Emplacement and High-Temperature Evolution of Gabbros of the 16.5°N Oceanic Core Complexes (Mid-Atlantic Ridge): Insights Into the Compositional Variability of the Lower Oceanic Crust

This study reports the composition of the oceanic crust from the 16.5 degrees N region of the Mid-Atlantic Ridge, a spreading ridge segment characterized by a complex detachment fault system and three main oceanic core complexes (southern, central, and northern OCCs). Lithologies recovered from the core complexes include both greenschist facies and weathered pillow basalt, diabase, peridotite, and gabbro, while only weathered and fresh pillow basalt was dredged from the rift valley floor. The gabbros are compositionally bimodal, with the magmatic crust in the region formed by scattered intrusions of chemically primitive plutonic rocks (i.e., dunites and troctolites), associated with evolved oxide-bearing gabbros. We use thermodynamic models to infer that this distribution is expected in regions where small gabbroic bodies are intruded into mantle peridotites. The occurrence of ephemeral magma chambers located in the lithospheric mantle enables large proportions of the melt to be erupted after relatively low degrees of fractionation. A large proportion of the dredged gabbros reveal evidence for deformation at high-temperature conditions. In particular, chemical changes in response to deformation and the occurrence of very high-temperature ultramylonites (>1000 degrees C) suggest that the deformation related to the oceanic detachment commenced at near-solidus conditions. This event was likely associated with the expulsion of interstitial, evolved magmas from the crystal mush, a mechanism that enhanced the formation of disconnected oxide-gabbro seams or layers often associated with crystal-plastic fabrics in the host gabbros. This granulite-grade event was soon followed by hydrothermal alteration revealed by the formation of amphibole-rich veins at high-temperature conditions (similar to 900 degrees C).