Petrogensis and tectonic setting of the Yemaquan granite from the iron-polymetallic ore area of Qimantag, Eastern Kunlun Mountains, Qinghai-Tibet Plateau

摘要

Being a part of the Paleo-Tethys Ocean, closing of the Buqingshan-Anyemaqen oceanic basin left a rich geologic record in the East Kunlun Orogenic Belt. The genesis and tectonic setting of the granites including quartz monzodiorite, granodiorite and mozogranite is discussed in light of the geochemical and U-Pb chronological data obtained. U-Pb dating studies on zircon from the quartz monzodiorite and monzogranite of the research area yielded ages of 220.11 +/- 0.49Ma ((Mean Square Weighted Deviates) MSWD=0.046) and 223.33 +/- 0.54Ma (MSWD=0.14), respectively, by Laser Ablation Multiple Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) method. According to sedimentological and structural investigations, the Paleo-Tethys Ocean in the Qimantag region began to close at about 235Ma, and completely disapperared at about 220Ma. The three types of granites in this study are considered to intrude the syn- to post-collisional stages. The quartz monzodiorite and granodiorite belong to the I-type granite whereas the monzogranite is of the S-type granite. These two types of granites were formed by different ways of partial melting: first, partial melting of the lower crust took place as a result of asthenosphere upwelling triggered by break-up of the leading edge or tearing of the descending oceanic slab. Subsequently partial melting of the middle-lower crust was caused by the underplating of basaltic magma formed by partial melting of the mantle wedge fluxed by fluids liberated by the oceanic slab dehydration. The magma responsible for the formation of S-type granites appears to have originated from partial melting of the upper crustal material at a shallower level with a clear signature of continental crust.