Plastic Deformation of Plagioclase in a Gabbro Pluton at a Slow-Spreading Ridge (IODP Hole U1473A, Atlantis Bank, Southwest Indian ridge)

Abstract:

The crustal architecture of slow-spread ocean crust results from complex interactions between magmatism, hydrothermalism and tectonics. IODP Hole U1473A (789m depth) was drilled at the summit of the Atlantis Bank, a gabbroic massif exhumed at the Southwest Indian Ridge (SWIR), during IODP Expedition 360. In this study, we identify and quantify plastic deformation processes in oceanic gabbros throughout Hole U1473A.

We describe deformed zones using petrographic observations made all along the core. Ductile deformation is widespread and is sometimes strongly localized. It initiated during accretion under magmatic conditions and continued until late brittle conditions. Porphyroclastic microstructures testify to post-magmatic, solid-state, high-temperature (HT) deformation. Plagioclase represents ~60% of rock’s volume and is the dominant phase accommodating deformation in the gabbro. It shows strong dynamic recrystallization, forming a fine-grained matrix. Strain localizes in mylonitic and ultramylonitic zones, and these shear zones are often overprinted by lower temperature deformation.

Electron Backscattered Diffraction (EBSD) analyses of plagioclase reveal weak to moderate crystallographic preferred orientations (CPO) first developed during early magmatic flow that has developed a primary fabric with a (010) foliation plane and a [100] lineation axis. This CPO is persistent during subsequent plastic deformation and strain localization, and is observed in almost all samples. However, internal misorientations measured at subgrains are consistent with the (010)[001] slip system, inconsistent with the CPO, but supported by the highest geometrically necessary dislocation densities calculated for this slip system. The strength of CPO is first increasing from slightly foliated gabbros to mylonites before decreasing significantly in ultramylonites, which could be explained by orientation scattering after subgrain rotation and grain boundary processes.