High‐ and Ultrahigh‐pressure Metamorphism and Retrogressive Textures of Gneiss in the Donghai Area

AbstractIn the gneisses from the drillhole ZK2304 of the Donghai area, there have been preserved high‐ and ultrahigh‐pressure metamorphic mineral assemblages, a series of complicated retrogressive textures and relevant metamorphic reactions. In addition to garnet, jadeititic‐clinopyroxene and rutile, other peak stage (M2) minerals in some gneisses include phengite, aragonite and coesite or quartz pseudomorphs after coesite. The typical peak‐stage mineral assemblages in gneisses are characterized by garnet + jadeitic‐clinopyroxene + 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 mol%. 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 recrystallization and exhibit a closewise P‐T evolutional path characterized by the near‐isothermal decompression. The inclusion assemblage (Hb+Ep+Bi+Pl+Qz) within garnet and other minerals has recorded a pre‐peak stage (M1) epidote amphibole fades metamorphic event. High‐ and ultrahigh‐pressure peak metamorphism (M2) took place at T=750–860°C and P>2.7 GPa. The symplectitic assemblages after garnet, jadeitic‐clinopyroxene and rutile imply a near‐isothermal decompression metamorphism (M3, M4) during the rapid exhumation. Several lines of evidence of petrography and metamorphic reactions indicate that both gneisses and eclogites have experienced ultrahigh‐pressure metamorphism in the Donghai area. This research may be of great significance for an in‐depth study of the metamorphism and tectonic evolution in the Su‐Lu ultrahigh‐pressure metamorphic belt.