calcium sulphates indicate that the brines are penesaline water, i.e., seawater concentrated below evaporate precipitation. Penesaline water is considered to be the most likely mechanism to provide the required magnesium [6,82]. The reflux system facilitated by basal and permeable siliciclastic aquifers was proposed in the past few years [9,14,24]. However, there is an absence of permeable sandstones in the underlying Kalashayi and Bachu formations (Figure 1). The penesaline water cannot flow through the underlying clastic aquifers. Although two deep fractures have developed, the fault-controlled dolomitization [9,24] could not be used to explain the origin of stratabound dolostones in the Xiaohaizi Formation.

The penecontemporaneous dolomitization of type A dolomite could be driven by the density-dependent convection of penesaline water. At the depositional period, limy allochems deposited on the carbonate shoals near the wave base. With the sea level fall, the relatively thick shoals were subjected to subaerial exposure, and the input of terrigenous materials was enhanced by river entrenchment and downcutting in the meantime. During the subaerial emergence, the meteoric water resulted in the dissolution of aragonitic allochems and chemical weathering of feldspar. The terrigenous quartz, feldspar, and clay minerals (e.g., kaolinite and illite) deposited together with limy allochems on the relatively thick shoal, and the shoal was adjacent to the hinterland (see Figure 13). Meanwhile, the restricted seawater among shoals would experience slight evaporation to produce penesaline water. During this evaporation, with water turbulence lessening, clay minerals were allowed to settle on the limy allochems, which could serve as nucleation centers for further dolomitization. After the sea level rise, these penesaline waters would have flowed through the permeable limy allochem sediments resulting from the density difference between penesaline and normal seawater. The proto-dolomites precipitated from the penesaline water with the catalysis of illite at low temperature, resulting in pervasive dolomitization of limy allochems on relatively thick shoals.

There is another dolomitization mechanism for the partially dolomitized limestone. Due to the absence of sufficient negative-charged clay minerals, the induction time of type B dolomites would have lasted for a long time, until the burial temperature increased and the connate seawater carried them, driven by compaction or tectonic uplift of the Bachu area in the Late Permian. Therefore, the partially dolomitized limestone bearing type B dolomites occurred in the late diagenesis on the relatively thin shoal without subaerial exposure and sufficient illite. Even in the lower part of the sequence on the relatively thick shoal, these granular limestone with very little terrigenous materials have been partially dolomitized in the process to some extent.

6. Conclusions

(1) The positive (La/La*)_SN and (Y/Y*)_SN anomaly, slightly positive (Gd/Gd*)_SN anomaly, and δ¹⁸O of seawater in fractionation equilibrium with granular dolostones of −2.8‰~1.7‰ PDB suggest that slightly modified seawater was the origin of dolomitization fluids responsible for the granular dolostones bearing a small amount of terrigenous materials from the Xiaohaizi Formation.

(2) The occurrence of vadose silt, detrital kaolinite, and fabric-retentive dissolution show that granular dolostones experienced subaerial exposure predating dolomitization. With the falling sea level, the subaerial exposure of shoal on the platform was accompanied by the enhanced input of terrigenous materials by river entrenchment and downcutting from the north of the study area.

(3) The input of negative-charged clay minerals could have facilitated the dolomitization of allochemical lime precursors under low temperature. Evidence comea from the weak positive correlation between Rb, Al, and Mg/Ca ratio of bulk dolostone, relatively higher ⁸⁷Sr/⁸⁶Sr values and ∑REE in granular dolostones than that in granular limestones, and the SEM images of illite-like clay minerals on the surface of dolomites.

(4) Early dolomitization was driven by the density-dependent convection of penesaline water, with the catalysis of illite. In the late diagenesis, the compaction fluid caused the partial dolomitization of limestone on the relatively thin shoal without subaerial exposure.