Nb–Ta fractionation induced by fluid‐rock interaction in subduction‐zones: constraints from UHP eclogite‐ and vein‐hosted rutile from the Dabie orogen,Central‐Eastern China

Niobium and Ta concentrations in ultrahigh‐pressure (UHP) eclogites and rutile from these eclogites and associated high pressure (HP) veins were used to study the behaviour of Nb–Ta during dehydration and fluid‐rock interaction. Samples were collected through a ～2 km profile at the Bixiling complex in the Dabie orogenic belt, Central‐Eastern China. All but one eclogite away from veins (EAVs) display nearly constant Nb/Ta ratios ranging from 16.1 to 19.2, with an average of 16.9 ± 0.8 (2 SE), similar to that of their gabbroic protolith from the Yangtze Block. Nb/Ta ratios of rutile from the EAVs range from 12.7 to 25.3 among different individual grains, with the average values close to those of the corresponding bulk rocks. These observations show that Nb and Ta were not significantly fractionated by prograde metamorphism up to eclogite facies when no significant fluid‐rock interaction occurs. In contrast, Nb/Ta ratios of rutile from eclogites close to veins (ECVs) are highly variable from 17.8 to 49.8, which are systematically higher (by up to 17) than those of rutile from the veins. These observations demonstrate that Nb and Ta were mobilized and fractionated during localized fluid flow and intensive fluid‐rock interaction. This is strongly supported by Nb/Ta zoning patterns in single rutile grains revealed by in situ LA‐ICP‐MS analysis. Ratios of Nb/Ta in the ECV‐hosted rutile decrease gradually from cores towards rims, whereas those in the EAV‐hosted rutile are nearly invariable. Furthermore, the vein rutile shows Nb/Ta zoning patterns that are complementary to those in rutile from their immediate hosts (ECVs), suggesting an internal origin for the vein‐forming fluids. The Nb/Ta ratios of such fluids evolved from low values at the early stage of subduction to higher values at later supercritical conditions with increased temperature and pressure. Quantitative modelling was conducted to constrain the compositional evolution of metamorphic fluids during dehydration and fluid‐rock interaction focusing on Nb–Ta distribution. The modelling results based on our proposed multistage fluid phase evolution path can essentially reproduce the natural observations reported in the present study.     

Small-scale transport of trace elements Nb and Cr during growth of titanite: an experimental study at 600?°C, 0.4 GPa

Trace element distribution in titanite overgrowths on rutile has been investigated experimentally at 600?°C, 400?MPa and f_O2 near NiNiO buffer. Compositionally homogenous Cr- or Nb-doped synthetic rutile single crystals or Nb-containing natural rutile crystals were the source of Cr, Nb and Ti to synthesize titanite using the double-capsule technique. All element exchange with the source of Si, Ca and Al occurred via a NaCl–H₂O fluid. Titanite forms quickly and exclusively around the rutile crystals. The titanite overgrowth separates rutile from the bulk fluid, and all elements from rutile dissolution have to pass through the titanite rim. Trace element concentrations in titanite show a considerable scatter in experiments with and without Al, although the average concentrations of Cr or Nb of titanite around compositionally homogeneous synthetic rutile approach the expected values for closed system conditions. Variability of Al with Cr or Nb in the titanite is not correlated. The Al zoning is irregular and patchy, and also the distribution of trace elements does not show systematic trends in the spatial distribution. In experiments using zoned natural rutile, the concentrations of Nb in titanite are related to the Nb zoning in rutile, but the contents also vary unsystematically. Under the controlled conditions of the experiment, the explanation for the strongly irregular spatial distribution is most likely due to variations in elemental concentrations during transport from the rutile along the titanite grain boundaries. The transport pathway is complex because grain boundary migration is important during titanite growth. Such irregular element distribution is also found in a natural sample of titanite overgrowth on rutile from an eclogite with retrograde overprint in the amphibolite facies. Transport of Ti and trace elements was focused on grain boundaries and shielded from the rutile as a source of these elements. We conclude that this type of zoning is not related to changes in P–T or composition in an open system, but solely controlled by transport in and through the titanite rim.     

金红石-榍石转变过程中元素地球化学行为——以雅鲁藏布江缝合带角闪岩为例

金红石边缘形成榍石冠状边结构在变质中-基性岩中普遍存在,是金红石与退变质流体携带的SiO_2与CaO作用的结果,反应形成的榍石微量元素特征受到金红石和流体的共同影响。雅鲁藏布江缝合带中角闪岩LZ06-04在抬升过程经历近等温降压退变质作用,石榴子石分解导致同一样品中含石榴子石部分与不含石榴子石部分的退变质流体成分的差异。两种流体分别与金红石反应,对应形成的榍石具有相似的Nb、Ta含量和Nb/Ta比值特征,但截然不同的REE特征。榍石的Nb、Ta来源于金红石,残余金红石与含水流体再平衡Nb、Ta的分配系数增大,且D_(Nb)~(Rt/Fluid)≥D_(Ta)~(Rt/Fluid);虽然Nb和Ta在含水流体中都表现为不活动元素,但相对于Nb,Ta在含水流体中活动性较高。榍石的Zr-Hf体系特征受到锆石、石榴子石等矿物的综合影响,并且Zr-Hf在含水流体中表现出比Nb-Ta更高的活动性。榍石的REE特征受流体中REE特征、榍石与流体配分系数以及共生矿物的影响。在岩浆或变质体系,榍石形成过程中,REE富集矿物(如石榴子石、锆石、褐帘石、独居石、磷灰石等)形成或分解将影响榍石的REE分布特征或形成REE环带结构。含水流体中金红石退变质形成榍石反应的进行受流体中TiO_2、CaO和SiO_2活度的影响。因此榍石常见于钙碱性岩浆岩、富Ca基性变质岩和矽卡岩中。流体中CaO活度的变化影响榍石的形成,进而影响Ti、Nb、Ta在流体中的运移能力。俯冲板片产生流体在交代上覆富Ca地幔楔物质过程中形成榍石残留同样可以造成部分熔融体具有亏损HFSE特征。     

Trace element geochemistry of scheelite and rutile from metaturbidite-hosted quartz vein gold deposits, Meguma Terrane, Nova Scotia, Canada: genetic implications

Scheelite and rutile from several metaturbidite-hosted gold-bearing quartz vein deposits of the Meguma Terrane of Nova Scotia were analyzed for trace elements including rare earth elements, niobium and tantalum. Scheelites have high concentrations of Sr, Nb, Y and rare earth elements (REE) with bell-shaped chondrite-normalized REE patterns accompanied by both positive and negative Eu anomalies. They also have high Nb/Ta ratios (80–300). Three distinct trace element types of the scheelites are interpreted to reflect chemical differences in the pulses of hydrothermal fluids. Hydrothermal rutiles have high contents of W (up to 4.2 wt.% WO₃), are rich in Ta compared to Nb and have a very low Nb/Ta ratio (~0.3). Hydrothermal fluids which produced both scheelite with a high Nb/Ta and rutile with a low Nb/Ta ratio are an efficient medium for fractionation of this ratio although these two minerals play an important role during the process.     

Natural and experimental constraints on formation of the continental crust based on niobium–tantalum fractionation

Fractionation between Nb and Ta, elements generally regarded as geochemical ‘identical twins’, is a key to deciphering the formation of the continental crust (CC). Here we show that Nb/Ta of rutile grains in eclogitic rocks from the Chinese Continental Scientific Drilling (CCSD) project are remarkably heterogeneous but overall subchondritic at core depths of 100–700 m, and are less variable and mainly suprachondritic at core depths of 700–3025 m, indicating clear Nb/Ta fractionation across a subducted slab. To understand the potential mechanism of Nb/Ta fractionation within the subducted plate, we analysed by laser ablation ICPMS a thermal migration experiment in which a wet andesite was placed in a large thermal gradient (300°C/cm with ends ranging from 950–350°C) at 0.5Gpa. Results show that Nb, Ta and Ti, driven by the thermal gradient, preferentially migrate by diffusion through supercritical fluids into the cooler end of the experiment (at 650–350°C). Due to contrasting Nb and Ta thermal migration patterns, dramatic fractionation between Nb, Ta, and Ti took place in the cooler end. Experimental results are consistent with the measured Nb, Ta in rutile from CCSD drillhole samples. We consider that major fractionation between Nb, Ta must occur before rutile appears, most likely during the prograde blueschist to amphibole–eclogite transformation, when Ti is also mobile. Before rutile appears, partitioning between Ti‐rich dominant minerals such as amphiboles and fluids in the hotter region where dehydration preferentially occurs, produces Nb–Ta–Ti‐rich fluids with subchondritic Nb/Ta, and dehydration residues with suprachondritic Nb/Ta. Meanwhile, owing to evolution of the thermal gradient within the subducting slab, thermal migration of Nb, Ta, and Ti in aqueous fluids result in Nb, Ta, and Ti enrichment in the cooler region and depletion in the hotter region. As a result of high‐pressure metamorphism, hydrous rutile‐rich eclogites with overall subchondritic Nb/Ta form in the cooler region, whereas relatively anhydrous rutile‐poor eclogites with suprachondritic Nb/Ta form in the hotter region. Subsequently, partial melting of hydrous rutile‐rich eclogites with initial subchondritic Nb/Ta at deeper levels transfers overall subchondritic Nb/Ta coupled with Nb, Ta, and Ti depletion characteristics to the CC, leaving dry rutile‐poor eclogites with suprachondritic Nb/Ta and rutile‐rich residual eclogites with overall, heterogeneous subchondritic Nb/Ta as a complementary reservoir to the CC.     

Nb/Ta fractionation observed in eclogites from the Chinese Continental Scientific Drilling Project

This paper reports detailed analyses of Nb and Ta concentrations of 19 eclogite samples and their principal mineral constituents from the main drill hole of the Chinese Continental Scientific Drilling Project (CCSD) and nearby outcrops. We observe highly fractionated and overall suprachondritic Nb/Ta values in minerals, e.g., rutile (4.8–87), titanite (12–62) and amphibole (2.0–67). Amphiboles in amphibolites (retrograded from eclogite) can be classified into two groups: a low Nb/Ta group that bears higher Al contents and is thus of higher pressure origin, and a high Nb/Ta, lower pressure group. The former group was likely formed during subduction; the latter may have formed during exhumation in the presence of rutile and titanite. The significant Nb/Ta fractionation in rutile and other minerals may reflect early dehydration of the subducted slab at shallow depths before the formation of rutile, which occurs at depths ≥50 km. The dehydration, with amphiboles existing as the main Nb–Ta-bearing phase, would lead to Nb/Ta fractionation, i.e., forming subchondritic Nb/Ta ratios in the released fluids and, complementarily, suprachondritic Nb/Ta ratios in the residual phases. While a large proportion of the fluids may escape from the slab to the mantle wedge, considerable amounts of the fluids can be retained in hydrous minerals within the descending slab, thus forming hydrated cold eclogites with subchondritic Nb/Ta characteristics. As subduction continues to depths over 50 km, rutile appears and consequently controls the Nb–Ta budget. In the presence of rutile, melting of the hydrated cold eclogites with very low Nb/Ta ratios would form magmas with negative Nb, Ta anomalies and subchondritic Nb/Ta. Further dehydration of the continuously descending slab results in even more fractionated Nb/Ta ratios in subsequently released fluids and residues, providing a feasible explanation for the large Nb/Ta variation observed in the modern arc magmas and residual eclogites.     

Zr-in-rutile thermometry of eclogites from the Karakaya Complex in NW Turkey: Implications for rutile growth during subduction zone metamorphism

Eclogites occur as a tectonic slice within a metabasite-phyllite-marble unit of the Karakaya Complex in northwest Turkey. The high-pressure mineral assemblage in eclogite is mainly composed of garnet + omphacite + glaucophane + epidote + quartz. Trace element characteristics of rutile and Zr-in-rutile temperatures were determined for eclogites from the Karakaya Complex. Core-rim analyses of rutile grains yield remarkable trace element zoning with lower contents of Zr, Nb and Ta in the core than in the rim. The variations in Zr, Nb and Ta can be ascribed to growth zoning rather than diffusion effects. The Nb/Ta and Zr/Hf ratios increase with a decrease in Ta and Hf contents, which could be ascribed to the effect of metamorphic dehydration in subduction zones on rutile Nb/Ta differentiation. The rutile grains from eclogites in the Karakaya Complex are dominated by subchondritic Nb/Ta and Zr/Hf ratios. It can be noted that subchondritic Nb/Ta may record rutile growth from local sinks of aqueous fluids from metamorphic dehydration.The Zr contents of all rutile grains range between 81 and 160 ppm with an average of 123 ppm. The Zr-in-rutile thermometry yields temperatures of 559–604 °C with an average temperature of 585 °C for eclogites from the Karakaya Complex. This average temperature suggests growth temperature of rutile before peak pressure during the subduction. However, some rutile grains have higher Zr contents in the outermost rims compared to the core. Zr-in-rutile temperatures of the rims are about 20 °C higher than those of the cores. This suggests that the outermost rims would have grown from a distinct fluid at higher temperatures than that of the cores. Moreover, Zr contents and calculated temperatures in both inclusion rutile and matrix rutile from eclogites are identical, which suggests that eclogites within the Karakaya Complex belong to the same tectonic slice and underwent similar metamorphic evolution.     

CCSD主孔榴辉岩中金红石微量元素特征：LA-ICPMS分析及其意义

对中国大陆科学钻探（CCSD）主孔200~1005m范围内8件榴辉岩样品的金红石进行了LA-ICPMS原位微区微量元素分析,结合前人已发表的全岩和金红石分析数据,研究结果发现：在不同类型榴辉岩中,金红石的微量元素与其全岩成分具有不同的相关关系。金红石中的Nb和Ta元素含量不同程度地受控于全岩Nb和Ta含量。在高钛和低镁钛榴辉岩中,金红石的Cr与全岩Cr/TiO₂正相关;在富镁榴辉岩中,金红石的Cr含量受全岩MgO含量的控制;在高钛和富镁榴辉岩中,全岩成分明显影响着金红石的Zr含量,金红石Zr温度计可能不适用。低镁钛榴辉岩的金红石的平衡温度普遍低于榴辉岩峰期变质温度,可能是变质流体参与下的扩散作用和退变质作用所致;多数情况下,单个样品中大部分金红石颗粒的Zr含量是均匀的,金红石Zr温度计所给出的温度可能代表着退变质再平衡的温度;CCSD榴辉岩的全岩Nb/Ta比值普遍低于其中金红石的Nb/Ta比值,不支持金红石榴辉岩可能是地球上超球粒陨石Nb/Ta比值储库的观点。     

Element mobility during continental collision: insights from polymineralic metamorphic vein within UHP eclogite in the Dabie orogen

Metamorphic dehydration and partial melting are two important processes during continental collision. They have significant bearing on element transport at the slab interface under subduction‐zone P–T conditions. Petrological and geochemical insights into the two processes are provided by a comprehensive study of leucocratic veins in ultrahigh‐pressure (UHP) metamorphic rocks. This is exemplified by this study of a polymineralic vein within phengite‐bearing UHP eclogite in the Dabie orogen. The vein is primarily composed of quartz, kyanite, epidote and phengite, with minor accessory minerals such as garnet, rutile and zircon. Primary multiphase solid inclusions occur in garnet and epidote from the both vein and host eclogite. They are composed of quartz ± K‐feldspar ± plagioclase ± K‐bearing glass and exhibit irregular to negative crystal shapes that are surrounded by weak radial cracks. This suggests their precipitation from solute‐rich metamorphic fluid/melt that involved the reaction of phengite breakdown. Zircon U–Pb dating for the vein gave two groups of concordant ages at 217 ± 2 and 210 ± 2 Ma, indicating two episodes of zircon growth in the Late Triassic. The same minerals from the two rocks give consistent δ¹⁸O and δD values, suggesting that the vein‐forming fluid was directly derived from the host UHP eclogite. The vein is much richer in phengite and epidote than the host eclogite, suggesting that the fluid is associated with remarkable concentration of such water‐soluble elements as LILE and LREE migration. Garnet and rutile in the vein exhibit much higher contents of HREE (2.2–5.7 times) and Nb–Ta (1.8–2.0 times) than those in the eclogite, indicating that these normally water‐insoluble elements became mobile and then were sunken in the vein minerals. Thus, the vein‐forming agent would be primarily composed of the UHP aqueous fluid with minor amounts of the hydrous melt, which may even become a supercritical fluid to have a capacity to transport not only LILE and LREE but also HREE and HFSE at subduction‐zone metamorphic conditions. Taken together, significant amounts of trace elements were transported by the vein‐forming fluid due to the phengite breakdown inside the UHP eclogite during exhumation of the deeply subducted continental crust.     

超高压榴辉岩金红石中高场强元素变化的控制因素及其地球动力学意义

利用LA-ICP-MS对CCSD-MH超高压榴辉岩中金红石进行了详细的原位微区微量元素组成分析.金红石中高场强元素Nb和Ta含量主要受全岩Nb、Ta和TiO₂含量控制, Zr、Hf含量比较稳定基本不受全岩含量影响.粒间金红石中, 同一颗粒金红石核部Zr含量系统高于边部, 而边部则出现了明显的Pb和Sr富集特征.CCSD-MH榴辉岩中金红石与全岩的Nb/Ta比值呈现明显的不一致性.全岩Nb/Ta比值明显低于金红石且与全岩TiO₂含量负相关, 而金红石的Nb/Ta比值与全岩Nb、Ta含量和Nb/Ta比值没有明显的相关关系.金红石和全岩之间非完全耦合的Nb/Ta组成表明, 金红石并非形成于原岩的结晶过程中而是在超高压变质作用过程中形成, 尽管金红石是榴辉岩中Nb、Ta含量的主要载体矿物, 但金红石的Nb/Ta比值并不一定能完全代表全岩的特征, 而与全岩Nb、Ta和TiO₂的含量有关.粒间金红石核部Zr含量所记录的温度与粒径之间具有明显的正相关性, 反映金红石中的Zr在其形成后没有封闭.粒间金红石所表现出的明显的边部富集Pb和Sr的特征, 反映了后期流体活动对金红石组成的影响.这些研究结果为金红石中Zr在高温下的扩散作用和后期流体活动的影响提供了重要证据, 这可能是利用金红石Zr含量地质温度计计算的苏鲁-大别榴辉岩变质温度(598~827℃) 偏低的主要原因.      

Redistribution of HFSE elements during rutile replacement by titanite

Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromsø eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry.     

Subduction channel fluid-rock interaction:Indications from rutile-quartz veins within eclogite from the Yuka terrane,North Qaidam orogen

High-pressure(HP)or ultrahigh-pressure(UHP)rutile-quartz veins that form at mantle depths due to fluid-rock interaction can be used to trace the properties and behavior of natural fluids in subduction zones.To explore the fluid flow and the associated element mobility during deep subduction and exhumation of the continental crust,we investigated the major and trace elements of Ti-rich minerals.Additionally,U–Pb dating,trace element contents,and Lu–Hf isotopic composition of zircon grains in the UHP eclogite and associated rutile-quartz veins were examined in the North Qaidam UHP metamorphic belt,Yuka terrane.The zircon grains in the rutile-quartz veins have unzoned or weak oscillatory zonings,and show low Th/U ratios,steep chondrite-normalized patterns of heavy rare earth elements(HREEs),and insignificant negative Eu anomalies,indicating their growth in metamorphic fluids.These zircon grains formed in 431
3 Ma,which is consistent with the 432
2 Ma age of the host eclogite.As for the zircons in the rutile-quartz veins,they showed steep HREE patterns on one hand,and were different from the zircons present in the host eclogite on the other.This demonstrates that their formation might have been related to the breakdown of the early stage of garnet,which corresponds to the abundance of fluids during the early exhumation stage.The core-rim profile analyses of rutile recorded a two-stage rutile growth across a large rutile grain;the rutile core has higher Nb,Ta,W,and Zr contents and lower Nb/Ta ratios than the rim,indicating that the rutile domains grew in different metamorphic fluids from the core towards the rim.The significant enrichment of high field strength elements(HFSEs)in the rutile core suggests that the peak fluids have high solubility and transportation capacity of these HFSEs.Furthermore,variations in the Nb vs.Cr trends in rutile indicate a connection of rutile to mafic protolith.The zircon grains from both the rutile-quartz veins and the host eclogite have similar Hf isotopic compositions,indicating that the vein-forming fluids are internally derived from the host eclogite.These fluids accumulated in the subduction channel and were triggered by local dehydration of the deeply subducted eclogite during the early exhumation conditions.     

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.     

Dehydration and melting during continental collision: Constraints from element and isotope geochemistry of low-T/UHP granitic gneiss in the Dabie orogen

A combined study of petrography, whole-rock major and trace elements as well as Rb?Sr and Sm?Nd isotopes, and mineral oxygen isotopes was carried out for two groups of low-T/UHP granitic gneiss in the Dabie orogen. The results demonstrate that metamorphic dehydration and partial melting occurred during exhumation of deeply subducted continent. Zircon δ¹⁸O values of ? 2.8 to + 4.7‰ for the gneiss are all lower than normal mantle values of 5.3 ± 0.3‰, consistent with ¹⁸O depletion of protolith due to high-T meteoric-hydrothermal alteration at mid-Neoproterozoic. Most samples have extremely low ⁸⁷Sr/⁸⁶Sr ratios at t₁ = 780 Ma, but very high ⁸⁷Sr/⁸⁶Sr ratios at t₂ = 230 Ma. This suggests intensive fluid disturbance due to the hydrothermal alteration of protoliths during Neoproterozoic magma emplacement and the metamorphic dehydration during Triassic continental collision. Rb–Sr isotopes, Th/Ta vs. La/Ta and Th/Hf vs. La/Nb relationships suggest that Group I gneiss experienced lower degrees of hydrothermal alteration, but higher degrees of dehydration, than Group II gneiss. The two groups of gneiss have similar patterns of REE and trace element partition. Group I gneiss displays good correlations between Nb and LREEs but no correlations between Nb and LILEs (Rb, Ba, Pb, Th and U), indicating differential mobilities of LILEs during the dehydration. Thus the correlation between Nb and LREEs is inherited from protolith rather than caused by metamorphic modification. Relative to Group I gneiss, Group II gneiss has stronger negative Eu anomaly, lower contents of Sr and Ba but higher contents of Rb, Th and U. In particular, Nb correlates with LILEs (e.g., Rb, Sr, Ba, Th and U), but not with LREEs (La and Ce). This may indicate decoupling between the dehydration and LILEs transport during continental collision. Furthermore, dehydration melting may have occurred due to breakdown of muscovite during “hot” exhumation. Group II gneiss has extremely low contents of FeO + MgO + TiO₂ (1.04 to 2.08 wt.%), high SiO₂ contents of 75.33 to 78.23 wt%, and high total alkali (Na₂O + K₂O) contents (7.52 to 8.92 wt.%), comparable with compositions predicted from partial melting of felsic rocks by experimental studies. Almost no UHP metamorphic minerals survived; felsic veins of fine-grain minerals occurs locally between coarse-grain minerals, resulting in a kind of metatexite migmatites due to dehydration melting without considerable escape of felsic melts from the host gneiss. In contrast, Group I gneiss only shows metamorphic dehydration. Therefore, the two groups of gneiss show contrasting behaviors of fluid–rock interaction during the continental collision.     

Zircon U-Pb Geochronology and Geochemical Characteristics of the Volcanic Host Rocks from the Tongyu VHMS Copper Deposit in the Western North Qinling Orogen and Their Geological Significance

Precise in situ zircon U-Pb dating and Lu–Hf isotopic measurement using an LA-ICP-MS system, whole-rock major and trace element geochemistry and Sr–Nd isotope geochemistry were conducted on the volcanic host rocks of the Tongyu copper deposit on the basis of further understanding of its geological characteristics. Three zircon samples from the volcanic host rocks yielded 206Pb/238 U weighted average ages ranging from 436±4 Ma to 440±5 Ma, which are statistically indistinguishable and coeval with the ca. 440 Ma northward subduction event of the Paleo-Qinling oceanic slab. The volcanic host rocks were products of magmatic differentiation that evolved from basalt to andesite to dacite to rhyolite, forming an integrated tholeiitic island arc volcanic rock suite. The primitive mantle-normalized trace element patterns for most samples show characteristics of island arc volcanic rocks, such as relative enrichment of LILE(e.g. Th, U, Pb and La) and depletion of HFSE(e.g. Nb, Ta, Ti, Zr and Hf). Discrimination diagrams of Ta/Yb vs Th/Yb, Ta vs Th, Yb vs Th/Ta, Ta/Hf vs Th/Hf, Hf/3 vs Th vs Nb/16, La vs La/Nb and Nb vs Nb/Th all suggest that both the volcanic host rocks from the Tongyu copper deposit and the volcanic rocks from the regional Xieyuguan Group were formed in an island arc environment related to subduction of an oceanic slab. Values of ISr(0.703457 to 0.708218) and εNd(t)(-2 to 5.8) indicate that the source materials of volcanic rocks from the Tongyu copper deposit and the Xieyuguan Group originated from the metasomatised mantle wedge with possible crustal material assimilation. Most of the volcanic rock samples show good agreement with the values of typical island arc volcanic rocks in the ISr-εNd(t) diagram. The involvement of crustal-derived material in the magma of the volcanic rocks from the Tongyu copper deposit was also reflected in the zircon εHf(t) values, which range from-3.08 to 10.7, and the existence of inherited ancient xenocrystic zircon cores(2616±39 Ma and 1297±22 Ma). The mineralization of the Tongyu copper deposit shows syn-volcanic characteristics such as layered orebodies interbedded with the volcanic rock strata, thus, the zircon U-Pb age of the volcanic host rocks can approximately represent the mineralization age of the Tongyu copper deposit. Both the Meigou pluton and the volcanic host rocks were formed during the ca. 440 Ma northward subduction of the Paleo-Qinling Ocean when high oxygen fugacity aqueous hydrothermal fluid released by dehydration of the slab and the overlying sediments fluxed into the mantle wedge, triggered partial melting of the mantle wedge, and activated and extracted Cu and other ore-forming elements. The magma and ore-bearing fluid upwelled and erupted, and consequently formed the island arc volcanic rock suite and the Tongyu VHMS-type copper deposit.     

Vanadium and niobium behavior in rutile as a function of oxygen fugacity: evidence from natural samples

Vanadium occurs in multiple valence states in nature, whereas Nb is exclusively pentavalent. Both are compatible in rutile, but the relationship of V–Nb partitioning and dependence on oxygen fugacity (expressed as fO₂) has not yet been systematically investigated. We acquired trace-element concentrations on rutile grains (n = 86) in nine eclogitic samples from the Dabie-Sulu orogenic belt by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) and combined them with published results in order to assess the direct and indirect effects of oxygen fugacity on the partitioning of V and Nb into rutile. A well-defined negative correlation between Nb (7–1,200 ppm) and V concentrations (50–3,200 ppm) was found, documenting a competitive relationship in the rutile crystal that does not appear to be controlled by bulk rock or mineral compositions. Based on the published relationship of ^RtD_V and V valence with ?QFM, we suggest that the priority order of V incorporation into rutile is V⁴⁺ > V³⁺ > V⁵⁺. The inferred Nb–V competitive relationship in rutile from the Dabie-Sulu orogenic belt could be explained by decreasing fO₂ due to dehydration reactions involving loss of oxidizing fluids during continental subduction: The increased proportion of V³⁺ (expressed as V³⁺/∑V) and attendant decrease in ^RtD_V is suggested to lead to an increase in rutile lattice sites available for Nb⁵⁺. A similar effect may be observed under more oxidizing conditions. When V⁵⁺/∑V increases, ^RtD_V shows a dramatic decline and Nb concentration increases considerably. This is possibly documented by rutile in highly metasomatized and oxidized MARID-type (MARID: mica–amphibole–rutile–ilmenite–diopside) mantle xenoliths from the Kaapvaal craton, which also show a negative V–Nb covariation. In addition, their Nb/Ta covaries with V concentrations: For V concentrations <1,250 ppm, Nb/Ta ranges between 35 and 45, whereas for V > 1,250 ppm, Nb/Ta is considerably lower (5–15). This relationship is mainly controlled by a change in Nb concentrations, suggesting that the indirect dependence of ^RtD_Nb on fO₂, which is not mirrored in ^RtD_Ta, can exert considerable influence on rutile Nb–Ta fractionation.     

Trace-element mobilization during Ca-metasomatism along a major fluid conduit: Eclogitization of blueschist as a consequence of fluid-rock interaction

In the subduction complex of the Tianshan mountains, western China, massive blueschist is cross-cut by an eclogite-facies major fluid conduit surrounded by a reaction zone which is mainly composed of omphacite and garnet. Petrological as well as geochemical evidence suggest that formation of the vein and the eclogitic selvage around the vein was caused by fluid infiltration under peak metamorphic conditions of 21 ± 1.5 kbar and 510 ± 30 °C. The combination of whole-rock with mineral trace-element data as well as mass-balance calculations indicate that substantial differences exist between the unaltered host rock and the part of the system which was altered by fluid-rock interaction. These differences include: (1) depletion of mainly large-ion lithophile elements (LILE) and Li of up to 60% relative to their concentrations in the unaltered host rock; (2) an extreme enrichment of CaO (∼115%), Sr and Pb (>300%) in the altered parts of the vein-wall-rock system; (3) redistribution of heavy rare earth elements (HREE) from partly replaced rutile and recrystallized titanite in the blueschist-eclogite transition zone into newly grown garnet rims in the eclogitic selvage around the vein; (4) transformation of high Nb/Ta rutile into low Nb/Ta titanite which is associated with preferred mobilization of Nb over Ta; and (5) decoupling of Zr and Hf from Nb and Ta; the latter are depleted by ∼30% relative to the unaltered blueschist host rock whereas the former are depleted by only ∼10%. The prerequisite for the transformation of Ca-poor blueschist (6-7 wt.% CaO) into Ca-rich eclogite (up to 13 wt.% CaO) was the infiltration of a Ca-rich fluid. The release of trace elements can be attributed to partitioning of these elements into the passing fluid phase during dissolution-reprecipitation processes in the course of eclogitization. The reactivity of the precursor mineral assemblage and the chemical gradients between the reacting and passing fluid of the conduit are mainly responsible for trace-element mobilization in the studied samples. The suite of trace elements released upon fluid-induced eclogitization of the reactive wall-rock resembles that in island arc magmas showing strong enrichment of LIL elements, strong depletions in HFS elements and intermediate concentrations of REE.     

Exsolution intergrowths of titanian ferrocolumbite and niobian rutile from the Weinebene Spodumene Pegmatites,Carinthia, Austria

Summary Titanian ferrocolumbite is a rare accessory mineral in the spodumene-bearing pegmatites at Weinebene, Carinthia, Austria. It contains abundant exsolved niobian rutile and scarce inclusions of cassiterite that may be primary. The titanian ferrocolumbite is relatively homogeneous with Mn/(Mn + Fe) 0.24–0.33, Ta/(Ta + Nb) 0.09–0.13 (atomic ratios) and 0.47–0.88 Ti per 12 cations (2.7–5.0 wt.% TiO₂). Natural specimens are considerably disordered but become more ordered on heating. Niobian rutile has Mn/(Mn + Fe) 0.00–0.04 and Ta/(Ta + Nb) 0.26–0.38; it concentrates Fe, Ta, Ti and Sn relative to the Mn- and Nb-enriched ferrocolumbite. The overall scarcity of Nb, Ta-oxide minerals in the spodumene-bearing pegmatites of southern Ostalpen conforms to their general features ranking them with the albite-spodumene type of rare-element pegmatites.With 4 Figures     

In situ analyses of micas in the Yashan granite,South China: Constraints on magmatic and hydrothermal evolutions of W and Ta–Nb bearing granites

Most rare-metal granites in South China host major W deposits with few or without Ta–Nb mineralization. However, the Yashan granitic pluton, located in the Yichun area of western Jiangxi province, South China, hosts a major Nb–Ta deposit with minor W mineralization. It is thus important for understanding the diversity of W and Nb–Ta mineralization associated with rare-metal granites. The Yashan pluton consists of multi-stage intrusive units, including the protolithionite (-muscovite) granite, Li-mica granite and topaz–lepidolite granite from the early to late stages. Bulk-rock REE contents and La/Yb ratios decrease from protolithionite granite to Li-mica granite to topaz–lepidolite granite, suggesting the dominant plagioclase fractionation. This variation, together with increasing Li, Rb, Cs and Ta but decreasing Nb/Ta and Zr/Hf ratios, is consistent with the magmatic evolution. In the Yashan pluton, micas are protolithionite, muscovite, Li-mica and lepidolite, and zircons show wide concentration ranges of ZrO₂, HfO₂, UO₂, ThO₂, Y₂O₃ and P₂O₅. Compositional variations of minerals, such as increasing F, Rb and Li in mica and increasing Hf, U and P in zircon are also in concert with the magmatic evolution from protolithionite granite to Li-mica granite to topaz–lepidolite granite. The most evolved topaz–lepidolite granite has the highest bulk-rock Li, Rb, Cs, F and P contents, consistent with the highest contents of these elements and the lowest Nb/Ta ratio in mica and the lowest Zr/Hf ratio in zircon. Ta–Nb enrichment was closely related to the enrichment of volatile elements (i.e. Li, F and P) in the melt during magmatic evolution, which raised the proportion of non-bridging oxygens (NBOs) in the melt. The rims of zoned micas in the Li-mica and topaz–lepidolite granites contain lower Rb, Cs, Nb and Ta and much lower F and W than the cores and/or mantles, indicating an exotic aqueous fluid during hydrothermal evolution. Some columbite-group minerals may have formed from exotic aqueous fluids which were originally depleted in F, Rb, Cs, Nb, Ta and W, but such fluids were not responsible for Ta–Nb enrichment in the Yashan granite. The interaction of hydrothermal fluids with previously existing micas may have played an important role in leaching, concentrating and transporting W, Fe and Ti. Ta–Nb enrichment was associated with highly evolved magmas, but W mineralization is closely related to hydrothermal fluid. Thus these magmatic and hydrothermal processes explain the diversity of W and Ta–Nb mineralizations in the rare-metal granites.     

Significance of Nb/Ta as an indicator of geochemical processes in the crust-mantle system

A mantle value of 17.5 for Nb/Ta appears well established; less well established are crustal values of 11–12, although it appears that Nb/Ta for crustal-derived melts is less than mantle Nb/Ta, demonstrating fractionation of these two elements during crustal evolution, and suggesting that Nb/Ta variation may be indicative of a particular chemical process within the crust-mantle system.

Experimental studies on silicate and carbonatitic liquids at high pressure indicate that, although silicate minerals such as garnet, amphibole and clinopyroxene do fractionate Nb and Ta, the partition coefficients (D's) for both elements are very low. Thus involvement of these minerals may explain relatively small changes in Nb/Ta, but appears inadequate to explain the crust-mantle variation. However, high-quality data for Nb, Ta may be used to provide information on mantle melting or metasomatic processes (e.g., amphibole in the source region decreases Nb/Ta in derived melts, while carbonatitic metasomatism will increase Nb/Ta in affected mantle). Titanate minerals have high D's for Nb and Ta, and do fractionate these elements (e.g., D_Nb/D_Ta rutile/liquid of 0.5–0.8), and their involvement in crystal fractionation would increase Nb/Ta in derivative liquids. In contrast, D_Nb/D_Ta for rutile/fluid is 1.25, so that rocks affected by fluid equilibrated with residual rutile will show a decrease in Nb/Ta

Some Archaean gneisses appear to have high Nb/Ta, and may be a complementary component to that part of the crust which has a relatively low Nb/Ta, such as crustal-derived magmas (e.g., A- ad I-type granites and silicic volcanics). Within the crustal system pegmatites are known to have extremely high and variable Nb, Ta contents, often with low Nb/Ta. A fluid is generally considered to be involved in the generation of these rocks. Thus it is possible that fluid/melt partitioning may be the key to fractionating Nb and Ta, with preference for Ta in the fluid, and enrichment of Ta relative to Nb into the mid-upper crustal system, as the crust evolved, through upward movement of fluid.