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 Ocean Drilling Program Hole 735B (Leg 176) in the Southwest Indian Ridge. 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 magmas, rather than partial melting products of gabbros. At the late stage of mid‐ocean ridge basaltic 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.

Silica‐Rich Vein Formation in an Evolving Stress Field, Atlantis Bank Oceanic Core Complex

“…However, the normal shear-sense gabbros associated with the Type II veins have a highly variable range from ΔFMQ 0 to +5.4 and low titanomagnetite-ilmenite ratio (Koepke et al, 2004(Koepke et al, , 2005 of oceanic gabbros at Atlantis Bank and MARK area. Magmatic amphiboles in MARK gabbros (Coogan et al, 2001) and in Atlantis Bank plagiogranites (Chen et al, 2019) are also included. D. L. stands for "detection limit.…”

“…(e) Comparison of amphibole Ti in formula with those generated by hydrous partial melting (Koepke et al, 2004(Koepke et al, , 2005 of oceanic gabbros at Atlantis Bank and MARK area. Magmatic amphiboles in MARK gabbros (Coogan et al, 2001) and in Atlantis Bank plagiogranites (Chen et al, 2019) are also included. D. L. stands for "detection limit."…”

“…Felsic veins including anorthosite, diorite, quartz diorites, tonalities, and trondhjemites, also termed oceanic plagiogranites, are ubiquitous in abyssal gabbros in oceanic core complexes (OCCs) at slow‐spreading and ultraslow spreading ridges, and ophiolites (Blackman et al, 2006; Coleman & Peterman, 1975; Robinson et al, 1989). They are products of late‐stage magmatic evolution in the lower crust, generated by differentiation of mid‐ocean ridge basalt (MORB) (Berndt et al, 2005; Chen et al, 2019; Coleman & Peterman, 1975; Feig et al, 2006; Grimes et al, 2011; Nguyen et al, 2018; Niu et al, 2002), partial melting of hydrated gabbros (Dick, Meyer, et al, 1991; Flagler & Spray, 1991; Koepke et al, 2004, 2005, 2007), or liquid immiscibility (Dixon & Rutherford, 1979; Natland et al, 1991). As such, they can provide critical information on the evolution of the lower ocean crust, constraining the final stages of its accretion and emplacement, reflecting the relationship between the late‐stage magmatic and internal structural evolution of the lower ocean crust.…”

“…These correspond to our Types II and I veins, respectively. Chen et al (2019), however, concluded that all the felsic veins in Hole 735B formed magmatically by fractional crystallization. Their sample set consisted of 21 veins, with none from the upper 500 m of Hole 735B.…”

Fe-Ti-Zr metasomatism in the oceanic mantle due to extreme differentiation of tholeiitic melts (Moa-Baracoa ophiolite, Cuba)

“…It is then conceivable that the Potosí pegmatitic gabbro dikes formed from derivative liquids after the crystal fractionation of primitive MORB similar to those that built the Moa-Baracoa lower oceanic crust and the more primitive olivine gabbro dikes intruding Potosí chromitites. Like oceanic Fe-Ti gabbros in slow-spreading mid-ocean ridges (Chen et al, 2019;Dick et al, 2000), Potosí gabbros have an assemblage composed of clinopyroxene and plagioclasewith multiple oxide inclusionsthat is distinctive of gabbroic rocks derived from MOR-like basaltic melts (with BABB-like trace element signature, Fig. 3).…”