大兴安岭中部乌兰浩特地区花岗岩锆石U-Pb年龄、地球化学特征及构造意义

Zircon U-Pb Age,Geochemical Characteristics and Tectonic Implications of Granites in the Wulanhaote Area,Central Daxing'an Mountains

对内蒙古乌兰浩特地区出露的花岗岩侵入体进行LA-ICP-MS锆石U-Pb定年和岩石地球化学研究,以探讨该区花岗岩形成时代、成因类型和区域构造演化。结果显示,其锆石206Pb/238U年龄的加权平均值为(214.4±2.6) Ma,反映岩体的结晶年龄为晚三叠世。岩石主量元素具高硅、高碱、富钾,贫钙、贫镁及低钛的特征;稀土元素配分曲线呈"海鸥式"较弱的右倾分布特征,具有强烈的Eu负异常;微量元素Rb、Th、Zr、Y和Yb含量较高,Ba、Sr含量较低,岩石具有高的Rb/Sr、Rb/Nb值,显示为低Sr、高Yb型花岗岩的特征。综合分析显示花岗岩形成的压力较低,应为减薄地壳物质熔融的产物。综合区域构造演化和K2O-Na2O、(Zr+Nb+Ce+Y)-(Na2O+K2O)/CaO、SiO2-TFeO/MgO、(Y+Nb)-Rb、(Yb+Ta)-Rb、Nb-Y-Ce图解分析,认为该岩体形成于造山后伸展构造背景,研究区由中三叠世的碰撞造山构造体制向晚三叠世造山后伸展构造体制转变,同时西伯利亚板块和华北板块的碰撞时限持续到中三叠世。

Zircon U-Pb geochronological and geochemical data are presented on the granites in Wulanhaote(Inner Mongolia),and we discussed the granite ages,types and regional tectonic evolution.The Wulanhaote granites are monzogranite,and zircon U-Pb dating yielded a weighted mean 206Pb/238U age of(214.4±2.6)Ma,indicating a Late Triassic granite emplacement.The rocks are characterized by having high SiO2 and alkali,but low CaO,MgO and TiO2 contents.Chondrite-normalized REE diagram shows a weak right-inclining,low-MREE pattern with strong negative Eu anomaly.The rocks show enrichments in Rb,Th,Zr,Y and Yb,but depletions in Ba and Sr.They have high Rb/Sr and Rb/Nb values and show features of low-Sr-and high-Yb-type granites.Our results show that the granites were formed under relatively low pressure by partial melting of the delaminated crust.Integrating regional tectonic evolution and tectonic discrimination diagrams,we suggest that the granites were emplaced in a post-orogenic extensional setting.This indicates that the regional tectonics has changed from Middle Triassic collisional orogenic to post-Triassic orogenic extensional,and that the Siberia-North China collision may have lasted till the Middle Triassic.