Magmatic history during late Carboniferous to early Permian in the North of the central Xing'an-Mongolia Orogenic Belt: a case study of the Houtoumiao pluton, Inner Mongolia

摘要

We undertake zircon U-Pb dating, Hf isotopes, and geochemical analyses of the Houtoumiao pluton in the Xilinhot microcontinent (XLMC) in the central Inner Mongolia with an aim of determining their ages, petrogenesis, and sources, which are important for understanding the late Palaeozoic tectonic evolution of the Xing'an-Mongolia Orogenic Belt. The Houtoumiao pluton consists of medium-grained granodiorite, coarse- and medium-grained syenogranite. Mafic microgranular enclaves (MMEs) are common in the Houtoumiao pluton. Zircon U-Pb dating has yielded ages of 303 +/- 2 and 301 +/- 2 Ma for the granodiorite, 295 +/- 2 Ma for the syenogranite, and 292 +/- 1 Ma for the MMEs. The granodiorite and syenogranite have features of high-K high silica content, rich in Rb, U, and Th, but low content of HFSE, belong to calc-alkaline series. The P2O5 concentration decreases with the increasing SiO2 content, suggesting I-type affinity. The MMEs, which are characterized by low SiO2, relatively high and variable TiO2, Al2O3, FeOT, MgO, CaO, Ni, and Cr contents, also have much higher total rare earth element concentrations that the REE patterns are subparallel to those of the host rocks. Zircons from the host rocks have epsilon(Hf)(t) values from +3.91 to +7.73 and T-DM2 values of 820-1067 Ma, suggesting that the granitoids were probably dominated by remelting of juvenile crust materials. The MMEs are of epsilon(Hf)(t) value ranging from +6.23 to +11.04 and T-DM1 values from 490 to 693 Ma, suggesting that the primary magma probably was derived from partial melting of a depleted lithospheric mantle, the mafic mineral fractional crystallization and crustal contamination occurred during the magma evolution. Combined with previous studies on the contemporaneous magma-tectonic activities in the Uliastai Continental Margin and XLMC, we suggest that the Houtoumiao pluton formed in a post-orogenic setting.