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

Key words: 
• Dacaotan Group

Abstract: 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. 