Protracted thrusting followed by late rapid cooling of the Greater Himalayan Sequence,Annapurna Himalaya,Central Nepal: Insights from titanite petrochronology

The Greater Himalayan Sequence (GHS) has commonly been treated as a large coherently deforming high‐grade tectonic package, exhumed primarily by simultaneous thrust‐ and normal‐sense shearing on its bounding structures and erosion along its frontal exposure. A new paradigm, developed over the past decade, suggests that the GHS is not a single high‐grade lithotectonic unit, but consists of in‐sequence thrust sheets. In this study, we examine this concept in central Nepal by integrating temperature–time (T–t) paths, based on coupled Zr‐in‐titanite thermometry and U–Pb geochronology for upper GHS calcsilicates, with traditional thermobarometry, textural relationships and field mapping. Peak Zr‐in‐titanite temperatures are 760–850°C at 10–13 kbar, and U–Pb ages of titanite range from c. 30 to c. 15 Ma. Sector zoning of Zr and distribution of U–Pb ages within titanite suggest that diffusion rates of Zr and Pb are slower than experimentally determined rates, and these systems remain unaffected into the lower granulite facies. Two types of T–t paths occur across the Chame Shear Zone (CSZ). Between c. 25 and 17–16 Ma, hangingwall rocks cool at rates of 1–10°C/Ma, while footwall rocks heat at rates of 1–10°C/Ma. Over the same interval, temperatures increase structurally upwards through the hangingwall, but by 17–16 Ma temperatures converge. In contrast, temperatures decrease upwards in footwall rocks at all times. While the footwall is interpreted as an intact, structurally upright section, the thermometric inversion within the hangingwall suggests thrusting of hotter rocks over colder from c. 25 to c. 17–16 Ma. Retrograde hydration that is restricted to the hangingwall, and a lithological repetition of orthogneiss are consistent with thrust‐sense shear on the CSZ. The CSZ is structurally higher than previously identified intra‐GHS thrusts in central Nepal, and thrusting duration was 3–6 Ma longer than proposed for other intra‐GHS thrusts in this region. Cooling rates for both the hangingwall and footwall of the CSZ are comparable to or faster than rates for other intra‐GHS thrust sheets in Nepal. The overlap in high‐T titanite U–Pb ages and previously published muscovite ⁴⁰Ar/³⁹Ar cooling ages imply cooling rates for the hangingwall of ≥200°C/Ma after thrusting. Causes of rapid cooling include passive exhumation driven by a combination of duplexing in the Lesser Himalayan Sequence, and juxtaposition of cooler rocks on top of the GHS by the STDS. Normal‐sense displacement does not appear to affect T–t paths for rocks immediately below the STDS prior to 17–16 Ma.     

Constraining the conditions of Barrovian metamorphism in Sikkim,India: P–T–t paths of garnet crystallization in the Lesser Himalayan Belt

Garnet crystallization in metapelites from the Barrovian garnet and staurolite zones of the Lesser Himalayan Belt in Sikkim is modelled utilizing Gibbs free energy minimization, multi‐component diffusion theory and a simple nucleation and growth algorithm. The predicted mineral assemblages and garnet‐growth zoning match observations remarkably well for relatively tight, clockwise metamorphic P–T paths that are characterized by prograde gradients of ～30 °C kbar^?1 for garnet‐zone rocks and ～20 °C kbar^?1 for rocks from the staurolite zone. Estimates for peak metamorphic temperature increase up‐structure toward the Main Central Thrust. According to our calculations, garnet stopped growing at peak pressures, and protracted heating after peak pressure was absent or insignificant. Almost identical P–T paths for the samples studied and the metamorphic continuity of the Lesser Himalayan Belt support thermo‐mechanical models that favour tectonic inversion of a coherent package of Barrovian metamorphic rocks. Time‐scales associated with the metamorphism were too short for chemical diffusion to substantially modify garnet‐growth zoning in rocks from the garnet and staurolite zones. In general, the pressure of initial garnet growth decreases, and the temperature required for initial garnet growth was reached earlier, for rocks buried closer toward the MCT. Deviations from this overall trend can be explained by variations in bulk‐rock chemistry.     

The P–T–t evolution of the exhumed Himalayan metamorphic core in the Likhu Khola region,East Central Nepal

The recent identification of multiple strike‐parallel discontinuities within the exhumed Himalayan metamorphic core has helped revise the understanding of convergence accommodation processes within the former mid‐crust exposed in the Himalaya. Whilst the significance of these discontinuities to the overall development of the mountain belt is still being investigated, their identification and characterization has become important for potential correlations across regions, and for constraining the kinematic framework of the mid‐crust. The result of new phase equilibria modelling, trace element analysis and high‐precision Lu–Hf garnet dating of the metapelites from the Likhu Khola region in east central Nepal, combined with the previously published monazite petrochronology data confirms the presence of one of such cryptic thrust‐sense tectonometamorphic discontinuities within the lower portion of the exhumed metamorphic core and provides new constraints on the P–T estimates for that region. The location of the discontinuity is marked by an abrupt change in the nature of P–T–t paths of the rocks across it. The rocks in the footwall are characterized by a prograde burial P–T path with peak metamorphic conditions of ~660°C and ~9.5 kbar likely in the mid‐to‐late Miocene, which are overlain by the hanging wall rocks, that preserve retrograde P–T paths with P–T conditions of >700°C and ~7 kbar in the early Miocene. The occurrence of this thrust‐sense structure that separates rock units with unique metamorphic histories is compatible with orogenic models that identify a spatial and temporal transition from early midcrustal deformation and metamorphism in the deeper hinterland to later deformation and metamorphism towards the shallower foreland of the orogen. Moreover, these observations are comparable with those made across other discontinuities at similar structural levels along the Himalaya, confirming their importance as important orogen‐scale structures.     

Coupled garnet Lu–Hf and monazite U–Pb geochronology constrain early convergent margin dynamics in the Ross orogen,Antarctica

The Ross orogen of Antarctica is an extensive (>3000 km‐long) belt of deformed and metamorphosed sedimentary rocks and granitoid batholiths, which formed during convergence and subduction of palaeo‐Pacific lithosphere beneath East Gondwana in the Neoproterozoic–early Palaeozoic. Despite its prominent role in Gondwanan convergent tectonics, and a well‐established magmatic record, relatively little is known about the metamorphic rocks in the Ross orogen. A combination of garnet Lu–Hf and monazite U–Pb (measured by laser‐ablation split‐stream ICP‐MS) geochronology reveals a protracted metamorphic history of metapelites and garnet amphibolites from a major segment of the orogen. Additionally, direct dating of a common rock‐forming mineral (garnet) and accessory mineral (monazite) allows us to test assumptions that are commonly used when linking accessory mineral geochronology to rock‐forming mineral reactions. Petrography, mineral zoning, thermobarometry and pseudosection modelling reveal a Barrovian‐style prograde path, reaching temperatures of ~610–680 °C. Despite near‐complete diffusional resetting of garnet major element zoning, the garnet retains strong rare earth element zoning and preserves Lu–Hf dates that range from c. 616–572 Ma. Conversely, monazite in the rocks was extensively recrystallized, with concordant dates that span from c. 610–500 Ma, and retain only vestigial cores. Monazite cores yield dates that overlap with the garnet Lu–Hf dates and typically have low‐Y and heavy rare earth element (HREE) concentrations, corroborating interpretations of low‐Y and low‐HREE monazite domains as records of synchronous garnet growth. However, ratios of REE concentrations in garnet and monazite do not consistently match previously reported partition coefficients for the REE between these two minerals. High‐Y monazite inclusions within pristine, crack‐free garnet yield U–Pb dates significantly younger than the Lu–Hf dates for the same samples, indicating recrystallization of monazite within garnet. The recrystallization of high‐Y and high‐HREE monazite domains over >50 Ma likely records either punctuated thermal pulses or prolonged residence at relatively high temperatures (up to ~610–680 °C) driving monazite recrystallization. One c. 616 Ma garnet Lu–Hf date and several c. 610–600 Ma monazite U–Pb dates are tentatively interpreted as records of the onset of tectonism metamorphism in the Ross orogeny, with a more robust constraint from the other Lu–Hf dates (c. 588–572 Ma) and numerous c. 590–570 Ma monazite U–Pb dates. The data are consistent with a tectonic model that involves shortening and thickening prior to widespread magmatism in the vicinity of the study area. The early tectonic history of the Ross orogen, recorded in metamorphic rocks, was broadly synchronous with Gondwana‐wide collisional Pan‐African orogenies.     

Evidence for protracted prograde metamorphism followed by rapid exhumation of the Zermatt‐Saas Fee ophiolite

Major and trace‐element zoning in garnet, in combination with Rb–Sr, Sm–Nd and Lu–Hf geochronology, provide evidence for a protracted garnet growth history for the Zermatt‐Saas Fee (ZSF) ophiolite, western Alps. Four new Lu–Hf ages from Pfulwe (c. 52–46 Ma) and one from Chamois (c. 52 Ma) are very similar to a previously published Lu–Hf age from Lago di Cignana. Overall, the similarity of geochronological and garnet zoning patterns suggests that these three localities had a similar prograde tectonic history, commensurate with their similar structural position near the top of the ZSF. Samples from the lower part of the ZSF at Saas Fee and St. Jacques, however, produced much younger Lu–Hf ages (c. 41–38 Ma). Neither differences in whole‐rock geochemistry, which might produce distinct garnet growth histories, nor rare‐earth‐element zoning in garnet, can account for the age differences in the two suites. This suggests a much later prograde history for the lower part of the ZSF, supporting the idea that it was subducted diachronously. Such a model is consistent with changes in subduction vectors based on plate tectonic reconstructions, where early oblique subduction, which produced long prograde garnet growth, changed to more orthogonal subduction, which corresponds to shorter prograde garnet growth. Six new Rb–Sr phengite ages range from c. 42 to 39 Ma and, in combination with previously published Rb–Sr ages, constrain the timing of the transition from eclogite to upper greenschist facies P–T conditions. The proximity of the ZSF in the Saas Fee region to the underlying continental Monte Rosa unit and the similarity of peak‐metamorphic ages suggest these two units were linked for part of their tectonic history. This in turn indicates that the Monte Rosa may have been partly responsible for rapid exhumation of the ZSF unit.     

P–T–t–D paths of Everest Series schist, Nepal

U(–Th)–Pb geochronology, geothermobarometric estimates and macro‐ and micro‐structural analysis, quantify the pressure–temperature–time–deformation (P–T–t–D) history of Everest Series schist and calcsilicate preserved in the highest structural levels of the Everest region. Pristine staurolite schist from the Everest Series contains garnet with prograde compositional zoning and yields a P–T estimate of 649 ± 21 ° C, 6.2 ± 0.7 kbar. Other samples of the Everest Series contain garnet with prograde zoning and staurolite with cordierite overgrowths that yield a P–T estimate of 607 ± 25 ° C, 2.9 ± 0.6 kbar. The Lhotse detachment (LD) marks the base of the Everest Series. Structurally beneath the LD, within the Greater Himalayan Sequence (GHS), garnet zoning is homogenized, contains resorption rinds and yields peak temperature estimates of ～650 ± 50 ° C. P–T estimates record a decrease in pressure from ～6 to 3 kbar and equivalent temperatures from structurally higher positions in the overlying Everest Series, through the LD and into GHS. This transition is interpreted to result from the juxtaposition of the Everest Series in the hangingwall with the GHS footwall rocks during southward extrusion and decompression along the LD system. An age constraint for movement on the LD is provided by the crystallization age of the Nuptse granite (23.6 ± 0.7 Ma), a body that was emplaced syn‐ to post‐solid‐state fabric development. Microstructural evidence suggests that deformation in the LD progressed from a distributed ductile shear zone into the structurally higher Qomolangma detachment during the final stages of exhumation. When combined with existing geochronological, thermobarometric and structural data from the GHS and Main Central thrust zone, these results form the basis for a more complete model for the P–T–t–D evolution of rocks exposed in the Mount Everest region.     

The timing of migmatization in the northern Arabian–Nubian Shield: Evidence for a juvenile sedimentary component in collision‐related batholiths

Collision‐related granitoid batholiths, like those of the Hercynian and Himalayan orogens, are mostly fed by magma derived from metasedimentary sources. However, in the late Neoproterozoic calcalkaline (CA) batholiths of the Arabian–Nubian Shield (ANS), which constitutes the northern half of the East African orogen, any sedimentary contribution is obscured by the juvenile character of the crust and the scarcity of migmatites. Here, we use paired in situ LASS‐ICP‐MS measurements of U–Th–Pb isotope ratios and REE contents of monazite and xenotime and SHRIMP‐RG analyses of separated zircon to demonstrate direct linkage between migmatites and granites in the northernmost ANS. Our results indicate a single prolonged period of monazite growth at 640–600 Ma, in metapelites, migmatites and peraluminous granites of three metamorphic suites: Abu‐Barqa (SW Jordan), Roded (S Israel) and Taba–Nuweiba (Sinai, Egypt). The distribution of monazite dates and age zoning in single monazite grains in migmatites suggest that peak thermal conditions, involving partial melting, prevailed for c. 10 Ma, from 620 to 610 Ma. REE abundances in monazite are well correlated with age, recording garnet growth and garnet breakdown in association with the prograde and retrograde stages of the melting reactions, respectively. Xenotime dates cluster at 600–580 Ma, recording retrogression to greenschist facies conditions as garnet continued to destabilize. Phase equilibrium modelling and mineral thermobarometry yield P–T conditions of ~650–680°C and 5–7 kbar, consistent with either water‐fluxed or muscovite‐breakdown melting. The expected melt production is 8–10 vol.%, allowing a melt connectivity network to form leading to melt segregation and extraction. U–Pb ages of zircon rims from leucosomes indicate crystallization of melt at 610 ± 10 Ma, coinciding with the emplacement of a vast volume of CA granites throughout the northern ANS, which were previously considered post‐collisional. Similar monazite ages (c. 620 Ma) retrieved from the amphibolite facies Elat schist indicate that migmatites are the result of widespread regional rather than local contact metamorphism, representing the climax of the East African orogenesis.     

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

The 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‐P_max 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.     

Carboniferous eclogite and garnet–omphacite granulite from northeastern Hainan Island,South China: Implications for the evolution of the eastern Palaeo‐Tethys

High‐P (HP) eclogite and associated garnet–omphacite granulite have recently been discovered in the Mulantou area, northeastern Hainan Island, South China. These rocks consist mainly of garnet, omphacite, hornblende, quartz and rutile/ilmenite, with or without zoisite and plagioclase. Textural relationships, mineral compositions and thermobarometric calculations demonstrate that the eclogite and garnet–omphacite granulite share the same three‐stage metamorphic evolution, with prograde, peak and retrograde P?T conditions of 620–680°C and 8.7–11.1 kbar, 820–860°C and 17.0–18.2 kbar, and 700–730°C and 7.1–8.5 kbar respectively. Sensitive high‐resolution ion microprobe U–Pb zircon dating, coupled with the identification of mineral inclusions in zircon, reveals the formation of mafic protoliths before 355 Ma, prograde metamorphism at c. 340–330 Ma, peak to retrograde metamorphism at c. 310–300 Ma, and subsequent pegmatite intrusion at 295 Ma. Trace element geochemistry shows that most of the rocks have a MORB affinity, with initial ε_Nd values of +2.4 to +6.7. As with similar transitional eclogite–HP granulite facies rocks in the thickened root in the European Variscan orogen, the occurrence of relatively high P?T metamorphic rocks of oceanic origin in northeastern Hainan Island suggests Carboniferous oceanic subduction leading to collision of the Hainan continental block, or at least part of it, with the South China Block in the eastern Palaeo‐Tethyan tectonic domain.     

Anticlockwise pressure–temperature paths record Variscan upper‐plate exhumation: Example from micaschists of the Porto Vecchio region,Corsica

To better understand the evolution of deep‐seated crust of the Variscan orogen in the Sardinia‐Corsica region, we studied garnet‐bearing micaschists which were sampled 3 km east and 15 km northeast of Porto Vecchio, south‐eastern Corsica. After a careful investigation of the textural relations and compositions of minerals, especially of zoned garnet, a P–T path was reconstructed using contoured P–T pseudosections. U–Th–Pb dating of monazite in the micaschists was undertaken with the electron microprobe. The micaschists from both localities were formed along similar anticlockwise P–T paths. The prograde branch of these paths starts at 3 kbar close to 600°C in the P–T field of sillimanite and reaches peak conditions at 7 kbar and 600 (15 km NE of Porto Vecchio) to 630°C (3 km E of Porto Vecchio). The metamorphism at peak P–T conditions happened c. 340 Ma based on low‐Y (<0.65 wt% Y₂O₃) monazite. Ages of monazite with high‐Y contents (>2 wt% Y₂O₃), which probably have formed before garnet, scatter around 362 Ma. The retrograde branch of the P–T paths passes through 4 kbar at ~550°C. We conclude that the micaschists belong to a common metasedimentary sequence, which extends over the Porto Vecchio region and is separated from other metamorphic rock sequences in the north and the south by major tectonic boundaries. This sequence had experienced peak pressures which are lower than those determined for metamorphic rocks, such as micaschist and gneiss, from north‐eastern Sardinia. At present, we favour a continent–continent collisional scenario with the studied metasedimentary sequence buried during the collisional event as part of the upper plate. The contemporaneous high‐P metamorphic rocks from NE Sardinia were part of the upper portion of the lower plate. The addressed rocks from both plates were exhumed in an exhumation channel.     

Timing of metamorphism,melting and exhumation of the Leo Pargil dome,northwest India

The Leo Pargil dome, northwest India, is a 30 km‐wide, northeast‐trending structure that is cored by gneiss and mantled by amphibolite facies metamorphic rocks that are intruded by a leucogranite injection complex. Oppositely dipping, normal‐sense shear zones that accommodated orogen‐parallel extension within a convergent orogen bound the dome. The broadly distributed Leo Pargil shear zone defines the southwest flank of the dome and separates the dome from the metasedimentary and sedimentary rocks in the hanging wall to the west and south. Thermobarometry and in‐situ U–Th–Pb monazite geochronology were conducted on metamorphic rocks from within the dome and in the hanging wall. These data were combined with U–Th–Pb monazite geochronology of leucogranites from the injection complex to evaluate the relationship between metamorphism, crustal melting, and the onset of exhumation. Rocks within the dome and in the hanging wall contain garnet, kyanite, and staurolite porphyroblasts that record prograde Barrovian metamorphism during crustal thickening that reached ～530–630 °C and ～7–8 kbar, ending by c. 30 Ma. Cordierite and sillimanite overgrowths on Barrovian assemblages within the dome record dominantly top‐down‐to‐the‐west shearing during near‐isothermal decompression of the footwall rocks to ～4 kbar by 23 Ma during an exhumation rate of 1.3 mm year^?1. Monazite growth accompanied Barrovian metamorphism and decompression. The leucogranite injection complex within the dome initiated at 23 Ma and continued to 18 Ma. These data show that orogen‐parallel extension in this part of the Himalaya occurred earlier than previously documented (>16 Ma). Contemporaneous onset of near‐isothermal decompression, top‐down‐to‐the‐west shearing, and injection of the decompression‐driven leucogranite complex suggests that early crustal melting may have created a weakened crust that was proceeded by localization of strain and shear zone development. Exhumation along the shear zone accommodated decompression by 23 Ma in a kinematic setting that favoured orogen‐parallel extension.     

Metamorphic patterns and SHRIMP zircon ages of medium‐to‐high grade rocks from the Tongbai orogen,central China: implications for multiple accretion/collision processes prior to terminal continental collision

The Qinling‐Tongbai‐Dabie‐Sulu orogenic belt comprises a Palaeozoic accretion‐dominated system in the north and a Mesozoic collision‐dominated system in the south. A combined petrological and geochronological study of the medium‐to‐high grade metamorphic rocks from the diverse Palaeozoic tectonic units in the Tongbai orogen was undertaken to help elucidate the origins of Triassic ultrahigh‐pressure metamorphism and collision dynamics between the Sino‐Korean and Yangtze cratons. Peak metamorphic conditions are 570–610 °C and 9.3–11.2 kbar for the lower unit of the Kuanping Group, 630–650 °C and 6.6–8.9 kbar for the upper unit of the Kuanping Group, 550–600 °C and 6.3–7.7 kbar for the Erlangping Group, 770–830 °C and 6.9–8.5 kbar for the Qinling Group and 660–720 °C and 9.1–11.5 kbar for the Guishan complex. Reaction textures and garnet compositions indicate clockwise P–T paths for the amphibolite facies rocks of the Kuanping Group and Guishan complex, and an anticlockwise P–T path for the granulite facies rocks of the Qinling Group. Sensitive high‐resolution ion microprobe U–Pb zircon dating on metamorphic rocks and deformed granite/pegmatites revealed two major Palaeozoic tectonometamorphic events. (i) During the Silurian‐Devonian (c. 440–400 Ma), the Qinling continental arc and Erlangping intra‐oceanic arc collided with the Sino‐Korean craton. The emplacement of the Huanggang diorite complex resulted in an inverted thermal gradient in the underlying Kuanping Group and subsequent thermal relaxation during the exhumation. Meanwhile, the oceanic subduction beneath the Qinling continental arc produced magmatic underplating and intrusion, leading to granulite facies metamorphism followed by a near‐isobaric cooling path. (ii) During the Carboniferous (c. 340–310 Ma), the northward subduction of the Palaeo‐Tethyan ocean generated a medium P/T Guishan complex in the hangingwall and a high P/T Xiongdian eclogite belt in the footwall. The Guishan complex and Xiongdian eclogite belt are therefore considered to be paired metamorphic belts. Subsequent separation of the paired belts is inferred to be related to the juxtaposition of the Carboniferous eclogites with the Triassic HP metamorphic complex during continental subduction and exhumation.     

Metamorphic petrology of a high‐T/low‐P granulite terrane (Damara belt,Namibia) – Constraints from pseudosection modelling and high‐precision Lu–Hf garnet‐whole rock dating

Migmatites comprise a minor volume of the high‐grade part of the Damara orogen of Namibia that is dominated by granite complexes and intercalated metasedimentary units. Migmatites of the Southern Central Zone of the Damara orogen consist of melanosomes with garnet+cordierite+biotite+K‐feldspar, and leucosomes, which are sometimes garnet‐ and cordierite‐bearing. Field evidence, petrographic observations, and pseudosection modelling suggest that, in contrast to other areas where intrusion of granitic magmas is more important, in situ partial melting of metasedimentary units was the main migmatite generation processes. Pseudosection modelling and thermobarometric calculations consistently indicate that the peak‐metamorphic grade throughout the area is in the granulite facies (~5 kbar at ~800°C). Cordierite coronas around garnet suggest some decompression from peak‐metamorphic conditions and rare andalusite records late, near‐isobaric cooling to <650°C at low pressures of ~3 kbar. The inferred clockwise P–T path is consistent with minor crustal thickening through continent–continent collision followed by limited post‐collisional exhumation and suggests that the granulite facies terrane of the Southern Central Zone of the Damara orogen formed initially in a metamorphic field gradient of ~35–40°C/km at medium pressures. New high‐precision Lu–Hf garnet‐whole rock dates are 530 ± 13 Ma, 522.0 ± 0.8 Ma, 520.8 ± 3.6 Ma, and 500.3 ± 4.3 Ma for the migmatites that record temperatures of ~800°C. This indicates that high‐grade metamorphism lasted for c. 20–30 Ma, which is compatible with previous estimates using Sm–Nd garnet‐whole rock systematics. In previous studies on Damara orogen migmatites where both Sm–Nd and Lu–Hf chronometers have been applied, the dates (c. 520–510 Ma) agree within their small uncertainties (0.6–0.8% for Sm–Nd and 0.1–0.2% for Lu–Hf). This implies rapid cooling after high‐grade conditions and, by implication, rapid exhumation at that time. The cause of the high geothermal gradient inferred from the metamorphic conditions is unknown but likely requires some extra heat that was probably added by intrusion of magmas from the lithospheric mantle, i.e., syenites that have been recently re‐dated at c. 545 Ma. Some granites derived from the lower crust at c. 545 Ma are the outcome rather than the cause of high‐T metamorphism. In addition, high contents of heat‐producing elements K, Th, and U may have raised peak temperatures by 150–200°C at the base of the crust, resulting in the widespread melting of fertile crustal rocks. The continuous gradation from centimetre‐scale leucosomes to decametre‐scale leucogranite sheets within the high‐grade metamorphic zone suggests that leucosome lenses coalesced to form larger bodies of anatectic leucogranites, thereby documenting a link between high‐grade regional metamorphism and Pan‐African magmatism. In view of the close association of the studied high‐T migmatites with hundreds of synmetamorphic high‐T granites that invaded the terrane as metre‐ to decametre‐wide sills and dykes, we postulate that crystallization of felsic lower crustal magma is, at least partly, responsible for heat supply. Late‐stage isobaric cooling of these granites may explain the occurrence of andalusite in some samples.     

The initial garnet‐in reaction involving siderite–rhodochrosite,garnet re‐equilibration and P–T–t paths of graphitic schists in the Black Hills orogen,South Dakota,USA

It is generally thought that garnet in metapelites is produced by continuous reactions involving chlorite or chloritoid. Recent publications have suggested that the equilibrium temperatures of garnet‐in reactions may be significantly overstepped in regionally metamorphosed terranes. The growth of small spessartine–almandine garnet crystals on Mn‐siderite at the garnet isograd in graphitic metapelites in the Proterozoic Black Hills orogen, South Dakota, demonstrates that Mn‐siderite was the principal reactant that produced the initial garnet in the schists. Moreover, the positions of garnet compositions in isobaric, T–(C/H) pseudosections for the schists show that the temperature of the garnet‐in reaction from Mn‐siderite was overstepped minimally at the most. In the Black Hills, garnet was initially produced during regional metamorphism beginning at c. 1755 Ma due to the collision of Wyoming and Superior cratons, and was subsequently partially or fully re‐equilibrated at more elevated temperatures and pressures during intrusion of the Harney Peak Granite (HPG) at c. 1715 Ma. Garnet occurs in graphitic schists in garnet, staurolite and sillimanite zones, the latter being a product of contact metamorphism by HPG. During metamorphism, coexisting fluid contained both CO₂ and CH₄. In the garnet zone, garnet crystals contain petrographically distinct cores with inclusions of quartz, graphite and other minerals. Centres of the cores have distinctly elevated Y concentrations that mark the positions of garnet nucleation. The elevated Y is thought to have come from the Mn‐siderite onto which Y was probably absorbed during precipitation in an ocean. In the upper garnet and staurolite zones, the cores were overgrown by inclusion‐poor mantles. Mantles are highly zoned and have more elevated Fe and Mg and lower Mn and Ca than cores. The growth of mantles is attributed to late‐orogenic heating by leucogranite magmas and attendant influx of H₂O that caused consumption of graphite in rock matrices. A portion of the Proterozoic terrane that includes the HPG is surrounded by four large faults. In this ‘HPG block’, garnet is inclusion‐poor and its composition does not preserve its early growth history. This garnet appears to have re‐equilibrated by internal diffusion of its major components and/or recrystallization of an earlier inclusion‐rich garnet. It has equilibrated within the kyanite stability range, and together with remnant kyanite in the high‐strain aureole of the HPG, indicates that the HPG block had a ≥6 kbar history. The HPG block has undergone decompression during emplacement of the HPG. The decompression is evident in occurrences of retrograde andalusite and cordierite in the thermal aureole of the HPG. The data support a polybaric metamorphic history of the Black Hills orogen with different segments of the orogen having their own clockwise P–T–t paths.     

Phase equilibria modelling and LASS monazite petrochronology: P–T–t constraints on the evolution of the Priest River core complex,northern Idaho

Phase equilibria modelling, laser‐ablation split‐stream (LASS)‐ICP‐MS petrochronology and garnet trace‐element geochemistry are integrated to constrain the P–T–t history of the footwall of the Priest River metamorphic core complex, northern Idaho. Metapelitic, migmatitic gneisses of the Hauser Lake Gneiss contain the peak assemblage garnet + sillimanite + biotite ± muscovite + plagioclase + K‐feldspar ± rutile ± ilmenite + quartz. Interpreted P–T paths predict maximum pressures and peak metamorphic temperatures of ~9.6–10.3 kbar and ~785–790 °C. Monazite and xenotime ²⁰⁸Pb/²³²Th dates from porphyroblast inclusions indicate that metamorphism occurred at c. 74–54 Ma. Dates from HREE‐depleted monazite formed during prograde growth constrain peak metamorphism at c. 64 Ma near the centre of the complex, while dates from HREE‐enriched monazite constrain the timing of garnet breakdown during near‐isothermal decompression at c. 60–57 Ma. Near‐isothermal decompression to ~5.0–4.4 kbar was followed by cooling and further decompression. The youngest, HREE‐enriched monazite records leucosome crystallization at mid‐crustal levels c. 54–44 Ma. The northernmost sample records regional metamorphism during the emplacement of the Selkirk igneous complex (c. 94–81 Ma), Cretaceous–Tertiary metamorphism and limited Eocene exhumation. Similarities between the Priest River complex and other complexes of the northern North American Cordillera suggest shared regional metamorphic and exhumation histories; however, in contrast to complexes to the north, the Priest River contains less partial melt and no evidence for diapiric exhumation. Improved constraints on metamorphism, deformation, anatexis and exhumation provide greater insight into the initiation and evolution of metamorphic core complexes in the northern Cordillera, and in similar tectonic settings elsewhere.     

Early Carboniferous HP metamorphism in the Hida Gaien Belt,Japan: Implications for the Palaeozoic tectonic history of proto‐Japan

We report two new eclogite localities (at Kanayamadani and Shinadani) in the high‐P (HP) metamorphic rocks of the Omi area in the western most region of Niigata Prefecture, Japan, which form part of the Hida Gaien Belt, and determine metamorphic conditions and pressure–temperature (P–T) paths. The metamorphic evolution of the eclogites is characterized by a tight hairpin‐shaped P–T path from prograde epidote–blueschist facies to peak eclogite facies and then retrograde blueschist facies. The prograde metamorphic stage is characterized by various amphibole (winchite, barroisite, glaucophane) inclusions in garnet, whereas the peak eclogite facies assemblage is characterized by omphacite, garnet, phengite and rutile. Peak P–T conditions of the eclogites were estimated to be ~600°C and up to 2.0 GPa by conventional cation‐exchange thermobarometry, Ti‐in‐zircon thermometry and quartz inclusion Raman barometry respectively. However, the Raman spectra of carbonaceous material thermometry of metapelites associated with the eclogites gave lower peak temperatures, possibly due to metamorphism at different conditions before being brought together during exhumation. The blueschist facies overprint following the peak of metamorphism is recognized by the abundance of glaucophane in the matrix. Zircon grains in blueschist facies metasedimentary samples from two localities adjacent to the eclogites have distinct oscillatory‐zoned cores and overgrowth rims. Laser ablation inductively coupled plasma mass spectrometry U–Pb ages of the detrital cores yield a wide range between 3,200 and 400 Ma, with a peak at 600–400 Ma. In the early Palaeozoic, proto‐Japan was located along the continental margin of the South China craton, providing the source of the older population of detrital zircon grains (3,200–600 Ma) deposited in the trench‐fill sediments. In addition, subduction‐related magmatism c. 500–400 Ma is recorded in the crust below proto‐Japan, which might have been the source for the younger detrital zircon grains. The peak metamorphic age was constrained by SHRIMP dating of the overgrowth rims, yielding Tournaisian ages of 347 ± 4 Ma, suggesting subduction in the early Carboniferous. Our results provide clear constraints on the initiation of subduction, accretion and the development of an arc‐trench system along the active continental margin of the South China craton and help to unravel the Palaeozoic tectonic history of proto‐Japan.     

Integrated garnet and zircon–titanite geochronology constrains the evolution of ultra‐high–pressure terranes: An example from the Sulu orogen

Dating ultra‐high–pressure (UHP) metamorphic rocks provides important timing constraints on deep subduction zone processes. Eclogites, deeply subducted rocks now exposed at the surface, undergo a wide range of metamorphic conditions (i.e. deep subduction and exhumation) and their mineralogy can preserve a detailed record of chronologic information of these dynamic processes. Here, we present an approach that integrates multiple radiogenic isotope systems in the same sample to provide a more complete timeline for the subduction–collision–exhumation processes, based on eclogites from the Dabie–Sulu orogenic belt in eastern China, one of the largest UHP terranes on Earth. In this study, we integrate garnet Lu–Hf and Sm–Nd ages with zircon and titanite U–Pb ages for three eclogite samples from the Sulu UHP terrane. We combine this age information with Zr‐in‐rutile temperature estimates, and relate these multiple chronometers to different P–T conditions. Two types of rutile, one present as inclusions in garnet and the other in the matrix, record the temperatures of UHP conditions and a hotter stage, subsequent to the peak pressure (‘hot exhumation') respectively. Garnet Lu–Hf ages (c. 238–235 Ma) record the initial prograde growth of garnet, while coupled Sm–Nd ages (c. 219–213 Ma) reflect cooling following hot exhumation. The maximum duration of UHP conditions is constrained by the age difference of these two systems in garnet (c. 235–220 Ma). Complementary zircon and titanite U–Pb ages of c. 235–230 Ma and c. 216–206 Ma provide further constraints on the timing of prograde metamorphism and the ‘cold exhumation' respectively. We demonstrate that timing of various metamorphic stages can thus be determined by employing complementary chronometers from the same samples. These age results, combined with published data from adjacent areas, show lateral diachroneity in the Dabie–Sulu orogeny. Three sub‐blocks are thus defined by progressively younger garnet ages: western Dabie (243–238 Ma), eastern Dabie–northern Sulu (238–235 Ma) and southern Sulu terranes (225–220 Ma), which possibly correlate to different crustal slices in the recently proposed subduction channel model. These observed lateral chronologic variations in a large UHP terrane can possibly be extended to other suture zones.     

Linking thermodynamic modelling,Lu–Hf geochronology and trace elements in garnet: new P–T–t paths from the Sevier hinterland

Major element, trace element and Lu–Hf geochronological data from amphibolite facies pelitic schist in the Raft River and Albion Mountains of northwest Utah and southern Idaho indicate that garnet grew during increasing pressure, interpreted to be the result of tectonic burial and crustal thickening during Sevier orogenesis. Garnet growth was interrupted by hiatuses interpreted from discontinuities in major element zonation. Pressure–temperature paths were determined from the pre‐hiatus portions of the garnet chemical zoning profiles and indicate an increase of ~2 kbar and ~50 °C in the western Raft River Mountains. Garnet Lu–Hf dates of 150 ± 1 Ma in the western Raft River Mountains and 138.7 ± 0.7 Ma and 132 ± 5 Ma in the southern Albion Mountains indicate the timing of garnet growth. Lutetium garnet zoning profiles indicate that the Lu–Hf ages are biased towards the post‐hiatus or outer pre‐hiatus segments, indicating that the determined ages likely post‐date the recorded P–T path history or date the tail end of the paths. Crustal thickening associated with Sevier orogenesis in the western Raft River Mountains thus began slightly before 150 ± 1 Ma, in the Late Jurassic. This study shows that integrating P–T paths determined from garnet growth zoning with Lu–Hf garnet geochronology and in situ garnet trace element analyses is an effective approach for interpreting and dating deformation events in orogenic belts.     

Synchronous peak Barrovian metamorphism driven by syn-orogenic magmatism and fluid flow in southern Connecticut, USA

Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T–t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.     

Petrology and zircon U–Pb dating of well‐preserved eclogites from the Thongmön area in central Himalaya and their tectonic implications

The discovery of eclogites is reported within the Great Himalayan Crystalline Complex in the Thongmön area, central Himalaya, and their metamorphic evolution is deciphered by petrographic studies, pseudosection modelling, and zircon dating. For the first time, omphacite has been found in the matrix of eclogites taken from a metamorphic mafic lens. Two groups of garnet have been identified in the Thongmön eclogites on the basis of major and rare earth elements and mineral inclusions. Core and intermediate sections of garnet represent Grt I, in which the major elements (Ca, Mg, and Fe) show a nearly homogenous distribution with little or weak zonation. This Grt I displays an almost flat chondrite‐normalized HREE pattern, and the main inclusions are amphibole, apatite, quartz, and abundant omphacite. Grt II, forms thin rims on large garnet grains, and is characterized by rim‐ward Ca decrease and Mg increase and MREE enrichment relative to HREE and LREE. No amphibole inclusions are found in Grt II, indicating the decomposition of amphibole contributed to its MREE enrichment. Two metamorphic stages, recorded by matrix minerals and inclusions in garnet and zircon, outline the burial of the Thongmön eclogites and progressive metamorphic processes to the pressure peak: (a) the assemblage of amphibole–garnet–omphacite–phengite–rutile–quartz, with the phengite interpreted as having been replaced by Bt+Pl symplectites, represents the prograde amphibole eclogite facies stage M1₍₁₎, (b) in the peak eclogite facies [stage M1₍₂₎], amphibole was lost and melting started. Based on the compositions of garnet and omphacite inclusions, M1₍₁₎ is constrained to 19–20 kbar and 640–660°C and M1₍₂₎ occurred at >21 kbar, >750°C, with appearance of melt and its entrapment in metamorphic zircon. SHRIMP U–Pb dating of zircon from two eclogite samples yielded consistent metamorphic ages of 16.7 ± 0.6 Ma and 17.1 ± 0.4 Ma respectively. The metamorphic zircon grew concurrently with Grt II in the peak eclogite facies. Thongmön eclogites characterized by the prograde metamorphism from amphibolite facies to eclogite facies were formed by the continuing continental subduction of Indian plate beneath the Euro‐Asian continent in the Miocene.