Indonesian Journal on Geoscience Vol. 8 No. 1 April 2021: 39-58 INDONESIAN JOURNAL ON GEOSCIENCE Geological Agency Ministry of Energy and Mineral Resources Journal homepage: hp://ijog.geologi.esdm.go.id ISSN 2355-9314, e-ISSN 2355-9306 Komatiitic Lamprophyre in West Sulawesi: First Evidence for >1350°C and 3.5 - 3.8 GPa Mantle Melts Shaban Godang1, Fadlin2,3, Bambang Priadi4, Arifudin Idrus5, and I Gde Sukadana5,6 1Geochemistry, Rare Minerals, and REE Researcher 2Geological Engineering, Jenderal Soedirman University (UNSOED); 3Akita University (Japan) 4Department of Geology, Institute of Technology Bandung (ITB) 5Department of Geological Engineering, Gadjah Mada University (UGM) 6Centre for Nuclear Mineral Technology, National Nuclear Energy Agency (BATAN) Corresponding authors: [email protected]; [email protected] Manuscript received: June, 25, 2018; revised: October, 16, 2019; approved: April, 27, 2020; available online: February, 09, 2021 Abstract - The presence of lamprophyric lavas of Late Cenozoic in Talaya Volcanic Formation at the boundary be- tween 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 composi- tion 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 par- tial 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. Keywords: komatiitic lamprophyre, mantle melts, soshhonitic, suprasubduction alkaline-arc © IJOG - 2021. All right reserved How to cite this article: Godang, S., Fadlin, Priadi, B., Idrus, A., and Sukadana I.G., 2021. Komatiitic Lamprophyre in West Sulawesi: First Evidence for >1,350°C and 3.5 - 3.8 GPa Mantle Melts. Indonesian Journal on Geoscience, 8 (1), p.39-58. DOI: 10.17014/ijog.8.1.39-58IJOG Introduction spinifex texture or a variety of needle-like texture (Arndt and Nisbet, 1982; Dostal, 2008); whereas Komatiite is a type of ultramafic mantle- the komatiitic is representative of high-Mg with derived volcanic rock with high magnesium, the ratio of MgO/Al2O3 above 0.7906 (in wt. %) which contains more than 18 % MgO, and the (Jensen, 1976). The komatiitic magma is probably Indexed by: SCOPUS (Q3) 39 Indonesian Journal on Geoscience, Vol. 8 No. 1 April 2021: 39-58 produced in very hot mantle upwellings or plumes kimberlites is usually released by using the geo- (Schmeling and Arndt, 2017). chemical plotting on ternary diagram as proposed Lamprophyres are mesocratic to melanocratic by Bergman, 1987. igneous rocks, usually hypabyssal, with a panidio- The lamprophyric rocks primarily occur as morphic texture and abundant mafic phenocrysts dikes, lopoliths, laccoliths, stocks, and small in- of dark mica (biotite or Fe-phlogopite) and/or trusions. On a purely chemical basis, an extrusive amphibole, with or without pyroxene, with or lamprophyre (e.g. minette or monchiquite) might without olivine, and sometimes melilite and/or be classified as potassic trachybasalt, shoshonite, feldspathoid, set in a groundmass of the same or latite using the total alkali-silica diagram (see minerals. Any feldspar, usually alkali feldspar, TAS classification), or as absarokite, shoshonite, is restricted to the groundmass (Gillespie and or banakite using a classification sometimes Styles, 1999). The lamprophyres are known as applied to potassium-rich lavas. Such chemical exotic rocks, because they are difficult to classify classifications ignore the distinctive textures and unambiguously using the existing criteria. They mineralogies of lamprophyres. are not amenable to classification according to Potassium-rich rocks have intensively been modal proportions, such as the QAPF system, studied for the purpose of understanding not nor compositional discrimination diagrams, only the differences between ultrapotassic such as TAS (Le Maitre et al., 1989). It seems magmas (K2O/Na2O>2.0), potassic (1.52<K2O/ unlikely that a simple taxonomic system will be Na2O<2.0), transition alkaline from shoshonitic found unless the appropriate genetic criteria are to potassic (1.24<K2O/Na2O<1.52), shoshonitic applied, that is, unless the classification takes into (0.5<K2O/Na2O<1.24), and sodic magmas (K2O/ account the genesis of the rocks (Woolley et al., Na2O<0.5)(Turner et al., 1996; Godang et al., 1996). Furthermore, lamprophyres are a complex 2016), but also those between various types of group of rocks that have mineralogical similari- K-rich lavas from different tectonic settings (e.g. ties to some kimberlites and lamproites which are Bergman, 1987; Foley et al., 1987; Muller et grouped into lamprophyric rocks (Rock, 1991). al., 1992). Therefore, the petrogenesis either of The classification of lamprophyric rocks is figured orogenic or anorogenic potassic lavas has been in the hierarchical chart below (Figure 1). The a subject of high interest, because they are com- discrimination for lamprophyres, lamproites, and monly emplaced in an environment with complex Lamprophyric Rocks Clan Alkaline lamprophyres Calc -alkaline lamprophyres Ultramafic {shoshonitic to Kimberlites Lamproites Branch (sodic to shoshonitic) lamprophyres (ultra-)potassic} [KIL] [LL] [CAL] [UML] [AL] Non-micaceous * Fitzroites (LF), Lamproites Family Volcanic/ Olivine- Plutonic kimberlites * Jumillites (LJ), transitional hypabyssal lamproites (K1; Group -1) * Wyomingites to CAL (LW), Micaceous kimberlites IJOG(K2; Group -2) Appinite Suite : Transitional Rock names * Minette (CM), * Camptonite (AC), * Aillikite (UA), Olivine-lamproite Lamproite Cocite * Appinite (CA), kimberlites -aillikites * Kersantite (CK), * Monchiquite (AM), * Alnoite (UL), (LO) (undiff) (LC) * Kentallenite (CE), (KU) * Sannaite (AS). * Damkjernite (UD), * Spessartite (CS), etc. * Vogesite (CV). * Ouachitite (UO), * Polzenite (UP). Vaugnerite series * Durbachite (CD), * Redwitzite (CR), * Vaugnerite (CG). Figure 1. Hierarchical classification of lamprophyric rocks (adopted from Rock, 1991). 40 Komatiitic Lamprophyre in West Sulawesi: First Evidence for >1350°C and 3.5 - 3.8 GPa Mantle Melts (S. Godang et al.) tectonic histories, which would facilitate better Varne, 1985; Rogers et al., 1987; Foley, 1992a,b; understanding and evaluating the role of vari- Edwards et al., 1994; Luhr, 1997; La Fleche et ous geological processes considered responsible al., 1998; Peccerillo, 1999; Carlson and Nowell, for the origin of enrichment in potassium (and 2001; Abdel-rahman, 2002). other highly incompatible elements) of K-rich Large parts of the West Sulawesi Province magmas. These processes include the differ- are covered by thick (up to 5,000 m) piles of entiation of primitive magma, fractionation of Upper Cenozoic shoshonitic to ultrapotassic Mg to Fe in olivine and pyroxene, fractionation and subordinate sodic volcanic rocks together of crystalline minerals (such as fractionation with associated intrusive and volcaniclastics. of K-feldspar mineral from plagioclase), sedi- The volcanic rocks occurring in the central part ment subduction, crustal contamination, melt/ of the province have been subdivided into four fluid-related metasomatism, involvement of units: Sekala Formation, and Sesean, Adang, continental lithospheric mantle or asthenospheric and Talaya Volcanics (Ratman and Atmawinata, mantle (e.g. Jensen, 1976; Schilling et al., 1983; 1993) (Figure 2). The tectonic setting of Adang 1360000 1380000 1400000 120oE 124oE a b North Sulawesi T rench 9720000 9720000 ( o Fault ench r 0 10 20 kilometer Palu - Kor Northern Sulawesi Sangihe T Strait rust Batui T Neogene 9700000 9700000 NW sedimentary rocks Makassar Eastern Sulawesi Paleogene 2oS sedimentary rocks Mesozoic (Adang Volcanics) sedimentary rocks Pliocene-Recent Matano Fault volcanics Neogene CW calc-alkaline volc. Neogene potassivc volc. ench Paleogene r volcanics 9680000 9680000 olo T SW T Intrusive rocks Metamorphic rocks (Talaya Volcanics) W alanae Fault Ophiolite Major thrust o 6 S ( Major strike- slip fault 100 km 1360000 1380000 1400000 c SURFICIAL SEDIMENTARY AND INTRUSIVE DEPOSITS VOLCANIC ROCKS ROCK HOLOCENE Qa Qf Qb Ql Y Qbt PLEISTOCENE QUARTENAR PLIOCENE Tmpi Tmmt Tmtv Tmps Tmm Tma Late Tmb Middle Y MIOCENE Early TERTIAR OLIGOCENE Tma : Adang volcanic rocks EOCENE Tmtv : Talaya volcanic rocks PALEOCENE IJOGKls CRETACEOUS MESOZOIC Figure 2. (a). Geological map of central Western Sulawesi (Ratman and Atmawinata, 1993) showing the location