Formation history and protolith characteristics of granulite facies metamorphic rock in Central Cathaysia deduced from U-Pb and Lu-Hf isotopic studies of single zircon grains

摘要

The petrochemical as well as zircon U-Pb and Lu-Hf isotopic studies of granulite fades metamorphic rock from the Taoxi Group in eastern Nanling Range, Central Cathaysia Indicate that its protolith is the sedimentary rock with low maturation index. The clastic materials are mostly from middle Neoproterozoic (approx 736 Ma) granitoid rocks with minor Neoarchaean and Paleoproterozoic rocks. The timing of this Neoproterozoic magmatism is in agreement with the second period of magmatism widespread surrounding the Yangtze Block. Hf Isotopic data indicate that the Neoproterozoic granitoids resulted from the recycled Paleoproterozoic mantle-derived crustal materials. The sedimentary rock was deposited in Late Neoproterozoic Era, and carried into low crust in Early Paleozoic. The partial melting of the meta-sedimentary rock took place at about 480 Ma and subsequently granulite facies metamorphism occurred at ca. 443 Ma. The zircons forming during this time interval (Early Paleozoic) show large Hf isotope variations, and their epsilon_(Hf)(t) values increase from -13.2 to +2.36 with decreasing age, suggesting the injection of mantle-derived materials during partial melting and metamorphism processes In the Early Paleozoic. Calculation results show that this metamorphic rock, if evolved to Mesozoic, has similar isotopic composition to the nearby Mesozoic high Si peraluminous granites, implying that this kind of granulite facies metamorphic rock is probably the source material of some Mesozoic peraluminous granitoids in eastern Nanling Range.