Jurnal Internasional Petrogenesis ODP Hole 735B (Leg 176) Oceanic Plagiogranite: Mencairnya sebagian Gabbros atau Tingkat Lanjut Kristalisasi Fraksional? – Chen – – Geokimia, Geofisika, Geosystems
Ocean plagiogranite was first defined by Coleman in 1975 as a collection of felsic rocks in the oceanic crust and is the only rock type in ophiolites for age dating. However, petrogenesis of felsic rocks is still controversial. Some consider it a product of smelting partly from gabbros, while others interpret it as representing a highly developed melt of sea ridge magmatism. To resolve this debate, we focused our study on the felsic vein of the Ocean Drilling Program Hole 735B (Leg 176) at Southwest Indian Ridge. We carry out detailed petrographics, analysis of mineral compositions, bulk-rock trace elements, and Sr-Nd-Pb-Hf isotope analysis and petrological modeling. These data and observations lead to the conclusion that this felsic venous lithology (eg, oceanic plagiogranite) is a solidified melt residue after the expansion of fractional crystallization of the sea ridge basaltic magma, rather than the gabbros parabolic smelting product. In the final stage of evolution of the mid-ocean ridge basaltic magma, Fe-Ti oxide appears in liquidus to crystallize, resulting in melting of rapidly enriched residues in SiO 2 . Such melt silicates are buoyant and can be transported by “cracking” in the lithology of the gabbroic parts of the oceanic crust in the form of dikes, veins and felsic veins. Small volumetric veins and plagiogranite veins (~ 0.5%) but widespread throughout the gabro provide evidence that some small intrusion and melting batch differentiation are the main mechanisms and the way accretion of oceanic crust on the back spreads slowly.