信阳周庄变辉长岩LA-ICPMS U-Pb定年与华北南缘复杂演化过程

Complex evolution of the southern margin of the North China Craton:Evidence from LA-ICPMS U-Pb dating of zircons in Zhouzhuang meta-gabbro.

河南信阳周庄变基性侵入岩体出露于华北克拉通南缘,主要由斜长石+角闪石(+绿帘石)组成,可见残余辉长结构,属经历了绿帘角闪岩相-低角闪岩相变质的变辉长岩。对变辉长岩的两个样品中的锆石进行了LA-ICPMS U-Pb同位素测年分析。这两个样品都给出了859Ma的谐和上交点年龄,但分别给出了356±82Ma和234±36Ma的谐和下交点年龄。其中一个样品还给出了另外两组谐和的年龄,它们的^(206)Pb/^(238)U表面年龄分别是616±10Ma和442±6.5Ma。我们解释共同的谐和上交点年龄(859Ma)代表着岩体的侵位时代,是Rodinia超大陆裂解事件在华北克拉通南缘的早期记录(如基性岩浆侵入作用);其他四组年龄是岩体后期所经历改造事件的年龄记录,包括新元古代晚期的再次裂解(如变质重结晶作用)和古生代、早中生代时期扬子克拉通向北的俯冲-碰撞对华北南缘的强烈改造过程。

Zhouzhuang meta-basic pluton in Xinyang, Henan Province, locates at the southern margin of the North China Craton. It is mainly composed of plagioclase + amphibole ( + epidote). The relict-gabbro texture can be found in some samples, suggesting that the rocks belong to meta-gabbro experienced epidote-amphibolite facies-low-amphibolite facies metamorphism. Zircons of two samples were analyzed, using LA-ICPMS for U-Pb ages. Both samples give concordant upper intercept age of 859Ma, but concordant lower intercept ages of them are 356 +/- 82Ma and 234 +/- 36Ma, respectively. One sample also gives another two groups of the concordant ages with the Pb-206/U-238 ages of 616 +/- 10Ma and 442 +/- 6. 5Ma. We interpret the upper intercept age of 859Ma as the intruding age of the basaltic magmatism, corresponding to the breakup of the Rodinia Supercontinent, which recorded at the southern margin of the North China Craton; the others record that the pluton underwent subsequent metamorphism and modifications, including the rifting event at the southern margin of the North China Craton in Late Neoproterozoic, and intense modifications due to subduction and collision of the Yangtze Craton in Paleozoic and in Early Mesozoic.