The Preliminary Studies On The Origin And Tectonic Setting Of Early Mesozoic Granites In The Southwest Margin Of Alax

The studied area is located in the northwest of Dashankou region of Alax Youqi in Inner Mongolia, which belongs to the southern margin of west in the North China plate, and is adjacent to North Qilian orogenic belt. Based on the field research and references in this region, distribution characteristics, nature, age and geochemical characteristics of Mesozoic granitic plutons have been studied using comprehensive analysis of the petrology, geochemistry, isotope geochronology and structural geology etc. in order to discuss the genesis and diagenetic tectonic setting of the early Mesozoic granites in Alax, and to provide a reliable petrologic, isotope chronological evidence to build crustal evolution and tectonic framework of the western part of the Mesozoic North China plate.Granites are distributed extensively in this area, among which Yaoquan granite is monzogranite, and LA-ICP-MS U-Pb age of zircon ranges from239Ma±4Ma to231Ma±3Ma with the weighted average of235Ma±4.4Ma, representing the crystallization age of rocks, and indicate it formed in the late of Middle Triassic. The geochemical results show that Yaoquan granite is characteristic of high Si(the content of SiO2varies from72.72wt.%to78.13wt.%with the average value of74.81wt.%), low Al (the content of Al2O3varies from11.56wt.%to14.07wt.%), Mg, Ti, Ca (the content of MgO, TiO2and CaO varies from0.20wt.%to0.56wt%, from0.12wt.%to0.26wt%, and from0.93wt.%to2.32wt%, respectively) and high-alkali(the content of K2O varies from3.61wt.%to4.31wt.%, and Na2O from3.15wt%to3.80wt%with the average values of Na2O+K2O of7.50%, and K2O/Na2O ranges from0.97to1.28) and A/CNK ratio ranges from093to1.04, and A/NK ranges from1.21to1.33, which belongs to quasi-aluminum high-K calc-alkaline series. CIPW standard minerals calculation indicate samples include diopside and corundum (C) and which content ranged from0.21%to0.57%, which is lower than1%,104Ga/Al <2.6, within the characteristic of I-type granites, and the samples fall into the I-type granite in ACF diagram. The samples is enriched in Rb, Cs, Th, U, K and Pb, and depleted in Ba, Nb, Ta, Sr, P and Ti, and slightly enriched in LREE, and heavy rare earth elements displays flat pattern,(La/Yb) equates to4.4, and significant negative Eu anomaly, and the δEu ranged from0.39to0.61with an average of0.5, revealed that the granitic magma derived from partial melting of middle and lower crustal material, and presence of fractional crystallization of plagioclase during magma ascent or residual plagioclase in the source. Samples fall into the post collision in the granite region in the Rb-Yb+Ta, Rb-Yb+Nb and Hf-Rb/10-Ta×3diagram, and indicate that monzogranite formed in post-collisional tectonic environment, and further inferring that arc-back basin represented by Chaganchulu ophiolite in the north of studied area have collided and closed, and then entered into post-collisional tectonic evolution stages.Based on the data and regional tectonic setting, the Phanerozoic plate tectonic evolution and granite emplacement mode of Alax have been built.