Petrology, phase equilibria modeling and zircon U-Pb geochronology of charnockites from the Badu Complex in southwestern Zhejiang: Implications for Indosinian orogenic event of the Cathaysia Block, South China

Abstract

As the oldest basement rocks in the Cathaysia Block of South China, the Paleoproterozoic Badu Complex underwent high-grade metamorphism and preserved a suit of charnockites that their petrogenesis is still controversial. Comprehensive studies of petrology, mineral chemistry, metamorphic P-T calculation, and zircon U-Pb geochronology trend to discuss the petrogenesis of garnet-bearing charnockites from the Badu Complex in the Suichang-Dazhe area of southwestern Zhejiang Province in detail. The charnockites contain an assemblage of garnet, clinopyroxene, orthopyroxene, biotite, plagioclase, anti-perthite, K-feldspar, quartz, and ilmenite. Three metamorphic stages can be recognized: The prograde (M1) mineral assemblage is mainly composed of garnet (core) + biotite + clinopyroxene + plagioclase + quartz occurring as inclusions in garnet porphyroblast or pyroxene; The peak metamorphic (M2) mineral assemblage is identified as garnet (mantle) + clinopyroxene + orthopyroxene + biotite + plagioclase + K-feldspar + anti-perthite + ilmenite + quartz + melt, typical of granulite-facies mineral assemblage; minerals in plagioclase symplectic corona around garnet porphyroblast and replacing orthopyroxene constitute the retrograde assemblage (M3), including garnet (rim) + clinopyroxene + biotite + plagioclase + quartz ± Ilm. Conventional geothermobarometry in combination with phase equilibria modeling employing the NCKFMASHTO system yield metamorphic P-T conditions for each metamorphic stage as 810–825 °C, 7.5–8.3kbar (M2), 570–640 °C, 5.9–6.4 kbar (M3). It suggests that the charnockites experienced granulite-facies metamorphism and overprinted decompression and cooling along a clockwise P-T path. LA-ICP-MS zircon U-Pb data show a protolith age of 1860 ± 40 Ma and metamorphic age of 229.1 ± 5.7 Ma from the charnockites respectively. Combined with previous studies, we infer that the protolith of charnockites could be the Paleoproterozoic A-type granites and underwent granulite-facies metamorphism during the Indosinian orogenic event. Considering high-grade metamorphism and clockwise P-T path in charnockites with other metamorphic rocks in the Badu Complex, we suggest that the Indosinian orogenic event in the Cathaysia block could be attributed to continental collisions of South China with Indochina and North China. Significant underthrusting of Badu Complex along plate margins with subsequent rapid exhumation might be involved in this process, leading to granulite-facies metamorphism and later decompression and cooling in Paleoproterozoic basement rocks such as charnockites in the Suichang-Dazhe area.