西藏那木如辉绿岩岩脉的年代学和地球化学特征

那木如辉绿岩具有较高的TiO2和P2O5含量,Mg#是0.6～0.7,表明岩石经中等程度或轻微的斜长石分离结晶作用；富集Rb、K、Th、Ba,亏损Nb、Ti、Ta和Zr、Hf,轻稀土富集,且具有负Eu的异常,表明岩石具有岛弧岩浆的性质.对那木如辉绿岩进行SHRIMP锆石U-Pb定年,其形成年龄值为64±1 Ma(2σ,MSWD=2.4),即辉绿岩结晶年龄.本次定年的结果正好处于西雅鲁藏布北亚带洋盆向北对拉萨地体南缘之下的俯冲(晚白垩世至早第三纪)时期内,说明那木如辉绿岩为此次俯冲形成的岛弧岩浆岩.位于冈底斯弧南侧的西雅鲁藏布北亚带蛇绿岩被第三系磨拉石不整合覆盖,说明蛇绿岩侵位时间早于第三系,与那木如辉绿岩的侵位年龄接近.由此可见,正是形成那木如岛弧岩浆岩的俯冲构造事件导致了西雅鲁藏布北亚带蛇绿岩的构造侵位.

Chronology and Geochemical Characteristics of Namuru Island Arc Volcanics in Tibet

The Mg# of the Namuru diabase is 0.6-0.7 indicates that the Namuru diabase crystallized from primitive magma.The Namuru diabase has relatively high contents of TiO2 and P2O5,enriches large ion lithophile elements(such as Rb,K,Th,Ba and light rare earth elements),depletes high field strength elements(such as Nb,Ti,Zr,and Hf),and appears negative Eu anomaly.These geochemical characters show that the Namuru diabase resulted from typical island arc magma.The SHRIMPII U-Pb dating gave a weighted average age of 64±1 Ma(2σ,MSWD = 2.4) for the crystallization time of the Namuru diabase(Sample 3X-423),showing that the Namuru diabases are island arc igneous rocks formed in the subduction of the north sub-zone of WYZS to south margin of Lhasa block during the late Cretaceous to the early Tertiary.Tertiary molasse covers the north sub-zone of west WYZS of south Gangdise arc unconformably indicates that the emplacement time of ophiolites is earlier than Tertiary and is similar to the emplacement age of the Namuru diabases.Therefore,we think that the subduction tectonic event of developing the Namuru island igneous rocks is the reason for the tectonic emplacement of the ophiolite in the north sub-zone of WYZS.