藏东碧土地区古特提斯主洋盆中的亚速尔型洋岛玄武岩.pdf

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第25卷第7期
地质通报
Vol 25. No.7
2006年7月
GEOLOGICAL BULLETIN OF CHINA
Jul.,2006
基础地质?
藏东碧土地区古特提斯主洋盆中的
亚速尔型洋岛玄武岩
吴根耀
WU Gen-yao
中国科学院地质与地球物理研究所,北京100029
I stitute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
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。常量元肃的判別图解表明
其构造背景是洋岛。微量元素含量显示出大洋板内玄式岩的特点。稀土元素配分型式表现为一组近于平行的右倾曲线,(Ce/
Yb)値约为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,稀土配分曲线和徴量元素蛛网曲线均落
入OIB与Eー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个亚遠尔型洋岛的厘定,结合以前报道的夏成夷型洋岛(瓦浦火山岩),充分说明藏东的古特提斯主洋盆曾是
个开阔的多岛洋,与滇西南昌宁一孟连多岛洋的构造面貌相同。
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;蔑东碧土地区
中阳分类号:P542+.4;P588.14+5
文献标识码:A
文章编号:1671-2552(2006)07-0772-10
Wu G Y. Azores-type oceanic sland basalts in the Paleo-tethyan major oceanic basin in the Bitu area, eastern Ti-
bet, China. Geological Buletin of China, 2006, 25(7): 772-781
Abstract: The subducted major Paleo-tethyan ocean in eastern Tibet is represented by the Bitu ophiolitic melange
great majority of volcanic rocks at the Jushui, Xueba and East Dengba sections in the Bitu area are alkali basalt. The discrim
inant diagrams of major elements show that the tectonic setting of the alkali basalt was oceanic island in the Major Paleo
Tethyan oceanic basin. The trace element contents show the characteristics of within-plate basalt. The REE patterns of the
basalt are a group of subparallel, right-inclined cures, with (Ce/Y)N6, and the total REE content increases with increas-
ing degrees of magmatic diferen aion Generally, these oceanic islands occurred during the Carboniferous when the develop
ment 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 an formation, with oceanic tholeiite intercalations in the lower part of the vo
canic succession, and its REE distribution patterns and trace element spidergram fall between OIB and E-MORB. The East
Dengba section is a record of the late stage of oceanic iland development, with interbeds of acid lavas at the top, overlain
by continuous deposits of limestone and sands one. 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 easter Tibet may have been an open archipelagic ocean, whose tectonic framework is the same as that of the
Changning-menglian archipelagic ocean in southwestern Yunnan
Key words: Azores-type oceanic island; alkali basalt; petrochemistry; Paleo-tethyan major oceanic basin; Bitu area in east-
em Tibet
收稿日期:2006-02-18;修订日期:2006-03-22
金项目:国家自然科学基金重点项目(编号:49232030和49732100)成果之ー。
作者简介:吴根耀(1946-),男,研究员,从事岩石大地构造和区域大地构造研究。E-mail:dzkx@mail.igcas,ac.cn