The U-Pb ages of zircons from the gabbro in the Kalamaili ophiolite, North Xinjiang and its tectonic significances
Fang Aimin1,2, Wang Shigang3, Zhang Junmin1, Zang Mei1, Fang Jiahu3, Hu Jianmin4
1. Xinjiang Institute of Engineering, Vrümqi 450016;
2. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029;
3. China University of Mining & Technology, Beijing 100083;
4. Institute of Geomechanics, Chinese Academy of Geological Science, Beijing 100081
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.