Igneous, Alteration and Exhumation Processes Recorded in Abyssal Peridotites and Related Fault Rocks from an Oceanic Core Complex along the Central Indian Ridge

摘要

This paper presents the first detailed studies on the petrology of abyssal peridotites and related fault rocks recovered from an oceanic core complex (OCC) in the southern part of the Central Indian Ridge using the submersible SHINKAI 6500 of the Japan Agency for Marine–Earth Science and Technology. Less deformed, statically serpentinized peridotites were recovered from the ridge-facing slope, whereas highly deformed rocks were recovered from sheet-like structures on the top surface of the OCC. The top surface of the OCC is interpreted to be the main detachment fault. The serpentinized peridotites are consistent with an origin as residues after moderate degrees (13–15%) of partial melting; these were later chemically modified as a result of the infiltration of evolved melts of probable granitic composition resulting in the formation of leucocratic veins. The deformed rocks from the detachment fault are divided into talc-rich and chlorite-rich parts, probably formed as a result of interaction of hydrothermal fluids with peridotite and gabbro precursors along the detachment fault, respectively. Deformation and alteration were locally concentrated along the detachment fault, resulting in mechanical mixing of both altered gabbros and serpentinized peridotites in the deformed rocks during the exhumation of the OCC associated with long-lived fault activity. Our results reveal that gabbros and peridotites are tectonically exposed in oceanic core complexes on the seafloor along the intermediate-spreading CIR, as well as in slow-spreading regions. Fluid-mobile elements such as Li, Rb, Ba, Pb, Sr and U are higher in serpentines than their precursor mantle minerals. The uranium content in serpentine is variable but is abundant in the outermost margin of the precursor minerals. The trace element compositions of serpentine appear to have been continuously changed along with changes in the chemistry of the hydrothermal fluids as temperature decreased. Fluid-mobile elements were thus added and/or leached out during the serpentinization of the peridotite combined with later seafloor weathering.