摘要:秦—祁构造结合部位以新阳—元龙大型剪切带为界，北侧的北祁连造山带和南侧的西秦岭造山带的构造线呈斜截关系，致使两侧岩石单元的对比存在困难。陇山岩群位于秦—祁结合部位的北祁连构造单元东端，是一套岩性复杂的中深程度变质杂岩，其形成年代和构造属性一直存在较大的争论。本文重点以陇山岩群中黑云母石英片岩为研究对象，对其中的碎屑锆石进行LA-ICP-MS U-Pb测年。其测年结果明显分为4组，两个主要区间为1 097～795 Ma （49%，峰期年龄约为929 Ma）和2 713～2 265 Ma （21.5%，峰期年龄约为2 435 Ma），另有两个次级年龄组为575～471 Ma （12%，峰期年龄约为541 Ma）与1 864～1 539 Ma （14%，峰期年龄约为1 717 Ma）。通过最小岩浆峰期年龄和陇山岩群内侵入体的最老年龄初步限定其形成时代介于寒武纪早期—早-中奥陶世（539～454 Ma），与已报道的晚太古代—早元古代陇山岩群TTG片麻岩形成于不同时代，不属于华北板块南缘基底的沉积岩系。通过与周边构造单元年龄特征峰值的对比研究发现，其新太古代—古元古代（2 713～2 265 Ma）和中元古代（1 864～1 539 Ma）的碎屑物质主要来自华北板块南缘基底，新元古代（1 097～7 95 Ma）的碎屑物质主要来自于北秦岭造山带和中祁连陆块，早古生代（575～471 Ma）的碎屑物质来自于天水—武山构造带，与该洋盆形成过程有关。因此，陇山岩群中黑云母石英片岩的碎屑物源既包括北侧的华北板块南缘基底，又包括南侧的秦岭—祁连造山带，可能形成于早古生代红土堡弧后盆地的形成扩展阶段，与北秦岭东段宽坪岩群副变质岩和二郎坪岩群变沉积岩形成构造环境相似。
关键词: 秦—祁结合部位/
陇山岩群/
碎屑锆石/
年代学
Abstract:The Qinling and Qilian orogenic belts have been separated by the the Xinyang-Yuanlong ductile shear zone. The relationship between the Qinling and Qilian orogenic belts remained disputed, because the tectonic lineation of the both orogens is at an oblique angle to each other. Longshan complex is located in the tectonic conjunction of the Qinling and Qilian orogens, consistsing of middle to high grade metamorphic assemblages. It still remained controversial about the formation age and tectonic settings. In this study, detrital zircons collected from the biotite quartz schist of the Longshan complex have been dated by the LA-ICP-MS, yielding 4 age ranges including two major age populations of 1 097～795 Ma（49%, age peak at ～929 Ma）and 2 713～2 265 Ma（22%, age peak at ～2 435 Ma）, and two minor age populations of 575～471 Ma（12%, age peak at ～541 Ma）and 1 864～1 539 Ma（14%, age peak at ～1 717 Ma）. Confined by the youngest peak age and the oldest intrusion age of the Longshan complex, the meta-sediments were deposited between the Early Cambrian and the Early Ordovician. Therefore, the meta-sediments was formed obviously later than the Late Archean-Paleoproterozoic TTG rocks of the Longshan complex, indicating that the meta-sediments are not ourcrops of the Archean-Paleoproterozoic basements along southern margin of the North China Block（NCB）. Compared with the characteristic age peaks of the surrounding tectonic terrains, the age populations of detrital zircons, ranging from the Archean to Paleoproterozoic（2 713～2 265 Ma）and from the Late Paleoproterozoic to Early Mesoproterozoic（1 864～1 539 Ma）, are thought to be derived from the southern margin of the NCB. The age population of detrital zircons ranging from the Early to Middle Neoproterozoic（1 097～795 Ma）is mainly derived from the North Qinling orogenic belt and central Qilian block, while that ranging from the Late Neoproterozoic to Early Paleozoic（575～471 Ma）are derived from the Wushan-Tianshui tectonic belt. Therefore, the biotite quartz schist from the Longshan complex is derived from the southern margin of the NCB in the north as well as the Qilian-Qinling orogenic belt in the south, indicating that the meta-sediments deposited in a rifting or extending Hongtubu back-arc basin during the Early Paleozoic period, which is similar to the tectonic environment of those parametamorphic rocks from the Kuanping complex and Erlangping complex in the eastern North Qinling orogenic belt.
Key words:Tectonic conjunction of the Qinling-Qilian orogen/
Longshan complex/
Detrital zircon/
Geochronology
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