Garnet Zone Research Articles

Mineralogy, isotope geochemistry and ore genesis of the miocene Cuonadong leucogranite-related Be-W-Sn skarn deposit in Southern Tibet

The Himalayan orogenic belt is characterized by widespread leucogranites, which are mostly peraluminous with less than 5% mafic minerals. Previous studies mainly focused on the petrological characteristic, genetic mechanism, magmatic evolution and tectonic significance of these leucogranites. However, recently, a skarn-type Be–W–Sn polymetallic deposit, which is spatially related to the Cuonadong leucogranites of the Himalayan leucogranites belt, was discovered. The orebodies occur in the inner and outer contact zones between the Cuonadong leucogranites and their surrounding marble. This study focuses on the the characteristics of the skarn mineral assemblages. The deposit shows a typical skarn mineral zonation, from inside to outside, including dark pyroxene-garnet skarn zone, red garnet zone, dark green amphibole-specularite zone, light-green vesuvianite-epidote ribbon, and vesuvianite-chlorite zone. Zircon U-Pb dating shows that the ore-related leucogranite was emplaced at ca. 18 Ma. Geochemically, the leucogranite has high SiO2, Al2O3, Na2O, K2O and low CaO contents belonging to peraluminous granite. The Cuonadong leucogranite is characterized by right-dipping chondrite-normalized REE patterns with distinct negative Eu anomalies. Importantly, all samples show Lanthanide tetrad effect on chondrite-normalized REE patterns, indicating significant melt-fluid interaction has occurred due to strong fractional crystallization. Additionally, the Cuonadong leucogranite is enriched in large ion lithophile elements (such as Rb) and relatively depleted in high field strength elements (such as Nb, Zr, Hf), suggesting a crustal source region. High F and B contents in the magma could have significantly increased the solubility of water in the magma, while the magmatic-hydrothermal solution enriched in F and B should have remarkably enhanced the metal-carrying capacity and increased the permeability of surrounding rocks, which in turn effectively facilitated water–rock reaction.

Finite pattern of Barrovian metamorphic zones: interplay between thermal reequilibration and post-peak deformation during continental collision—insights from the Svratka dome (Bohemian Massif)

The Barrovian inverted metamorphism of the Svratka dome developed within two nappes derived from the Brunia continent that was thrust beneath the Moldanubian orogenic root. The metamorphism increases from biotite–chlorite zone in the basement to very closely spaced staurolite, kyanite and sillimanite zones at the top of the nappe pile. The sequence of mineral growth, chemical zoning of garnet, and pseudosection modelling indicate prograde paths from 4.5 kbar/510 °C to 5.5 kbar/540 °C in the garnet zone, from 6 kbar/530 °C to 7 kbar/600 °C in the staurolite zone, and from 3.5 kbar/510 °C to 8.5 kbar/650 °C in the kyanite zone. The age of monazite inclusions in garnet and staurolite is interpreted to reflect prograde metamorphism at 338 ± 7 Ma and 336 ± 7 Ma, respectively. An older matrix monazite crystal is interpreted as dating prograde crystallization at 345 ± 7 Ma, whereas a younger monazite group records recrystallization at/or down to 334 ± 7 Ma. While these petrological and geochronological data are consistent with data from an inverted metamorphic sequence of the southern Thaya dome, the spacing and distribution of metamorphic zones, nappe thicknesses, and late structures are different in the two domes. An antiformal stack of imbricated basement sheets and the extreme attenuation of metamorphic isograds at the top of the nappe pile in the Svratka dome are explained by a relatively cold overthrusting Moldanubian domain, formed mainly of middle orogenic crust. The homogeneous thickening of the hinterland-dipping basement duplexes and the regular spacing of metamorphic isograds in the Thaya dome are explained by a hot overriding Moldanubian domain, which in this region has a high proportion of exhumed lower orogenic crust and formed a hot mid-crustal channel.

Release of oxidizing fluids in subduction zones recorded by iron isotope zonation in garnet

Subduction zones are key regions of chemical and mass transfer between the Earth’s surface and mantle. During subduction, oxidized material is carried into the mantle and large amounts of water are released due to the breakdown of hydrous minerals such as lawsonite. Dehydration accompanied by the release of oxidizing species may play a key role in controlling redox changes in the subducting slab and overlying mantle wedge. Here we present measurements of oxygen fugacity, using garnet–epidote oxybarometry, together with analyses of the stable iron isotope composition of zoned garnets from Sifnos, Greece. We find that the garnet interiors grew under relatively oxidized conditions whereas garnet rims record more reduced conditions. Garnet δ56Fe increases from core to rim as the system becomes more reduced. Thermodynamic analysis shows that this change from relatively oxidized to more reduced conditions occurred during lawsonite dehydration. We conclude that the garnets maintain a record of progressive dehydration and that the residual mineral assemblages within the slab became more reduced during progressive subduction-zone dehydration. This is consistent with the hypothesis that lawsonite dehydration accompanied by the release of oxidizing species, such as sulfate, plays an important and measurable role in the global redox budget and contributes to sub-arc mantle oxidation in subduction zones. Lawsonite dehydration and release of oxidizing fluids could play an important role in sub-arc mantle oxidation in subduction zones, suggest measurements of changing oxygen fugacity in zoned garnets from Sifnos, Greece.

Concordant pulse in Mn, Y and HREEs concentrations during UHP eclogitic garnet growth: Transient rock dynamics along a cold subduction plate interface

We herein report strikingly concordant Mn, Y, and HREE garnet zoning in ultrahigh-pressure low-temperature oceanic eclogites found in the fossil Devonian-Carboniferous subduction plate interface in SW-Tianshan. Zr-in-rutile thermometry, thermodynamic modeling and diffusion-controlled garnet growth modeling show that this type of zoning can be explained by a short-lived transient, most likely a ∼80°C thermal pulse (from 490 to 570°C). This event could be as transient as ∼300 Ka, assuming a prograde to peak burial duration of 5 Ma. Such garnet zoning is observed for grains sampled meters apart (Fig. 1d) and was detected in other eclogites in the region. This transient event could reflect the short-lived movement of UHP oceanic slices, once detached, towards the hotter mantle wedge, and/or deformation-induced remobilization of fluids enhancing diffusion-controlled garnet growth.

Implications of overstepping of garnet nucleation for geothermometry, geobarometry and P–T path calculations

The composition of a porphyroblast such as garnet is not governed by equilibrium relations when it nucleates and grows after considerable overstepping. The approach followed here is to assume the composition is controlled by the maximum driving force (MDF) or parallel tangent model and to examine the potential pitfalls for assuming equilibrium. The MDF model ensures Fe-Mg partitioning equilibrium between the matrix phases and growing porphyroblast, but the net transfer equilibrium controlling grossular content has a different stoichiometry from the equilibrium relations. An important result of this study is that garnet zoning profiles generated using this model and assuming chemical fractionation are nearly identical to zoning profiles grown assuming continuous equilibrium and it is impossible to discern whether garnet grew under near-equilibrium conditions or only after considerable overstepping by examination of the zoning profile alone.Pressure–temperature paths calculated using the method of intersecting isopleths from simulations in which garnet was grown after overstepping and assuming the MDF model typically display nearly isothermal loading or loading with minor heating paths, even when the garnet was grown under isothermal and isobaric conditions. Garnet-plagioclase barometry on the garnet core and rim also suggests loading during garnet growth, even for crystals grown isothermally and isobarically. Garnet-biotite Fe-Mg exchange thermometry does, however, recover the correct temperature to within 10 °C for both the garnet core and rim.Comparison of natural samples to the theoretical calculations suggests that the local effective bulk composition in the vicinity of garnet is depleted in Na2O, in addition to FeO, MnO and CaO, thus rendering calculation schemes that rely on knowledge of the bulk composition difficult to administer. To the extent that garnet has nucleated and grown after considerable overstepping, application of methods of extracting P–T paths from the chemical zoning in garnet will yield incorrect results if those methods assume continuous equilibration with the matrix. The major discrepancy comes from calculation of the P–T conditions of the garnet core. Simulations and natural samples suggest that under typical scenarios the garnet rim is in near equilibrium with the matrix assemblage so equilibrium approaches (thermobarometry or intersecting isopleths) should recover the peak metamorphic conditions with reasonable assuredness.

The significance of Mn‐rich ilmenite and the determination of P–T paths from zoned garnet in metasedimentary rocks from the western Cape Breton Highlands, Nova Scotia

AbstractThe Jumping Brook Metamorphic Suite in the western Cape Breton Highlands of Nova Scotia is part of an inverted Barrovian sequence that formed during a Late Silurian–Early Devonian promontory–promontory collision in the Canadian Appalachians. In this study, systematic discrepancies between geochemical observations and thermodynamic model predictions led to the discovery of a systematic relationship linking the style of garnet core isopleth intersection (GCII) to the pyrophanite (MnTiO3) component of co‐existing ilmenite. Samples that yielded tight GCIIs at or near the garnet‐in curve were found to contain ilmenite with negligible pyrophanite components, whereas samples yielding GCIIs far removed (up to 105°C) from the garnet‐in curve were found to contain ilmenite with significant pyrophanite and/or ecandrewsite (ZnTiO3) components. Based on petrographic and geochemical observations, Mn(±Zn)‐rich ilmenite are interpreted to have sequestered Mn throughout prograde metamorphism due to sluggish intracrystalline diffusion. The amount of reactive Mn input into the thermodynamic models from whole‐rock analyses were, in some cases, overestimated, resulting in garnet‐in curve topologies that extend to erroneously low P–T conditions. Modifications to the whole‐rock chemistry that account for Mn sequestration into ilmenite, however, yielded robust model results. Our results show that, in addition to uncertainties in thermodynamic data sets and phenomenon related to reaction kinetics, Mn‐rich ilmenite may superimpose additional complexities related to the interpretation of predicted equilibria involving garnet. Numerical simulations of garnet crystallization were used to infer P–T paths of metamorphism for one sample from the garnet zone (Mn corrected) and two samples from the staurolite zone (Mn uncorrected) of the inverted sequence. Model results are remarkably similar among the three samples and indicate that garnet crystallization occurred along relatively steep (31–37°C/km) clockwise P–T paths. The peak conditions of garnet crystallization and metamorphism (560–590°C, 7.4–8.0 kbar) are interpreted to have been attained approximately simultaneously, such that the paths are characterized by tight prograde‐to‐retrograde transitions. The hairpin nature of the P–T paths is interpreted to represent the onset of thrust‐related exhumation and isograd inversion along ductile shear zones, consistent with available field and geochronological constraints.

Factors controlling the crystal morphology and chemistry of garnet in skarn deposits: A case study from the Cuihongshan polymetallic deposit, Lesser Xing'an Range, NE China

Abstract The grossular-andradite solid solutions in garnet from skarn deposits in relation to hydrothermal processes and physicochemical conditions of ore formation remain controversial. Here we investigate garnet occurring in association with calcic and magnesian skarn rocks in the Cuihongshan polymetallic skarn deposit of NE China. The calcic skarn rocks contain three types of garnets. (1) Prograde type I Al-rich anisotropic garnets display polysynthetic twinning and a compositional range of Grs18–80Adr10–75. This type of garnet shows markedly low rare earth element (REE) contents (3.27–78.26 ppm) and is strongly depleted in light rare earth elements (LREE, 0.57–44.65 ppm) relative to heavy rare earth elements (HREE, 2.31–59.19 ppm). They also display a significantly negative Eu anomaly (Eu/Eu* of 0.03–0.90). (2) Fe-rich retrograde type II garnets are anisotropic with oscillatory zoning and own wide compositional variations (Grs1–47Adr30–95) with flat REE (13.73–377.08 ppm) patterns. (3) Fe-rich retrograde type III isotropic garnets display oscillatory zoning and morphological transition from planar dodecahedral {110} crystal faces to {211} crystal faces in the margin. Types III garnets exhibit relatively narrow compositional variations of Grs0.1–12Adr85–97 with LREE-enrichment (0.80–51.87 ppm), flat HREE patterns (0.15–2.46 ppm) and strong positive Eu anomalies (Eu/Eu* of 0.93–27.07 with almost all >1). The magnesian skarn rocks contain euhedral isotropic type IV Mn-rich garnet veins with a composition of Grs10–23Sps48–62Alm14–29. All calcic garnets contain considerable Sn and W contents. Type II garnet containing intermediate compositions of andradite and grossular shows the highest Sn contents (64.36–2778.92 ppm), albeit the lowest W range (1.11–468.44 ppm). Birefringence of garnet is probably caused by strain from lattice mismatch at a twinning boundary or ion substitution near intermediate compositions of grossular-andradite. The fine-scale, sharp, and straight garnet zones are probably caused by self-organization, but the compositional variations of zones from core to rim are probably caused by external factors. The zoning is likely driven by external factors such as composition of the hydrothermal fluid. REE concentrations are probably influenced by the relative proportion and temperature of the system. Moreover, the LREE-HREE fractionation of garnet can be attributed to relative compositions of grossular-andradite system. The W and Sn concentrations in garnet can be used as indicators for the exploration of W-Sn skarn deposits.

Two-stage garnet growth in coesite eclogite from the southeastern Papua New Guinea (U)HP terrane and its geodynamic significance

Mineral compositions and textures of Late Miocene coesite eclogite from Tomagabuna Island were investigated to constrain P–T conditions during UHP metamorphism and subsequent exhumation. Two stages of garnet growth (core and rim), at eclogite-facies conditions, were documented. In addition to core garnet (I), peak assemblages include omphacite, coesite, phengite, and rutile. Rim garnet (II), amphibole, paragonite, plagioclase, quartz, and accessory biotite and spinel were formed after peak pressure conditions. Although a compositional gradient is present at the core–rim garnet interface, the garnet core has a relatively flat compositional profile, suggesting its crystallization in the coesite stability field. Together with minerals formed subsequent to peak pressure (UHP) conditions, the composition of the garnet rim indicates heating of the eclogite during its decompression into the amphibolite facies mineral stability field. Based on compositional zoning in garnet and application of diffusion modelling, we propose that eclogite-facies metamorphism of the mafic protolith and its host lithologies occurred at, or near, UHP conditions. The core garnet (I) formed at 650 °C at 8 Myr in the coesite stability field and was partially resorbed during the onset of exhumation. We infer that garnet rim growth at < 7.1 Ma occurred at P–T conditions corresponding to the boundary between the eclogite and amphibolite facies fields. Diffusion modelling for the garnet core–rim boundary compositions suggests a transient heating event (0.3 Myr) occurred at 1.5 GPa that we infer resulted from heat transport within the Australian—Woodlark plate boundary zone as the coesite eclogite was exhumed. Results caution that apparent P–T paths documented for many UHP terranes may not result from isothermal decompression corresponding to peak temperatures. Instead, paths may link P–T points’ set during transient mineral growth events when the UHP rocks form within the subduction channel, and during subsequent heating events, when UHP rocks are exhumed from mantle depths.

Metamorphism of Jhyallaphay-Barpak-Bhachchek area of Gorkha District, Lesser Himalaya and Higher Himalaya, Central Nepal

The study is focused on geological mapping, petrography and metamorphism of Jhyallaphat–Barpak–Bhachchek area, a part of Gorkha District, Central Nepal using a base of 1:25000 scale covering an area of 139.80 sq. km. The rocks of the study area can be broadly divided into two tectonic zones; the Lesser Himalaya consisting of fie lithological units, and the Higher Himalaya consisting of Formation I of the Tibetan Slab. Three metamorphic zones can be distinguished in the study area; biotite zone, garnet zone and kyanite zone. The biotite zone of the mineral assemblage in pelitic rocks consists of biotite+muscovite+chlorite+quartz, in psammitic rocks comprises of biotite+muscovite+chlorite+feldspar+quartz and in carbonate rocks comprises of biotite+muscovite+calcite/dolomite+feldspar+quartz, respectively. These mineral assemblages show that the area belongs to the greenschist facies. The mineral assemblage of the garnet zone in pelitic rocks constitutes garnet+biotite+muscovite+chlorite+quartz, and in psammitic rocks constitutes of garnet+biotite+muscovite+feldspar+quartz. The minerals assemblages found within the biotite and garnet zones represent the well-known inverted metamorphism in the Lesser Himalaya. Mineral assemblage of the kyanite zones constitutes of kyanite+garnet+biotite+muscovite+feldspar+quartz. The mineral assemblages of the both garnet and kyanite zones show that the area belongs to the epidote amphibolite facies.&#x0D; The bedding and foliation planes are almost parallel, showing that isograds also cut across the foliation. Therefore, the main metamorphic event should have followed development of foliation in the area. The rocks of the area show at least two metamorphic events: syntectonic prograde and post-tectonic retrograde. Syn-tectonic prograde metamorphism (M1), which has grown during a single phase of deformation and most frequently encountered garnet prophyroblast. Metamorphic deformation is represented by the presence of metamorphic foliation, stretching lineation, and S-C fabric. Post-tectonic retrograde metamorphism (M2), which is followed by retrograde mineral formation changing its P-T condition from high to low grade minerals, such as the formation of the biotite and chlorite minerals around the rims of the garnet porphyroblasts.

Geology and mineral resources of Khudi-Bahundanda area of west-central Nepal along Marshyangdi Valley

Geological study was carried out along the Khudi-Bahundanda area of the Marshyangdi Valley in the west central Nepal. The area lies partly in the Main Central Thrust (MCT) zone and partly in the Higher Himalayan Crystalline Zone. The aim of the study was to prepare a detail geological map and cross section in the scale of 1:25,000 to work out on stratigraphy, metamorphism and mineral resource potential of the area. The rocks of the Higher Himalaya have been mapped under a single unit as Formation I. This unit consists of kyanite-garnet para-gneiss. The lithological units of the MCT zone are mapped into three units as the Benighat Slate, the Malekhu Formation and the Robang Formation from the bottom to the top, respectively. The Benighat Slate consists of dark grey to black schist with some carbonate beds as members. The Malekhu Formation consists of creamy white siliceous dolomite marble with parting of schist. The Robang Formation comprises of light grey psammitic schist with garnet and white micaceous quartzite in various proportion.&#x0D; Many secondary structures are observed in the study area, but primary structures are missing due to extreme metamorphism. The large-scale structures are the MCT, which separates the Lesser Himalayan rocks to the south from the Higher Himalaya to the north, and the Bahundanda Thrust (BT). Numerous outcrop-scale structures like meso-scale folds, quartz veins, boudinage and ptygmatic folds are abundant. Folds in the MCT zone are mostly E-W trending, and rocks have experienced multiple metamorphism and dynamic crystallization of minerals. The Lesser Himalayan rocks resemble the garnet zone while the Higher Himalayan rocks resemble to the kyanite grade of metamorphism. As in the other sections of the Himalaya, the present section also clearly shows the inverted metamorphism in the MCT zone. The MCT zone is considered as the potential site for precious and semi-precious stones, of which the most potential ones are the garnet and kyanite.

Geology and micro-structure analysis of the MCT zone along Khudi- Bahundanda area of Lamjung District, west-central Nepal

Geological mapping was carried out along the Marsyangdi Valley in the Khudi-Bahundanda area of west-central Nepal covering the Main Central Thrust (MCT) zone. The main objectives of the study were to draw a clear picture of lithology, geological structures and micro-tectonics in the rocks. A detail survey on stratigraphy and correlation with central Nepal reveals geological rock units such as the Benighat Slate, the Malekhu Formation and the Robang Formation of the Lesser Himalaya and the Formation I of the Higher Himalaya. Both regional and small-scale geological structures have been studied. The MCT zone has been mapped as a major regional structure in the region. The Bahundanda Thrust (BT), which has brought the older Malekhu Formation over the younger Robang Formation, is an another significant structure mapped. The BT is marked on the basis of fault breccia, slickensides as well as large deposits of debris mass at the fault zone.&#x0D; The study area has undergone poly-metamorphism and dynamic crystallization of minerals. The Lesser Himalayan rocks resemble the garnet zone while the Higher Himalaya rocks resemble to the kyanite grade of metamorphism. The present section clearly shows the inverted metamorphism in the MCT zone as in the other sections of the Himalaya. Microscopic features like ribbon-quartz, polygonization of quartz crystals, grain boundary reduction, mica-fish and rotated garnet grains indicates the ductile shearing in the MCT zone suggesting the dynamic recrystallization during thrust propagation. Numerous outcrop-scale structures like meso-scalefolds, quartz veins, boudinage and ptygmatic folds are abundant folds in the MCT zone and these are mostly E-W trending.

New evidence for the prograde and retrograde PT-path of high-pressure granulites, Moldanubian Zone, Lower Austria, by Zr-in-rutile thermometry and garnet diffusion modelling

Compositional zoning in garnet, mineral inclusions and the application of the Zr-in-rutile thermometry on rutile inclusions in garnet in combination with conventional geothermobarometry and thermodynamic modelling allows a reconstruction of the prograde pressure-temperature evolution in felsic and mafic high-pressure granulites from the Moldanubian Zone, Bohemian Massif, Lower Austria. Most garnets in these rocks show homogeneous core compositions with high grossular contents (~30 mol%), while their rim zones have a markedly reduced grossular content. Rutile inclusions in the grossular rich garnet cores have low Zr concentrations (400 to 1300 ppm) indicating a formation temperature of ~810–820 °C which implies that the garnet host grew at these temperature conditions as well. Based on numerous polycrystalline melt inclusions, high Ti-biotite relics and a generally high Ti concentration in garnet cores, the peritectic biotite breakdown reaction is considered to be responsible for a first garnet growth, now observed as high-grossular garnet cores. The corresponding pressure is estimated to be in the range of 1.6 to 2.5 GPa, based on experimentally determined biotite breakdown reactions, thermodynamic modelling and the occurrence of high-Ti biotite in garnet cores. Rutile inclusions in low-Ca garnet rims contain significantly higher Zr concentrations (1700 to 5800 ppm) resulting in ultrahigh temperatures of ~1030 °C. Similar temperature as well as corresponding pressure estimates of 1000 ± 50 °C and 1.60 ± 0.10 GPa were obtained by geothermobarometry and thermodynamic modelling using garnet rim and re-integrated ternary feldspar compositions. These high pressure and ultrahigh temperature conditions are well known from literature for these granulites. The proposed two-phase garnet growth is not only seen in different temperatures obtained from rutile inclusions in garnet core and rim areas, but also in discontinuous trace (Cr, Ga, P, Ti, V, Zr) and heavy rare earth element profiles across garnet porphyroblasts, implying a different reaction mechanism for garnet rim growth. This second phase of garnet growth must have occurred during near isobaric heating to the ultrahigh temperature peak, most likely even at slightly lower pressures compared to the garnet core growth.By applying a binary Fe-Mg diffusion model to strongly zoned garnet grains a maximum timescale of 5–6 million years was estimated for the exhumation and cooling process, assuming a linear cooling path from 1000 °C at 1.6 GPa to 760 °C at 0.8 GPa. This short-lived ultrahigh temperature event corresponds to cooling and exhumation rates of 40–50 °C Ma−1 and 5.3–6.6 mm y−1, respectively.

Timing and style of high-temperature metamorphism across the Western Gawler Craton during the Paleo- to Mesoproterozoic

Combined in situ monazite dating, mineral equilibria modelling and zircon U–Pb detrital zircon analysis provide insight into the pressure–temperature–time (P–T–t) evolution of the western Gawler Craton. In the Nawa Domain, pelitic and quartzo-feldspathic gneisses were deposited after ca 1760 Ma and record high-grade metamorphic conditions of ∼7.5 kbar and 850 °C at ca 1730 Ma. Post-peak microstructures, including partial plagioclase coronae and late biotite around garnet, and subtle retrograde garnet compositional zoning, suggest that these rocks cooled along a shallow down-pressure trajectory across an elevated dry solidus. In the northwest Fowler Domain (Colona Block), monazite grains from pelitic gneisses record two stages of growth/recrystallisation interpreted to represent discrete parts of the P–T path: (1) ca 1710 Ma monazite growth during prograde to peak conditions, and (2) ca 1690 Ma Y-enriched monazite growth/recrystallisation during partial garnet breakdown and cooling towards the solidus. Relict prograde growth zoning in garnet suggests rocks underwent a steep up-P path to peak conditions of ∼8 kbar at 800 °C. The new P–T–t results suggest basement rocks of the southwestern Nawa and northwestern Fowler were buried to depths of 20–25 km during the Kimban Orogeny, ca 10 Myrs after the sedimentary precursors were deposited. The P–T path for the Kimban Orogeny is broadly anti-clockwise, suggesting that at least the early phase of this event was associated with extension. Exhumation of rocks from both the southwestern Nawa and northwestern Fowler domains may have occurred during the waning stages of the Kimban Orogeny (<ca 1690 Ma). The limited low-grade overprint in these rocks may be explained by a mid-to-upper crustal position for these rocks during the subsequent Kararan Orogeny. Aluminous quartz-feldspathic gneiss of the Nundroo Block in the eastern Fowler Domain records peak conditions of ∼7 kbar at 800 °C. Monazite grains from the Nundroo Block are dominated by an age peak at ca 1590 Ma, although the presence of some older ages up to ca 1690 Ma, possibly reflect partial resetting of older monazite domains. The P–T–t conditions suggest these rocks were buried to 20–25 km at ca 1590 Ma during the Kararan Orogeny. This high-grade metamorphism in the Nundroo Block is a mid-crustal expression of the same thermal anomaly that caused magmatism in the central-eastern Gawler Craton. Juxtaposition of rocks affected by the Kimban and Kararan orogenic events in the western Gawler Craton was controlled by lithospheric-scale shear zones, some of which have facilitated ∼20 kilometres of exhumation.

Two‐stage Variscan metamorphism in the Canigou massif: Evidence for crustal thickening in the Pyrenees

AbstractThe Variscan metamorphism in the Pyrenees is dominantly of the low‐pressure–high‐temperature (LP‐HT) type. The relics of an earlier, Barrovian‐type metamorphism that could be related to orogenic crustal thickening are unclear and insufficiently constrained. A microstructural and petrological study of micaschists underlying an Ordovician augen orthogneiss in the core of the Canigou massif (Eastern Pyrenees, France) reveals the presence of two syntectonic metamorphic stages characterized by the crystallization of staurolite (M1) and andalusite (M2), respectively. Garnet is stable during the two metamorphic stages with a period of resorption between M1 and M2. The metamorphic assemblages M1 and M2 record similar peak temperatures of 580°C at different pressure conditions of 5.5 and 3 kbar, respectively. Using chemical zoning of garnet and calculated P–T pseudosections, a prograde P–T path is constrained with a peak pressure at ~6.5 kbar and 550°C. This P–T path, syntectonic with respect to the first foliation S1, corresponds to a cold gradient (of ~9°C/km), suggestive of crustal thickening. Resorption of garnet between M1 and M2 can be interpreted either in terms of a simple clockwise P–T path or a polymetamorphic two‐stage evolution. We argue in favour of the latter, where the medium‐pressure (Barrovian) metamorphism is followed by a period of significant erosion and crustal thinning leading to decompression and cooling. Subsequent advection of heat, probably from the mantle, leads to a new increase in temperature, coeval with the development of the main regional fabric S2. LA‐ICP‐MS U–Th–Pb dating of monazite yields a well‐defined date at c. 300 Ma. Petrological evidence indicates that monazite crystallization took place close to the M1 peak pressure conditions. However, the similarity between this age and that of the extensive magmatic event well documented in the eastern Pyrenees suggests that it probably corresponds to the age of monazite recrystallization during the M2 LP‐HT event.

The evolution of quartz veins during the tectonometamorphic development of the Brusque Metamorphic Complex, Brazil

Microstructural analysis and characterization of quartz veins hosted in rocks from the Brusque Metamorphic Complex in the region of Brusque, State of Santa Catarina, have been performed to establish the relations between fluid regimes, tectonic styles and deformation/recrystallization mechanisms. The analysis was based on observations of structural overprinting, spatial structural relations, the origin of the quartz veins and microtectonics studies. Five types of veins were identified: meter-long veins parallel to the regional foliation (V1-veins); massive, meterwide veins present in thermal aureoles (V2-veins); millimeter-wide, erratic veins, also present in thermal aureoles (V3-veins); tabular, undeformed and NW striking (V4-veins); millimeter-wide erratic veins restricted to brittle reactivation of strike-slip shear zones (V5-veins). The regional foliation, developed under garnet zone metamorphic conditions, was more effective in vein production when compared to the steeply dipping mylonitic foliation corridors developed under chlorite zone conditions. Pressure solution was the main deformation mechanism during the regional foliation development. However, granoblastic and decussate textures in hornfels reveal an influence of grain boundary area reduction mechanism. The granoblastic texture in hornfels would have inhibited fluid circulation during dehydration reactions, increasing fluid pressure and promoting massive (V2-veins) and erratic hydraulic fracture-related (V3) veins. In dextral NEtrending strike-slip shear zones, tabular quartz veins (V4-veins) are parallel to tension gashes. Reactivation of strike-slip shear zones under brittle conditions has produced local brecciated millimeter-wide quartz veins (V5-veins). This study underscores the important role of fluids during orogenic evolution near the brittle-plastic transition of the crust, and demonstrates how combined vein and microtextural analysis can reveal the tectonometamorphic history of lowgrade metamorphic rocks.

Compositional zoning in garnet and its modification by diffusion during pressure and temperature changes in metamorphic rocks; an approach and software

We present an approach and software for modelling the Compositional Zoning in Garnet and its Modification by diffusion (CZGM), which combines pseudosection modelling using Perple_X and multicomponent diffusion in garnet during metamorphism. CZGM uses the initial compositional profile, obtained by Perple_X with application of garnet fractionation, to generate a population of garnets of prescribed size-distribution and to apply multicomponent diffusion along a selected P-T path during prograde metamorphism and subsequent decompression and/or cooling. As an example, CZGM was applied on average pelitic rocks subjected to metamorphism at different temperatures (600 to 800 °C) and time duration of 1–10 Ma. CZGM provides an easy-to-handle tool for petrologists to examine hypothetical scenarios of garnet development along prograde and retrograde paths and to constrain the P-T path of the whole metamorphic event. By treatment of laboratory measured garnet-bearing metamorphic rocks, it allows to improve the accuracy of pressure and temperature conditions, usually estimated only from compositional isopleths of garnet.

The partial equilibration of garnet porphyroblasts in pelitic schists and its control on prograde metamorphism, Glen Roy, Scotland

AbstractGarnet porphyroblasts in sillimanite‐bearing pelitic schists contain complex textural and compositional zoning, with considerable variation both within and between adjacent samples. The sillimanite‐bearing schists locally occur in regional Barrovian garnet zone assemblages and are indicative of a persistent lack of equilibrium during prograde metamorphism. Garnet in these Dalradian rocks from the Scottish Highlands preserves evidence of a range of metamorphic responses including initial growth and patchy coupled dissolution–reprecipitation followed by partial dissolution. Individual porphyroblasts each have a unique and variable response to prograde metamorphism and garnet with mainly flat compositional profiles co‐exists with those containing largely unmodified characteristic bell‐shaped Mn profiles. This highlights the need for caution in applying traditional interpretations of effective volume diffusion eliminating compositional variation. Cloudy garnet with abundant fluid inclusions is produced during incomplete modification of the initial porphyroblasts and these porous garnet are then particularly prone to partial replacement in sillimanite‐producing reactions. The modification of garnet via a dissolution–reprecipitation process releases Ca into the effective whole‐rock composition, displacing the pressure–temperature positions of subsequent isograd reactions. This represents the first report of internal metasomatism controlling reaction pathways. The behaviour of garnet highlights the importance of kinetic factors, especially deformation and fluids, in controlling reaction progress and how the resulting variability influences subsequent prograde history. The lack of a consistent metamorphic response, within and between adjacent schists, suggests that on both local and regional scales these rocks have largely not equilibrated at peak metamorphic conditions.

High-pressure, ultrahigh-temperature 1.9 Ga metamorphism of the Kramanituar Complex, Snowbird Tectonic Zone, Rae Craton, Canada

The high-pressure (P) granulite–facies Kramanituar Complex, dominated by a metagabbroic–anorthositic suite with subordinate sillimanite- and kyanite-bearing diatexite and charnockite, is one of several complexes that demarcate the 1000 km-long Snowbird Tectonic Zone. Ti-in-quartz (TiQ) and Zr-in-rutile (ZiR) thermometry on inclusions in garnet in mafic and pelitic rocks establish that temperatures (T) exceeded 990–1000 °C, ~ 100 °C higher than recorded by Fe–Mg exchange thermometers. At 1000 °C, thermobarometry and forward modelling of both mafic and pelitic rocks from the complex define 14–15 kbar pressures, where TiQ and ZiR temperatures are in good agreement. The occurrence in diatexite of sparse prograde F-bearing biotite and high-T rutile inclusions in garnet rims, not cores, can be accounted for with a high-Ti solubility biotite model and the presence of fluorine. Forward modelling suggests that heterogeneities in both the composition and zoning of garnet reflect the effects of melt loss on bulk composition. Gabbroic rocks with coronitic and symplectitic reaction textures record high-T decompression to 8 kbar and 800 °C, also reflected by matrix quartz ribbons in diatexite which yield TiQ values between 655 and 824 °C at 8 kbar. Combined results define a clockwise P–T path for the complex at 1910–1896 Ma. South of the complex, a contemporaneous clockwise P–T path is recorded in semipelitic rocks that reached peak conditions of 690 °C and 8.1 kbar. Overall, results support tectonic reworking of the thickened (> 50 km) Rae margin from ca. 1910 Ma, culminating in ca. 1902 Ma mantle-derived mafic magmatism and rapid exhumation, potentially triggered by slab breakoff or lithospheric delamination.

High-pressure medium-temperature metamorphism of semi-pelitic rocks in the Scotia Metamorphic Complex, Powell Island, South Orkney Islands, Antarctica

The Antarctic continent constituted the southwestern margin of Gondwana until its break-up in the early Cretaceous, when new margins were created along the separating fragments of South America and Antarctica, forming the Scotia Arc. In the Jurassic, part of this passive continental margin became active with subduction of oceanic lithosphere, leading to the introduction of ocean floor material into the accretionary wedge accompanied by deformation and metamorphism. One of these margins is preserved in the South Orkney Microcontinent, and crops out at the South Orkney Islands. At Powell Island, situated in the center of the South Orkney Islands, a gradual transition from very low-grade metarenite, interlayered with metasiltite and slate of the Greywacke Shale Formation, in the south, to biotite-garnet schist, in the north, belonging to the Scotia Metamorphic Complex, is present. The metamorphic map presents from south to north a pumpellyite muscovite chlorite zone, a garnet zone, a biotite-garnet zone and an abundant biotite-garnet zone. Thermobarometric calculations yielded for the garnet and garnet-biotite zones temperatures between 498 and 517 °C with pressures of 9–11 kbar and for the abundant biotite-garnet zone temperatures between 522 and 550 °C and pressures between 11.8 and 13 kbar. These results align well with earlier obtained data for the lower grade rocks and confirm the idea of metamorphism in an accretionary wedge. The relatively high-pressure is interpreted to be responsible for the inversion of the biotite and garnet isograds, for the albitic composition of plagioclase and for the relatively Ca-rich garnet. P-T conditions fall in the transitional field between greenschist, amphibolite, blueschist and eclogite facies.

Metamorphic pressure–temperature conditions of the Lützow–Holm Complex of East Antarctica deduced from Zr–in–rutile geothermometer and Al&lt;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;5&lt;/sub&gt; minerals enclosed in garnet

The metamorphic pressure–temperature (P–T) conditions of high–grade pelitic gneisses from Akarui Point, Skarvsnes, Skallen, and Rundvågshetta in the Lützow–Holm Complex (LHC), East Antarctica are re–examined by applying the Zr–in–rutile geothermometer to rutile inclusions in garnet enclosing Al2SiO5 minerals and to matrix rutile grains. By utilizing the P zoning of garnet to indicate isochronous surface, samples from Akarui Point, Skarvsnes, and Skallen were shown to have experienced almost the same P–T conditions around the kyanite/sillimanite transition boundary (~ 830–850 °C/~ 11 kbar). From Rundvågshetta, higher–T condition (850 ± 15 °C/0.1 kbar to 927 ± 16 °C/12.5 kbar) was confirmed from rutile inclusions in garnet enclosing sillimanite. Matrix rutile yielded similar temperature as inclusion rutile for Akarui Point, Skarvsnes, and Skallen samples. Therefore, the traditional metamorphic zone mapping based on matrix mineral assemblages, which classified Akarui Point as belonging to the transitional zone between the upper–amphibolite and the granulite facies zones, does not reflect the highest metamorphic conditions attained. The P–T–t evolution of the LHC needs to be re–evaluated utilizing detailed petrochronological approaches.