The Petrology & Geochemistry of REE-Enriched, Alkaline Vol-Canic Rocks of Ambitle Island, Feni Island Group, Papua New Guinea

1. Introduction

The Feni Island group is part of the Tabar-Lihir-Tanga-Feni (TLTF) volcanic arc within the New Ireland Basin (NIB), New Ireland Province (NIP), northeast Papua New Guinea (PNG). NIP has a total population of 228,000-243035 [1,2] and is made up of several islands: New Ireland, New Hanover, Mussau, Djaul and the TLTF islands. The TLTF and NIB are bounded by New Ireland island to the west and Kilinailau Trench (KT) in the east. The TLTF, KT, Bougainville and Solomon Islands are part of a series of arcs and trenches within the seismically and volcanically active SW Pacific region.

Feni Island Group consists of Babase and Ambitle islands located between geographic coordinates 3° 45’S, 153°30’E and 4° 15’, 154° 00’ E, forming the southern end of the Pliocene to Holocene alkaline TLTF chain (refer to accompanying paper). The TLTF arc hosts significant gold reserves associated with alkaline magmatism at Lihir (~23 Moz Au) and Simberi (~2.2 Moz Au) mines [3,4]. Recent papers by Lindley [5,6,7] and Brandl et al [8] focus on the geology, mineral deposits, and the geotectonic evolution of Feni and the TLTF respectively.

In this paper, we present the petrography, mineralogy, and geochemistry of selected igneous rocks from Feni, particularly focusing on Ambitle Island, in conjunction with the TLTF regional geological framework in order to understand the magmato-tectonic processes that produced these rocks. This paper is preceded by a sister article by the same authors titled “PNG Geotectonics and the geological evolution of Feni” which discusses the major geotectonic terranes and geology of PNG, the Melanesian Arc, New Britain, New Ireland, TLTF, and the Feni Islands

The Feni islands were targeted as a natural laboratory of Au-rich alkaline to calc-alkaline volcanism. Fieldwork examined the structural and geological setting of Feni (Ponyalou et al, in review) and sampled a representative suite of rocks for petrographic, petrological, and geochemical research. Comparisons were also made with lavas from neighboring arcs which include the Quaternary New Britain arc, Manus back-arc basin, and Gallego in the Solomon arc. Gallego volcanic field (GVF) on Guadalcanal was also visited in 2009 and sampled as it is an adjacent arc continuing on from the NW-trending Manus-Kilinailau trench.

2. Background on alkalic magmatism in PNG & Oceania

Shoshonitic or alkaline rocks also occur in other parts of PNG. These include the Highlands (ie. Porgera, Crater Mountain, Mt Hagen, Mt Giluwe), Oro (ie. 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 [9,10,11]. Within the SW Pacific Region, shoshonitic alkalic rocks also occur in North Fiji and the Lau back-arc basin [12,13], Indonesia in the Sunda arc and Cadia, Australia [14]. Unlike the voluminous calc-alkaline mafic, intermediate and 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 [13,15]. Post-subduction processes include slab roll back or slab delamination or slab tear [15,16].

Previous work published on the geology, alkalic magmatism, and porphyry-epithermal Cu-Au prospectivity of Ambitle include Johnson et al [17], Heming [18], Wallace et al [19] and Licence et al [20]. Heming [18] 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 [18] suggested that the undersaturated Ambitle lavas were most closely related to those from the Lesser Sunda arc in Indonesia. Wallace et al [19] 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. Wallace et al [19] noted that most of the Ambitle volcanic geochemical signatures were typical of island arcs, including the high concentration of incompatible (LIL) elements (ie Rb, Ba, Sr, K) and low Ti (which is generally high for most silica-undersaturated rocks from intercontinental arcs). In summary, the outcropping rocks from 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 this study (Figure 3).

Figure 1. Geology & sample location map of Feni Island Group. Geology modified after Wallace et al (1983) [24] and Esso (1983) [52].

Figure 1. Geology & sample location map of Feni Island Group. Geology modified after Wallace et al (1983) [24] and Esso (1983) [52].

Figure 2. SRTM Digital Elevation Model (DEM) of Feni Islands.

Figure 2. SRTM Digital Elevation Model (DEM) of Feni Islands.

Figure 3. Geology map of Ambitle modified after Esso (1983) [52]. Red circles represent samples collected for this study.

Figure 3. Geology map of Ambitle modified after Esso (1983) [52]. Red circles represent samples collected for this study.

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 September and November 2009 (Figure 3). A third field trip was carried out in January 2011 where outcrop samples were collected from the Nanum and Kabang areas (Figure 3) but these were not analysed. The 52 samples from the 2009 trips include 41 volcanic and volcaniclastic rock samples, 3 volcanogenic mudstone-siltstone samples and 8 geothermal sinter and clay deposits. During the first trip (abbreviated as Feni 1), 21 igneous float samples were collected from both Babase and Ambitle islands, whilst 8 insitu silica sinter and hydrothermal clay samples were collected from the geothermal fields of Waramung and Kapkai on Ambitle Island. During the second field trip (abbreviated Feni 2), several outcrops on Ambitle Island were mapped and sampled at the following localities: Natong, Niffin, Balangus and Olu Creek. The third trip in January 2011 was conducted to Nanum and Kabang on Ambitle with E. Scott who had collected a wood sample from a resedimented ash deposit and carbon-dated it to 2100 years [22](Scott, 2011). For the purpose of this paper and limited space, a representative suite has been selected for whole rock geochemical data presentation (Table 2 and Table 3). The full analytical data of all 52 samples are appended.

The samples collected were analysed petrographically and geochemically in Papua New Guinea and the United Kingdom (UK). Thin sections and polished sections were prepared at the Earth Sciences Division (ESD), University of Papua New Guinea (UPNG), and at the Geology Department of the University of Leicester (UOL) in the UK. The sample preparation and geochemical analytical techniques are described herein. All rock samples, including a few with weathered surfaces, were crushed to rock powder using agate balls in a Retsch Planetary Mill at the UOL.

Inductively Coupled Plasma Mass Spectrometry. The crushed material of the 31 samples (i.e. F1-F36) 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.

X-ray Fluorescence. 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 a pressed powder pellet for trace element analysis and a glass bead made from the fusion of the rock powder with lithium metaborate or tetraborate for major elemental analysis. 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 were also submitted to OMAC Laboratories for whole rock geochemical and gold analysis. See Supplementary Data for all XRF and ICP MS results. Both the ICP-MS and XRF datasets were used for whole rock geochemical analysis in the creation of bivariate plots, ratios and discrimination diagrams and are labelled accordingly in the relevant figures. In addition, the same datasets 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 fresh igneous samples from Feni. The Feni rocks geochemical data are compared with the volcanic and intrusive rocks from the Gallego Volcanic Field (GVF) in Guadalcanal, Solomon Islands sampled by the same authors in 2009 and previously published by Petterson et al (2011) [23]. Comparisons are also made with rocks from other arcs in PNG using published and unpublished data.

Electron Microprobe. 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 most 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 done using the electron microprobe but is usually achieved via laser ablation ICPMS (LA ICPMS) and secondary ion mass spectrometry (SIMS).

4.1. Alkali vs silica volcanic rock classification

The Feni volcanic rocks are mainly shoshonitic or alkaline in chemistry and include basalt, trachybasalt, trachyandesite, phonotephrite and trachydacite (Figure 4). In comparison, Gallego rocks in the Solomon arc are mainly calc-alkaline andesite and basaltic andesite with minor gabbro (plotting in the basalt field in Figure 4). Pyroclastic and resedimented ash flow deposits were also noted on Ambitle at Niffin and near Kabang but were not plotted in Figure 4. Data from the New Britain volcanic arc (labelled NBT for New Britain Trench), Solomon Slab Basalt (from the sea floor that is subducting at the New Britain Trench) and the Manus MORB were adopted from [24] Woodhead et al (1998) and plotted for comparison.

The Feni shoshonitic volcanic rocks have SiO2 values ranging between 45% and 67% and K2O+Na2O 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 with SiO2 values ranging between 40% and 62% and K2O+Na2O values of 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 [24] (Woodhead et al., 1998).

4.2. Petrography & Mineral Chemistry

The primitive suites are the mafic or basic volcanics which include basalt (F4, F7, F11, F15, F16, F19, OPN6, OPN7, OPN8A and OPOL1) and trachybasalt (F2, F5, F8, F17, and OPBA1) as depicted in Figure 6 and Figure 7. Some samples look basaltic (OPN3, F6) and trachyandesitic (OPN8B) but plot in the phonotephrite field of the TAS plot (Figure 4). The intermediate rocks are trachyandesite porphyries from Natong (OPNA1, OPNA3A, OPNA3B, OPNA4, OPNA5 and OPN5) and an evolved phonotephrite from Niffin (OPN8B) as shown in the polished sections in Figure 8. The most felsic magmas are the trachydacite porphyries (F12, F33) also depicted in Figure 8. The primitive basaltic and fine-grained phonotephrite magmas contain feldspathoids, alkali feldspars, clinopyroxene and minor olivine (Figure 5 and Figure 7). Hornblende occurs in trachyandesite and phonotephrite from Natong and Niffin respectively (Figure 5). Biotite is most abundant in the felsic trachydacite porphyry but also occurs minimally in trachyandesite and phonotephrite almost as an accessory mineral in the hornblende-rich suites (Figure 5). Remnant clinopyroxene appears to have been replaced by hornblende in the trachyandesite and phonotephrite samples. Feldspathoid minerals, mainly leucite and nepheline, is notable in all the basalt and trachybasalt rocks and the mafic or more primitive phonotephrite samples (ie OPN3, OPN8A). Nosean was potentially noted in the Natong trachyandesite. Apatite is present throughout all rock types but is most abundant in the hornblend-bearing trachyandesite at Natong and the evolved hornblende-phonotephrite at Niffin. Apatite abundance is strongly associated with hornblende clusters (Figure 5).

Figure 4. TAS classification diagram (Le Maitre et al., 1989) [25] & subdivision of Subalkalic Rocks using K2O vs SiO2 (Peccerillo and Taylor, 1976) [26] of Feni volcanic rocks (black circles) vs Gallego rocks (blue circles), New Britain volcanic arc field (pink), Solomon Slab basalt (green), and Manus MORB (red). NBT and Manus MORB data are taken from Woodhead et al. (1998) [24] with permission.

Figure 4. TAS classification diagram (Le Maitre et al., 1989) [25] & subdivision of Subalkalic Rocks using K2O vs SiO2 (Peccerillo and Taylor, 1976) [26] of Feni volcanic rocks (black circles) vs Gallego rocks (blue circles), New Britain volcanic arc field (pink), Solomon Slab basalt (green), and Manus MORB (red). NBT and Manus MORB data are taken from Woodhead et al. (1998) [24] with permission.

Figure 5. Modal abundance of minerals observed in thin section, as determined from gridding of thin section scans (minimum 200 points). Samples are ordered from felsic to mafic from left to right. F12 is trachydacite. OPNA5 is hornblende trachyandesite. OPN8B, F6, OPN3 are phonotephrite samples. Basalt samples are F7, OPBA1 and OPN8A.

Figure 5. Modal abundance of minerals observed in thin section, as determined from gridding of thin section scans (minimum 200 points). Samples are ordered from felsic to mafic from left to right. F12 is trachydacite. OPNA5 is hornblende trachyandesite. OPN8B, F6, OPN3 are phonotephrite samples. Basalt samples are F7, OPBA1 and OPN8A.

Figure 6. Polished section scans of mafic volcanic rocks from Ambitle. A) OPBA1 trachybasalt from Balangus. B) OPN3 aphanitic, grey phonotephrite outcrop from Niffin C) F6 phonotephrite float sample.

Figure 6. Polished section scans of mafic volcanic rocks from Ambitle. A) OPBA1 trachybasalt from Balangus. B) OPN3 aphanitic, grey phonotephrite outcrop from Niffin C) F6 phonotephrite float sample.

4.3. Whole-rock Geochemistry

The whole rock geochemical analysis is split into major (Table 5) and trace elements (Table 6). These are a representative summary of the geochemical data which is fully presented in the supplementary material.

4. 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 vs Gallego Volcanics of the Solomon Islands volcanic arc (Petterson et al., 2011) [23].

The igneous rocks of New Britain, Bougainville, and the Gallego Volcanics of the Solomon arc are dominantly calc-alkaline whilst those of the Manus spreading center show MORB or tholeiitic signatures (Figure 4, 19). In contrast, the TLTF volcanic islands are alkaline, shoshonitic lavas ranging from undersaturated alkali basalts to intermediate trachytes (Figure 4).

Primitive versus evolved rocks The main volcanic rock types on Ambitle are either primitive basic lavas bearing olivine, pyroxene, plagioclase, and feldspathoid or evolved lavas with hornblende, plagioclase, feldspar, and biotite phenocrysts. The alkali basalt, olivine-pyroxene phonotephrite, and trachybasalt rocks are generically classified as primitive whilst the hornblende-biotite Natong trachyandesite, Niffin 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 evolved rock types. This pattern was also observed in the Lihir lavas by [47] Kennedy et al (1990). Similar rock types were also described in a petrographic and mineralogical report by Ellis (1975) [48] who analyzed 6 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 possibly crustal melts [24] (Wallace et al., 1983).

Olivine Olivine xenocrysts from basalt sample OPN7 are forsteritic in composition ranging from Fo₈₇ to Fo₉₄ together with a high Ni content of 778 to 2546 ppm. Olivine analysis from basaltic and basanite samples in the Ellis (1975) [48] 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 (Gill, 2010). Given the abundance of 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 spider plots of Feni (Figure 20, 21) indicate hornblende and clinopyroxene fractionation in the source region but does 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 30 from two different basalt samples: olivine xenocrysts in OPN7 and olivine phenocryst in F5. F5 has lower 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. Kennedy et al (1990^a) [47]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.

Figure 27. Olivine in basalt F5 and OPN7: Mg# vs Ni plot indicating a high-pressure, mantle source for olivine xenocryst in OPN7 and a lower pressure or shallow depth of formation for olivine in F5.

Figure 27. Olivine in basalt F5 and OPN7: Mg# vs Ni plot indicating a high-pressure, mantle source for olivine xenocryst in OPN7 and a lower pressure or shallow depth of formation for olivine in F5.

Feldspathoids 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 setting such as the TLTF. Volcanism in the TLTF arc is related to the extension of oceanic lithosphere overlying a subduction-modified mantle region [41] (McInnes and Cameron, 1994). [41] McInnes and Cameron (1994) also propose that production of silica-undersaturated arc magmas may be related to partial melting of upwelled zones of hybrid phlogopite clinopyroxenite. Prior to upwelling the sub-arc mantle was hybridized and enriched in CO₂ and alkali metals via a carbonate-rich slab melt. Hauyne is present in hornblende trachyandesite porphyry samples from Natong. It is a foid mineral indicative of a sulfur-rich magma (Baldridge et al., 1981)[49]. This correlates with the pyrite mineralization observed in the trachyandesite porphyry at Natong.

In relation to feldspathoid fractionation in Lihir lavas, Kennedy et al (1990^a) [47] observed that fractionation at low-pressure ~ <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. Feni magmas also show the same pattern as Lihir magmas whereby feldspathoid crystallization is suppressed as hornblend and Ti-magnetite fractionate to form the trachyandesite suite at Natong.

Clinopyroxene Clinopyroxene crystals occur in basaltic and phonotephritic rock typically shows zoning and hour-glass extinction. The chemically zoned diopside crystals in phonotephrite (OPN3) shows increasing MgO and CaO composition from rim to core but decreasing values of MnO, Al₂O₃, FeO and TiO₂ (Figure 12)_. The Mg-rich cores and Fe-rich rims are indicative of chemical substitution between Fe-Mg (Ellis, 1975) [48] and slow cooling during crystal growth possibly during ascent or residence time in magma chamber(s). Aluminium values are unusually high (Al₂O₃ up to 6.33%) and are similar to pyroxene aluminium values for Feni rocks by Ellis (1975) [48] which he attributed to being a Ca Tschermak component due to Si-Al substitutions at low pressures.

Hornblende Hornblende analyses of Natong trachyandesite (OPNA5) and Niffin phonotephrite (OPN8B) from eastern Ambitle evaluated in the Amph-TB spreadsheet by [27] Ridolfi et al. (2010) showed the latter formed at greater depths and temperatures. Hornblende compositions were magnesian-rich or 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 replacing clinopyroxene in phonotephrite (OPN8B) while biotite forms inclusions within the hornblende. The hornblende-altered clinopyroxene grains are corroded or remelted. These are evidences of recrystallization or retrograde alteration of phonotephrite caused by the introduction of a hydrous melt. Smith (2014) [50] and Cooper et al (2016) [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.

Biotite. 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. Apatite and possibly other phosphate minerals are the likely phases carrying the REE. Although REE and other trace elements could not be measured in the microprobe, it was observed in the whole rock geochemical bivariate plots of REEs vs P2O5 (Figure 28-29) that the REEs showed a positive correlation with P2O5. Generally, the Feni magmas also contained higher values of P2O5 and REEs relative to the other arcs. This is a common characteristic of most alkalic deposits which are abundant in phosphate minerals and REE. Thus, in addition to gold and copper, the TLTF also has the potential for REE exploration, research, and development under the Critical Minerals banner for green energy technology.

Figure 28. Bivariate plots of P2O5 vs REE (Ce, Nd, Pr).

Figure 28. Bivariate plots of P2O5 vs REE (Ce, Nd, Pr).

Figure 29. Bivariate plots of P2O5 vs REE (Sm, Eu).

Figure 29. Bivariate plots of P2O5 vs REE (Sm, Eu).

5. Conclusions

The TLTF is a Pleistocene-Holocene alkalic or shoshonitic magmatic event post-dating the older Eocene-Miocene Melanesian Arc. Its presence in the Melanesian Arc after the cessation of subduction along the Manus-Kilinailau Trench implies the involvement of post-subduction, extension-related magmatism related to low degrees of partial melting of a subduction-modified upper mantle. This is supported by the arc signatures of the TLTF magma geochemistry such as LILE enrichment and depletion in HFSE.

The main process controlling the major elemental variation in the alkaline lavas of Ambitle is fractional crystallization. High LILE (i.e. Ba, Sr, K) and depleted HFSE (Nb, Ti, Y, Th) values are typical arc signatures of the Feni rocks. The presence of LREE enrichment and depletions in MREE and HREE are signatures indicative of hornblende, clinopyroxene, or garnet fractionation in the source region. Pyroxene fractionation is dominant in the mafic lavas whereas hornblende fractionation is a major petrologic process within the phonotephrite and trachyandesite suites. Due to the presence of more feldspathoids in the primitive lavas relative to the evolved rocks, it is concluded that the addition of hornblende to the fractionating assemblage of trachyandesite (and phonotephrite) suites inhibits the crystallization of nepheline or foids. Although plagioclase is a dominant mineral phase, REE plots lack a negative Eu anomaly implying that there was limited or no plagioclase fractionation in the source region or that it fractionated later potentially as a result of high-water content in the melt. Earlier higher pressure fractionation would inhibit plagioclase fractionation.

The typical signatures of olivine fractionation are not observed in the REE spidergram plots of Feni. We interpret olivine as a largely xenocrystic phase with forsteritic or Mg-rich compositions and appreciable concentrations of Ni and Cr. This signifies a mantle origin for the olivine crystals in the Feni basalts and that most of the Feni rock types analysed in this study originated from melts that had little or no olivine in solution and more clinopyroxene in the source region.

The presence of clinopyroxene is also an important precursor to the formation of amphibole as observed texturally under the optical microscope with hornblend replacing clinopyroxene. This also implies open system processes whereby fluids are percolating into the magma chamber and interacting with clinopyroxene crystals to form amphibole. The presence of OH-bearing mafic minerals such as amphibole and biotite, and the late fractionation of plagioclase signifies a hydrous or high-water magma content.

Hornblende fractionation was polybaric as deduced from the compositional variation of amphibole in trachyandesite and phonotephrite. Magnesiohastingsite is the main amphibole species in both rock types but phonotephrite has pargasitic composition and formed at greater depths and higher temperatures with less melt H₂O whereas the trachyandesite primarily contained magnesiohastingsite and was formed at shallower depths and lower temperatures with higher H₂O content. Hornblende, Ti-magnetite and apatite are observed in thin section to occur together in clusters within the trachyandesite suite.

The abundance of apatite and/or other phosphate mineral phases in the alkalic Feni magmas also has an influence on REE fractionation and enrichment. REE is elevated in Feni and the TLTF in contrast to the other neighbouring arcs and poses an opportunity for REE mineral exploration in the TLTF for critical minerals. Thus, apatite fractionation also controls the behavior of trace and rare earth elements in alkaline lavas.