Garnet Growth Zoning Research Articles

In-situ Lu-Hf garnet dating of Archean deep crust granulites from the polymetamorphic Grenville Front Tectonic Zone

Abstract Recent advances in geochronological techniques now allow the ability to efficiently decipher the timing and duration of geological processes in complex high-grade polymetamorphosed orogenic terranes. This is the case of the Grenville Front Tectonic Zone, which truncates the Superior Craton to the southeast. The zone exposes parautochtonous Archean rocks that underwent mid- to high-pressure granulite facies metamorphism of uncertain age. The metamorphic assemblages have been either interpreted as Archean and associated with the final stages of the Superior craton assembly, or as the result of Mesoproterozoic Grenvillian metamorphism, based on cross-cutting relationships and traditional geochronology methods such as U-Pb zircon and 40Ar-39Ar mica dating. Herein, we revisit the extent of the Grenvillian metamorphic overprint in the parautochtonous domain and provide new age constraints for granulite-facies metamorphic assemblages through in-situ garnet dating within migmatitic paragneiss, migmatitic orthogneiss, and mafic granulites, combined with in-situ trace element mapping. Six samples, which show bell-shaped and occasionally sharp and oscillatory lutetium growth zoning in garnet, yield garnet Lu-Hf isochrons with identical Archean dates of c. 2.6 Ga. Sparce analyses of material trend toward Grenvillian ages (c. 1 Ga) in one sample from which garnet shows lutetium zoning consistent with post-growth fluid-assisted disturbance. Overall, our results indicate that the widespread granulite-facies metamorphism within the Grenville Front Tectonic Zone is dominantly late Neoarchean in age, unveiling a rare exposure of Archean lower crust in the southeastern Superior Craton. Our results also point towards a limited Grenvillian metamorphic overprint though the spatial extent and precise thermal conditions of this metamorphism are still unknown. The results presented herein demonstrate the potential of in-situ isotopic geochronology on rock-forming minerals like garnet in polymetamorphic terranes.

H2O budget and metamorphic re-equilibration in polycyclic rocks as recorded by garnet textures and chemistry

Massive dehydration is expected to occur during oceanic subduction. The situation is quite different during continental subduction, where often a large amount of crustal material has experienced a first orogenic cycle before the burial during the second subduction/collision cycle (rocks are therefore polycyclic). The amount and timing of dehydration and/or hydration episodes in polycyclic rocks strongly controls the extent of metamorphic re-equilibration during the second orogenic cycle. This study aims at estimating the fluid budget in polycyclic metapelites from the Muret Unit (Dora-Maira Massif, Western Alps). The excellent preservation, in a kilometre-scale low-strain domain, of pre-Alpine minerals and structures allows the comparison with the textures and structures observed in the pervasively foliated adjacent rocks. In the low-strain domain, the main foliation is pre-Alpine and defined by high-temperature minerals whereas the Alpine high-pressure overprint is static. Pre-Alpine garnet porphyroblasts were fractured and partially dissolved before the growth of the Alpine garnet over pre-Alpine garnet fragments. The preservation of the overall shape of the original pre-Alpine porphyroblasts suggests that pre-Alpine garnet crystals were pseudomorphically replaced by chlorite during the late Variscan retrogression. This process was likely triggered by an episode of fluid-rock interaction and moderate hydration (⁓ 1–2 wt% H2O) which is also responsible of the growth of metamorphic zircon at ~304 Ma. In the high-strain domain, the dominant fabric is Alpine and developed at high-pressure conditions. Despite the difference in the strain intensity, metapelites from both low- and high-strain domains developed the same peak Alpine assemblage garnet-chloritoid-glaucophane-muscovite-rutile in the presence of a free H2O phase at 21–22 kbar and 530–560 °C. Thermodynamic modelling indicates that after the peak pre-Alpine metamorphism, a minimum re-hydration of at least 1 wt% was needed in order to reach H2O-saturation during the Alpine cycle. Alpine garnet mainly sealed fractures and formed thin discontinuous overgrowth on the partially dissolved pre-Alpine garnet. Its growth occurred during the Alpine prograde to peak evolution, involving progressive consumption of chlorite and lawsonite and resulting in a prograde growth zoning. The preservation of the Alpine garnet growth zoning and the absence of complex compositional modifications suggest that the rock did not record massive pulses of fluid infiltration during subduction. Instead, the main fluid-rock interaction episode was limited and occurred before the Alpine re-equilibration during the late Variscan evolution.

Garnet growth and diffusion zoning during the late Paleoproterozoic metamorphism, North Altyn area, SE Tarim Craton, NW China: Implications for a long-lived hot orogen

Metamorphic rocks encode invaluable information on the burial and exhumation history of rocks that is critical to our understanding of orogens. In this work, zircon U−Pb dating, pseudosection, and garnet zoning modeling were performed on samples from the North Altyn area, SE Tarim Craton, NW China. Zircon U−Pb geochronological data indicate that the peak metamorphism occurred at ca. 2.0 Ga. Pseudosection isopleth thermobarometry using garnet growth zoning of the sample from the northern part of the North Altyn area (Kalatashtagh) reveals a decompression and heating path to a peak temperature of 735 °C and 0.74 GPa under the equilibrium crystallization model. The preservation of growth zoning under high temperature requires relatively rapid cooling. In contrast, garnet from the southern part of the North Altyn area (Aktashtagh) preserves diffusion zoning. Metamorphic peak occurs at 670−735 °C and 0.67−0.73 GPa, which suggests a high dT/dP-type metamorphism. Diffusion zoning modeling suggests an extremely slow cooling rate of 1−5 °C/m.y., which indicates prolonged (∼100 m.y.) crustal residence time at high temperature. This requires a long-term heat supply, most likely from radiogenic heat from buried sediments or heat conducted from the asthenosphere. The high dT/dP and slow cooling rate, combined with extensive crustal anatexis from previous studies, indicate that the late Paleoproterozoic North Altyn Orogen is a long-lived hot orogen rather than a subduction−accretionary complex. We propose a tectonic model that involves a ca. 2.1−2.0 Ga arc−back-arc system in the SE Tarim Craton. The back-arc basin was subsequently closed by northward underthrusting due to arc−arc/continental collision at ca. 2.0−1.95 Ga during the integration of the Tarim Craton into the Columbia/Nuna supercontinent.

P–T–t Path of Unusual Garnet–Kyanite–Staurolite– Amphibole Schists, Ellesmere Island, Canada—Quartz Inclusion in Garnet Barometry and Monazite Petrochronology

Abstract Garnet–kyanite–staurolite assemblages with large, late porphyroblasts of amphibole form garbenschists in Ordovician volcaniclastic rocks lying immediately south of the Pearya terrane on northernmost Ellesmere Island, Canada. The schist, which together with carbonate olistoliths makes up the Petersen Bay Assemblage (PBA), displays a series of parallel isograds that mark an increase in metamorphic grade over a distance of 10 km towards the contact with Pearya; however, a steep, brittle Cenozoic strike-slip fault with an unknown amount displacement disturbs the earlier accretionary relationship. The late amphibole growth, probably due to fluid ingress, is clear evidence of disequilibrium conditions in the garbenschist. In order to recover the P–T history of the schists, we construct isochemical phase equilibrium models for a nearby garnet–mica schist that escaped the fluid event and compare the results to quartz inclusion in garnet (QuiG) barometry for a garbenschist and the metapelitic garnet schist. Quartz inclusions are confined to garnet cores and the QuiG results, combined with Ti-in-biotite and garnet–biotite thermometry, delineate a prograde path from 480 to 600°C and 0.7 to 0.9 GPa. This path agrees with growth zoning in garnet deduced from X-ray maps of the spessartine component in garnet. The peak conditions obtained from pseudosection modelling using effective bulk composition and the intersection of garnet rim with matrix biotite and white mica isopleths in the metapelite are 665°C at ≤0.85 GPa. Three generations of monazite (I, II and III) were identified by textural characterization, geochemical composition (REE and Y concentrations) and U–Pb ages measured by ion microprobe. Monazite I occurs in the matrix and as inclusions in garnet rims and grew at peak P–T conditions at 397 ± 2 Ma (2σ) from the breakdown of allanite. Monazite II forms overgrowths on matrix Monazite I grains that are oriented parallel to the main schistosity and yield ages of 385 ± 2 Ma. Monazite III, found only in the garbenschist, is 374 ± 6 Ma, which is interpreted as the time of amphibole growth during fluid infiltration at lower temperature and pressure on a clockwise P–T path that remained in the kyanite stability field. These results point to a relatively short (≈12 Myr) Barrovian metamorphic event that affected the schists of the PBA. An obvious heat source is lacking in the adjacent Pearya terrane, but we speculate it was large Devonian plutons—similar to the 390 ± 10 Ma Cape Woods granite located 40 km across strike from the fault—that have been excised by strike-slip. Arc fragments that are correlative to the PBA are low grade; they never saw the heat and were not directly involved in Pearya accretion.

Garnet perspectives on the metamorphic history and tectonic significance of Paleoproterozoic high-pressure mafic granulites from the northern Hengshan, North China Craton

Reconstructing the pressure-temperature (P-T) evolution of early Precambrian high-pressure granulites and eclogites is critical for decoding the tectono-metamorphic evolution of the early Earth. However, debates continue regarding the P-T history of these rocks because of various modifications by peak and retrograde rehydration metamorphism. As a significant rock-forming mineral in high-grade metamorphic rocks, garnet not only grows with increasing pressure or temperature and develops variably zoned patterns along given P-T loops, but its properties are stable subsquent to its formation when compared to other minerals in the matrix. Consequently, garnet is excellent for monitoring the thermo-metamorphic history of its host rock during orogeny. Selected garnet grains from the Paleoproterozoic high-pressure mafic granulites in the northern Hengshan area of the North China Craton, which exhibit variable microstructural and chemical zoning, present an excellent example for extracting metamorphic evolution of the host rocks. Microstructural zoning displays a zonal distribution of frozen inclusion-type minerals in garnet. Chemically, the garnet shows pronounced compositional zoning, with bell-shaped XSps, initially flat and then increasing/decreasing XPyr and Fe#, initially increasing and then decreasing XGrs from core to mantle, and resorbed rims. Both zoning patterns document multiple generations of radial growth history. The progressive evolution of the host rocks can be further divided into early prograde (M1–1), late prograde (M1–2), near peak (M2–1), and peak (M2–2) stages. The resorption textures indicate the succeeding decompression (M3) and cooling (M4) stages.The pseudosection modeling indicates P-T conditions from M1–1 to M2–1 of 6–9 kbar/600–700 °C, 9–11.2 kbar/680–740 °C and 12–15 kbar/735–785 °C, respectively. Although the peak metamorphic information of M2–2 is lost because of the breakdown of the garnet rim, plausible P–T results are estimated at ~15–17 kbar/~790–810 °C. These conditions prevailed to the transitional eclogite-granulite facies through qualitative reconstruction of the original composition of clinopyroxene, modal proportion, and rim composition of the garnet. The subsequent conditions of decompression (M3) and cooling (M4) stages were roughly estimated at ~7–10 kbar/~800–850 °C and 3–5 kbar/ 650–680 °C. Together, they define a clockwise P-T path in response to the Paleoproterozoic orogeny involved in the assembly of the North China Craton. The zoned garnet suggests that the host mafic granulite documents a relatively complete progressive metamorphic history, which probably correlates with subduction processes. The resorbed rim sequentially records exhumation and cooling processes. Furthermore, the preservation of growth zoning in granulitic garnet and near-isothermal decompression P-T path suggests that the host rocks must have experienced a very short peak metamorphism and a rapid exhumation process to the middle crust. These findings suggest that the tectonic processes involved in the formation of the Paleoproterozoic northern Hengshan granulite terrane might be similar to those of the Phanerozoic continental collisional orogens under modern plate tectonic regimes; however, they record relatively higher apparent thermal gradients (~14–16 °C /km).

Prograde and retrograde P–T evolution of a Variscan high-temperature eclogite, French Massif Central, Haut-Allier

The P–T evolution of a mafic eclogite sample from the Haut-Allier was studied in order to constrain the dynamic of the Variscan subduction in the eastern French Massif Central. Three successive metamorphic stages M1, M2 and M3, are characterized by assemblages comprising garnet1-omphacite-kyanite, garnet2-plagioclase, and amphibole-plagioclase, respectively, and define a clockwise P–T path. These events occurred at the conditions of eclogite (M1; ∼ 20 kbar, 650 °C to ∼ 22.5 kbar, 850 °C), high-pressure granulite (M2; 19.5 kbar and 875 °C) and high-temperature amphibolite facies (M3; < 9 kbar, 750–850 °C), respectively. Pseudosection modelling of garnet growth zoning and mineralogy of the inclusions reveal a prograde M1 stage, first dominated by burial and then by near isobaric heating. Subsequent garnet1 resorption, prior to a renewed growth of garnet2 is interpreted in terms of a decompression during M2. High-pressure partial melting is predicted for both the M1 temperature peak and M2. M3 testifies to further strong decompression associated with limited cooling. The preservation of garnet growth zoning indicates the short-lived character of the temperature increase, decompression and cooling cycle. We argue that such P–T evolution is compatible with the juxtaposition of the asthenosphere against the subducted crust prior to exhumation driven by slab rollback.

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.

Population-wide garnet growth zoning revealed by LA-ICP-MS mapping: implications for trace element equilibration and syn-kinematic deformation during crystallisation

The strong partitioning of many trace elements into garnet and their slow diffusivities in both garnet and the rock matrix means that their distribution may record valuable petrogenetic information not documented by major elements in metamorphic rocks. Complex trace element growth zoning in garnet porphyroblasts from a garnet-grade metapelite from the Barrovian sequence of the Sikkim Himalaya is assessed using quantified LA-ICP-MS raster mapping coupled with X-ray micro-computed tomography. The data document systematic changes in the zoning patterns from early- to late-nucleated crystals, and also suggest that the REE+Y chemistry incorporated into garnet is dependent on persistent disequilibrium in the rock volume. There is evidence for HREE+Y diffusion haloes surrounding growing garnets and a heterogeneous HREE+Y distribution in the rock matrix. Annuli superimposed on oscillatory zoning are not consistent with formation during some rock-wide event, but are dependent on the spatial disposition of the garnet. Annuli are interpreted to reflect an integrated history of varying growth rates and the incorporation of pre-existing heterogeneities due to relatively slow matrix diffusivities. Conversely, smooth zoning of many transition metals indicate that their distribution in garnet may be controlled by equilibrium partitioning between garnet and the rock matrix. Significant rotation of garnet porphyroblasts during growth is revealed due to immobility of Cr over the duration of the crystallisation interval and overprinting of the heterogenous precursor Cr distribution. Strain rate estimates derived from this zoning are on the order of $$10^{-11}$$ – $$10^{-12}\, \hbox {s}^{-1}$$ .

Characterisation of a garnet population from the Sikkim Himalaya: insights into the rates and mechanisms of porphyroblast crystallisation

The compositional zoning of a garnet population contained within a garnet-grade metapelitic schist from the Lesser Himalayan Sequence of Sikkim (India) provides insight into the rates and kinetic controls of metamorphism, and the extent of chemical equilibration during porphyroblast crystallisation in the sample. Compositional profiles across centrally sectioned garnet crystals representative of the observed crystal size distribution indicate a strong correlation between garnet crystal size and core composition with respect to major end-member components. Systematic steepening of compositional gradients observed from large to small grains is interpreted to reflect a progressive decrease in the growth rate of relatively late-nucleated garnet as a result of an increase in interfacial energies during progressive crystallisation. Numerical simulation of garnet nucleation and growth using an equilibrium approach accounting for chemical fractionation associated with garnet crystallisation reproduces both the observed crystal size distribution and the chemical zoning of the entire garnet population. Simulation of multicomponent intracrystalline diffusion within the population indicates rapid heating along the pressure–temperature path, in excess of 100 $$^{\circ }$$ C Myr $$^{-1}$$ . Radial garnet growth is correspondingly rapid, with minimum rates of 1.4 mm Myr $$^{-1}$$ . As a consequence of such rapid crystallisation, the sample analysed in this study provides a close to primary record of the integrated history of garnet nucleation and growth. Our model suggests that nucleation of garnet occurred continuously between incipient garnet crystallisation at $$\sim$$ 520 $$^{\circ }$$ C, 4.5 kbar and peak metamorphic conditions at $$\sim$$ 565 $$^{\circ }$$ C, 5.6 kbar. The good fit between the observed and predicted garnet growth zoning suggests that the departure from equilibrium associated with garnet nucleation and growth was negligible, despite the particularly fast rates of metamorphic heating. Consequently, rates of major element diffusion in the intergranular medium during garnet crystallisation are interpreted to have been correspondingly rapid. It is, therefore, possible to simulate the prograde metamorphic history of our sample as a succession of equilibrium states of a chemical system modified by chemical fractionation associated with garnet crystallisation.

Pseudosection modelling and garnet Lu–Hf geochronology of HP amphibole schists constrain the closure of an ocean basin between the northern and southern Lhasa blocks, central Tibet

AbstractThe P–T–t path of high‐P metamorphic rocks in subduction zones may reveal valuable information regarding the tectonic processes along convergent plate boundaries. Herein, we present a detailed petrological, pseudosection modelling and radiometric dating study of several amphibole schists of oceanic affinity from the Lhasa Block, Tibet. The amphibole schists experienced an overall clockwise P–T path that was marked by post‐Pmax heating–decompression and subsequent isothermal decompression following the attainment of peak high‐P and low‐T conditions (~490°C and 1.6 GPa). Pseudosection modelling shows that the amphibole schists underwent water‐unsaturated conditions during prograde metamorphism, and the stability field of the assemblage extends to lower temperatures and higher pressures within the water‐unsaturated condition relative to water‐saturated model along the prograde path. The high‐P amphibole schists were highly reduced during retrograde metamorphism. Precise evaluation of the ferric iron conditions determined from the different compositions of epidote inclusions in garnet and matrix epidote is crucial for a true P–T estimate by garnet isopleth thermobarometry. Lu–Hf isotope analyses on garnet size separates from a garnet‐bearing amphibole schist yield four two‐point garnet–whole‐rock isochron ages from 228.2 ± 1.2 Ma to 224.3 ± 1.2 Ma. These Lu–Hf dates are interpreted to constrain the period of garnet growth and approximate the timing of prograde metamorphism because of the low peak metamorphic temperature of the rock and the well‐preserved Mn/Lu growth zoning in garnet. The majority of zircon U–Pb dates provide no constraints on the timing of metamorphism; however, two concordant U–Pb dates of 222.4 ± 3.9 Ma and 223.3 ± 4.2 Ma were obtained from narrow and uncommon metamorphic rims. Coexistence of zircon and sphene in the samples implies that the metamorphic zircon growth was likely assisted by retrogression of rutile to sphene during exhumation. The near coincident radiometric dates of zircon U–Pb and garnet Lu–Hf indicate rapid burial and exhumation of the amphibole schists, suggesting a closure time of c. 224–223 Ma for the fossil ocean basin between the northern and southern Lhasa blocks.

P–T evolution of Precambrian eclogite in the Sveconorwegian orogen, SW Sweden

AbstractConditions of the prograde, peak‐pressure and part of the decompressional P–T path of two Precambrian eclogites in the eastern Sveconorwegian orogen have been determined using the pseudosection approach. Cores of garnet from a Fe–Ti‐rich eclogite record a first prograde and syn‐deformational stage along a Barrovian gradient from ~670 °C and 7 kbar to 710 °C and 8.5 kbar. Garnet rims grew during further burial to 16.5–19 kbar at ~850–900 °C, along a steep dP/dT gradient. The pseudosection model of a kyanite‐bearing eclogite sample of more magnesian bulk composition confirms the peak conditions. Matrix reequilibration associated with subsequent near‐isothermal decompression and partial exhumation produced plagioclase‐bearing symplectites replacing kyanite and clinopyroxene at an estimated 850–870 °C and 10–11 kbar. The validity of the pseudosections is discussed in detail. It is shown that in pseudosection modelling the fractionation of FeO in accessory sulphides may cause a significant shift of field boundaries (here displaced by up to 1.5 kbar and 70 °C) and must not be neglected. Fast burial, exhumation and subsequent cooling are supported by the steepness of both the prograde and the decompressional P–T paths as well as the preservation of garnet growth zoning and the symplectitic reaction textures. These features are compatible with deep tectonic burial of the eclogite‐bearing continental crust as part of the underthrusting plate (Eastern Segment, continent Baltica) in a collisional setting that led to an effectively doubled crustal thickness and subsequent exhumation of the eclogites through tectonic extrusion. Our results are in accordance with regional structural and petrological relationships, which demonstrate foreland‐vergent partial exhumation of the eclogite‐bearing nappe along a basal thrust zone and support a major collisional stage at c. 1 Ga. We argue that the similarities between Sveconorwegian and Himalayan eclogite occurrences emphasize the modern style of Grenvillian‐aged tectonics.

Dynamics of Saxothuringian subduction channel/wedge constrained by phase‐equilibria modelling and micro‐fabric analysis

AbstractSubduction and exhumation dynamics can be investigated through analysis of metamorphic and deformational evolution of associated high‐grade rocks. The Erzgebirge anticline, which forms at the boundary between the Saxothuringian and Teplá‐Barrandian domains of the Bohemian Massif, provides a useful study area for these processes owing to the occurrence of numerous meta‐basites preserving eclogite facies assemblages, and coesite and diamond bearing quartzofeldspathic lithologies indicating subduction to deep mantle depths. The prograde and retrograde evolution of meta‐basite from the Czech portion of the Erzgebirge anticline has been constrained through a combination of thermodynamic modelling and conventional thermobarometry. Garnet growth zoning indicates that the rocks underwent burial and heating to peak conditions of 2.6 GPa and at least 615 °C. Initial exhumation occurred with concurrent cooling and decompression resulting in the growth of amphibole and zoisite poikiloblasts overgrowing and including the eclogite facies assemblage. The development of clinopyroxene–plagioclase–amphibole symplectites after omphacite and Al‐rich rims on matrix amphibole indicate later heating at the base of the lower crust. Omphacite microstructures, in particular grain size analysis and lattice‐preferred orientations, indicate that the prograde evolution was characterized by a constrictional strain geometry transitioning into plane strain and oblate fabrics during exhumation. The initial constrictional strain pattern is interpreted as being controlled by competing slab pull and crustal buoyancy forces leading to necking of the subducting slab. The transition to plane strain and flattening geometries represents transfer of material from the subducting lithosphere into a subduction channel, break‐off of the dense slab and rebound of the buoyant crustal material.

Metamorphic evolution of LT-UHP eclogite from the south Dabie orogen, central China: An insight from phase equilibria modeling

Low-temperature ultrahigh pressure (LT-UHP) eclogites from the south Dabie orogen are distributed in the Zhujiachong and Anqiling zones and their peak mineral assemblages and P–T paths are still disputable. The rocks consist commonly of garnet, omphacite, epidote, phengite, amphibole, quartz/coesite and rutile with or without glaucophane, kyanite, talc and paragonite. Garnet porphyroblasts in the representative sample (D901) from the Zhujiachong zone exhibit core–mantle zoning with slightly increasing pyrope (Xpy) and decreasing grossular (Xgr) contents, and mantle–rim zoning with rapidly increasing Xpy and decreasing Xgr. Garnet shows diverse rim compositions which can be classified into group-1 that show high Xpy and low Xgr values, being an indicative character of LT-UHP eclogite, and group-2 that show lower Xpy and higher Xgr than in group-1. Phengite shows higher Si contents in the Anqiling zone than in the Zhujiachong zone. Epidote, paragonite and amphibole usually occur as porphyroblasts that contain inclusions of garnet, omphacite and rutile. Pseudosections calculated using THERMOCALC for the representative samples suggest that the mineral assemblage in the pressure peak stage would be garnet+omphacite+lawsonite+talc+phengite+coesite in the model system. In this assemblage, Xpy steadily increases as temperature rises and Si in phengite increases with pressure rising, whereas Xgr is very insensitive as pressure changes. The peak P–T conditions for low-T UHP eclogites can be determined using the isopleths of maximum Xpy and Si in phengite in P–T pseudosections. Using this approach, the peak P–T conditions of eclogite were estimated to be ∼30kbar/615°C for the representative sample in the Zhujiachong zone, and ∼32kbar/623°C or ∼36kbar/640°C for samples in the Anqiling zone. The pre-peak prograde metamorphic evolution during subduction was modeled according to the garnet growth zoning in sample D901, where the core–mantle zoning suggests a flat P–T vector dominated by heating, representing a slower subduction process, and the mantle–rim zoning indicates a steep P–T vector dominated by increasing pressure, pointing to a faster subduction process. The post-peak exhumation of the LT-UHP eclogite involves two stages: the early-stage exhumation is dominated by lawsonite dehydrations, forming the porphyroblasts of epidote, paragonite and glaucophane under fluid-present conditions, and the late-stage exhumation occurs under fluid-absent conditions, forming hornblendic amphibole and plagioclase triggered by fluid-infiltration. As a consequence of the metamorphic evolution especially during the early-stage exhumation, the observed mineral assemblages in LT-UHP eclogites cannot well match those at their peak stages.

Prograde and retrograde evolution of eclogite from Adrar Izzilatène (Egéré-Aleksod terrane, Hoggar, Algeria) determined from chemical zoning and pseudosections, with geodynamic implications

Adrar Izzilatène in the Egéré-Aleksod terrane of the LATEA metacraton (Hoggar, Algeria) exposes one of the best preserved examples of eclogite facies metamorphism in Hoggar. Three distinct stages of metamorphic development are recognised, namely, the pre-peak stage (M1), characterised by garnet, amphibole, epidote, quartz and rutile, the peak eclogite facies stage (M2), consisting of omphacite, garnet, edenite, epidote, quartz and rutile, and the retrograde stage (M3), where initial decompression resulted in the appearance of plagioclase, the development of pargasite+plagioclase kelyphites and finally the formation of anhydrous plagioclase+diopside coronas. Porphyroblastic omphacite has a jadeite content of up to XJd=0.36, which is the highest yet observed for eclogite facies rocks from the Tuareg Shield. Garnet growth zoning patterns are characterised by flat profiles in the cores (XAlm=0.55–0.60; XPrp=0.12–0.16; XGrs=0.26–0.30) before showing a decrease in almandine to XAlm=0.45, coupled to an increase in pyrope to XPrp=0.29 and decrease in grossular to XGrs=0.26 at the rims. Calculated P–T–MH2O pseudosections show that the prograde M1 assemblage equilibrated at 13–14kbar and 580°C, before pressure and temperature increased to 19kbar and 650–700°C at fluid-saturated conditions during peak metamorphism. Retrogression involved near-isothermal decompression to 8–9kbar and 700–750°C at fluid-undersaturated conditions. Prograde-to-peak metamorphism of the Izzilatène eclogite could have involved either oceanic or continental subduction, followed by exhumation as the area was obducted towards the LATEA metacraton during the Pan-African orogeny and the assembly of Western Gondwana.

Metamorphic evolution and Zircon ages of Garnet–orthoamphibole rocks in southern Hengshan, North China Craton: Insights into the regional Paleoproterozoic P–T–t history

Garnet–orthoamphibole rock (GOR) from southern Hengshan of the North China Craton (NCC) shows a prominent porphyroblastic texture involving garnet, orthoamphibole, kyanite, sillimanite, chlorite, rutile and quartz, with or without talc, hornblende and staurolite. Garnet develops coronae or symplectite assemblages of cordierite, anthophyllite, ilmenite and quartz. The P–T and P–H2O evolution for three representative samples were determined by using pseudosections calculated with THERMOCALC. Generalized prograde paths are inferred from subtle garnet growth zoning, chlorite compositions and inclusion assemblages in garnet. The peak mineral assemblages are predicted to be characteristic of presence of garnet, gedrite, chlorite, kyanite and rutile under P–T conditions estimated to be 9–12kbar and 650–670°C. The post-peak history is characterized by isothermal decompression, developing coronae and symplectite assemblages under a mix of fluid-present and fluid-absent conditions. An idealized P–XMg [=Mg/(Mg+Fe) in mole] pseudosection drawn at T=655°C and a P–M(H2O) pseudosection at T=630°C suggest that the garnet-free cordierite–orthoamphibole rock (COR) can form from decompression of GOR assemblages when the rock is high-in XMg value or the decompression temperature is low. Zircon U–Pb dating yields a metamorphic age of 1964±25Ma (∼1.95Ga) and a protolith age of 2533±14Ma, where the former was interpreted to represent the prograde- or peak stages of metamorphism. This paper validates an argument that the amalgamation of the NCC via a crust thickening event occurred at ∼1.95Ga, followed by an isothermal decompression and a prolonged nearly isobaric cooling process from 1.90 to 1.78Ga.

Metamorphism as Garnet Sees It: The Kinetics of Nucleation and Growth, Equilibration, and Diffusional Relaxation

Research Article| December 01, 2013 Metamorphism as Garnet Sees It: The Kinetics of Nucleation and Growth, Equilibration, and Diffusional Relaxation Jay J. Ague; Jay J. Ague 1Department of Geology and Geophysics, Yale UniversityP.O. Box 208109, New Haven, Connecticut 06520-8109, USAE-mail: jay.ague@yale.edu2Peabody Museum of Natural History, Yale UniversityNew Haven, Connecticut 06511, USA Search for other works by this author on: GSW Google Scholar William D. Carlson William D. Carlson 3Department of Geological Sciences, University of Texas at AustinAustin, Texas 78712, USAE-mail: wcarlson@jsg.utexas.edu Search for other works by this author on: GSW Google Scholar Elements (2013) 9 (6): 439–445. https://doi.org/10.2113/gselements.9.6.439 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Jay J. Ague, William D. Carlson; Metamorphism as Garnet Sees It: The Kinetics of Nucleation and Growth, Equilibration, and Diffusional Relaxation. Elements 2013;; 9 (6): 439–445. doi: https://doi.org/10.2113/gselements.9.6.439 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyElements Search Advanced Search Abstract Garnet bears witness to the importance of kinetics during metamorphism in its microstructural features, compositional zoning, and diffusional response to thermal events. Porphyroblastic textures carry quantitative signals of protracted nucleation and sluggish intergranular diffusion, key impediments to reaction progress that may result in crystallization under conditions well removed from equilibrium. Growth zoning in garnet reveals partial chemical equilibration with matrix minerals: intergranular transport keeps pace with garnet growth for some elements but not for others, leading to variable degrees and length scales of chemical equilibration. Partial relaxation of compositional zoning by intracrystalline diffusion is a sensitive and quantitative indicator of thermal history, constraining rates and timescales of peak metamorphic heating, processes of burial and exhumation, and retrogression on cooling. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Modeling of zoning patterns in garnet: Thermodynamic and kinetic aspects

Results from the modeling of compositional zoning patterns in garnet porphyroblasts from the medium-grade metapelitic schist of northern Ladoga area are considered. The P-T pseudosections in the model KMnFMASH system were calculated for this purpose using THERMOCALC software (Powell et al., 1998). Particular emphasis is placed upon the effect of garnet growth kinetics on the model zoning profiles for Mn (Gulbin, 2013). They fit the observed profiles if intergranular diffusion-controlled growth is assumed for porphyroblasts. Additionally, a model of metamorphic fractional crystallization is used to characterize the oscillations in both the garnet core and rim. Starting from the assumption that a reservoir, where garnet grows, consists of chlorite, and that this mineral is intensely replaced with biotite and staurolite at the onset of crystallization, a partial release of Mn from the chlorite structure and the concentration of this component in intergranular space is inferred. In terms of the model under consideration, the coefficient of the Mn partition between garnet and reservoir temporarily increases at the early stage of garnet growth, giving rise to the enrichment of the intermediate zone of porphyroblasts in Mn. In addition to the modeling of garnet growth zoning, its subsequent diffusion modification is estimated on the basis of intracrystalline diffusion profile simulation. The reverse zoned, Mn-rich and Mg-poor garnet rims are related to retrograde growth of garnet at the late stage of porphyroblast formation. The data obtained are used to constrain metamorphic evolution and the P-T-t path of staurolite-bearing rocks in the northern domain of the studied area.

High-pressure garnet amphibolite from the Funaokayama unit, western Kii Peninsula and the extent of eclogite facies metamorphism in the Sanbagawa belt

Quantitative analyses of garnet-bearing epidote amphibolite (hereafter garnet amphibolite) from the Funaokayama unit, western Kii Peninsula of the Sanbagawa belt reveal a previously unrecognized high-grade part of the Sanbagawa metamorphism. Petrographic observations suggest that the garnet amphibolite underwent three stages of metamorphism (M1, M2 and M3). Records of M1 are preserved as syn-tectonic prograde-zoned garnet and its inclusions. Pseudosection modeling reproduces the observed M1 assemblage garnet + amphibolite + epidote + phengite + quartz and the growth zoning of garnet records the pressure (P)-temperature (T) evolution from 0.8 GPa, 570 °C to 1.3 GPa, 590 °C. Therefore, this garnet growth and associated deformation (D1) took place during subduction up to eclogite facies conditions. The garnet rim compositions and inclusions of epidote, titanite, rutile and quartz (5titanite + 2clinozoisite = 5rutile + 3grossular + 2quartz + H2O) give consistent peak-P estimates of 1.3-1.9 GPa. The garnet contains quartz inclusions retaining residual pressures of up to ∼ 0.7 GPa, which is as high as the values reported in well-characterized eclogite samples in central Shikoku. The inferred peak-P conditions in the eclogite facies is further supported by the occurrence of aragonite in associated pelitic schist. The assemblage hornblende + epidote + titanite + quartz + albite in the matrix and its equilibrium conditions of ∼ 0.8 GPa, 580 °C suggest decompression to the epidote-amphibolite facies (M2). The M2 minerals locally show replacement by the mineral assemblage actinolite + epidote + chlorite + titanite + calcite + quartz + albite, suggesting a partial re-equilibration in the greenschist facies (M3). M2 and M3 are synchronous with the main phase of ductile deformation during exhumation (D2). Despite the absence of the omphacite + quartz assemblage, it is likely that eclogite facies metamorphism in the Sanbagawa belt can be extended to western Kii Peninsula.

The structural, metamorphic and temporal evolution of the country rocks surrounding Venetia Mine, Limpopo Belt, South Africa: Evidence for a single palaeoproterozoic tectono-metamorphic event with implications for a tectonic model

This paper presents the results of a detailed and fully integrated pressure–temperature–time-deformation (P–T–t-D) study of the country rocks around Venetia Mine in the Central Zone of the Limpopo Belt, South Africa. Detailed structural mapping around Venetia Mine delineates four deformation events (D1–D4). Relict S1 comprises quartzofeldspathic bands in biotite gneiss and amphibolite. S2, defined by biotite in biotite schist, gneissic banding in biotite gneiss, long axes of quartzofeldspathic augen, and the long axes of amphibolite lenses/boudins, is axial planar to F2. F3 occurs predominantly at the contacts between biotite gneiss and biotite schist, forming open to closed, upright to inclined, E-W- to ENE-trending, shallowly plunging folds. D4, which was constrictional-prolate in nature, refolded S1, S2, F2 and F3. F4 folds and an L4 mineral lineation, defined predominantly by sillimanite in metapelitic schist, plunge moderately NE to NNE and overprint all previous fabrics. Poles to refolded S2 foliations show a characteristic great circle distribution. In turn, the pole to this great circle coincides with the orientation of L4. Such patterns are also found in the Avoca, Bellevue and Ha-Tshansi sheath folds in the Central Zone, albeit that the plunge of the lineations at Venetia are predominantly NE-wards, rather than SW-wards. The moderately NE- to NNE-plunging F4 sheath folds and L4 lineations accord with the NE-SW trend of sheath folds and associated mineral elongation lineations observed elsewhere in the Central Zone. However, PbSL dating of syntectonic garnets constrain the minimum and maximum age of D2 and D4 structures respectively to c. 2037–2040 +/− 22Ma. The metamorphism developed in metapelitic lithologies is characterized by peak amphibolite facies conditions of ∼6.5kbar and 680°C. Pseudosection modelling of growth zoning in garnet provides evidence of a prograde pressure and temperature increase from ∼5kbar at 600°C to ∼6–6.5kbar at 650–680°C, which is interpreted to be a consequence of tectonic thickening. Collectively, the integrated P–T–t-D data unequivocally demonstrates that the country rocks around Venetia experienced a structural–metamorphic event in the palaeoproterozoic. Coupled with published data we suggest a simple two-fold tectonic model, which involves the Central Zone as separate terrane docking with the Kaapvaal during the Neoarchean and later, during the Palaeoproterozoic this Central Zone-Kaapvaal amalgam collides with the Zimbabwe Craton.

Preservation of Garnet Growth Zoning and the Duration of Prograde Metamorphism

Chemically zoned garnet growth and coeval modification of this zoning through diffusion are calculated during prograde metamorphic heating to temperatures of up to 850°C. This permits quantification of how the preservation or elimination of zoning profiles in garnet crystals of a given size is sensitive to the specific burial and heating (P–T) path followed, and the integrated duration spent at high temperature (dT/dt). Slow major element diffusion in garnet at T < 600°C means that centimetre-scale zoning profiles may remain for >30 Myr at amphibolite-grade conditions, but small-scale (tens of micrometres) zoning features will be lost early in the prograde stage unless this is ‘rapid’ (⁠5 Myr for rocks reaching c. 600°C). Calculations indicate that preservation of unmodified growth compositions in even relatively large (up to 3 mm diameter) pelitic garnet crystals requires prograde and exhumational events to be <10 Myr for rocks reaching c. 600°C. This timescale can be 5 Myr for garnet in rocks reaching 650°C or hotter. It is likely, therefore, that most natural prograde-zoned crystals record compositions already partially re-equilibrated between the time of crystal growth and of reaching maximum temperature. However, a large T–t window exists within which crystals begin to lose their growth compositions but retain evidence of crystal-scale zoning trends that may still be useful for thermobarometry purposes. The upper limit of this window for 500 μm diameter crystals can be as much as several tens of millions of years of heating to c. 700°C. Absolute re-equilibration timescales can be significantly different for garnet growing in different rock compositions, with examples of a granodiorite and a pelite given.