Zircon U-Pb ages of the Qinling Group in Danfeng area: Recording Mesoproterozoic and Neoproterozoic magmatism and Early Paleozoic metamorphism in the North Qinling terrain
丹凤地区秦岭岩群片麻岩锆石U-Pb年龄: 北秦岭地体中-新元古代岩浆作用和早古生代变质作用的记录

The Qinling Group is considered as the oldest basement rocks of Precambrian exposed in the North Qinling terrain and records crustal formation and evolution of North Qinling orogenic belt. In this study, zircon U-Pb ages of five gneiss samples collected from Danfeng-Xixia area are reported in order to constrain the time of formation and metamorphism of this rock group and discuss the tectonic provenance of the North Qinling terrain. Analytical results of the zircon dating show that zircon grains of magmatic origin yield ages clustering in about 1400~1600Ma and about 850~950Ma, recording two major magmatic activities. Six zircon grains of metamorphic origin with low Th/U ratios of <0.03 give 206Pb/238U ages ranging from 510 Ma to 465 Ma with a weighted mean value of 477±18Ma, recording Early Paleozoic metamorphic overprint. This metamorphism is temporally coincident with the high pressure metamorphism distinguished in both the northern and southern margins of the Qinling terrain, indicating that Qinling Group underwent metamorphism during the North Qinling orogenic process in Early Paleozoic. Qinling Group formed mainly in Late Mesoproterozoic to Early Neoproterozoic and no magmatic events of Paleoproterozoic and Archean were recorded in the basement rocks. In the view of the formation time of the magmatic activities, the North Qinling terrain can be discriminated from the Yangtze block characterized of strong magmatism of Late Neoproterozoic along the northern margin and also from the North China block that is composed of Late Archean to Paleoproterozoic basement rocks along the southern margin. The North Qinling terrain would be a microcontinent with distinguishable evolution in Mesoproterozoic to Neoproterozoic.