西昆仑冰山地区白垩纪E-MORB型辉长岩:新特提斯弧后伸展的新证据

Early Cretaceous E-MORB gabbros in the Bingshan area,West Kunlun orogenic belt:New evidence for back-arc extension of Neo-Tethys

青藏高原西北缘的西昆仑—喀喇昆仑造山带是研究特提斯构造演化的重要地区之一。近期在西昆仑南部冰山地区新识别出白垩纪辉长岩脉。LA-ICP-MS锆石U-Pb定年结果显示,冰山辉长岩最年轻锆石集中在109~103 Ma,加权平均年龄为106.2±1.7 Ma,表明冰山辉长岩形成于早白垩世。岩石含有前寒武纪年龄锆石,指示区内地壳深部存在对应时代的基底物质。辉长岩低硅(48.52%~50.69%)、高铝(14.38%~16.34%)、高镁(MgO=6.88%~8.02%,Mg#=52~58)特征,以及类似E-MORB的稀土配分模式((La/Yb)N=1.36~1.81),不相容元素原始地幔标准化的蛛网图和亏损的全岩Nd同位素组成(εNd(t)=2.54~4.04),表明母岩浆形成于相对亏损的地幔源区,且在上升过程中受到地壳混染程度有限。综合考虑以上特征并结合西昆仑—帕米尔地区晚中生代研究进展,认为冰山辉长岩脉形成于受俯冲板片释放流体交代形成的软流圈地幔部分熔融,其成因与白垩纪中期新特提斯北向平板俯冲引起的弧后伸展作用有关。

The West Kunlun-Karakorum orogenic belt in the northwestern margin of the Tibetan Plateau is one of the key areas to understand the Tethyan evolution.Recently,we recognized Early Cretaceous E-MORB type gabbro dykes in the Bingshan area,West Kunlun orogenic belt.The youngest ages of LA-ICP-MS zircon U-Pb dating results ranging from 109 Ma to 103 Ma,with a weighted mean age was 106.2±1.7 Ma,demonstrating that the gabbros were emplaced in the Early Cretaceous.Several Precambrian zircon ages were found in the Bingshan gabbro,indicating for the existence of Precambrian basements in the deep crust under the Bingshan area.Geochemically,the Bingshan gabbros show relatively low contents of SiO_(2)(48.52%~50.69%),high contents of Al2O3(14.38%~16.34%)and Mg(MgO=6.88%~8.02%,Mg#=52~58),with E-MORB-like rare earth element patterns((La/Yb)N=1.36~1.81)and trace element spider diagram and depleted whole-rockεNd(t)values(2.54~4.04).We prefer that the parental magma of the Bingshan gabbros was originated from a relatively depleted mantle source and underwent insignificantly assimilation and fractional crystallization before their final emplacement.In combination of the above characteristics and the Late Mesozoic breakthrough in the West Kunlun-Pamir region,it is concluded that the Cretaceous Bingshan gabbro dikes were formed by partial melting of the asthenospheric mantle metasomatized by fluid released by the subducted Neo-Tethys oceanic slab,and its origin is related to the back-arc extension caused by the northward subduction of the Neo-Tethys in the Middle Cretaceous.