大兴安岭吉峰地区中元古代科马提岩及成因类型

THE MESOPROTEROZOIC KOMATIITES AND THEIR GENETIC TYPE IN THE JIFENG AREA OF DAXINGANLING

大兴安岭吉峰地区的橄榄质科马提岩具有良好的显微鬣刺结构,在橄榄石和辉石晶体间隙中充填有基质物质,说明其为火山岩.通过研究,对其初步划分了冷凝带和鬣刺带,它们构成一个不完整的冷凝单元.吉峰科马提岩在岩石化上表现了良好的科马提岩属性,与南非及西澳大利亚橄榄质科马提岩一致.岩石地球化学研究表明,科马提岩及与其伴生的玄武岩具有紧密的成因联系,它们的演化符合科马提岩—拉斑玄武岩的演化趋势,具良好的正相关特点.在主量元素、微量元素及REE特征上,本区科马提岩具有较好的II类科马提岩属性,与南非巴伯顿、印度、西格陵兰II类科马提岩极为相似.吉峰科马提岩的εNd(T)=+7.51,说明源于亏损地幔,并且以LREE轻度富集为特征,显示它们系亏损地幔经较小程度部分熔融产生的.吉峰拉马提岩的形成时代为中元古代,其形成环境可能是地壳拉伸减薄,上涌的科马提质岩浆在淬火快速冷却的条件下形成.这一岩石类型的发现为研究大兴安岭地区元古宙大地构造演化提供了一个新线索.

The peridotitic komatiites in Jifen g area of Daxinganling are with fine microspinifex texture.The matrix materials existing between olivine and augite crystals indicate a volcanic origin.Systematic research has initialy iden-tified a chill zone and a spinifex zon e,which constitute an incomplete condensate unit.The petrochemical ch aracteristics of the Jifeng komatiites are in accordance with the peridotitic komatiit es in South Africa and Western Austra lia.The geochemistry study shows a close gen etic relationship between the komatiites and the associated basalts ,wi th a positive correlation,conforming to the evolutionary trend of komatiite-tholeiite.The cardinal elements,trace elements and REE of the komatiites in the area present a p erfect attribute of type II komatiit e,which is compatible with the type I I komatiite in South Africa,India,and West Gree nland.The Jifeng komatiites are characterized by low LREE affluence,wi thε Nd (T)=+7.51,showing that the komatiites are derived from partially melted depleted mantle.The komatiites were f ormed in Mesoproterozoic,in a tectonic environment of tensile crust,while the r ising komatiite magma was quenched a nd rapidly cooled.The Jifeng komatiites provide a new key for research of Mesoprote rozoic tectonics in Daxinganling area.