High- and ultrahigh-pressure metamorphism and retrogressive textures of gneiss in the Donghai area - Evidence from gneisses in drillhole ZK2304

In the gneisses from the drillhole ZK2304 of the Donghai area, there have b een preserved high- and ultrahigh-pressure metamorphic mineral assemblages, a series of complicated retrogressive textures and relevant metamorphic re actions. In addition to garnet, jadeititic-clinopyroxene and rutile, other peak stage (M-2) minerals in some gneisses include phengite, aragonite and coesite or quartz pseudomorphs after coesite. The typical peak-stage minera l assemblages in gneisses are characterized by garnet + jadeitic-clinopyrox ene + rutile + coesite, garnet + jadeitic-clinopyroxene + phengite + rutile +/- coesite and garnet + jadeitic-clinopyroxene: + aragonite + rutile +/- coesite. The grossular content (Gro) in garnet is high and may reach 50.1 m ol%. The SiO2 content of phengite ranges from 54.37% to 54.84% with 3.54-3. 57 p.f.u. Quartz pseudomorphs after coesite occur as inclusions in garnet. The gneisses of the Donghai area have been subjected to multistage recrysta llization and exhibit a closewise P-T evolutional path characterized by the near-isothermal decompression. The inclusion assemblage (Hb+Ep+Bi+PI+Qz) w ithin garnet and other minerals has recorded a pre-peak stage (M-1) epidote amphibole facies metamorphic event. High- and ultrahigh-pressure peak meta morphism (M-2) took place at T=750-860 degrees C and P>2.7 GPa The symplect itic assemblages after garnet, jadeitic-clinopyroxene and rutile imply a ne ar-isothermal decompression metamorphism (M-3,M- M-4) during the rapid exhu mation. Several lines of evidence of petrography and metamorphic reactions indicate that both gneisses and eclogites have experienced ultrahigh-pressu re metamorphism in the Donghai area. This research may be of great signific ance for an in-depth study of the metamorphism and tectonic evolution in th e Su-Lu ultrahigh-pressure metamorphic belt.