Geochronology,Geochemistry and Geological Significance of the Granite Veins in the Bailongjiang Group, South Qinling

An EW-trending Indosinian granitoide belt is occurred at the north side of the Mianlue suture zone in the southern margin of the Qinling Orogenic Belt. The granite veins in Silurian Bailongjiang Group are parts of the Indosinian granitoids in South Qingling, with width range from 1 to 8m and extend direction of NWW-SEE. They consist of granodiorite porphyry and quartz diorite porphyry. In this paper, the intensive granite veins from Silurian Bailongjiang Group in South Qinling have been detailedly studied from the aspects of LA-ICP-MS zircon U-Pb ages and geochemistry characteristics. The results show that the crystallization age of the granodiorite porphyry veins in Bailongjiang Group is (210.44±0.47)Ma, belonging to the Late Triassic. Geochemical data show that these veins have relatively high K₂O and Sr contents, Zr/Y ratios, they are enriched in LREE and LILE, but poor in HFSE, and their Ritterman indexes (σ) are 1.41~2.73. These samples are similar to thehigh-K calc-alkaline I-type garnitoids. Moreover, they are characterized by low Y and Yb abundances, high (La/Yb)_N ratios and depleted in Nb, Ta and Ti, suggesting that they possibly originated mainly from the partial melting of thinned crustal basic rocks trigged by lower crustal delamination, with minor mantle-derived component, and they were formed in the geological underground of crustal thickening. Combined with regional geological data, it's proposed that the granite veins in Bailongjiang Group were formed in a post-collisional tectonic environment, and were formed in the transitional stage from extrusion to extensional regime after collision between the North China plate and the Yangtze plate.