Structure and petrology of hydrothermal veins in gabbroic rocks from Sites 921 to 924, MARK area (Leg 153): alteration history of slow-spread lower oceanic crust

Abstract

Gabbros recovered during Ocean Drilling Program (ODP) Leg 153 from Sites 921 to 924 in the Mid-Atlantic Ridge at the Kane Transform (MARK) area display metamorphic and structural features that record a complex alteration and deformational history of lower crustal rocks formed in a slow-spreading ridge environment. Exposures of gabbroic rocks on the western wall of the Mid-Atlantic Ridge, nearly 10 km west of the spreading axis and about 6 km south of the Kane Fracture Zone, indicate their rapid unroofing within 500,000 to 750,000 yr via faulting and block uplifting near the ridge-transform intersection. Hydrothermal vein assemblages in the gabbroic rocks reflect an alteration chronology that has accompanied crystal-plastic and brittle deformational episodes, as the lower crust was emplaced on the median valley wall. Hydrothermal alteration began preferentially within and along locally developed extensional discrete shear zones, which acted as high-permeability pathways for Na-rich fluids that caused albite enrichment in the plagioclase and Fe enrichment in the clinopyroxene. The concentration of Fe-Ti oxides along some of these discrete shear zones suggests that the shear zones also controlled late magmatic (evolved) and subsolidus fluid flow in the gabbroic rocks (...)