The Granite Origin Of Dahuabei In The Wula Mountain Area, Inner A Mongolia

The Dahuabei granite Plutons, in the east of the Wula Mountain area in the west part of north margin of North China Craton, has been closely related to several mineral resources and an important research topic of interaction between plaeo-Asia ocean plate and North China Craton, the accretion of North China, crust-mantle evolution and orogenic mineralization. There were many problems in the previous research such as many researchers only emphasized on the mineral resources and ignored the foundational petrogenesis study and regarded it as a whole instead of discussing concrete magma evolutionary series, while geochemistry research lacked system consideration and many lacked sufficient field evidences and so on.Based on studying and concluding the former research achievement, focusing on the above problems, the study by virtue of geological survey on 1:5000 scale set enough routes and sections to cover the entire Dahuabei plutons:(1)on the term of field and indoor evidence abtained and relational analysis and measurement results, we divide Dahuabei plutons into six units: medium to coarse grained syenogranite(ξγzc), medium to coarse grained porphyric monzogranite(ηγbzc), fine to medium grained monzogranite(ηγzx), medium to coarse grained monzogranite(ηγzc), coarse grained syenogranite(ξγc), and syenogranite porphyry(ξγπ).(2) we find that the entire Dahuabei granite plutons have a nearly equiaxial round shape and different rock units spreads as concentric rings which locate at the junction of Wulateqianqi-Hohhot Fault and Dahuabei-Hejiao Fault and between two ductile shear belts, the macroaxis orientation of single rock unite or compound units is parallel to the strike orientation of Dahuabei-Hejiao Fault, suggesting that strong emplacement existed in the petrogenetic course of Dahuabei granite plutons, whose formation, development and evolution were controlled by these brittle and ductile faults.Given that field survey can’t distinguish the order of several rock units formation, we conduct targeted sampling and single grain zircon LA-ICP-MS U-Pb dating results show the crystallization age of medium to coarse grained syenogranite is 333.8±1.3Ma, that of medium to coarse grained porphyric monzogranite(ηγbzc) is 332.8±1.2Ma, and fine to medium grained monzogranite(ηγzx), 332. 6±1.1Ma. Finally, we can determinate the evolution series of Dahuabei granite plutons: C1ξγzcâ†'C1ηγbzcâ†'C1ηγzxâ†'C1ηγzc â†'C1ξγcâ†'C1ξγπ.In order to testify the classification scheme of Dahuabei granite plutons, and give deep insights into their petrogenesis and tectonic environment characteristics, according to the statistic rules and plutons distribution characteristics, this research, based on regional geology and field research, conducts systematic geochemical study focusing on Dahuabei granite plutons, including rock geochemistry, microelement and REE and disposes and calculates the previous data from isotope Pb, we determinate(1) that different rock units divided from Dahuabei plutons have common in several aspects but different special geochemical characteristics exist, which testifies the field classification scheme further,(2) that there are two different type granites: S-type and fractional I type in this area, and S-type granite exist in the units of C1ηγbz and C1ηγzx, others belong to fractional I-type granite,(3) that the source rocks mainly derived from middle-lower crust, besides mature arc and ocean arc basalt, in which unkown proportion of upper crust constitutes and oceanic sedimentary and little mantle material participated,(4) that Dahuabei granite plutons formed in the extrusion to tension back-arc background where paleo-Asian ocean plate subduct under North China Craton.To further explain the concrete tectonic-magma mechanism of Dahuabei granite plutons, we divide the states and systems of joints based on the statistic analysis of conjugated joint, and we maintain that Dahuabei granite plutons recorded four period tectonic movements, Hercynian period, Indo-China period, Yanshanian Period and Himalyan period, and simulate the Hercynian palaeotectonic stress and strain field in this area that controlled the formation of Dahuabei granite plutons, and we find that on the trigger of nearly South-North extrusion, partial upheaval and several faults movement cause the abnormal 1st principal stress and strain area, which provide the heat source, force and space for the magma’s formation, transportation, and final location. Serval extrusion-interval extrusion led to the formation of granite compound plutons of today scale in Dahuabei area.Synthesizing the above factors, we conclude the petrogenesis of Dahuabei granite plutons and put forth their synthetic petrogenesis model.