Petrographical and geochemical characteristics of the Cretaceous granitoids in the Tianshuihai area, western Qinghai-Tibet Plateau, China and its geological significance. Geological Bulletin of China

The Cretaceous intermediate-acid intrusive rocks are widely distributed in Tianshuihai, those rocks can be divided into three categories as per different age: early Cretaceous (123Ma), the early stage of Late Cretaceous (100-93Ma) and the late stage of Late Cretaceous. The early Cretaceous intrusive bodies are sporadic and dominated by orthoclase granite, two-mica granite and granite containing garnet, which belong to high-K calc-alkaline type. The A/CNK ratios are more than 1.1, the contents of K2O are more than those of Na2O, negative Eu anomalies are obvious, the molecular numbers of corundum are more than 1.1, all these facts indicate that they belong to the typical peraluminum S type granitoids. Granitoids from the early stage of late Cretaceous are also widely distributed dominated by tonalite and granodiorite, which mainly belong to medium-high potassium and calcium alkaline types, some of them belong to potassium basalt. Generally A/CNK ratios are less than 1.15, negative Eu anomalies are medium, molecular numbers ofcorundum are less than 1.1 and they mainly belong to I type granitoids. Granitoids from the Late stage of Late Cretaceous are widely distributed which dominated by granite diorite and ultimate-granite, mainly belong to high-potassium calcium alkaline type, some of them belong to medium-potassium calcium type. A/CNK ratios are over 1.05 with noticeable Eu negative anomalies, the molecular numbers of corundum are more than 1.1 and generally belong to S type. It is concluded after comprehensive analysis that the Cretaceous intrusive rocks in the Tianshuihai area are closely linked to the reduction and closure of Bangonghu-Nujiang residual ocean basin, and their formation may keep record of those syn-collision or post-collison structural magmatic evolution between Qiangtang-Gangdisi belt.