四川盆地北部早白垩世剑门关组泥质岩地球化学特征及其地质意义

早白垩世剑门关组是研究四川盆地北部沉积环境演化的良好地层,对该地层开展系统的地球化学研究,对探讨四川盆地北部早白垩世剑门关组物源区性质、构造背景、古风化作用及古环境具有重要意义。系统分析了剑门关组泥质岩主量、微量和稀土元素地球化学特征,发现早白垩世剑门关组泥质岩富CaO、MgO,贫Al_2O_3、Fe_2O_3、K_2O、Na_2O、TiO_2、P_2O_5、MnO,富Cr、Cs、V,贫Sr、Nb、U、Hf,稀土元素总量为(164.96～234.35)×10^-6,轻、重稀土元素比值为11.77～15.87,轻、重稀土元素分馏程度高,轻稀土元素相对富集,具弱的正Eu异常。综合分析认为:剑门关组为同一物源的近物源再旋回沉积岩;源岩为沉积岩,可能富含斜长石、重晶石等富Eu矿物;源岩在沉积前经历了中等程度的化学风化作用,并发生了钾交代作用;剑门关组沉积期为温暖、湿润的气候,物源区具有由活动大陆边缘向大陆岛弧转化的特征。

Geochemical characteristics and geological significance of argillaceous rocks of Early Cretaceous Jianmenguan Formation in Northern Sichuan Basin

The Early Cretaceous Jianmenguan Formation is a good formation to study the sedimentary environment evolution in Northern Sichuan Basin. The systematical study on its geochemical characteristics is significantly important for studying the provenance, tectonic background, ancient weathering and paleoenvironment of Early cretaceous Jianmenguan Formation. In this paper, the geochemical characteristics of major elements, trace elements and rare earth elements (REE) of the argillaceous rocks of Jianmenguan Formation were systemically analyzed. The results show that the argillaceous rocks of Jianmenguan Formation are rich in CaO,MgO, lack Al₂O₃,Fe₂O₃,K₂O,Na₂O,TiO₂,P₂O₅,MnO, are rich in Cr, Cs, V, and lack Sr, Nb, U, Hf. The total amount of REE(∑REE) ranges from 164.96×10^-6 to 234.35×10^-6. LREE/HREE is in the range of 11.77~15.87. The fractionation between LREE and HREE is obvious, and LREE content is higher than HREE, with a weak positive Eu anomaly. Based on the analyses above, Jianmenguan Formation is recycling sedimentary rocks from the same sources. Its source rocks are sedimentary rocks, and it may be rich in europium-rich minerals such as plagioclase and barite. The source rocks underwent moderate chemical weathering before sedimentation and K-metasomatisim during the weathering process. The climate during the sedimentary period of Jianmenguan Formation was warm and humid. Tectonic evolution characteristics of the source area appeared to have a transformation from the continental margin to the continental island arc.