【摘要】 阿拉善北部宗乃山地区出露少量前寒武纪岩石,主要为一套变质表壳岩和变质深成侵入岩组成的变质杂岩,其形成时代和同位素特征对于了解该地区的基底性质和探讨阿拉善北部构造格架具有重要意义。本文对宗乃山西南缘阿尔滕托亚副片麻岩和嘎顺呼都格正片麻岩开展了详细的岩石学、锆石U-Pb年代学和Hf同位素研究,并对其构造意义进行了初步探讨。阿尔滕托亚副片麻岩包括黑云二长片麻岩和变斑状黑云斜长片麻岩,两套岩石中分选出来的锆石具有典型岩浆锆石特征,LA-ICPMS分析结果显示这些锆石的207Pb/206Pb年龄相对集中,并分别获得了1459±11Ma(MSWD=3.3)和1458±3Ma(MSWD=0.13)的年龄。初步研究认为阿尔滕托亚副片麻岩的源区存在~1.46Ga的岩浆事件,这些副片麻岩形成时代稍晚于该期岩浆事件。嘎顺呼都格正片麻岩以角闪黑云二长片麻岩为代表,其锆石结晶年龄为1433±17Ma(MSWD=0.25)。锆石Hf同位素分析结果显示,阿尔滕托亚副片麻岩和嘎顺呼都格正片麻岩具有一致的锆石εHf(t)值(+0.1~+11.9),二阶段模式年龄为2.19~1.44Ga,反映了片麻岩原岩岩浆可能来自当时相对年轻的地壳物质。结合前人研究成果,宗乃山前寒武纪岩石可能主要形成于中元古代1.4~1.5Ga,而不是以往认为的中太古代-古元古代。截至目前,宗乃山-沙拉扎山构造带暂未发现类似阿拉善地块内部的太古代-古元古代基底岩石,因而其与阿拉善地块在早前寒武纪的亲缘性存有疑问。更多还原

【Abstract】 Minor Precambrian rocks are exposed in the Zongnaishan area of northern Alxa,and they are mainly composed of metamorphosed supracrustal rocks and meta-intrusive rocks. Revealing the age and origin of the Precambrian rocks is essential for the understanding of the basement characteristic and tectonic architecture. We presented systematic petrology,zircon U-Pb and Hf isotopic studies for the paragneisses and orthogneiss in the southwestern Zongnaishan-Shalazhashan zone( ZSZ). The U-Pb ages of 1459 ± 11 Ma and 1458 ± 3Ma were obtained for the biotite-monzonitic gneisses and the augen biotite plagioclase gneiss in Aertengtuoya area respectively,suggesting a Mesoproterozoic magmatic event in the source area. The formation age of the paragneisses should be slightly later than magmatic event. U-Pb dating of the magma zircons from the orthogneiss( amphibole and biotite monzonitic gneisses in the Gashunhuduge area) yield an age of 1433 ± 17 Ma. The high and mainly positive zircon εHf( t) values between + 0. 1 and + 11. 9 and the zircon Hf model ages of 2. 19 to 1. 44 Ga,suggested that the igneous protoliths were mainly derived from the juvenile crustal material. These observations,in combination with previous work,indicated that the basement of ZSZ was composed of Mesoproterozoic( 1. 4 ~ 1. 5Ga) metamorphosed rocks. These U-Pb zircon age data do not support the existence of exposed Archean-Paleoproterozoic rocks in the ZSZ,and challenge the previous interpretation that the ZSZ was part of the Alxa block during Early Precambrian times.更多还原