浙东早白垩世火山岩组合的地球化学及其成因研究

Geochemistry and genesis of early Cretaceous volcanic rock assemblages in eastern Zhejiang.

浙东早白垩世含中性岩的火山岩组合和双峰式火山岩组合的主量元素和微量元素特征对比研究表明,中性火山岩的地球化学特征明显受到伴生的基性和酸性火山岩的影响。它们的微量元素协变关系证实:中性火山岩是由基性岩浆和酸性岩浆混合形成的。双峰式火山岩具有与含中性岩的火山岩组合相类似的主量元素特征,但两者的微量元素特征相差较大。根据东南沿海在早白垩世时期的构造背景是由挤压向伸展拉张转变,以及各种元素具有不同的扩散速度,提出了浙东早白垩世火山岩组合的形成模式:若基性岩浆和酸性岩浆仅进行了部分微量元素的交换,没有来得及发生主量元素的交换,就在伸展拉张的构造背景下喷出地表,则形成双峰式火山岩组合;若基性岩浆与酸性岩浆在地壳深处共存的时间较长,发生一定程度的主量元素交换,则形成偏基性和偏酸性的中性岩浆,若进一步发生化学成分的交换,则可形成典型的安山质岩浆,喷出地表就形成含中性岩的火山岩组合。

The comparison studies of major elements and trace elements of early Cretaceous intermediate-bearing volcanic rock assemblage and bimodal volcanic rock assemblage in eastern Zhejiang suggest that the geochemical characteristics of intermediate volcanic rocks are obviously controlled by those of coexisted basalts and rhyolites; The plots of trace elements of these volcanic rocks testify that intermediate volcanic rocks were generated by mixing of basaltic magma and rhyolitic magma. Intermediate-bearing volcanic rock assemblage and bimodal volcanic rock assemblage have similar characteristics of major elements and different characteristics of trace elements. Based on the early Cretaceous tectonic setting in the coastal area of southeast China which was changing from compression to extension, and the different diffusion rate of trace elements during magma mixing, a genetic model for early Cretaceous volcanic rock assemblages in eastern Zhejiang is proposed. If basaltic magma and rhyolitic magma coexisted only for a short time in lower crust before eruption, then they had no time to exchange their major elements except for some trace elements, they formed bimodal volcanic rock assemblage. If basaltic magma and rhyolitic magma coexisted in lower crust for a long period of time, and they had enough time to exchange their major elements except for trace elements, then intermediate magma can be generated at the interface of the magmas. A typical andesitic magma can be produced by further composition exchanges, forming intermediate-bearing volcanic rock assemblage.