云南潞西三台山超镁铁岩体Os-Nd-Pb-Sr同位素特征及地质意义

滇西地区特提斯造山带具有多块体拼合的构造特点,是东特提斯造山带的重要组成部分之一,其大地构造演化对理解全球特提斯造山带具有重要的地质意义.在腾冲和保山地块之间的龙陵-瑞丽断裂带中出露超镁铁质岩石,曾被解释为两个陆块缝合的证据.本文采用高灵敏度高精度超低本底Re-Os、Sm-Nd、Pb和Rb-Sr同位素分析技术,对采自该断裂带潞西三台山的超镁铁岩样品进行了Os-Nd-Pb-Sr同位素测定.分析结果表明,这些超镁铁岩石具有强烈富集特征.分析的5个岩石样品具有高的初始~(87)Sr/~(86)Sr值(0.71074～0.71444)、低的初始ε_(Nd)值(-6.2～-10.6)、低初始γ_(Os)值(-4.8～-8.8)和较高的初始铅同位素比值,其Os同位素模式年龄值(t_(RD))为0.97～1.71Ga.Nd-Os同位素组成特征表明,三台山超镁铁岩具有古老的富集大陆岩石圈地幔岩石的特征,不能作为腾冲和保山地块的缝合证据.

Os-Nd-Pb-Sr isotopic compositions of the Santaishan ultramafic rock in western Yunnan and its geological significances

The western Yunnan Tethyan terrain that is formed by the amalgamation of multiple continental blocks represents an important part of the eastern Tethyan tectonic belt. Its tectonic evolution bears essential significances for understanding the formation of the global Tethyan orogenic belt. Along the Longling-Ruili fault zone between the Tengchong and Baoshan blocks, many small outcrops of ultramafic rocks are exposed, which have ever been interpreted as evidence for the suturing of these two blocks. Os-Nd-Pb-Sr isotopic compositions of ultramafic rocks sampled from Santaishan massif were analyzed by using low procedural blank and high precision analytical techniques in this study. Analytical results show that these ultrmafic rocks exhibit isotopic characteristics of strong enrichment. Five analyzed samples of ultramafic rock yield high initial ~(87)Sr/~(86)Sr values (0.71074 ～0.71444), low initial ε_(Nd) values (-6.2 ～ -10.6), low initial γ_(Os) values (-4.8 ～ -8.8) and relative high initial Pb isotopic ratios. Os isotopic model ages (t_RD)) range from 0.97 to 1.71Ga. The Nd-Os isotopic compositions indicate that the Santaishan ultramafic rocks are characterized by theisotopic feature of old sub-continental lithospheric mantle and therefore, can not represent evidence for a suture between the Tengchong and Baoshan blocks.