阿尔金南缘鱼目泉岩浆混合花岗岩LA-ICP-MS测年与构造意义

阿尔金南缘鱼目泉花岗质岩体中含有大量暗色闪长质包体,岩相学及地球化学特征研究表明该岩体是由同期的幔源基性岩浆和酸性岩浆在近于液相(或"晶粥状")状态下发生不均匀混合作用的产物。LA-ICP-MS锆石U-Pb同位素测年结果显示其形成年龄(496.9±1.9Ma)与南阿尔金山大陆深俯冲超高压变质岩的峰期变质年龄(504～487Ma)相一致,且在地球化学特征上具有高Al2O3(平均15.88%),较高的K2O/Na2O值(平均1.26),高Sr(平均446×10-6),高(La/Yb)N值(平均24.04)和Sr/Y值(平均40),极低的Y(平均14.0×10-6)及Yb含量(平均1.5×10-6),类似于加厚地壳背景下形成的高Sr、低Y及Yb型花岗岩,反映出约500Ma时的南阿尔金造山带总体上处于地壳相互叠覆增厚的陆-陆碰撞造山作用阶段。分析认为,约500Ma时,南阿尔金山地区伴随着增厚地壳发生熔融作用产生大规模酸性岩浆活动的同时,还存在幔源基性岩浆的底侵,其原因可能与同时期大陆深俯冲作用所诱发的深部热地幔上升有关。

The LA-ICP-MS zircon dating and Tectonic significance of the Yumuquan magma mixing granite, southern Altyn Tagh

The Yumuquan granitoid complex, located in the south margin of Altyn Tagh, northwestern China, contain a large number of melanocratic magmatic enclaves (MME). Petrographic and geochemical study indicates that MME is the product of heterogenetic mixing of contemporaneous mantle-derived basic magma and felsic magma under the liquid phase or crystal mesh. The LA-ICP-MS zircon U-Pb isotopic dating show that the age (496.9±1.9 Ma (MSWD=0.68)) of Yumuquan granite is in agreement with the age (509~487 Ma) of ultrahigh-pressure metamorphic rocks exposed at south Altyn Tagh. The granitoid rock is characterized by high Al2O3 (ave.15.88%), high K2O/Na2O ratios (ave. 1.26), high Sr (ave.446×10-6), and high (La/Yb)N ratios (24.04) and Sr/Y ratios (avg. 40), and extremely low Y (ave. 14.0×10-6) and Yb (avg. 1.5×10-6).These characteristics are similar to those of high Sr and low Y(Yb) type potash granite derived from thickened lower crust, reflecting that the southern Altyn orogen was generally in the period of continental-continental collision orogeny resulting from crust superimposition at during ~500 Ma. Our research suggests that large-scale acid magmatic activity took place at at ~500 Ma as a result of melting of thickening crust in southern Altyn Tagh area, accompanied by underplating of mantle-derived mafic magma. The explanation for that is likely related to rising of deep hot mantle, which was triggered by contemporaneous continent subduction.