藏东南超镁铁质岩墙及其地质意义

Tectonics implications of the ultramafic dykes in southeastern Tibet

青藏高原东南部出露有大量近东西向展布的(超)镁铁质岩墙,锆石U-Pb年龄聚集在145～130 Ma之间。矿物学及岩石学结果显示:哲古错地区的亚碱性超镁铁苦橄质岩石源于150～180 km深处,5～6 GPa下石榴石二辉橄榄岩地幔高比率的部分熔融;碱性超镁铁质岩石源于石榴石—尖晶石二辉橄榄岩转换带,是低比率部分熔融的产物;镁铁质岩石的岩浆在演化过程中遭受了低压下结晶分离作用的改造,不能代表原始岩浆的成份。藏东南(超)镁铁质岩石是不同深度地幔物质不同比率部分熔融的产物,亚碱性超镁铁苦橄质岩墙的出现指示了晚侏罗—早白垩印度板块北缘存在的高比率部分熔融的岩浆活动事件,暗示了地幔柱的存在和影响。

The ages of the east-west trending mafic-uhramafic dykes of southeast Tibet Plateau range between 145 Ma and 130 Ma. In Zhegu area, the subalkaline ultramafic picritic dykes are interpreted to have been highly partial melted from garnet lherzolite mantle source in the depth of 150 - 180 km, 5 - 6 GPa; alkaline ultramafic dyke tends to be the result of low partial melting in the spinel-garnet transitional zone; the mafic dykes＇ magmas have experienced partial crystallization at relatively low pressures and no longer stand for the primary magmas. Subalkaline ultramafic picritic dykes in southeast Tibet suggest the high degree partial melting event,indicating the existing of a mantle plume carrying huge energy and/or volatile in the northern Indian continental margin during late Jurassic to early Cretaceous.