川西冕宁-德昌稀土矿带霓长岩的地球化学特征及地质意义

霓长岩化作用是指碳酸岩(或碱性岩)流体对围岩的交代蚀变,它是碳酸岩型稀土(REE)矿床常见的蚀变类型,其所形成的岩石即为霓长岩。对霓长岩的深入研究可以鉴别碳酸岩体的存在,厘定碳酸岩岩浆(或流体)的地球化学性质及源区特征,这对于找寻碳酸岩相关的矿产资源(尤其是REE)以及剖析矿床成因机制有着重要的地质意义。川西冕宁-德昌稀土矿带是中国最重要的轻稀土矿带之一,包括牦牛坪超大型、大陆槽大型、木落寨和里庄中小型REE矿床以及一系列矿点。REE矿化与碳酸岩-碱性岩杂岩体密切相关,受一系列新生代走滑断裂的控制。该矿带广泛发育霓长岩化蚀变带,尤以大陆槽及里庄矿床为显著。岩相学分析表明,大陆槽和里庄霓长岩中的矿物多呈他形粒状结构,主要由长石、黑云母、霓辉石以及少量副矿物组成;主微量元素分析表明,霓长岩的碱质(K_2O+Na_2O)、MgO、Fe_2O_3T含量较高,且富集REE、Sr、Ba等微量元素;电子探针分析表明,霓长岩中的霓辉石Fe OT含量较高,长石Na_2O及K_2O含量较高,Ca O含量极低。An-Ab-Or三角图解显示长石主要为透长石和钠长石,属碱性长石系列;黑云母的地球化学成分图解表明云母的成因类型为交代型且具有相对富镁、贫铁等特征,属镁质黑云母。霓长岩化作用的交代流体含有较高的CO_2组分,且富含碱质、Mg、Fe及REE、Sr、Ba等元素。对比霓长岩与原岩的主微量元素发现:相比于正长岩原岩,在主量元素中,霓长岩的Fe、Mg、Ca等元素含量增加,Si、Al等元素含量降低;微量元素中,霓长岩的REE及Sr、Ba等元素显著增加。这意味着交代流体含有的Fe_2O_3T、MgO、CaO等组分在霓长岩化过程中被带进了围岩,而SiO_2和Al_2O_3等从围岩中被逐出。大陆槽及里庄矿区发育的角砾岩指示了矿区曾经历过频繁的角砾岩化事件,这提高了霓长岩作用的强度,并且为矿脉的穿插及REE矿物的沉淀提供了空间。在霓长岩化过程中,流体-围岩的组分交换反复发生,这削弱了REE络合物的稳定性,伴随多期次的热液活动及构造事件,最终完成REE活化→迁移→沉淀的过程。     

Petrogenesis of garnet-bearing ultramafic rocks and associated eclogites in the Su-Lu ultrahigh-P metamorphic terrane, eastern China

Ultramafic blocks that themselves contain eclogite lenses in the Triassic Su-Lu ultrahigh-P terrane of eastern China range in size from hundreds of metres to kilometres. The ultramafic blocks are enclosed in quartzofeldspathic gneiss of early Proterozoic age. Ultramafic rocks include garnetiferous lherzolite, wehrlite, pyroxenite, and hornblende peridotite. Garnet lherzolites are relatively depleted in Al₂O₃ (<3.8wt%), CaO (<3.2%) and TiO₂ (<0.11 wt%), and are low in total REE contents (several p.p.m.), suggesting that the rocks are residual mantle material that was subjected to low degrees of partial melting. The eclogite lenses or layers within the ultramafic rocks are characterized by higher MgO and CaO, lower Al₂O₃ and TiO₂ contents, and a higher CaO/Al₂O₃ ratio compared to eclogites enclosed in the quartzofeldspathic gneiss. Scatter in the plots of major and trace elements vs. MgO, REE patterns and La, Sm and Lu contents suggest that some eclogites were derived from melts formed by various degrees (0.05–0.20) of partial melting of peridotite, and that other eclogites formed by accumulation of garnet and clinopyroxene ± trapped melt in the upper mantle. Both ultramafic and eclogitic rocks have experienced a complex metamorphic history. At least six stages of recrystallization occurred in the ultramafic rocks based on an analysis of reaction textures and mineral compositions. Stage I is a high temperature protolith assemblage of Ol + Opx + Cpx + Spl. Stage II consists of the ultrahigh-pressure assemblage Ol + Cpx + Opx + Grt. Stage III is manifested by the appearance of fine-grained garnet after coarse-grained garnet. Stage IV is characterized by formation of kelyphitic rims of fibrous Opx and Cpx around garnet, and replacement of garnet by spinel and pargasitic-hornblende. Stage V is represented by the assemblage Ol + Opx + Prg-Hbl + Spl. The mineral assemblages of stages VI_A and VI_B are Ol + Tr-Amp + Chl and Serp + Chl ± talc, respectively. Garnet and orthopyroxene all show a decrease in MgO with retrogressive recrystallization and Na₂O in clinopyroxene also decreases throughout this history. Eclogites enclosed within ultramafic blocks consist of Grt + Omp + Rt ± Qtz ± Phn. A few quartz-bearing eclogites contain rounded and oval inclusion of polycrystalline quartz aggregates after coesite in garnet and omphacite. Minor retrograde features include thin symplectic rims or secondary amphiboles after Cpx, and ilmenite after rutile. P-T estimates indicate that the ultrahigh-metamorphism (stage II) of ultramafic rocks occurred at 820-900d? C and 36-41 kbar and that peak metamorphism of eclogites occurred at 730-900d? C and >28 kbar. Consonant with earlier plate tectonic models, we suggest that these rocks were underplated at the base of the continental crust. The rocks then underwent ultrahigh-pressure metamorphism and were tectonically emplaced into thickened continental crust during the Triassic collision between the Sino-Korean and Yangtze cratons.     

江西南部震旦—寒武纪寻乌岩组变沉积岩地球化学特征及构造意义

寻乌岩组变沉积岩发育在江西南部新元古代—早古生代南城—寻乌盆地内,时代为震旦—寒武纪。寻乌岩组的Si O2含量变化较大,除1个含量较低的样品外,平均为63.01%,具有相对较高的K2O、Al2O3及(TFe O+Mg O)值,较低的Na2O、Ca O。相容元素含量与后太古代澳大利亚页岩(PAAS)十分接近,高于中国东部上地壳,与其较高的(TFe O+Mg O)特征相吻合,说明源区具有一定数量的中基性组分。稀土元素总量高于上地壳及PAAS,但其球粒陨石标准化配分模式与典型的上地壳及PAAS相似,表现为明显向右倾斜,轻稀土富集,重稀土平坦,铕负异常显著,铈负异常不明显。岩石地球化学特征显示其原岩属于一种中等成熟度的陆源碎屑岩,物源主要为被改造的上地壳长英质和花岗质物源区,少部分物源为中基性火山岩。高场强元素及稀土元素等不活动元素地球化学特征表明其沉积环境为浅海—半深海,其构造环境属于发育有裂谷系的被动大陆边缘,该认识从沉积岩地球化学方面为华南早古生代构造属性提供了新资料。     

Trace element geochemistry of the Mt Vulture carbonatites,southern Italy

The Mt Vulture carbonatites are the only carbonatite occurrence in the southern Apennines. We present new trace element data for these rocks in order to evaluate the factors influencing rare earth element (REE) and other trace element fractionations and their REE grade. This study focuses on massive hyalo-alvikites from two lava flows and one dike, which have different relative abundances of silicate and carbonate (i.e. Si/Ca). These differences are also evident from CaO/(CaO + MgO + FeO(T) + MnO) and Sr/Ba ratios. The REE grade of the Mt Vulture carbonatites is very similar to that of the global average for calcio-carbonatites. R-mode factor analysis shows that most of the trace element variance reflects the relative roles of carbonate and silicate minerals in influencing trace element distributions. Silicates largely control heavy rare earth element (HREE), transition metal, Zr, and Th abundances, whereas carbonate minerals control light rare earth element (LREE), Ba, and Pb abundances. In addition, apatite influences LREE concentrations. Increasing silica contents are accompanied by decreases in (La/Yb)N and (La/Sm)N ratios and less marked LREE enrichment. In contrast, higher carbonate contents are associated with increases in (La/Yb)N and (La/Sm)N. The Si/Ca ratio has little influence on Eu anomalies and middle rare earth element (MREE) to HREE fractionations. Apatite has a negligible effect on inter-REE fractionations amongst the carbonatites.     

华北克拉通新太古代板块俯冲作用:来自山西云中山地区变质高镁火成岩组合的证据

在华北克拉通中部的山西云中山地区,新太古代花岗闪长质片麻岩中存在一些超镁铁质岩-镁铁质岩块及由斜长角闪岩、角闪变粒岩、石英岩和石榴夕线黑云片岩等岩石类型构成的变质表壳岩残片,其中的超镁铁质-镁铁质岩、斜长角闪岩和角闪变粒岩构成一套高镁火成岩组合。超镁铁质岩已变质为橄榄绿泥阳起片岩等岩石类型,呈变余斑状结构,橄榄石斑晶仍有保存;岩石SiO_2含量为39.22%～44.99%,Al_2O_3为8.82%～13.47%,Mg O为19.24%～22.13%,Na_2O+K_2O=0.71%～1.11%,CaO为5.75%～8.42%;Al_2O_3/TiO_2=14.8～17.4,CaO/Al_2O_3=0.60～0.84;化学成分上与科马提岩有一定的相似性。与之紧密伴生的斜长角闪岩也具有高镁特征,Mg O含量为11.28%～15.09%,铝、硅和碱质均偏低,具正铕异常,显示堆晶辉长岩的特征。非高镁斜长角闪岩有相对高的铝、硅和碱质,其原岩应为钙碱性玄武岩。角闪变粒岩样品的SiO_2含量为54.21%～55.71%,Al_2O_3为14.24%～15.49%,Mg O为6.26%～8.28%,Fe OT/Mg O=1.11～1.58,高钠低钾,Na_2O+K_2O=3.7%～4.78%,Na_2O/K_2O=5.15%～13.13,Mg#=53.0～61.5,属于高镁安山岩。由超镁铁质质岩-斜长角闪岩-角闪变粒岩构成的变质高镁火山岩组合具有钙碱性系列趋势。超镁铁质岩稀土元素含量总量较低,具有轻稀土富集和重稀土亏损的稀土型式;斜长角闪岩与超镁铁质岩比较,除富集大离子亲石元素和Cr、Ni明显较低外,具有相似的微量元素图谱形态。三种岩石类型在微量元素蛛网图上均显示出Ta、Nb、Ti负异常和Pb正异常。野外产状和岩石地球化学特征表明超镁铁质岩和高镁斜长角闪岩属于阿拉斯加型杂岩体,角闪变粒岩属于赞岐岩质高镁安山岩。在Zr/Nb-Nb/Th和Nb/Y-Zr/Y构造环境判别图解上显示出与俯冲相关的演化趋势,在Hf-Th-Ta、Nb/La-(La/Sm)N和Th/Yb-Nb/Yb图解上也落在岛弧钙碱性岩石区域。以上特征表明高镁火成岩组合形成于与板块俯冲相关的岛弧构造背景。野外地质关系和锆石U-Pb年龄限定高镁火成岩组合形成时代在～2.5Ga。云中山地区阿拉斯加型镁铁质-超镁铁质杂岩与赞岐岩质高镁安山岩共生,表明该地区存在新太古代的板块俯冲作用,为太古宙存在板块构造机制提供了新证据。     

北京周边二叠系山西组泥岩地球化学特征及其地质意义

对采自北京周边的二叠系山西组露头及钻井泥岩样品进行了岩石学和地球化学研究,结果表明：岩石主要由粘土矿物和石英组成,兼有少量碳酸盐岩和长石;岩石中碳酸盐岩含量的高低与CaO、MgO含量有很好的对应关系;SiO₂、Na₂O和K₂O亏损,TiO₂和Fe₂O₃^T富集,反映了基性物源的存在;微量元素特征比值显示源区母岩为非单一物源,稀土元素总量较高,轻稀土元素富集,重稀土元素平坦,铕大部分具明显负异常、少部分微弱正异常,Ce基本无异常,显示为多物源,各样品稀土元素配分模式与大陆上地壳一致,显示了沉积物具有同源性。源岩应为来自于华北地块北缘的大陆上地壳的沉积岩、花岗岩和碱性玄武岩的混合。二叠系山西组泥岩沉积于覆水较深的还原环境,源区构造背景为大陆岛弧与活动大陆边缘。     

Geochemical Constraints of Sediments on the Provenance, Depositional Environment and Tectonic Setting of the Songliao Prototype Basin

The geochemistry of sediments is primarily controlled by their provenances, and different tectonic settings have distinctive provenance characteristics and sedimentary processes. So, it is possible to discriminate provenances, depositional environments and tectonic settings in the development of a sedimentary basin with the geochemistry of the clastic rocks. The analytical results of the present paper demonstrate that sediments in the Songliao prototype basin are enriched in silica (SiO2=66.48-80.51 %), and their ΣREE are 30-130 dmes of that of chondrite with remarkable Eu anomalies. In discriminating diagrams of Eu/Eu vs eeeeeREE and (La/Yb)N vs ΣREE, most samples locate above the line Eu/ Eu=l, on the right of the line Eu/Eu/ΣREE=1 and under the line La/Yb)N/eeeeeREE=1/8, which indicates that the depositional environment of sediments in the basin was oxidizing. In addition, variations of MgO, TiO2, A12O3, FeO+Fe2O3, Na2O and CaO vs SiO2 reflect a tendency of increasing mineral maturity of sediments     

Geochemical systematics of the Mauranipur-Babina greenstone belt,Bundelkhand Craton,Central India:Insights on Neoarchean mantle plume-arc accretion and crustal evolution

The Neoarchean Bundelkhand greenstone sequences at Mauranipur and Babina areas within the Bundelkhand Gneissic Complex preserve a variety of magmatic rocks such as komatiitic basalts, basalts,felsic volcanic rocks and high-Mg andesites belonging to the Baragaon, Raspahari and Koti Formations.The intrusive and extrusive komatiitic basalts are characterized by low SiO_2(39-53 wt.%), high MgO(18-25 wt.%).moderately high Fe_2O_3(7.1-11.6 wt.%), Al_2O_3(4.5-12.0 wt.%), and TiO_2(0.4-1.23 wt.%)with super to subchondritic(Gd/Yb)N ratios indicating garnet control on the melts. The intrusive komatiitic suite of Ti-enriched and Al-depleted type possesses predominant negative Eu and positive Nb, Ti and Y anomalies. The chemical composition of basalts classifies them into three types with varying SiO_2, TiO_2, MgO, Fe_2O_3, Al_2O_3 and CaO. At similar SiO_2 content of type Ⅰ and Ⅲ basalts, the type II basalts show slightly high Al_2O_3 and Fe_2O_3 contents. Significant negative anomalies of Nb, Zr, Hf and Ti, slightly enriched LREE with relatively flat HREE and low ∑REE contents are observed in type Ⅰ and Ⅱ basalts. TypeⅢ basalts show high Zr/Nb ratios(9.8-10.4), TiO_2(1.97-2.04 wt.%), but possess strikingly flat Zr, Hf, Y and Yb and are uncontaminated. Andesites from Agar and Koti have high SiO_2(55-64 wt.%), moderate TiO_2(0.4-0.7 wt.%), slightly low Al_2O_3(7-11.9 wt.%), medium to high MgO(3-8 wt.%) and CaO contents(10-17 wt.%). Anomalously high Cr, Co and Ni contents are observed in the Koti rhyolites. Tholeiitic to calc alkaline affinity of mafic-felsic volcanic rocks and basalt-andesite dacite-rhyolite differentiation indicate a mature arc and thickened crust during the advanced stage of the evolution of Neoarchean Bundelkhand greenstone belt in a convergent tectonic setting where the melts were derived from partial melting of thick basaltic crust metamorphosed to amphibolite-eclogite facies. The trace element systematics suggest the presence of arc-back arc association with varying magnitudes of crust-mantle interaction. La/Sm, La/Ta,Nb/Th, high MgO contents(20 wt.%), CaO/Al_2O_3 and(Gd/Yb)_N 1 along with the positive Nb anomalies of the komatiite basalts reflect a mantle plume source for their origin contaminated by subductionmetasomatized mantle lithosphere. The overall geochemical signatures of the ultramafic-mafic and felsic volcanic rocks endorse the Neoarchean plume-arc accretion tectonics in the Bundelkhand greenstone belt.     

Confirmation and Preliminary Study of Aluminous A—Type Granites in the Wuyi Mountain Area,Eastern Jiangxi Province,China

Widespread in the Wuyi Mountain area of eastern Jiangxi are petrochemically peraluminous granites and they are characterized as being high in silica (SiO₂ > 75% ) and highly alkaline (ALK=6.9% -7.5%) with K₂O > Na₂O and rather high ratios of FeO^T/MgO (11.3-17.9). The rocks have low contents of CaO, MgO, TiO₂ and P₂ O₅. The granites are enriched in REE (ΣREE =210.3 - 496. 8 μg/g) with remarkable negative Eu anomalies, but depleted in Eu, Ba, Sr, V, Co, and Ni, with 10000 x Ga/Al ratios, varying from 6. 1 to 9. 8. It is clear that these granites are obviously different from the I- and S-type granites, but are quite similar to those typical A-type granites such as aluminous A-type granites in the coastal areas of Fujian Province. State Geological Survey Project: supported by the Regional Geological Survey Project (No: 20001300002091 ) on the basis of the maps (scale 1: 250000) of Jingdezheng City, Nanchang City and Shangrao City.     

Metasomatism of garnet peridotite from Jiangzhuang,southern Sulu UHP belt: constraints on the interactions between crust and mantle rocks during subduction of continental lithosphere

The Jiangzhuang ultrahigh‐pressure (UHP) metamorphic peridotite from south Sulu, eastern China occurs as a layer within gneiss with eclogite blocks, and consists of coarse‐grained garnet porphyroblasts and a fine‐grained matrix assemblage of garnet + forsterite + enstatite + diopside ± phlogopite ± Ti‐clinohumite ± magnesite. Both types of garnet are characterized by high MgO content and depletion of light rare earth element (LREE) and enrichment of heavy rare earth element, but the matrix garnet has lower MgO, TiO₂ and higher Cr₂O₃ and REE contents. Diopside displays LREE enrichment, and has low but variable large‐ion lithophile element (LILE) contents. Phlogopite is a major carrier of LILE. Ti‐clinohumite contains high Nb, Ta, Cr, Ni, V and Co contents. The P–T conditions of 4.5–6.0 GPa and 850–950 °C were estimated for matrix mineral assemblages. Most peridotites are depleted in Al₂O₃, CaO and TiO₂, and enriched in SiO₂, K₂O, REE and LILE. In contrast to phlogopite‐free peridotites, the phlogopite‐bearing peridotites have higher K₂O, Zr, REE and LILE contents. Zircon occurs only in the phlogopite‐bearing peridotites, shows no zoning, with low REE contents and Th/U ratios, and yields tight U–Pb ages of 225–220 Ma, indicating the peridotites experienced consistent Triassic UHP metamorphism with subducted supercrustal rocks. These data demonstrate that the Jiangzhuang peridotites were derived from the depleted mantle wedge of the North China Craton, and experienced various degrees of metasomatism. The phlogopite‐free peridotites may have been subjected to an early cryptic metasomatism at UHP conditions of the mantle wedge, whereas the phlogopite‐bearing peridotites were subjected to a subsequent strong metasomatism, characterized by distinctly enrichment in LILE, LREE, Zr and K as well as the growth of zircon and volatile‐bearing minerals at UHP subduction conditions. The related metasomatism may have resulted from the filtration of fluids sourced mainly from deeply subducted supracrustal rocks.     

Mafic–intermediate plutonic rocks of the Salmas area,northwestern Iran: their source and petrogenesis significance

The Salmas area, in the northernmost part of the Sanandaj–Sirjan zone of Iran, contains a crystalline mafic–intermediate complex that intrudes into the Precambrian metamorphic basement complex and is composed of gabbroic and gabbrodiorite cumulates and fine-grained non-cumulate gabbronorites and diorites. These rocks have fine- to coarse-grained texture and are mainly composed of plagioclase, pyroxenes, and amphibole. Major element geochemistry indicates that the pluton has a low-K with tholeiitic affinity. Variations of major and trace elements on Harker diagrams, including negative correlations MgO, Fe₂O₃, CaO, and Co and positive correlations Na₂O, K₂O, Rb, Ba, and La, with increasing SiO₂ and chondrite-normalized REE patterns, suggest that fractional crystallization of gabbroic rocks could have played a significant role in the formation of evolved rocks. The chondrite-normalized REE patterns are not fractionated (La_N /Lu_N = 1.3–5.4) and display strong Eu anomalies (Eu/Eu* = 1.15–1.76) in cumulate rocks, which we attributed to cumulus plagioclase. Sr and Nd isotopic ratios vary from 0.704698 to 0.705866 and from 0.512548 to 0.512703, respectively. Gabbronorites with high ¹⁴³Nd/¹⁴⁴Nd ratios, low ⁸⁷Sr/⁸⁶Sr ratios, and high MgO, Ni, and Cr contents indicate that they were generated from relatively primitive magmas. We used petrogenetic modelling to constrain sources. Trace element ratio modelling indicates that the gabbroic rocks were generated from a spinel-peridotite source via 5–20% degrees of fractional melting at a depth of ～52 km. Major and REE modelling shows that the diorites are the products of fractional crystallization of gabbronorites.     

Rare earth elements in phoscorites and carbonatites of the Devonian Kola Alkaline Province,Russia: Examples from Kovdor,Khibina, Vuoriyarvi and Turiy Mys complexes

The Devonian (ca. 385–360 Ma) Kola Alkaline Province includes 22 plutonic ultrabasic–alkaline complexes, some of which also contain carbonatites and rarely phoscorites. The latter are composite silicate–oxide–phosphate–carbonate rocks, occurring in close space-time genetic relations with various carbonatites. Several carbonatites types are recognized at Kola, including abundant calcite carbonatites (early- and late-stage), with subordinate amounts of late-stage dolomite carbonatites, and rarely magnesite, siderite and rhodochrosite carbonatites. In phoscorites and early-stage carbonatites the rare earth elements (REE) are distributed among the major minerals including calcite (up to 490 ppm), apatite (up to 4400 ppm in Kovdor and 3.5 wt.% REE₂O₃ in Khibina), and dolomite (up to 77 ppm), as well as accessory pyrochlore (up to 9.1 wt.% REE₂O₃) and zirconolite (up to 17.8 wt.% REE₂O₃). Late-stage carbonatites, at some localities, are strongly enriched in REE (up to 5.2 wt.% REE₂O₃ in Khibina) and the REE are major components in diverse major and minor minerals such as burbankite, carbocernaite, Ca- and Ba-fluocarbonates, ancylite and others. The rare earth minerals form two distinct mineral assemblages: primary (crystallized from a melt or carbohydrothermal fluid) and secondary (formed during metasomatic replacement). Stable (C–O) and radiogenic (Sr–Nd) isotopes data indicate that the REE minerals and their host calcite and/or dolomite have crystallized from a melt derived from the same mantle source and are co-genetic.     

华北克拉通怀安陆块新太古代低铝和高铝TTG片麻岩的地球化学特征与成因

英云闪长岩-奥长花岗岩和花岗闪长岩(简称TTG)是太古宙高级变质地体的主要物质组成,对深入理解早期大陆生长及其机制具有重要的科学意义。目前,人们对其成因过程与机制仍有不同认识。本文以怀安陆块中广泛分布的TTG片麻岩为例,探讨其成因演化和机制。研究区位于华北克拉通中北部,主要由新太古代英云闪长岩及少量奥长花岗岩、花岗闪长岩组成。我们从该区识别出富硅富重稀土和负铕异常的低铝奥长花岗质片麻岩,形成时代与广泛分布的高铝TTG质片麻岩一致(锆石SHRIMP U-Pb年龄2.53Ga)。岩石地球化学数据显示,低铝奥长花岗质片麻岩的主量元素具有富SiO 2(76%~79%),低Al2O3(11.01%~12.61%)、CaO(1.27%~1.59%)、MgO(0.74%~0.24%)和Mg#(18~53)等特征,而广泛分布的高铝TTG岩系的主量元素含量变化大,例如,SiO 2=63%~77%、Al2O3=13.2%~17.77%、CaO=1.8%~5.78%、MgO=0.18%~3.84%和Mg#=35~64。微量元素方面,低铝奥长花岗质片麻岩具有Eu/Eu*负异常(除1个样品为弱正异常1.38外,其余样品分布在0.59~0.44),富集重稀土((La/Yb)N=4~7,(Gd/Yb)N=0.36~1.27),而高铝TTG岩系从弱负铕异常到正异常(Eu/Eu*=0.8~5.35),轻重稀土分馏明显((La/Yb)N=10~103、(Gd/Yb)N=1.97~5.72)。在微量蛛网图中二者的区别除重稀土明显存在区别外,低铝奥长花岗质片麻岩显示出Ba、Sr的相对亏损,而高铝TTG岩系则相反。二者Lu/Hf比值差异明显,低铝奥长花岗质片麻岩变化在0.1~0.16,而高铝TTG岩系变化在0.01~0.07。在Lu/Hf与相关元素以及SiO 2与相关元素哈克图解中,二者差异更加明显,表明它们之间不存在直接的成因联系。综合锆石U-Pb、Lu-Hf同位素特征以及岩石地球化学特征,我们认为低铝奥长花岗质片麻岩是低压下由新太古代新生基性地壳物质低程度部分熔融而成,源区残留矿物相以辉石+斜长石为主,岩浆可能存在过独居石的分异作用。高铝TTG岩系主要由新生基性地壳在相对高压下部分熔融而成,源区残留相以石榴石+辉石+角闪石以及无或少量斜长石为特征。岩浆经历过角闪石和辉石分离结晶作用,铕正异常增大的现象可能与斜长石堆晶有关。本区同时形成高铝和低铝TTG岩系的机制还需深入研究。俯冲机制、地幔柱机制以及二者共同作用等机制均能解释TTG的成因。依据本区同期还形成大量辉长质-闪长质岩浆和稍晚(2.5~2.45Ga)形成的钾质花岗岩类岩浆的侵入活动,我们认为本区高铝和低铝TTG岩系分别来自底侵作用导致的下地壳不同深度不同程度的部分熔融有关。引起底侵作用的机制可能与地幔柱或地幔柱与板块俯冲共同作用有关。     

The Volch’etundrovsky Massif of the autonomous anorthosite complex of the Main Range, the Kola Peninsula: Geological, petrogeochemical, and isotope-geochronological studies

The Volch??etundrovsky Massif occupies the middle part of the autonomous anorthosite complex of the Main Range, has a sheet morphology and marks the tectonic suture between the Kola block and the Belomorian mobile belt. The massif is characterized by homogenous structure and consists of the volumetrically dominant Main Zone including leucogabbro, leucogabbronorites, and anorthosites, and Marginal Zone made up of leuconorites and gabbronorites with subordinate plagioclasites and orthopyroxenites. Chemically, the rocks of the Volch??etundrovsky Massif are ascribed to the normal (tholeiitic and calc-alkaline) petrochemical series with typomorphic high Al₂O₃ contents (11.71?C29.32 wt %). With Al₂O₃ increase in the leuconorite-anorthosite series, the SiO₂ and TiO₂ contents show weak variations, CaO and alkalis insignificantly increase, whereas the MgO and FeO contents sharply decrease. The rocks of the Volch??etundrovsky Massif reveal significant REE fractionation and increase in total REE content in the leuconorite-anorthosite series, most approximating the Paleoproterozoic (Sumian) anorthosites of the Kola region. The anorthosites and leucogabbro are characterized by flat HREE, while the leuconorites is strongly depleted in HREE due to garnet fractionation. All rocks of the massif have significant positive Eu anomalies caused by the plagioclase accumulation. Zircons are characterized by LREE depletion and enrichment in HREE. This defines the steep positive slope of the plots complicated by the negative Eu and positive Ce (in zircons from leucogabbro) anomalies, which is typical of the REE distribution patterns in the unaltered zircons from igneous rocks. In zircons from anorthosites, the Ce anomaly is weak to absent. The trace-element distribution in the rocks of the Volch??etundrovsky Massif show positive Ba, Ta, Pb, Sr, Sc, and V anomalies, being controlled by the mineral specifics of the massif and the presence of definite accessory minerals. New U-Pb zircon data on the rocks of the Volch??etundrovsky Massif indicate that the leuconorites from the Marginal Zone were formed 2473 ± 7 Ma and 2463 ± 2.4 Ma ago, and the leucogabbro from the Main Zone, 2467 ± 8 Ma. These rocks have negative ?_Nd(T) from -1.54 up to -3.10, which indicates their derivation from enriched mantle reservoir variably contaminated by crustal material. The anorthosites of the Main Zone define an U-Pb age of 2407 ± 3 Ma and ?_Nd(T) = ?3.78, which presumably reflect the timing of hydrothermal-metasomatic alterations in the upper part of the magmatic chamber accompanied by significant crustal contamination.     

Depositional conditions,characteristics and source of rare earth elements in carbonate strata of the Albian Asu River Group,Middle Benue Trough,north central Nigeria

Major, trace and rare earth elements (REE) concentrations in limestone beds of the Asu River Group within the Middle Benue Trough were measured to understand the depositional conditions, characteristics and source of REE. The limestone has high content of CaO (Average of 46.55%), followed by SiO₂ (Average of 7.90 %), Fe₂O₃(t), MgO and Al₂O₃. The limestones are depleted in most of the trace elements (Co, V, Rb, Ba, Zr, Y, Nb, Hf and Th) when compared with the Post-Archean Australian Shale (PAAS). The observed large variations in ΣREE contents among various limestones of the present study (12.22 to 142.53ppm) are mainly due to the amount of terrigenous matter present in them. The characteristics of non-seawater-like REE patterns, elevated REE concentrations, high La_N/Yb_N ratios and low Y/Ho ratios, suggest that the observed variations in ΣREE contents are mainly controlled by the amount of detrital sediments in the limestones of the Asu River Group in the middle Benue trough. The observed variations in Ce contents and Ce anomalies in the studied samples resulted from detrital input. The limestones show positive Mn* values (0.30 to 0.78) and low contents of U (～0.60–3.20 ppm) suggesting that they were deposited under oxygen-rich environment.     

同源岩浆不同期次之间混合产生的暗色包体——以北拉萨地块中部晚白垩世桑心日岩体为例

本文在研究西藏北拉萨块体中段桑心日岩体中的暗色包体时发现了一种具有特殊岩石成因的暗色包体。暗色包体呈椭球状,在暗色包体和寄主岩的接触面上通常形成一个明显的可能由风化作用造成的间隙面。暗色包体为二长玢岩-花岗闪长玢岩,寄主岩为花岗岩,暗色包体明显较寄主岩更基性,更富Na_2O、CaO、MgO和Fe_2O_3~T。暗色包体和寄主岩具有明显不同的稀土元素特征,暗色包体的稀土元素含量变化较大,最基性样品具有最高的稀土含量,随着基性程度的降低稀土元素含量明显下降。随着岩浆的进一步演化,岩浆向花岗岩方向演化,稀土含量又逐步升高。寄主岩和暗色包体具有基本一致的微量元素组成,具有典型的弧岩浆岩的特征,富集Rb、Cs、K等大离子亲石元素和Th、U,亏损Nb、Ta、Ti等高场强元素。此外,暗色包体和寄主岩具有明显的Ba、Sr的负异常。暗色包体成岩年龄为75. 6±1. 2Ma,寄主岩的成岩年龄为71. 8±0. 6Ma,暗色包体成岩年龄较寄主岩早约4Myr。两者具有一致的锆石原位Lu-Hf同位素特征。综合以上岩相学、年代学、元素地球化学和同位素地球化学证据,我们认为桑心日暗色包体和寄主岩来源于同源母岩浆,初始岩浆在母岩浆房中经历了不同程度的含钾角闪石结晶分离作用,并沿早期较弱的构造裂隙侵入到地壳的某一层位,随着构造活动进一步加剧,经过进一步分异母岩浆大规模上侵,并将早先侵位处于半塑性状态的暗色包体侵吞、裹挟至近地表。桑心日暗色包体最可能的成因模式可以解释为同源岩浆不同期次间的物理混合。     

河南舞阳铁山庙式BIF铁矿的矿物学与地球化学特征及对矿床成因的指示

河南舞阳铁矿位于华北克拉通南缘.铁山庙式铁矿是舞阳铁矿的一部分,赋存于新太古界太华杂岩铁山庙组表壳岩中.本文根据铁山庙式铁矿中三种不同类型矿石(条带状石英-辉石-磁铁矿、块状辉石-磁铁矿、块状石英-磁铁矿)中磁铁矿的矿物成分、全岩/矿的主量元素及微量元素特征,探讨铁山庙式铁矿床的成因.磁铁矿单矿物成分分析表明,条带状石英-辉石-磁铁矿矿石中磁铁矿的FeOT含量90.6％ ～93.1％,平均91.8％;块状辉石-磁铁矿矿石中磁铁矿的FeOT含量90.7％～91.2％,平均91.0％;块状石英-磁铁矿矿石中磁铁矿的FeOT含量92.0％～93.0％,平均92.4％.上述平均值均与磁铁矿FeOT的理论值(93.1％)接近.三种类型矿石的其它元素如TiO2、MgO、MnO、CaO、Al2O3 Cr2O3 NiO等含量均＜0.1％,无明显区别,表明该区磁铁矿为含杂质极少的纯磁铁矿,表现出沉积变质成因磁铁矿的特征.矿石中斜方辉石-单斜辉石及近矿围岩紫苏辉石-长石-石英矿物组合,表明铁山庙式矿床经受了高级变质作用,石英、磁铁矿等矿物普遍发生变质重结晶,颗粒粗大,但仍保存原有的地球化学组成.元素地球化学分析显示,三种类型矿石中SiO2 、TiO2 Al2O3、P2O5的含量相近;块状辉石-磁铁矿较其它二者相对贫铁、富钙、镁,这是由于块状辉石-磁铁矿石中富含铁普通辉石和铁次透辉石所致;矿石中TiO2、Al2O3含量都极低,说明该区成岩成矿过程中未受到碎屑物质的混染.三种不同类型矿石的主量元素含量总体上都与世界典型BIF的相近.对于稀土元素,三种类型矿石均具有轻稀土亏损、重稀土富集((La/Yb)PAAS=0.29～0.995＜1),La、Eu、Y的正异常(La/La*=1.10～1.89;Eu/Eu* =1.30～2.23;Y/Y* =1.47～1.84),较高的Y/Ho比值(39.7 ～51.3),具有现代海水及高温热液混合特征.因此,我们认为铁山庙式铁矿三种不同类型的矿石是极少受到陆源碎屑混染的化学沉积成因,虽遭受后期变质作用,但仍属BIF型铁矿.     

THE PETROCHEMISTRY OF ANCIENT GRANITOIDS OF THE SOUTHERN SIBERIAN CRATON AND ITS MARGINAL FOLD BELTS

The geochemistry of Laramide granitic rocks from central Sonora was studied to relate possible compositional variations to the assimilation of different crustal sources. Most of the studied rocks are granodiorites collected near the southern boundary of the Paleozoic North American continent. North of this boundary, the Laramide plutons intruded a thick section of Upper Proterozoic and Paleozoic miogeoclinal strata, whereas south of it, the intrusives were emplaced in Lower Ordovician to Permian eugeoclinal rocks accreted to the continental margin between Late Permian and Middle Triassic times. Whole-rock Na₂O, TiO₂, and P₂O₅ contents are slightly higher in plutons intruded in cratonic and miogeoclinal rocks, whereas MgO and CaO seem to be higher in plutons emplaced in eugeoclinal rocks. The samples located in the north are characterized by steeper chondrite-normal-ized REE slopes and generally well developed negative europium anomalies, whereas the group of granitoids in the south have flatter REE slopes and generally smaller negative europium anomalies. Available isotope data show ⁸⁷Sr/⁸⁶Sr initial ratios above 0.7070, and ε_Nd initial values between ?4.6 and ?4.2 in plutons to the north. In a narrow E-W region just south of the Paleozoic continental margin, the samples yielded similar ⁸⁷Sr/⁸⁶Sr ratios but unexpectedly low ε_Nd values between ?5.4 and ?5.1. South of this region, the isotope signatures are slightly more primitive, with ⁸⁷Sr/⁸⁶Sr initial ratios between 0.7067 and 0.7057 and initial ε_Nd values between ?3.9 and ?3.7. The results of this study suggest that the nature of the assimilated crust may have influenced the final composition of the Laramide granitic rocks of central Sonora.     

The alleged kimberlite-carbonatite relationship: evidence from ilmenite and spinel from Premier and Wesselton mines and the Benfontein sill,South Africa

Carbonate-rich, SiO₂-poor residua are developed in some kimberlites solidifying as ocelli, layers, or discrete dikes which satisfy petrographic definitions of carbonatite. Arguments that these rocks have mineralogies, antecedents, and comagmatic rocks differing from those of the carbonatites in alkaline rock complexes, including the specific observation that kimberlites and carbonatites contain ilmenites and spinels of different composition, have been used to refute the alleged kimberlite-carbonatite relationship. New microprobe analyses of ilmenites and spinels from carbonate-rich rocks associated with kimberlites in three South African localities correspond to spinels and ilmenites of carbonatites from alkalic complexes, or have characteristics intermediate between those of carbonatites and kimberlites. The ilmenites are distinguished from kimberlite ilmenites by higher MnO, FeTiO₃, and Nb₂O₅, and by negligible Cr₂O₃. The spinels are distinguished from kimberlite spinels by their Al₂O₃ and Cr₂O₃ contents. There is clearly a genetic relationship between the kimberlites and the carbonate-rich rocks, despite the observation that their ilmenites and spinels are distinctly different, which indicates that the same observation is not a valid argument against a petrogenetic relationship between kimberlites and carbonatites. These rocks are among the diverse products from mantle processes influenced by CO₂, and we believe that the petrogenetic links among them are forged in the upper mantle. We see insufficient justification to deny the name carbonatite to carbonate-rich rocks associated with kimberlites if they satisfy the petrographic definition in terms of major mineralogy.