Prograde Path Research Articles

Monazite petrochronology of polymetamorphic granulite‐facies rocks of the Larsemann Hills, Prydz Bay, East Antarctica

AbstractThe Prydz Bay coast, including the Larsemann Hills, features relatively extensive bedrock exposures of interest because of the proximity to a hypothesized suture associated with Gondwana assembly. Critical units are the basement Søstrene Orthogneiss (1,126 ± 11 Ma protolith) and cover Brattstrand Paragneiss (maximum depositional age 1,023 ± 19 Ma). The two units share a polymetamorphic history with events at ~900 Ma (D1) and ~530 Ma (D2‐4). Here we present electron microprobe dates of monazite growth zones and Perple_X pseudosection models of granulite‐facies rocks from the Søstrene Orthogneiss, Brattstrand Paragneiss, and D2‐4 pegmatites of the Larsemann Hills. We propose a scenario for Cambrian metamorphism involving a peak stage at 6–7.5 kbar and 800–860°C (D2 convergence), melt crystallization and garnet breakdown during decompression to early retrograde conditions of 3–4.5 kbar and 700–750°C (D2 convergence, D3 extension), and a late retrograde stage with decompression and cooling to 3–3.5 kbar and 550–650°C (D4). We combine monazite chemistry with phase assemblages predicted by pseudosection modelling to link specific monazite growth domains to individual tectonic stages. Monazite domains containing moderate Th and low to moderate Y are interpreted to be preserved from the prograde path when garnet was stable, and constrain the timing of prograde metamorphism at 536 ± 4 Ma. High‐Th, low‐Y domains, dated at 527 ± 2 Ma, represent the earliest stages of post‐peak melt crystallization. Monazite domains with elevated Y and low‐moderate Th are interpreted to represent monazite growth during garnet breakdown at 514 ± 2 Ma. Our monazite ages, combined with published biotite Ar–Ar cooling ages, yield a two‐stage history of cooling at 3–8°C/Myr from ~530 Ma to ~510 Ma followed by cooling at 18–25°C/Myr from ~510 Ma to ~490 Ma, corresponding to 0.2–0.6 mm/yr of exhumation. This duration of granulite‐facies metamorphism in the Larsemann Hills is consistent with estimates for Precambrian granulite facies metamorphic complexes elsewhere.

Identification of continental-type eclogites in the Paleo-Tethyan Changning–Menglian orogenic belt, southeastern Tibetan Plateau: Implications for the transition from oceanic to continental subduction

As a record of ancient subduction, eclogites preserve critical information for exploring geodynamic processes at convergent plate boundaries. The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is situated between the Gondwana-derived Baoshan–Tengchong Block and the Eurasia-derived Lanping–Simao Block. The belt has long been accepted as a typical cold subduction–accretion belt associated with the closure of the main Paleo-Tethys Ocean, owing to the development of oceanic-type lawsonite-bearing eclogites. However, this viewpoint is challenged by the presence of newly discovered continental-type eclogites hosted by metasedimentary rocks in the CMOB. The continental-type eclogites are depleted in high field-strength elements (e.g., Nb, Ta, and Zr), enriched in Th and U, and preserve highly negative ɛNd(t) values varying from −15.26 to −13.04, indicating derivation from continental mafic protoliths that underwent extensive crustal contamination. Inherited zircons from both the continental-type eclogites and their host schists have characteristic detrital age distributions, with dominant Pan-African age peaks at 630–505 Ma that match well with those in the Baoshan–Tengchong Block. Combined with published age data, our new geochemical and geochronological data provide convincing evidence for the presence of significant volumes of subducted continental fragments derived from the Baoshan–Tengchong Block. Thermodynamic modeling of the continental-type eclogites indicates a clockwise P–T path with a steep prograde path consistent with a geothermal gradient of 5–8 °C/km, which was followed by near-isothermal decompression. Calculations suggest the eclogite-facies assemblage of garnet + omphacite + glaucophane ± talc ± lawsonite ± phengite records peak conditions of 28.5–29.6 kbar and 580–594 °C, equivalent to depths of 85–90 km. U–Pb dating of metamorphic zircon rims from the eclogites gives an age of 234–233 Ma, which is interpreted to reflect the timing of continental subduction. The continental-type eclogites show mineral associations, paragenetic sequences and metamorphic P–T–t evolutions similar to those of low-temperature oceanic-type eclogites in the CMOB. As a result, we propose that rapid, cold subduction of continental crust, and its subsequent rapid exhumation, occurred shortly after the closure of the dense and long-lived Paleo-Tethys oceanic lithosphere. The discovery of continental-type eclogites imparts significant insights into the dynamics of Gondwana–Eurasia convergence in the Paleo-Tethyan domain. The CMOB provides a well-exposed example of the tectonic transformation from final oceanic subduction to the onset of continental subduction and collision in a cold geothermal regime.

Tectono-metamorphic evolution of the proto-Andean margin of Gondwana: Evidence of internal high-grade metamorphism along the northern portion of the Famatinian orogen, Sierra de Aconquija, Sierras Pampeanas Orientales, Argentina

The present work gives a detailed analysis of the metamorphic and structural evolution of the back-arc portion of the Famatinian Orogen exposed in the southern Sierra de Aconquija (Cuesta de La Chilca segment) in the Sierras Pampeanas Orientales (Eastern Pampean Sierras). The Pampeanas Orientales include from north to south the Aconquija, Ambato and Ancasti mountains. They are mainly composed of middle to high grade metasedimentary units and magmatic rocks.At the south end of the Sierra de Aconquija, along an east to west segment extending over nearly 10 km (Cuesta de La Chilca), large volumes of metasedimentary rocks crop out. The eastern metasediments were defined as members of the El Portezuelo Metamorphic–Igneous Complex (EPMIC) or Eastern block and the western ones relate to the Quebrada del Molle Metamorphic Complex (QMMC) or Western block. The two blocks are divided by the La Chilca Shear Zone, which is reactivated as the Río Chañarito fault.The EPMIC, forming the hanging wall, is composed of schists, gneisses and rare amphibolites, calc– silicate schists, marbles and migmatites. The rocks underwent multiple episodes of deformation and a late high strain-rate episode with gradually increasing mylonitization to the west. Metamorphism progrades from a M1 phase to the peak M3, characterized by the reactions: Qtz + Pl + Bt ± Ms → Grt + Bt2 + Pl2 ± Sil ± Kfs, Qtz + Bt + Sil → Crd + Kfs and Qtz + Grt + Sil→ Crd. The M3 assemblage is coeval with the dominant foliation related to a third deformational phase (D3).The QMMC, forming the foot wall, is made up of fine-grained banded quartz – biotite schists with quartz veins and quartz-feldspar-rich pegmatites. To the east, schists are also overprinted by mylonitization. The M3 peak assemblage is quartz + biotite + plagioclase ± garnet ± sillimanite ± muscovite ± ilmenite ± magnetite ± apatite.The studied segment suffered multiphase deformation and metamorphism. Some of these phases can be correlated between both blocks. D1 is locally preserved in scarce outcrops in the EPMIC but is the dominant in the QMMC, where S1 is nearly parallel to S0. In the EPMIC, D2 is represented by the S2 foliation, related to the F2 folding that overprints S1, with dominant strike NNW – SSE and high angles dip to the E. D3 in the EPMIC have F3 folds with axis oblique to S2; the S3 foliation has striking NW – SE dipping steeply to the E or W and develops interference patterns. In the QMMC, S2 (D2) is a discontinuous cleavage oblique to S1 and transposed by S3 (D3), subparallel to S1. Such structures in the QMMC developed at subsolidus conditions and could be correlated to those of the EPMIC, which formed under higher P-T conditions. The penetrative deformation D2 in the EPMIC occurred during a prograde path with syntectonic growth of garnet reaching P-T conditions of 640°C and 0.54 GPa in the EPMIC. This stage was followed by a penetrative deformation D3 with syn-kinematic growth of garnet, cordierite and plagioclase. Peak P-T conditions calculated for M3 are 710°C and 0.60 GPa, preserved in the western part of the EPMIC, west of the unnamed fault.The schists from the QMMC suffered the early low grade M1 metamorphism with minimum PT conditions of ca 400°C and 0.35 GPa, comparable to the fine schists (M1) outcropping to the east. The D2 deformation is associated with the prograde M2 metamorphism. The penetrative D3 stage is related to a medium grade metamorphism M3, with peak conditions at ca 590°C and 0.55 GPa.The superimposed stages of deformation and metamorphism reaching high P-T conditions followed by isothermal decompression, defining a clockwise orogenic P-T path. During the Lower Paleozoic, folds were superimposed and recrystallization as well as partial melting at peak conditions occurred. Similar characteristics were described from the basement from other Famatinian-dominated locations of the Sierra de Aconquija and other ranges of the Sierras Pampeanas Orientales.

What can high‐P sheared orthogneisses tell us? An example from the Curinga–Girifalco Line (Calabria, southern Italy)

AbstractHigh‐P (HP) mineral parageneses are usually poorly developed within metagranitoids, as these rocks are commonly affected by fluid‐deficient conditions when experiencing a metamorphic cycle. However, since ductile shear zones can act as preferential pathways for fluids in the Earth's crust, if metagranitoids are involved in ductile shear under HP conditions, the presence of fluids during deformation can induce recrystallization and equilibration in these rocks. With this in mind, we investigate the formation and evolution of mineral assemblages in the orthogneisses of the Castagna Unit from the northern Serre Massif (Calabria, southern Italy). During Alpine tectonics, the thrusting along the Curinga–Girifalco Line juxtaposed these rocks, representative of the Hercynian intermediate crust, below the lithologies of the Hercynian lower crust. A detailed microstructural study of the orthogneisses, sampled along a progressively increasing ductile deformation gradient, revealed a variation in the mineral assemblage between the weakly deformed orthogneisses and those in the shear zone. Phase diagram calculations in the MnNCKFMASHTO system indicate that the progressive replacement of relict minerals by new, Alpine minerals in the shear zone, was related to the presence of fluids during deformation. This allowed equilibration of the sheared orthogneisses up to metamorphic peak conditions of ~0.9–1.0 GPa and ~560–590°C. Our integrated study highlights that both weakly deformed and mylonitic orthogneiss share the same peak metamorphic conditions, and that the new equilibrium mineral assemblage was stabilized in the mylonitic orthogneisses along a fluid‐conservative prograde path, where no fluid was added or lost. After metamorphic peak, the fluid was channelled towards the inner part of the shear zone, with fluid‐present conditions that were restricted to the mylonitic orthogneisses close to the tectonic contact. These mylonitic orthogneisses record cooling and exhumation to 0.6–0.7 GPa and 360–400°C, showing an overall anticlockwise P–T path. By comparing our findings with existing structural studies, we highlight that the Castagna Unit was under‐thrusted to lower‐crustal depths during the Alpine orogeny, before the re‐activation of the Curinga–Girifalco Line during the Oligocene to Miocene extensional tectonic phase, that enabled the exhumation of this unit.

Petrological and Lu–Hf age constraints for eclogitic rocks from the Pam Peninsula, New Caledonia

We report new petrological and geochronological data for eclogitic rocks of the Pouébo Eclogite Mélange (PEM) in the New Caledonian high-pressure/low-temperature (HP/LT) belt. Pseudosections were calculated to link newly determined Lu–Hf dates with P-T conditions corresponding to garnet growth, which started on the prograde path and continued into the eclogite facies to approximately 560 °C and 2.05 GPa, coinciding with omphacite formation. The Lu–Hf garnet dates of 38.27 ± 0.14, 36.5 ± 0.7, and 38.42 ± 0.15 Ma are weighted towards the early part of the garnet growth interval, and thus represent maximum ages of eclogite-facies conditions. These dates are indistinguishable from those reported earlier from Ar–Ar in white mica, which had been interpreted as cooling ages, and they are about 6 Myr younger than those from U–Pb in metamorphic zircon, which had been previously considered to date peak eclogite-facies metamorphism at 44 Ma. The new Lu–Hf dates overlap with Rb–Sr white mica ages of nearby metasomatic veins related to the blueschist-to-eclogite transition. The Lu–Hf and Rb–Sr data imply that eclogite-facies metamorphism in parts of the PEM lasted at least until 38 Ma and is thus younger than previously suggested. This prograde, near-peak event cannot be reconciled with concurrent (41–36 Ma) cooling previously inferred from the Ar–Ar dates. We conclude that both the 38.4–36.5 Ma Lu–Hf and Rb–Sr dates as well as similar Ar–Ar results record near-peak conditions, while the 44 Ma U–Pb zircon rims probably are related to prograde mineral reactions. The HP stage is followed by rapid exhumation and cooling at minimum rates of ~0.7 cm/yr and ~50 °C/Myr.

Differences in decompression of a high-pressure unit: A case study from the Cycladic Blueschist Unit on Naxos Island, Greece

Determining the tectonic evolution and thermal structure of a tectonic unit that experiences a subduction-related pressure temperature (P-T) loop is challenging. Within a single unit, P-T conditions can vary from top to bottom which can only be revealed by detailed petrological work. We present micropetrological data from the middle section of the Cycladic Blueschist Unit (CBU) in Naxos, Greece, which indicates a different P-T loop than that for the top of the sequence. Using Zr-in-rutile and Ti-in-biotite thermometry coupled with quartz-in-garnet elastic barometry and phase equilibrium thermodynamic modeling, we identify a prograde path from 15.4 ± 0.8 kbar to 19.9 ± 0.6 kbar and from 496 ± 16 °C to 572 ± 7 °C (2σ uncertainty), equilibration during decompression at 8.3 ± 1.5 kbar and 519 ± 12 °C followed by near-isobaric heating to 9.2 ± 0.8 kbar and 550 ± 10 °C (or even 584 ± 19 °C), and a final greenschist-facies equilibration stage at 3.8 ± 0.3 kbar and 520 ± 4 °C. We compare these P-T estimates with published data from the top and also the bottom of the CBU section and find that the bottom half of the CBU on Naxos records higher peak high-pressure (HP) of about 4 kbar than the top of the unit, defining the thickness of the CBU section on Naxos to about 15 km in the Eocene. We determine that crustal thickening of up to ~15% occurs in the upper half of the CBU section during exhumation of the HP rocks in an extrusion wedge in a convergence setting. At about 30 Ma, the bottom half of the CBU was finally thrust onto the radiogenic Cycladic basement. Subsequently this bottom half of the CBU section underwent isobaric heating of 9–96 °C between c. 32–28 and 23–21 Ma. Isobaric heating occurred below the upper CBU section that thickened during decompression and commenced when HP metamorphism in the Cyclades ended. This suggests that thermal relaxation following tectonic accretion in the Cyclades controlled heating of the evolving Cycladic orogen during a tectonically quiescent period before lithospheric extension commenced by 23–20.5 Ma.

Can mineral equilibrium modelling provide additional details on metamorphism of the Barberton garnet amphibolites?

AbstractThe Stolzburg domain to the south of the Barberton Greenstone Belt preserves evidence for a 3.23 Ga subduction–collision tectonic event. Garnet amphibolite greenstone remnants have previously yielded conventional thermobarometric P-T estimates of 12 to 15 kbar at 600 to 650°C, 8 to 11 kbar at 650 to 700°C and 7.5 to 8.5 kbar at 560 to 640°C from, respectively, the Inyoni shear zone along the western margin of the Stolzburg domain, the central part of the domain and from the Tjakastad schist belt on the boundary with the main body of the Barberton Greenstone Belt. Pseudosection calculations constrain the stability conditions of the peak metamorphic assemblages at the three localities to be 10 kbar at 675 to 690°C, ~10 kbar at 700°C and ~7 and 10 kbar at 660°C respectively. Although it is possible that the peak metamorphic assemblages may be displaced to somewhat lower conditions if Mn is considered in the calculations, these estimates are generally in good agreement with existing estimates, and confirm that the Stolzburg domain exposes an intact mid- to lower-crustal section that was metamorphosed in a relatively cool environment at 3.23 Ga. Our results do not support previously documented higher-pressure conditions, and we contend that the mineral assemblages used to derive these estimates can equally reflect the conditions determined here. The presence of albite-epidote inclusion assemblages in garnet indicates that the likely prograde path involved a component of heating at depth, which is typical of subduction–collision environments and markedly different from the heating–burial paths expected for sinking greenstones in a vertical tectonic model.

Structural and chemical resetting processes in white mica and their effect on K-Ar data during low temperature metamorphism

White mica has been widely used to date microstructures and tectonic events in faults, shear-zones and folds because of its suitability for radiogenic dating. However, complex (i) microstructural evolution, (ii) individual chemical evolution of the K-bearing phases, (iii) mixing of ‘detrital grains’ with newly formed and/or recrystallized or chemically reset grains as well as (iv) volume diffusion may result in apparent K-Ar ages. Here, specimens from a prograde sediment sequence of the exhumed fossil European Alpine accretionary wedge were used to investigate resetting processes of white mica by the type and intensity of deformation as well as peak metamorphic conditions. We combine the K-Ar system with mass and mineral quantities from grain size fractions to calculate the amount of recrystallized white mica in each grain size fraction along the metamorphic gradient. Increasing recrystallization with increasing metamorphic grade is related to thermally activated pressure solution and dissolution-precipitation creep, as seen by the formation of a spaced cleavage of recrystallized phyllosilicates documented through Synchrotron X-ray Fluorescence Microscopy and Scanning Electron Microscope imaging techniques. Increasing recrystallization by dissolution-precipitation processes induces chemical resetting of the isotopic system, resulting in a prograde decrease of apparent K-Ar ages. We demonstrate that Ar volume diffusion does not play a significant role for the low-temperature samples, promoting recrystallization as the important physico-chemical process for age resetting. However, white mica chemistry reveals that no simple relation between isotopic resetting and grain size exists along the prograde path. Reliable age information can therefore only be obtained in the case of (nearly) complete resetting, which accounts only for the smallest grain size fraction at the highest metamorphic temperature. These findings could shed new light on accurate dating of mica-rich fault rocks, where the time constraints depend not only on the temperature, but also on the amount and type of deformation.

Establishing the P-T path of UHT granulites by geochemically distinguishing peritectic from retrograde garnet

Abstract The P-T evolution (and particularly the prograde path segment) of ultrahigh-temperature (UHT) granulites is commonly ambiguous, hampering our understanding of deep crustal processes. Here, we establish the P-T path by distinguishing garnet origin (peritectic or retrograde) based on the combined Ca, Ti, Zr, and Y+REE chemical signatures, using the residual UHT granulites of the Khondalite Belt, North China Craton, as a test case. In these rocks, peritectic garnet is characterized by rare inclusions, whereas retrograde garnet has overprinted the main foliation and is characterized by abundant biotite and sillimanite inclusions, which are interpreted to have grown together with retrograde garnet during cooling. Zirconium in peritectic garnet increases from 10 to 50 ppm with garnet growth. In contrast, Zr in retrograde garnet generally decreases from 60 to 10 ppm with garnet growth. A similar trend is observed for Ti. Temperatures calculated from the Ti-in-garnet geothermometer increase from 830 to 980 °C based on Ti in peritectic garnet, indicating prograde partial melting, whereas decrease from 900 to 700 °C based on Ti in retrograde garnet, indicating post-peak cooling. Peritectic and retrograde garnets show distinct Eu/Eu* (0.2–0.5 vs. 0.05–0.2, respectively) and Ca contents (6000–12 000 vs. 4000–6000 ppm, respectively), which generally decrease with progressive garnet crystallization. The pressures calculated from the Ca-in-garnet geobarometer in peritectic and retrograde garnet are 9–11 and 7–9 kbar, respectively. Peritectic garnet shows a bell-shaped Y (80–340 ppm) pattern, whereas retrograde garnet shows an increase in Y content (20–100 ppm) toward rims. Taken together, these results establish a P-T path comprised of an earlier high-pressure peritectic garnet formation during prograde partial melting before the UHT peak and a late abundant retrograde formation during post-peak cooling stage. We conclude that change of Zr and other elements (e.g., Ti, Ca, Y, and Eu/Eu*) can well distinguish different garnet formation events in UHT granulites, which is critical for the P-T path establishment, and further sheds light on the cause of UHT metamorphism and the geodynamic evolution.

Geochemistry of the (meta-)mafic rocks from the Gonzalito mining district, northern Patagonia

In spite of hosting one of the most important Pb–Ag–Zn mineralizations in Patagonia, the metamorphic history of the rocks of the Mina Gonzalito Complex (MGC; east of the North Patagonian Massif) is still unclear. The complex consists of schists, para- and ortho-derived gneisses, ranging from greenschist to amphibolite facies, and metamafic rocks. Leucogranites and pegmatites were intruded synkinematically. Field, petrological and thermochronological evidence indicates that the MGC experienced an early prograde path and metamorphic peak during the Early Ordovician (ca. 472 Ma), magmatism and localized post-peak deformation and re-equilibrium at lower pressure, followed by uplift during the Late Permian. The MGC is intruded by the calc-alkaline Santa Rosa Diorite (SiO2 = 58.7–60.4 wt%; LaN/YbN = 7.2–10.5) and trachyte dike swarms in the Late Permian- Early Triassic. The mafic intrusives of the MGC form small schistose, massive and banded bodies interlayered within the gneisses and granites and recorded recrystallization of hornblende + plagioclase + quartz + titanite ± clinopyroxene ± biotite ± ilmenite. The metamafic rocks are mostly tholeiitic gabbros having SiO2 (45.4–52.1 wt%), TiO2 (0.62–2.88 wt%), flat REE patterns (LaN/YbN = 0.48–2.76), although some pyroxene-banded varieties show higher ratios. Initial P–T modelling in the NCKFMASHTO system for the metamafic rocks defined P-T conditions between 550 and 730 °C and 1–4 kbar. Our data suggest that the protolith of the metamafic rocks was emplaced in a shallow environment, associated with underplating of mantle-derived magmas slightly modified by crustal contamination. The intrusion of mantle-derived magmas may have been related either to a magmatic arc or to a continental rift environment. The model involving an Ordovician intracontinental back-arc basin is favored herein because it can reasonably explain many other geological features of Early Paleozoic basement rocks from northern Patagonia.

Hercynian subduction‐related processes within the metamorphic continental crust in Calabria (southern Italy)

AbstractLinking the deformation history of mylonitized continental rocks to the progress of devolatilization reactions that trigger reaction softening is critical for the understanding of crustal scale processes. We have analysed the field geometries and microstructures of deformed rocks within the southern Hercynian belt in Calabria, as well as modelled the pressure–temperature–deformation (P–T–d) trajectory of a main ductile shear zone that tectonically coupled the deeper crustal Mammola Paragneiss Unit with the upper crustal Stilo–Pazzano Phyllite Unit. P–T modelling of the mylonitic Mammola Paragneiss Unit was performed through calculation of phase equilibrium diagrams with the software thermocalc in the MnNCKFMASHTO model system. The prograde P–T–d trajectory is based on the zoning profiles of garnet porphyroblasts and their mineral inclusions, primarily barroisite and epidote. P–T modelling shows that peak metamorphic conditions of ~0.9 GPa and 585°C were reached during a Dn‐1 under‐thrusting event. The following exhumation during the Dn mylonitic event, and contact metamorphism during Dn+1 and Dn+2 folding events, have also been modelled because they are essential to restore the previous tectono‐metamorphic history. The exhumation trajectory was modelled down to 0.3 GPa with temperatures of 440–460°C, under fluid‐deficient conditions, as well as the final late Carboniferous contact metamorphism up to Tmax of 680–720°C. The prograde path shows clear evidence for thermal buffering during garnet growth at the expense of chlorite, with a heating‐dominated stage after chlorite breakdown. Subsequently, a rheological change associated with epidote breakdown (i.e. reaction softening) occurred, highlighted by a net steepening of the P/T trajectory towards the pressure peak. On the basis of the barroisite inclusions within garnet porphyroblasts as well as the ‘hairpin’ shape of the reconstructed P–T–d path (before contact metamorphism), we infer that the unusual low T/P gradient for the Hercynian crust exposed in the Mammola Paragneiss Unit records its involvement in the Palaeotethys–Gondwana subduction beneath Laurussia during Dn‐1 under‐thrusting. We present a new palaeotectonic interpretation along the southern Hercynian belt in Calabria during the Upper Mississippian–Lower Pennsylvanian, that is consistent with previous geochronology studies.

Deciphering the Jurassic\u2013Cretaceous evolution of the Hamadan metamorphic complex during Neotethys subduction, western Iran

The Hamadan high-grade metapelites in the northwestern part of the Sanandaj–Sirjan zone, Iran, show a polymetamorphic evolution with relics of a garnet-bearing metamorphic mineral assemblage (M1), a contact metamorphic overprint (M2) related to the emplacement of the Middle to Late Jurassic Alvand composite pluton and a Buchan-type regional metamorphic event (M3) marked by 40Ar/39Ar ages in the 80–70 Ma range that is associated with penetrative ductile deformation producing a foliation and a thermal overprint onto the M2 assemblages. The M1 event is exclusively preserved as small garnet grains and mineral inclusions contained therein, incorporated into M2-stage cordierite porphyroblasts. Distinct metamorphic zones are developed over a region of ~ 600 km2, which are partly correlated with distance to the composite pluton: zones (1) cordierite + K-feldspar hornfels, and (2) andalusite ± cordierite hornfels that surround the Alvand composite pluton at a distance of up to 5 km. These two zones are clearly related to M2 metamorphism associated with pluton emplacement. Zones (3) staurolite schist, (4) andalusite schist, and (5) sillimanite schist are found outside of the contact aureole and are considered to be the result of regional M3 metamorphism in the eastern part distant to the Alvand composite pluton. Conventional thermobarometry shows that temperatures in the area vary between ~ 560 and 660 °C for zones 1 and 2 and ~ 490 and 690 °C for zones 3–5. Phase equilibria modelling in the MnNCKFMASHT system indicates two distinct isobaric prograde paths at low pressures, at ~ 2.7 kbar for zones 1 and 2 and slightly higher pressures of around 3.5–5.5 kbar for zones 3–5. U–Th–Pb monazite geochronology revealed overlapping ages of 168 ± 11 Ma and 149 ± 19 Ma in the hornfels (1 and 2) and schistose (3–5) zones, respectively. These ages are similar to the intrusion age of the Alvand composite pluton (153.3 ± 2.7 to 166.5 ± 1.8 Ma) and are interpreted to reflect heating due to the emplacement of the composite pluton (M2 contact metamorphic event). However, 40Ar/39Ar dating of white mica and amphibole yielded plateau ages ranging from 80 to 69 Ma over the entire transect. The formation of schistosity in zones 3–5 postdates the intrusion and is thus related to M3 metamorphism. The white mica fabric indicates formation of the foliation during M3 garnet growth, which is followed by local retrogression of garnet to chlorite during exhumation. Consequently, the 40Ar/39Ar white mica and amphibole ages likely indicate reheating during M3 to more than ca. 500 ± 25 °C (argon retention temperature in amphibole). These data establish the occurrence of a Cretaceous, Buchan-style regional metamorphic event that had not been firmly identified before. Subsequent Late Cretaceous exhumation of the Hamadan complex with its high-grade metapelites is due to extension along the Tafrijan–Mangavi–Kandelan fault, which represents a major ductile low-angle normal fault. Metamorphic temperatures coupled with mineral ages from this and published work suggest a fast stage of cooling with a rate of ~ 6 °C/Ma during exhumation after M3 metamorphism.

Primary CO2-bearing fluid inclusions in granulitic garnet usually do not survive

CO2-bearing fluid inclusions (FI) from granulites have been the main advocate for an active role of fluids during high-temperature metamorphism, yet the fluid regime of the deep crust and the role of fluids remain largely debated topics. In this dispute, one of the most controversial issues is the timing of FI entrapment relative to peak metamorphic conditions. Here we investigate three world-renowned high- to ultra-high temperature metamorphic terranes to evaluate the fate of primary CO2 and COH fluid inclusions certainly trapped during the prograde path. Fluid inclusions coexist along with nanogranitoids in peritectic garnet. Combination of cutting-edge techniques indicates that the FI are composed of fluid and aggregates of solid phases. The latter usually comprises siderite, ferroan magnesite, pyrophyllite, calcite, corundum, quartz, and in some cases kaolinite, dolomite, biotite and muscovite. In the fluid phase, low-density CO2 is the most common component and no free H2O has been detected. Methane and N2 may be also present. The high proportion of solids in the FI with carbonates and OH-bearing phases cannot have precipitated as daughter phases directly from a simple COH-fluid as daughter phases. These minerals require additional cations (e.g. Fe, Mg, Ca, Al, Si) which may be dissolved in very small amounts in crustal fluids and yet are very abundant in the host. The solids, therefore, imply that the initial composition of high-density carbonic inclusions must have been changed by the interaction with the host garnet during cooling. Thermodynamic modelling of such fluid-garnet interaction demonstrates that the assemblages found in FI are metastable and that, whatever retrograde path is followed by the host rock, primary C-bearing FI must change their nature to a multiphase assemblage of stepdaughter minerals as a natural consequence of cooling. It follows that supposed primary unmodified FI in garnet previously reported in the literature are, in most cases, secondary, retrograde features whose low temperature of entrapment inhibited fluid-host interaction. Our finding undermines the main pillar of the theory of carbonic fluid-assisted metamorphism (the presence of superdense CO2 inclusions in granulites), but at the same time it offers a novel perspective to identify prograde COH fluids in the deep hot crust.

The coupling of high-pressure oceanic and continental units in Alpine Corsica: Evidence for syn-exhumation tectonic erosion at the roof of the plate interface

The subduction of continental crust is now a matter of fact but which are the mechanisms and the factors controlling the exhumation of continental units and their coupling with oceanic units are still a matter of debate. We herein present the tectono-metamorphic study of selected continental units belonging to the Alpine Corsica (Corte area, Central Corsica, France). The tectonic pile in the study area features thin slices of oceanic units (i.e. Schistes Lustrés Complex) tectonically stacked between the continental units (i.e. the Lower Units), which record a pressure–temperature-deformation (P-T-d) evolution related to their burial, down to P-T-peak conditions in the blueschist facies and subsequent exhumation during the Late Cretaceous – Early Oligocene time span. The metamorphic conditions were calculated crossing the results of three different thermobarometers based on the HP-LT metapelites. The continental units only recorded the P-peak conditions of 1.2 GPa-250 °C, up to the T-peak conditions of 0.8 GPa-400 °C, and the retrograde path up to LP-LT conditions. The metamorphic record of the oceanic units includes part of the prograde path occurring before the peak conditions reached at 1.0 GPa-250 °C followed by the last metamorphic event related to LP-LT conditions. The results indicate that each unit experienced a multistage independent pressure–temperature-deformation (P-T-d) evolution and suggest that the oceanic and continental units were coupled during the rising of the last ones at about 10 km of depth, where the oceanic units were stored at the base of the wedge. Subsequently they were deformed together by the last ductile deformation event during exhumation. We propose a mechanism of tectonic erosion at the base of the wedge, by which slices of Schistes Lustrés Complex were removed at the roof of the plate interface during the exhumation of the Lower Units.

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.

On the petrology of brittle precursors of shear zones – An expression of concomitant brittle deformation and fluid–rock interactions in the ‘ductile’ continental crust?

AbstractThe inherited localization model for shear zone development suggests that ductile deformation in the middle and lower continental crust is localized on mechanical anisotropies, like fractures, referred to as shear zone brittle precursors. In the Neves area (Western Tauern Window, Eastern Alps), although the structural control of these brittle precursors on ductile strain localization is well established, the relative timing of the brittle deformation and associated localized fluid flow with respect to ductile deformation remains in most cases a matter of debate. The present petrological study, carried out on a brittle precursor of a shear zone affecting the Neves metagranodiorite, aims to determine whether brittle and ductile deformations are concomitant and therefore relate to the same tectonic event. The brittle precursor consists of a 100–500 µm wide recrystallized zone with a host mineral‐controlled stable mineral assemblage composed of plagioclase–garnet–quartz–biotite–zoisite±white mica±pyrite. Plagioclase and garnet preserve an internal compositional zoning interpreted as the fingerprint of Alpine metamorphism and fluid–rock interactions concomitant with the brittle deformation. Phase equilibrium modelling of this garnet‐bearing brittle precursor shows that metamorphic garnet and plagioclase both nucleated at 0.6 ± 0.05 GPa, 500 ± 20°C and then grew along a prograde path to 0.75 ± 0.05 GPa, 530 ± 20°C. These amphibolite facies conditions are similar to those inferred from ductile shear zones from the same area, suggesting that both brittle and ductile deformation were active in the ductile realm above 500°C for a depth range between 17 and 21 km. We speculate that the Neves area fulfils most of the required conditions to have hosted slow earthquakes during Alpine continental collision, that is, coupled frictional and viscous deformation under high‐fluid pressure conditions ~450°C. Further investigation of this potential geological record is required to demonstrate that slow earthquakes may not be restricted to subduction zones but are also very likely to occur in modern continental collision settings.

Sulfide and silicate anatexis in the Balmat zinc deposit (New York, USA) and its implications for ore remobilization

The Cross-Cutting zone of the Fowler ore body is one of the many ore bodies of the Balmat, NY massive sulfide zinc deposit that underwent remobilization during upper amphibolite metamorphism and associated deformation. The kilometer-scale remobilization is difficult to reconcile with purely solid-state processes. Given that other polymetallic ores of similar metamorphic grade have been shown to have undergone varying degrees of anatexis, SEM-EDS micro-petrographic analysis of ores was undertaken to determine if melts were present in Balmat ores during metamorphism and deformation. Low melting temperature micro-inclusions of sulfosalt and sulfide assemblages occur predominantly in Qtz-Py and Kfs-Py domains that cross-cut peak metamorphic assemblages. These results indicate that localized anatexis occurred on the prograde path, producing low volumes of melts of varying composition. Polymetallic sulfide melts were initiated by the prograde breakdown of minor phases containing low melting temperature chalcophile elements including As, Sb, Pb, and Cu. Alkaline silicate ± carbonate ± sulfide melts were fluxed by halogens, sulfur and other volatile components, released during metamorphism of evaporitic and organic-rich horizons in the marble-dominated host sequence. These low-volume melts and residual fluids would have wetted grain boundaries, affecting the rheology of the bulk ore, and facilitating remobilization of the ores through a combination of solid- and liquid-state processes.

Can we extract ultrahigh-temperature conditions from Fe-rich metapelites? An example from the Khondalite Belt, North China Craton

In this study, garnet–sillimanite gneisses at Hongshaba in the eastern segment of the Khondalite Belt, North China Craton (NCC) are interpreted to have experienced ultrahigh-temperature (UHT) metamorphism (980–1040 °C) followed by post-Tmax cooling at pressures of 8–9 kbar to the solidus (810–830 °C), consistent with rare sapphirine-bearing assemblages in surrounding regions. This interpretation is mainly based on the combination of P–T fields and garnet Xgr (=Ca/(Ca + Mg + Fe2+)) isopleths on the pseudosection of three garnet–sillimanite gneiss samples. Spinel tends to be enclosed in the outer margins of garnet, commonly closely associated with quartz. We interpret this to reflect the partial break down of garnet along the prograde path during heating with decompression followed by new garnet growth during cooling along an overall clockwise P–T evolution. Although Fe-rich UHT metapelites tend to contain neither diagnostic mineral assemblages nor orthopyroxene from which to extract T via Al-in-orthopyroxene thermometry, isopleths of Ca in garnet may aid in retrieving UHT conditions from these compositions. This is attributed to Ca diffusion in garnet being much slower than Fe and Mg diffusion, leading to little change in Ca contents during post-Tmax cooling. LA-ICP-MS U-Pb dating of metamorphic zircon in one garnet–sillimanite gneiss sample yields a mean 207Pb/206Pb age of ca. 1.91 Ga, which is interpreted to record the timing of cooling of the UHT rocks to the solidus. This UHT metamorphism is interpreted to have been generated by mantle-derived magma during a tectonic extension from ca. 1.95 to 1.92 Ga within a post-orogenic setting.

Post-Variscan metamorphism in the Apuseni and Rodna Mountains (Romania): evidence from Sm–Nd garnet and U–Th–Pb monazite dating

The Tisza and Dacia mega-units constitute a central part of the Alps-Carpathians-Dinarides orogenic system. Polyphase medium-grade metamorphism observed in mineral assemblages from the crystalline basement is often correlated with Variscan and pre-Variscan events. However, a mid-Cretaceous Sm–Nd garnet age (103.6 ± 1.8 Ma) from the Apuseni Mountains is at odds with this interpretation. Electron-microprobe U-Th–Pb dating of monazite in samples from the Apuseni Mountains, the Rodna Mountains, as well as the Şimleu Silvaniei, Ticău and Preluca inselbergs revealed a complex pattern of Alpine and pre-Alpine age clusters. Pre-Variscan and Variscan ages were obtained from the core of zoned monazite grains and from samples that apparently escaped Alpine overprinting. Relic monazite in the latter is often replaced by rhabdophane and/or surrounded by allanite coronas. Permian to Early Triassic monazite ages correlate with the intrusion of granitic melts and pegmatites. Early Cretaceous ages from rims of chemically zoned grains and from monazite inclusions in garnet, biotite and staurolite represent newly formed metamorphic grains that crystallized on the prograde path during Alpine metamorphism. Petrographic observations of prograde allanite breakdown reactions, Sm–Nd garnet analyses and thermobaric estimates (500–550 °C/5–8 kbar) from parts of the Tisza and Dacia mega-units constrain medium-grade conditions during Early Cretaceous times. Exclusively mid-Cretaceous monazite ages from the inselbergs and the Rebra-Unit of the Rodna Mountains, allow extending the Alpine prograde overprint across the Transylvanian basin. Together with other studies from the basement of the Pannonian basin, this implies that the Dacia Mega-Unit and parts of the Tisza Mega-Unit experienced a medium-grade metamorphic overprint and synkinematic garnet-growth during late Early Cretaceous times. The Alpine prograde medium-grade overprint is pronounced in the contact zone between the Tisza and Dacia mega-units and forms a continuous belt with the Cretaceous metamorphic imprint in the Eastern Alps, when back-rotated to its original position during the Cretaceous.

A new record of deeper and colder subduction in the Acatlán complex, Mexico: Evidence from phase equilibrium modelling and Zr-in-rutile thermometry

Variable pressure–temperature (P–T) conditions and paths have previously been reported from eclogites and blueschists from the Acatlán complex, Mexico; yet, the large range of P–T estimates, and uncertainties related with conventional thermobarometry suggest that the P–T evolution of the Acatlán complex remains incompletely understood hindering reliable tectonic interpretations. In this paper, we focus on two high-pressure samples: a phengite eclogite and a garnet-epidote blueschist. We couple phase equilibrium modelling and Zr-in-rutile thermometry to re-evaluate the P–T conditions from these rocks and discuss their implications for the Acatlán complex. The phengite eclogite displays a granoblastic texture with a peak mineral assemblage of garnet + omphacite + phengite + rutile + quartz, and an early retrograde mineral assemblage of amphibole + zoisite + plagioclase + titanite. The garnet-epidote blueschist display a nematoblastic texture with a peak mineral assemblage glaucophane + phengite + epidote + quartz + plagioclase + chlorite + rutile + garnet. Our results indicate that the phengite eclogite followed a clockwise P–T path from peak conditions at ~22 kbar and ~690 °C to re-equilibration conditions at ~14.5 kbar and ~660 °C. Similarly, the garnet-epidote blueschist followed a clockwise P–T path. Phengite zoning indicates two different possibilities for the P–T evolution: (1) from peak conditions at ~19 kbar and ~505 °C or (2) from ~13 kbar and ~480 °C, to re-equilibration conditions at ~8.5 kbar and ~487 °C. Epidote + phengite + albite aggregates (pseudomorphs after lawsonite) and phengite zoning constrain the prograde path within the lawsonite stability field, indicating that the complex evolved through a colder subduction zone (gradient of ~6 °C km−1) than pre-existing estimates. Our results indicate that the eclogite and blueschist reached depths higher than previously thought, ~70 and ~50 km respectively, and experienced a near-isothermal exhumation (~0.15–0.60 °C km−1 for the blueschist, and ~0.9 °C km−1 for the eclogite). These new results, combined with published geochronology, result in a burial rate of ~5.4 km Myr−1, and an exhumation rate of ~3.4 km Myr−1.