Individual Garnet Research Articles

Directed growth of a sector-zoned garnet in a pegmatoid from the Bohemian Massif, Austria

Almandine-spessartine garnet from the Moldanubian Zone in the Bohemian Massif (Austria) records microstructural and compositional features of crystal growth imprinted during the transition from pegmatoid magmatic to subsolidus state. Directed garnet growth results in asymmetric compositional, morphological and microstructural zoning. In addition, the garnet core shows sector zoning reflected by the predominance of phosphate inclusions in {110} sectors, and rutile inclusions in {112} sectors of garnet. These sector-specific abundances of inclusions are also expressed in the electron probe microanalytical data of the non-equivalent garnet sectors. The contribution of the submicron and nano-inclusions of phosphates and rutile is of crucial importance for thermobarometric methods based on P, Na and Ti in garnet. The sector-specific abundance of inclusions is referred to the surface configuration of individual garnet facets. Contrastingly, the microstructural growth banding observed in the {112} garnet sectors supposedly results from compositional fluctuations of a boundary layer at the garnet-melt interface. These fluctuations control the details of the inclusion microstructures, which differ even in crystallographically equivalent sectors.

Evolution of pegmatite recorded by zoned garnet from the No. 9 dike in the Jiajika Li polymetallic deposit, eastern Tibetan plateau

Granitic pegmatites can be extremely rich in Li, an important metal for modern industries. Being the Asia’s largest hard-rock Li polymetallic deposit, the Jiajika Li deposit is associated with numerous pegmatite dikes in the eastern Tibetan Plateau. The major Li resources in Jiajika are mainly hosted in fine-grained phases of pegmatite dikes. In this study, samples were collected from the typically zoned No. 9 dike, which inwardly consists of fine-grained albite phase, medium-grained albite phase, coarse-grained albite-microline phase, massive microline phase and massive quartz phase. Garnet is one of the common accessory minerals in the fine-grained albite phase. We report the first in-situ LA-ICP-MS U-Pb age of garnet from pegmatite dikes, yielding an age of 216.7 ± 5.9 Ma, which is consistent with the age of the associated granitic pluton. Combined with published isotopic dating results in Jiajika, two age groups can be identified: ∼210 to ∼ 220 Ma and ∼ 190 to ∼ 200 Ma, which are suspected to be resulted from two stages of thermal events, probably separate pulses of pegmatitic melts. The late pulse of melts might be responsible for the weak deformation of garnet grains and Fe-enriched rims along the deformed boundaries. The early pulse of melts derived from the Majingzi-Jiajijiami granitic pluton shows an indistinct time path from granite, to fine-grained phases, to medium- to coarse-grained phases, which resulted in the trace elemental core-rim textures within individual garnet grains. The garnet cores have typically higher trace element concentrations than the rims, and the boundaries between cores and rims are usually sharp and sinuous, which are proposed to record the amalgamation, dissolution and reprecipitation processes. The growth of cores was not in equilibrium with other phases by strong undercooling, and thus inherited the initial Li signature of the incipient pegmatitic melts, which can be up to 373 ppm. Accordingly, it implies that Li and other incompatible element contents of the incipient pegmatitic melts in Jiajiaka could be enriched in a significant amount at the early stage, which may, to some extent, explain why major Li resources are hosted in very-fine to fine-grained phases in Jiajika.

Lu–Hf Geochronology on Single Garnets in a Micaschist from the North Qilian Orogenic Belt

The acquisition of the timing and duration of metamorphic mineral growth is the key to understanding the evolution of metamorphic belts. Garnet Lu–Hf geochronology is becoming increasingly powerful in this aspect. It is believed that garnet Lu–Hf radiometric systems are preserved during low-temperature metamorphism. However, this hypothesis has not been systematically tested. To examine the Lu–Hf systematics of garnets during low-temperature metamorphism, we conducted radiometric dating on individual garnet crystals of different sizes from a single micaschist in the North Qilian orogenic belt. The garnet Lu–Hf dates correlate well with the grain sizes and their core Mn concentrations. The positive correlation between the ages and grain sizes suggests that grain size is a proxy for preserving garnet nucleation and growth history. Small grains nucleated and grew later than large crystals. The Lu–Hf date of the individual garnet crystals faithfully recorded the total growth time span. The date difference of ~16 Myr is the minimum duration of the total garnet growth. The age discrepancy between the micaschist and the eclogite indicates they may not have experienced the same subduction and exhumation in the North Qilian orogenic belt.

Lithium, Phosphorus, and rare earth elements in magmatic garnets from granitic Pegmatites: Coupled substitution and petrogenetic implications

The general chemical formula of garnet can be expressed as X3Y2(ZO4)3 where X, Y, and Z represent dodecahedral, octahedral, and tetrahedral sites, respectively. Lithium, phosphorus, and rare earth elements (including yttrium) in garnets from the Luanzhuang-Guanpo and Ziyugou (ZYG) pegmatite fields in the East Qinling orogen and the Kalu’an (KLA) Li pegmatites in the Chinese Altay orogen have been investigated both at a mineral scale and in the context of pegmatite types to identify the substitution mechanisms for these elements. Our results show that Y + REE or P can enter the garnet lattice through coupled substitution with Li (+Na) that can compensate the charge differences due to Y + REE in the X-site or P in the Z-site of the garnet. Two substitutions, namely [X(Li,Na)+1X(Y,REE)3+1(R2+)–2] and [X(Li,Na)1ZP1XR2+-1ZSi-1], are likely the most important and can account for incorporation of the majority of Y + REE, P, and Li + Na in pegmatite garnets. Whichever substitution becomes predominant in garnets is strongly dependent on the bulk chemistry of pegmatite-forming melts. Consequently, the first substitution dominates in garnets from NYF (niobium-yttrium-fluorine) pegmatites and mixed-type pegmatites with NYF affinity whereas the other one dominates in garnets from relatively P-rich LCT (lithium-cesium-tantalum) and barren pegmatites. Incorporation of Sc and Ti in garnet may also need Li + Na, although Sc and Ti are not as much as Y and REE in pegmatite garnet. The zonings of Y, REE, and Sc in garnet are mainly controlled by the composition of pegmatite-forming melt and consumption of these elements due to the growth of garnet itself and other minerals enriched in Y + REE (e.g., xenotime, fergusonite, monazite, etc.) or Sc (e.g., biotite). Additionally, our results, combined with the data in the literature, show that garnets from NYF pegmatites and mixed-type pegmatites with NYF affinity commonly contain elevated Y (up to ∼ 20000 ppm), REE (up to ∼ 20000 ppm), and Sc (up to ∼ 2400 ppm) but low P (<100 ppm) compared to those from LCT and barren pegmatites. Nevertheless, individual garnets with low concentrations of Y, REE, Sc, and P can occur in any types of pegmatites. Therefore, the coupled substitution between Li + Na and Y + REE or P along with zonings and concentrations of Y, REE, and Sc in garnet should be investigated on a regular basis to interpret pegmatite petrogenesis.

Industrial garnet as an unconventional heavy rare earth element resource: Preliminary insights from a literature survey of worldwide garnet trace element concentrations

Current global supply of the critically important Heavy Rare Earth Elements (HREE) may be insufficient to meet future demand unless new sources are identified. This study utilizes trace element data in the literature to evaluate the potential of garnet, a common industrial mineral, to be further developed as a HREE-enriched mineral resource. A compilation of garnet REE data (n > 1,900 individual garnet analyses) demonstrates that garnet in metapelitic host rocks can have higher concentrations of the HREE dysprosium (Dy) through lutetium (Lu) in comparison to many of the existing and prospective sources of the Rare Earth Elements (REE). Furthermore, consideration of a smaller dataset containing garnet REE in addition to uranium and thorium concentrations indicates that many of the garnets with elevated HREE content also have uranium and thorium concentrations below those of the prospective and existing sources. As a case study, data from an actively mined garnet placer in northern Idaho (Emerald Creek), indicates that its proven garnet reserve contains ∼ 12 Mt of Lu alone despite its relatively small size. In total, the data indicate that experiments focused on the economics of extracting REE from garnet are warranted.

Distribution and characterization of heavy minerals in Meghna River sand deposits, Bangladesh

Heavy mineral (HM) concentrations in sand bar deposits of the Meghna River were evaluated using optical microscopy, X-Ray Diffraction (XRD), X-Ray Fluorescence spectrometry (XRF), Scanning Electron Microscopy (SEM) and Electron Probe Microanalysis (EPMA) to determine the mineralogy, geochemistry and likely source rocks of the sands, and to examine the distribution and HM potential of the valuable heavy minerals, ilmenite and garnet. The Meghna sands were dominated by the light minerals quartz and feldspar and grainsize analysis results showed that>78 wt.% of the sand population fell within the size range of fine to very fine sand (−250 + 63 μm). The HM component made up around 9 wt.% and consisted mainly of amphibole (∼17–46 wt.%), epidote (∼7–30 wt.%) and garnet (∼3–21 wt.%). Fe-Ti oxide minerals including ilmenite, rutile, and titanomagnetite were present at levels of 0.7–7 wt.% with the magnetite content at 0.9–7 wt.%. The textures (moderate sorting, sub angular) and significant feldspar (6–15 wt.%) content in the sands indicated low degrees of hydraulic sorting and low amounts of alteration/weathering. The HM assemblage indicated a likely mixed source area of varying metamorphic grade (low to high) facies and igneous rocks. EPMA results from the detrital ilmenites and garnets showed that for ilmenite, TiO2 contents were between 42.9 and 54.5 wt.% (ave. 51 wt.%), with overall low impurities, while the garnet compositions were dominated by Fe-rich, almandine types. From the chemical analysis of individual garnet and ilmenite grains, it is likely the Meghna River sands have good potential for commercial extraction of garnet and ilmenite.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ МЕХАНИЧЕСКИХ СВОЙСТВ И ХИМИЧЕСКОГО СОСТА-ВА ГРАНАТОВЫХ ПЕСКОВ НА ИХ АБРАЗИВНУЮ И РЕЖУЩУЮ СПОСОБНОСТИ

In this work the complex of physical and mechanical properties of garnet sand produced by the Australian company GMA Garnet PTY Ltd. and two prototypes of garnet concentrates produced by the Russian organization IMGRE with different mineral content was studied. The values of such characteristics as roughness, hardness, modulus of elasticity, strength, abrasive and cutting abilities, as well as bulk density were measured. A comparison of the obtained data was carried out in order to solve technical and economic issues related to the extraction of garnet sands and their use in specific types of material processing. Since abrasive and cutting ability are considered the main indicators of the quality of abrasive garnet sands, based on the obtained results, an analysis was made of the correlation of abrasive and cutting ability values with other measured properties. It is shown that, taking into account the presence of impurities, the average hardness of all three sand samples coincides within the error and is (17.2 ± 3.5) GPa, the elastic modulus was (274 ± 43) GPa. The roughness value for all three samples coincided within the error. At the same time, the obtained values of the hardness of individual garnet particles are comparable with the results of studies of garnet particles from various deposits or produced by other companies. It is shown that the main parameters of the studied, which are in direct correlation with the abrasive and cutting ability, are the strength and modulus of elasticity, as well as the percentage of garnet particles in the sand mixture. The content of impurity particles with lower hardness in the investigated sands nonlinearly underestimates the strength, abrasive and cutting abilities.

Constraining the ductile deformation mechanisms of garnet across pressure-temperature space

Garnet is a common mineral at elevated pressures, and how it deforms plays an important role in the strength of lower crustal shear zones, subduction thrusts, and the mantle. Strong shape preferred orientations in elongate garnets at elevated temperatures and pressures attests to garnet's ability to deform through ductile mechanisms; however, how individual garnets deformed is frequently ambiguous or disputed. Garnet microstructures from the Morin shear zone and the Sulu ultra-high pressure terrane are revisited using fine-scale electron backscattered diffraction and wavelength dispersive spectroscopy mapping. The dominant deformation mechanism at each site is re-interpreted, and we show that garnet deformed through dissolution-precipitation creep at Sulu and through dislocation-assisted diffusion creep at Morin. These observations are integrated into a compilation of ductile deformation microstructures for garnet which reveals domains where dissolution-precipitation creep, recovery-accommodated dislocation creep (through subgrain rotation dynamic recrystallization), and dislocation-assisted diffusion creep dominate in P-T space. Garnet may deform through ductile deformation at temperatures as low as ~500 °C and deforms under low differential stresses in both eclogite and granulite facies conditions. The efficacy of deformation indicates that 1) garnet may not be the strongest phase in these environments, and 2) that at elevated temperatures, inclusion barometry may be affected by garnet's inability to maintain high differential stress.

Chemická zonálnosť Ti-andraditového granátu v Ca-skarne z oblasti Magnetový vrch pri Tisovci (Slovenská republika)

Analyzed garnets from the Magnet hill area near Tisovec (Slovak Republic) are part of a Ca-skarn mineral association consisting of diopside, clinochlore and calcite. Compositionally they correspond to Ti-rich andradite (Adr50.9-73.7) with minor grossular (Grs3.8-44.5) and schorlomite (Sch0.5-41.5) components. Garnets contain up to 13 wt. % TiO2 and in all of them YFe3+&gt;YTi ratio prevails. Both sector and oscillatory chemical zoning were observed, which is primarily caused by variable distribution of Ti contents within individual garnet crystals. The three principal zones were distinguished in BSE imaging. The brightest are Ti-enriched zones with Ti content ranging from 0.85 to 0.50 apfu. Titanium gradually decreases in transitional zone (0.17 - 0.40 apfu) and reaches the minimum values in the dark zones (0.01 - 0.13 apfu). In the Ti-enriched zones the content of Si4+ and Al3+ is decreased due to substitution of Ti4+ and Fe3+ and assumed hydrogarnet substitution (SiO4)4- ↔ (O4H4)4-.

Pyroxene exsolution microstructures in garnet from the Almklovdalen peridotite, SW Norway

We report the occurrence of oriented pyroxenes in garnets within a garnetite and an orthopyroxenite from the Almklovdalen peridotite body located in the central-western part of the Western Gneiss Region (WGR), SW Norway. The garnetite sample is a non-deformed clot or aggregate that contains optically strongly birefringent purple garnets up to several centimetres in size. The microstructure is defined by millimetre-sized olivine and orthopyroxene grains that partially decorate garnet–garnet grain boundaries, but also occur as inclusions within individual garnets. Additionally, garnet has acicular to lamellar shaped inclusions that are dominantly 40–60 μm thick, up to 600 μm long and seem to have their longest crystal-shape axis oriented in four directions within the garnet host mineral. Individual lamellae are composed of orthopyroxene (dominantly) or clinopyroxene or both. Usually, these early minerals (interstitials, inclusions and host) are severely replaced by kelyphite, amphibole and chlorite. The garnet-orthopyroxenite sample is less altered and shows substantial dynamic recrystallisation. Porphyroclastic orthopyroxenes have abundant chromite and clinopyroxene lamellae while porphyroclastic garnets preserve oriented lamellae of pyroxene that are similar to those of the garnetite sample. These lamellae in cogenetic garnet and pyroxene indicate an origin by exsolution during cooling from temperatures ≥1500 °C at lithospheric mantle depth. Dynamically recrystallised pyroxenes lack oriented inclusions. Garnets of both samples are pyrope-rich (55–79 mol%) and have moderate CaO (4.0–5.7 wt%) and Cr2O3 (0.95–2.90 wt%) contents. Application of classical thermobarometry on the lamellae-bearing mineral assemblages suggests equilibration at metamorphic conditions of circa 2.9 GPa and 730 °C. This P–T estimate is consistent with a cratonic geotherm with 38mWm−2 surface heat flow. In contrast, the dynamically recrystallised mineral assemblage has orthopyroxene with a very low Al2O3 content (0.28 wt%) indicating equilibration at circa 4.1 GPa and 770 °C, i.e. within the stability field of diamond. The microstructures, the chemistry and metamorphic conditions of the minerals in the Almklovdalen garnetite and garnet-orthopyroxenite resemble those of similar rocks with Archaean majoritic garnet precursors that occur in the northwestern part of the WGR. Accordingly, these mantle fragments are interpreted to have been derived from the same depth interval (circa 90-110 km) within the mantle hanging wall of cratonic Laurentia before they have been incorporated into the subducted Baltic crust. Subsequent shared prograde diamond-grade partial recrystallisation implies that the early Scandian tectonic history across the WGR had much more in common than previously recognised. Another implication is that former majoritic garnet was once spatially widespread throughout the E Greenland lithosphere.

Andradite skarn garnet records of exceptionally low δ18O values within an Early Cretaceous hydrothermal system, Sierra Nevada, CA

Skarn garnets in the Mineral King roof pendant of the south–central Sierra Nevada within Sequoia National Park, California, USA reveal variable fluid chemistry with a significant component of meteoric water during metasomatism in the Early Cretaceous Sierra Nevada Batholith. We focus on andradite garnet associated with Pb–Zn mineralization in the White Chief Mine. Laser fluorination oxygen isotope analyses of δ18O of garnet (δ18O(Grt)) from sites along the skarn show a large range of values (− 8.8 to + 4.6‰ VSMOW). Ion microprobe (SIMS) analyses elucidate that individual andradite crystals are strongly zoned in δ18O(Grt) (up to 7‰ of variation). Total rare-earth element concentrations (∑REE) across individual garnets show progressive depletion of skarn-forming fluids in these elements during garnet growth. These findings support a skarn model of earliest red high-δ18O grandite garnet consistent with a magmatic-dominated equilibrium fluid (δ18O(H2O) as high as ≈ + 8‰). Later, green andradite crystallized in equilibrium with a low-δ18O fluid indicating a significant influx of meteoric fluid (δ18O(H2O) ≈ − 6 to − 5‰), following a hiatus in garnet growth, associated with late-stage Pb–Zn mineralization. Latest orange overprint rims have higher δ18O values (δ18O(H2O) ≈ 0–2‰), and depleted total REEs, suggesting influx of high-δ18O, trace-element depleted fluid derived from regional metamorphism of the carbonate host. Remarkably, low δ18O(Grt) values in the White Chief canyon skarn require a significant proportion of meteoric fluid available during > 400 °C andradite-forming metasomatism. Fluid flow was channelized at the pluton–wallrock contact, evidenced by the narrow extent of skarn.

Plagioclase disequilibrium induced during fluid‐absent biotite‐breakdown melting in metapelites

AbstractIn the Bandelierkop Quarry (South Marginal Zone of the Limpopo belt), two generations of leucosome contain peritectic garnet produced by incongruent melting of biotite‐bearing assemblages, either in the presence or absence of sillimanite, respectively. This study focuses on mineral inclusions in large garnet crystals from both leucosome generations, which, in rare cases, host an abundance of euhedral plagioclase inclusions with variable compositions. Plagioclase in the residuum adjacent to the leucosomes ranges from An32 to An39, while the euhedral plagioclase inclusions are considerably more calcic with compositions that range from An50 to An83. These grains are also significantly zoned, with inclusions displaying both Na‐ and Ca‐enriched rims. Garnet zoning directly adjacent to the inclusions demonstrates that the Ca‐rich nature of the inclusions is not a consequence of Ca‐uptake from garnet. The euhedral plagioclase inclusions are interpreted to reflect disequilibrium during anatexis due to slow diffusion in plagioclase. Thus, the kinetic pathway followed by the incongruent melting reaction appears to be one where only the outer portions of the reactant plagioclase grains were involved, via dissolution, not diffusion, and with the precipitation of new Ca‐rich peritectic plagioclase as a consequence. This is predictable, as the coupled substitutions involved in plagioclase solid solution result in very slow intracrystalline diffusion of the major elements, while plagioclase dissolution in granitic melt is faster by several orders of magnitude. Phase equilibrium modelling demonstrates that the Ca‐content of the peritectic plagioclase is inversely proportional to the amount of plagioclase that participates in the fluid‐absent incongruent melting reaction in biotite‐bearing metapelites, with the specific compositions of the peritectic grains, reflecting local variations in effective bulk composition as the melting reaction progressed. The variation in composition and zoning patterns within plagioclase inclusions from individual garnet grains is interpreted to reflect melt segregation and flow simultaneously with garnet growth, allowing individual garnet grains to trap euhedral peritectic plagioclase grains that had formed within a range of effective bulk compositions. The preservation of this unusual garnet, which was most likely due to rapid and efficient melt loss from the residual source, as well as rapid cooling, provides a rare insight into the details of the anatectic process.

Garnet composition as an indicator of skarn formation: LA‐ICP‐MS and EPMA studies on oscillatory zoned garnets from the Haobugao skarn deposit, Inner Mongolia, China

Oscillatory zoned garnets are widespread in the Haobugao skarn‐type copper–lead–zinc–iron polymetallic deposit, and they can record garnet growing process in the early stages of metallogenesis. In order to investigate the skarn‐forming process and hydrothermal fluid evolution of the Haobugao deposit, major, trace, and rare earth element (REE) contents of oscillatory zoned garnets were analysed by electron probe microscope analysis (EPMA) and laser‐ablation inductively‐coupled plasma mass spectrometry (LA‐ICP‐MS) techniques. Three distinct generations of garnets were identified: the first generation garnets (Grt I) are Fe‐rich, euhedral, fine‐ to coarse‐grained, isotropic, show characteristic concentric oscillatory zoning, and have light rare earth element (LREE)‐enriched and heavy rare earth element (HREE)‐depleted REE patterns, with strong positive Eu anomalies and low ΣREE concentrations. The second generation garnets (Grt II) are Al‐rich, anhedral to subhedral, anisotropic, with abundant oscillatory zoning alone the growth lines, and have LREE‐depleted and HREE‐enriched REE patterns, with negligible Eu anomalies and relatively higher ΣREE concentrations. The third generation garnets (Grt III) are anhedral, anisotropic and generally occurred at the rim of pre‐existing Grt I or beside fractures that cut through the pre‐existing Grt I crystals. All these three generations garnets show oscillatory zoning under an optical microscope and have different compositions from each other, but there's limited chemical zoning (such as bell‐shaped zoning of major elements) in each individual garnet crystal.The texture and composition characteristics of the garnets indicate that the Grt I is precipitated rapidly from high temperature and oxidized magmatic fluids by advective metasomatism, in a high water/rock ratio condition; the Grt II is precipitated from low temperature residual fluids that were in equilibrium with the host rock by diffusive metasomatism, in a low water/rock ratio condition; and the Grt III is formed by retrograde hydrothermal‐metasomatic alteration of pre‐existing garnets. The incorporation of REE into garnet is controlled by its crystal chemistry and fluid composition, dominated by the YAG (yttrium aluminium garnet) ‐type substitution mechanism .

Archaean and Proterozoic diamond growth from contrasting styles of large-scale magmatism

Precise dating of diamond growth is required to understand the interior workings of the early Earth and the deep carbon cycle. Here we report Sm-Nd isotope data from 26 individual garnet inclusions from 26 harzburgitic diamonds from Venetia, South Africa. Garnet inclusions and host diamonds comprise two compositional suites formed under markedly different conditions and define two isochrons, one Archaean (2.95 Ga) and one Proterozoic (1.15 Ga). The Archaean diamond suite formed from relatively cool fluid-dominated metasomatism during rifting of the southern shelf of the Zimbabwe Craton. The 1.8 billion years younger Proterozoic diamond suite formed by melt-dominated metasomatism related to the 1.1 Ga Umkondo Large Igneous Province. The results demonstrate that resolving the time of diamond growth events requires dating of individual inclusions, and that there was a major change in the magmatic processes responsible for harzburgitic diamond formation beneath Venetia from the Archaean to the Proterozoic.

Metasomatic changes during periodic fluid flux recorded in grandite garnet from the Weondong W-skarn deposit, South Korea

The Weondong skarn deposit primarily consists of grandite (andradite-grossular) garnets, which exhibit poikilitic/intergrowth textures and compositional zoning in metasomatism. The garnets can be classified into three main types based on their textural characteristics and rare earth element (REE) concentrations: (1) exoskarn garnet near quartz porphyry with a poikilitic texture and oscillatory zoning (core: And31–50, rim: And54–63) (T1 garnet), (2) exoskarn garnet near quartz porphyry with intergrowths (core: And55–83, mantle: And55–59, rim: And77–97) and poikilitic textures (T2 garnet), and (3) vein-hosted garnet with reverse zoning (core: And70–96, rim: And31–36) and epitaxial growth on distinct cores (T3 garnet). The textural features of the garnets are considered to have been caused by periodic fluctuations. The chondrite-normalized REE patterns of the garnet types show that the Al-rich garnets are HREE-enriched and formed at equilibrium with slow growth rates according to the kinetics of the garnet's growth. In contrast, Fe-rich garnets are slightly LREE-enriched and HREE-depleted. In terms of the kinetics of the garnet's growth, the Fe-rich garnets grew rapidly from externally derived fluids during infiltration metasomatism. The enriched W contents in the grandite garnets also indicate the type of growth mechanism, the infiltration process, and a specific skarn environment (such as advective metasomatism). Furthermore, each textural type of garnet was influenced differently by fluid/rock interactions, the magnitude of the fluid flow and the fluid compositions in the skarn system. Consequently, periodic fluid fluctuation in the skarn system affected the morphology and composition of individual garnet grains. Thus, the geochemical features of the grandite garnet from the Weondong polymetallic deposit provide key information to understand the evolution of the skarn deposit and its metasomatic history.

Lu–Hf geochronology on cm-sized garnets using microsampling: New constraints on garnet growth rates and duration of metamorphism during continental collision (Menderes Massif, Turkey)

This study shows Lu–Hf geochronology of zoned garnet crystals contained in mica schists from the southern Menderes Massif, Turkey. Selected samples are four 3–5 cm large garnet megacrysts of which several consecutive garnet shells have been sampled with a micro-saw and analyzed for dating. The results are used to extract growth rates of garnet, and also to improve the time constraint for Alpine-aged overprint of the Pan-African basement in the Menderes Massif.Lu–Hf ages of the sampled garnet shells are determined by two-point garnet-only isochrons using the garnets' Lu-depleted rim compositions. This yields a consistent decrease of age information from core to rim segments of individual garnet crystals and the calculated isochron ages propose a time frame of growth between 42.6±1.9 and 34.8±3.1 Ma. Major element profiles in the investigated garnets characterize zoning patterns indicative of prograde conditions: Rayleigh fractionated bell-shaped Mn and decreasing Fe/(Fe + Mg) are recorded by the garnets' core to rim compositions. Therefore the obtained Lu–Hf ages record timing of early prograde growth for the cores of the garnets. Two of the large garnet crystals also yield isochron ages of 58.83±0.69 and 50.16±0.84 Ma in their innermost cores, which appear to record an early nucleation event. This view, however, is not in concordance with the observed major element profiles of these garnets, and therefore is interpreted with caution. Termination of the garnet growth period is determined through the calculation of radial growth rates based on the size of the garnets and the Lu–Hf ages obtained for consecutive shells. Extrapolation of these rates potentially constrains the total duration for garnet growth terminating at 31±6 Ma. Comparison of the growth rates calculated for individual crystals shows a variety of slow and fast growing garnets, and similar results have been previously obtained with the Rb–Sr and Sm–Nd isotope systems.The new data provides a precise age determination for prograde Barrovian metamorphism in the southern Menderes Massif, which so far was placed between 63 and 27 Ma on the basis of mica Rb–Sr and Ar–Ar dating. This study provides new constraints crucial to the understanding of the tectonic evolution of southwest Anatolia and the Aegean realm, as it yields a shorter outline for Alpine aged continental collision.

Evaluating chemical equilibrium in metamorphic rocks using major element and Sm–Nd isotopic age zoning in garnet, Townshend Dam, Vermont, USA

A quantitative assessment of metamorphic chemical equilibrium derived from correlation of spessartine content and garnet Sm–Nd ages suggests that major element matrix equilibrium was maintained (to a first order) throughout a ca. 40cm-wide rock sample during garnet growth; however cm-scale Sm–Nd isotopic heterogeneity limits the Sm–Nd age precision required to evaluate more subtle age differences within individual garnet crystals. Central wafers from 1–3cm diameter garnet grains within a 1.21×104cm3 block of pelitic schist were used to document concentric growth zoning of major elements, with decreasing Mn and Ca and increasing Fe and Mg from cores to rims. Garnets also preserve growth zoning patterns for HREE and MREE and show evidence for resorption and partial recrystallization of the outermost rims. Similar garnet core compositions and identical garnet rim compositions for large like sized porphyroblasts throughout the sample suggest that garnet growth occurred at near equilibrium P–T–X conditions for major elements over the sample volume.Comparison of 28 rock Sm–Nd isotope values from the sample indicates substantial cm-scale heterogeneity, which precludes meaningful use of local garnet rock isotope pairs for isochron age calculation. Therefore, Sm–Nd isotopic compositions of thirty-eight concentric core to rim garnet segments from ten large (1–3cm) garnets and two small (1–4mm) bulk garnets, with narrow ranges of Mn content, are paired with sixteen matrix/whole-rock Sm–Nd isotopic compositions collected over the rock volume to define a range of isochron ages from 383.1±6.8Ma to 324.5±3.3Ma. Four of the garnets have anomalously young rims that likely result from post-growth alteration. Chlorite, quartz, and xenotime haloes around garnet suggest that anomalously young garnet rim ages reflect post-growth resorption/recrystallization effects. Excluding these young rims yields a range of ages from 383.1±6.8 (oldest core) to 374.9±1.8Ma (youngest rim). Sm–Nd age precisions >1.5m.y. (and high MSWD) result primarily from isotopic heterogeneity in the finely layered metasedimentary rock matrix. However, garnet cores with high Mn (n=7), mantles with intermediate Mn (n=14), and rims with low Mn (n=8; including the 2 smaller bulk garnet analyses), define three distinct multi-grain isochrons of 380.3±2.0Ma (n=23, MSWD=14), 377.3±1.4Ma (n=30, MSWD=18), and 376.5±1.0Ma (n=24, MSWD=18), respectively, yielding an average garnet growth duration of 3.8±2.2m.y. These three composite Mn-age zones define a Mn vs. age relationship that reflects depletion of Mn in the rock matrix as it is sequestered by growing garnet. Correlation of garnet major element compositions throughout the sample suggests that major element matrix equilibrium was generally maintained (to a first order) throughout the ca. 4m.y. duration of garnet growth.

Decoding polyphase migmatites using geochronology and phase equilibria modelling

AbstractIn this study, in situ U–Pb monazite ages and Lu–Hf garnet geochronology are used to distinguish mineral parageneses developed during Devonian–Carboniferous and Cretaceous events in migmatitic paragneiss and orthogneiss from the Fosdick migmatite–granite complex in West Antarctica. SHRIMP U–Pb monazite ages define two dominant populations at 365–300 Ma (from cores of polychronic grains, dominantly from deeper structural levels in the central and western sectors of the complex) and 120–96 Ma (from rims of polychronic grains, dominantly from the central and western sectors of the complex, and from monochronic grains, mostly from shallower structural levels in the eastern sector of the complex). For five paragneisses and two orthogneisses, Lu–Hf garnet ages range from 116 to 111 Ma, c. 12–17 Ma older than published Sm–Nd garnet ages of 102–99 Ma from three of the same samples. Garnet grains in the analysed samples generally have Lu‐enriched rims relative to Lu‐depleted cores. By contrast, for three of the same samples, individual garnet grains have flat Sm concentrations consistent with high‐T diffusive resetting. Lutetium enrichment of garnet rims is interpreted to record the breakdown of a Lu‐rich accessory mineral during the final stage of garnet growth immediately prior to the metamorphic peak, and/or the preferential retention of Lu in garnet during breakdown to cordierite in the presence of melt concomitant with the initial stages of exhumation. Therefore, garnet is interpreted to be part of the Cretaceous mineral paragenesis and the Lu–Hf garnet ages are interpreted to record the timing of close‐to‐peak metamorphism for this event. For the Devonian–Carboniferous event, phase equilibria modelling of the metasedimentary protoliths to the paragneiss and a diatexite migmatite restrict the peak P–T conditions to 720–800 °C at 0.45–1.0 GPa. For the Cretaceous event, using both forward and inverse phase equilibria modelling of residual paragneiss and orthogneiss compositions, the P–T conditions after decompression are estimated to have been 850–880 °C at 0.65–0.80 GPa. These P–T conditions occurred between c. 106 and c. 96 Ma, determined from Y‐enriched rims on monazite that record the timing of garnet and biotite breakdown to cordierite in the presence of melt. The effects of this younger metamorphic event are dominant throughout the Fosdick complex.

Thermal evolution of the Tseel terrane, SW Mongolia and its relation to granitoid intrusions in the Central Asian Orogenic Belt

AbstractThe timing and thermal effects of granitoid intrusions into accreted sedimentary rocks are important for understanding the growth process of continental crust. In this study, the petrology and geochronology of pelitic gneisses in the Tseel area of the Tseel terrane, SW Mongolia, are examined to understand the relationship between igneous activity and metamorphism during crustal evolution in the Central Asian Orogenic Belt (CAOB). Four mineral zones are recognized on the basis of progressive changes in the mineral assemblages in the pelitic gneisses, namely: the garnet, staurolite, sillimanite and cordierite zones. The gneisses with high metamorphic grades (i.e. sillimanite and cordierite zones) occur in the central part of the Tseel area, where granitoids are abundant. To the north and south of these granitoids, the metamorphic grade shows a gradual decrease. The composition of garnet in the pelitic gneisses varies systematically across the mineral zones, from grossular‐rich garnet in the garnet zone to zoned garnet with grossular‐rich cores and pyrope‐rich rims in the staurolite zone, and pyrope‐rich garnet in the sillimanite and cordierite zones. Thermobarometric analyses of individual garnet crystals reveal two main stages of metamorphism: (i) a high‐P and low‐T stage (as recorded by garnet in the garnet zone and garnet cores in the staurolite zone) at 520–580 °C and 4.5–7 kbar in the kyanite stability field and (ii) a low‐P and high‐T stage (garnet rims in the staurolite zone and garnet in the sillimanite and cordierite zones) at 570–680 °C and 3.0–6.0 kbar in the sillimanite stability field. The earlier high‐P metamorphism resulted in the growth of kyanite in quartz veins within the staurolite and sillimanite zones. The U–Pb zircon ages of pelitic gneisses and granitoids reveal that (i) the protolith (igneous) age of the pelitic gneisses is c. 510 Ma; (ii) the low‐P and high‐T metamorphism occurred at 377 ± 30 Ma; and (iii) this metamorphic stage was coeval with granitoid intrusion at 385 ± 7 Ma. The age of the earlier low‐T and high‐P metamorphism is not clearly recorded in the zircon, but probably corresponds to small age peaks at 450–400 Ma. The low‐P and high‐T metamorphism continued for c. 100 Ma, which is longer than the active period of a single granitoid body. These findings indicate that an elevation of geotherm and a transition from high‐P and low‐T to low‐P and high‐T metamorphism occurred, associated with continuous emplacement of several granitoids, during the crustal evolution in the Devonian CAOB.

Geothermobarometric history of subduction recorded by quartz inclusions in garnet

Burial histories of subduction zone rocks are often difficult to accurately constrain, owing to a lack of robust mineral geobarometers applicable to high pressure mineral assemblages. Knowledge of the depth‐histories of subduction is, however, required for our understanding of global geochemical cycles, subduction‐related seismicity, and the evolution of destructive tectonic boundaries. The high spatial resolution of quartz inclusion geobarometry can be used to determine pressure evolution during metamorphic growth of individual garnet crystals. Quartz inclusions in garnet from Sifnos, Greece, preserve such a record of the pressure of garnet growth, allowing detailed reconstruction of the metamorphic evolution of these rocks. Pressure‐dependent Raman spectra of quartz inclusions were combined with elastic modeling to infer the conditions at which they were trapped during garnet growth. All measured inclusions suggest that garnet growth occurred between 19 and 20.5 kbars, with little evidence for significant pressure variation during the garnet growth interval, which is interpreted to record ∼100°C of heating. Coupled with thermometry and geochronology, these results show that early, cold burial was followed by a phase of rapid heating, which immediately preceded exhumation. Garnet growth occurred primarily during this heating phase.