Petrogenesis and dating of the Kangding complex,Sichuan Province
Abstract An analysis of major and trace element geochemistry and SHRIMP U-Pb zircon dating of mafic, intermediate, and felsic rocks from the Mianning metamorphic complex of Sichuan Province was conducted. One of our conclusions is that a majority of the zircons crystallized between 721 and 773 Ma in a magmatic regime. The complexes contain early to late Paleo- proterozoic relict zircons from ancient continental crust. The oldest age, 2468 Ma, may represent the basement of the Yangtze block. Overgrowth zircon rims and palingenetic zircons have been observed and dated, and are believed to have formed after crystallization, during Paleozoic and Mesozoic metamorphic-anatexis. We also conclude that depletion of Nb, Ta, HREE, in a spider-diagram normalized to primitive mantle and calc-alkaline association, implies that they formed in island-arc or under-plating settings.
Paleozoic sedimentary record of the Xing-Meng Orogenic Belt, Inner Mongolia Implications for the provenances and tectonic evolution of the Central Asian Orogenic Belt
The Xing-Meng Orogenic Belt is the eastern extension of the Central Asian Orogenic Belt, which marks the boundary between the North China and Siberian blocks. Key information on the collision between these two blocks and the evolution of this orogenic belt at the end of the Paleozoic are preserved in the widely distributed Paleozoic sedimentary sequences. Petrographic studies of the clastic sedimentary rocks from the Ordovician to the Permian in this region have shown that the rocks are mainly greywackes and arkosic sandstones characterized by low maturity, poor sorting and a large number of lithic fragments. Provenances of Paleozoic sandstones are mainly newly accreted crustal materials. Nd model ages range from the Neoproterozoic to the Late Mesoproterozoic.These ages are very different from those of the North China Block, but the change of εNd(t) (from the initialεNd value) is similar to that of igneous rocks in the Xing-Meng Orogen. This result indicates that provenances of these Paleozoic sedimentary rocks mainly arise from the Xing-Meng Orogen itself. These clastic sedimentary rocks mainly consist of inputs of juvenile accreted crustal materials and limited recycled old crusts. Their provenance is controlled by the following three end members: a felsic basement, a partially melted mafic, and a mafic end member. From the point view of lithology, these three end members must have mixed in a ratio of 53:41:5 to form the average composition of the clastic sedimentary rocks. The depositional environment began to change from an arc to an orogen during the Permian, and therefore the collision was complete before the Triassic. This interpretation is consistent with the argument that the final collision time was at the end of the Permian or beginning of the Triassic.
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