新疆北部卡拉麦里蛇绿岩中辉长岩的锆石U-Pb年龄及其构造意义

新疆东准噶尔卡拉麦里蛇绿岩是一套由以镁铁-超镁铁岩为主的构造岩块和不同岩性的基质组成的蛇绿混杂带,被认为是哈萨克斯坦板块和西伯利亚板块之间的缝合带。由于其特殊的大地构造位置,该蛇绿岩对恢复区域构造演化具有重要意义。然而,在过去20多年里,有关其形成及就位时代的争议一直很大,其原因在于不同学者采用来自化石资料和同位素年龄方面的不同证据。本文选择该蛇绿岩套中保存相对完整的辉长岩进行了LA-ICP-MS锆石U-Pb年龄测定,获得了3个峰值年龄:406±1.8 Ma,514±5.1Ma和332±6.4 Ma。结合该区野外地质特征和前人资料分析,本文认为除514±5.1 Ma的年龄可能来自古老围岩中的残余锆石之外,另外两组年龄分别与蛇绿岩的形成和就位有关,前者代表卡拉麦里蛇绿岩的形成时间,后者则代表卡拉麦里蛇绿岩的构造侵位时间,即卡拉麦里洋盆形成于泥盆纪初期,最终闭合于早石炭世早期。

The U-Pb ages of zircons from the gabbro in the Kalamaili ophiolite,North Xinjiang and its tectonic significances

The Kalamaili ophiolite in North Xinjiang, composed mainly of structurally blocks of ultramafic rocks and mafic rocks mixed with some matrix components of basic eruptive rocks and ocean-floor sediments, is one of the most important tectonic units in reconstruction of regional tectonic evolution since it was considered as the suture-zone between Sibiria plate and Hasakestain plate. However, quite different opinions upon its formation time were proposed based on different types of evidence of fossil assemblages in the sedimentary strata and chronological data of the volcanic units in the past two decays, which greatly influenced the understanding of the formation of central Asian orogenic belt. In this paper, zircons from the gabbro which is relatively well preserved in the ophiolite were picked out for LA-ICP-MS age dating, and three groups of U-Pb ages were obtained, namely, 406±1.8 Ma, 332±6.4 Ma and 514±5.1 Ma, respectively. Among these data, the age of 406±1.8 Ma indicated the formation time of the Kalamaili ophiolite, the age of 332±6.4 Ma is correspondent to the regional re-crystallization during the dynamic metamorphism in this area after the plate collision in the later stages, while the age of 514±5.1 Ma might represent the residual zircons captured from the old strata. By further discussion, the formation time and emplacements of the Kalamai ophiolite was concluded that, the Kalamaili oceanic basin was formed in the Early Devonian and finally closed in the Early Carboniferous.