藏东碧土地区古特提斯主洋盆中的亚速尔型洋岛玄武岩  

Azores-type oceanic island basalts in the Paleo-Tethyan major oceanic basin in the Bitu area, eastern Tibet, China

作　　者：吴根耀[1] 

机构地区：[1]中国科学院地质与地球物理研究所,北京100029

出　　处：《地质通报》2006年第7期772-781,共10页Geological Bulletin of China

基　　金：国家自然科学基金重点项目(编号:49232030和49732100)成果之一。

摘　　要：采自藏东碧土蛇绿混杂岩带的据水、学巴和登巴东3条剖面的火山岩样品,主体是碱性玄武岩。常量元素的判别图解表明其构造背景是洋岛。微量元素含量显示出大洋板内玄武岩的特点。稀土元素配分型式表现为一组近于平行的右倾曲线,(Ce/Yb)N值约为6,稀土总量随岩浆分异程度的增高而增加。洋岛总体发育的时间为石炭纪(古特提斯洋盆的全盛期),属亚速尔型洋岛。其中,学巴剖面代表了洋岛形成的初期阶段,其下部有大洋拉斑玄武岩的夹层,稀土配分曲线和微量元素蛛网曲线均落入OIB与E-MORB之间;登巴东剖面则代表了洋岛火山岩发育的晚期阶段,顶部有酸性熔岩的夹层,火山岩之上连续沉积有灰岩和砂岩。对这3个亚速尔型洋岛的厘定,结合以前报道的夏威夷型洋岛(瓦浦火山岩),充分说明藏东的古特提斯主洋盆曾是一个开阔的多岛洋,与滇西南昌宁-孟连多岛洋的构造面貌相同。The subducted major Paleo-Tethyan ocean in eastern Tibet is represented by the Bitu ophiolitic melange. The great majority of volcanic rocks at the Jushui, Xueba and East Dengba sections in the Bitu area are alkali basalt. The discriminant diagrams of major elements show that the tectonic setting of the alkali basalt was oceanic island in the Major PaleoTethyan oceanic basin. The trace dement contents show the characteristics of within-plate basalt. The REE patterns of the basalt are a group of subparaUd, right-inclined curves, with （Ce/Yb）N≈6, and the total REE content increases with increasing degrees of magrnatic differentiation. Generally, these oceanic islands occurred during the Carboniferous when the development of the Paleo-Tethyan oceans reached their culmination, and they should belong to the Azores-type ones. The Xueba section represents an early stage of oceanic island formation, with oceanic tholeiite intercalations in the lower part of the volcanic succession, and its REE distribution patterns and trace dement spidergram fall between OIB and E-MORB. The East Dengba section is a record of the late stage of oceanic island development, with interbeds of acid lavas at the top, overlain by continuous deposits of limestone and sandstone. The three Azores-type oceanic islands, combined with the Hawaiian-type oceanic island （the Wapu volcanic rocks） reported previously by the author, indicate that the major Paleo-Tethyan oceanic basin in eastem Tibet may have been an open archipelagic ocean, whose tectonic fi＇amework is the same as that of the Changning-Menglian archipelagic ocean in southwestem Yurman.

关 键 词：亚速尔型洋岛 碱性玄武岩 岩石地球化学 古特提斯主洋盆 藏东碧土地区 

分 类 号：P542.4[天文地球—构造地质学] P588.145[天文地球—地质学]