Zircon SHRIMP U-Pb Geochronology of the Yaoshan Group, Southeastern Yunnan, China and Its Geological Significance

Abstract

The Yaoshan Group locates in the southeastern Ailao Shan-Red River strike-slip fault zone. It has experienced high-amphibolite facie metamorphism and chorismitization, thus it was thought as the Precambrian basement of the South China block in SE Yunnan region. In order to determine the ages of the protolithos, later magmatic, metamorphic and tectonic events of the Yaoshan Group, the zircon SHRIMP U-Pb dating was investigated. According to CL images and optical microscope analyses, the zircons derived from the Yaoshan Group, which are very complicated with the metamorphosed re-crystal and over-growthing, could be divided into inherited, metamorphic, anatectic or magmatic types. Besides a few analyzed spots with the Neo-Proterozic ages, the inherited zircons mainly yielded 206 pb/ 238 U ages in the range from 235 ± 1. 7 Ma to 261 ± 3. 6 Ma, with a weighted mean value of 250. 8 ± 9. 8 Ma (N-4, MSWD=4. 1). The 206 Pb/ 238 U ages of anatectic or magamatic zircons represented by two peaks of ~85 Ma and ~75 Ma, respectively. The 206 pb/ 238 U ages of metamorphic zircons are in the range from 49 Ma to 30 Ma, with three peaks of 32, 37 and 42 Ma, respectively. Based on the regional geology, this study suggested that the peak age of inherited zircons (250. 8 ± 9. 8 Ma) indicated that the major protolithos of the Yaoshan Group might be formed later than the Permian, and that metamorphic, anatectic/magmatic zircons showed five 206 Pb/ 238 U peak a-ges of ~85 Ma, ~75 Ma, ~43 Ma, ~37 Ma and ~32 Ma might indicate five important tectonic, magmatic and metamorphic events in the south part of Red River fault since the later Yanshannian.