Geochronology,Petrogenesis and Tectonic Implications of Devonian High-K Acid Magmatic Rocks from Yemajing Area in Beishan Orogen
Xu Wei;Xu Xueyi;Lu Jincai;Niu Yazhuo;Chen Gaochao;Shi Jizhong;Dang Ben;Song Bo;Zhang Yuxuan;Zhang Qiao;Xi'an Center of China Geological Survey;School of Earthscience & Resources,Chang'an University;Centre for Orogenic Belt Geology,CGS;China Geological Survey;
In order to further clarify the tectonic evolution of the southern Beishan orogen during the Late Paleozoic, a comprehensive study including whole rock geochemistry and zircon U-Pb-Hf isotopes has been carried out on the monzonitic granites and rhyolites collected from the Yemajing area. The results of zircon LA-ICP-MS U-Pb dating for the monzonitic granites and rhyolites are 402.7±2.4 Ma and 392.9±2.5 Ma(206 Pb/238 U ages) respectively, showing these rocks formed during the Devonian. The monzonitic granites and rhyolites in the research area were characterized by potassic, peraluminous to strongly peraluminous, relatively enriched in LREE and depleted in HREE with weak to moderate fractionation between LREE and HREE. The monzonitic granites display no or small negative Eu anomalies, whereas the rhyolites show more obviously negative Eu anomalies. They exhibit similar geochemical compositions including positive Rb, Th, U and Pb, but negative Nb, Ta, Ba, Sr and Ti anomalies, and belong to I-type granites. The zircon εHf(t) values of the monzonitic granites and rhyalites vary from-2.2 to+6, and the Hf model ages(tDM2) vary from 962 Ma to 1533 Ma. Consequently,it can be concluded that the potassic igneous rocks from this area were mainly derived from partial melting of the Middle Proterozoic continental crust. Combining with regional geological evidence, we suggest that the monzonitic granites and rhyolites were probably emplaced or erupted in a post-collision extensional setting. This study provides new crucial evidence for understanding the tectonic evolution of the southern CAOB during the Late Paleozoic.