秦岭造山带东段秦岭岩群的年代学和地球化学研究
Geochronology and geochemistry of the Qinling Group in the eastern Qinling Orogen
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摘要: 对东秦岭地区的陕西省洛南县、宁陕县、长安县和河南省淅川县出露的四个秦岭岩群变质岩进行的岩石学和地球化学研究表明,样品主要由变质火山岩和变质沉积岩组成。详细的锆石U-Pb定年结果显示三个正变质岩均形成于新元古代早期(971~843Ma),而副变质岩中富集大量新元古代碎屑锆石,根据最年轻的谐和年龄(859Ma)和早古生代的变质年龄,推测其沉积时代为新元古代中晚期。因此,北秦岭南部的秦岭岩群的变质岩主要由新元古代早期的火成岩和新元古代中晚期的沉积岩组成。变质作用主要发生在加里东期,局部有燕山期的变质作用叠加。指示北秦岭的造山作用主要发生在早古生代。岩石地球化学研究还显示秦岭岩群的新元古代火山岩均形成于火山弧构造环境,沉积岩沉积于大陆弧-活动大陆边缘环境,指示秦岭造山带在新元古代早期是一个火山弧。秦岭岩群的火山岩和沉积岩在形成时代和构造环境方面与扬子克拉通西缘的特征非常相似,表明位于北秦岭造山带的秦岭岩群应归属于扬子克拉通陆块,是扬子北缘的一个大陆边缘弧。Abstract: Petrology and geochemistry of the basement metamorphic rocks from the Qinling Group in the Luonan, Ningshan and Changan County from Shanxi Province and Xichuan County from Henan Province of the Eastern Qinling Orogen indicate that the samples are mainly composed of metavolcanic rocks and metasedimentary rocks. LA ICP-MS zircon U-Pb dating results suggest that three orthometamorphites were all formed in early Neoproterozoic (971~843Ma) and metasedimentary rock contains a number of Neoproterozoic detrital zircons. The depositional time of the sedimentary rock should be middle-late Neoproterozoic on the basis of the youngest age (859Ma) of concordant detrital zircons and early Paleozoic metamorphic age. Therefore, the Qinling Group probably is a complex consisting mainly of early Neoproterozoic igneous rocks and middle-late sedimentary rocks. The metamorphism of the Qinling Group occurred mainly in Caledonian period and locally in Yanshanian period, suggesting that the major orogeny in the northern Qinling area took place in early Paleozoic time. Geochemical characteristics also indicate that Neoproterozoic volcanic rocks in the Qinling Group formed in a volcanic arc tectonic setting and the sedimentary rock deposited on continental arc to active continental margin environment, suggesting the Qinling orogenic belt was a volcanic arc during early Neoproterozoic. The volcanic and sedimentary rocks of the Qinling Group are very similar to those in the western Yangtze craton in the formation time and tectonic setting. Therefore, the Qinling Group distributing along southern margin of northern Qinling Orogen should belong to the Yangtze craton and it was an island arc in the northern margin of Yangtze craton.
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