关于冀东太古宙蛇绿岩的几个问题  

COMMENT ON ARCHEAN OPHIOLITE IN EASTERN HEBEI

作　　者：张旗[1] 倪志耀[2] 翟明国[1] 

机构地区：[1]中国科学院地质与地球物理研究所,北京100029 [2]成都理工大学地球科学学院,四川成都610059

出　　处：《地学前缘》2003年第4期429-437,共9页Earth Science Frontiers

基　　金：国家自然科学基金委员会(49902005);中国科学院知识创新工程(KZCX 1-07)

摘　　要：最近美国学者Kusky博士和北京大学李江海教授在各种国际会议和Science等报章杂志上,报道他们在冀东发现了世界上最古老的25亿年的东湾子蛇绿岩以及遵化和辽西地区的蛇绿混杂岩和豆荚状铬铁矿。冀东是否存在蛇绿岩存在争论。我们的研究表明,所谓的太古宙东湾子蛇绿岩尚难成立。原因是他们所报道的蛇绿岩的所有组成岩石均多多少少存在问题:(1)东湾子蛇绿岩是否存在地幔橄榄岩还不能确定,Kusky和李江海等所说的地幔橄榄岩或方辉橄榄构造岩(harzburgite tectonite),经研究主要由富铁质的辉石岩和角闪石岩组成,且与辉长岩单元之间未直接接触,因此,上述超基性岩是否是太古宙的也不能肯定。(2)辉长岩的矿物组合中有少量原生黑云母出现,是蛇绿岩组合中罕见的。(3)席状岩墙群不成立,既无对称的冷凝边,也无不对称的冷凝边;所谓的岩墙群主要由辉石岩和角闪石岩组成,并非基性岩浆:辉石岩和角闪石岩的地球化学特征暗示其可能来自板块内部构造环境,也非蛇绿岩的特点。(4)与超基性岩伴生的铬铁矿富Fe和Cr,类似于产于古老地块的、与富铁质超基性岩有关的铬铁矿的特征,而非蛇绿岩的豆荚状铬铁矿。(5)缺少能够支持东湾子为蛇绿岩的详细和系统的地球化学资料。因此,根据目前的认识,冀东太古宙是否存在蛇绿岩仍然是一个不确?Dr. Kusky and Prof. Jianghai Li recently reported the discovery of the oldest worldwide ophi-olite (2. 5 Ga) in the Dongwanzi area of eastern Hebei Province and ophiolite melange and podiform chromites in Zunhua area and western Liaoning Province on several international congresses and on Science as well as other Chinese and international journals. However, it is still doubtful of the existence of ophiolite in the eastern Hebei. Our recent research for these rocks indicates that the so called Archean Dongwanzi ophiolite is not recognizable. The rock assemblages of the reported ophiolite are very questionable. Firstly, it cannot be confirmed yet the existence of mantle peridotite. The mantle peridotite or harzburgite tectonite which is described by Kusky and Li is virtually composed of Fe-rich pyroxenite and amphibolite, without direct contact with gabbro unit. Therefore, it cannot be confirmed that the above ultramafic rocks formed in Archean. Secondly, it is quite rare for an ophiolite section that primary bio- tite occurs in the gabbro. Thirdly, the reported sheeted dyke swarms are not actually sheeted dyke swarms, because they do not show either symmetrical chilled border or asymmetrical chilled border. The so called dyke swarms are composed of pyroxenite and amphibolite, which is not a mafic magma. Moreover, the geochemistry of these pyroxenite and amphibolite indicate that they are probably formed in within-plate tectonic setting, which is different from that of the ophiolite. Fourthly, unlike the characteristics of podiform chromite in the ophiolite sections, the reported chromite associated with the ultra-mafic rocks is enriched in Fe and Cr, similar to that of the chromite from Fe-rich ultramafic rocks in the old terranes. And finally, there is no comprehensive geochemical data to demonstrate whether the rocks occurred in the Dongwaizi area is ophiolite. It is therefore suggested here that it is too early to affirm that Archean ophiolite occurred in the eastern Hebei.

关 键 词：冀东 太古宙 蛇绿岩 质疑 

分 类 号：P588.1[天文地球—岩石学]