Paleoproterozoic ultrahigh-temperature metamorphism and anatexis of the pelitic granulites in the Kongling terrane, South China

Abstract

Crustal anatexis and ultrahigh-temperature (UHT) metamorphism provide crucial insights into metamorphic evolution and tectonic process at the convergent plate margins. Pelitic granulites in the Kongling terrane record both Paleoproterozoic UHT metamorphism and anatexis, providing an ideal window to reveal the model of plate tectonic during early earth. The pelitic granulites from the Kongling terrane are mostly composed of the coarse-grained unfoliated granulite and the fine-grained foliated or migmatized granulite. The unfoliated granulite preserves a peak granulite-facies metamorphic mineral assemblage of coarse-grained garnet + sillimanite + plagioclase + K-feldspar + biotite + quartz + rutile. The foliated granulite, displaying banded structure of melanosome and leucosome, is composed of fine-grained garnet + biotite + plagioclase + quartz + rutile/ilmenite + felsic melt. The coarse-grained garnets have core-mantle-rim structures. The clean cores are rich in grossular contents and contain oriented needle-like rutile exsolution. Ti-in-garnet thermometer on the garnet cores reveals temperatures ranging from 837 to 1030 °C with a mean ± 1σ uncertainty of 943 ± 12 °C, which are consistent with ternary feldspar temperature (940 °C), suggesting a UHT granulite-facies metamorphism. The garnet ‘dirty’ mantles and rims are almandine-pyrope solutions with inclusions of felsic melt, biotite and ilmenite. Zircon grains have two groups: most of them are anatectic grains, and few of them exhibit granulite-facies core with anatectic rim. The granulite-facies zircons yield ∼2.05 Ga ²⁰⁷Pb/²⁰⁶Pb ages for peak UHT metamorphism, and the anatectic zircons give ∼2.0 Ga ²⁰⁷Pb/²⁰⁶Pb ages for anatexis. Ti-in-zircon thermometer on the anatectic zircons yields temperatures ranging from 595 to 828 °C (692 ± 4 °C), which are consistent with temperatures of the low-temperature perthite grains (670–740 °C, 705 ± 7 °C) but are lower than Ti-in-garnet temperatures of the peritectic garnets (820–916 °C, 855 ± 4 °C), suggesting a process from anatexis to melt crystallization. These results suggest that the pelitic granulites underwent crustal UHT metamorphism and subsequent anatexis during Paleoproterozoic. Together with the contemporaneous low-temperature and high-pressure metapelites from the Shuiyuesi mélange belt in the Kongling terrane, our study suggests the presence of a paired metamorphic belt at a convergent plate margin during the Paleoproterozoic orogeny.