Ultrahigh‐pressure metamorphism and decompressional P‐T paths of eclogites and country rocks from Weihai, eastern China

Abstract

Abstract Altered quartz‐rich and nearly quartz‐free eclogitic rocks and completely retrograde quartz‐rich garnet amphibolites occur as blocks or lenses in gneisses at Weihai, northeastern tip of the Sulu ultrahigh‐P belt. Eclogitic rocks with assemblage garnet ± clinopyroxene ± coesite + rutile have experienced three‐stage metamorphic events including ultrahigh‐pressure eclogite, granulite and amphibolite facies. Granulite metamorphic event is characterized by formation of the hypersthene + salite + plagioclase ± hornblende corona between garnet and quartz + clinopyroxene. P‐T conditions for the three‐stage recrystallization sequence are 840 ± 50°C, >28 kbar, about 760±50°C, 9 kbar, and ~650°C, <8 kbar respectively. Most country rock gneisses contain dominant amphibolite‐facies assemblages; some garnet‐bearing clinopyroxene gneisses recrystallized under granulite‐facies conditions at about 740±50°C and 8.5 kbar; similar to granulite‐facies retrograde metamorphism of the enclosed eclogitic blocks. Minor cale‐silicate lenses within gneisses containing an assemblage grossular + salite + titanite + quartz with secondary zoisite and plagioclase may have formed within a large pressure range of 14‐35 kbar. Eclogitic boudins and quartzo‐feldspathic country rocks may have experienced coeval in situ UHP and subsequent retrograde metamorphism. The established nearly isothermal decompression P‐T path suggests that this area may represent the interior portion of a relatively large subducted sialic block. The recognized UHP terrane may extend eastward across the Yellow Sea to the Korean Peninsula.