Komatiitic Lamprophyre in West Sulawesi: First Evidence for >1350°C and 3.5-3.8 GPa Mantle Melts

The presence of lamprophyric lavas of Late Cenozoic in Talaya Volcanic Formation at the boundary between the subregencies of Mamuju and Tabulahan (Western Sulawesi) associated with the mantle enrichment rocks of the Adang Volcanics is the subject of this study. Petrologically, lamprophyre is composed of orthopyroxene (enstatite), clinopyroxene (augite), biotite, leucite, amphibole, magnetite, and autometasomatism of chlorite in grain minerals and groundmass. The lamprophyre is classified into monchiquite shoshonitic lamprophyre, and it has a komatiitic composition with the ratio of MgO/Al2O3 > 0.7906 (in wt %). The komatiitic monchiquite lamprophyre is characterized by high MgO (10.02 12.67 %), relatively low alumina (Al2O3= 10.98 11.70 %), SiO2= 46.43 47.8 %, TiO2 (0.84 1.00 %), FeOt (7.75 7.88 %), and relatively high content of alkaline (Na2O: 2.20 2.59 %; K2O: 1.58 2.45 %; Total alkali: 4.00 4.89 %, and CaO (9.29 10.71 %). The geochemical trace element plots using various diagrams suggests the geotectonic setting of the lamprophyric rock was formed in suprasubduction alkaline continental-arc, and the proposed source of magmatism comes from the suprasubduction activities from the east. The protolith of magma was originated from partial melting of depleted MORB mantle (DMM), composed of pyroxene-peridotite (garnet-lherzolite). The partial melting conditions are suggested to occur at high pressure (3.5 3.8 GPa) and the depth of ~120 km with melting temperature of >1350°C, and the magma is dominantly controlled by olivine fractional crystallization.


Introduction
Komatiite is a type of ultramafic mantlederived volcanic rock with high magnesium, which contains more than 18 % MgO, and the spinifex texture or a variety of needle-like texture (Arndt and Nisbet, 1982;Dostal, 2008); whereas the komatiitic is representative of high-Mg with the ratio of MgO/Al 2 O 3 above 0.7906 (in wt. %) (Jensen, 1976). The komatiitic magma is probably produced in very hot mantle upwellings or plumes (Schmeling and Arndt, 2017).
Lamprophyres are mesocratic to melanocratic igneous rocks, usually hypabyssal, with a panidiomorphic texture and abundant mafic phenocrysts of dark mica (biotite or Fe-phlogopite) and/or amphibole, with or without pyroxene, with or without olivine, and sometimes melilite and/or feldspathoid, set in a groundmass of the same minerals. Any feldspar, usually alkali feldspar, is restricted to the groundmass (Gillespie and Styles, 1999). The lamprophyres are known as exotic rocks, because they are difficult to classify unambiguously using the existing criteria. They are not amenable to classification according to modal proportions, such as the QAPF system, nor compositional discrimination diagrams, such as TAS (Le Maitre et al., 1989). It seems unlikely that a simple taxonomic system will be found unless the appropriate genetic criteria are applied, that is, unless the classification takes into account the genesis of the rocks (Woolley et al., 1996). Furthermore, lamprophyres are a complex group of rocks that have mineralogical similarities to some kimberlites and lamproites which are grouped into lamprophyric rocks (Rock, 1991). The classification of lamprophyric rocks is figured in the hierarchical chart below (Figure 1). The discrimination for lamprophyres, lamproites, and kimberlites is usually released by using the geochemical plotting on ternary diagram as proposed by Bergman, 1987. The lamprophyric rocks primarily occur as dikes, lopoliths, laccoliths, stocks, and small intrusions. On a purely chemical basis, an extrusive lamprophyre (e.g. minette or monchiquite) might be classified as potassic trachybasalt, shoshonite, or latite using the total alkali-silica diagram (see TAS classification), or as absarokite, shoshonite, or banakite using a classification sometimes applied to potassium-rich lavas. Such chemical classifications ignore the distinctive textures and mineralogies of lamprophyres.

I J O
G tectonic histories, which would facilitate better understanding and evaluating the role of various geological processes considered responsible for the origin of enrichment in potassium (and other highly incompatible elements) of K-rich magmas. These processes include the differentiation of primitive magma, fractionation of Mg to Fe in olivine and pyroxene, fractionation of crystalline minerals (such as fractionation of K-feldspar mineral from plagioclase), sediment subduction, crustal contamination, melt/ fluid-related metasomatism, involvement of continental lithospheric mantle or asthenospheric mantle (e.g. Jensen, 1976;Schilling et al., 1983;Varne, 1985;Rogers et al., 1987;Foley, 1992a,b;Edwards et al., 1994;Luhr, 1997;La Fleche et al., 1998;Peccerillo, 1999;Carlson and Nowell, 2001;Abdel-rahman, 2002). Large parts of the West Sulawesi Province are covered by thick (up to 5,000 m) piles of Upper Cenozoic shoshonitic to ultrapotassic and subordinate sodic volcanic rocks together with associated intrusive and volcaniclastics. The volcanic rocks occurring in the central part of the province have been subdivided into four units: Sekala Formation, and Sesean, Adang, and Talaya Volcanics (Ratman and Atmawinata, 1993) (Figure 2). The tectonic setting of Adang  (Ratman and Atmawinata, 1993) showing the location of study area. The coordinates of geological map use UTM zone 50S. The white circles are high-Mg lamprophyre sampling point, the blue circle is the other Adang Volcanic peralkaline dyke which exposed in the Talaya Volcanics. (b). Simplified geological map of Sulawesi {modified after Sukamto, 1975b;Hamilton, 1979;Silver et al., 1983;Parkinson, 1991, (in Van Leeuwen andPieters, 2011) Volcanics were formed in a postsubduction, within-plate continental extension/initial rift tectonic setting, which consist of (ultra-) potassic to sodic series and were generated by minor (< 0.1 %) partial melting of depleted MORB mantle (DMM) material (garnet-lherzolite) with the silicate melt having undergone strong metasomatism (Godang et al., 2016). The magmatic process is also influenced by the continental crust component (Sukadana et al., 2015), whereas the tectonic of Talaya Volcanics has yet to be studied in detail. In general, the Talaya Volcanics is composed of andesitic-basaltic volcanic breccia, tuff and lava, with intercalation of sandstone and marl, local coal (Ratman and Atmawinata, 1993); whereas the Adang Volcanics is predominantly composed of leucite/pseudoleucitebearing trachytic tuff, lapilli-tuff, agglomerate, volcanic breccia, volcanic-sedimentary products (volcaniclastics consisting of trachytic weathering residue, trachytic fragments), volcaniclastics and lava intercalations of basalt/basaltic to intermediate composition (consisting of leucite/ pseudoleucite, diopside/aegirine and high temperature phlogopite) (Godang et al., 2016). In the research zone, Talaya Volcanics oppresses the Adang Volcanics, wherein the peralkaline dyke of Adang Volcanics and high-Mg lamprophyre lava were found in Talaya Volcanics. The presence of high-Mg lamprophyric lava in Talaya Unit which associated with the mantle enrichment rocks of the Adang Volcanics is a subject to be studied.
The aim of this study is to understand the genesis, geotectonic, and the melting conditions of high-Mg lamprophyre. The studied area is located at the boundary between Subregencies of Mamuju (Mamuju Regency) and Tabulahan (Mamasa Regency), West Sulawesi.

Materials And Methods
Geochemical analyses of major oxides, trace elements, and fifteen REE elements were carried out at Intertek Laboratories in Jakarta on July 2012 and August 2016, by using XRF (X-ray fluorescence), and ICP-MS (Inductively Coupled Plasma Emission Mass Spectrometry) with four acid digestions. Petrographic analysis was conducted at Mineral Resources Laboratory (Gadjah Mada University) on August 2016.

Geochemical Features
The results of major and trace element analyses of five komatiitic lamprophyric samples are shown in Table 2 (Jensen, 1976; Figure 7). The komatiitic lamprophyre has a high Mg#= 69 -74 which proves that the protolith was originated from mantle melt (after Schilling et al., 1983; Figure 8). The relatively low content of TiO 2 < (-1.1610 + 0.1935 x Al 2 O 3 ; in wt. %) indicates the typical of geotectonic is more towards Arc-related  Figure 9).

Interpretation of Results
The analytical results of various diagrams have been plotted on a diagram to classify high-Mg komatiitic lamprophyric rock and to explain       Figure 11). The approach with multigeotectonic basalt diagrams developed from Hollocher et al., 2012a(after Sun et al., 2006, modified by Godang et al. (2016; Pearce, 2008) (Figures 12 -14) shows that the whole diagram presents the tectonic setting of komatiitic shoshonitic lamprophyre is in the form of 'alkaline continental-arc' or 'continental within-plate'. The overlay between the diagram of Figure 9 and Figures 12 -14 could be proposed the geotectonic was formed in 'alkaline continental-arc'. As a comparison, the geochemical data for shoshonitic alkaline lamprophyre has been plotted from Lamprophyres textbook (Rock, 1991) into the same Figures 12 -14 and Figure 9, which shows the rock fall within a mantle plume field and withinplate. These reveal the alkaline lamprophyres could be generated from different tectonic setting.
The correlation between Ni and MgO concentrations is displayed in Figure 17 (after Wang et al., 2007). The diagram shows that the komatiitic lamprophyric magma is dominantly controlled by olivine fractional crystallization. The plot in Figure 18 (diagram after Aldanmaz et al., 2000 and suggests the komatiitic lamprophyre is derived from the enriched mantle source (Nb/Zr > 0.0627), and the magma was generated from partial melting of DMM (depleted MORB mantle) which composed of pyroxene-peridotite (garnet-lherzolite). This finding is consistent with the previous paper from Godang et al., 2016. As a comparison, the plot data for primary magma of Galunggung   Komatiitic lamprophyre (data_2012: this study) Ghiorso and Sack (1995) (Kersting and Arculus, 1994) MgO (after Wang et al., 2007) show Western Sulawesi komatiitic lamprohyre and Galunggung basalts form a tight linear trend dominantly controlled by olivine fractional crystallization. Symbols in Figure 7. Figure 18. Partial melting curves of mantle source (after Aldanmaz et al., 2000 and. Magmatic affinity: the ratio of La/Yb (MacLean and Barrett, 1993), the ratio of Nb/Zr for discriminating depleted--slightly--enriched mantle is adopted from Le Roex et al. (1983) and Sun et al. 2006.

Discussion
The results show the West Sulawesi monchiquite lamprophyre is represented by shoshonitic magma series with komatiitic composition, which generated from the lithospheric mantle melts formed in a suprasubduction alkaline continental-arc tectonic setting (see also in Table  3). The source of magmatism proposed comes from the suprasubduction activities from the east. Furthermore, the partial melting of monchiquite lamprophyre estimating was formed in high pressure of 3.5 -3.8 GPa with melting temperature of >1350°C.
Referring to the spidergram pattern in Figure  19, West Sulawesi komatiitic lamprophyre has a similar trend with China Yunnan Donggualin lamprophyre (Huang et al., 2002), but has a little bit higher of Th content, and the difference in the ratio of MgO/Al 2 O 3 . The komatiitic lamprophyre (this study) has a ratio of MgO/Al 2 O 3 > 0.7901, whereas the China Yunnan Donggualin lamprophyre has a ratio of MgO/Al 2 O 3 = 0.64 which indicates nonkomatiitic lamprophyre (Figure 7). The komatiitic lamprophyre has also a different pattern with the textbook of alkaline lamprophyre (Rock, 1991). Furthermore, the alkaline lamprophyre (Rock, 1991) has a similar trend with Hawaiite mantle plume (sodic series; Chakraborty, 2007). The similarities or inequality patterns of lamprophyres may be due to the differences in genesis and/or protoliths. West Sulawesi komatiitic lamprophyre in spidergram pattern ( Figure 20) shows the negative Eu anomaly (dEu = 0.68 -0.73), and it is characterized by distinctly negative spikes in Nb-Ta, Ti, and has enriched Figure 19. Incompatible to compatible multi-trace elements diagram Normalized to Primitive Mantle (the incompatibility sequence is referred from Zhang, 2014). The description of weakly-moderately-strongly mantle metasomatism is only used for the determination of metasomatism level of mafic rocks (modified from Godang et al., 2016). Primitive Mantle (PM) values are taken from McDonough and Sun (1995) and Depleted Mantle (DM) from Salters and Stracke (2004), showing plots of Western Sulawesi komatiitic lamprophyre have a similar pattern with China Yunnan Donggualin lamprophyre. The alkaline lamprophyre (Rock, 1991) have a similar trend with Hawaiite mantle plume (sodic series). Taiwan Tsaolingshan high-Mg potassic rock has a different pattern with Sulawesi komatiitic lamprophyre (this study), Yunnan Donggualin lamprophyre, Alkaline lamprohyre (Rock, 1991), and Hawaii mantle plume. The Galunggung basalt (tholeiitic primary magma) has a lower value of trace elements and REE (in TREY).   Figure 20. The overlay of Normalized to Primitive Mantle diagram between Rare Earth Elements (REEs) and trace elements. The grey solid line is copied from Figure 19, orange solid line is REEs. The chondrite values are taken from McDonough and Sun (1995) and Depleted Mantle (DM) from Salters and Stracke (2004). The diagram shows Western Sulawesi komatiitic lamprophyre has a negative Eu anomaly (dEu = 0.68 -0.73) followed by the enrichment of light-REE (La, Ce, Pr, Nd, Sm) and TREY. Furthermore, it is characterized by distinctly negative spikes in Nb-Ta, Ti, and has enriched in other High Field Strength Elements (HFSE), i.e. Th-U, Zr-Hf.  (McDonough and Sun, 1995) deple ted mant le chondrite in Th-U, Zr-Hf, light-REE (La, Ce, Pr, Nd, Sm), and TREY.

Conclusion
The integrated petrographic, mineralogical, and geochemical studies of komatiitic shoshonitic lamprophyre have obtained the following conclusions: • West Sulawesi lamprophyre is classified into monchiquite lamprophyre with needle-like texture, indicating the unfractionated komatiitic primitive magma with shoshonitic alkalinity magma series. • Geotectonic setting of the lamprophyric rock was formed in suprasubduction alkaline continental-arc, which the magmatism came from the suprasubduction activities from the east.
• The protoliths of komatiitic lamprophyric magma was generated from partial melting of depleted MORB mantle (DMM), comprising pyroxene-peridotite (garnet-lherzolite). • The partial melting conditions are suggested to occur at high pressure (3.5 -3.8 GPa) and depth of about 120 km with the melting temperature of >1350°C, and the magma is dominantly controlled by olivine fractional crystallization.