Geological Characteristics And Genesis Of The Mesozoic Granitoids In Western Henan

The western region of Henan Province is located in the southern margin of the North ChinaCraton. There exists an evident transformation of tectonic regime in the Mesozoic resulting in theoccurrence of intense tectonic and magmatic activities and formation of many Mesozoic granitoidplutons in this region. A detailed geological, isotope chronological, petrological and geochemicalstudy combined with collection and sorting out of relevant data has been carried out on thegranitoids. Based on this study, a summarization has been made on the temporal and spatialdistribution regularity of the Mesozoic granitoids and their comprehensive characteristics, and adiscussion done not only on material source and geological and tectonic setting for the formationof the granitoids but also on their petrogenesis.The following are the achievements that have beenacquired in this study:1.The spatial distribution regularity of the Mesozoic granitoid plutons in thewestern region of Henan Province has been preliminarily ascertained. These plutons areequidistantly 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 beltswith six sub-belts of the Mesozoic tectonics and magmatism have been recognized in thisregion.2.The times of every pluton intrusion have been preliminarily defined on the basis of theisotopic age data of the Mesozoic granitoids. The formation process of the rocks is divided for thefirst 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 ofgranitoids.3. A detailed study on the petrological characteristics of different cycles of theMesozoic granitoids supports classification of the rocks into three lithological associationsincluding A-type, high-K calc-alkaline, and calc-alkaline granitoids, of which the first associationbelongs to alkaline series, and the others belong to calc-alkaline series. It is shown that thelithologies change from (quartz) diorite, quartz monzonite, and granodiorite to monzonitic graniteand syenogranite with periods from early to late in each cycle.4. A preliminary summarization hasbeen made on the evolution characteristics of the rocks. There occur three cycles of evolution incomposition of the Mesozoic granitoids from intermediate acidic to acidic, corresponding to themagmatic activities of J2, J3, and K1, respectively. The material for formation of the granitoids isderived from mixing sources of crust and mantle, with decreasing in crust-derived componentsfrom early to late.5. The tectonic setting for formation of the Mesozoic granitoids in the region isdefined as a post-collisional setting. The occurrence of a small part of rocks with characteristics ofcollisional or anorogenic granitoids provides evidence for the continuous process oftransformation of tectonic regime from collisional through post-collisional to anorogenic.6. Themultiple cycles of mantle-derived magma underplating caused by the multiple stages oflithospheric thinning in the Mesozoic result in the partial melting of middle to lower crust orcrust-mantle syntexis to form magmas. The crystallization of these magmas emplacing afterfractionation is responsible for the formation of the Mesozoic granitoids in this region.