峨眉山大火成岩省与玄武岩铜矿--以贵州二叠纪玄武岩分布区为例

贵州晚二叠世玄武岩是峨眉山大火成岩省的组成部分，并位于其东区。全属高钛玄武岩。它是地幔柱边部或消亡期局部熔融产物。产物我省玄武岩中的铜矿床（点），与北美大陆同类铜矿有相似之处，可统称为玄武岩铜矿，属于“与陆相镁铁质喷发岩有关的铜矿床成矿系列”。     

峨眉山玄武岩地幔源成分及其变化的微量元素标志

根据峨眉山玄武岩系岩石的稀土元素、不相容元素特征，估计了产生其母岩浆的地幔源成分。讨论了地幔平衡部分熔融和岩浆分离结晶过程中强不相容元素与一般不相容元素的比值变化后，提出用双对数图解来判别地幔成分、元素总分配系数及母岩浆形成时地幔熔融度的原理。根据Ｌａ，Ｃｅ和Ｓｃ，Ｙｂ在地幔－岩浆过程中地球化学特征，运用上述原理，讨论了峨眉山玄武岩系母岩浆的地幔成分及其变化，计算了地幔矿物相组成和部分熔融度。     

峨眉山玄武岩研究中的一些问题的讨论

峨眉山玄武岩是当前研究的特点，本文从以下方面论述了峨眉山玄武岩研究中存在的一些问题：时空分布；岩石组合；高Ti和低Ti玄武岩；与其它典型大陆溢流玄武岩的异同点；与地幔柱的关系和地幔柱的起因。对这些问题提出了一些新的认识和解释，或者提出了解决这些问题的途径。这些问题的解决，对研究娥眉山玄武岩本身以及地幔柱－岩石圈的相互作用及其成矿效应均具有重要意义。     

峨眉山高钛玄武岩物源的不均一性研究

对二滩地区高钛玄武岩物源研究表明,该区岩石可分为高钛-1和高钛-2两种岩石类型。其反映了二滩高钛玄武岩浆物源的不均一性,这种不均一性主要表现在:1)微量元素Zr,Sr含量明显不同;2)部分熔融程度和熔融压力不同;3)岩浆的结晶分异不同;4)Pb同位素比值不同。这可能是深源地幔柱(来自核-幔边界或下地幔)在上升过程中携带的不同物质发生部分熔融所致。     

峨眉山玄武岩Sr、Nd、Pb同位素特征及其物源探讨

选择峨眉山玄武岩区2个出露最全的云南永胜大迪里剖面和宾川上仓剖面进行了Sr、Nd、Pb同位素地球化学研究.结果表明, 少数样品的Pb同位素与Hanan和Graham定义的C组分相似, 而大多数样品则不在C组分范围之内, 说明除地幔柱物质外, 有岩石圈物质的加入.在多元同位素图解上, 峨眉山玄武岩位于EMⅠ、EMⅡ和DMM三端元之间, 表明其源区可以由地幔柱、富集的岩石圈地幔和地壳不同程度的混合来解释.结合已有的微量元素资料分析, 其中的地壳组分主要为下地壳, 而早期玄武质岩浆在上升过程中由于通道不畅通, 有较多的上地壳组分的混染.岩石圈地幔的富集作用可能与地幔柱释放出的小体积富Na、P而贫K的流体交代作用有关.粗面岩的同位素组成和玄武岩接近, 说明粗面岩是玄武质岩浆分离结晶作用形成的.      

峨眉山玄武岩的地幔热柱成因

根据峨眉山玄武岩的岩石组合、岩相学特征将峨眉火成岩省分为盐源-丽江岩区、攀西岩区、贵州高原岩区和松潘-甘孜岩区。通过对研究区二叠世的区域地质背景和古地理环境的分析，对峨眉山玄武岩喷发与地幔热柱的关系及其火山喷发的大地构造背景进行了进一步系统归纳和总结。根据地层学关系大致确定峨眉山玄武岩的主喷发期是阳新世（中二叠）晚期-乐平世（晚二叠）早期，时限大致为259Ma-257Ma。峨眉山玄武岩微量元素地幔标准化曲线特征与OIB基本一致，反映出其成因与地幔热柱活动有密不可分的关系。     

眉山玄武岩Sr、Nd、Pb同位素特征及其物源探讨

选择峨眉山玄武岩区2个出露最全的云南永胜大迪里剖面和宾川上仓剖面进行了Sr、Nd、Pb同位素地球化学研究。结果表明，少数样品的Pb同位素与Hanan和Graharn定义的C组分相似，而大多数样品则不在C组分范围之内，说明除地幔柱物质外，有岩石圈物质的加入。在多元同位素图解上，峨眉山玄武岩位于EMⅠ、EMⅡ和DMM三端元之间，表明其源区可以由地幔柱、富集的岩石圈地幔和地壳不同程度的混合来解释。结合已有的微量元素资料分析，其中的地壳组分主要为下地壳，而早期玄武质岩浆在上升过程中由于通道不畅通，有较多的上地壳组分的混染。岩石圈地幔的富集作用可能与地幔柱释放出的小体积富Na、P而贫K的流体交代作用有关。粗面岩的同位素组成和玄武岩接近，说明粗面岩是玄武质岩浆分离结晶作用形成的。     

喀喇昆仑岔路口地区玄武岩地幔柱信息

研究区以基性火山熔岩为主,主要岩性为玄武岩,局部发育苦橄岩、科马提岩,科马提岩具微观鬣刺结构.以TiO2=2.8为界,将玄武岩分为高钛玄武岩和低钛玄武岩,空间分布具明显分带性.岩石具与峨眉山玄武岩一致的近似平坦或轻稀土富集的稀土元素分布型式,相容性元素V,Co,Cr,Ni明显亏损,不相容元素和LREE富集.不相容元素在...     

峨眉山高钛玄武岩中主要的赋钛矿物——榍石的产状、特征及成因

峨眉山玄武岩中钛以什么矿物形式存在一直以来很少有学者提及。本文通过镜下观察、全岩化学分析、X-衍射、能谱、扫描电镜、电子探针、阴极发光对川西南部周公山-汉王场地区钻井岩心中峨眉山玄武岩进行了详细的分析,讨论了其主要赋钛矿物及成因。(1)SiO₂含量46.4%~48.3%和TiO₂>3%显示区内峨眉山玄武岩属于高钛峨眉山玄武岩系列。但多个层段榍石含量>5%,而极少见磁铁矿、钛铁矿;(2)榍石主要以隐晶质的云雾状、雪花状、芝麻点状、枝状等形态分布于微晶长石之间和溶孔、溶洞边缘及裂缝中,少量呈显晶质粒状分布于微晶长石、绿泥石之间。(3)电子探针分析显示:所有含钛矿物中,钛铁矿中TiO₂含量最高,为39.069%,榍石中TiO₂次之,TiO₂含量为17.143%~38.648%,磁铁矿中TiO₂含量最高为12.293%,平均在5%~10%左右,其他矿物基本上都少于1%。(4)扫描电镜及其能谱分析显示:榍石中的Ti含量(2.49%~24.97%)明显高于含钛磁铁矿(2.68%~9.21%)、含钛赤铁矿(3.64%)中Ti含量,与钛铁矿(19.51%)含量相当。分析结果认为:峨眉山玄武岩中大量出现的隐晶榍石可能是岩浆后期产物或期后蚀变的产物。在峨眉山玄武岩中首次鉴别出的大量隐晶质榍石是高钛峨眉山玄武岩中最主要的赋钛矿物。隐晶榍石在玄武岩中的含量是区分"高钛"和"低钛"玄武岩的主要标志之一。     

峨眉山大火成岩省“高钛玄武岩”和“低钛玄武岩”成因探讨

大量的岩石化学资料分析表明，峨眉山大火成岩省玄武岩的TiO2含量是连续变化的，不存在明显的间断。野外地质特征表明高钛和低钛玄武岩既不存在空间分带，也不存在时间分带。其Sr、Nd和Pb同位素组成也没有明显的区别，推测它们可能是同源岩浆分离结晶的产物。根据MgO和TiO2的相关关系，可将苦橄岩和玄武岩的演化划分为4个趋势，并采用分离结晶模式对其进行了成因模拟，表明高钛和低钛玄武岩是同—母岩浆(苦橄—玄武岩浆)通过不同矿物相分离结晶的产物。     

Geochemistry of Two Types of Basalts in the Emeishan Basaltic Province: Evidence for Mantle Plume-Lithosphere Interaction

Based on the temporal-spatial distribution and geochemical characteristics, the Emeishan basalts can be divided into two types: high-P2O-TiO2 basalt (HPT) and low-P2O5-TiO2 basalt (LPT), which differ distinctly in geochemistry: the LPTs are characterized by relatively high abundances of MgO, total FeO and P2O5 and compatible elements (Cr, Ni, Sc), and relatively low contents of moderately compatible elements (V, Y, Yb, Co), LREE and other incompatible elements compared with the HPT. On the diagrams of trace element ratios, they are plotted on an approximately linear mixing line between depleted and enriched mantle sources, suggesting that these two types of basalts resulted from interactions of varying degrees between mantle plume and lithospheric mantle containing such volatile-rich minerals as amphibole and apatite. The source region of the LPT involves a smaller proportion of lithospheric components, while that of the HTP has a larger proportion of lithospheric components. Trachyte is generated by pa     

The Principles for Discrimination of the Source Composition of Mantle-Derived Igneous Rocks and the Nature of the Mantle Source Region of the Emeishan Basalts

This paper discusses the discrimination principles. deduction and methods for probing into the source composition of mantle-derived magma. The magmatophile (incompatible) source elements are not all optimal tracers for mantle source composition. The ratios of two strong magmatophile elements (D<1) or the ratios of two trace elements with the same D value are not controlled by the formation mode and evolution degree of a magma, but maintain the characteristics of their composition in mantle source region prior to the magma formation. The ratios are related to different mantle-crust structures and dynamics. The mantle source composition of the Emeishan Basalt series is similar to that of the South Atlantic Rio Grande Rise-Walvis Ridge Basalts and Brazil continental-margin basalts. This may indicate that these basalt series might have similar source regions and tectonic environments.     

Platinum-Group Element Geochemical Characteristics of the Picrites and High-Ti Basalts in the Binchuan Area,Yunnan Province

The Binchuan area of Yunnan is located in the western part of the Emeishan large igneous province in the western margin of the Yangtze Block.In the present study,the Wuguiqing profile in thickness of about 1440 m is mainly composed of high-Ti basalts,with minor picrites in the lower part and andesites,trachytes,and rhyolites in the upper part.The picrites have relatively higher platinum-group element（PGE） contents（ΣPGE=16.3-28.2 ppb）,with high Cu/Zr and Pd/Zr ratios,and low S contents（5.03-16.9 ppm）,indicating the parental magma is S-unsaturated and generated by high degree of partial melting of the Emeishan large igneous province（ELIP） mantle source.The slightly high Cu/Pd ratios（11 000-24 000） relative to that of the primitive mantle suggest that 0.007%sulfides have been retained in the mantle source.The PGE contents of the high-Ti basalts exhibit a wider range（ΣPGE=0.517-30.8 ppb）.The samples in the middle and upper parts are depleted in PGE and haveε_Nd（260 Ma） ratios ranging from -2.8 to -2.2,suggesting that crustal contamination of the parental magma during ascent triggered sulfur saturation and segregation of about 0.446%-0.554% sulfides,and the sulfide segregation process may also provide the ore-forming material for the magmatic Cu-Ni-PGE sulfide deposits close to the studied basalts.The samples in this area show Pt-Pd type primitive mantle-normalized PGE patterns,and the Pd/Ir ratios are higher than that of the primitive mantle（Pd/Ir=1）,indicating that the obvious differentiation between Ir-group platinum-group elements（IPGE） and Pd-group platinum-group elements（PPGE） are mainly controlled by olivine or chromites fractionation during magma evolution.The Pd/Pt ratios of most samples are higher than the average ratio of mantle（Pd/Pt=0.55）,showing that the differentiation happened between Pt and Pd.The differentiation in picrites may be relevant to Pt hosted in discrete refractory Pt-alloy phase in the mantle;whereas the differentiation in the high-Ti basalts is probably associated with the fractionation of Fe-Pt alloys,coprecipitating with Ir-Ru-Os alloys.Some high-Ti basalt samples exhibit negative Ru anomalies,possibly due to removal of laurite collected by the early crystallized chromites.     

峨眉地幔柱-岩石圈的相互作用:来自低钛和高钛玄武岩的Sr-Nd和O同位素证据

蛾眉山玄武岩总体具有较高的^87Sr／^86Sr比值和较低的εNd(t)值，并具有富集地幔源区的特点。而低钛玄武岩(LT)与高钛玄武岩(HT)间又表现出一定的差异性，即早期低钛玄武岩(LTl)的^87Sr/^86Sr比值最高(0．7063—0．7078)，而其εNd(t)最低(—6．74-—0．34)：晚期高钛玄武岩(HT)具有最低的^87Sr/^86Sr比值(0．7049—0．7064)和最高的εNd(t)值(—0．71—1．5)。蛾眉山低钛玄武岩中单斜辉石的氧同位素变化范围为6．2‰-7．86‰，高于洋岛拉斑玄武岩的平均值5．4‰。研究样品较地幔岩石偏高的δ^18O值说明在其形成和演化过程中有壳源物质的参与。结合前人的研究成果和对元素地球化学的研究认为，壳源物质可能主要来自于新元古代富集的扬子西缘次大陆岩石圈地幔。地幔柱—岩石圈的相互作用过程中表现在时间和空间的系统变化，即早期西岩区形成含大量壳源组分的低钛玄武岩，晚期为壳源组分相对较少的高钛玄武岩。空间上低钛玄武岩仅分布在西岩区，而中、东岩区皆为高钛玄武岩。壳源组分随着时间演化逐渐减少，在空间上由西而东也逐渐减少。表明蛾眉山火成岩省形成早(主)期地幔柱头卷入并熔融了较多交代富集的次大陆岩石圈物质，晚期则有较少的壳源物质参与。建立了蛾眉地幔柱与大陆岩石圈作用的工作模型。     

Geochemistry of Picritic and Associated Basalt Flows of the Western Emeishan Flood Basalt Province, China

Picritic lava flows near Lijiang in the late Permian Emeishanflood basalt province are associated with augite-phyric basalt,aphyric basalt, and basaltic pyroclastic units. The dominantphenocryst in the picritic flows is Mg-rich olivine (up to 91·6%forsterite component) with high CaO contents (to 0·42wt %) and glass inclusions, indicating that the olivine crystallizedfrom a melt. Associated chromite has a high Cr-number (73–75).The estimated MgO content of the primitive picritic liquidsis about 22 wt %, and initial melt temperature may have beenas high as 1630–1690°C. The basaltic lavas appearto be related to the picritic ones principally by olivine andclinopyroxene fractionation. Age-corrected Nd–Sr–Pbisotope ratios of the picritic and basaltic lavas are indistinguishableand cover a relatively small range [e.g. _Nd(t) = –1·3to +4·0]. The higher _Nd(t) lavas are isotopically similarto those of several modern oceanic hotspots, and have ocean-island-likepatterns of alteration-resistant incompatible elements. Heavyrare earth element characteristics indicate an important rolefor garnet during melting and that the lavas were formed byfairly small degrees of partial melting. Rough correlationsof isotope ratios with ratios of alteration-resistant highlyincompatible elements (e.g. Nb/La) suggest modest amounts ofcontamination involving continental material or a relativelylow-_Nd component in the source. Overall, our results are consistentwith other evidence suggesting some type of plume-head originfor the Emeishan province. KEY WORDS: Emeishan; flood basalts; picrites; mantle plumes; late Permian     

峨眉山大火成岩省中的苦橄岩:地幔柱活动证据

峨眉山大陆溢流玄武岩省中新发现的苦橄质熔岩中的橄榄石和铬尖晶石分别以富镁和富铬为特征。利用橄榄石熔体平衡原理恢复得到原生岩浆的MgO含量约为22％．表明该地区既有代表原生岩浆成分的苦橄岩,也有代表演化和堆晶成因的苦橄岩,根据有关实验估算其形成的T=1600℃,p=4．5GPa。如此高的温度指示了苦橄岩的形成与地幔柱作用有关。其稀土和微量元素配分模式以轻稀土富集和高场强元素(HFSE)相对亏损．不存在Nb、Ta负异常,而以P和K负异常为特征。其La／Ta、La／Sm、(La／Nb)PM、(Th／Ta)PM值变化范围小,均指示其地幔柱成因,且上升过程中很少或没有受到岩石圈地幔或地壳物质的混染,是石榴二辉橄榄岩经大约7％的部分熔融的产物。地幔柱的轴部位置可能位于现今云南丽江县城一带。     

二滩玄武岩的水含量：峨眉山大火成岩省地幔源区水含量的区域性分布特征

近期对科马提岩以及许多大火成岩省中的苦橄岩进行的水含量分析以及地幔潜能温度的研究表明,无论是太古宙还是显生宙的大火成岩省的形成都和含水的地幔柱有关。晚二叠纪的峨眉山大火成岩省（ELIP）位于扬子克拉通西缘,目前主流观点认为其是由地幔柱形成的。前人根据大量的岩石地球化学工作将ELIP分为西区、中区和东区;证明了位于西区的大理、宾川的苦橄岩和玄武岩地幔源区的水含量高于2500×10^-6。然而对于其他区域玄武岩源区的含水性还不清楚。文章以位于中区的二滩剖面底部高钛型玄武岩为研究对象,采用单斜辉石斑晶反演的方法研究恢复了其原始岩浆的水含量。结果表明,单斜辉石斑晶水含量范围为76×10^-6 ~424×10^-6, 对应的平衡熔体水含量为3.01 wt%。在考虑分离结晶影响后,恢复的原始岩浆水含量达到2.71±0.95 wt%。该水含量略低于大理苦橄岩水含量,与宾川苦橄岩相当。而计算的地幔源区水含量最低估计为1357×10^-6,该值低于大理、宾川苦橄岩的源区水含量,但仍显著高于正常洋中脊玄武岩和洋岛玄武岩源区。ELIP中不同区域的苦橄岩和玄武岩都存在高水含量,这表明在ELIP的形成和演化过程中水都扮演了很重要的角色。     

Petrology, Mineralogy and Geochemistry of the Emeishan Continental Flood Basalts, SW China: Evidence for Activity of Mantle Plumes

Electronic microprobe analyses for olivine, clinopyroxene and Cr-spinel in picrites, which we have discovered recently in the Emeishan continental flood basalt province (ECFBP), show that the olivine is rich in Mg, and that Cr-spinel is rich in Cr. Based on the olivine-melt equilibrium, the primary parental melt compositions are calculated. The high-Mg olivine-hosted picrite can be regarded as parental melt. Thus, the melting temperature and pressure are estimated: T=1600℃ and P=4.5 GPa. It suggests that the picrites are connected with the activity of mantle plumes. Their major element composition is comparable to many other CFBs by their high Fe8, (CaO/Al2O3)8 and low Na8, indicating a high pressure. All rocks display a similar chondrite-normalized REE patterns, i.e., enrichment of LREE, relative depletion of HFSE and absence of negative Nb and Ta but depletion in P and K. Some incompatible element ratios, such as La/Ta, La/ Sm, (La/Nb)PM, (Th/Ta)PM, are in a limited range, show that they were derived