Tectonic significance of the Xilin Gol Complex, Inner Mongolia, China: Petrological, geochemical and U-Pb zircon age constraints

The Xilin Gol Complex, consisting of strongly deformed and metamorphosed rocks, is exposed as a large tectonic unit within the Central-Asian Orogenic Belt (CAOB). It is located on the Xilinhot-Sonidzuoqi north-dipping thrust belt and near the Solonker suture zone that is widely regarded to record the terminal evolution of the CAOB. Its genesis and tectonic origin are a key to test the existing tectonic models as to whether there was a single arc chain by successive accretion or multiple amalgamation involving some microcontinents. This study reports petrological, geochemical, and zircon U-Pb age data of biotite-plagioclase gneisses from the complex. As migmatization is common in the gneiss, restitic and magmatic components are identified in the gneiss by their petrographic features. Mineral chemistry indicates that the Xilin Gol Complex was subjected to amphibolite facies metamorphism. Three groups of zircons are identified by cathodoluminescence (CL) images and Th/U ratios: (1) the first and most abundant group is of magmatic origin and yields a weighted mean Pb-206/U-238 age of 452 +/- 5 Ma; (2) the second, scarcest group is of metamorphic origin and yields a weighted mean Pb-206/U-238 age of 339 +/- 4 Ma; (3) the third group is minor, of apparent detrital origin and yields Pb-207/U-206 ages from 1902 +/- 11 to 886 +/- 54 Ma for the cores, with a peak at 1150 to 900 Ma. Whole-rock major and trace element contents are representative for the upper continental crust, with Rb, Pb, U, Th, K, LREE enrichment and Nb, Ta, Sr depletion. This suggests the gneisses are sourced in the upper continental crust with some old sediment contamination. The protolith of the biotite-plagioclase gneiss is a sedimentary rock with detrital source ages between 1902 +/- 11 Ma and 886 +/- 54 Ma that was deposited at the southern active continental margin of the Siberia Craton during the amalgamation of the Rodinia supercontinent. Partial melting and migmatization occurred at 452 +/- 5 Ma in response to north-dipping oceanic subduction in the Sonidzuoqi-Xilinhot area. Many new magmatic materials formed in the Xilin Gol Complex during this time, while it resided in a continental marginal arc tectonic setting. Oceanic subduction started at 490 +/- 8 Ma and lasted at least to 440 +/- 4 Ma. In the Early-Paleozoic, the CAOB was an advancing accretionary orogen. The complex was mildly affected by a 320-310 Ma magmatic event in the Xilinhot-Sonidzuoqi area. Therefore, the Xilin Gol Complex recorded a long-lived successive accretion along the southern margin of the Siberia Craton. (C) 2010 Elsevier Ltd. All rights reserved.