Cretaceous high‐temperature rapid loading and unloading in the Abukuma metamorphic terrane, Japan

Abstract

The Cretaceous Abukuma metamorphic terrane consists of the oceanic Gosaisyo Series overthrust onto the terrigenous Takanuki Series. Although the dominant mineralogy defines one of the classic areas of andalusite–sillimanite type progressive metamorphism, there are several lines of evidence suggesting an earlier higher‐pressure (up to c. 12 kbar) history of the Takanuki Series and the nearby Gosaisyo Series. These are: (1) the occurrence of rare although widespread relic kyanite in sillimanite+K‐feldspar zone‐grade pelitic rocks; (2) the high grossular content of garnet interiors (up to c. 30 mol %) overgrown by Ca‐poor rims (c. 2 mol % grossular) in pelitic rocks containing Al2SiO5 minerals (sillimanite±relic kyanite±retrograde andalusite), plagioclase and quartz; (3) the occurrence of rutile as inclusions in garnet in pelitic rocks; and (4) the occurrence of relic corundum+almandine association in silica‐poor and Al–Fe‐rich rocks. Garnet in the Takanuki Series pelitic rocks commonly shows textural sector zoning and preserves growth zoning despite the high metamorphic grade, suggesting rapid changes in P–T conditions and a relatively short duration of high‐temperature conditions. Combined with radiometric dating, these observations suggest that the Abukuma sillimanite+K‐feldspar zone‐grade rocks underwent a clockwise P–T path with very fast (>4 mm y−1) average burial and exhumation rates.