浙东南中生代晚期火山岩Nd同位素组成及其地层学意义

Nd Isotopic Systematics of the Late Mesozoic Volcanic Rocks from Southeasterm Zhejiang Province, China: Implications for Stratigraphic Study

浙江东南部中生代晚期火山地层可划分为三个火山旋回 ,通过对不同层位火山岩Nd同位素组成的研究 ,得出结果 :第一至第三旋回火山岩 (自下而上 ) εNd(t)值依次为 - 14.1～ - 6.2 ,平均 - 9.9;- 7.7～ - 6.4 ,平均 - 7.0及 - 7.5～ +0 .1,平均 - 4 .6。t2DM值依次为 1.6～ 1.8Ga ,平均 1.7Ga ;1.4～ 1.6Ga ,平均 1.5Ga及 0 .9～ 1.5Ga ,平均 1.3Ga。由此可见 ,在不同的火山旋回之间具有明显的Nd同位素组成特征差异 ,它反映了该区中生代晚期火山活动随时间的推移 ,在构造环境和岩浆来源上所发生的有序变化 ,从而 ,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。

Late Mesozoic volcanic rocks in southeastern Zhejiang Province can be subdivided into three stratigraphic cycles. The ε Nd (t) values for these three cycles, from bottom to top, are -14.1 to -6.2, with an average of -10.0; -7.7 to -6.3, with an average of -7.0; and -7.5 to 0, with an average of -4.6, respectively. T 2DM for the three cycles are 1.6 to 1.8, with an average of 1.72Ga; 1.4 to 1.6, with an average of 1.48Ga; and 0.9 to 1.5, with an average of 1.31Ga, respectively. Volcanic rocks from different cycles show significant variation in Nd isotopic compositions, indicating that the tectonic environment of formation and the magma source of volcanic rocks varied with time. Therefore, Nd isotopic compositions of the volcanic rocks are very important in stratigraphic division and comparison.