SHRIMP U-Pb ZIRCON GEOCHRONOLOGY OF THE HUAI'AN COMPLEX: CONSTRAINTS ON LATE ARCHEAN TO PALEOPROTEROZOIC MAGMATIC AND METAMORPHIC EVENTS IN THE TRANS-NORTH CHINA OROGEN

摘要

The Huai'an Complex is situated in the northern segment of the Trans-North China Orogen (TNCO), a continent-continent collisional belt along which the discrete Archean Eastern and Western Blocks amalgamated to form the basement of the North China Craton. The complex consists of six distinct lithologic units: the Huai'an TTG gneisses, the Manjinggou high-pressure mafic granulites, the Khondalite Series, the Dongjiagou granitic gneiss, the Huai'an charnockite, and the Dapinggou K-feldspar granite. SHRIMP U-Pb geochronology, combined with Th and U data and cathodoluminescence (CL) imaging of zircons, enables resolution of magmatic and metamorphic events that can be directed towards understanding the late Archean to Paleoproterozoic history of the TNCO. CL images reveal the coexistence of magmatic and metamorphic zircons in most lithologies of the Huai'an Complex, of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating oscillatory-zoned magmatic zircon cores. SHRIMP U-Pb analyses on magmatic zircons reveal that the tonalitic, trondhjemitic and granodioritic protoliths of the Huai'an TTG gneisses were emplaced at 2515 + - 20 Ma, 2499 + - 19 Ma and 2440 + - 26 Ma, respectively, much earlier than the emplacement of the Dongjiagou granitic gneiss dated at 2036 + - 16 Ma. However, their metamorphic zircons yield similar concordant ~(207)Pb/~(206)Pb ages of 1847 + - Ma, 1842 + - 10 Ma and 1847 + - 11 Ma for the tonalitic, trondhjemitic and granodioritic gneisses, respectively, and 1839 + - 46 Ma for the Dongjia granitic gneiss. These ages demonstrate that the Huai'an Complex underwent a regional metamorphic event at approx 1850 Ma, which is further supported by a mean ~(207)Pb/~(206)Pb age of 1848 + - 19 Ma for metamorphic zircons in the Manjinggou high-pressure mafic granulite and 1849 ±10 Ma and 1850 + - 17 Ma for igneous zircons in the anatectic Huai'an charnockite and Dapinggou garnet-bearing S-type granite, respectively. The timing of late Archean to Paleoproterozoic magmatism and regional metamorphism in the Huai'an Complex is in general agreement with recent SHRIMP zircon data for other metamorphic complexes in the TNCO. These data prove that the high-grade gneiss complexes were not the basement to the low-grade granite-greenstone terranes in the TNCO. Furthermore, the lithologies of the orogen are considered to have developed as a long-lived magmatic arc that was subsequently tectonically disrupted and juxtaposed during the collision of the Eastern and Western Blocks at approx 1.85 Ga, leading to final assembly of the North China Craton.