Metamorphic P–T–t path retrieved from metapelites in the southeastern Taihua metamorphic complex, and the Paleoproterozoic tectonic evolution of the southern North China Craton

Abstract

The Taihua metamorphic complex in the southern part of the North China Craton is composed of tonalite-trondhjemite-granodiorite (TTG) gneisses, amphibolites, metapelitic gneisses, marbles, quartzites, and banded iron formations (BIFs). The protoliths of the complex have ages ranging from ∼2.1 to ∼2.9Ga and was metamorphosed under the upper amphibolite to granulite facies conditions with NWW-SEE-striking gneissosity. Metapelitites from the Wugang area have three stages of metamorphic mineral assemblages. The prograde metamorphic mineral assemblage (M1) includes biotite+plagioclase+quartz+ilmenite preserved as inclusions in garnet porphyroblasts. The peak mineral assemblage (M2) consists of garnet porphyroblasts and matrix minerals of sillimanite+biotite+plagioclase+quartz+K-feldspar+ilmenite+rutile+pyrite. The retrograde mineral assemblage (M3), biotite+plagioclase+quartz, occurs as symplectic assemblages surrounding embayed garnet porphyroblasts. Garnet porphyroblasts are chemically zoned. Pseudosection calculated in the NCKFMASHTO model system suggests that mantles of garnet porphyroblasts define high-pressure granulites facies P–T conditions of 12.2kbar and 830°C, whereas garnet rims record P–T conditions of 10.2kbar and 840°C. Integrating the prograde mineral assemblages, zoning of garnet porphyroblasts with symplectic assemblages, a clockwise metamorphic P–T path can be retrieved. High resolution SIMS U−Pb dating and LA−ICP−MS trace element measurements of the metamorphic zircons demonstrate that metapelites in Wugang possibly record the peak or near peak metamorphic ages of ∼1.92Ga. Furthermore, 40Ar/39Ar dating of biotite in metapelites suggests that the cooling of the Taihua complex may have lasted until ∼1.83Ga. Therefore, a long-lived Palaeoproterozoic metamorphic event may define a slow exhumation process. Field relationship and new metamorphic data for the Taihua metamorphic complex does not support the previous model in which the Tran-North China Craton (TNCO) was formed through the collision between the East and West blocks.