滇西洱海东第三纪超K质火成岩系的Nd-Sr-Pb同位素特征与西南大陆地幔演化

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

洱海东部第三纪火山岩与同时代基性岩为超K岩系。Nd、Sr、Pb同位素与微量元素研究表明地幔源区区别于中国东部新生代火山作用源区,相近于腾冲,与东非和南意大利的火山作用有相似之处。该区火山作用存在三个主要组分端元:(1)低SiO_2,低K,LIL富集程度低的端元;(2)低SiO_2高K,LIL高度富集端元;(3)高SiO_2高K,LIL中等程度富集的端元。三个端元的Nd、Sr、Pb同位素组成分别为:(1)0.51255,0.7067,18.65;(2)0.51260,0.7060,18.71;(3)0.51230,0.7071,18.47。三个端元的形成与构造环境从古俯冲带地幔源向裂谷地幔源转化有关。

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.