内蒙古大青山北麓蓝晶石榴长英质片麻岩的发现:岩相学、地球化学和锆石SHRIMP定年

摘要

最近1:5万地质填图发现,在华北克拉通西部阴山地块西乌兰不浪地区的高级变质杂岩中,存在一套由蓝晶石榴长英质片麻岩、石墨蓝晶石榴长英质片麻岩、石墨石榴长英片麻岩和石榴蓝晶石英岩构成的岩石组合(蓝晶石榴长英质片麻岩组合).它们以构造残片形式产于由太古宙兴和岩群和紫苏石英闪长质-紫苏斜长花岗质-紫苏花岗质片麻岩(狼牙山片麻岩)为主构成的高级变质杂岩之上,以富Al2O3、TiO2,低MgO、FeO、CaO、MnO,贫K2O、Na2O为特征.与工作区南部孔兹岩带中的富铝片麻岩相比,明显贫K2O、Na2O,亏损大离子亲石元素Rb、Ba、Sr和高场强元素Nb、Ta,稀土总量低和轻稀土相对亏损.岩石中主要存在2461～2542Ma和2349～2399Ma两组碎屑锆石,其结构和组成特征表明既有来自岩浆岩物源区也有来自变质岩物源区.可靠的最年轻锫石年龄为约2.15Ga,但其成因难以准确确定(碎屑锆石或变质锆石),然而,结合其它方面的综合研究,蓝晶石榴长英质片麻岩组合的原岩很可能形成于古元古代晚期,与研究区南侧孔兹岩带广泛分布的孔兹岩系变质岩一道,构成古元古代晚期西部造山带的不同构造岩片.%A set of rock assemblage(Kyanite garnet quartzfeldspathie gneiss assemblage), composed ofkyanite garnet quartzfeldspathic gneiss, graphite kyanite garnet quartzfeldspathic gneiss, graphite garnet felsic gneiss and graphite kyanite garnet quartzite was discovered in high-grade metamorphic complex in West Ulanbulang area, located at Yinshan block,Western block, North China craton during the Recently 1:50000 geological mapping. The rock assemblage was occurred on the high-grade metamorphic complex made up by Archean Xinghe Group complex and hypersthene quartz dioritic--enderbitic--charnockitic gneiss( Langya Mountain gneiss) as a relict slice, and is characterized by being riched in Al2O3and TiO2,lower in MgO, TFeO, CaO, MnO, poored in K2O and Na2O. Compared with Al-riched gneiss in the khondalite belt south of studied area. it is obviously poored in K2 O and Na2 O. depleted in large-ion lithophile elements Rh. Ba. Sr and high field strength elements Nb. Ta. It is also lowered in total REE and relatively depleted in light REE. The textures and components of two groups detrital zircon respectively 2461 ～2542Ma and 2349～399Ma in the rocks show that they are not only from igneous rock source area but also from metamorphic source area. Although the origin (detrital zircon or metamorpbic zircon) of the most young zircon.of which the reliably age is 2. 15Ga.is difficult to exactly determine, it is proposed that the protoliths nf the kyanite garnet quarlzfeldspathic gneiss assemblage is most likely formed in the late Paleoproterozoic with other studies, and that the assemblage and the widespread khondalite series mctamorphic rocks in khondalite belt south of the studied area are of the different tectonic slices of late Paleoprotcrozoic orogenic helt in the Western North China craton.