内蒙古土牧尔台地区黑云母二长花岗岩及其暗色包体的成因及构造意义

Petrogenesis and tectonic implications of the biotite monzogranite and associated mafic microgranular enclaves in Tumuertai area,Inner Mongolia

内蒙古中部土牧尔台地区位于华北克拉通北缘,区内广泛发育黑云母二长花岗岩及暗色闪长岩包体,绝大多数包体与寄主岩石呈渐变过渡的接触关系。锆石LA-ICP-MS U-Pb定年结果显示,寄主岩石年龄为273~271Ma,包体年龄为268±2Ma,两者均形成于早-中二叠世。其中,寄主岩石具有高钾钙碱性、弱过铝质、高分异I型花岗岩的地球化学特点,且富集K、Rb、Th等大离子亲石元素和LREEs,亏损Nb、P、Ti等高场强元素和HREEs,具有典型的大陆弧岩浆特征。花岗岩中锆石εHf(t)值为-13.0~-6.1,二阶段Hf模式年龄(t_(DM2))为2.12~1.68Ga,ε_(Nd)(t)值为-11.3~-10.7,亏损地幔模式年龄(t_(DM))为2.44~1.73Ga,指示岩浆源自华北克拉通下地壳的部分熔融。暗色包体多呈塑性形态,镁铁质矿物含量明显高于寄主岩石,且发育大量针状磷灰石,同位素(ε_(Nd)(t)值为-11.1~-10.2,t_(DM)为2.13~1.85Ga)特点与寄主岩石相近。本文对黑云母二长花岗岩及其暗色包体的研究表明,区域上这些具有弧岩浆性质的早-中二叠世岩浆岩,很可能源自古亚洲洋俯冲洋壳及下地壳的部分熔融,幔源基性岩浆与下地壳长英质岩浆的混合形成寄主岩石及暗色包体,揭示了早-中二叠世时期区域上岩浆具有壳-幔相互作用的特点,且印证了该时期古亚洲洋洋壳尚未完全消失,中亚造山带的造山作用为区域岩浆岩及包体的形成提供地球动力学条件。

We present detailed geochronological,geochemical and Sm-Nd isotopic data for the host biotite monzogranite with mafic microgranular enclaves(MMEs)in the Tumuertai of the central Inner Mongolia,northern North China Craton(NCC).LA-ICP-MS UPb zircon ages from host biotite monzogranite yielded ages ranging from 273 Ma to 271 Ma,and 268±2 Ma for the MMEs.The biotite monzogranite exhibit uniform LREE-enriched patterns,and enrichment in K,Rb,Th and LREEs,but depletion in Nb,P,Ti and HREEs,with features of I-type granite,showing typical geochemical signatures of continental arc rocks.ε_(Hf)(t)isotope from the biotite monzogranite range from -13.0 to-6.1,with t_(DM2) model ages of 2.12~1.68 Ga.The results are coupled with the Nd isotope,varying from-11.3 to-10.7,with t_(DM)ages from 2.44 Ga to 1.73 Ga.Similar results were observed in the MMEs,i.e.,the ε_(Nd)(t)values range from -11.1 to -10.2,with t_(DM)ages from 2.13 Ga to 1.85 Ga.Content of apatite and associated mafic minerals in the MMEs are higher than those in the host biotite monzogranite.These arc-related magmatic rocks were likely derived from crust-mantle interaction induced by deep subduction of the Paleo-Asian Ocean,following the partial melting of lower crust.We favor that the PaleoAsian Oceanic plate has not vanished completely in the northern NCC,at least in part,until in the Early-Middle Permian.