Abstract
Some metamorphosed pelites (commonly called as kondalites) expose over the Archean crystal basement in the Kongling terrane of the Yangtze Block. These sedimentary rocks mainly contain biotite – plagioclase gneisses, garnet bearing biotite – plagioclase – silimanite gneisses, garnet bearing biotite – plagioclase – silimanite – cordierite gneisses, graphite schists, and some marbles and quartzites. Amphibolites and mafic granulites always occur as lenses or layers in these metasedimentary rocks. In this study, three kinds of metasedimentary rocks, which are Grt-Bi-Pl gneiss, Grt-Sil-Crd-Bi-Pl gneiss and Grt-Sil-Bi-Pl gneiss, are chosen for petrographic, zircon U-Pb, and Lu-Hf isotopic analysis. The mineral assemblages of these three rocks are Grt + Bi + Pl + Q, Grt + Sil + Crd + Bi + Pl + Q + Spl + Kf and Grt + Sil + Bi + Pl + Q respectively, which imply granulite facies metamorphism. In addition, garnet porphyroblasts in both of these metasedimentary rocks and the mafic granulites which are enclaves in these meta-sedimentary rocks develop decompressional coronas, indicating isothermal decompression (ITD) processes during retrograde metamorphism. Core-Rim structures in zircons can be observed in all these three samples based on cathodoluminescence images. Morphological characteristics, Th/U ratios and trace element compositions indicate that zircon cores are of magmatic origin while zircon rims and some single grain zircons are of metamorphic origin. Ages of magmatic zircon cores in Grt-Bi-Pl gneiss are between 3532 ± 20 Ma to 2286 ± 47 Ma while those of metamorphic zircons are 2022 ± 22 Ma. Most zircons in the Grt-Sil-Crd-Bi-Pl gneiss are of metamorphic origin and they have a weighted mean 207Pb/206Pb age of 2017 ± 15 Ma. Magmatic zircon cores in Grt-Sil-Bi-Pl gneiss have a weighted mean 207Pb/206Pb age of 2160 ± 31 Ma and metamorphic zircons have a weighted mean 207Pb/206Pb age of 2028 ± 43 Ma. One zircon core (3199 ± 25 Ma) in Grt-Bi-Pl gneiss possesses the oldest two-stage Hf model age of 3676 ± 85 Ma. Two-stage Hf model (TDM2) ages of all the other analyzed zircon cores in this sample range from 3452 ± 40 to 3277 ± 43 Ma, and the metamorphic zircons give a weighted mean TDM2 age of 3471 ± 14 Ma. Metamorphic zircons in the Grt-Sil-Crd-Bi-Pl gneiss have a weighted mean two-stage Hf model (TDM2) age of 3468 ± 16 Ma. These TDM2 ages indicate that the sources of these metasedimentary rocks are obviously older than those documented by previous studies and they also suggest that protolith of these metasedimentary rocks belong to Archean crustal materials of the Kongling terrane, but with obviously different sedimentary sources. The widespread decompressional textures around garnet in various samples in the study area suggest that the Kongling area underwent a high-pressure granulite facies metamorphism and an isothermal decompression process at ∼2.0 Ga, corresponding to the assemblage of the Paleoproterozoic Nuna Supercontinent.
Published Version
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