Petrogenesis And Their Geological Significance Of The Granites From Shangzhi Area, Southern Section Of Zhangguangcai Range

The Zhangguangcailing, eastern margin of the Mongolian Orogenic belt, located in the intersection part of Paleoasian ocean and the circum-pacific Mesozoic tectonic domain. It plays a significant part in Northeast China tectonic units. The indosinian granitic rocks are prevalent in this region, which were characterized by the occurrence of huge granitic batholith, particularly, Yimianpo, Weihe and Changlin. This paper was systemic study the batholith of Yimianpo, Weihe and Changlin. We take advantage of petrologic, geochemical, whole rock Sr-Nd and zircon in-situ Lu-Hf isotopic, and geochrologic to constrain the petrogenesis, origin, and the evolution of the granitic rocks in study region. The conclusions are listed as follows:1. Analyses of zircon U-Pb yielded the formation age of 211±4Ma from Yimianpo batholith that belong to Late Triassic intrusion. These granitic rocks have relatively depleted in zircon Hf isotopes with εHf (t) of+1.75～+13.81. Weihe and Changlin batholith are coeval with ages of 186±2 Ma and 186±4 Ma, respectively, and the corresponding values of sHf (t) are-1.67～+11.28 and+2.42～+10.43.2. The granitic rocks are predominated by biotite granite in Yimianpo, Weihe, and Changlin batholith. The granites in Yimianpo have relatively high SiO2 and enrich in alkaline, which belong to high-K calc-alkaline series and metaluminou to slightly peraluminous, high differentiation I-type granite. Weihe granitic rocks also belong to high-K, calc-alkaline serires, metaluminou to slightly peraluminous I-type granite, but the low degree fractional crystallization of plagioclase, hornblende, and accessory minerals such as apatite and sphene play an important part during the evolution of the magmas. Therefore, the Weihe granitic rocks are not high differentiation I-type granite. Changlin granitic rocks belong to calc-alkaline to high-K calc-alkaline series, metaluminou to slightly peraluminous, I-type granite without obvious differentiation.3. Coupled with the previous researches and the history of the region tectonic evolution, it can be proposed that the granites from Shangzhi area in Zhangguangcai range can be subdivided into two stages. The first-stage granites are derived from the partial melting of lower crustal juvenile mafic-rocks in the extensional setting after the closure of Paleo-Asian Ocean and then underwent the highly fractionational evolution. The granites certainly belong to the I-type granite. The formation of second-stage granites related to the subdction of the Paleo-Pacific Ocean and then the tectonic setting transform from compression to extension and the lithosphere thinning, which made the basaltic melting upwelling and underplated into the lower crust, resulting in the partial melting of the thickened lower crustal materials.