松辽盆地火山岩的同位素年代、地球化学特征及意义

Geochronology and geochemistry of volcanic rocks from the Songliao basin and their implications

沉积盆地内部的火山岩是研究盆地形成的构造环境以及探讨盆地发展演化的动力学特征的重要线索。松辽盆地具有二元结构特征,断陷构造层发育大量的火山岩。但是,前人尚未对此进行深入系统的研究。根据采自钻井岩芯的火山岩样品,通过同位素测年(K/Ar)和地球化学分析等结果,讨论了松辽盆地晚中生代火山岩的岩石学、岩石化学和地球化学特征,探讨了盆地发育早期的构造环境及其地球动力学背景。指出松辽盆地在其形成早期(即断陷期)是一个发育陆缘岩浆弧构造背景上的走滑拉张盆地,火山岩岩浆源区具有多成分、复合性的演化特点。

Volcanic rocks in basins are significant for us in understanding the tectonic settings and geodynamics of the basins. The Songliao basin is characterized vertically by double structural layers, which are the faulted and the depressed layers in the lower and the upper, respectively. The lower sequence developed bulk volcanic rocks. However, previous work ignored the systematic study on these volcanic rocks. The purpose of this paper is to explore the tectonic setting and the geodynamics of volcanic rocks from the Songliao basin, based on dating results of K/Ar and geochemical analyses for the volcanic rocks collected from the drilling cores. The results show that the volcanic samples from the Songliao basin define a melt evolution trend typical for calc alkaline suites. Compatible and incompatible major elements (e.g. MgO, Fe2O3, Al2O3, and K2O vs. SiO2) delineate continuous melt differentiation trends as observed along the subduction zones. Additionally, the volcanic rocks from the Songliao basin is characterized by high Al2O3, Cr, Ni and low MgO contents. The ratios of Nb/Th (2～20) and Ce/Pb (2～30) of basalts and basaltic andesites demonstrate that the igneous genesis of the Songliao basin was related to the subduction, and also is characterized by rocks of oceanic and continental crust affinity. As a consequence, it is suggested that the Songliao basin developed on a magmatic arc in an active continental margin during Late Mesozoic (ca. 150～70 Ma), and the arc was related to the oblique subduction of the Izanaqi plate. The magma genesis of volcanic rocks was complex, and characterized geochemically by multiple resources.