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Geochemical Signatures of Potassic to Sodic Adang Volcanics, Western Sulawesi: Implications for Their Tectonic Setting and Origin

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Geochemical Signatures of Potassic to Sodic Adang Volcanics, Western Sulawesi: Implications for Their Tectonic Setting and Origin Indonesian Journal on Geoscience Vol. 3 No. 3 December 2016: 197-216 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 Geochemical Signatures of Potassic to Sodic Adang Volcanics, Western Sulawesi: Implications for Their Tectonic Setting and Origin Godang Shaban1, Fadlin2, and Bambang Priadi3 1Geochemist, Rare Minerals and REEs Researcher 2Geological Engineering, Jenderal Soedirman University (UNSOED) Jln. Mayjen. Sungkono Km 05, Blater, Purbalingga 3Department of Geology, Institute of Technology Bandung (ITB) Jln. Ganesha No.10, Bandung Corresponding author: [email protected] Manuscript received: July 26, 2016; revised: October 24, 2016; approved: November 18, 2016; available online: December 7, 2016 Abstract - The Adang Volcanics represent a series of (ultra) potassic to sodic lavas and tuffaceous rocks of predominantly trachytic composition, which forms the part of a sequence of Late Cenozoic high-K volcanic and associated intrusive rocks occurring extensively throughout Western Sulawesi. The tectonic setting and origin of these high-K rocks have been the subject of considerable debates. The Adang Volcanics have mafic to mafitic-intermediate characteristics (SiO2: 46 - 56 wt%) and a wide range of high alkaline contents (K2O: 0.80 - 9.08 %; Na2O: 0.90 - 7.21 %) with the Total Alkali of 6.67 - 12.60 %. Al2O3 values are relatively low (10.63 - 13.21 %) and TiO2 values relatively high (1.27 - 1.91 %). Zr and REE concentrations are also relatively high (Zr: 1154 - 2340 ppm; Total REE (TREY = TRE): 899.20 - 1256.50 ppm; TRExOy: 1079.76 - 1507.97 ppm), with an average Zr/TRE ratio of ~ 1.39. The major rock forming minerals are leucite/pseudoleucite, diopside/aegirine, and high temperature phlogopite. Geochemical plots (major oxides and trace elements) using various diagrams suggest the Adang Volcanics formed in a postsubduction, within-plate continental extension/initial rift tectonic setting. It is further suggested magma was generated by minor (< 0.1 %) partial melting of depleted MORB mantle material (garnet-lherzolite) with the silicate melt having undergone strong metasomatism. Melt enrichment is reflected in the alkaline nature of the rocks and geochemical signatures such as Nb/Zr > 0.0627 and (Hf/Sm)PM > 1.23. A comparison with the Vulsini ultrapotassic volcanics from the Roman Province in Italy shows both similarities (spidergram pattern indicating affinity with Group III ultrapotassics volcanics) and differences (nature of mantle metasomatism). Keywords: Adang Volcanics, sodic and potassic/ultrapotassic, within-plate continental extension/initial rift, metasomatized silicate melts, leucite/pseudoleucite © IJOG - 2016. All right reserved How to cite this article: Godang, S., Fadlin, and Priadi, B., 2016. Geochemical Signatures of Potassic to Sodic Adang Volcanics, West- ern Sulawesi: Implications for Their Tectonic Setting and Origin. Indonesian Journal on Geoscience, 3 (3), p.197-216. DOI: 10.17014/ijog.3.3.197-216 Introduction with associated intrusives and volcaniclastics. he volcanic rocks occurring in the central part Large parts of the Western Sulawesi Prov- of the province have been subdivided into four ince are covered by thick (up to 5000 m) piles units, these are Sekala Formation, Sesean, Adang, of Upper Cenozoic shoshonitic to ultra-potassic and Talaya Volcanics (Ratman and Atmawinata, and subordinate sodic volcanic rocks together 1993). The Adang Volcanics are the subject of AccreditedIJOG/JGI by:(Jurnal - LIPI, Geologi valid AugustIndonesia) 2016 - -Acredited August 2021 by LIPI No. 547/AU2/P2MI-LIPI/06/2013. valid 21 June 2013 - 21 June 2016 - RISTEKDIKTI, valid May 2016 - May 2021 197 Indonesian Journal on Geoscience, Vol. 3 No. 3 December 2016: 197-216 this paper. Their locations are shown in Figure ous subduction processes (e.g. Yuwono et al., 1. The unit consists of a sequence about 400 m 1985; Leterrier et al., 1990); thick of poorly bedded leucite-bearing lapilli 2. The volcanic rocks were formed in an Active tuff, volcanic breccias consisting of leucite basalt Continental Margin (ACM) setting in which fragments embedded in leucite-bearing tuffaceous magma was generated from mantle melting in matrix, and leucite basalt flows (Ratman and At - the final stage of the subduction process (Pus - mawinata, 1993). Major element analyses carried pita et al., 2005), which involved subduction of out previously on a number of samples suggest microcontinental crust (Sukadana et al., 2015); the Adang Volcanics are mafic to intermediate in 3. The volcanics developed as the result of col- composition, ranging from trachyte to tephrite lision between the Banggai-Sula fragment and phonolite (Sukadana et al., 2015). Waele and and western Sulawesi (Bergman et al., 1996). Muharam (2014) describe the rocks as phlogopitic The goal of the research is to deduce from and leucitic volcanic rocks. The Adang Volcanics the magmatic typology of the Adang Volcanics, interfinger with the marine sedimentary Mamuju their tectonic setting, and the nature of submantle Formation, which has a latest Miocene to Early enrichment, using the geochemical approach. Pliocene age (Ratman and Atmawinata, 1993), The studied area is defined by the following and have yielded K/Ar ages of 5.4 and 2.4 Ma coordinates 118°45’11.10”E - 119°5’13.41”E (Bergman et al., 1996). The morphology of the and 2°33’28.27”S - 2°59’34.52”S, covering the volcanics is youthful, showing a volcanic centre Subregencies of Kalukku, Mamuju, Tabulahan, and several domal structures (Sukadana et al., Simboro, Tapalang, and Malunda. 2015). Opinions differ regarding the origin of these and other high-K rocks in the province: Materials and Methods 1. Magmatism took place in a postsubduction, continental-margin rift setting with the source For this purpose, six outcrop samples (loca- mantle having been metasomatized by previ- tions shown on Figures 1a, b, and 2) were sub- b 120o E 124 o E a 2o N 2o N North Sulawesi Trench 9.715.000 9.710.000 Northern Sulawesi Palu - Koro Fault Koro - Palu Mamuju Sangihe Trench City 9.705.000 Batui Trust 9.700.000 Neogene NW sedimentary rocks Makassar Strait o Paleogene o 2 S sedimentary rocks 2 S Mesozoic 9.695.000 Tma (Adang Volcanic), Rifting fault sedimentary rocks (Mid -- Late Miocene) Pliocene-Recent volcanics CW Matano Fault Neogene calc-alkaline volcanics 9.690.000 Kls Neogene potassive volcanics (Latimojong Easthern Sulawesi Formation) Paleogene volcanics Tolo Trench 9.685.000 Intrusive rocks SW N Tmtv Metamoorphic rocks (Talaya Volcanic) Walanae Fault Tmpi Ophiolite 9.680.000 (Granitoid N Major thrust 0 6 12 km complex) 6o S 6o S Major strike-slip fault 695.000 700.000 705.000 710.000 715.000 720.000 725.000 730.000 735.000 120o E 100 km 124 o E Figure 1. (a). Geological map of central Western Sulawesi (Ratman and Atmawinata, 1993) showing the location of the researched area and the six samples described in the text. Coordinate system UTM Zone 50S; (b). Simplified geological map of Sulawesi (modified after Sukamto, 1975b; Hamilton, 1979; Silver et al., 1983; Parkinson, 1991, in Van Leeuwen and Pieters, 2011). Western Sulawesi (dashed line) subdivided into NW (northwest; north part of Western Sulawesi), CW (central-west), and SW (southwest) Sulawesi. 198 Geochemical Signatures of Potassic to Sodic Adang Volcanics, Western Sulawesi: Implications for Their Tectonic Setting and Origin (Godang, S. et al.) 0 4.5 9 km Sample for Petrographic (GS1-0016-17) 9.715.000 N 9.710.000 Mamuju City 9.705.000 Rifting fault 9.700.000 1600 m 9.695.000 Tma 1500 m 1400 m 1300 m 1200 m 9.690.000 1100 m Kls 1000 m 900 m 800 m 9.685.000 700 m Sample for Petrographic 600 m (GS1-0607-10) Tmtv 500 m 400 m 9.680.000 300 m Sample for Petrographic Tmpi 200 m (GS1-0926-28) 100 m 0 m 695.000 700.000 705.000 710.000 715.000 720.000 725.000 730.000 735.000 Figure 2. Image of DEM-SRTM (ASTER 30m) and the position of sampling points. jected to major oxide, trace element, and rare weathering residue, trachytic fragments), vol- earth element (REE) analyses (Tables 1 and 2). caniclastics and lava intercalations of mafic/ The rocks were petrographically also studied in mafitic-intermediate composition (consisting of order to support the interpretation of the geo- leucite/pseudoleucite, diopside/aegirine and high chemical data. Geochemical analyses involved temperature phlogopite). Other rocks observed XRF and four acids digest (ICP-OES/MS, ICP- locally include peralkaline dyke, a mafic trachyte REE) assay methods, which were carried out at body, multiple dykes of mafic and leucitic intru- Intertek Laboratories in Jakarta and Perth. In sions, subvolcanic sanidine trachyte intrusion, addition, trace element data have been plotted intercalations of crystalline limestone, and other on various diagrams previously obtained from carbonate rocks. The limestone intercalations and 31 company drill core samples, and major oxide the presence of pillow lavas indicate the Adang data for 36 previously analyzed outcrop samples Volcanics were deposited in a marine environ- for which LOI contents were not determined. ment. Figures 3 and 4 show how the volcanics These samples were also analyzed in the Intertek occur in the field. Laboratories. Field Relationships Analytical Results The field and petrologic observations indicate that the Adang Volcanics, covering an area of The mineralogy of three samples in thin secton ~820 km2 (Ratman and Atmawinata, 1993), are is shown in Figures 5 - 7. Moreover, major and composed predominantly of leucite/pseudoleu- trace element results for the six samples presented cite-bearing trachytic tuff, lapilli-tuff, agglom- in Tables 1 and 2 respectively have a Mafic Index erate, volcanic breccia, volcanic-sedimentary of Alteration (MIA(o)) ranging from 31.6 to 45.9 products (volcaniclatics consisting of trachytic (Figure 8).
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