The Geologic Characteristics And Tectonic Significance Of Late Mesozoic Granites Of Hexigten In Inner Mongolia

Magmatic and intrusive activities happened in Hexigten of Inner Mongolia since the Mesozoic. Granites have been considered as a direct reflection of the deep magmatic activity. The author study and analysis macro features, petrography, geochemistry, rock series of Chenjiayingzi, shenjingou, etc. granite rock in Hexigten, based on the field investigation and microscopic identification, geochemical testing, aim to study the issues and the magma source, and discuss the rock formation and dynamics of the tectonic setting background.Hexigten granite is located in Early Paleozoic continental margin of Kedan Mountain-Xar Moron River-Wenduer Temple which belongs to Early Paleozoic suture zone between northern margin North China plate and the southern margin of the Siberian plate. Through field survey and investigation, the author identified that different granite all intruded Zhesi formation of Permian period. According to interspersed,cutting,wrapping relation among the granite, we can determine their order of formation is syenodiorite to medium-coarse biotite orthogranite, long fine-grained biotite granite, coarse grained porphyritic granite, granite porphyry. Coarse-grained biotite granite syenite zircon U-Pb LA-ICP dating to 138.4±3.2Ma in Shibangou Southeast at about 300m show that invasion time is Late Jurassic period, while coarse-grained biotite long granite zircon U-Pb LA-ICP dating to 132.4±1.5Ma in Nailingou show that invasion time is Early Cretaceous period. The rock mineral composition characteristics of Late Jurassic and Early Cretaceous in Hexigten are same, such orthoclase, plagioclase, quartz as major minerals. The major element characteristics show that it is a aluminum-stronger aluminum high-K calc-alkaline rocks, with high SiO2, ALK, K2O/Na2O, low MgO, TFe2O3, CaO as the characteristics and a Early Cretaceous rock with more alkali, aluminum than a Late Jurassic rock.Correlation with Na2O, Al2O3, TFe2O3, CaO, TiO2, MgO, MnO, P2O5 and SiO2 show obviously negative. According to standard mineral characteristics, differentiation index between 84-98, indicate that the higher fractional crystallization of magma higher, and the larger acid magma degree.REE content of the various rock show high enrichment of LREE, with Ba, Cr, Ni, Hf loss and Rb, Sr, Co, Cu enrichment, and Th, U, Ta, Nb, Zr display weak-rich set state. REE distribution curves showed the typical "V" shaped right-wing shape, and they are roughly parallel to one another similarly, which indicats that the magma of Late Jurassic and Early Cretaceous rocks may come from the same types of magma area. Eu anomalies index of two periods granite are less than 1, with a strong negative Eu anomalies, indicating that magma crystallization differentiation occurred during magmatic processes. Most 8Eu value between 0.1～0.38 in the study area, suggesting that the granites feature is of crustal rocks. Ta/Hf-Th/Hf derived tectonic setting show that Hexigten granite of the Late Jurassic and Early Cretaceous granite magma come from the lower crust of lithospheric in continental plate.We can see that various types'granites in Hexigten formation in the Late Jurassic and Early Cretaceous is the A-type granite rock series, which is the product of post-orogenic extensional tectonic environment. The formation of granite is controlled by dynamic background of subduuction and collision with the western Pacific plate and the Eurasion plate in the late Mesozoic.Research on petrology, petrography, geochemistry, structural setting, and so on, in Hexigten Granite of Inner Mongolia has provided important evidence for revealling the late Mesozoic tectonic regime changes, and played a great significance in discussing on its tectonic setting.