Occurrence of Felsic Rocks in Oceanic Gabbros from IODP Hole U1473A: Implications for Evolved Melt Migration in the Lower Oceanic Crust

Abstract

Felsic rocks are minor in abundance but occur ubiquitously in International Ocean Discovery Program Hole U1473A, Southwest Indian Ridge. The trace element abundances of high-Ti brown amphibole, plagioclase, and zircon in veins, as well as the presence of myrmekitic texture in the studied felsic rocks support crystallization origin from highly-evolved melts, probably controlled by fractional crystallization. Based on geochemical criteria and texture of the mineral assemblage in felsic rocks and their relationship with host gabbros, they can be divided into three types: (1) Felsic rock with sharp boundaries is formed when felsic melt intrudes into fractures of host gabbros, resulting in minimal interaction between the melt and the wall minerals. (2) Replacive felsic rock, which is characterized by a pseudomorphic replacement of minerals in the host gabbro. This vein type is caused by the replacement of the host mineralogy by minerals in equilibrium with the felsic melts. (3) Felsic rock with diffused boundaries is formed either by infiltration of felsic melt into the solidifying gabbro body or crystallization of interstitial melts. Infiltration modes of felsic melts are likely controlled by the temperature condition of the cooling host gabbros.

Magma Reservoir Formation and Evolution at a Slow-Spreading Center (Atlantis Bank, Southwest Indian Ridge)

“…Closer to the veins, minerals present interaction textures with amphibole rims around Cpx, mostly dissolved Ol grains and strongly zoned Pl. Similar lithologies have been reported from Atlantis Bank IODP Hole U1473A; those are referred to as "felsic rocks with diffused boundaries (vein-D)" by Nguyen et al (2018), and interpreted as felsic melt infiltration into solidifying gabbros. More generally in olivine gabbros, mineral textures such as intergrown Cpx, Cpx with symplectite textures, Pl crystals with discordant polysynthetic twins on irregular grain boundaries and/ or complex optical zoning are common ( Figures 4A,B).…”

“…The reservoir model presented herein eventually covers most of the differentiation sequence of primitive MORB-type melts in a slow-spreading environment, apart from the ultimate stages of evolution documented by the oxide gabbros and felsic veins (e.g., Koepke et al, 2018;Nguyen et al, 2018). One of the questions arising is if this reservoir evolution and associated processes impact the differentiation of the MORBs erupted on the seafloor.…”

Early-Stage Melt-Rock Reaction in a Cooling Crystal Mush Beneath a Slow-Spreading Mid-Ocean Ridge (IODP Hole U1473A, Atlantis Bank, Southwest Indian Ridge)

“…The felsic veins locally crosscut the foliation, although they may themselves be internally deformed. Felsic material makes up ∼1.5 vol% of the core and tends to be concentrated in specific zones, within which it appears to have been formed either as intrusions of a late-stage melt or by crystallization of melts produced by local gabbro anatexis (e.g., Koepke et al, 2007;Nguyen et al, 2018).…”

Petrogenesis of ODP Hole 735B (Leg 176) Oceanic Plagiogranite: Partial Melting of Gabbros or Advanced Extent of Fractional Crystallization?

“…They are chemically characterized by high SiO 2 , Na 2 O, and CaO and low K 2 O and FeO T (Coleman & Donato, 1979;Coleman & Peterman, 1975). They have been well documented in the present-day ocean crust (e.g., Bortnikov et al, 2005;Dick et al, 2000;Nakamura et al, 2007;Nguyen et al, 2018;Silantyev et al, 2010) and many ophiolites (e.g., Amri et al, 1996;Coleman & Peterman, 1975). Though oceanic plagiogranites are volumetrically minor, accessory minerals such as zircon, apatite, and titanite they contained allow for dating the emplacement ages of the ocean crust and ophiolite complexes (e.g., Mukasa & Ludden, 1987;Schwartz et al, 2005).…”