The duration of near‐peak metamorphism from diffusion modelling of garnet zoning

Garnet in a staurolite–kyanite zone sample from central Vermont displays a bell‐shaped Mn growth zoning with diffusional modification over the outer 100 μm. The diffusion is driven by the prograde net transfer reaction garnet + chlorite = kyanite + biotite as is evidenced by a well‐defined resorption zone on the rim. Analysis of the reaction history and resorbed garnet composition suggests that the peak temperature attained was 620–660 °C. Diffusional modelling of the rim diffusion provides an estimate of the duration of the metamorphic episode over which significant garnet diffusion occurs. The duration is a function of the assumed peak temperature and garnet diffusivities and range from a few hundred thousand years to a few million years. Such short durations require rapid tectonic burial and exhumation of relatively thin tectonic slices.     

Rutile inclusions in garnet from a dissolution‐reprecipitation mechanism

Metamorphic garnet commonly contains needle‐like rutile inclusions as well as equant rutile inclusions that surround quartz inclusions and range in size from submicrometer to nanometer. Although the origin of these equant rutile inclusions, that is, exsolution or non‐exsolution, has important implications for petrological and tectonic processes, the crystallographic characteristics of these inclusions have rarely been studied because of the small sizes and analytical difficulties involved. Here, we report the crystallographic characteristics pertinent to the genetic origin of minute equant rutile inclusions in cloudy, nearly spherically shaped garnet domains with Ti‐depleted compositions surrounding quartz inclusions in ultrahigh‐pressure garnet from several diamondiferous Erzgebirge quartzofeldspathic gneissic rock samples. TEM analyses show that the equant rutile crystals in cloudy garnet domains are partially bounded by the low‐energy {100}_rt ± {110}_rt ± {101}_rt facets and have rather random crystallographic orientation relationships (CORs) with the garnet host, with preferential alignment of low‐energy lattice planes, for example, {100}_rt//{112}_grt, for some rutile crystals. Although the rather random CORs are unlikely to be attributed to solid‐state exsolution subjected to the stringent topotactic garnet lattice constraints, the characteristic subhedral {100}_rt ± {110}_rt ± {101}_rt crystal forms of rutile can be rationalized by a metasomatic dissolution‐reprecipitation mechanism via a fluid phase. In this scenario, the quartz+fluid inclusions in garnet were first subjected to decompression microcracking during rock exhumation, followed by dissolution of Ti‐bearing garnet matrix at the crack tips or along the crack surfaces and subsequent reprecipitation of rutile, apatite, gahnite, akdalaite, and Ti‐depleted garnet. The rapid coalescence between rutile and garnet crystals in fluid or direct attachment of rutile crystals onto the dissolving crack surfaces would then yield the rather random CORs as reported here. These results, along with previous work on rutile needles, indicate rather diverse genesis of rutile inclusions in various crystal forms, thus shedding light on the controversial exsolution origin for other inclusion suite/microstructure in minerals.     

Coexistence of garnet blueschist and eclogite in South Tianshan,NW China: dependence of P–T evolution and bulk‐rock composition

Coexisting garnet blueschist and eclogite from the Chinese South Tianshan high‐pressure (HP)–ultrahigh‐pressure (UHP) belt consist of similar mineral assemblages involving garnet, omphacite, glaucophane, epidote, phengite, rutile/sphene, quartz and hornblendic amphibole with or without paragonite. Eclogite assemblages generally contain omphacite >50 vol.% and a small amount of glaucophane (<5 vol.%), whereas blueschist assemblages have glaucophane over 30 vol.% with a small amount of omphacite which is even absent in the matrix. The coexisting blueschist and eclogite show dramatic differences in the bulk‐rock compositions with higher X(CaO) [=CaO/(CaO + MgO + FeO_total + MnO + Na₂O)] (0.33–0.48) and lower A/CNK [=Al₂O₃/(CaO + Na₂O + K₂O)] (0.35–0.56) in eclogite, but with lower X(CaO) (0.09–0.30) and higher A/CNK (0.65–1.28) in garnet blueschist. Garnet in both types of rocks has similar compositions and exhibits core–rim zoning with increasing grossular and pyrope contents. Petrographic observations and phase equilibria modelling with pseudosections calculated using thermocalc in the NCKMnFMASHO system for the coexisting garnet blueschist and eclogite samples suggest that the two rock types share similar P–T evolutional histories involving a decompression with heating from the P_max to the T_max stage and a post‐T_max decompression with slightly cooling stage, and similar P–T conditions at the T_max stage. The post‐T_max decompression is responsible for lawsonite decomposition, which results in epidote growth, glaucophane increase and omphacite decrease in the blueschist, or in an overprinting of the eclogitic assemblage by a blueschist assemblage. Calculated P–X(CaO), P–A/CNK and P–X(CO₂) pseudosections indicate that blueschist assemblages are favoured in rocks with lower X(CaO) (<0.28) and higher A/CNK (>0.75) or fluid composition with higher X(CO₂) (>0.15), but eclogite assemblages preferentially occur in rocks with higher X(CaO) and lower A/CNK or fluid composition with lower X(CO₂). Moreover, phase modelling suggests that the coexistence of blueschist and eclogite depends substantially on P–T conditions, which would commonly occur in medium temperatures of 500–590 °C under pressures of ~17–22 kbar. The modelling results are in good accordance with the measured bulk‐rock compositions and modelled temperature results of the coexisting garnet blueschist and eclogite from the South Tianshan HP–UHP belt.     

Genesis of the Kamioka Skarn Deposits: An Important Role of Clinopyroxene Skarn and Graphite-bearing Limestone in Precipitating Sulfide Ore

Abstract: A genetical relationship between skarn formation and mineralization is investigated for the Kamioka skarn deposits which are the largest Zn-Pb producer in Japan. In the Mozumi deposit, one of main deposits in the Kamioka mining area as well as Tochibora and Maruyama, clinopyroxene skarn was generally subjected to later replacement by garnet or magnetite–calcite–quartz during the Zn-Pb mineralization. The replacement of hedenbergitic clinopyroxene by andraditic garnet resulted in the formation of diopsidic clinopyroxene relicts. With the progress of replacement, the S/So value (So: an estimated area occupied by an original clinopyroxene grain in a thin section, S: a total area of relict clinopyroxene fragments) which is an index of the degree of replacement decreases from 0. 7 to 0. 1, and the hedenbergite mole percent of relict clinopyroxene decreases drastically from about 65 to less than 40. A close association of andraditic garnet and sphalerite suggests that heden-bergitic clinopyroxene skarn played an important role to reduce the relatively oxic ore-forming fluid enriched in Zn²⁺ and SO4^2– and to precipitate sphalerite from the fluid. Ferrous iron in the hedenbergitic clinopyroxene skarn was oxidized to form andraditic garnet. Besides this garnet formation, the mineral assemblage of magnetite–calcite–quartz replaced the clinopyroxene skarn at the time of mineralization. In both cases, the reduction of relatively oxic ore-forming fluid by hedenbergitic clinopy-roxene skarn at the later stage brought about the precipitation of sulfide minerals. In contrast, these types of later replacement are not found in the Tochibora deposit. Instead, graphite-bearing crystalline limestone and relatively fresh clinopyroxene skarn are common. Mineralized clinopyroxene skarn has high graphite carbon contents relative to barren one, suggesting that the amount of graphite in the skarn was an important controlling factor for mineralization. It is very likely that the graphite played a role of reducing agent during the mineralization in the Tochibora deposit.     

Metamorphic charnockite in contact aureoles around intrusive enderbite from Natal, South Africa

In the Port Edward area of southern Kwa-Zulu Natal, South Africa, charnockitic aureoles up to 10 m in width in the normally garnetiferous Nicholson's Point Granite, are developed adjacent to intrusive contacts with the Port Edward Enderbite and anhydrous pegmatitic veins. Mineralogical differences between the country rock and charnockitic aureole suggest that the dehydration reaction Bt + Qtz → Opx + Kfs + H₂O and the reaction of Grt + Qtz → Opx + Pl were responsible for the charnockitization. The compositions of fluid inclusions show systematic variation with: (1) the Port Edward Enderbite being dominated by CO₂ and N₂ fluid inclusions; (2) the non-charnockitized granite by saline aqueous inclusions with 18–23 EqWt% NaCl; (3) the charnockitic aureoles by low-salinity and pure water inclusions (<7 EqWt% NaCl); (4) the pegmatites by aqueous inclusions of various salinity with minor CO₂. As a result of the thermal event the homogenization temperatures of the inclusions in charnockite show a much larger range (up to 390 °C) compared to the fluid inclusions in granite (mostly <250 °C). Contrary to fluid-controlled charnockitization (brines, CO₂) which may have taken place along shear zones away from the intrusive body, the present “proximal” charnockitized granite formed directly at the contact with enderbite. The inclusions indicate contact metamorphism induced by the intrusion of “dry” enderbitic magma into “wet” granite resulting in local dehydration. This was confirmed by cathodoluminescence microscopy showing textures indicative for the local reduction of structural water in the charnockite quartz. Two-pyroxene thermometry on the Port Edward Enderbite suggests intrusion at temperatures of ∼1000–1050 °C into country rock with temperature of <700 °C. The temperature of aureole formation must have been between ∼700 °C (breakdown of pyrite to form pyrrhotite) and ∼1000 °C. Charnockitization was probably controlled largely by heat related to anhydrous intrusions causing dehydration reactions and resulting in the release and subsequent trapping of dehydration fluids. The salinity of the metamorphic fluid in the contact zones is supposed to have been higher at an early stage of contact metamorphism, but it has lost its salt content by K-metasomatic reactions and/or the preferential migration of the saline fluids out of the contact zones towards the enderbite. The low water activity inhibited the localized melting of the granite. Mineral thermobarometry suggests that after charnockite aureole genesis, an isobaric cooling path was followed during which reequilibration of most of the aqueous inclusions occurred. Received: 8 November 1998 / Accepted: 21 June 1999     

The clustered nucleation and growth processes of garnet in regional metamorphic rocks from north-west Connecticut,USA

Serial sectioning and imaging with a flatbed scanner yielded the three-dimensional size and spatial distribution of garnet porphyroblasts in two garnet schists and one staurolite-bearing schist from the Everett Formation, north-west Connecticut. The dominant garnet-producing reaction in all samples was chlorite+quartz=garnet+H₂O. The appearance of staurolite, and additional garnet growth in the staurolite-bearing sample, was due to the reaction chloritoid=garnet+staurolite+chlorite. Statistical measures of garnet spatial distributions, using the pair correlation function (PCF), indicate that garnet crystals are weakly to strongly clustered at length scales between 2 and 10 mm. Such clustered nucleation may reflect minor bulk compositional variations. Covariance measures between garnet size and nearest-neighbour distance, using the mark covariance function (MCF), suggest a very weak correlation between crystal size and nearest-neighbour distance for length scales of 2 mm or less. These statistical data suggest that if diffusional gradients were present around growing garnet crystals, they did not influence nucleation and growth patterns at length scales greater than c. 2 mm. Compositional maps, through the garnet centres, show that the smaller crystals have lower Mn core compositions relative to larger crystals, consistent with progressive nucleation during pro-grade metamorphism. Radius-rate plots calculated from compositional X-ray maps show similar growth rates for garnet crystals of different size, consistent with an interface-controlled growth model for garnet. The presence of minor diffusional gradients around growing garnet cannot be entirely dismissed, but the lack of observable reaction rims, the clustered spatial distribution and the radius-rate data are most consistent with an interface-controlled garnet growth model.     

Water in garnet of garnetite (metarodingite) and eclogite from the Erzgebirge and the Lepontine Alps

Little is known about water in nominally anhydrous minerals of orogenic garnet peridotite and enclosed metabasic rocks. This study is focused on peridotite-hosted eclogite and garnetite (metarodingite) from the Erzgebirge (EG), Germany, and the Lepontine Alps (LA), Switzerland. Newly discovered, peridotite-hosted eclogite in the Erzgebirge occurs in the same ultra-high pressure (UHP) unit as gneiss-hosted coesite eclogite, from which it is petrologically indistinguishable. Garnet is present in all mafic and ultramafic high pressure (HP) rocks providing for an ideal proxy to compare the H₂O content of the different rock types. Garnet composition is very similar in EG and LA samples and depends on the rock type. Garnet from garnetite, compared to eclogite, contains more CaO (garnetite: 10.5–16.5 wt%; eclogite: 5–11 wt%) and is also characterized by an anomalous REE distribution. In contrast, the infrared (IR) spectra of garnet from both rock types reveal the same OH absorption bands that are also identical to those of previously studied peridotitic garnet from the same locations. Two groups of IR bands, SW I (3,650 ± 10 cm⁻¹) and SW II (3,570–3,630 cm⁻¹) are ascribed to structural hydroxyl (colloquially ‘water’). A third, broad band is present in about half of the analysed garnet domains and related to molecular water (MW) in submicroscopic fluid inclusions. The primary content of structural H₂O, preserved in garnet domains without fluid inclusions (and MW bands), varies systematically—depending on both the location and the rock type. Garnet from EG rocks contains more water compared to LA samples, and garnet from garnetite (EG: 121–241 wt.ppm H₂O; LA: 23–46 wt.ppm) hosts more water than eclogitic garnet (EG: 84 wt.ppm; LA: 4–11 wt.ppm). Higher contents of structural water (SW) are observed in domains with molecular water, in which the SW II band (being not restricted to HP conditions) is simultaneously enhanced. This implies that fluid influx during decompression not only led to fluid inclusions but also favoured the uptake of secondary SW. The results signify that garnet from all EG and LA samples was originally H₂O-undersaturated. Combining the data from eclogite, garnetite and previously studied peridotite, H₂O and CaO are positively correlated, pointing to the same degree of H₂O-undersaturation at peak metamorphism in all rock types. This ubiquitous water-deficiency cannot be reconciled with the derivation of any of these rocks from the lowermost part of the mantle wedge that was in contact with the subducting plate. This agrees with the previously inferred abyssal origin for part of the rocks from the LA (Cima di Gagnone). A similar origin has to be invoked for the Erzgebirge UHP unit. We suggest that all mafic and ultramafic rocks of this unit not only shared the same metamorphic evolution but also a common protolith origin, most probably on the ocean floor. This inference is supported by the presence of peridotite-hosted garnetite, representing metamorphosed rodingite.     

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。     

福建云霄石榴子石宝石矿物学特征

福建云霄是我国重要的宝石级石榴子石产地,然而该区石榴子石的致色机理不清,制约了对其形成机制的理解及后续开发利用。本文选取7件福建云霄橙黄-橙红色石榴子石样品,利用傅立叶红外光谱、紫外-可见光光谱和拉曼光谱分析其谱学特征,使用电子探针及激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)分析限定其主量、微量元素组成。结果表明云霄石榴子石主要为锰铝榴石,其颜色主要与二价锰(Mn²⁺)和铁离子(Fe²⁺)对可见光的吸收有关,Mn²⁺导致其主体呈橙色,少量Fe²⁺控制其橙红色调,微量Ti⁴⁺使其呈褐色调。福建云霄石榴子石样品核部锰含量相对较低而铁、镁含量较高,锰元素含量由核部向边部逐渐升高,且具有重稀土元素富集、轻稀土元素亏损的左倾配分模式和Eu负异常,表明其形成于岩浆结晶作用晚期。     

Evolution of lithospheric mantle in the north of Nain‐Baft oceanic crust (Neo‐Tethyan ophiolite of Ashin,Central Iran)

This study is focused on a plagioclase‐bearing spinel lherzolite from Chah Loqeh area in the Neo‐Tethyan Ashin ophiolite. It is exposed along the west of left‐lateral strike‐slip Dorouneh Fault in the northwest of Central‐East Iranian Microcontinent. Mineral chemistry (Mg#^olivine < ~ 90, Cr#^{clinopyroxene} < ~ 0.2, Cr#^spinel < ~ 0.5, Al₂O₃^{orthopyroxene} > ~ 2.5 wt%, Al₂O₃^{clinopyroxene} > ~ 4.5 wt%, Al₂O₃^spinel > ~ 41.5 wt%, Na₂O^{clinopyroxene} > ~ 0.11 wt%, and TiO₂^{clinopyroxene} > ~ 0.04 wt%) confirms Ashin lherzolite was originally a mid‐oceanic ridge peridotite with low degrees of partial melting at spinel‐peridotite facies in a lithospheric mantle level. However, some Ashin lherzolites record mantle upwelling and tectonic exhumation at plagioclase‐peridotite facies during oceanic extension and diapiric motion of mantle along Nain‐Baft suture zone. This mantle upwelling is evidenced by some modifications in the modal composition (i.e. subsolidus recrystallization of plagioclase and olivine between pyroxene and spinel) and mineral chemistry (e.g. increase in TiO₂ and Na₂O of clinopyroxene, and TiO₂ and Cr# of spinel and decrease in Mg# of olivine), as a consequence of decompression during a progressive upwelling of mantle. Previous geochronological and geochemical data and increasing the depth of subsolidus plagioclase formation at plagioclase‐peridotite facies from Nain ophiolite (~ 16 km) to Ashin ophiolite (~ 35 km) suggest a south to north closure for the Nain‐Baft oceanic crust in the northwest of Central‐East Iranian Microcontinent.