海拉尔盆地乌尔逊凹陷南一段物源-沉积体系与构造背景

海拉尔盆地乌南凹陷南一段时期断裂和火山作用强烈, 导致岩石碎屑组成具有多样性。碎屑成分有火山碎屑、变质岩碎屑和其他剥蚀碎屑。碎屑多样性使本区岩性及其分区具有鲜明的特色。岩性丰度、矿物组合、阴极发光及其重矿物组合研究显示, 区内存在4个单岩性区和2个混合区。单岩性区是变质岩碎屑岩区、火山碎屑沉积岩区、熔结火山碎屑岩区和火山碎屑岩区。根据岩石中各碎屑组成的体积分数可将混合区分为正常沉积碎屑占优势的混合区和火山碎屑占优势的混合区。各单岩性区与构造单元相符合, 变质岩碎屑岩区对应乌西断阶带, 火山碎屑沉积岩区和熔结火山碎屑岩区对应巴彦塔拉构造带, 火山碎屑岩区为乌东弧形构造带的一部分。地球化学数据显示本区源岩岩浆属于壳源花岗质岩浆, 同时受到明显的幔源岩浆影响, 重稀土较富集。乌西断阶带具有高的稀土总量和不明显的Eu负异常, 变质源岩岩浆壳幔混染程度高;巴彦塔拉构造带具有低的稀土总量和明显的Eu负异常, 源岩岩浆壳幔混染程度中等;乌东弧形构造带具有高的稀土总量和明显的Eu负异常, 源岩岩浆壳幔混染程度低。Dickinson图解显示, 乌东弧形构造带源区没有深切割, 而巴彦塔拉构造带源区经过了深切割。德尔布干断裂的正滑移导致嵯岗构造片麻岩发育和嵯岗隆起相对隆升, 为乌西断阶带提供变质碎屑沉积物, 形成扇三角洲沉积体系;同时断裂又为幔源岩浆的上升提供了通道。巴彦塔拉断裂的伸展走滑使得幔源岩浆上升发生壳幔混染并发生火山喷发, 在巴彦塔拉构造带发育熔结火山碎屑岩-火山碎屑沉积岩, 塑造了火山碎屑扇三角洲沉积体系。巴彦山隆起区壳幔混染程度低, 但幔源岩浆的底垫作用导致壳源岩浆的强结晶分异作用、火山活动和地表隆升, 在乌东弧形构造带发育火山碎屑岩, 塑造了火山碎屑三角洲沉积体系。

Provenance-Sedimentary System and Tectonic Setting of First Member of Nantun Formation in Wuerxun Depression of Hailar Basin

Fracture and volcanism were intensive in First Member of Nantun Formation of Wunan depression in Hailar basin,due to the diversity of detritus.The composition of clastic were metamorphic clastic,pyroclastic and general erosion detrital.The diversity of detritus makes lithology and division have the distinct characteristics.The study of mineral association,cathode luminescence and heavy mineral assemblage shows that the study area can be divided into four single lithology areas and two mixed areas among them.Four single lithology areas are sedimentary rocks area with metamorphic rocks clastics,pyroclastic sedimentary rocks area,welded pyroclastic rocks area and pyroclastic rocks area. According to the contents and components of clastics in rocks,mixed area can be divided into two mixed areas:the area with normal sedimentary clastic was dominant and the area with pyroclastic was dominant.Each single lithology area corresponds with each tectonic unit.The sedimentary rocks area with metamorphic rocks clastics corresponds to Wuxi fault terrace belt,the pyroclastic rocks area correspond to Bayantala structural belt,and pyroclastic sedimentary rocks area is part of Wudong arc structural belt.Geochemical data reveal that magma of source rocks is granitoid magma of crust source, meanwhile be contaminated by magma of mantle-derived,and heavy rare earth elements are enrichment. In Wuxi fault terrace belt,total content of rare earth elements is high and negative anomaly of Eu is not obvious,so the crust-mantle contamination of source rocks magma is heavy.In Bayantala structural belt,total content of rare earth elements is low and negative anomaly of Eu is obvious,so the crust-mantle contamination of source rocks magma is medium.In Wudong arc structural belt,total content of rare earth elements is high and negative anomaly of Eu is obvious,so the crust-mantle contamination of source rocks magma is low.Dickinson graphics of sandstone composition reveal that source regions of Wudong arc structural belt has not deeply cut,and source region of Bayantala structural belt has deeply cut.Noral-slip of Derbgan fault led tectonic gneiss of Chaigang to be developed and Chaigang to uplift relatively,which become erosion area and provided Wuxi fault terrace belt with metamorphic sediments, shaped the sedimentary system of fan delta with metamorphic clastic,meanwhile the fracture provided a channel for the mantle-derived magma rising.Extension slip of Bayantala fault made the rise of mantle-derived magma and volcanic eruption,developed a set of rocks between fusion sintering pyroclastic rock and pyroclastic sedimentary rock,and shaped the sedimentary system of fan delta with pyroclastic.The crust-mantle contamination degree of Bayanshan uplift was low,but underplateing of mantle-derived magma made strong crystallization differentiation, volcanic activity and surface uplift, developed pyroclastic rocks and shaped sedimentary system of delta in Wudong arc structural belt.