1. Introduction
The Feni Island Group is part of the Tabar–Lihir–Tanga–Feni (TLTF) alkaline volcanic chain within the New Ireland Basin (NIB), New Ireland Province (NIP), northeast Papua New Guinea (PNG). Our first paper [
1], also published in this journal, presents an overview of the geological and tectonic evolution of PNG and Feni. NIP is made up of several islands: New Ireland, New Hanover, Mussau, Djaul, and the TLTF islands (
Figure 1). The TLTF and NIB are bounded by New Ireland to the west and the older Kilinailau Trench (KT) to the east (
Figure 1). The TLTF chain, KT, Bougainville, New Ireland, and New Britain form the New Guinea Islands Terrane within the greater Melanesian Arc and are part of a series of arcs and trenches within the seismically and volcanically active SW Pacific region (
Figure 1). Regional and location maps of the study area are presented in the first paper [
1].
Feni consists of Babase and Ambitle islands located between geographic coordinates 3°45′ S, 153°30′ E and 4°15′ S, 154°00′ E, forming the southern end of the Pliocene to Holocene alkaline TLTF chain [
1,
2,
3]. The TLTF chain hosts significant gold reserves associated with alkaline magmatism at Lihir (~23 Moz Au) and Simberi (~2.2 Moz Au) mines [
4,
5]. Recent papers by Lindley [
6,
7,
8] and Brandl et al. [
9] focus on the geology, mineral deposits, and geotectonic evolution of Feni and the TLTF, respectively.
The TLTF chain is proximal to two subduction zones: the older Kilinailau Trench directly east and the New Britain Trench (NBT) further south [
1]. The geochemical patterns of the TLTF magmatic rocks indicate island arc signatures (i.e., high LILE and depleted HFSE) [
2]. However, several authors attribute the formation of the TLTF to rifting associated with the opening of the Manus Basin ~3.5 Ma due to the fact that there is no seismically active slab under the TLTF [
1,
2]. Furthermore, the TLTF volcanism is relatively young and is not directly related to the older Eocene–Oligocene Kilinailau–Melanesian Arc subduction system. O’Kane [
10] noted a slab tear on the down-going Solomon Slab at the curved New Britain Trench proximal to Feni. Given the tectonic complexities of the region, the general research question arises: how did the alkaline magmatic rocks of Feni form? The purpose of this study, therefore, is to (1) present and interpret the petrographic, mineralogical, and geochemical data from Ambitle, Feni, within the context of the tectonic evolution of the TLTF chain, and (2) propose a geodynamic, petrogenetic model for the Feni volcanic rocks in order to understand the processes that produced these rocks.
The Feni islands were initially targeted as a natural laboratory of Au-rich alkaline to calc-alkaline volcanism. Fieldwork included mapping the structural and geological units of Feni and sampling a representative suite of rocks for petrographic, mineralogical, and geochemical research [
1]. Comparisons were also made with volcanics from neighboring arcs which include the Quaternary New Britain Arc, Manus back-arc basin, and Gallego in the Solomon Arc. The Gallego Volcanic Field (GVF) on Guadalcanal, Solomon Islands, was also visited in 2009 and sampled as it is an adjacent arc continuing on from the NW-trending Manus–Kilinailau Trench.
2. Alkaline Magmatism in PNG
In addition to the TLTF chain, shoshonitic or alkaline rocks also occur in other parts of PNG. These include the Highlands (i.e., Porgera, Crater Mountain, Mt Hagen, Mt Giluwe), Oro (i.e., Mt Lamington pre-1951 volcanics), southeast Papuan Peninsula (i.e., Cloudy Bay Volcanics), and Milne Bay (i.e., Kutu Volcanics, Gabahusuhusu Syenite) and are also closely linked to gold mineralization [
11,
12,
13]. Within the SW Pacific region, shoshonitic rocks also occur in North Fiji and the Lau back-arc basin [
14,
15], Sunda Arc in Indonesia, and Cadia in Australia [
16]. Unlike the voluminous calc-alkaline porphyry Cu-Au magmas formed during subduction in island and magmatic arc settings, alkaline or shoshonitic magmas are formed post-subduction from low-degree partial melting of subduction-modified lithospheric mantle during extension [
15,
17]. Post-subduction processes include slab rollback, slab delamination, or slab tear [
17,
18].
Feni is a group of alkaline stratovolcanoes built on an Oligocene oolitic limestone basement [
2]. Small outcrops of limestone and siltstone are noted on Ambitle and Babase islands (
Figure 2 and
Figure 3) which appear to be older than the volcanics [
1]. Miocene limestone units are also abundant on Simberi, New Ireland, and the New Ireland Basin. The emplacement of the volcanic and subvolcanic undersaturated, alkaline rocks of the TLTF are controlled by major NW extensional faults transected by later faults (
Figure 2 and
Figure 3). On Ambitle, Cu-Au mineralization and geothermal activity are also constrained within the NW-trending Niffin Graben along intersections with later NE and NS faults [
1].
Previous work published on the geology, alkaline magmatism, and porphyry–epithermal Cu-Au prospectivity of Ambitle includes Johnson et al. [
18], Heming [
19], Wallace et al. [
2], and Licence et al. [
20]. Heming [
19] described the undersaturated lavas on Ambitle as basanite, tephrite, trachyte, and ankaramitic (or pyroxene-rich) lavas. He observed olivine, clinopyroxene, plagioclase, and hauyne within the basanite, whereas in the tephrite, amphibole and mica were dominant in the absence of olivine and hauyne. He also noted that the lavas contained unusually high concentrations of incompatible trace elements relative to island arc basalts and andesites. Heming [
19] suggested that the undersaturated Ambitle lavas were most closely related to those from the Lesser Sunda Arc in Indonesia. Wallace et al. [
2] described the rocks of Feni as alkali basalt, tephrite, basanite, phonolitic tephrite, tephritic phonolite, trachybasalt, trachyandesite, transitional basalt, and quartz–trachyte. The quartz–trachyte cumulodome is observed throughout the TLTF chain as the youngest and the most silica-saturated volcanic rock [
2]. Wallace et al. [
2] noted that most of the Ambitle volcanic geochemical signatures were typical of island arcs, including the high concentration of the incompatible large ion lithophile elements or LILE (i.e., Rb, Ba, Sr, K) relative to the high field strength elements (HFSE) and low Ti (which is generally high for most silica-undersaturated rocks from intercontinental arcs).
The outcropping rocks of Ambitle and Babase are mostly feldspathoid-bearing mafic to intermediate alkaline volcanics and subvolcanic rocks with rare quartz–trachydacite porphyries. Drilling in the Kabang prospect on Ambitle by past companies intersected a syenite porphyry intrusive with prospective porphyry–epithermal alteration and mineralization at depth [
20]. Older company maps show a monzonite intrusive body at Matoff [
21] but this was not visited during our field excursions (
Figure 2 and
Figure 3).
Figure 2.
Geology and sample location map of Feni Island Group. Geology modified after Wallace et al. [
2] and Esso [
22] and includes our mapping and interpretations.
Figure 2.
Geology and sample location map of Feni Island Group. Geology modified after Wallace et al. [
2] and Esso [
22] and includes our mapping and interpretations.
Figure 3.
Geology map of Ambitle. Red circles represent samples collected for this study from the first two field trips. Sample IDs indicated for samples collected from outcrops. Yellow samples represent samples collected in the third field trip that were not analyzed.
Figure 3.
Geology map of Ambitle. Red circles represent samples collected for this study from the first two field trips. Sample IDs indicated for samples collected from outcrops. Yellow samples represent samples collected in the third field trip that were not analyzed.
3. Materials and Methods
Fifty-two (52) rock samples comprising twenty-one (21) float and thirty-one (31) outcrop samples were collected from Feni during two field trips in 2009 and labelled as Feni 1 and 2 (
Figure 3). Several outcrops on Ambitle Island were mapped and sampled at the following localities: Natong, Niffin, Balangus, and Olu Creek (
Figure 2). A third field trip (i.e., Feni 3) was carried out in January 2011 where outcrop samples were collected from Nanum and Kabang on Ambitle (
Figure 3) but these were not analyzed due to a lack of funding and limited time factor [
23]. For the purpose of this paper, a representative suite has been selected for whole-rock geochemical data presentation. The full analytical data of all 52 samples are appended.
The samples collected were prepared and analyzed in PNG and the United Kingdom (UK). Thin sections and polished sections were prepared at the Earth Sciences Division (ESD) of the University of Papua New Guinea (UPNG) and the Geology Department of the University of Leicester (UOL) in the UK. The sample preparation and geochemical analytical techniques are described below. All rock samples, including a few with weathered surfaces, were crushed into rock powder using agate balls in a Retsch Planetary Mill at the UOL for whole-rock geochemical analysis.
The crushed material of the 31 samples (i.e., F1–36) from the Feni 1 trip were submitted to OMAC Laboratories Ltd. in Ireland for inductively coupled plasma mass spectrometry (ICP-MS) analysis of major and trace elements using the lithium borate fusion dissolution technique. Aqua regia dissolution was also applied on samples F1–36 for base and precious metal analysis. QAQC analyses of blanks, duplicates, and standards are included in the online
Supplementary Information.
Twenty-three (23) samples (OPBA1-OPOL1) from the Feni 2 trip were analyzed for major and trace elements with the PANalytical Axios Advanced XRF X-ray fluorescence (XRF) spectrometer and PANalytical SuperQ software at the UOL Geology Department. The crushed samples were initially prepared into (1) a pressed powder pellet for trace element analysis and (2) a glass bead for major elemental analysis. The glass bead was made from the fusion of the rock powder with lithium metaborate or tetraborate. Instrument calibration and precision–accuracy measurements were conducted using various reference materials at the UOL Geology lab. Counting statistic errors are available within the raw XRF data spreadsheet under sheet “axrht322 cse”.
The left-over rock powder of the Feni 2 samples was also submitted to OMAC Laboratories for whole-rock geochemical and gold analysis. See
Supplementary Data for all XRF and ICP MS results. The results were cleaned to exclude all weathered and altered igneous rocks and those possessing a loss on ignition (LOI) > 5% resulting in a total of 26 representative samples from Feni. Both the ICP-MS and XRF datasets were then used in the creation of bivariate plots, ratios, spider plots, and discrimination diagrams.
Electron microprobe analysis (EMPA) was conducted using the JEOL 8600 Superprobe at UOL for the determination of major elemental oxides in the polished sections of selected igneous rocks. Prior to analysis, the polished sections were coated with carbon to prevent ionization and the subsequent damage of minerals by the electron beam. The microprobe uses a wavelength dispersive system where a 30 nA current and a 15 kV accelerating voltage were used for all analyses. A 10 µm beam was employed for the analysis of feldspars and micas whilst a 5 µm beam was used for other minerals. Precision for electron probe analysis, calculated from counting statistics, was generally better than ±1% for measurements > 10 wt%, and better than ±5% for contents > 0.5 wt%. Trace element analysis of mineral phases cannot be carried out using the electron microprobe but is usually achieved via laser ablation ICP MS (LA ICP MS) and secondary ion mass spectrometry (SIMS).
The composition for each mineral phase was then calculated from the electron microprobe analysis using appropriate conversion spreadsheets. Olivine composition (Te, Fa, Fo, Ca-Ol) was calculated from Mineral Formulae Recalculation (carleton.edu). Pyroxene compositions were calculated in the PYXCALC spreadsheet from Gabbrosoft. Olivine and pyroxene compositions were then plotted in ternary diagrams online (Ternary Plot Generator—Quickly create ternary diagrams). Amphibole classification was carried out using the Ca-amphibole classification scheme by Leake et al. [
24] and the AMPH13 spreadsheet by Gabbrosoft.org for stoichiometric calculations of electron microprobe analysis (
https://www.gabbrosoft.org/wp-content/uploads/2018/07/AMPH13.xls) accessed on 19 June 2022. The AMP_TB excel spreadsheet published by Ridolfi et al. [
25] was also utilized to interpret amphibole composition, pressure, temperature, and depth of formation.
5. Discussion
This section aims to interpret and describe a qualitative petrogenetic model for the Ambitle volcanic and subvolcanic rocks, based on petrography, mineralogy, and trace element geochemistry. We also make comparisons to the igneous mineralogy and geochemical trends of Lihir and Simberi (Tabar) within the TLTF chain vs. the Gallego Volcanics of the Solomon Islands volcanic arc [
26].
The igneous rocks of New Britain, Bougainville, and Gallego are dominantly calc-alkaline, whilst those of the Manus Spreading Centre show MORB or tholeiitic signatures (
Figure 1,
Figure 4 and
Figure 23). In contrast, the TLTF volcanic islands are alkaline, K-rich, shoshonitic lavas ranging from undersaturated alkali basalts to intermediate trachyandesite and felsic trachyte or trachydacite (
Figure 4). The Feni shoshonitic volcanic rocks have SiO
2 values ranging between 45% and 67% and K
2O + Na
2O values falling between 3% and 12%. In contrast, the Gallego volcanic rocks from Guadalcanal, Solomon Islands, and some NBT rocks are medium K calc-alkaline arc magmas with SiO
2 values ranging between 40% and 62% and K
2O + Na
2O values between 1 and 7%. Low K tholeiitic rocks include the Solomon slab (subducting under the New Britain trench), some New Britain arc volcanics, and Manus MORB [
27].
The main volcanic rock types on Ambitle are either primitive basic magmas bearing olivine, pyroxene, plagioclase, and feldspathoid or evolved lavas with hornblende, plagioclase, feldspar, and biotite phenocrysts. The alkali basalt, mafic phonotephrite, and trachybasalt rocks are generically classified as primitive, whilst the hornblende trachyandesite, hornblende phonotephrite, and trachydacite porphyry are more evolved in composition. Feldspathoids including leucite, nepheline, and hauyne are more abundant in primitive lavas and decrease with the appearance of hornblende and biotite in the evolved rock types. This pattern was also observed in the Lihir lavas by Kennedy et al. [
43]. Similar rock types were also described in a petrographic and mineralogical report by Ellis [
44] who analyzed six lavas from Feni and classified them as potassic phonolitic tephrite, potassic olivine nephelinite, sodic nepheline trachyandesite, and potassic tephritic phonolite. Trachydacite or quartz trachytes occur throughout the TLTF and are interpreted as late crustal melts [
2].
The Feni magmatic rocks are most enriched in REE relative to the neighboring arcs of New Britain, Solomon, and Manus MORB (
Figure 19 and
Figure 20). When the REE concentration of each rock type from Feni was plotted in the spidergram plots, the mafic alkaline volcanics were highly enriched in REE whilst the felsic trachydacite was strongly depleted in REE (
Figure 21 and
Figure 22). This is because there is minimal or no mineral phase to absorb the REE so it remains in the mafic melt. As hornblende and apatite appear in the intermediate and felsic rocks, REE behaves compatibly starting from SiO
2 52% as observed in Yb and Y (
Figure 18).
Olivine phenocrysts are not common in the Feni basaltic rocks but instead occur as xenocrysts. The olivine xenocryst from basalt sample OPN7 is forsteritic in composition ranging from Fo
87 to Fo
94 together with a high Ni content of 778 to 2546 ppm. Olivine analysis from basaltic and basanite samples in the Ellis [
44] study are also Mg-rich or forsteritic (up to 90%) in composition. These high Mg values are comparable to mantle olivine or primary basaltic melts with Mg numbers greater than 68 and Ni ≥ 250 ppm [
45]. Given the abundance of other mineral phenocryst phases in the Feni volcanics, the coarse olivine crystals are interpreted as xenocrysts or xenoliths formed at greater depths and pressures. In addition, the REE spidergram plots of Feni (
Figure 19 and
Figure 20) indicate hornblende and clinopyroxene fractionation in the source region but do not show signatures of olivine fractionation. We conclude that olivine fractionation occurred at greater depths or pressures in or near the mantle and that the lavas sampled in this study formed at shallower crustal magma chambers. This is supported by the Mg# vs. Ni (ppm) plot of olivine crystals in
Figure 27 from two different basalt samples: olivine xenocrysts in OPN7 and olivine phenocryst in F5. F5 has low Mg and Ni content, whereas OPN7 olivine xenocryst contains higher Mg and Ni values that will have formed at greater depths and pressures within the mantle (
Figure 27). Kennedy et al. [
43] also interpreted that olivine fractionation in Lihir occurred at high pressure or greater depths. This resulted in erupted lavas with low Ni and MgO contents.
The mafic alkaline volcanic rocks of Feni are rich in clinopyroxenes, accompanied by feldspathoids, Fe-Ti oxides, and minor olivine. This is very similar to the mineral composition of other mafic alkaline volcanics in other countries such as Uganda, Austria, and Italy [
46,
47,
48]. Augite crystals occur in basaltic and phonotephritic rocks and typically exhibit zoning and hour-glass extinction under the microscope. The chemically zoned xenocrystic diopside crystal in phonotephrite (OPN3) shows increasing MgO and CaO composition from rim to core but decreasing values of MnO, Al
2O
3, FeO, and TiO
2 (
Figure 12). The Mg-rich cores and Fe-rich rims are indicative of chemical substitution between Fe-Mg [
44] and slow cooling during crystal growth possibly during ascent or residence time in magma chamber(s). Aluminium values are unusually high (Al
2O
3 up to 6.33%) and are similar to pyroxene aluminium values for Feni rocks by Ellis [
44], which he attributed to being a Ca Tschermak component due to Si-Al substitutions at low pressures.
In order to investigate the petrogenesis of clinopyroxene-rich alkaline magmas in Italy, Iacono Marziano et al. [
48] conducted an experiment on basaltic melt by contaminating it with limestone (due to the abundance of carbonate-rich sedimentary country rock predating the alkaline magmas). They noted that increasing the amount of carbonate assimilation into high temperature olivine basaltic magma increased the crystallization of clinopyroxene which in turn used up most of the silica. This decreased the amount of silica in the residual melt which led to the formation of silica-undersaturated magmas such as the feldspathoid-bearing phonolite and tephrites. The presence of the negative Ce anomaly in the Feni and Simberi basalts further corroborates the hypothesis of sediment assimilation.
Similarly, McInnes and Cameron [
37] described a magmatic-derived sulphate-carbonate-H
2O-alkali-rich aluminosilicate melt (SCHARM) inclusion within olivine xenocrysts from Simberi in the Tabar Group within the TLTF. They proposed that SCHARM was not in equilibrium with mantle assemblages and may have formed from the partial melting of a feldspathic phase following the initial melting of a sea water-altered basaltic slab.
Thus, we propose that both mechanisms are plausible in forming the TLTF magmas as (1) SCHARM would introduce alkali elements into the melt [
37] and (2) limestone assimilation would result in the formation of clinopyroxene-rich, silica-undersaturated magmas [
48]. The presence of Oligocene- to Miocene-age carbonate-rich limestone units on New Ireland, the New Ireland Basin, and the entire TLTF further corroborates the possibility of limestone or carbonate assimilation during the petrogenetic evolution of the Feni magmas [
1].
Silica-undersaturated rocks bearing feldspathoids such as nepheline, leucite, and nosean are characteristic of continental rifts and intraplate hot spots. They are unusual occurrences for volcanic island arc settings such as the TLTF. McInnes and Cameron [
37] proposed that volcanism in the TLTF arc is related to the extension of oceanic lithosphere overlying a subduction-modified mantle region. They also suggested that the production of silica-undersaturated arc magmas may be related to the partial melting of upwelled zones of hybrid phlogopite clinopyroxenite. Prior to upwelling, the sub-arc mantle was hybridized and enriched in CO
2 and alkali metals via a carbonate-rich slab melt or SCHARM [
37]. Hauyne is also present in hornblende trachyandesite porphyry samples from Natong. It is a feldspathoid mineral indicative of a sulfur-rich magma [
49] which further supports the involvement of SCHARM as a source of sulfur, aluminium, and alkali elements.
In relation to feldspathoid fractionation in the Lihir lavas, Kennedy et al. [
43] observed that fractionation at low pressure of ~ <5 kb produced two evolutionary trends: (1) normative nepheline in the primitive lavas due to the separation of clinopyroxene, plagioclase, and minor olivine; and (2) reduction or disappearance of normative nepheline as Ti-magnetite and hornblende formed. Our study of the Feni magmatic rocks also shows the same pattern whereby feldspathoid crystallization is suppressed as hornblende and Ti-magnetite fractionate to form the trachyandesite suite at Natong.
Hornblende analyses of Natong trachyandesite (OPNA5) and Niffin phonotephrite (OPN8B) from eastern Ambitle were evaluated in the Amph-TB spreadsheet by Ridolfi et al. [
25] and showed the latter formed at greater depths and temperatures. Using the Amp-TB spreadsheet, melt H
2O content in trachyandesite was calculated as 4–5 ± 0.7 wt% with an estimated depth of crystallization at 15–19.4 km under oceanic crust and temperature ranges of 998–1025 °C [
25]. In contrast, the phonotephrite sample OPN8B had a melt H
2O content of 4.1–4.9 ± 0.7 wt% with an estimated depth of crystallization at 19.6–24.2 km under oceanic crust at temperature ranges of 1029–1056 °C.
Hornblende composition was predominantly the Mg-rich magnesiohastingsite in the Natong trachyandesite, whereas the composition of hornblende in the Niffin phonotephrite was both magnesiohastingsite and pargasite. This signifies that the hornblende species within the two magma types crystallized under polybaric conditions or two different pressure, depth, and temperature ranges. Texturally, hornblende is observed replacing clinopyroxene in phonotephrite (OPN8B), while biotite forms inclusions within the hornblende. The hornblende-altered clinopyroxene grains are corroded or remelted. This is evidence of the recrystallization or retrograde alteration of phonotephrite caused by the introduction of a hydrous melt. Smith [
50] and Cooper et al. [
51] also described clinopyroxene precursors that had reactive melts percolate through them in an open system causing the crystallization or fractionation of amphibole. Thus, the prior appearance of clinopyroxene is important for the subsequent formation of amphiboles.
The more evolved subvolcanic intermediate trachyandesite and trachydacite porphyry suites contain sodic plagioclase (i.e., andesine and albite), whereas mafic basalt and phonotephrite contain the Ca-rich labradorite. However, in the hornblende–clinopyroxene-bearing phonotephrite samples F9 and OPN8B, both high temperature labradorite and lower temperature plagioclase occur together signifying a change in temperature from high to low possibly due to the influx of water or crystal fractionation.
The biotite in the slightly hydrothermally altered trachydacite sample, F12, is the Mg-rich phlogopite, whereas the biotite in phonotephrite was slightly more Fe-rich and is thus interpreted as igneous biotite. Hydrothermal alteration and pyrite mineralization in F12 and OPNA5 signify the fertility of the hydrous Feni melts to carry metals and sulfide.
Apatite is ubiquitous in both primitive and evolved Feni magmas but it is most abundant and strongly associated with hornblende and magnetite clusters within the hydrous trachyandesite magmas. Apatite and other phosphate minerals are the known phases that host REE. Although REE and other trace elements could not be measured in the electron microprobe, whole-rock geochemical bivariate plots of REEs vs. P
2O
5 (
Figure 28 and
Figure 29) proved that the REEs positively correlated with P
2O
5. EPMA analysis of apatite crystals in
Table 4 showed that the Natong trachyandesite contained Ca-rich apatite relative to the other rock types, suggesting that this may also have an influence on the behavior of REE. Regionally, the concentrations of P
2O
5 and REEs within the K-rich, alkaline Feni magmas are higher than the tholeiitic and calc-alkaline magmas of New Britain and Gallego in the Solomon Islands. This is a common characteristic of most alkaline deposits which are abundant in phosphate minerals and REE. Thus, in addition to gold and copper, Feni and the entire TLTF also have the potential for REE exploration, research, and development under the Critical Minerals banner for green energy technology.
5.1. Major Element Geochemistry
The trends observed in the bivariate major elemental oxide plots are indicative of a change in fractionation of the mineral assemblages (
Figure 17). At SiO
2 contents ranging from 45% (i.e., basalt) to 52% (i.e., trachybasalt and phonotephrite composition), the declining MgO, CaO, Fe
2O
3, and TiO
2 compositions indicate olivine, pyroxene, magnetite, and anorthitic plagioclase +/− amphibole crystal fractionation. In contrast, Al
2O
3, Na
2O, K
2O, and P
2O
5 behave incompatibly with silica in the mafic rocks <52%. This is because during this phase of fractionation, there is little or no mineral phase to remove K
2O and Na
2O from the primitive basaltic melt except for minor alkali feldspar and feldspathoid. As the melt fractionates and becomes more felsic, the minerals that take up K
2O and Na
2O include feldspar, plagioclase, feldspathoid, and biotite in the more intermediate and felsic igneous rocks.
From ~53% to 70% SiO2, MgO and CaO decline gradually relative to the basic or low SiO2 rocks. K2O and Na2O behave incompatibly relative to silica with the former displaying more scatter possibly as a result of variable cumulus feldspar crystals and alteration. These geochemical patterns are consistent with a change in the crystal fractionation of the controlling mineral assemblage:
- (1)
Olivine, clinopyroxene, and amphibole fractionation removes MgO and Fe2O3 from the melt.
- (2)
Clinopyroxene, amphibole, anorthite, and apatite fractionation removes CaO from the melt.
- (3)
Titanite, magnetite, clinopyroxene, amphibole, and biotite fractionation removes TiO
2 and Fe
2O
3 from the melt (
Figure 12,
Table 2,
Table 3 and
Table 4)
Apatite is the main phosphate mineral that will use up P
2O
5 from the melt. It mainly occurs as a minor or accessory mineral in the mafic and felsic magmas of Feni. It is also strongly abundant in the mafic glomerocryst or cumulate phases of the Natong trachyandesite occurring alongside hornblende and magnetite. This explains the divergence or scatter in the Harker plots in
Figure 17 as a result of its abundance and mode of occurrence in the various rock types.
5.2. Trace Element Geochemistry
Sc (~0–60 ppm) behaves compatibly throughout the evolution of the Feni magmas because the metal substitutes for Fe in most ferromagnesian minerals, including olivine, pyroxene, amphibole, and biotite [
42]. Thus, the decreasing linear trend of Sc vs. SiO
2 is, therefore, an indication of the fractionation of these Fe-Mg silicate minerals. V is the most abundant of the trace elements in the Feni magmas assaying up to 400 ppm and also behaves similarly to Sc where it substitutes for Fe in pyroxene, amphibole, and biotite. However, V is most strongly associated with magnetite fractionation in oxidized magmas [
42]. Thus, magnetite fractionation is possibly the main controlling phase for V in the Feni magmas followed by clinopyroxene, amphibole, and biotite. In addition, note that the geochemical behavior of Sc and V relative to silica is similar to TiO
2 indicating fractionation of the same minerals whereby all elements decrease with increasing silica. These elements are now compatible, meaning that at certain temperatures and compositions, certain favorable minerals will start to crystallize, incorporating these trace elements into their structure.
Zr fractionation occurs when the mineral zircon starts to crystallize in felsic to intermediate rocks around temperatures greater than 750 °C [
52] and is particularly enhanced in the presence of hydrous, oxidized, and F-rich magmas [
42]. Thus, the inflection point of 52% is interpreted as the composition where the magma became hydrous and hornblende crystallization began. Y and Yb are usually taken up by hornblende fractionation. Zr (30–90 ppm), Y (0–32 ppm), Yb (0–3.5 ppm), Nb (1–5 ppm), and Th (0.4–2.7 ppm) are initially incompatible and remain in the melt [
53] where their concentration increases with increasing silica content from 42 to 52%. At the inflection point of 52%, all five elements decrease sharply towards 70% silica. The concentration for Eu, however, is quite low, ranging from 0 to 2 ppm. Its generally compatible trend is attributed to plagioclase and apatite fractionation which occurs throughout the Feni magma suite from the mafic to felsic composition. In general, a negative Eu anomaly is not observed in the Feni magmas and thus suggests that plagioclase did not fractionate in the source area, possibly due to the presence of water or that plagioclase crystals did not physically separate from the magma. The trachyandesite suite contains a slight positive Eu anomaly which correlates with the abundance of plagioclase phenocrysts.
Hornblende fractionation is observed in the REE spidergram plots where LREE is enriched relative to MREE and HREE forming a spoon-shaped profile (
Figure 20). It is also supported by the abundance of hornblende, specifically, magnesiohastingsite phenocrysts and mineral clusters in the Natong trachyandesite. Apatite also has a strong control on REEs, U, and Th (
Figure 28 and
Figure 29). In addition, Chelle-Michou [
54] also discovered that titanite fractionation causes Nb, Th, U, Zr, Y, Yb, and all REEs to behave compatibly. We also observe this pattern for Nb, Th, Zr, Y, and Yb in the Feni magmas around SiO
2 52% (
Figure 18) and attribute this to the appearance of titanite along with hornblende and apatite.