Ployphase Early Paleozoic metamorphism in the northern Qinling orogenic belt.
北秦岭造山带的早古生代多期变质作用

High-grade metamorphic rocks in North Qinling orogen, traditionally regarded as the Qinling Group, consist of minor eclogites, high pressure granutlite and widely distributed medium-low P/T granulite-amphibolite facies metamorphic rocks. Radiometric data indicate that ployphase Early Paleozoic metamorphisms are recorded in different rocks. The previous U-Pb datings of eclogites and associated rocks in the Guanpo area of the North Qinling orogen gave an age of ca.500Ma, interpreted as the time of eclogite facies metamorphism. In combination with petrological data, SHRIMP and LA-ICPMS U-Pb geochronology on two HP granulite samples in the Songshugou area of the North Qinling orogen yield ages of 504±7Ma and 506±3Ma, respectively, representing the time of high pressure granulite metamorphism. This implies that HP granulite-facies conditions in high pressure granulite and eclogite facies conditions in eclogites developed at the same time but they formed in different thermal environments. U-Pb datings of zircons of amphibole-two-pyroxene granulite from the Xixia area reveal two Early Paleozoic age groups: 440±2Ma represents the age of granulite metamorphism whereas the ca. 426±1Ma reflects the time of amphibolite facies metamorphism. The U-Pb data of zircons of garnet-two-pyroxene granulite from the North Tongbai area yield an age 436±1Ma, reflecting the time of granulite facies metamorphism. These data suggest that the North Qinling orogen experienced an Early Ordovician subduction and crustal thickening, followed by an Early Silurian widespread Barrovian-type metamorphic event as observed in many collisional orogen.