西天山巴尔盖提花岗岩体年龄、地球化学特征与构造意义

Geochronology,geochemical properties and tectonic significance of Baergaiti granitoid in western Tianshan Mountain,NW China

巴尔盖提花岗岩体位于伊犁盆地北缘西天山博罗霍洛构造带。锆石LA-ICP-MS U-Pb年龄(374±7.0)Ma显示岩体侵位于晚泥盆世。岩石为斑状黑云母花岗岩,其主量元素SiO_2、Al_2O_3、K_2O、Na_2O的含量分别为70.72%~79.12%,9.87%~14.51%,4.13%~5.54%,2.87%~4.35%,富碱、高钾、低镁钛,属高钾钙碱性系列;轻稀土相对富集,(La/Yb)N为7.61~16.31,具有显著的负Eu异常;相对富集大离子亲石元素Rb、U和Th等,亏损Nb、Ta和Ti等高场强元素,地球化学特征总体显示了I-A花岗岩过渡特征。区域上地层不整合关系、沉积特征及地球化学构造图解特征综合分析表明,巴尔盖提花岗岩体形成于晚泥盆世区域性造山后伸展背景,与石炭纪火山—沉积建造构成了晚古生代完整的开合构造演化旋回。

The Baergaiti granitoid is located in the Boluohuoluo tectonic belt in the western Tianshan Mountain of the Yili Basin. LA-ICP-MS zircon U-Pb shows age of 374±7.0 Ma, corresponding to the Late Devonian. Petrogeochemical analyses suggest that the intrusion belong to calc-alkaline metaliferous rocks characterized with high silica, potassium, and alkalinity, low magnesium and titanium. The contents of SiO2, Al2O3, K2O, Na2O range 70.72%-79.12%, 9.87%-14.51%, 4.13%-5.54%, 2.87%-4.35%, respectively. Meanwhile, the REE contents show an obvious rich LREE pattern （（La/Yb）N ranges 7.61-16.31） with negative Eu anomalies. Large ion lithophile elements such as Rb, Th and U are relatively enriched, whereas high strength field elements such as Nb, Ta and Ti are relatively depleted. Geochemical and isotope characteristics show a transition feature of high-K calc-alkaline I-A type granite. The analyses of a regional angular unconformity and sedimentary features suggest that the Baergaiti granitoid was deposited under a post-orogenic intra-continental extensional tectonic setting during the Late Devonian, and formed a complete tectonic evolution cycle coupling with the Carboniferous volcanic-sedimentary strata.