攀西三个镁铁-超镁铁质岩体的地球化学及其对源区的约束  

Geochemistry of three layered mafic-ultramafic intrusions in the Panxi area and constraints on their sources.

作　　者：张招崇[1] 李莹[1] 赵莉[1] 艾羽[1] 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,北京100083

出　　处：《岩石学报》2007年第10期2339-2352,共14页Acta Petrologica Sinica

基　　金：国土资源部百名优秀青年计划;教育部新世纪人才支持计划(NCET-04-0728)的部分成果;高等学校学科创新引智计划(B07011)资助。

摘　　要：在峨眉山大火成岩省的攀西地区的南北向断裂带内分布着一系列层状镁铁-超镁铁质岩体,它们在时空上与峨眉山玄武岩有着密切的联系,代表了岩浆管道系统。本文选择了3个代表性岩体,含钒钛磁铁矿的攀枝花岩体,含铜镍硫化物矿床的力马河岩体以及同时含钒钛磁铁矿和铜镍硫化物铂族元素矿床的新街岩体进行了主要元素、微量元素和Sr-Nd同位素地球化学研究。总体上新街岩体在地球化学特征上均介于攀枝花岩体和力马河岩体之间。在主要元素特征上,攀枝花岩体相对富Fe、Ti、Ca、Al,而力马河岩体则相对富Mg。3个岩体的稀土和微量元素标准化曲线均与丽江苦橄岩相似,均显示出LREE和LILE的相对富集,但是力马河岩体有明显的负Nb、Ta和Ti异常,反映了源区有地壳物质或岩石圈地幔的混染。在log(Th/Yb)_(PM)vs.log(Ta/Th)_(PM)图解上,3个岩体显示出线性关系,其中攀枝花岩体具有高的(Ta/Th)_(PM)和低的(TH/Yb)_(PM)值,而力马河岩体则显示出相反的特征,并且该图解也暗示了攀枝花岩体有岩石圈地幔的混染,力马河岩体有上地壳物质的混染,而新街岩体则有下地壳物质的混染。在Sr-Nd同位素特征上,3个岩体也有明显的区别,其中新街岩体具有较均一的(^(87)Sr/^(86)Sr)_t(0.70610～0.70636)和ε_(Nd)(t)值(1.01 to 1.75),力马河岩体则具有高的(^(87)Sr/^(86)Sr)_t(0.70631～0.70930)和低的ε_(Nd)(t)值(-0.85 to-4.13),而攀枝花岩体则与丽江苦橄岩的Sr-Nd同位索组成(0.70434～0.70510,1.1～3.2)相似。然而,3个岩体的Sr-Nd同位素成分(除力马河岩体部分样品外)均落在峨眉山玄武岩和洋岛玄武岩(OIB)内,因此这3个岩体可能均来自于地幔柱,但地幔柱熔体在上升过程中受到了不同物质的混染,其中,攀枝花岩体的源区为地幔柱和岩石圈地幔,新街岩体为地幔柱和下地壳,而力马河岩体则为地幔柱和上地壳,其源区的特征可能反映A series of layered mafic and ultramafic intrusions aligned along a N-S trending fault zone are exposed in the Panxi area of the Emeishan continental flood basalt province, SW China. They are temporally and spatially associated with flood basalts and therefore considered to represent the magma storage system. Three representative intrusions, i.e. Panzhihua (V-Ti ores), Limahe (Cu-Ni ores) and Xinjie (V-Ti and Cu-Ni-PGE), were chosen for major element, trace element and Sr-Nd isotope studies. The Xinjie intrusion is geochemically intermediate between the Panzhihua and Limahe intrusions. In terms of the features of major elements, the Panzhihua intrusion has relatively high Fe, Ti, Ca and Al contents, whereas the Limahe intrusion has relatively high Mg contents. All the three intrusions have similar chronrite-normalized REE patterns and primitive mantle-normalized trace element patterns, which resemble those of the Lijiang picrite that is characterized by slight enrichment of LREE and LILE. However, the Limahe intrusion displays negative Nb, Ta and Ti anomalies, suggesting modest contamination with crustal materials or the lithospheric mantle in the source area. They display an approximately linear trend on the diagram of log (Th/Yb)(PM) vs. log (Ta/Th)(PM), in which the Panzhihua intrusion displays higher (Ta/Th)(PM), and lower (Th/Yb)(PM), whereas the Limahe intrusion have reverse ratios as. compared with the former, possibly implying contamination with upper crustal materials, and the Xinjie intrusion falls between them, showing contamination with lower crust. Each intrusion has a quite distinct isotopic composition. The Xinjie intrusion has homogeneous (Sr-87/Sr-86), ratios (0.70610 - 0.70636) and epsilon(Nd) (t) values 1.01 to 1.75). The Limahe intrusion is characterized by high (Sr-87/Sr-86), ratios (0.70631 - 0.70930) and low epsilon(Nd) (t) values (-0.85 to -4.13), whereas Panzhihua has the similar isotopic composition to that of the Emeishan picrite: 0.70434 - 0.70510 for (Sr-87/Sr-86)(t) and 1.1 similar t

关 键 词：层状镁铁-超镁铁质岩体 地球化学 混染 地幔柱 攀西 

分 类 号：P588.125[天文地球—岩石学]