High-temperature gabbro mylonites and ultramylonites from Godzilla Mullion, the Parece Vela Basin

Godzilla Mullion, the world’s largest oceanic core complex, occurs in the Parece Vela Basin spreading ridge, Philippine Sea. Fault rocks derived from lithospheric mantle and lower oceanic crust occur on the surface of Godzilla Mullion, indicating the existence of a detachment fault at the seafloor surface (Harigane et al., 2005). Based on a study of deformed gabbroic rocks at dredge site KR03-01-D6 in the breakaway area, Harigane et al. (2008) reported the earliest deformation recognized in Godzilla Mullion, possibly related to the initiation of the detachment fault. This study presents microstructural and petrological characteristics of the deformed gabbroic rocks such as mylonites and ultramylonites sampled from the medial region of Godzilla Mullion (dredge site KH07-02-D18) developed in a ductile shear zone in the lower crust as a deeper part of the detachment fault. Mylonites are characterized by porphyroclastic textures consisting of plagioclase/clinopyroxene porphyroclasts in plagioclase/clinopyroxene/amphibole matrix. Mylonites can be further classified into two types based on plagioclase matrix grain size: coarse-type (70-75 µm) and medium-type (16-40 µm). Ultramylonites are characterized by lesser amount of plagioclase/amphibole porphyroclasts with very-fine grained plagioclase/amphibole matrix than those in the mylonites. Plagioclase crystal-preferred orientations vary from a (010)[100] pattern for the coarse-type mylonites, (010)[100] and (001)[100] patterns for the medium-type mylonites, and a weak (001)[100] pattern or random orientations for the ultramylonites. These patterns could result from a change in deformation mechanism of plagioclase from dislocation creep to grain-size-sensitive creep. Clinopyroxene and secondary amphibole occur ubiquitously within the mylonites and ultramylonites. Amphibole consists mainly of pargasite (i.e. brown hornblende) and of subordinate actinolite (i.e. green hornblende) around clinopyroxene grains. Amphibole in the coarse-type mylonites shows no evidence of deformation, whereas that in the medium-type mylonites and ultramylonites was plastically deformed. Formation of secondary amphibole within the deformed gabbroic rocks is considered to be related to the syn-deformation retrograde reaction, suggesting that hydrothermal alteration occurred within the mylonites and ultramylonites. The gabbroic rocks could be deformed during exhumation-related cooling of the gabbroic body, which is a similar process to that observed in the deformed gabbroic rocks from the breakaway area. In contrast to the breakaway area, however, the gabbroic rocks in the medial region appear to be deformed under higher-T conditions (650-880 degree) than those in the breakaway area, since relict clinopyroxene grains are still preserved among the hydrous minerals within the ultramylonites. The gabbroic rocks from the medial region may represent a ductile shear zone occurred at depth initially under anhydrous conditions at high temperatures above 880 degree, which was subsequently developed under relatively higher-T conditions despite of hydration during deformation.