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

DOI:10.17014/ijog.3.3.195-214The 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).


Introduction
Large parts of the Western Sulawesi Province are covered by thick (up to 5000 m) piles of Upper Cenozoic shoshonitic to ultra-potassic and subordinate sodic volcanic rocks together with associated intrusives and volcaniclastics. he volcanic rocks occurring in the central part of the province have been subdivided into four units, these are Sekala Formation, Sesean, Adang, and Talaya Volcanics (Ratman and Atmawinata, 1993). The Adang Volcanics are the subject of this paper. Their locations are shown in Figure  1. The unit consists of a sequence about 400 m thick of poorly bedded leucite-bearing lapilli tuff, volcanic breccias consisting of leucite basalt fragments embedded in leucite-bearing tuffaceous matrix, and leucite basalt flows (Ratman and Atmawinata, 1993). Major element analyses carried out previously on a number of samples suggest the Adang Volcanics are mafic to intermediate in composition, ranging from trachyte to tephrite and phonolite (Sukadana et al., 2015). Waele and Muharam (2014) describe the rocks as phlogopitic and leucitic volcanic rocks. The Adang Volcanics interfinger with the marine sedimentary Mamuju Formation, which has a latest Miocene to Early Pliocene age (Ratman and Atmawinata, 1993), and have yielded K/Ar ages of 5.4 and 2.4 Ma (Bergman et al., 1996). The morphology of the volcanics is youthful, showing a volcanic centre and several domal structures (Sukadana et al., 2015).
Opinions differ regarding the origin of these and other high-K rocks in the province: 1. Magmatism took place in a postsubduction, continental-margin rift setting with the source mantle having been metasomatized by previ-ous subduction processes (e.g. Yuwono et al., 1985;Leterrier et al., 1990); 2. The volcanic rocks were formed in an Active Continental Margin (ACM) setting in which magma was generated from mantle melting in the final stage of the subduction process (Puspita et al., 2005), which involved subduction of microcontinental crust (Sukadana et al., 2015); 3. The volcanics developed as the result of collision between the Banggai-Sula fragment and western Sulawesi (Bergman et al., 1996). The goal of the research is to deduce from the magmatic typology of the Adang Volcanics, their tectonic setting, and the nature of submantle enrichment, using the geochemical approach. The studied area is defined by the following coordi nates 118°45'11.10"E -119°5'13.41"E and 2°33'28.27"S -2°59'34.52"S, covering the Subregencies of Kalukku, Mamuju, Tabulahan, Simboro, Tapalang, and Malunda.  (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 andPieters, 2011). Western Sulawesi (dashed line) subdivided into NW (northwest; north part of Western Sulawesi), CW (central-west), and SW (southwest) Sulawesi. jected to major oxide, trace element, and rare earth element (REE) analyses (Tables 1 and 2). The rocks were petrographically also studied in order to support the interpretation of the geochemical data. Geochemical analyses involved XRF and four acids digest (ICP-OES/MS, ICP-REE) assay methods, which were carried out at Intertek Laboratories in Jakarta and Perth. In addition, trace element data have been plotted on various diagrams previously obtained from 31 company drill core samples, and major oxide data for 36 previously analyzed outcrop samples for which LOI contents were not determined. These samples were also analyzed in the Intertek Laboratories.

Field Relationships
The field and petrologic observations indicate that the Adang Volcanics, covering an area of ~820 km 2 (Ratman and Atmawinata, 1993)

Analytical Results
The mineralogy of three samples in thin secton is shown in Figures 5 -7. Moreover, major and trace element results for the six samples presented in Tables 1 and 2 respectively have a Mafic Index of Alteration (MIA(o)) ranging from 31.6 to 45.9 ( Figure 8). This suggests that all six samples were   Figure 9) Groves, 1993 and.

Interpretation of Results
The analytical results on various diagrams have been plotted in a diagram to classify the Adang Volcanics, and to elucidate their tectonic setting and the processes in the source mantle that led to their formation.
On the TAS diagram of Le Bas et al. (1986) most samples (including the 36 other samples)     As a comparison, data for a leucite phonolite have been plotted from the Vulsini Volcano, Roman Province, Italy (Santii et al., 2003) on Figure 11, which shows the rock belongs to the ultrapotassic sub-alkaline series (alkaline-calcic character, alumina oversaturated). It should be noted that its alumina content (Al 2 O 3 : 19.97 -20.37 %) is significantly higher than that of the Adang Volcanics (Al 2 O 3 : 10.63 -13.21 %).
Using the diagram designed by Muller and Groves (1994) to discriminate the tectonic settings of potassic to ultrapotassic rocks, the plotting result shows that the Adang Volcanic samples fall within the anorogenic within-plate field (Figures 9 and 14).
In contrast, the Vulsini Volcanics appear to have formed in an arc-related tectonic setting (Alkaline Continental Arc; Supra-subduction Continental Volcanic Arc). Similar settings for the two units are suggested by plots on tectonic discriminant diagrams for mafic rocks using trace elements shown   Nesbitt and Wilson, 1992), modified by Babechuk et al., 2014 (MIA(o)  . Geotectonic diagram for within-plate vs. Arc-related Groves, 1993 and suggesting the Adang Volcanics, others Adang trachytic rocks and Vulsini Volcanic rocks were formed in different tectonic settings. Figure 11. Volcanic rocks classification for orogenic zone (K 2 O vs. SiO 2 , wt%), (Peccerillo and Taylor, 1976). The depth to Benioff Zone 15 (modified by Fadlin and Godang, 2015(in Godang, 2015after Hatherton and Dickinson, 1969). The plotting result of Vulsini Volcanic (Table 3) shows ultrapotassic rocks whereas the Adang Volcanics and others Adang trachytic rocks showed medium-K calc-alkaline to potassic peralkaline.    Figure 15. Tectonic discrimination diagram for Basalts (after Hollocher et al., 2012b). Magmatic Affinity: ratio La\Yb for Tholeiitic--Transitional--Calc-alkaline to Alkaline (MacLean and Barrett, 1999), ratio Th/Nb (after Sun et al., 2006). Plot in Figure 15 and overlaying with Figure 14, the result showed Adang Volcanics were formed in within-plate continental extension zone/initial rift and Vulsini Volcanics formed in alkaline arcs, whereas Virunga Volcanics (East African Rift) were formed from within-plate oceanic island and the magma is possible generated from 'mantle plume'? Madupitic lamproite (Leucite Hills) shows a lower HREE signature (La/Yb ~ 800). Figure 14. Tectonic discrimination diagram for high-K igneous rocks (Muller and Groves, 1994) suggesting the Adang Volcanics, other Adang trachytic rocks were formed in different tectonic settings.   Figure 16. Tectonic discrimination diagram for basalts (after Wang et al., 2001; modified in this paper). Overlaying with Figure 14 suggesting the Adang Volcanics were formed in within-plate continental extension zone/initial rift, Vulsini Volcanics were formed in an arc-related (alkaline arcs; supra-subduction continental volcanic arc) and Virunga Volcanics were formed in mantle plume. Figure 17. Tectonic discrimination diagram for basalts (after Sun et al., 2006;modified in this paper). Overlaying with Figure  14 suggesting the Adang Volcanics were formed in within-plate continental extension zone/initial rift, Vulsini Volcanics was formed in an arc-related (alkaline arcs; supra-subduction continental volcanic arc) and Virunga Volcanics (East African Rift) were formed in mantle plume, but the plot result of Madupitic lamproite (Leucite Hills) showed the difference tectonic setting (after Wang et al., 2001 andafter Sun et al., 2006) is possible indicating to an interaction of two magma sources.  Figure 19. Plot in metasomatism of Mafic Rocks Diagram (after La Flèche et al., 1998; modified in this paper) shows Adang Volcanics ('Adang Volcanic Complex') were formed by the silicate melt mantle metasomatism + a hydrated mantle source metasomatism process and Vulsini Volcanics was formed by a hydrated mantle source metasomatized, whereas Madupitic lamproite (Leucite Hills) formed from a hydrated mantle source metasomatism + carbonatite metasomatism (very low HREE signature, Figure 19) and Virunga Volcanics (East African Rift) were formed from moderate to strongly of the hydrated mantle metasomatism process.   Figure 20. Partial melting curves of mantle source (after Aldanmaz et al., 2000;modified in this paper). magmatic affinity: ratio La\Yb (MacLean and Barrett, 1999), Depleted--slightly--enriched Mantle (after Le Roex et al., 1983 andafter Sun et al., 2006), showing the genesis of the Adang Volcanics is related enrichment of DMM (garnet-lherzolite), whereas Virunga Volcanics are related enrichment of WAM (spinel-lherzolite).  Table 3. Major Oxides (%) and Trace Elements (ppm) of Vulsini Volcanics -Roman, Italy (Santi et al., 2003)

Discussion
The results show that the Adang Volcanics are represented by potassic and sodic magma series, and consist predominantly of trachybasalt and basaltic trachyandesite. Furthermore, they indicate that they were formed in a within-plate continental extension/initial rifting tectonic setting after subduction had ceased.
The samples include ultrapotassic rocks. Foley et al., 1987(in Gupta, 2015  The Adang Volcanics clearly belong to Group III based on their spidergram pattern, which is similar to that of the Vulsini volcanic rocks (Figure 21a and b) and other Group III volcanics, which are characterized by distinctly negative spikes in Ba, Nb-Ta, Ti, and P, whereas Rb, Th, and K are strongly enriched (Wilson, 1989). Lamproites of Group I and Group II rocks show a very different pattern (Figure 21b; Wilson, 1989). These differences may be caused by different genesis or metasomatism processes. Lamproites of Group I (e.g. Leucite Hills rocks) were formed from mixing between melts from metasomatized hydrated mantle and carbonatite sources (the rocks have a lower Zr/TRE and very low HREE signatures) (Figures 19 and 21b) and Group II rocks {e.g. Virunga Volcanics (Nyiragongo, East African Rift)} have a genesis from related enrichment of the Western Anatolian Mantle (WAM) from mantle material spinel-lherzolite ( Figure 20). While the patterns shown by the Adang and Vulsini volcanics are very similar, it should be noted that the Adang rocks are more strongly enriched in Zr and Hf (~110-222x) than the Vulsini volcanic rocks (Zr 56x and Hf 38x). This difference can be explained by authors' interpretation that the high Zr-Hf signature displayed by the Adang Volcanics is the result of enrichment of the silicate melt from a source within the mantle itself, whereas the relatively low Zr-Hf signature of the Vulsini volcanic rocks is linked to metasomatism by a hydrated mantle source (see above; Figure 19).
Over the years a number of models have been proposed for the tectonic setting and origin of the Late Cenozoic potassic to ultrapotassic volcanics and associated intrusives in Western Sulawesi. Most favour a post-subduction, within plate extensional setting (cf. Maulana et al., 2016), consistent with authors' findings, which some authors based on similarities with the high-K volcanics in the Roman Province (Leterrier et al., 1990). It is generally assumed that the magmas were derived from mantle previously metasomatized by one ore more subduction events. As mentioned above, the result suggests, however, the magma that generated (at least part of) the Late Cenozoic volcanics in Western Sulawesi was metasomatized in a different way.

Conclusion and Suggestion
The main conclusion of this study, involving petrographic and geochemical analyses, the results of which have been plotted on a series of diagrams, is that the Adang Volcanics consist of (ultra) potassic to sodic rocks of predominantly trachytic composition, that were formed by minor partial melting (<0.1 % by volume) which has been enriched DMM (Depleted MORB Mantle) of garnet-lherzolite composition. Metasomatic processes accompanying silicate melting caused the strong enrichment in Na, K, Zr, and REEs (with a geochemistry signature of (Hf/Sm)PM > 1.23). The magmatism that gave rise to the Adang volcanic complex took place during a within-plate rifting event. The presence of both potassic and sodic volcanic rocks possibly indicates magma was generated from more than one mantle reservoir.
It would be of great interest to carry out a comparative study on the ultrapotassic leucite/ pseudoleucite-bearing volcanics from Gunung Muria and Bawean Island (Central Java) with the aim to establish whether or not those rocks were formed by similar processes as the Adang Volcanics and in a similar tectonic setting. It is further recommend to carry out U/P zircon dating on a suite of representative samples from the Adang Volcanics in order to better constrain the age of this unit, including the temporal relationship between the potassic and sodic series.