Metamorphism, P-T path and zircon U-Pb dating of Paleoproterozoic mafic and felsic granulites from the Kongling terrane, South China

Abstract

The Kongling terrane, the best Archean outcrops exposed in South China, is considered as an ideal target to reveal the crustal formation and evolution of the Yangeze Block in Precambrian. Since the ancient Archean zircon was reported, considerable studies on geochronology have been conducted. Except for the Archean magmatic events, Paleoproterozoic metamorphism in the Kongling terrane is also widespread. In this study, metamorphism in Paleoproterozoic is primarily investigated compared with previous studies. The studies on metamorphism and zircon U-Pb geochronology of four mafic granulite samples and one felsic granulite sample in the Kongling terrane were conducted. Multi-stage metamorphism is recognized through petrographic observation. In the mafic rocks, the mineral assemblage of peak metamorphic stage is represented by garnet + orthopyroxene + clinopyroxene + plagioclase + quartz, and the retrograde stage is represented by the coronas of orthopyroxene + plagioclase + hornblende and orthopyroxene + plagioclase around garnet. The P-T conditions of the two stages obtained from two samples of them are at 811–853 °C, ~11 kbar; 700–728 °C, 5.1–6.0 kbar and 837–869 °C, ~11 kbar, 749–764 °C, 4.5–5.6 kbar, respectively. The calculated P-T conditions reveal that the mafic rocks underwent a stage of granulite facies metamorphism and a subsequent stage of high-amphibolite to low-granulite facies retrograde metamorphism, which defines a near isothermal decompression P-T path and may be associated to a continental collision process. Metamorphic zircons from mafic granulite rocks have weighted mean 207Pb/206Pb ages of ~2.0 Ga. These suggest that the terrane may be influenced by the assemblage of the Paleoproterozoic Nuna Supercontinent. Besides, felsic granulite is recognized in this study, and studies on its metamorphism and geochronology are conducted for the first time. The mineral assemblages of the peak and retrograde metamorphic stage are garnet + orthopyroxene + plagioclase + quartz and symplectite of orthopyroxene + cordierite surrounding garnet, respectively. The peak P-T conditions are at 875–936 °C, 8.3–10.4 kbar. The weighted mean 207Pb/206Pb age of this felsic granulite is also at ~2.0 Ga. Moreover, a period of magmatic event is recorded in this sample, which have a weighted mean 207Pb/206Pb age of 2945 ± 18 Ma, corresponding to the widespread magmatism generating rocks at 2.9 Ga in the Kongling terrane. The Kongling terrane used to be thought of lacking orthopyroxene in both of the meta-felsic and metasedimentary rocks. But because of the high weathering rate in this area, good observations were not achieved in the field, so the geological implication of the felsic granulite also requires further studies.