Garnet Rims Research Articles

Petrochronological close-up on the thermal structure of a paleo-subduction zone (W. Alps)

We present the results from a study combining zoned geochronology, mineral trace element geochemistry and phase equilibria modeling of a mafic lithology from the Zermatt Saas Ophiolite in order to elucidate the tectonic rates and thermal structure of the Western Alps paleo-subduction interface. The Zermatt-Saas Ophiolite represents dismembered slices derived from an eclogitized, 60-km wide coherent fragment of Tethyan oceanic lithosphere. Two cm-sized garnet crystals from a pyrite-rich chlorite-talcschist (representative of sub-seafloor hydrothermally-altered basalts) from the Servette mine locality (St. Marcel Valley, Italy) were microdrilled to separate core and rim growth generations and dated using Sm-Nd geochronology to determine the overall duration of garnet growth. Garnet cores were dated to 46.9±1.6Ma, signifying the timing of the initiation of garnet growth. Garnet rims were dated to 43.5±1.3Ma, signifying the timing of burial to peak depths (constrained to ∼75 km). Major and trace element zoning in garnet suggest two distinct generations of garnet growth. Garnet crystal cores display evidence for rapid nucleation and growth, while garnet crystal rims suggest growth at relatively slow (tectonic) rates. The duration of garnet growth (3.4±2.1Ma), when coupled to constraints from phase equilibria modeling and Zr-in-rutile thermometry, provides estimates on the burial and heating rate of 4.7 (+7.9−2.1) km/Myr and 13.2 (+17.8−6.1)°C/Myr, respectively. Using the depth and temperature conditions recorded by laterally equivalent meta-ophiolites from across the Western Alps, a ‘snapshot’ of the geothermal gradient at the Alpine subduction interface is presented. This reveals a pressure-temperature trajectory highlighted by a significant decrease of the thermal gradient from ∼10°C/km down to ∼3°C/km at ∼45 Ma. We further present geometric calculations to identify the tectonic parameters that best fit this dual-segmented thermal gradient. Using convergence rates from paleogeographic reconstructions during Eocene times (on the order of 1.5-2 cm/yr), our calculations require a paleo-slab angle of 15-20°, in line with dip estimates from most modern subduction zones. This study highlights how a detailed petrochronology-based approach enables assessing the thermal structure and geometry of a paleo-subduction zone, with implications on seismicity, stress distribution, and devolatilization potential of deeper portions of the subduction interface.

Evidence for ultrahigh-pressure metamorphism discovered in the Appalachian orogen

AbstractThe Appalachian orogen has long been enigmatic because, compared to other parts of the Paleozoic orogens that formed following the subduction of the Iapetus Ocean, direct evidence for ultrahigh-pressure (UHP) metamorphism has never been found. We report the first discovery of coesite in the Appalachian orogen in a metapelite from the mid-Ordovician (Taconic orogeny) Tillotson Peak Complex in Vermont (USA). Relict coesite occurs within a bimineralic SiO2 inclusion in garnet. In situ elastic barometry and trace-element thermometry allow reconstruction of the garnet growth history during prograde metamorphism. The data are interpreted to indicate garnet nucleation and crystallization during blueschist- to eclogite-facies subduction zone metamorphism, followed by garnet rim growth at UHP conditions of > 28 kbar and > 530 ° C. Results provide the first direct evidence that rocks of the Appalachian orogen underwent UHP metamorphism to depths of > 75 km and warrant future studies that constrain the extent of UHP metamorphism.

Multiple P–T–d–t paths reveal the evolution of the final Nuna assembly in northeast Australia

AbstractThe final assembly of the Mesoproterozoic supercontinent Nuna was marked by the collision of Laurentia and Australia at 1.60 Ga, which is recorded in the Georgetown Inlier of NE Australia. Here, we decipher the metamorphic evolution of this final Nuna collisional event using petrostructural analysis, major and trace element compositions of key minerals, thermodynamic modelling, and multi‐method geochronology. The Georgetown Inlier is characterised by deformed and metamorphosed 1.70–1.62 Ga sedimentary and mafic rocks, which were intruded by c. 1.56 Ga old S‐type granites. Garnet Lu–Hf and monazite U–Pb isotopic analyses distinguish two major metamorphic events (M1 at c. 1.60 Ga and M2 at c. 1.55 Ga), which allows at least two composite fabrics to be identified at the regional scale—c. 1.60 Ga S1 (consisting in fabrics S1a and S1b) and c. 1.55 Ga S2 (including fabrics S2a and S2b). Also, three tectono‐metamorphic domains are distinguished: (a) the western domain, with S1 defined by low‐P (LP) greenschist facies assemblages; (b) the central domain, where S1 fabric is preserved as medium‐P (MP) amphibolite facies relicts, and locally as inclusion trails in garnet wrapped by the regionally dominant low‐P amphibolite facies S2 fabric; and (c) the eastern domain dominated by upper amphibolite to granulite facies S2 foliation. In the central domain, 1.60 Ga MP–medium‐T (MT) metamorphism (M1) developed within the staurolite–garnet stability field, with conditions ranging from 530–550°C at 6–7 kbar (garnet cores) to 620–650°C at 8–9 kbar (garnet rims), and it is associated with S1 fabric. The onset of 1.55 Ga LP–high‐T (HT) metamorphism (M2) is marked by replacement of staurolite by andalusite (M2a/D2a), which was subsequently pseudomorphed by sillimanite (M2b/D2b) where granite and migmatite are abundant. P–T conditions ranged from 600 to 680°C and 4–6 kbar for the M2b sillimanite stage. 1.60 Ga garnet relicts within the S2 foliation highlight the progressive obliteration of the S1 fabric by regional S2 in the central zone during peak M2 metamorphism. In the eastern migmatitic complex, partial melting of paragneiss and amphibolite occurred syn‐ to post‐ S2, at 730–770°C and 6–8 kbar, and at 750–790°C and 6 kbar, respectively. The pressure–temperature–deformation–time paths reconstructed for the Georgetown Inlier suggest a c. 1.60 Ga M1/D1 event recorded under greenschist facies conditions in the western domain and under medium‐P and medium‐T conditions in the central domain. This event was followed by the regional 1.56–1.54 Ga low‐P and high‐T phase (M2/D2), extensively recorded in the central and eastern domains. Decompression between these two metamorphic events is ascribed to an episode of exhumation. The two‐stage evolution supports the previous hypothesis that the Georgetown Inlier preserves continental collisional and subsequent thermal perturbation associated with granite emplacement.

Eocene migmatite formation and diachronous burial revealed by petrochronology in NW Himalaya, Zanskar

AbstractIn this contribution, we highlight the importance of in‐situ monazite geochronology linked to P−T modelling for identification of timescales of metamorphic processes. Barrovian‐type micaschists, migmatites and augengneiss from the Gumburanjun dome in the southeastern extremity of the Gianbul dome, NW Himalaya, have been studied in order to correlate the early stages of Himalayan metamorphism at different crustal levels and infer the timing of anatexis. P−T−t paths are constrained through combined pseudosection modelling and in‐situ and in‐mount monazite and xenotime laser ablation–split‐stream inductively coupled plasma‐mass spectrometry. Petrography and garnet zoning combined with pseudosection modelling show that garnet‐staurolite schists record burial from ~530 to 560°C and 5.5 kbar to ~630 to 660°C and 7 kbar; staurolite‐kyanite schists from ~530 to 560°C and 5 kbar to ~670 to 680°C and 7−9 kbar; and garnet‐kyanite migmatites from 540−570°C and 5 kbar to ~680 to 750°C and 7−10 kbar, probably also to >750°C and >9 kbar above the muscovite stability field. The decompression paths of garnet‐staurolite schists indicate cooling on decompression, while garnet rim chemistry and local sillimanite growth point to a stage of re‐equilibration at ~600 to 670°C and 4−6 kbar in some of the staurolite‐kyanite schists, and at ~670 to 700°C and 6 kbar in garnet‐kyanite migmatites. Some of the staurolite‐kyanite schists and garnet‐kyanite migmatites also contain andalusite or andalusite‐cordierite. Monazite and xenotime were analysed in thin sections in garnet, staurolite and kyanite, and in the matrix; and in mounts. BSE images and compositional maps of monazite (xenotime was too small) show variable internal structures from homogeneous through patchy zoning with embayed to sharp boundaries. Two groups of samples can be identified on the basis of the presence or absence of c. 44 − 37 Ma ages. The first group of samples—two garnet‐staurolite schists—recorded only c. 31 − 27 Ma ages in porphyroblasts and no c. 40 Ma ages. The second group (samples of staurolite‐kyanite schist, garnet‐kyanite migmatites, augengneiss) have both the older, c. 44 − 37 Ma monazite ages in porphyroblasts and younger ages down to c. 22 Ma. These significantly different ranges of ages from porphyroblasts of 44−37 Ma, and 31−27 Ma, are interpreted as the duration of prograde P−T paths in Eocene and Oligocene, and indicate diachronous two‐stage burial of rocks. Early migmatization occurred at 38 Ma. The c. 29 Ma is interpreted as the time when rocks from the lower and middle crustal levels were partially exhumed and came in to contact with rocks that were downgoing at this time. Localized monazite recrystallization is as young as 26−24 Ma. The youngest ages of 23−22 Ma are related to leucogranite emplacement.

Paleoproterozoic postcollisional metamorphic and igneous activities in the Jinan area of the Jiao-Liao-Ji Belt in the North China Craton and their tectonic implications

The Jiao-Liao-Ji Belt (JLJB) is a Paleoproterozoic collisional belt between the Longgang Block in the northeastern North China Craton and the Nangrim Massif on the Korean Peninsula. The Jinan area, located in the northeastern JLJB, was intruded by monzogranite at 2151 ± 6 Ma and then intruded again by porphyritic granites at 1842 ± 5 Ma in a postcollisional tectonic setting. The monzogranite and an associated gabbro were metamorphosed into metamonzogranite and metabasite at 1868 ± 11 Ma–1846 ± 3 Ma. Pelitic granulites occur together with the porphyritic granites. The pelitic granulites have peak metamorphic mineral assemblages of garnet + biotite + plagioclase + K-feldspar + sillimanite + quartz + ilmenite and underwent retrograde metamorphism, as evidenced by the presence of cordierite and muscovite. Pseudosection analysis of the granulites yields peak metamorphic conditions of ~8.5–7 kbar and 800–760 °C, and conventional geothermobarometry analysis of garnet rims and adjacent minerals yields retrograde metamorphic conditions of ~5–3.5 kbar and 740–680 °C. Dating of monazite from the two granulites demonstrates that peak metamorphism occurred between 1840 ± 2 Ma and 1847 ± 4 Ma. These results, together with the findings of previous studies, indicate that in the Liaodong and Jinan areas of the JLJB, igneous and metamorphic activities occurred together at ~1.85 Ga in a postcollisional tectonic setting, suggesting that collision between the Longgang Block and the Nangrim Massif occurred before ~1.85 Ga.

Metamorphic evolution of metasedimentary rocks of the Feira Nova region: Tectonic implications for the Brasiliano Orogeny in eastern Borborema Province, Northeast Brazil

Thermobarometric estimations and X-ray maps were combined to evaluate the metamorphic evolution of metasedimentary rocks (Surubim Complex) in the Feira Nova region, eastern Borborema Province (NE Brazil). Sillimanite-bearing biotite schists, garnet-biotite schists and gneisses were studied. X-ray maps show different growing phases and retrograde zoning patterns for biotite and garnet. High Ca and Mg contents in garnet cores along with high Ti in biotite are interpreted as equilibrium compositions at peak metamorphism. Lower contents of Ca and Mg at garnet rims and of Ti in biotite rims suggest cooling and decompression. Garnet-biotite geothermometer and garnet-Al2SiO5-quartz-plagioclase (GASP) geobarometer constrain high temperature (∼650–760 °C) and medium pressure (∼0.6–0.9 GPa; burial to ∼23–34 km) conditions for the metamorphic peak, which was synchronous to development of a flat-lying foliation and local anatexis. Garnet rims indicate exhumation to ∼19-11 km (∼0.4–0.3 GPa) at ∼590-520 °C, probably during a transpressional regime. The data points to low exhumation and cooling rates. The lack of extensional fabrics in the region right after the Brasiliano Orogeny supports erosional unroofing rather than extensional collapse. Based on kyanite occurrences nearby, we infer a clockwise P-T path related to thickening in a predominant intracontinental setting, followed by exhumation with low erosional and cooling rates.

Evolution of Mineral Composition during Eclogite Metamorphism in the Belomorian Mobile Belt: Data from Vichennaya Luda Island

The detailed mineralogical-geochemical study of eclogites and their retrograde products (amphibolites in a rim of eclogite boudin) from Vichennaya Luda Island (Keret Archipelago, White Sea) revealed systematic variations in major-, trace and rare-earth element composition of rock-forming minerals, which should be taken into account in geochronological and thermobarometric studies. Garnets from garnet–amphibole interlayers in the eclogites demonstrate a prograde zoning. In addition, garnet rims differ from their cores in a “hump-like” REE pattern owing to the elevated Sm, Eu, Gd and Dy contents and a negative slope of HREE pattern, as well as in an increase of Sm/Nd and a decrease of Lu/Hf ratio. It has been established that the eclogites contain clinopyroxene with depleted (to chondritic level) REE content, a positive Eu anomaly, and lowered Ti, V, Cr, Y, Zr, and Hf contents. Based on these geochemical features, the Cpx can be ascribed to relict that have preserved during peak eclogite metamorphism. Amphiboles in amphibolite rim of boudin differ sharply from amphiboles in eclogite in the lowered contents of LREE and some HREE. In addition to almost a two-fold decrease of Ti content in the eclogite–amphibolite sequence, amphiboles demonstrate a significant decrease of V, Sr, Y, Nb, and Hf contents. All biotites have sinusoidal REE pattern, which is typical of minerals formed through fluid-induced disequilibrium processes. Biotite from eclogites has higher Ti content and elevated contents of REE, Nb, V, Cr, Ba, and Hf as compared to biotites from the amphibolization rim. The eclogites and amphibolites developed after them are similar in major, trace, and rare-earth element composition.

Monazite behaviour during metamorphic evolution of a diamond-bearing gneiss: a case study from the Seve Nappe Complex, Scandinavian Caledonides

Abstract Monazite is a common mineral in metapelitic rocks including those which underwent ultra-high pressure (UHP) metamorphism. During metamorphic evolution monazite adapts its composition to the changing mineral assemblage, especially in its heavy rare earth element contents. We studied this process in diamond-bearing gneiss containing monazite, from Saxnäs in the Seve Nappe Complex of the Scandinavian Caledonides. Although the rock has been re-equilibrated under granulite facies and partial melting conditions, it still preserves minerals from the UHP stage: garnet, kyanite, rutile, and especially diamond. Microdiamonds occur in situ as inclusions in garnet, kyanite and zircon, either as single-crystals or polyphase inclusions with Fe-Mg carbonates, rutile and CO2. Both monazite and diamond occur in the rims of garnet showing the highest pyrope content and a secondary peak of yttrium. Such a position indicates thermally activated diffusion under high temperature at the end of prograde metamorphism. Monazite compositions show negative Eu anomalies, which we interpret to be inherited from the source rock, not reflecting the coexistence with plagioclase and/or K-feldspar which are unstable at UHP conditions. Our results suggest that the effect of whole-rock composition may be more important than that of coexisting phases. The UHP monazite was most likely formed from allanite during subduction and prograde metamorphism. The monazites included in garnet and kyanite are mostly unaltered, whereas those in the matrix show breakdown coronas consisting of apatite, REE-epidote/allanite and REE carbonate, likely formed due to pressure decrease and cooling. U-Th-Pb chemical age dating of monazites yields an isochron centroid age of 472 ±3 Ma. We interpret this age as monazite growth under UHP conditions related to subduction of the Baltica continental margin in Early Ordovician time.

Refining the timing and mechanism of the Triassic partial melting in the Sulu UHP orogen, China: Zircon and garnet evidence from a felsic vein and its host granitic gneiss

Partial melting of high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks in subduction zones has great influence on both the chemical and physical properties of deeply subducted continental crust. Felsic veins in HP-UHP metamorphic rocks are products of partial melting and thus would be a key target to trace partial melting in HP-UHP terranes. This contribution is an integrated study of petrology, zircon and titanite U-Pb ages, as well as zircon Hf-O and major minerals O isotope compositions for a felsic vein and its host granitic gneiss in the Sulu UHP terrane to decipher the timing and mechanism of partial melting. Zircon U-Pb dating for the inherited magmatic zircon cores of the granitic gneiss constrains the protolith age at ca. 750 Ma. The inherited zircons had an average δ18O value of 0.03 ± 0.15‰ and yielded a weighted mean εHf(t) value of −1.16 ± 0.54 and two-stage Hf model ages of 1707 ± 65 Ma. These results suggest that the protolith was derived from reworking of Paleoproterozoic crust in a Neoproterozoic rift. In the two samples, broad garnet core/mantle domains are of metamorphic origin and narrow garnet rims of peritectic origin. There are large differences in δ18O values between the protolith magmatic zircon (δ18O = 0.03 ± 0.15‰) and metamorphic garnet (−4.14 ± 0.21‰ for the felsic vein and −4.13 ± 0.11‰ for the host granitic gneiss), suggesting that the negative δ18O fingerprints might result from hydrothermal alteration or multi-episodes of rift magmatism after the formation of the protolith. Zircon from the felsic vein yielded a U-Pb age of 212 ± 3 Ma. Titanites from the two studied samples show undistinguishable U-Pb ages of 227 ± 14 Ma and 212 ± 6 Ma within the analytical uncertainties. These ages register the timings of the partial melting. The approximately consistent δ18O values of the metamorphic zircon (−4.78 ± 0.15‰) and metamorphic garnet indicate that the melt was sourced from the surrounded gneiss in a closed system, which might be triggered by breakdown of phengite. According to a compilation of partial melting in the Sulu UHP terrane, there exists a clear spatio-temporal variation. The ca. 237-224 Ma partial melting in the northern UHP zone might be due to its relatively high temperature metamorphism; the most extensive partial melting occurred at ca. 220-215 Ma in both the southern and northern UHP zones, which might be triggered by the detachment of the preceding subducted slab; and partial melting of the northern UHP zone continued to ca. 210 Ma due to heat supply from the underneath mantle. Such a variation might be dictated by the tectonic evolution of the different UHP slices. Accordingly, the whole partial melting process of UHP rocks in the Sulu UHP terrane might have lasted for about 27 Myr with two major episodes at ca. 230–224 and 220-215 Ma.

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.

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.

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.

Deciphering the tectono-metamorphic evolution of the Nevado-Filábride complex (Betic Cordillera, Spain) – A petrochronological study

The Nevado-Filábride metamorphic complex (Betic Cordillera, Spain) exhibits a succession of Paleozoic and Mesozoic metasediments, orthogneisses and metabasites. The complex has been divided into three tectono-metamorphic units, from bottom to top: the Ragua, Calar-Alto and Bédar-Macael units. The petrochronological evolution of the complex is not well constrained as P-T-t conditions differ between authors mainly because of the investigated rock types and the thermobarometric and geochronological techniques applied. Five garnet-bearing mica-schists from various areas of the complex were investigated for constraining and comparing the shape of the P-T paths recorded by the three units. Quantitative compositional mapping of garnet and K-white mica was combined with iterative thermodynamic models. The resulting P-T trajectories suggest that the units experienced similar clockwise P-T trajectories during Alpine metamorphism. The three units reached high-pressure and low-temperature conditions of ~2.0 GPa and ~520 °C in the Bédar-Macael unit; ~2.0–2.2 GPa and ~470–490 °C in the Calar-Alto unit; and ~2.2 GPa and ~480 °C in the Ragua unit. All samples recorded a temperature increase of ~130 °C during exhumation as shown by the successive stages of white mica re-equilibrations. Heating during exhumation was probably triggered by the hot hanging wall over the Nevado-Filábride complex. The garnet rims reflect a high-temperature and low-pressure stage which was dated at ~13 Ma using allanite U-Th-Pb geochronology (LA-ICP-MS). Following the temperature peak, exhumation continued associated to cooling. Similarities in the shapes of P-T path throughout the units suggest a continuous metamorphic sequence rather a tectonically divided complex.

Mesoarchaean clockwise metamorphic P-T path from the Western Dharwar Craton

In this study, we utilize high-alumina pelitic schist samples from the Holenarsipur Greenstone Belt, a component of the ca. 3.30–3.15 Ga Sargur Supergroup in the Western Dharwar Craton, South India to establish a Mesoarchaean metamorphic history in the terrane. On the basis of integrated textural, mineral compositional, metamorphic reaction history and P-T pseudosection modelling studies, garnet-monazite trace element thermometry and coupled monazite electron probe micro-analyzer and sensitive high-resolution ion microprobe U-Pb dating results, we constrain for the first time a Mesoarchaean metamorphism along a hairpin clockwise metamorphic P-T path in the Dharwar Craton. The path reveals a two-stage prograde segment with an initial phase of heating accompanying burial (dP/dT gradient ~ 30 bar/°C) and a later phase of steep burial (∆P ~ 1.1 kbar, dP/dT gradient ~110 bar/°C), peak metamorphism at 7.5 kbar, 640 °C and a post-peak, steep decompression stage (P~6.3 kbar at 635 °C). Chemically zoned monazite grains shielded within garnet or occurring in low-strain matrix domains were formed in two stages during the Mesoarchaean metamorphism: (a) Monazite cores with relatively lower Y concentrations, higher Th/U ratios, a strong negative Eu anomaly; their REE partitioning relationships with the garnet inner rim reveal their crystallization was synchronous with the growth of garnet. (b) Monazite rims with higher Y and uraniferous compositions and lower Th/U ratios which, combined with Y and HREE enrichments in the outer rim of garnet, indicate their formation during the post-peak decompression stage associated with coupled decompositions of garnet and zircon. We correlate U-Pb monazite core and rim ages at 3144 ± 7 Ma and 3105 ± 10 Ma respectively with the timings of peak and retrograde metamorphism. Our findings, when evaluated with the available geological, isotopic, geochronological data of Meso- to Palaeoarchaean extant and extinct felsic crust as well as thermo-mechanical models, predict the ca.3.14–3.11 Ga metamorphism in the Holenarsipur Greenstone Belt as part of a widespread tectono-thermal event in the Western Dharwar Craton. The event marks late stages of sagduction tectonics in the craton as it evolves from a predominantly vertical to a horizontal shortening mode.

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.

Oxygen isotope speedometry in granulite facies garnet recording fluid/melt–rock interaction (Sør Rondane Mountains, East Antarctica)

AbstractIn situ analysis of a garnet porphyroblast from a granulite facies gneiss from Sør Rondane Mountains, East Antarctica, reveals discontinuous step‐wise zoning in phosphorus and large δ18O variations from the phosphorus‐rich core to the phosphorus‐poor rim. The gradually decreasing profile of oxygen isotope from the core (δ18O = ~15‰) to the rim (δ18O = ~11‰) suggests that the 18O/16O zoning was originally step‐wise, and modified by diffusion after the garnet rim formation at ~800°C and 0.8 GPa. Fitting of the 18O/16O data to the diffusion equation constrains a duration of the high‐T event (~800°C) to c. 0.5–40 Ma after the garnet rim formation. The low δ18O value of the garnet rim, together with the previously reported low δ18O values in metacarbonates, indicates regional infiltration, probably along a detachment fault, of low δ18O fluid/melt possibly derived from meta‐mafic to ultramafic rocks.

Metamorphism and Oceanic Crust Exhumation—Constrained by the Jilang Eclogite and Meta-Quartzite from the Sumdo (U)HP Metamorphic Belt

The Sumdo eclogite-bearing (U)HP metamorphic belt extends over 100 km across the middle part of the Lhasa terrane in southern Tibet, which forms a Permian-Triassic oceanic subduction zone between the south and the north Lhasa sub-terranes, leading to the reinterpretation of the tectonic evolution of the Lhasa terrane in the Tibetan-Himalayan orogeny. Previous studies show that there are significant differences in temperature and pressure conditions of the eclogites in four areas, e.g., Sumdo, Xindaduo, Bailang and Jilang areas. Studying the peak metamorphic P-T conditions and path of eclogite in the Sumdo belt is of great significance to reveal the subduction and exhumation mechanism of Paleo-Tethys Ocean in the Lhasa terrane. In this contribution, eclogite in the Jilang area of the Sumdo belt is chosen as an example to study its metamorphic evolution. The mineral assemblage of the eclogite is garnet, omphacite, phengite, hornblende, epidote, quartz and minor biotite. Garnet has a "dirty" core with abundant inclusions such as epidote, amphibole, plagioclase and a "clear" rim with few inclusions of omphacite and phengite. From the core to the rim, pyrope content in garnet increases while grossular content decreases, showing typical growth zoning. The rim of garnet is wrapped by the pargasite+plagioclase corona, showing amphibolite facies overprint during retrogression. Three stages of metamorphism are inferred as (1) prograde stage, represented by the core of garnet and mineral inclusions therein; (2) peak stage, represented by the garnet rim, omphacite, lawsonite, phengite, and quartz; (3) retrograde stage characterized by decomposition of lawsonite to zoisite, followed by symplectite of omphacite and corona rimmed garnet. A P-T pseudosection contoured with isopleths of grossular and pyrope contents in garnet is used to constrain the near peak P-T condition at 2.85 GPa, 575 C. In general, the Jilang eclogite shows a clockwise P-T path with a near isothermal decompression process during exhumation. Combined with the age peaks of 583, 911, and 1 134 Ma from the detrital zircons of the country metaquartzite, a continental margin material involving exhumation process at shallow depth after the subduction channel exhumation is inferred for the Jilang eclogite and may further indicate that the subduction direction of the Sumdo eclogite belt is from north to south.

Implications for Thrust‐Related Shortening Punctuated by Extension From P‐T Paths and Geochronology of Garnet‐Bearing Schists, Southern (Çine) Menderes Massif, SW Turkey

AbstractThe Menderes Massif, Turkey, is a type locality for deciphering the plate tectonic response from collision‐ to extension‐driven exhumation. Conventional thermobarometry and garnet pressure‐temperature (P‐T) paths from isochemical phase diagrams were calculated across a major fault (Selimiye Shear Zone, SSZ) bounding the southern edge of the Menderes Massif. Both approaches yield similar garnet rim temperatures (from 555 to 671 °C), but estimated P differs by between 8 and 15 kbar. Three garnets north of the SSZ reveal N‐shaped P‐T paths, whereas paths from three samples south of the SSZ show a simple increase in P‐T. Monazite and zircon were dated in thin section from the same rocks using Secondary Ion Mass Spectrometry and Laser Ablation Inductively Coupled Plasma‐Mass Spectrometry, respectively. Textural relationships of monazite within garnet appears indicative of post‐garnet growth. The amount of monazite common 204Pb and 137Ba+/Th+ significantly exceeds what is observed for the monazite age standard, suggesting their ages mark fluid‐driven events, loosely constrained to Late Eocene‐Early Miocene. Some zircon ages are consistent with Cambro‐Ordovician ages reported elsewhere in the region, and other ages are Neoproterozoic and Permian‐Triassic, a period not previously recognized in this area. Despite the lack of age constraints for the duration of garnet growth, we present a thermal model to understand the meaning of the N‐shaped path. These paths are best reproduced by thermal models incorporating SSZ thrusting before and after denudation. This paper presents an example of the insight from high‐resolution P‐T paths, and an example of denudation within a prograde metamorphic event.

Multi-Stage Metamorphism of the UHP Pelitic Gneiss from the Southern Altyn Tagh HP/UHP Belt, Western China: Petrological and Geochronological Evidence

The kyanite-bearing garnet pelitic gneiss from the Jianggalesayi area in southern Altyn Tagh high pressure/ultra-high pressure belt was proved to have been experienced UHP metamorphism (>12 GPa) by the discovery of kyanite and spinel exsolution microstructure in quartz (precursor stishovite). In this study, three stages of retrograded metamorphism (M2–M4) after the UHP metamorphism (M1) were identified for the UHP pelitic gneiss. The HP granulite-facies stage (M2) was characterized by the mineral assemblage of garnet+kyanite+K-feldspar+rutile+quartz±ilmenite, recording the P-T condition of >1.12 GPa and ≈850–930 oC. The granulite-facies stage (M3) was represented by the mineral assemblage of garnet rim+K-feldspar+sillimanite (Sill1)+biotite (Bt1)+plagioclase (Pl1)+ilmenite+quartz, and confined under P-T conditions of 0.5–0.8 GPa and ≈770–795 oC. The late cooling stage M4 was accompanied by the appearance of fine-grained Pl2, Sill2 and Bt2 in the matrix, and the P-T conditions were 0.4–0.6 GPa and 350 km) to the lower crust depth (≈40–20 km), resulting in an average exhumation rate of 9.11–9.70 mm/yr. In the southern Altyn Tagh region, the HP and UHP rocks from different areas had identical peak metamorphic ages. Therefore, contemporary UHP and HP rocks with different metamorphic evolutions were recognized coexisting in the same orogenic belt, which can be interpreted by the model of subduction channel. The continental crustal were subducted to different depths along the direction of the subduction channels at ≈500 Ma, suffered different grade metamorphism, and then returned to the surface along the subduction channel.

Disequilibrium REE compositions of garnet and zircon in migmatites reflecting different growth timings during single metamorphism (Aoyama area, Ryoke belt, Japan)

Chemical disequilibrium of coexisting garnet and zircon in pelitic migmatites (Aoyama area, Ryoke belt, SW Japan) is shown by microtextural evidence and their heavy rare earth element (HREE) patterns. In zircon, two stages of metamorphic rim growth is observed under cathodoluminescence image, although their SHRIMP UPb zircon ages are similar at ca. 92 Ma. Inner and outer rims of zircon tend to show steep HREE patterns irrespective of the UPb age. The inner rims tend to give higher U content than the outer rims; some rim analyses give various Th/U ratios of 0.02–0.07 compared to the very low (<0.02) values seen in the rest of rim analyses. The higher-Th/U values are ascribed to the mixed analyses between thin prograde domains and thick retrograde overgrowths. Zircon grains with inclusions similar to previously-reported melt inclusions are further enclosed in garnet, supporting the growth of thin zircon domains coexisting with garnet during the prograde metamorphism.Garnet rims are commonly replaced by biotite-plagioclase intergrowths, indicating a back reaction with partial melts. Garnet exhibits decrease in HREE and Y concentrations towards the rim, pointing to its prograde growth. The garnet cores have prograde xenotime inclusions, show steep HREE patterns, and yield growth temperature of ~530–570 °C by a YAG-xenotime thermometer. On the other hand, the garnet rims have no xenotime inclusion and show flat HREE patterns. Rare garnet domains including sillimanite needles also show flat HREE patterns and low Y concentrations, which is interpreted as a product of dehydration melting consuming biotite and sillimanite at near-peak P-T conditions (~800 °C and ~0.5 GPa). One such garnet domain gives nearly-equilibrium REE distribution pattern when paired with the matrix zircon rims.Retrograde xenotime is present in the cracks in garnet and in the biotite-plagioclase intergrowths, suggesting that retrograde breakdown of garnet released HREE and Y to form it. Considering the availability of HREE and Zr and presence of melt inclusions in zircon rims, most part of the zircon rims with positive HREE patterns likely grew during the melt crystallization stage, meaning that the zircon rims and presently-preserved garnet domains did not grow in equilibrium. The above scenario was tested by the array plot analysis and it gave a result consistent with microtextural and traditional REE distribution constraints. Combination of microtextural and the array plot analyses may become a powerful tool to reliably correlate the zircon ages to the P-T evolution of the high-grade metamorphic rocks.