南堡凹陷新近系馆陶组砂岩地球化学、构造背景和物源探讨

对南堡凹陷馆陶组(粉)砂岩的主量、微量及稀土元素的地球化学特征研究表明:多数主量元素含量中等,SiO2、Al2O3等主元素含量及其相关比值反映大多数砂岩属于岩屑砂岩和长石砂岩,个别为杂砂岩;镁铁族元素(Co,Cr,Ni,Sc,V)和大离子亲石元素(Rb,Cs,Ba,Sr)含量类似于上地壳。砂岩十分相似的REE分布模式显示了相似的物源和构造背景。SiO2-K2O/Na2O图解、主元素双变量判别图解及Th-Sc-Zr/10和Th-Co-Zr/10等构造背景判别图解均表明,馆陶组物源区属于活动大陆边缘构造背景。根据主元素物源判别方程图、La/Sc-Co/Th和Th-Hf-Co图投影及(Gd/Yb)N值,表明馆陶组物源主要来自由长英质岩石组成的后太古宙上陆壳。通过与燕山地区不同时代(期)中酸性火山岩和花岗岩REE的对比,表明母岩主要来自燕山褶皱带地区于燕山期发育的中酸性火山岩和/或花岗岩体。

Tectonic Background and Provenance of the Neogene Sandstones Evidenced from Their Chemical Composition, Guantao Formation of Nanpu Sag, Bohai Bay Basin, China

Compositions of major,trace and rare earth elements of the Neogene sandstones and siltstones in Guantao Formation of Nanpu sag are analyzed.The results reveal that most contents of major elements are moderate,SiO2 and Al2O3 contents and their ratios show that most of sandstone samples are arkose or lithic arkose as well as other major elements,with only a few graywackes existing.Contents of mafic elements(Co,Cr,Ni,Sc,V) and large-ion lithophile elements(LILE) like Rb,Cs,Ba,Sr are similar to that in the upper crust.The unanimous chondrite-normalized REE distribution patterns suggest that sandstones have similar provenance and be formed in the same tectonic settings.Most discrimination diagrams of tectonic settings,such as diagrams of log(K2O/Na2O) versus SiO2,double variants distinction diagrams of major elements,diagrams of Th-Sc-Zr/10 and Th-Co-Zr/10 etc.,show that the provenance of Guantao Formation in Nanpu sag belongs to active continental margin tectonic setting.According to the diagrams of provenance discrimination(such as Th-Hf-Co plot and La/Th-La/Yb plot) and(Gd/Yb)N ratios,it can be known that the primary materials of sedimentary rocks of Guantao Formation came from the upper continental crust of post-Archaean that were mainly composed of felsic rocks.Compared with REE distribution patterns of intermediate-acidic volcanic rocks and granites from Yanshan orogeny during different times,the results show that source rocks were mainly from intermediate-acidic volcanic rocks and/or granites which were formed during Yanshan movement period inYan-shan orogeny.