浙江白垩纪火山岩的地层划分及钕同位素特征

浙江晚中生代火山岩系的划分和对比一直存在不同认识。根据近年来获得的新资料,结合前人的研究成果,对有关浙江白垩纪火山岩的地层划分和对比提出浙江白垩纪火山岩Nd同位素组成主要变化规律为:不同大地构造单元(浙西扬子地块和浙东华夏地块)的火山岩Nd同位素组成差异较大,显示出古老变质基底岩石对火山岩Nd同位素组成的制约;建德群下部劳村组和黄尖组以及磨石山群下部的大爽组、高坞组、西山头组火山岩εNd值变化不大,而与上部的寿昌组以及九里坪组火山岩εNd值有明显的不同,表明两个旋回火山岩物质来源有一定差异;火山岩的εNd值随时代变新有增大的趋势。Nd同位素示踪信息也就有可能作为火山地层划分和对比的重要依据之一。

STRATIGRAPHIC SUBDIVISION AND NEODYMIUM ISOTOPE CHARACTERISTICS OF THE CRETACEOUS VOLCANIC ROCKS IN ZHEJIANG PROVINCE

There is a long-standing controversy over the stratigraphic subdivision and correlation of the Late Mesozoic volcanic rocks in Zhejiang Province. On the basis of previously published data and new data collected in recent years, this paper discuss some tentative proposals with regard to the stratigraphic subdivision and correlation of the Late Mesozoic volcanic rocks in Zhejiang Province. Several patterns of Neodymium (Nd) isotope variation in these volcanic rocks have been recognized: the Late Mesozoic volcanic rocks in different tectonic units (e.g. Yangtze Block in western Zhejiang and Cathaysia Block in eastern Zhejiang) are characterized by different Nd isotope values, reflecting possible control on Nd isotope signature by basement rocks; the ε_Nd values of volcanic rocks in the Laocun and Huangjian formations of the lower Jiande Group are comparable to those in the Dashuang, Gaowu and Xishantou formations of the lower Moshishan Group, but they are distinct from those in the Shouchang and Jiuliping formations in the upper Jiande Group, the distinct Nd isotope values indicate different magma sources for the volcanic rocks in the lower and upper Jiande Group; the ε_Nd values of the Late Mesozoic volcanic rocks in Zhejiang Province appear to increase with time. Thus, Nd isotopes may serve as an important basis for stratigraphic subdivision and correlation of the Late Mesozoic volcanic rocks in Zhejiang Province.