Granitoid Geochemical Characteristics And Genesis In The Mesozoic Resulting In The Western Region Of Henan Province

The western region of Henan Province is located in the southern margin of the North China Craton. There exists an evident transformation of tectonic regime in the Mesozoic resulting in the occurrence of intense tectonic and magmatic activities and formation of many Mesozoic granitoid plutons in this region. A detailed geological, isotope petrological and geochemical study combined with collection and sorting out of relevant data has been carried out on the granitoids. Based on this study, a summarization has been made on the temporal and spatial distribution regularity of the Mesozoic granitoids and their comprehensive characteristics, and a discussion done not only on material source and geological and tectonic setting for the formation of the granitoids but also on their petrogenesis.The following are the achievements that have been acquired in this study:1.The spatial distribution regularity of the Mesozoic granitoid plutons in the western region of Henan Province has been preliminarily ascertained. These plutons are equidistantly distributed as tracts in the region, belts in E-W direction, and strings in S-N direction. Based on this regularity combined with regional geological and tectonic characteristics, two belts with six sub-belts of the Mesozoic tectonics and magmatism have been recognized in this region.2.The times of every pluton intrusion have been preliminarily defined on the basis of the isotopic age data of the Mesozoic granitoids. The formation process of the rocks is divided for the first time into three cycles corresponding to J₂, J₃, and K₁, respectively, and each cycle into early, middle, and late periods based on intruding contact relationship among different types of granitoids.3. A detailed study on the petrological characteristics of different cycles of the Mesozoic granitoids supports classification of the rocks into three lithological associations including A-type, high-K calc-alkaline, and calc-alkaline granitoids, of which the first association belongs to alkaline series, and the others belong to calc-alkaline series. It is shown that the lithologies change from (quartz) diorite, quartz monzonite, and granodiorite to monzonitic granite and syenogranite with periods from early to late in each cycle.4. A preliminary summarization has been made on the evolution characteristics of the rocks. There occur three cycles of evolution in composition of the Mesozoic granitoids from intermediate acidic to acidic, corresponding to the magmatic activities of J₂, J₃, and K₁, respectively. The material for formation of the granitoids is derived from mixing sources of crust and mantle, with decreasing in crust-derived components from early to late.5. The tectonic setting for formation of the Mesozoic granitoids in the region is defined as a post-collisional setting. The occurrence of a small part of rocks with characteristics of collisional or anorogenic granitoids provides evidence for the continuous process of transformation of tectonic regime from collisional through post-collisional to anorogenic.6. The multiple cycles of mantle-derived magma underplating caused by the multiple stages of lithospheric thinning in the Mesozoic result in the partial melting of middle to lower crust or crust-mantle syntexis to form magmas. The crystallization of these magmas emplacing after fractionation is responsible for the formation of the Mesozoic granitoids in this region.