DEFORMATION TEXTURES AND ALERATION MINERALOGY OF DEFORMED META-PERIDOTITE FROM OCEAN DRILLING PROGRAM, SITE 1271

Extensional faulting at slow spreading mid-ocean ridges denudes mantle peridotite containing gabbroic intrusions to the seafloor surface. Denudation has been hypothesized to occur along low-angle normal (detachment) faults during periods of amagmatic spreading. Near the intersection of the Mid-Atlantic Ridge with the 15°20' Fracture Zone, however, peridotite exposures occur in zones of low relief and linear abyssal hill morphology, suggesting that multiple fault geometries may result in lithospheric extension and mantle denudation. We present results of a deformation texture and metamorphic petrology study of core samples recovered from Ocean Drilling Program Site 1271 (Leg 209) at the intersection of the Mid-Atlantic Ridge with 15°20' Fracture Zone. These data are used to help constrain the conditions and geometry of extensional faulting. Site 1271 core consists dominantly of dunnite and harzburgite with small (<10 m wide) gabbroic intrusions and veins. Granulite facies deformation was concentrated into the margins of gabbro intrusions and gabbroic veins within which strain remained localized during down-temperature metamorphism to sub-greenschist facies. Samples analyzed in this study were collected dominantly from within or near the margins of tremolite/chlorite schist that appears to occupy former gabbroic veins. Mineralogical and micro-textural observations indicate a highly complex history of deformation. Tremolite/chlorite schist shear zones with variable to steep dips contain remnant porphyroclasts of pale brown tremolite with asymmetric pressure shadow tails. Tremolite/chlorite schists cut statically serpentinized peridotite and zones of chlorite and amphibole with texture indicative of cataclastically deformed gabbro affected by static greenschist grade alteration. Tremolite chlorite schists contain paracrystalline micro-boudinage textures indicating shear strain accommodation via a combination of cataclasis and diffusive mass transfer under greenschist conditions. All deformation fabrics are cut by calcite/chlorite-filled tensile fractures. These textures could potentially have been formed in a moderately to steeply dipping normal fault active over temperatures ranging from amphibolite to greenschist facies conditions.