Approach To Late Yanshanian Granitoids In South Hainan

The Late Yanshanian is a crucial period in the tectonic evolution of the SouthChina. In this period, together with the upcoming early Cenozoic, it involves theconversion of the continental margin of South China. subduction of Paleo-SouthChina Sea, thickening and thinning of continental lithosphere, continental marginrupture, early continental rift activities and the opening of the South China Sea andsome other major regional tectonic events all took place in the late Yanshanian toOligocene period. More importantly, the tectonic events, represented by geologicalprocesses are different in nature, but there is the relationship between inheritance andconversion in time, so far, there are divergent views on the nature and the conversiontime, and there’s no unified understanding being formed. This article widelydistributed in southern Hainan Island the Late Mesozoic granite System, worked hard,through their basic characteristics, determined by the age and tectonic environmentdiscrimination, in the late Hercynian evolution of the northern margin of the SouthChina Sea (South China continental margin) provide certain basis.In this paper, Ithroughly studied the widely distributed granitoids in Hainan trying to analyze thebasic characteristics of the rock samples,determine the age of these rocks andeventually help us understand the tectonic environment.Hainan Island is located in the southwest of the continental margin of SouthChina which is one of the large land mass wedged into the South China Sea.Significantly different from the island of Taiwan located in the northeastern part of theSouth China Sea, Hainan Island has a very long geological evolution history andcomplex geological units. It’s made of Proterozoic continental crystalline basementand Paleozoic sedimentary cover. Based on available information, Mesozoicgranitoids is widespreaded in Hainan Island which is an important way tounderstanding the nature of the northern continental margin of the South China Sea.The author collected more than10pieces of granite and granodiorite samples in Qianjia, Sheng Yu, Sanya and other places, Isotope Geochronology, petrographic,geochemical analysis are performed indoor. The following understandings areachieved:(1) Most of the granite and granodiorite are aged around the late EarlyCretaceous and early Late Cretaceous(110Ma-90Ma), a small amount of graniteemplaced in the Jurassic and Triassic, but the tectonic environment clearly indicatedCretaceous granite. Previous calibrated Cretaceous granite(Shengyu) proved to formduring239±15-267.1±9.9Ma using zircon U-Pb method,which obviously does notbelong to the scope of this study, but add a valuable information to geological studiesin Hainan Island.(2) Metaluminous high-K calc-alkaline granite, peraluminum high-Kcalc-alkaline granite and other types of acid intrusive rocks are present in the studyarea. Using A/CNK data, we find most of the Cretaceous granite are I-type granites,and Triassic-Jurassic granites present A-type and S-type characteristics, This showsthat they are formed in a completely different tectonic environment and experiencedifferent geological processes.(3) A variety of trace elements discrimination diagrams show that the Cretaceousgranite samples fall into the the VAG volcanic arc region, Pre-Cretaceous granites arenot in this case. There should be a typical continental arc somewhere in South Hainanduring Late Yanshanian, and this continental arc become a geological remnants withthe demise of Paleo-south China Sea based on currently available information, thisshift,according to the current findings should began in the middle of Late Cretaceous.