西秦岭大草滩群的形成时代和构造意义探讨

大草滩群a岩组中最年轻的一组锆石206Pb/238U年龄加权平均值为404.9±4.8 Ma（MSWD=0.11），b岩组中最年轻的锆石206Pb/238U年龄值为385±5 Ma，c岩组样品中最年轻的锆石206Pb/238U年龄值为375±6 Ma，这一结果将大草滩群的最大沉积时代有效地限定在405~375 Ma之间。结合生物化石组合资料，大草滩群的时代归属于晚泥盆世。根据岩石组合、沉积相、构造变形、形成时代和物源分析等资料综合研究表明，晚泥盆世大草滩群河湖相粗粒碎屑岩形成时的大地构造背景是位于安第斯型活动大陆边缘的弧前拉张—裂陷盆地，作为北秦岭微陆（+岩浆弧）与华北大陆西南缘碰撞的沉积响应，是碰撞造山作用以后南部洋壳持续俯冲阶段同火山—岩浆活动的弧前沉积体系。

Discussion on the Formation Age and Tectonic Implications of Dacaotan Group in West Qinling

The youngest discrete population of three zircons from the a Formation yielded a weighted mean 206Pb/238U age of 404.9±4.8 Ma(MSWD=0.11), the youngest discrete one zircon from the b Formation yielded a 206Pb/238U age of 385 5 Ma, and the youngest discrete one zircon from the c Formation yielded a 206Pb/238U age of 375 6 Ma, which can be constrained the maximum depositional age of the Dacaotan Group as 405~375 Ma. This result and the biota fossils data from Dacaotan Group, provide reliable evidence that its depositional age is Late Devonian. According to its lithologic assemblage, sedimentary facies, depositional age, deformation features and sediment provenance, the authors discuss the tectonic implications of Dacaotan Group. In the middle Devonian, an early microcontinent (+magmatic arc) continent collisional orogen was overprinted by Andean type magmatic arc along the southern convergent margin of North China continent. The large volumes of molasse sediments have been shed into extensional basin in the forearc area at this time, presumably resulted from the combined effects of the collision and the southern oceanic crust subduction northwards. Siliciclastic rocks of the Upper Devonian Dacaotan Group record rapid tectonism denudation and sedimentary response during this period. 