Petrology Of Early Paleozoic Granites And Their Relation To Tectonic Evolution Of Orogen In The North Qinlian Orogenic Belt

North Qilian Orogenic Belt (NQOB) underwent complex multi-phase and multi-stage tectonic evolution which yielded different types of granitoids. Thirteen granite bodies in the eastern part of Qilian Mount Area(QMA) were selected in this paper and their petrology, geochemistry, zircon geochronology and in situ zircon Hf isotope as well as whole rock Rb-Sr and Sm-Nd isotope were systematically analyzed,. On the basis of previous works, the granitic magma episodes for NQOB were divided in terms of geochemical characteristics and geochronology results, then petrogenesis of granitoids from NQOB, their source material and geodynamic setting were thereafter illustrated.Granitic magma activities of NQOB can be subdivided into four periods(or five stages altogether):The first period (505.4-498.6Ma),occurred in Late Cambrian, mainly consists quartz diorite and granodiorite which belongs to VAG(Volcanic Arc Granite) exposed in the southeast part of NQOB and has the characteristics of type-I granite. The second period (464.6-440.9Ma) belongs to the Early and Middle Ordovician which occurred the largest and most widely distributed granitoids with I-S transitional characteristics. Among which, the earlier stage (464.6-450.6Ma) mainly formed the peraluminous quartz diorite-granodiorite-granite with low SiO2and high FeOT,∑REE, Y content, while the Late stage (450.3-440.9Ma) predominantly generated two types of granitoids with low content of∑REE and Y:one type has the characteristics of the high Sr, low Y; another has low Sr and Y. The third period (453-421.3Ma) granitoids in Late Ordovician are widely distributed in the QMA with low content of SiO2, high Al2O3, Sr, Y and K2O+Na2O content. It is generated by the delamination of the orogen roots, as well as orogenic belt stretching and slump. The fourth period (422-418.1Ma) mainlyn formed A-type granites in the Late Silurian. It has the characteristics of high content of SiO2, low Al2O3, Na2O/K2O and Sr content, merely weak fractionation of LREE/HREE, and strong negative Eu anomalies (δEu=0.11-0.49) which demonstrated the late orogenic process of NQOB, and then entering the intracontinental evolution stage.Zircon Lu-Hf isotope and whole rock Sr-Nd isotopic results show that:the εHf) value of granitoids zircons in first period ranging from-5.7to2.5, their TDM2are1.31-1.83Ga, implying that the protolith of granitoids is mainly Neoproterozoic crust with small amount of mantle-derived component. The εHf(t) value of grantoids zircons from first stage of second period which mainly distributed in the south part of NQOB ranging from-1.4to12, their TDM2are0.68-1.53, suggesting that the protolith is mainly mantle-derived component with small amount of Mesoproterozoic crustal material. Futthermore the εND(t) value of sample 07CL113/114range from-12--8.2, their TDM2are1.86-2.17, implying that the protolith is mainly the ancient continental crust. The εHf(t)(-1.8-4.8) and εNd(t)(-5.5--0.1) value of grantoids zircons from second stage of second period indicated the protolith is crust-mantle mixture. The εHf(t) value of granitoids zircons from the third period range from-12.2to1.8, their TDm2are1.3-2.19Ga, suggesting that the protolith is mainly Paleoproterozoic crust with small amount of Neoproterozoic mantle-derived component. The protolith of fourth period A-type granites might be the metamorphic igneous material from the continent-continent collisional environment.Based on the investigations mentioned above, the characteristics of tectonic evolution of NQOB were discussed:(1)North Qilian Ocean(NQO) southward subduction in Cambrian(512-501Ma);(2) northward Subduction of NQO, and ultimately the closure of the ocean basin in the stage of Early-Middle Ordovician (464.6-440.9Ma);(3) Post-orogenic stage in435Ma-421.3Ma;(4) the end of the orogenic process and then came the intracontinental evolution stage(422-418.1Ma).