CRETACEOUS HIGH-TEMPERATURE RAPID LOADING AND UNLOADING IN THE ABUKUMA METAMORPHIC TERRANE, JAPAN

The Cretaceous Abukuma metamorphic terrane consists of the oceanic Gos aisyo Series overthrust onto the terrigenous Takanuki Series. Although the dominant mineralogy defines one of the classic areas of andalusit e-sillimanite type progressive metamorphism, there are several lines o f evidence suggesting an earlier higher-pressure (up to c. 12 kbar) hi story of the :Takanuki Series and the nearby Gosaisyo Series. These ar e: (1) the occurrence of rare although widespread relic kyanite in sil limanite + K-feldspar zonegrade pelitic rocks; (2) the high grossular content of garnet interiors (up to c. 30 mol %) overgrown by Ca-poor r ims (c. 2 mol % grossular) in pelitic rocks containing Al2SiO5 mineral s (sillimanite+/-relic kyanite +/- retrograde andalusite), plagioclase and quartz; (3) the occurrence of rutile as inclusions in garnet in p elitic rocks; and (4) the occurrence of relic corundum + almandine ass ociation in silica-poor and Al-Fe-rich rocks. Garnet in the Takanuki S eries pelitic rocks commonly shows textural sector zoning and preserve s growth zoning despite the high metamorphic grade, suggesting rapid c hanges in P-T conditions and a relatively short duration of high-tempe rature conditions. Combined with radiometric dating, these observation s suggest that the Abukuma sillimanite + K-feldspar zone-grade rocks u nderwent a clockwise P-T path with very fast (>4 mm y(-1)) average bur ial and exhumation rates.