南天山南缘早二叠世酸性火山岩的地球化学、同位素年代学及其构造意义

Geochemical and geochronological characteristics and its tectonic significance of Early Permian acid volcanic rocks of Xiaotikanlike Formation in the southern margin of South Tianshan orogen,NW China

南天山南缘小提坎里克组酸性火山岩高碱(Na_2O+K_2O=6.03%～7.70%)且富钾(K_2O/Na_2O=2.35～5.03),铝饱和指数A/CNK=0.89～1.59(平均1.24),里特曼指数(σ)为1.48,属于过铝质高钾钙碱性火山岩系。轻稀土强烈富集,亏损Ba、Cs、Nb、Ta、Sr等大离子亲石元素,Hf、Zr等高场强元素基本无亏损,具有明显的负铕异常(δ_(Eu)=0.44～0.66),总体具有碰撞晚期花岗岩类的地球化学特点。岩石的Nd/Th(1.78～3.50)、Th/U(4.10～9.79)、La/Ta(17.69～35.77)和Nb/Ta(10.48～11.84)比值显示了壳源特征。对火山岩中锆石进行的激光探针等离子体质谱(LA-ICP-MS)U-Pb微区测定获得了289.4±5.5Ma的形成年龄。结合区域构造分析认为,早二叠世初南天山地区处于碰撞晚期冲断造山作用阶段,小提坎里克组火山岩是对南天山造山带南缘碰撞晚期冲断变形和前陆盆地发育的响应。

The acid volcanic rocks of Xiaotikanlike Formation in the southern margin of the South Tianshan orogen are characterized by high-alkali （K2O ＋ Na2O = 6. 07% - 7.70% ） and high-K （K20/Na20 = 2. 35 N 5.03 ）, with A/CNK values between 0. 89 - 1.59 and Rittman index （σ） 1.48, so the rocks belong to high-K calc-alkaline volcanic series. LREE enrichments, LILE depletion, normal HFSE and obvious δEu （0.44 -0. 66 ） of the volcanic rocks reveal the geochemical features of granitoid in late-stage of collisional orogeny. Nd/Th, Th/U, La/Ta and Nb/Ta values suggest its crust-derived character. In situ zircon U-Pb LA-ICP-MS dating on the magmatic genetic zircon yielded an age of the 289. 4 ± 5.5Ma, representing the emplacement age of the volcanic rocks. Combined with regional tectonic analysis, it is concluded that the South Tianshan is under late-stage of collisional orogeny in initial stage of Early Permian, and volcanic rocks of Xiaotikanlike Formation occur as respondence to thrusting and foreland basin developing in the southern margin of the South Tianshan in the late-stage of collisional orogeny.