冀北小张家口基性-超基性杂岩的成因:岩石学、地球化学和Nd-Sr同位素证据  

Petrogenesis of the Xiaozhangjiakou Mafic-Ultramafic Complex, North Hebei: Constraints from Petrological, Geochemical and Nd-Sr Isotopic Data

作　　者：陈斌[1] 田伟[1] 刘安坤[1] 

机构地区：[1]北京大学地球与空间科学学院,造山带与地壳演化重点实验室,北京100871

出　　处：《高校地质学报》2008年第3期295-303,共9页Geological Journal of China Universities

基　　金：科技部973项目(2006CB403501);国家自然科学基金(40625005)共同资助

摘　　要：冀北小张家口基性-超基性杂岩体主要由辉石岩和次要的纯橄岩组成,并含有方辉橄榄岩透镜状捕掳体。岩石学和元素地球化学数据表明,辉石岩和纯橄岩可能是堆晶成因,而方辉橄榄岩可能是来自地幔源区的地幔岩捕掳体(抽取玄武质熔体之后的难熔残余)。辉石岩和纯橄岩都表现出相当富集的Nd-Sr同位素特征,εNd(t)=-2^-10,ISr=0.7045~0.7081,表明它们的母岩浆来自富集的岩石圈地幔的部分熔融。化学成分和同位素成分一致表明,母岩浆在以橄榄石和辉石为主的分离结晶过程中,遭受下地壳不同程度的混染。辉石岩受显著混染,不能代表岩石圈地幔的同位素成分;而纯橄岩是最初的分离结晶体,仅受轻微下地壳混染,可近似反应岩石圈地幔源区的同位素成分。小张家口岩体的形成可能与古亚洲洋的缝合(240~250Ma)有关。The Xiaozhangjiakou mafic-uhramafic complex, north Hebei, is composed mainly of pyroxenites and subordinate dunite. Lenses of harzburgites can be seen in the complex. Penological and geochemical data suggest a cumulate origin for the pyroxenites and dunites, while harzburgites represent mantle xenoliths （mantle residues after extraction of basaltic melts）. The pyroxenites and dunites show significantly enriched Nd-Sr isotopic compositions, with εNa（t）=-2～-10 and Isr=0.7045～0.7081, suggesting that their parental magma was derived from partial melting of enriched mantle portions. Geochemical and isotopic data consistently demonstrate that variable proportions of lower continental crustal components were incorporated in the marieultramafic complex during the processes of olivine- and pyroxene-dominated fractional crystallization. The pyroxenites experienced the most significant crustal contamination, and cannot be representative of genuine isotopic features of the mantle source. The dunites, however, was the first cumulate crystallized from magma and was contaminated only slightly by crustal components. Therefore, the Nd isotopic composition （εNa（t）=-1.9） of the dunites could be close to that of the mantle source. Formation of the Xiozhangjiakou complex was probably related to the closure of the Ancient Asian Ocean at about 240～250 Ma.

关 键 词：堆晶体 辉石岩 纯橄岩 地球化学 ND-SR同位素 冀北 

分 类 号：P588[天文地球—岩石学] P591[天文地球—地质学]