Non-ultrahigh-pressure unit bordering the Sulu ultrahigh-pressure terrane, eastern China: Transformation of Proterozoic granulite and gabbro to garnet amphibolite

The Haiyangsuo area of the NE Sulu ultrahigh-pressure terrane, eastern China, consists of gneisses with minor granulite and amphibolite layers, metagabbros, and granitic dikes. The peak-stage assemblages of the granulites (garnet + orthopyroxene + clinopyroxene + plagioclase ± pargasite ± biotite ± quartz) formed at >750 °C and 9-11 kbar and were overprinted by amphibolite-facies phases characterized by welldeveloped corona layers of | garnet | amphibolite + quartz | at contacts between plagioclase and clinopyroxene or orthopyroxene, as well as by the exsolution of (orthopyroxene + ilmenite + amphibole) from clinopyroxene. These textures indicate a near-isobaric cooling history of the granulite-bearing gneiss terrane. The metagabbro preserves a relict igneous assemblage (orthopyroxene + clinopyroxene + plagioclase + pargasite ± ilmenite ± quartz) in its core, but in its margins has a metamorphic corona texture similar to the granulite that formed at ̃600-700 °C and 7-10 kbar. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating of zircons indicates that the protolith age of the garnet-biotite gneiss is older than 2500 Ma, whereas the granulitefacies metamorphism (the first regional metamorphic event) occurred at 1846 ± 26 Ma. Gabbro intrusion took place at 1734 ± 5 Ma, and the formation of amphibolite assemblages in both metagabbro and granulite occurred at ca. 340-370 Ma. Both gneiss and metagabbro were intruded by granitic dikes, with one dated at 158 ± 3 Ma. These data, together with a lack of eclogitic assemblages, suggest that this granuliteamphibolite- facies complex is exotic relative to the Triassic Sulu high-pressure- ultrahigh-pressure terrane; juxtaposition took place in Jurassic time.