The volcanics, cumulates, and residual peridotites of the Yakuno ophiolite were thoroughly metamorphosed before its emplacement into Permian sediments. Metamorphic grade increases downward through the ophiolite succession, and reaches granulite facies in the middle of the cumulates. The facies series, including prehnite-pumpellyite and epidote amphibolite facies, suggests a lower geothermal gradient than that of ocean-floor metamorphism. The granulite-facies metagabbros and peridotites closely resemble xenoliths in alkali basalts derived from deeper parts of the crust. Metamorphism involved penetrative solid flow to form preferred orientation of coarse-grained crystals and subsequent local shear to form fine-grained blastomylonites. Temperatures ranging from 750 to 920 °C are recorded in the cores of coarse-grained crystals, and lower temperatures in the blastomylonites. Plagioclase-olivine reaction and the absence of pyropic garnet define the pressure range of the metamorphism between 5 and 10 kb. These pressures require an original thickness of 15-30 km for the overlying cumulate-volcanic sequence, which is much thicker than normal oceanic crust. The lithology and chemistry of the metavolcanics and metacumulates suggests that the Yakuno ophiolite may be a fragment of unusually thick oceanic crust formed beneath a small oceanic basin.
ASJC Scopus subject areas
- Geochemistry and Petrology