Petrology, geochemistry and tectonic significance of serpentinized ultramafic rocks from the South Arm of Sulawesi, Indonesia

Abstract

Serpentinized ultramafic rocks occur in two separate basement complexes in the South Arm of Sulawesi, the Bantimala and Barru Blocks. We present petrographic, mineral chemical and geochemical data for these rocks, and interpret them in terms of petrogenesis and tectonic setting.The rocks of both blocks show strong serpentinization of original anhydrous silicates. The Bantimala ultramafics consist mainly of peridotite (harzburgite and dunite) and clinopyroxenite, with lenses of podiform chromitite. Metamorphism is evidenced by the occurrence of tremolite schists. In contrast, the Barru ultramafics consist of harzburgite peridotite and podiform chromitite.They also show an amphibolite-facies overprint that in this case may be related to intrusion by a large dacite/granodiorite body. The absence of garnet indicates that the ultramafics in neither block experienced high-pressure conditions. Whole-rock trace element analyses and spinel compositions show that the Barru harzburgite is depleted relative to primitive mantle, and has had some melt extracted. In contrast, the Bantimala dunite, harzburgite and clinopyroxenite are cumulates. Both are derived from a supra-subduction zone environment, and were obducted during the closure of small back-arc basins. If there has been no rotation of the blocks, then the Bantimala ultramafics were emplaced from an ENE direction, while the Barru ultramafics were emplaced from the WNW. The ultramafic suites from these two blocks are juxtaposed with metamorphic assemblages which were later intruded by younger volcanics, particularly in the Barru Block.

Chromite Composition and Accessory Minerals in Chromitites from Sulawesi, Indonesia: Their Genetic Significance

“…According to the diagrams presented in Figure 10, we can argue that the Sulawesi chromitites formed in an SSZ from a single batch of magma with an initial high-Cr boninitic composition, which was undergoing differentiation in the deep mantle section and above to the Moho transition zone. This observation is in agreement with the conclusions reported by [34] that investigated serpentinized ultramafic rocks in the South Arm of Sulawesi. These authors, on the basis of whole-rock trace element analyses and spinel compositions, suggested that dunite, harzburgite and clinopyroxenite located in the Bantimala Block in the South Arm of Sulawesi are cumulates.…”

“…However, as documented by [37], low-Ca content is not unique to mantle olivine. Since the presence of cumulate ultramafics rocks has been reported in the Bantimala Block located the in the South Arm of Sulawesi [34], we cannot exclude the possibility that the Alekale and Kalamasse chromitites formed above the Moho-transition zone.…”

“…However, smaller occurrences of the ophiolite complex have been also described in the South Arm ( Figure 1). The presence of serpentinized ultramafic rocks in the Bantimala and Barru Block, located in the South Arm of Sulawesi, has been described by [34], who used geochemical and mineralogical data to conclude that both of these blocks formed in an SSZ environment. Furthermore, they recognized that the harzburgite of the Barru Blocks is hosted in the mantle sequence of the ophiolite.…”

An Overview on the Possibility of Scandium and REE Occurrence in Sulawesi, Indonesia

“…As Sc is a compatible element, mafic and ultramafic rocks generally have higher Sc contents. Sc is incorporated into pyroxene (or amphibole), but is rarely contained in olivine (Leeman and Scheidegger, 1977 (Onuma et al, 1968;Sanematsu et al, 2014;Maulana et al, 2015).…”

Petrogenetic Significance of Spinels From Serpentinised Peridotites From the Verianaousa Ophiolite

“…Furthermore, the formation of ferrian-chromite and chromian magnetite alteration rims around the Al-spinels, Cr-spinels and magnesiochromites is assigned to secondary processes that typically occur during serpentinisation (Khedr and Arai, 2011;Singh et al, 2013;Banerjee et al, 2015;Maulana et al, 2015). Notable Mn-erichments in the ferrian chromite zones involve interaction of hydrothermal fluids that were likely rich in Mn, similarly to other ophiolite complexes (e.g., Gahlan and Arai, 2007).…”