Petrogenesis of ODP Hole 735B (Leg 176) oceanic plagiogranite: Partial melting of gabbros or advanced extent of fractional crystallization?

Abstract

Oceanic plagiogranite was first defined by Coleman in 1975 as an assemblage of felsic rocks in the ocean crust and is the sole rock type in ophiolites for age dating. However, the petrogenesis of these felsic rocks remains controversial. Some consider them as partial melting products of gabbros, while others interpret them as representing highly evolved residual melt of ocean ridge magmatism. To resolve this debate, we focus our study on felsic veins from ODP Hole 735B (Leg 176) in the Southwest Indian Ridge (SWIR). We carried out detailed petrography, mineral compositional analysis, bulk‐rock trace element and Sr‐Nd‐Pb‐Hf isotope analysis, and petrological modeling. These data and observations lead to the conclusion that these felsic vein lithologies (i.e., oceanic plagiogranite) are solidified residual melts after advanced extents of fractional crystallization of ocean ridge basaltic (MORB) magmas, rather than partial melting products of gabbros. At the late stage of MORB magma evolution, Fe‐Ti oxides appear on the liquidus to crystallize, resulting in the residual melts rapidly enriched in SiO2. Such silicic melts are buoyant and can transport through “cracking” within the lithologies of the gabbroic sections of the ocean crust in the form of felsic dykes, veins and veinlets. The volumetrically small (~ 0.5%) but widespread plagiogranite veins and veinlets throughout the gabbroic sections provide evidence that multiple small melt batch intrusion and differentiation are primary mechanism and mode of ocean crust accretion at slow‐spreading ridges.