Adakitic granites within the NE Jiangxi ophiolites, South China: geochemical and Nd isotopic evidence
Adakitic granites within the NE Jiangxi ophiolites, South China: geochemical and Nd isotopic evidence
复制标题
DOI：
10.1016/s0301-9268(02)00206-1
复制
发表时间：
2003-04
影响因子：
3.8
通讯作者：
Wenjie Li
中科院分区：
文献类型：
--
作者：
Wenjie Li
报道了中国南部新元古代赣东北蛇绿岩中约970 Ma的钠长石花岗岩的地球化学和Nd同位素资料。这些花岗岩富含Al_2O_3(15-17%)、Na_2O(7.3-8.5%)、Mg#(43-62)、Sr(383-920ppm)、Sr/Y(166-444)和Y/Yb(12-14)，但K2O(0.2-0.6%)、TiO(0.3%)、P_2O_5(<；0.1%)含量较低。此外，它们在约970 Ma时显示出一致的高的、类似MORB的初始εND值+4.6至+6.7.这些地球化学和同位素特征与现代埃达克岩相似。与现代埃达克岩相比，西湾钠长石花岗岩亏损中重稀土，Y/Yb比值为12~14，源区残留角闪石和石榴石。地质和地球化学特征表明，花岗岩是在足以稳定石榴石和角闪石的压力下，俯冲的细粒状洋壳在低程度的部分熔融作用下形成的。这是赣东北蛇绿岩中首次发现的埃达克质花岗岩。它们的存在表明，侵入蛇绿岩的花岗岩不仅可以像以前认为的那样在海底扩张、洋壳剪切和碎裂的大洋岩石圈的俯冲过程中产生，也可以在大洋岩石圈的俯冲过程中产生。西湾埃达克质花岗岩的SHRIMP锆石U-Pb年龄为968±23 Ma[Geochimica 23(1994)117]，应重新解释为洋壳俯冲的时间，而不是先前解释的洋壳形成年龄。
Geochemical and Nd isotopic data are reported for the ca. 970Ma albite granites within the Neoproterozoic NE Jiangxi ophiolite, South China. These granites are high in Al2O3(15–17%), Na2O (7.3–8.5%), Mg# (43–62), Sr (383–920ppm), Sr/Y (166–444) and Y/Yb (12–14), but low in K2O (0.2–0.6%), TiO2(<0.3%), P2O5(<0.1%). Furthermore, they display consistently high, MORB-like initial εNd values of +4.6 to +6.7 at ca. 970Ma. These geochemcial and isotopic characters are similar to those of the modern adakites. Compared with the modern adakites, the Xiwan albite granites are depleted in middle to heavy REE with Y/Yb ratio of 12–14, which can be attributed to residual amphibole and garnet in the source. Geological and geochemical features suggested that the granites were generated by low degrees of partial melting of subducted, spilitized oceanic crust at pressures high enough to stabilize garnet and amphibole. These are the first adakitic granites identified within the NE Jiangxi ophiolite. Their presence shows that granitic rocks that intrude ophiolites can be generated not only during the processes of sea-floor spreading, oceanic crust shearing and obduction of fragmented oceanic lithosphere as previously thought, but also during the process of subduction of oceanic lithosphere. The SHRIMP U–Pb zircon age of 968±23Ma for the Xiwan adakitic granite [Geochimica 23 (1994) 117] should be re-interpreted as the timing of subduction of oceanic crust, rather than as the age of oceanic crustal formation as previously interpreted.