阿拉善北大山乌布日布特地区晚古生代侵入岩的地球化学、年龄、Hf同位素特征及意义

Geochemistry,age and zircon Hf isotope characteristics for the Late Paleozoic intrusive rocks in Ubud area of Beidashan,Alxa and their significances

内蒙古阿拉善西部的北大山北部出露一些古生代侵入岩,这些侵入岩的时空分布和源区物质组成特征对于确定北大山北部的构造属性以及探讨阿拉善西部的构造划分和构造演化具有重要意义。本文对北大山北部乌布日布特地区腰泉和沙枣泉两个花岗质岩体开展了详细的年代学、地球化学和锆石Hf同位素研究。锆石LA⁃ICP⁃MS U⁃Pb测年结果显示,腰泉黑云母正长花岗岩年龄为261.8±1.2 Ma,沙枣泉黑云母花岗岩年龄为303.1±1.6 Ma,均形成于晚古生代。腰泉黑云母正长花岗岩与沙枣泉黑云母花岗岩的稀土元素组成相似,均表现出轻稀土元素相对富集,重稀土元素相对亏损的特征,并显示有微弱—中等的Eu负异常。腰泉黑云母正长花岗岩与沙枣泉黑云母花岗岩均具有正的ε_(Hf)(t)值,腰泉黑云母正长花岗岩的ε_(Hf)(t)值为+1.93~+7.40,沙枣泉黑云母花岗岩的εHf(t)值为+0.52~+10.00,这与北大山北部其它古生代酸性侵入岩的锆石Hf同位素相似,表明北大山北部酸性岩浆的深部源区以新生地壳物质为主。北大山北部深部地壳物质组成特征与阿拉善中东部宗乃山—沙拉扎山地区相似,而与北大山南部和阿拉善中东部雅布赖—诺尔公—红古尔玉林地区具有明显差别。结合区域研究资料,推测北大山北部可能不属于阿拉善地块,而与阿拉善中东部的宗乃山—沙拉扎山地区具有相同的构造属性,是中亚造山带的组成部分。

Paleozoic intrusive rocks are widely exposed in northern Beidashan,western Alxa League,Inner Mongolia.The spatial⁃temporal distribution and source characteristics of these intrusive rocks are of great significances not only for understanding the tectonic affinity of northern Beidashan,but also in extending our knowledge on the division and geological evolution of western Alxa.In this paper,geochronological,whole rock geochemistry and zircon Hf isotope analysis have been performed on Yaoquan and shazaoquan two granitic bodies in the Ubud area,northern Beidashan.Based on zircon LA⁃ICP⁃MS U⁃Pb dating results,the Yaoquan biotite syenogranite formed at 261.8±1.2 Ma while the Shazaoquan biotite granite emplaced at 303.1±1.6 Ma,showing that both of the granites were emplaced in the Late Paleozoic.The Yaoquan and Shazaoquan biotite granites share similar chondrite normalized REE patterns,which show enriched light REE and depleted heavy REE with weak⁃medium Eu negative anomalies.The zirconεHf(t)values for the Yaoquan biotite syenogranite are in the ranges of+1.93~+7.40 and for the Shazaoquan biotite granite in the ranges of+0.52~+10.00.Both of the zircon Hf isotopes are similar to those of felsic intrusive rocks from the northern Beidashan,indicating the source of the origin for the felsic plutons from the northern Beidashan are dominated by juvenile crust.The juvenile characteristics of the deep crust for the northern Beidashan area is similar to the Zongnaishan⁃Shalazashan area,but different from the southern Beidashan and Yabulai⁃Nurgong⁃Honggueryulin area in the mid⁃eastern part of the Alxa.Combined with previous regional studies,it is speculated that the northern Beidashan may not be part of the Alxa block,but could be part of Central Asian orogenic belt,showing the same tectonic affinity as the Zongnaishan⁃Shalazashan area in mid⁃eastern Alxa.