T023-04
Crustal accretion in a slow-spreading back-arc basin: Insights from the Mado Megamullion oceanic core complex (Shikoku Basin, Philippine Sea)
Thursday, 10 December 2020: 04:12
Virtual
Valentin Basch1, Alessio Sanfilippo1, Camilla Sani1, Yasuhiko Ohara2, Jonathan E Snow3, Osamu Ishizuka4, Yumiko Harigane5, Atlanta Sen6, Kyoko Okino7, Masakazu Fujii8, Norikatsu Akizawa9, Katsuyoshi Michibayashi10 and Hiroyuki Yamashita11, (1)University of Pavia, Pavia, Italy, (2)JAMSTEC Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan, (3)Louisiana State University, Baton Rouge, LA, United States, (4)Tsukuba, Ibaraki, Japan, (5)Geological Survey of Japan, Tsukuba, Japan, (6)University of Houston, Houston, TX, United States, (7)Univ Tokyo, Atmosphere and Ocean Research Institute, Kashiwa, Chiba, Japan, (8)Atmosphere and Ocean Research Institute University of Tokyo, Tokyo, Japan, (9)University of Tokyo, Atmosphere and Ocean Research Institute, Kashiwa, Japan, (10)Nagoya University, Nagoya, Japan, (11)Kanagawa Prefectural Museum of Natural History, Odawara, Japan
Abstract:
Oceanic core complexes (OCCs) represent tectonic windows into the oceanic lower crust and mantle; they are key structures in understanding the tectono-magmatic processes shaping the oceanic lithosphere. We present a petrological and geochemical study of gabbros collected at the Mado Megamullion, a recently discovered OCC located in the extinct Shikoku back-arc basin (Philippine Sea). Bathymetric surveys of the Mado Megamullion reveal spreading-parallel corrugations extending 25 km from the breakaway to the termination, with a total extent of the corrugated surface of ~550 km2. Samples were collected in several locations and include peridotites, gabbros, dolerite and rare pillow basalts. Gabbros range from granular olivine gabbros to varitextured gabbros and oxide gabbros, all widely crosscut by felsic veins. The emplacement of these gabbroic rocks within the oceanic lithosphere was followed by a multiphase tectono-metamorphic evolution including: (i) dynamic recrystallization within shear zones, developed under granulite- to upper-amphibolite-facies conditions (900-750°C) and (ii) intrusion of highly evolved melts forming felsic segregations with sharp to diffuse contacts with the host rock.
This tectono-metamorphic evolution recalls that of the lower crust from other OCCs worldwide, demonstrating that this OCC exposes deep-seated intrusions progressively exhumed by detachment faulting. Nonetheless, the Mado Megamullion lower crustal gabbros show an unusual crystal line of descent, different from what is reported from mid-ocean ridge lower crustal rocks. We infer that the water-bearing character of the primary melts in this back-arc basin triggered the early precipitation of clinopyroxene, soon followed by amphibole and Fe-Ti oxides. Such modifications in phase saturation are likely to be directly related to the back-arc setting of the Mado Megamullion. If so, the phase assemblages of oceanic gabbros may be a diagnostic for the tectonic setting of lower crustal rocks in ophiolites.
