玄武岩类形成的大地构造环境Ta、Hf、Th、La、Zr、Nb比值对比判别

Discrimination of Ratios of Ta,Hf,Th,La,Zr and Nb for Tectonic Settings in Basalts

玄武质岩浆见于板块构造演化的全过程和各个重要阶段,是所有火成岩中最主要的构造地球化学指示剂,因此,玄武岩类相关元素图解和比值成为研究火成岩成岩作用和形成构造环境判别的首选对象。Ta、Hf、Th、La、Zr、Nb都是高场强元素和较强不相容元素,由于地球化学性质的相似性和在玄武岩类成岩过程中的规律性变化,所以它们是玄武岩类成因研究和构造环境判别图解中使用频率最高的元素,而且两两元素含量之间的比值能较好地区分其形成时的构造环境。在前人发明的玄武岩类Th/Hf-Ta/Hf、Th/Zr-Nb/Zr及La/Zr-Nb/Zr双对数判别图解基础上,运用这6个元素两两组对的Ta/Hf、Th/Ta、Th/Hf、Nb/Zr、Th/Nb、La/Nb值,建立比值判别表,极好地区分了几乎最常见的各种构造环境中有关的玄武岩类。该比值判别表是对双对数判别图解的极好补充。

Basaltic magma,which is found in whole process of plate tectonic evolution and the critical stages,is a most important tectonogeochemical indicator in all igneous rocks.Therefore, the diagrams and ratios of related elements in basalts become the first choice to investigate the igneous diagenesis and tectonic setting.Ta,Hf,Th,La,Zr and Nb,which are high-field strength elements and stronger incompatible elements,have similar geochemical properties and regular changes in basaltic diagenesis,thus,they are most frequently used in basalt genesis and discrimination diagrams of tectonic setting.Moreover,the ratios of contents between the each one of elements can better distinguish the tectonic settings of formation.Based on the double-log discrimination diagrams of Th/Hf-Ta/Hf,Th/Zr-Nb/Zr and La/Zr-Nb/Zr in basalt yielded by previous research,the ratios of Ta/Hf,Th/Ta,Th/Hf,Nb/Zr,Th/Nb,La/Nb values were proposed to distinguish the basalts in various common tectonic settings, and the ratio discrimination table was put forward.The table extremely distinguishes basalt in most common variety of tectonic environments and is a great supplement for the double-log discrimination diagrams.