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

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 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

Geochemistry Study of Cross-castic Magma Alkalinity Evolution

“…The plot results in TAS diagram (after Le Bas, 1986) show RCVR, Fossa delle Felci, and Salak volcanics fall in sub-alkaline field (calc-alkaline series) (Figure 14), but the plot result in 'K 2 O vs. SiO 2 ' diagram (after Peccerillo and Taylor, 1976;Figure 15) shows the magmatic alkalinity of RCVR and Fossa delle Felci evolved from medium-K to high-K (cross-castic); whereas the Salak volcanics (basaltic-andesite to andesite) has a trend of cross-castic evolution. The result of plotting in the overlay diagram between Ishihara and Murakami (2004) and Godang et al (2016), Trapezoid model (Godang et al, 2016), and New ternary magmatic alkalinity evolution trend diagram that created in this study (Figures 16,17,and 18) are presented in Table 2.…”

“…The red dash-line is converted from Ishihara and Murakami (2004) which clearly explained the crosscastic magma alkalinity evolution. (afte r Cala nchi et al, 2002 ) Sh os ho nit e ser ies low -K (T ho lei ite ser ies ) hig h-K (C alc -al ka lin e ser ies ) me diu m-K (Tr an sit ion al/ Ca lc-alk ali ne ser ies ) uration of alumina as proposed by Shand (1927) and exposed in the Trapezoid model (Godang et al, 2016) (Figure 6 and Figure 17). In general, S-type granitoids from sedimentary protoliths or plagiogranite (or albitites/peralkaline granites/ peralkaline rhyolites) from igneous protoliths are not suitable in the plot using the TAS diagram.…”

“…The new magmatic alkalinity evolution trend diagram presented here has been designed for non-alkaline/peralkaline magma in the form of a ternary model using components of the feldspar group (plagioclase and K-feldspar) namely in the form of variables of CaO, K 2 O, and Na 2 O, minus Al 2 O 3 (in wt.%). This ternary diagram is based on a conversion (modification) of the Trapezoid model (Godang et al, 2016), referring to the modified of continuous branch from Bowen's 1922 (Figure 4), and adopting the affirmation of the diagram 'K 2 O vs. SiO 2 ' (Harker, 1909) where there is an increase in K 2 O along the stages of magma differentiation (from basalt/gabbro to rhyolite/granite). The evolutionary trend of magmatic alkalinity from the Trapezoid Model is shown in Figure 5.…”

“…The discrimination of magmatic alkalinity has been the subject of ongoing debate over the past nine decades. Various models have been developed since Shand's classification for peralkaline against calc-alkaline magma (Shand, 1927), including the recent Trapezoid model which was developed by Godang et al (2016) for the classification of (sodic/potassic) calc-alkaline, (sodic/ shoshonitic/potassic) alkaline-calcic, alkaline, and peralkaline igneous rocks.…”

“…Magmatic alkalinity evolution by fractional crystallization diagram. The ratio of K 2 O/Na 2 O is adopted from Godang et al (2016). The red dash-line is converted from Ishihara and Murakami (2004) which clearly explained the crosscastic magma alkalinity evolution.…”