内蒙古索伦地区花岗斑岩岩石成因及构造背景

Petrogenesis and tectonic setting of granite porphyry in Suolun area in Inner Mongolia

通过对内蒙古索伦地区花岗斑岩的野外地质特征、锆石U-Pb测年结果及相关地球化学元素特征等方面的研究,确定了该区花岗斑岩的形成时代,探讨了花岗斑岩成因及构造背景。索伦地区花岗斑岩多呈岩株或岩脉产出,LA-ICP-MS锆石U-Pb同位素测年结果表明其形成于早白垩世晚期(119. 9±1. 6 Ma)。岩石具有富硅、富碱和富铝质的特点,属于铝质-过铝质高钾钙碱性系列。矿物组合及地球化学特征显示索伦地区花岗斑岩为高分异Ⅰ型花岗岩,富集大离子亲石元素(Rb、Th、U)和轻稀土元素(La、Ce、Pr),具有Eu的明显负异常,显示壳源岩浆或岩浆被地壳物质混染的地球化学特点。花岗斑岩具有正的εHf(t)值(+7. 5～+10. 8),二阶段模式年龄为489～695 Ma,反映其源区物质可能形成于早古生代—新元古代基性下地壳物质的熔融。索伦地区花岗斑岩形成于造山后伸展环境,这种构造背景可能与蒙古—鄂霍茨克缝合带闭合后加厚陆壳的拆沉作用有关。

Through the study of field geology,Zircon U-Pb dating results,and characteristics of relevant geochemical elements of granite porphyry in Suolun area in Inner Mongolia,the formation age of granite porphyry was determined,petrogenesis and tectonic background of granite porphyry are discussed. Granite porphyry in Suolun area usually occurs as beads or dikes. The LA-ICP-MS Zircon U-Pb isotope dating results show the granite porphyry formed in late Early Cretaceous( 119. 9 ± 1. 6 Ma). The rocks are rich in silicon,alkali and aluminum,and belong to the series of aluminum-peraluminous high-potassium calc-alkaline. Mineral assemblages and geochemical element analysis exhibit that the granite porphyry is highly fractionated I type granite,and is characterized by the enrichment of large ion lithophilic elements( Rb,Th,U) and light rare earth elements( La,Ce,Pr) with obvious negative anomalies of Eu,indicating geochemical characteristics of crust-sourced magma or magma mixed with crustal materials. Granite porphyry in Suolun area has positive εHf( t) values of + 7. 5 ~ + 10. 8,and two-stage model ages of 489 Ma to 695 Ma,which indicated that the source materials may have formed from the melting of the basic lower crust material in the Early Paleozoic and Neoproterozoic. The granite porphyry was formed in the postorogenic extensional environment in Suolun area,which may be related to the delamination of the thickened continental crust after the closure of the Mongol-Okholsk suture zone.