北京密云放马峪基性岩墙群LA-MC-ICP-MS锆石U-Pb年龄及其意义

太古宙基性岩墙群在北京密云地区广泛分布,其年代学研究一直备受关注,多年来取得了一些数据,但对太古宙变质片麻岩控制作用未做论述。利用LA-MC-ICP-MS U-Pb同位素测定技术,测得放马峪石榴辉石岩中锆石的年龄为2497±15Ma,此年龄被解释为区内变质片麻岩成岩年龄应早于基性岩墙群成岩年龄,其为研究区内太古宙片麻岩成岩年龄提供了重要的参考数据。     

新疆西天山查岗诺尔铁矿床环带石榴子石和绿帘石的发现及意义

查岗诺尔铁矿是新疆西天山阿吾拉勒铁矿带内的重要大型铁矿床之一。矿体赋存在下石炭统大哈拉军山组安山质火山岩中,与普遍发育的石榴子石化、阳起石化和绿帘石化时空关系密切。石榴子石和绿帘石分属不同热液成矿阶段,它们均发育丰富的环带结构,具体表现为明显地颜色、干涉色、背散射图像及成分（FeO、Al₂O₃、SiO₂、MnO、TiO₂）等差异性。石榴子石具有2个世代、3个类型。早世代石榴子石（Grt1和Grt2）产于块状石榴子石-磁铁矿蚀变岩,呈褐黄色,粒度较细,发育核-边结构,呈非均质性,显示异常干涉色,其核部（Grt1-c）均匀相对富钙铝榴石（Gro_51-53And_41-43Spr_4-8）,而边部（Grt1-r）发育振荡成分环带,总体相对富钙铁榴石（Gro_18-35And_60-77Spr_4-6）;Grt2核部（Grt2-c）呈均质性,为钙铁榴石（And_99-100Spr_0-1）,边部显异常干涉色,发育振荡成分环带,为钙铝铁榴石（Gro_34-54And_38-61Spr_6-9）。晚世代的石榴子石（Grt3）以细脉状或角砾胶结物形式分布,呈红褐色,自形粗粒结构,显非均质性,发育振荡成分环带,端员组分总体以钙铁榴石为主,次为钙铝榴石（Gro_27-43And_50-68Spr_3-8）。石榴子石结构和元素含量变化表明,早期石榴子石形成于弱氧化-氧化、中性-碱性流体体系,其中向边部生长过程,由于新注入流体以及周期性压力汇聚和释放,体系的氧逸度、pH值呈振荡变化;晚期石榴子石形成于弱氧化、弱碱性、动荡的开放流体环境。绿帘石发育3个世代（Ep1、Ep2和Ep3）。Ep1发育核-边结构,核部（Ep1-c）均匀无环带,X_Fe值（X_Fe=Fe³⁺/（Al+Fe³⁺）,原子比值）为0.19~0.21,w（MnO）为0.05%~0.18%,w（TiO₂）为0.10%~0.12%,生长边（Ep1-r）多发育振荡环带,X_Fe值为0.26~0.29,w（MnO）为0.01%~0.14%,w（TiO₂）为0.19%~0.26%。Ep2沿Ep1-r边缘生长,不均匀且经历了溶解-再沉淀过程,X_Fe值为0.15~0.20,w（MnO）为0.42%~1.19%,w（TiO₂）为0.02%~0.07%。Ep3呈柱状或不规则粒状交代Ep2、贴近或穿切Ep1-r生长,较均匀、无环带结构,X_Fe值为0.28~0.37,w（MnO）为0.12%~0.77%,w（TiO₂）为0.02%~0.10%。绿帘石成分变化表明,从Ep1-c到Ep1-r,到Ep2,再到Ep3,流体体系氧逸度经历了先增加,后降低,再升高的变化过程。同时,流体成分也在变化,先从相对贫Ti和Mn向相对富Ti贫Mn演化,而后又变为富Mn贫Ti。因此,在热液磁铁矿矿化阶段,查岗诺尔铁矿的成矿热液的物理-化学环境是不断变化的。研究显示,石榴子石和绿帘石结构和成分研究可以刻画热液成矿系统的流体演化历史。     

五河杂岩的变质岩石学及P-T条件分析钻孔的研究

五河杂岩是华北克拉通东部陆块东南缘最重要的岩石单元。然而,直至今日,人们对于该组杂岩一直缺乏足够的研究,特别是对于其形成的变质条件分歧较大。本文通过对蒙城南ZK02钻孔中五河杂岩的构成和地质特征详细研究表明五河杂岩分为上、下两段。上段主要为一套由云母片岩、大理岩和花岗片麻岩组成的变质表壳岩系,下段为一套基性变质岩,主要岩性为斜长角闪岩、角闪岩、石榴斜长角闪岩和少量的混合岩。同时,针对下段特征岩石（石榴斜长角闪岩）,进行了较为详细的岩相学和矿物化学分析,并应用不同的温压计进行P-T条件评价和比较,结合岩相学限定和前人的研究成果,认为五河杂岩的主体普遍经历了高压角闪岩相变质作用,其温压范围为：T=671 ℃～700 ℃和P=0.82～0.95 GPa,地温梯度约为25～28 ℃/km,与典型陆壳地温梯度类似。与同邻区主要岩石单元的变质级别相比,五河杂岩应当形成于板内变质环境,并不具备造山带变质特征。     

滇西马厂箐铜钼多金属矿床成矿流体来源及演化过程——来自石榴子石原位微量元素及H- O同位素组成的限定

滇西马厂箐铜钼多金属矿床位于三江特提斯成矿域,是一个与喜马拉雅期富碱斑岩侵入有关的多金属矿床。前人研究表明,马厂箐铜钼多金属矿床形成于斑岩-矽卡岩成矿系统,但由于缺乏系统矿物学研究,目前对矽卡岩矿化过程和成矿效应仍不清楚,限制了对该矿床成矿过程的全面认识。因此,本文以马厂箐矿床矽卡岩型矿化中的石榴子石为研究对象,利用H-O同位素、电子探针(EPMA)以及LA-ICP-MS原位微区技术开展了同位素及成分分析,限定成矿流体来源以及反演成矿流体演化过程。石榴子石可分为早期自形石榴子石(Grt I)和晚期他形石榴子石(Grt II)。Grt I与辉石共生,并伴有黄铁矿、黄铜矿等金属矿物组合,属于钙铝-钙铁榴石固溶体(And_{49.37～99.58}Gro_0～49.79);Grt II更富Fe,属钙铁榴石系列(And_{67.50～99.85}Gro_0～31.84)。两期石榴子石均富含Th、U、Nd,亏损Ba、Sr、Hf、Nb,富轻稀土。基于石榴子石矿物化学特征认为,Grt I可能是在弱酸性、氧化、低水岩比(W/R)条...     

Origin of Grandite Garnet in Calc-Silicate Granulites: Mineral-Fluid Equilibria and Petrogenetic Grids

The role of clinopyroxene in producing grandite garnet is evaluatedusing data from an ultrahigh-temperature metamorphosed calc-silicategranulite occurrence in the Eastern Ghats Belt, India. ‘Peak’pressure–temperature conditions of metamorphism were previouslyconstrained from associated high Mg–Al granulites as c.0·9 GPa, >950°C, and the rocks were near-isobaricallycooled to c. 750°C. Grandite garnet of variable compositionwas produced by a number of reactions involving phases suchas clinopyroxene, scapolite, plagioclase, wollastonite and calcite,in closely spaced domains. Compositional heterogeneity is preservedeven on a microscale. This precludes pervasive fluid fluxingduring either the peak or the retrograde stage of metamorphism,and is further corroborated by computation of fluid–rockratios. With the help of detailed textural and mineral compositionalstudies leading to formulation of balanced reactions, and usingan internally consistent thermodynamic dataset and relevantactivity–composition relationships, new petrogenetic gridsare developed involving clinopyroxene in the system CaO–Al₂O₃–FeO–SiO₂–CO₂–O₂in T–aCO₂–fO₂ space to demonstrate the importanceof these factors in the formation of grandite garnet. Two singularcompositions in garnet-producing reactions in this system arededuced, which explain apparently anomalous textural relations.The possible role of an esseneite component in clinopyroxenein the production of grandite garnet is evaluated. It is concludedthat temperature and fO₂ are the most crucial variables controllinggarnet composition in calc-silicate granulites. fO₂, however,behaves as a dependent variable of CO₂ in the fluid phase. Externalfluid fluxing of any composition is not necessary to producechemical heterogeneity of garnet solid solution. KEY WORDS: grandite garnet; role of clinopyroxene; internal buffering; oxidation–decarbonation equilibria     

Roles for fluid and/or melt advection in forming high-P mafic migmatites, Fiordland, New Zealand

A series of striking migmatitic structures occur in rectilinear networks through western Fiordland, New Zealand, involving, for the most part, narrow anorthositic dykes that cut hornblende‐bearing orthogneiss. Adjacent to the dykes, host rocks show patchy, spatially restricted recrystallization and dehydration on a decimetre‐scale to garnet granulite. Although there is general agreement that the migration of silicate melt has formed at least parts of the structures, there is disagreement on the role of silicate melt in dehydrating the host rock. A variety of causal processes have been inferred, including metasomatism due to the ingress of a carbonic, mantle‐derived fluid; hornblende‐breakdown leading to water release and limited partial melting of host rocks; and dehydration induced by volatile scavenging by a migrating silicate melt. Variability in dyke assemblage, together with the correlation between dehydration structures and host rock silica content, are inconsistent with macroscopic metasomatism, and best match open system behaviour involving volatile scavenging by a migrating trondhjemitic liquid.     

高喜马拉雅中段基性捕虏体的变质作用及其构造意义

西藏南部聂拉木—樟木剖面出露的高喜马拉雅变质带主要由副变质片麻岩和花岗质片麻岩组成,其次为伟晶岩和淡色花岗侵入体,区域变质程度为角闪岩相。我们对其中的变质基性捕虏体进行详细的变质作用研究,内容包括变质矿物组合,矿物变质反应结构和变质作用的温度—压力条件分析。基性捕虏体中的石榴子石角闪片麻岩和斜长角闪片麻岩均保存了两期变质矿物组合。温度与压力计算结果表明,石榴子石角闪片麻岩早期变质阶段(M1)温度约为829 ℃,压力为7.3 kbar; 晚期(M2)变质温度为625 ℃,压力为4.3 kbar。斜长角闪片麻岩所经历的早期变质阶段(M1)温度约为776 ℃、压力约为10.6 kbar; 晚期(M2)变质温度超过692 ℃,压力为7.4 kbar。石榴子石角闪片麻岩和斜长角闪片麻岩捕虏体均记录了典型的顺时针P-T轨迹,表明高喜马拉雅变质带曾向北俯冲到下地壳深度,之后被抬升到地表剥蚀出露。变质基性捕虏体的研究说明高喜马拉雅结晶岩系经历过较高温度—压力的变质作用,支持了其沿着藏南拆离系和主中央逆冲断裂系向南挤出的大地构造模型。     

Ultrahigh-pressure metamorphism in the forbidden zone: the Xugou garnet peridotite, Sulu terrane, eastern China

The Xugou garnet peridotite body of the southern Sulu ultrahigh‐pressure (UHP) terrane is enclosed in felsic gneiss, bounded by faults, and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite. The peridotite is composed of variable amounts of olivine (Fo₉₁), enstatite (En_92?93), garnet (Alm_20?23Prp_53?58Knr_6?9Grs_12?18), diopside and rare chromite. The ultramafic protolith has a depleted residual mantle composition, indicated by a high‐Mg number, very low CaO, Al₂O₃ and total REE contents compared to primary mantle and other Sulu peridotites. Most garnet (Prp_44?58) clinopyroxenites are foliated. Except for rare kyanite‐bearing eclogitic bands, most eclogites contain a simple assemblage of garnet (Alm_29?34Prp_32?50Grs_15?39) + omphacite (Jd_24?36) + minor rutile. Clinopyroxenite and eclogite exhibit LREE‐depleted and LREE‐enriched patterns, respectively, but both have flat HREE patterns. Normalized La, Sm and Yb contents indicate that both eclogite and garnet clinopyroxenite formed by high‐pressure crystal accumulation (+ variable trapped melt) from melts resulting from two‐stage partial melting of a mantle source. Recrystallized textures and P–T estimates of 780–870 °C, 5–7 GPa and a metamorphic age of 231 ± 11 Ma indicate that both mafic and ultramafic protoliths experienced Triassic UHP metamorphism in the P–T forbidden zone with an extremely low thermal gradient (< 5 °C km^?1), and multistage retrograde recrystallization during exhumation. Develop of prehnite veins in clinopyroxenite, eclogite, felsic blocks and country rock gneiss, and replacements of eclogitic minerals by prehnite, albite, white mica, and K‐feldspar indicate low‐temperature metasomatism.     

Oscillatory zoning in garnet from the Willsboro Wollastonite Skarn, Adirondack Mts, New York: a record of shallow hydrothermal processes preserved in a granulite facies terrane

Oscillatory zoning in low δ¹⁸O skarn garnet from the Willsboro wollastonite deposit, NE Adirondack Mts, NY, USA, preserves a record of the temporal evolution of mixing hydrothermal fluids from different sources. Garnet with oscillatory zoning are large (1–3 cm diameter) euhedral crystals that grew in formerly fluid filled cavities. They contain millimetre‐scale oscillatory zoning of varying grossular–andradite composition (X_Adr = 0.13–0.36). The δ¹⁸O values of the garnet zones vary from 0.80 to 6.26‰ VSMOW and correlate with X_Adr. The shape, pattern and number of garnet zones varies from crystal to crystal, as does the magnitude of the correlated chemistry changes, suggesting fluid system variability, temporal and/or spatial, over the time of garnet growth. The zones of correlated Fe content and δ¹⁸O indicate that a high Fe³⁺/Al, high δ¹⁸O fluid mixed with a lower Fe³⁺/Al and δ¹⁸O fluid. The high δ¹⁸O, Fe enriched fluids were likely magmatic fluids expelled from crystallizing anorthosite. The low δ¹⁸O fluids were meteoric in origin. These are the first skarn garnet with oscillatory zoning reported from granulite facies rocks. Geochronologic, stable isotope, petrologic and field evidence indicates that the Adirondacks are a polymetamorphic terrane, where localized contact metamorphism around shallowly intruded anorthosite was followed by a regional granulite facies overprint. The growth of these garnet in equilibrium with meteoric and magmatic fluids indicates an origin in the shallow contact aureole of the anorthosite prior to regional metamorphism. The zoning was preserved due to the slow diffusion of oxygen and cations in the large garnet and protection from deformation and recrystallization in zones of low strain in thick, rigid, garnetite layers. The garnet provide new information about the hydrothermal system adjacent to the shallowly intruded massif anorthosite that predates regional metamorphism in this geologically complex, polymetamorphic terrane.     

Polymetamorphism in garnet micaschists of the Saualpe Eclogite Unit (Eastern Alps,Austria), resolved by automated SEM methods and EMP–Th–U–Pb monazite dating

Polymetamorphic garnet micaschists from the Austroalpine Saualpe Eclogite Unit (Kärnten, Austria, Eastern Alps) display complex microstructural and mineral–chemical relationships. Automated scanning electron microscopy routines with energy dispersive X‐ray (EDX) spectral mapping were applied for monazite detection and garnet mineral–chemical characterization. When the Fe, Mg, Mn and Ca element wt% compositions are used as generic labels for garnet EDX spectra, complex zonations and porphyroblast generations can be resolved in complete thin sections for selective electron‐microprobe analyses. Two garnet porphyroblast generations and diverse monazite age populations have been revealed in low‐Ca and high‐Al‐metapelites. Garnet 1 has decreasing Mn, constant Ca and significantly increasing Mg from cores to rims. Geothermobarometry of garnet 1 assemblages signals a crystallization along a M1 prograde metamorphism at ~650 °C/6–8 kbar. Sporadic monazite 1 crystallization started at c. 320 Ma. Subsequent pervasive 300–250 Ma high‐Y and high‐Gd monazite 1 formation during decompression coincided with the intrusion of Permian and Early Triassic pegmatites. Monazite 1 crystallized along the margin of garnet 1. Coronas of apatite and allanite around the large 320–250 Ma monazite signal a retrogressive stage. These microstructures suggest a Carboniferous‐to‐Early‐Permian age for the prograde M1 event with garnet 1. Such a M1 event at an intermediate‐P/T gradient has not yet been described from the Saualpe, and preceded a Permo‐Triassic low‐P stage. The M2 event with garnet 2 postdates the corona formation around Permian monazite. Garnet 2 displays first increasing X_Ca at decreasing X_Mg, then increasing X_Ca and X_Mg, and finally decreasing X_Ca with increasing X_Mg, always at high Ca and Mg, and low Mn. This records a P–T evolution which passed through eclogite facies conditions and reached maximum temperatures at ~750 °C/14 kbar during decompression‐heating. A monazite 2 population (94–86 Ma) with lower Y and Gd contents crystallized at decreasing pressure during the Cretaceous (Eo‐Alpine) metamorphism M2 at a high‐P/T gradient. The Saualpe Eclogite Unit underwent two distinct clockwise metamorphic cycles at different P–T conditions, related to continental collisions under different thermal regimes. This led to a characteristic distribution pattern of monazite ages in this unit which is different from other Austroalpine basement areas.