Granulite-Facies Metacumulates of the Yakuno Ophiolite, Japan: Evidence for Unusually Thick Oceanic Crust

The volcanics, cumulates, and residual peridotites of the Yakunoophiolite were thoroughly metamorphosed before its emplacementinto Permian sediments. Metamorphic grade increases downwardthrough the ophiolite succession, and reaches granulite faciesin the middle of the cumulates. The facies series, includingprehnite-pumpellyite and epidote amphibolite facies, suggestsa lower geothermal gradient than that of ocean-floor metamorphism.The granulite-facies metagabbros and peridotites closely resemblexenoliths in alkali basalts derived from deeper parts of thecrust. Metamorphism involved penetrative solid flow to formpreferred orientation of coarse-grained crystals and subsequentlocal shear to form fine-grained blastomylonites. Temperaturesranging from 750 to 920 ?C are recorded in the cores of coarse-grainedcrystals, and lower temperatures in the blastomylonites. Plagioclase-olivinereaction and the absence of pyropic garnet define the pressurerange of the metamorphism between 5 and 10 kb. These pressuresrequire an original thickness of 15-30 km for the overlyingcumulate-volcanic sequence, which is much thicker than normaloceanic crust. The lithology and chemistry of the metavolcanicsand metacumulates suggests that the Yakuno ophiolite may bea fragment of unusually thick oceanic crust formed beneath asmall oceanic basin.