Paleoproterozoic (ca. 1.7 Ga) magmatism in Chifeng, Inner Mongolia: implications for the tectonic evolution of the Trans-North China Orogen

摘要

The Meilin porphyritic biotite granite is located along the northern margin of the North China Craton and the northern central orogenic belt. The Meilin granite is complex but is dominated by a porphyritic biotite granite. Isotopic dating using zircon U-Pb LA-ICP-MS analyses indicates that the porphyritic biotite granite was emplaced at 1715.6 +/- 9.6 Ma during the Late Paleoproterozoic, rather than during the Permian as previously thought. The Meilin granite is an A-type, and all samples from this granite are characterized by relatively high contents of silica (SiO2 = 69.86-71.70%), alkalis ((Na2O + K2O) = 8.69-9.40%), alumina (Al2O3 = 13.71-14.59%), high ratios of FeOt/MgO, low contents of calcium (CaO = 0.26-0.39%), and a negative Eu anomaly (Eu = 0.47-0.57). Additionally, all samples display strong enrichment in Th, K, La, Ce, and P and depletion in U, Ti, Ta, and Nb. These characteristics indicate that the granite formed in a rift environment, where rifting caused mantle decompression and the formation of basic magma. Underplating of the basic magma provided a heat source, leading to the partial melting of the lower crust. Sr isotopes of the Meilin porphyritic biotite granite suggest that the magma source was the remelting of the metamorphic basement. The granite was therefore emplaced in a non-orogenic extensional tectonic setting, which may have been related to the initial breakup of the Columbia supercontinent during the Late Paleoproterozoic.