Tectonic Setting And Geological Characteristics Of The Mesozoic Granites Of Wushijiazi In Inner Mongolia

The Great Xing’an Ranges experienced strongly magmatic activity during Mesozoic.Formed the main body of the southern of the Great Xing’an Ranges. With large research about granites has been done in the northern segment. Few systematie geochronological study is still absent in the soutnern segments. It hamper our further studing of the mesozoic tectonic evolution in the Great Xing’an Ranges and the eastern of china. On the basic research of the Linxi Wushijiazi Mesozoic Granites. Combined the geochemistry,zircon U-Pb dating and Hf isotopic analyzing we can discussed the geodynamic background, rock characteristics and the formation of source granites of Linxi Wushijiazi.There are three Mesozoic granites exposed in the research area : Chaoyanggou granites, Hudougeshaorong Granites and bayanhushuo granites. Through the petrology,petrography, geochronology, geochemistry. we can further study the Mesozoic tectonic evolution of the sourthern Great Xing’an Ranges. In this paper, the main results were following :1. LA-ICP-MS zircon U-Pb dating of Chaoyanggou pluton indicates that the zircon age was 126.2±2.5Ma. meanwhil,the zircon age of Hudougeshaorong pluton was 129.9±1.4Ma,the zircon age of Bayanhushuo pluton was 150±1Ma. That show the granite emplacement is in the Jurassic-Early Cretaceous. Mesozoic magmatism in Great Xing’an Ranges mainly in the late Jurassic and early cretaceous.2. Compared with the major elements trace and rare elements geochemicalcharacteristics between the three granites pluton. All of them show high SiO2, high alkali and poor in water. On the plots of A/CNK-A/NK and SiO2-K2 O. The threeplutons are belong to aluminum and high-K calc-allaline granites. The pattern of trace elements, slope and tilt direction are sumilar. The plutons show relatively enriched in LREE and loss in HREE, it have evident negative Eu anomalies. Their trace elements are characterized distinct negative Ba, Sr, P, and with enrichment of LILE such as Rb, Th. All these indicated that there existed good internal relations in these three plutons.3. The result of the Hf isotopic analysis in the Bayabhushuo and Hudougeshaorong pluton show that the later have positive εHf(t)ratio(+2.2~+9.4),the two-stage model ages range from 1030 Ma to 570 Ma, with an average age of 808.7Ma; Pluton of Bayanhushuo have positive εHf(t)ratio(6.1~12.3),the two-stage model ages range from810 Ma to 415 Ma, with an average age of 666.9Ma.By combining the Th/U, Nb/Ta between the two plutons, The author thought the primary magma of Mesozoic granite in research area was derived from the Neoproterozoic Depleted mantle material, and then hybridism with the newly crust in the later emplacement which occured partial melting.4. The three Mesozoic plutons in research area are belonging to A-style granites.Considering the wide spread of mafic-felsic dyke swarms. The tectonic background of the southern Great Xing’an Ranges in early cretaceous should be extensional settings.The Jurassic-Early Cretaceous is the transition from extrusion to stretch. The late Jurassic granites pluton was effected by the Mongolia-Okhotskorogen.Formed in the tectonic setting from cornpressron to extension. The early Cretaceous granites pluton the was affected by the west subduction of the Pacific plate.Due to the collision formed the thickened crust that induced the occurrence delamination lithosphere.The early Cretaceous granites is the product of partial melting of stretch crust.