内蒙古察哈尔右翼后旗元山洼A型花岗岩年代学及地球化学特征

Geochronological and Geochemical Characteristics of Yuanshanwa A-type Granite in Chahar Right Back Banner,Inner Mongolia

以内蒙古中部察哈尔右翼后旗元山洼二长花岗岩为对象,对其开展了LA-ICP-MS锆石U-Pb年代学和主、微量元素地球化学研究。元山洼二长花岗岩主要由正长石、微斜长石、更长石、石英和少量黑云母组成,其锆石U-Pb测年结果为(232.1±2.6)Ma,属晚三叠世侵入岩。该岩体富SiO_(2)、K_(2)O、F、Rb、Ga、Th、U、Zr、Hf、LREE,亏损FeO^(T)、MgO、CaO、P_(2)O_(5)、Zn、Ce、Sr、Ba、Nb、Ta、Ti、HREE,具较高的10000 Ga/Al、LREE/HREE、La_(N)/Yb_(N)比值和较低的Y/Nb、Rb/Nb、Sc/Nb、Ce/Nb、Sr/Y比值。稀土元素球粒陨石标准化配分曲线整体右倾,具中等—较强的负Eu异常和“W”型四分组效应。地球化学分析结果表明,该岩体属于铝质A型花岗岩,可进一步归为A1型花岗岩。通过综合分析认为,晚三叠世时期华北板块北缘处于后碰撞晚期伸展拉张阶段,受幔源物质底侵加热、地壳减薄和乌兰哈达—高勿素深大断裂引张切割等因素影响,下地壳镁铁质麻粒岩相发生部分熔融,熔体在经历较强结晶分异和富F流体相交代后,形成了元山洼铝质A型花岗岩。

Research is conducted on Yuanshanwa monzogranite through LA-ICP-MS zircon U-Pb geochronology and geochemistry.Yuanshanwa monzogranite is dominantly composed of orthoclase,microcline,oligoclase,quartz and minor dark mineral such as biotite.Zircon U-Pb dating result is(232.1±2.6)Ma,which implies that the pluton emplaced during late-Triassic.Yuanshanwa monzogranite is enriched in SiO_(2)、K_(2)O、F、Rb、Ga、Th、U、Zr、Hf and LREE,depleted in FeO^(T)、MgO、CaO、P_(2)O_(5)、Zn、Ce、Sr、Ba、Nb、Ta、Ti and HREE,with high 10000 Ga/Al、LREE/HREE、La_(N)/Yb_(N) ratios and low Y/Nb、Rb/Nb、Sc/Nb、Ce/Nb、Sr/Y ratios.The normalized distribution curve of rare earth element chondrite is overall right-leaning,with moderate-strong negative Eu anomaly and“W”type four-group effect.The geochemical analysis results show that the rock mass belongs to Al-A type granite,which can be further classified as A1 type granite.Through comprehensive analysis,it is believed that the northern margin of the North China Plate during the late Triassic was in the late post-collision extensional stage.It mainly is influenced by the mantle underplating and heating,the crust attenuation and decompression,and the tension and dissection of Wulanhada-Gaowusu fault.After highly crystal fractionation and metasomatism by F-rich fluid phase,the granitic melt emplaced and formed Yuanshanwa aluminous A-type granite.