PETROLOGY, GEOCHEMISTRY, AND GEODYNAMICS SIGNIFICANCE OF GRANITIC ROCKS FROM SOUTH SULAWESI, INDONESIA

Petrology and geochemical characteristics of granitic rocks from South Sulawesi, especially from Polewali and Masamba area and discussion on related magmatic processes and possible source material as well as geodynamic significance are presented. The granitic rocks in the Polewali area consist of granodiorite, monzodiorite and quartz monzonite whereas those from Masamba area consist of granite, granodiorite and diorite in composition. The former has lower silica content (63 wt% in average) compared to the latter (67 wt%). The petrological characteristic showed that quartz, K-feldspar and plagioclase occur as major phases with hornblende and biotite as major ferromagnesian mineral. Titanite, zircon, apatite and magnetite found as accessory mineral either as inclusion or in the groundmass. All of the samples were plotted in calc-alkaline field, show the metaluminous affinity and classified into I-type granitic rocks. Primitive-mantle normalized trace elements pattern of both granitic rock groups displayed an extreme enrichment of LILE (Rb, Ba, Th and U) and depleted HFSEs, particularly Nb and Ta and resembles the upper continental crust affinity. Chondrite-normalised REE pattern of the granitic samples show enrichment in LREE with La_N/Yb_N = 15 and a slight negative Eu anomaly (Eu* = 0.21) with relatively flat HREE pattern and resembles the upper continental composition pattern. ∑ REE in Polewali granitic rocks ranging from 191 – 279 with average of 249 and decreases to the average of 194 in Masamba granitic rocks. The systematic decreases of Sr concentration with increasing SiO₂ and the Eu anomaly and in all granitic rocks indicate continuous plagioclase fractionation during evolution from most of the samples. Early fractionations of apatite and titanite were evidenced by the P₂O₅ and TiO₂ along with FeO systematic decreasing trend with increasing SiO₂ respectively. Based on the geochemical characteristic, it is assumed that these two granitic groups were produced from partial melting of upper continental crust and both have close relationship of sources composition within a suite. The Ta and Nb depletion pattern of almost all the samples explained the arc related subduction environment which later give an evidence of continental derived material subducted to form continental arc environment.