南秦岭白龙江群中花岗岩脉群年代学、地球化学特征及地质意义

秦岭造山带南缘勉略缝合带北侧发育近东西向展布的印支期花岗岩带，出露于南秦岭志留系白龙江群中花岗岩脉群，属于该花岗岩带的一部分，其宽度多为1~8m不等，呈北西西-南东东向延伸，主要岩性为花岗闪长斑岩-石英闪长斑岩组合。笔者对南秦岭志留系白龙江群中密集出露的花岗岩脉体进行了详细的LA-ICP-MS锆石U-Pb年代学及岩石地球化学研究，结果表明，白龙江群中花岗闪长斑岩脉结晶年龄为（210.44±0.47）Ma，属晚三叠世。地球化学特征显示这些脉体相对高K2O、Sr含量及Zr/Y值，富集LREE和LILE，亏损HFSE，里特曼指数（σ）为1.41~2.73，具高钾钙碱性I型花岗岩特征。此外，它们明显亏损Nb、Ta及Ti，低Y、Yb，具有较高的（La/Yb）N值，表明它们可能为地壳增厚背景下，下地壳拆沉作用导致减薄地壳中以基性岩为主的源岩部分熔融的产物，可能存在幔源岩浆的贡献。综合区域地质资料研究认为，出露于南秦岭白龙江群中的花岗岩脉群形成于后碰撞构造环境，是华北板块与扬子板块碰撞造山之后挤压体制向伸展体制转换阶段的产物。

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.