一个开放的岩浆房系统:攀西新街镁铁-超镁铁质层状岩体  

Open-system magma chamber:An example from the Xinjie mafic-ultramafic layered intrusion in Panxi region,SW China.

作　　者：赵莉[1] 张招崇[1] 王福生[2] 郝艳丽[2] 艾羽[1] 杨铁铮[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 [2]中国地质科学院地质研究所,北京100037

出　　处：《岩石学报》2006年第6期1565-1578,共14页Acta Petrologica Sinica

基　　金：教育部新世纪优秀人才支持计划(NCET-04-0728);国土资源部百名优秀青年计划国家自然科学基金(Nos.40273020 and 40172026);973项目(No.G1999043205);美国国家自然科学基金(EAR98-05318)的部分成果。

摘　　要：新街镁铁-超镁铁岩体是与峨眉山溢流玄武岩同期共生的层状岩体之一,可分为三个火成堆积旋回,包括六个岩相带,各旋回下部主要由超镁铁岩组成,上部主要由辉长岩、橄长岩等组成。对第一旋回和第二旋回下部主量元素、微量元素和同位素分析结果表明,其总体特征与峨眉山玄武岩中的低钛玄武岩特征相似,其^(87)Sr/^(86)Sr、ε_(Nd)(t)、^(206)Pb/^(204)Pb、^(207)Pb/^(204)Pb和^(208)Pb/^(204)Pb分别为0．70610-0．70636、1．01-1．75、17．895-18．935、15．563-15．639、38．345-39．037。这种相似性显示了新街岩体与峨眉山玄武岩具有成因联系,并且其同位素比值和高场强元素的原始地幔标准化图解和洋岛玄武岩(OIB)的相似性说明岩浆的源区可能为地幔柱。MgO和其它氧化物的相关性指示了橄榄石、斜方辉石和少量磁铁矿的分离结晶作用。另外,部分样品的(La/Nb)_(PM)和(Th/Ta)_(PM)大于1说明有地壳物质的混染或岩石圈地幔的混染。同时对O同位素值的测定结果表明,第二旋回和第一旋回上部的样品具有高δ^(18)O的特征(＞6‰),说明岩浆房的上部遭受了地壳物质的混染,而下部则没有受到地壳物质的混染。另外δ^(18)O与SiO_2不存在正相关关系,指示了混染物不是上地壳,而是SiO_2较低的下地壳。其Nd同位素和O同位素模拟计算结果表明,其平均混染程度大约为12%～15%。可能正是由于下地壳物质的混染导致了岩浆中FeO含量的降低,从而导致硫化物发生熔离作用,并由此导致铂族元素的富集。The Xinjie mafic-uhramafic layered intrusion is one of those that are associated with the Emeishan flood basalts. There are three igneous cycles, including six petrofacies. The lower part of each cycle consists of ultramafic rocks with a cumulus texture, and the upper part is composed of gabbro and troctolite. The major elements, trace elements and isotope analyses of the first cycle and the lower part of the second cycle show that their geochemistry resembles the Emeishan basalts, and their ^87Sr/^86Sr,εNd（t）,^206Pb/^204Pb,^207Pb/^204Pb和^208Ph/^204Pbvaluce range from 0.70610-0.70636,1.01-1.75,17.895-18.935,15.563-15.639,38.345-39.037 respectively. This similarity implies the genetic relation between the Xinjie intrusion and the Emeishan basalts, whereas the similar isotope ratios and primitive mantle-normalized high strength element patterns with oceanic island basalts （OIB） suggest that the magmas were derived from a mantle plume. The correlation between MgO contents and major elements reflect the fractional crystallization of olivine, orthopyroxene and minor magnetite. In addition, some samples with （La/Nb）PM and （Th/Ta）PM 〉 1 indicate the contamination of crustal materials or lithospheric mantle. The O isotope data show that some samples of the upper part of the first cycle and the second cycle have high δ^18O （ 〉 6‰）, suggesting that crustal contamination occurred at the upper part of the magma chamber, not at the lower part. No correlation between ^δ18O and SiO2 implies that the contaminants are not upper crust but lower crust with lower SiO2 contents. The modeling calculation of Nd isotope and O isotope indicates that the mean contaminated degree is about 12%-15%. It is likely that the contamination of lower crust caused decreasing FeO contents of contaminated magams, which gave rise to immiscibile sulfide liquid, thus enrichment of platinum-group elements at the bottom of the Xinjie intrusion.

关 键 词：镁铁-超镁铁质岩体 地球化学 同化混染 地幔柱 新街 攀西 

分 类 号：P588.125[天文地球—岩石学]