中国东部中新生代玄武岩及其与大地构造演化的关系

THE MESOZOIC-CENOZOIC BASALTS IN EASTERN CHINA AND RELATIONSHIP WITH GEOTECTONIC EVOLUTION

中国东部中新生代火山岩带,记录了自中生代以来陆缘内侧地壳构造发展的演化过程和特征。也反映了构造岩浆的形成机制。其中特别是玄武岩类,在整个演化过程中,在空间分布和成分变化上反映出了很明显的规律性。本文就作者所获得和已知的地质、岩石化学等方面的资料,着重探讨玄武岩类的形成特点及其与大地构造演化的相关性。

A Mesozoic-Cenozoic volcanic belt in Eastern China is controlled by fault systems represented by gravity gradient belts. One of the fault systems is Daxinganling-Taihang shan-Wulingshan abyssal fault system and the other is known as Tancheng-Lujiang abyssal fault system. Along them, the Mesozoic- Ccnozoic volcanic rocks occur intermittently and between them is a huge belt of fault-basin from the north to the south, including the basins of Songliao, Xialiaohe, Huabci and Jianghan.The Jurassic rocks are characterized by andesitc and andesite-basalt and no true basalt was found. In Early Cretaceous occurred volcanic association represented by basalt (alkali basalt) -andesite-trachandesite-liparite assemblage. The basalts in the basins and between the two faults appear to be similar to tholeiite and to be higher and higher alkali towards the lateral sides. This case is more considerable in Early Tertiary. These were in a tectonic environment characterized by most enten-sience stress in the Early Cretaceous-Early Tertiary. The earth's crust was thinning, and the thinnest part is only 28-32Km. In this case, the SiO2 saturation of the basalt appears to become slightly unsatured. These features indicate that a rift mechanism seems to exist in course of forming the double fault-volcanic eruption and the faulted basins in Eastern China. This inference is supported by strontium isotopic data of which the initial Sr37/Sr36 ratios from the exhibit only a small range (0.705-0.709) that seems to be the same to that of typical rifts, such as Oslo and West African Rifts.The combination of the step like faulted basins arranged northeast between two faults in the formation of a huge fossil rift basin in Early Cretaceous-Early Tertiary. Because of the spreading of the Japan Sea in Late Tertiary-Quaterary, the development of the rift was interurrupted by compressional stress and tend to end, while the basalts appear to be of highly SiO2 contents and to be highly enriched in alkaline.On all accounts, the continental volcanic belt in Eastern China has possibly not an origin of subduction zone type, but a genetic mechanism of rift. This hypothesis may be supported by existing Early Mesozoic uplift, the intrusion of alkaline and granite, the deposits of molasse (190-240 m. y), volcanic eruptions and formation of faulted basins, esecially the change of basalt in composition, strontium isotopic characters, and the formation of fossil rift basin.