Identification of the Permian Arc-Related Magmatic Rocks and Its Significance in Panjiajingzi Area, Southern Margin of Beishan Orogenic Belt

The Beishan Orogenic Belt, located at the southern margin of the Central Asian Orogenic Belt, is important for understanding the tectonic affinity and evolutionary history, in terms of the accurate determination or disintegration of the namely Precambrian basement rocks. The originally defined Dunhuang Group in Panjiajingzi area is composed mainly of leptynite, leptite, meta-conglomerate, minor amphibolite, and mica quartz schist, and is characterized by strong deformation and low-grade metamorphosed pyroclastic rocks, based on regional geological investigation. Zircon U-Pb ages of 294~285 Ma for leptynite, mica quartz schist, and leptite samples, as well as the minimum age of (272±6) Ma for gravel sample from meta-conglomerate have been obtained with no ancient geochronological information. All these dating data indicate that these mata pyroclastic rocks were deposited later than Middle Permian. In addition, the meta basic volcanic samples have high TiO₂ and Na₂O contents with obvious Nb-Ta trough, and display right-sloping chondrite-nomalized REE patterns, resembling that of the arc basalts. Combining our data and regional geology, it is reasonable to believe that these meta pyroclastic rocks were formed in a local extension setting during the subduction process during Early to Middle Permian, and thus these rocks should be disintegrated from the Dunhuang Group.