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中文摘要：
秦岭造山带商丹缝合带的西延部分在天水地区以出露的关子镇蛇绿岩为标志，该蛇绿岩的形成时代是探讨秦岭造山带构造格局与演化的关键。在室内外综合研究的基础上，从该蛇绿岩的辉长岩中精选锆石，通过锆石的CL显微图像进行锆石形态学研究，结合U、Th含量特征，证明本次用于定年的锆石为岩浆成因。辉长岩中的锆石LA—ICP—MS定年给出U—Pb年龄为471Ma±1．4Ma（MSWD=1．03，2σ），代表了辉长岩的结晶时代，也限定了关子镇蛇绿岩的形成时代。结合已有的地质、地球化学研究成果，认为秦岭商丹古洋盆存在的时限至少可延续至470Ma。该年龄的确定为深入研究和认识秦岭造山带古生代的构造格局及其演化提供了新的证据。
英文摘要：
The westward extension of the Shangdan suture （SDS） in the West Qinling is marked by the exposure of the Guanzizhen ophiolite in the Tianshui area. The age of this ophiolite is a key to understanding the tectonic framework and evolution of the Qinling orogenic belt. Based on a field investigation and geochemical analysis, some zircons were selected from gabbro of the ophiolite. The study Of zircon CL images and U and Th contents indicate that the zircons used for dating are of magmatic origin. Zircon LA-ICP-MS dating of gabbro yields a U-Pb age of 471±1.4 Ma （MSWD=1.03,2σ）, representing the time of crystallization of the gabbro. This age also constrains the age of the Guanzizhen ophiolite. According to this age, combined with the available regional geological and geochemical data, it is suggested that the time span for the existence of the Shangdan paleo-ocean between the North China block and Yangtze block continued fill 470 MaBP at least. The determination of this age provides new evidence for an in-depth study and understanding of the tectonic framework and evolution of the Qinling orogenic belt.
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