原地-半原地深熔花岗岩特征:以华北克拉通北缘包头地区石榴花岗岩为例

在华北克拉通北缘大青山地区,广泛的深熔作用导致新太古代晚期石榴花岗岩发育。石榴花岗岩空间上与新太古代晚期大青山表壳岩(主要为石榴黑云母片麻岩)共生,渐变过渡。宏观上岩性具有不均一性,在包头哈德门沟一条实测地质剖面上可以观察到石榴混合闪长岩、石榴混合石英闪长岩和石榴混合花岗闪长岩等不同岩石类型。岩相学研究表明,石榴花岗岩与源岩矿物组合相近,但含量却大不相同。石榴花岗岩和源岩中的石榴石特征十分相似。石榴石周围存在放射状黑云母,是典型的逆反应结构。研究表明深熔作用可能发生在具有顺时针P-T轨迹的变质作用演化峰期及峰期后近等温降压阶段,主要发生钾长石熔融和黑云母熔融。地球化学特征研究表明石榴花岗岩与石榴黑云母片麻岩具有亲缘关系,均表现出高Al2O3、CaO含量,K2O/Na2O比值存在较大变化,富集轻稀土和大离子亲石元素,亏损Nb、Ta、P和Ti。但是,石榴花岗岩存在组成变化,Eu富集型和亏损型稀土模式为原地-半原地深熔花岗岩的标志性特征。SHRIMP锆石年代学研究表明深熔作用发生在2.43Ga之前,与区内新太古代晚期水平顺层滑脱变形有关,揭示华北克拉通北缘新太古代晚期构造热事件。

The characteristics of autochthonous and hypautochthonous anatectic granite: A case study of garnet granite in Baotou area, northern margin of the North China Craton

Late Neoarchean garnet granites were developed due to wide anatexis in the Baotou area, northern margin of the North China Craton (NCC). Garnet granites are spatially associated with the Late Neoarchean Daqingshan supracrustal rocks (mainly garnet-biotite gneisses), as indicated by garnet-biotite gneiss and residual minerals occurring in the garnet granites and garnet leucosomes occurring in garnet-biotite gneisses. The garnet granites are heterogeneous and can be further divided into garnet diorite, garnet quartz diorite and garnet granodiorite which can be identified in a geological section near Hadmengou. Petrographic studies show that the garnet granites and anatectic source rocks are similar in mineral association, but are different in their contents. The garnets from the garnet granite and garnet-biotite gneiss are similar in features; they are surrounded by radial biotite flakes, a typical structural indicator of inverse reaction. The anatexis may occur during a period from metamorphic peak isothermal depressurization in a clockwise P-T process, mainly resulting in melting of k-feldspar and biotite. The garnet granites are high in Al₂O₃ content and have a large variation in CaO content and K₂O/Na₂O ratio, characterized by enrichment of light REE and large ion lithophile elements and depletion of Nb, Ta, P and Ti, being similar in composition to the garnet-biotite gneisses. However, the garnet granites show variations in compositions, and Eu-enriched and Eu-depleted patterns are considered as a prominent geochemical characteristic of autochthonous and hypautochthonous anatectic granite. Geochronology studies constrainthe anatexis occurring before 2.43Ga, as a result of subhorizontal detachment structures in the area due to Late Neoarchean tectonothermal event along the northern margin of the NCC.