藏北中生代火山岩的岩石化学与地球化学初步研究

在西藏冈底斯山以北、唐古拉山以南地区,分布着侏罗纪到白垩纪的基性和中性火山岩。它们与本区的超基性岩空间上接近,或者呈断层接触。对这些岩石组合的成因有不同的解释。有人认为是大陆碰撞缝合带的蛇绿岩(常承法与郑锡澜,1974),有人认为是岛弧与边缘海的产物。近来,张旗(1983)在丁青发现了比较连续的蛇绿岩组合。要正确地对这些岩石的构造环境进行解释,必须进行详细的岩石学、岩石化学和微量元素地球化学研究。本文对采自拉弄沟、罗布中(图1)两地的玄武岩和采自切里湖的安山岩的岩石化学和微量元素地球化学做了初步研究,这里将初步结果及讨论作一报道。

PRELIMINARY STUDY ON THE PETROCHEMISTRY AND GEOCHEMISTRY OF MESOZO1C VOLCANIC ROCKS FROM NORTHERN XIZANG

There are some exposures of Mesozoic volcanic rocks spreading in Northern Xizang between Kangdese Range and Tanggula.Some pillow basalts are spatially close to or contact in fault with ultramafic bodies.Therefore,several different explanations on the tectonic environment and origin of these rock associations have suggested.Some authors proposed that the rock associations represent the ophiolite complex in a collision suture and others considered that they were the products of arc and marginal sea magmatism.In order to examine the availability of these interpretations,the major and trace element compositions of the volcanic rocks from Lanonggou and Luobuzhong were analysed with wet chemical and instrument neutron activation methods.For the sake of comparation,the trace elements of andesites from Qielihu and some gabbros and peridotites from adjacent districts were also analysed.Through the petrochemical and geochemical studies,some ideas can be suggested as follow:(1) According to the Fe₂O₃ + FeO +1/2(MgO + CaO)-Al₂O₃/SiO₂ variation diagram,most of Lanongguo and Luobuzhong volcanic rocks belong to basalts.The major and trace element compositions of these basalts indicate that they have transitional characteristics from arc basalt to oceanic tholeiite.REE patterns of Lanonggou basalts are similar to those of MORE and those of Luobuzhong to that of island arc tholeiite.(2) The chondrite-normalized transitional metal patterns of Lanongguo and Luobuzhong basalts lie between those of arc and oceanic tholeiite,and are more similar to that of MORB.(3) The chondrite-normalized REE patterns of Northern Xizang basalts,gabbros and peridotites show a very apparent sequence of variation which implies that these rocks may be the pillow lava and cumulates of an ophiolite complex.(4) The trace element compositions of Qielihu andesites exhibit the characteristics of island arc andesite,(5) Based on the petrochemistry and geochemistry,it can be proposed that the Northern Xizang basalts and mafic-ultramafic cumulates are the component parts of an ophiolite origined in an interarc basin which separated the remain arc in north from the front arc in Kangdese Range.