碳酸盐岩风化成土过程中REE超常富集及Ce强烈亏损的地球化学机理

碳酸盐岩的REE含量极低，但最近在贵州一些碳酸盐岩风化表土层底部首次发现了REE的超常富集层，REE总量最高可达近31000μg/g;Ce强烈亏损，δCe值最低达0.007。这种低背景、高富集、强分异的REE富集现象在贵州碳酸盐岩风化壳中具有一定的普遍性。选取发育在下三叠统花溪灰岩和平坝白云岩之上的两个较具代表性的碳酸盐岩风化壳进行研究，结果表明，这种富集现象与碳酸盐岩风化成土的两阶段性密切相关。（1）在残积土形成阶段（风化早期），碳酸盐的快速溶解导致风化前缘形成一个垂直方向相对狭窄、突变的碱性障(pH值为8左右）。此障既能有效地将碳酸盐岩分解释放的REE以及下渗水携带的REE分解沉淀和以吸附于粘土矿物上的方式富集，也容易使Ce^3 氧化成Ce^4 ，与HCO3^-形成稳定的可溶性络合物随下渗雨水流走，从而使Ce进一步亏损。（2）残积土演化阶段，轻、重稀土发生明显分异，Ce^3 氧化成Ce^4 并发生水解沉淀，致使下渗水富重稀土而贫Ce，最终使REE在剖面上显示出一般风化壳共有的分异特征。另外，根据对碳酸盐岩中的酸不溶物、可溶性稀土的提取以及质量平衡计算，碳酸盐岩能够提供足够的REE物源；以可溶态为主的赋存状态有利于REE的淋滤（活化）。     

新疆恰尔墩巴斯希铁-铜-金矿矿床地质研究

新疆恰尔墩巴斯希铁-铜-金矿床中磁铁矿化和铜矿化与中基性侵入岩体密切相关。磁铁矿化在辉长岩体与中基性火山岩的内外接触带发育,由内带的磁铁矿+透辉石组合变化到外带的磁铁矿+石英+钙铁榴石+方解石组合。辉长岩和闪长岩的轻、重稀土元素分异明显〔(La/Yb)N为3.19～7.81〕,富集大离子亲石元素,亏损Nb、Ta,具岛弧岩浆岩特征。辉长岩氧逸度较高,明显富钾。铜矿化主要集中于闪长岩体的外接触带,大部分铜矿化充填在热液角砾岩中。可划分出4个铜矿化阶段:①钠长石-石英阶段;②黄铜矿-黄铁矿-自然金-绢云母-石英阶段;③黄铜矿-黄铁矿-绿帘石-葡萄石阶段;④闪锌矿-方铅矿-石英-方解石阶段。自然金主要出现在糜棱岩化之后的黄铁矿-石英脉中。     

河南省方城县双山碱性正长岩型铌矿综合找矿方法及找矿模型

通过双山铌矿的发现,提出了适合于碱性正长岩型"稀有稀土"矿的综合找矿方法,即以水系沉积物测量发现靶区、以岩石测量圈出矿化带、以特征矿物填图厘定含矿地质体、以槽探工程控制地表矿体、以钻探工程验证深部矿体的方法.实践证明,该找矿方法组合具有经济、快捷、找矿效果显著等特点.通过对各种找矿方法的分析研究,建立了找矿模型,对国内碱性正长岩型铌 稀土多金属找矿具有重要意义.     

Contents of Fixed-Ammonium (NH_4~+) in Lamprophyres in the Zhenyuan Gold Orefield, Yunnan Province, China: Implications for Its Characteristics of the Source Region

This paper determined the fixed-ammonium (NH 4) contents of lamprophyres in the Zhenyuan gold orefield, Yunnan Province, China. The results show that the NH 4 contents of minettes in the orefield range from 120×10 -6 to 469×10 -6 and those of kersantites from 74.3 ×10 -6 to 136×10 -6 . These values are higher than those of other mantle-derived rocks (less than 50×10 -6 ), but lower than those of carbonaceous wall rocks in the orefield (from 1200×10 -6 to 1343×10 -6 ). Combining with the Sr isotopic composition, this paper suggested that lamprophyres in the orefield with high NH 4 contents relative to other mantle-derived rocks would not have resulted from the primary magma contaminated by crustal materials in the process of rising or in the magma chamber, but from mantle metasomatism.     

内蒙古科尔沁右翼中旗碱长花岗岩锆石U-Pb年代学、岩石地球化学及其动力学意义

对内蒙古科尔沁右翼中旗碱长花岗岩进行了同位素年代学及岩石地球化学研究。碱长花岗岩LA-ICP-MS锆石U-Pb加权平均年龄为(166±1)Ma,表明该侵入体是中侏罗世侵位形成的。岩石学及地球化学成分显示其属于碱性、具A型花岗岩特征。岩石高硅(w(SiO₂)=74.80%~76.34%)、富碱(w(Na₂O+K₂O)=7.94%~8.71%)、高铁镁比(TFeO/MgO=13.54~24.28)、贫钙(w(CaO)=0.10%~0.21%)、贫镁(w(MgO)=0.08%~0.16%)和低钛(w(TiO₂)=0.07%~0.10%);稀土配分曲线呈现"海鸥式"分布特征,显示强烈的Eu负异常(δEu=0.09~0.17);微量元素特征显示具有较高质量分数的有Zr(128.95×10^-6~156.32×10^-6)、Yb(4.93×10^-6~5.35×10^-6)和Y(40.93×10^-6~56.75×10^-6),较低质量分数的有Sr(23.16×10^-6~37.14×10^-6)、Ba(186.13×10^-6~231.31×10^-6),在微量元素原始地幔标准化蛛网图上显示明显的Sr、Ba和Ti的负异常。以上特征表明,碱长花岗岩为A型花岗岩。岩石具有高的Rb/Sr值(4.26~7.81,平均为6.12)和Rb/Nb值(10.2~14.7,平均为12.7),显示出壳源岩浆的成分特征。综合分析表明,碱长花岗岩为低压下长英质地壳部分熔融的产物。w(Rb)-w(Yb+Ta)图解、w(Rb)-w(Y+Nb)图解、w(Ta)-w(Yb)图解、w(Nb)-w(Y)图解、Ce/Nb-Y/Nb图解、Ce/Nb-Yb/Ta图解及结合区域构造演化研究表明,碱长花岗岩形成于造山后伸展的构造环境,并与松辽盆地及其周围的花岗岩一起暗示松辽盆地是在中侏罗世造山作用之后伸展的构造环境下形成的陆内盆地。     

Role of chemical processes on shear zone formation: an example from the Grimsel metagranodiorite (Aar massif,Central Alps)

Alpine deformation in the Grimsel granodiorite (Aar massif, Central Alps) at greenschist facies conditions (6.5 ± 1 kbar for 450°C ± 25°C) is characterized by the development of a network of centimetre to decametre localized shear zones that surround lenses of undeformed granodiorite. Localization of deformation is assumed to be the result of a first stage of extreme localization on brittle precursors (nucleation stage) followed by a transition to ductile deformation and lateral propagation into the weakly deformed granodiorite (widening stage). A paradox of this model is that the development of the ductile shear zone is accompanied by the crystallization of large amounts of phyllosilicates (white mica and chlorite) that maintains a weak rheology in the localized shear zone relative to the host rock so that deformation is localized and prevents shear zone widening. We suggest that chemical processes, and more particularly, the metamorphic reactions and metasomatism occurring during re‐equilibration of the metastable magmatic assemblage induced shear zone widening at these P–T–X conditions. These processes (reactions and mass transfer) were driven by the chemical potential gradients that developed between the thermodynamically metastable magmatic assemblage at the edge of the shear zone and the stable white mica and chlorite rich ultramylonite formed during the first stage of shear zone due to localized fluid infiltration metasomatism. P–T and chemical potential projections and sections show that the process of equilibration of the wall rocks (μ–μ path) occurs via the reactions: kf + cz + ab + bio + MgO + H₂O = mu + q + CaO + Na₂O and cz + ab + bio + MgO + H₂O = chl + mu + q + CaO + Na₂O. Computed phase diagram and mass balance calculations predict that these reactions induce relative losses of CaO and Na₂O of ～100% and ～40% respectively, coupled with hydration and a gain of ～140% for MgO. Intermediate rocks within the strain gradient (ultramylonite, mylonite and orthogneiss) reflect various degrees of re‐equilibration and metasomatism. The softening reaction involved may have reduced the strength at the edge of the shear zone and therefore promoted shear zone widening. Chemical potential phase diagram sections also indicate that the re‐equilibration process has a strong influence on equilibrium mineral compositions. For instance, the decrease in Si‐content of phengite from 3.29 to 3.14 p.f.u, when white mica is in equilibrium with the chlorite‐bearing assemblage, may be misinterpreted as the result of decompression during shear zone development while it is due only to syn‐deformation metasomatism at the peak metamorphic condition. The results of this study suggest that it is critical to consider chemical processes in the formation of shear zones particularly when deformation affects metastable assemblages and mass transfer are involved.     

东昆仑造山带中灶火地区早中生代镁铁质岩墙群的成因及地质意义

东昆仑造山带中灶火地区镁铁质岩墙群以闪长玢岩为主,含少量闪斜煌斑岩、辉绿玢岩及辉绿岩,LA-ICPMS锆石U-Pb年代学指示该套岩墙群结晶侵位年龄为(249±1)Ma。稀土元素含量整体较高,富集轻稀土元素(∑REE=99.9×10-6~173.9×10-6,(La/Yb)N=3.5~9.3);微量元素表现出富集大离子亲石元素(LILE),亏损高场强元素(HFSE)的特征;源区分析表明,镁铁质岩浆为俯冲洋壳析出的流体交代富集地幔的结果,且在岩石成因中部分熔融起到主导作用,地壳混染和分离结晶作用对岩浆成分分异起到的作用有限。构造环境分析表明,岩石的形成与俯冲作用有关,结合区域构造演化认为镁铁质岩墙群的成因为:早三叠世,在古特提斯洋向北俯冲的环境下,俯冲板片释放的流体交代富集地幔,诱发地幔部分熔融形成镁铁质岩浆,受弧后伸展的动力学背景影响,岩浆最终上升侵位形成镁铁质岩墙群。     

Final-stage Southward Subduction of the Eastern Paleo-Asian Ocean: Evidence from the Middle Permian Mafic Intrusions in the Northern Margin of the North China Craton

The northern margin of the North China Craton(NCC)contains widespread Permian magmatic rocks,but the origin of these rocks remains controversial.This uncertainty hampers us from better understanding of tectonic framework and evolution of the eastern Paleo-Asian Ocean,particularly with respect to its final-stage subduction and closure time.To address these questions,this study presents petrological,zircon U-Pb geochronological,whole-rock geochemical and in situ zircon Hf isotopic data for these Permian mafic intrusions in the northern margin of the NCC.Precise zircon U-Pb dating results indicate that these mafic intrusions were emplaced in the Middle Permian(ca.260 Ma).Geochemically,the studied mafic intrusions have high MgO and transition metals element contents,with high Mg^# values,indicating a mantle origin.These mafic intrusions are characterized by enrichments in large ion lithophile elements(LILEs;e.g.,Rb,Ba,and K)and light rare earth elements(LREEs),and depletions in high field strength elements(HFSEs;e.g.,Nb,Ta,and Ti)and heavy rare earth elements(HREEs),indicating that they were formed in a subduction-related setting.These geochemical features,together with zircon ε_Hf(t)values(-1.1 to+11.2),indicate that their parental magmas were derived from partial melting of heterogeneous mantle wedge metasomatized by subduction-related fluids,with the contributions of slab sediments.The studied mafic intrusions also show wide range of major and trace elements contents,and variable Mg^# values,Eu and Sr anomalies,suggesting that their parental magmas had undergone variable degrees of fractional crystallization.Together with the E-W trending Permian continental arc along the northern margin of the NCC,we confirm that the generation of the Middle Permian mafic intrusions was related to southward subduction of the Paleo-Asian oceanic lithosphere beneath the NCC and the Paleo-Asian Ocean had not closed prior to the Middle Permian.     

Geochemistry and petrogenesis of lamprophyric dykes and the associated rocks from Eslamy peninsula, NW Iran: Implications for deep-mantle metasomatism

Eslamy peninsula in NW of Iran is formed by a strato-volcano with collapsed calderon, which is intruded by lamprophyric dykes with minette composition. Also trachytic and microsyenitic dykes have intruded the volcanic rocks. The oldest volcanic activity includes eruption of leucite basanite, leucite tephrite, basanite and tephrite, which are associated with pyroclastic rocks. Lamprophyric dykes are distinguishable with large mica phenocrysts. Mica-clinopyroxenite xenoliths can be found in the rocks. The source magma of the rocks had a ultrapotassic to shoshonitic nature, rich in LREE and LILE. Eslamy peninsula lamprophyres are between alkaline and calc-alkaline lamprophyres in terms of REE patterns and spider diagrams for trace elements, but are closer to clac-alkaline lamprophyres. The behaviour of trace elements studied by the means of spider diagrams show that the magma, producing the lamprophyres, is generated from deep-mantle probably from a garnet-bearing source (garnet lherzolite) with high CO₂/H₂O content. The resulted magma had interacted with crustal materials and had formed Eslamy peninsula lamprophyres in a post-collisional tectonic setting. Geochemistry of rare elements indicate an extensive rutile-rich metasomatism in the source magma of the lamprophyres.     

南海新生代碱性玄武岩中斜长石矿物的化学成分及意义

南海新生代碱性玄武岩中存在两种不同粒径的斜长石矿物.其一为斜长石斑晶,常见熔蚀麻点,是岩浆上升、压力降低时发生熔蚀作用并在骤冷条件下形成的;其二斜长石微晶,半定向或杂乱分布于火山玻璃中,其中空骸晶结构表明斜长石微晶是在淬冷条件下迅速结晶形成的.斜长石斑晶具弱成分环带,斑晶边部的An值稍高或接近于斜长石微晶.微晶斜长石An值与岩浆喷出后的水深以及喷发位置距离岩浆主通道的远近存在一定联系.本区的斜长石斑晶形成温度明显低于冲绳海槽地区,而类似于东海陆架地区;斜长石微晶的结晶温度类似于冲绳海槽,表明两地区在岩浆喷出海底后淬火结晶的物理化学条件相似.结合同样品中橄榄石斑晶研究结果以及已有的地球物理学和岩石学方面的资料,可能反映了地幔柱快速上涌使早期部分熔融及结晶分异作用较弱,岩浆本身温度高提供了早期结晶形成的斑晶与寄主岩浆进一步充分反应的热量.计算的斜长石斑晶温度不能反映源区温度特征,后者应高于本文所计算的斜长石斑晶的结晶温度.