Formation and alteration of plagiogranites in an ultramafic-hosted detachment fault at the Mid-Atlantic Ridge (ODP Leg 209).

Jöns, Niels; Bach, Wolfgang; Schroeder, Timothy

We examined small-scale shear zones in drillcore samples of abyssal peridotites from the Mid-Atlantic Ridge. These shear zones are associated with veins consisting of chlorite actinolite/tremolite assemblages, with accessory phases zircon and apatite, and they are interpreted as altered plagiogranite melt impregnations, which originate from hydrous partial melting of gabbroic intrusion in an oceanic detachment fault. Ti-in-zircon thermometry yields temperatures around 820°C for the crystallization of the evolved melt. Reaction path modeling indicates that the alteration assemblage includes serpentine of the adjacent altered peridotites. Based on the model results, we propose that formation of chlorite occurred at higher temperatures than serpentinization, thus leading to strain localization around former plagiogranites during alteration. The detachment fault represents a major pathway for fluids through the oceanic crust, as evidenced by extremely low δ18O of altered plagiogranite veins ( 3.0–4.2‰) and adjacent serpentinites ( 2.6–3.7‰). The uniform oxygen isotope data indicate that fluid flow in the detachment fault system affected veins and adjacent host serpentinites likewise.

MID-Atlantic Ridge; SHEAR zones; PERIDOTITE; GEOLOGIC faults; SCHISTS; CHLORITE minerals; SERPENTINITE; ZIRCON; APATITE; IGNEOUS intrusions

Contributions to Mineralogy & Petrology, 2009, Vol 157, Issue 5, p625

0010-7999

Academic Journal

10.1007/s00410-008-0357-2