Nd, Sr AND Pb ISOTOPIC CHARACTERISTICS OF CENOZOIC ULTRA-POTASSIC VOLCANIC ROCKS FROM EASTERN ERHAI, YUNNAN AND THEIR IMPLICATIONS FOR SUBCONTINENT-MANTLE EVOLUTION IN SOUTHWESTERN CHINA
Zhu Bingquan Zhang Yuquan Xie Yinwen(Institute of Geochemistry, Academta Sinica, Guangzhou 510640)
The Cenozoic volcanic and mafic rocks in eastern Erhai, western Yunnan, constitute an ultra-potassic complex. It is indicated from the studies of Nd, Sr and Pb isotopes and trace elements that its mantle source is evidently distinguished from that of eastern China, but somewhat similar to that of Tengchong calc-alkaline volcanics and southern Italy ultra-potassic vo-lcanics. The source can be divided into three end-members: (1) low SiO2 and K, and slightly enriched in LIL; (2) low SiO2 and high K, strongly enriched in LIL; (3) high SiO2 and K, moderately enriched in LIL. Their Nd, Sr and Pb isotopic compositions are respectively: (1) (1)0.51255,0.7067, 18.65; (2)0.51260, 0.7060, 18.71; and(3)0.5130, 0.7071, 18.47. The mechanism of formation can be attributed to the transition of tectonic environments from mantle sources associated with the ancient subduction zone to those associated with the ancient uction zone to those associated with rifts.