Pressure Temperature Time Research Articles

Syn-collapse eclogite metamorphism and exhumation of deep crust in a migmatite dome: The P–T–t record of the youngest Variscan eclogite (Montagne Noire, French Massif Central)

In many orogens, high-pressure (HP) metamorphic rocks such as eclogite occur as lenses in quartzofeldspathic gneiss that equilibrated at much lower pressures. The pressure–temperature–time (P–T–t) history of eclogite relative to host gneiss provides information about mechanisms and timescales of exhumation of orogenic crust. The Montagne Noire of the southern Massif Central, France, is an eclogite–bearing gneiss (migmatite) dome located at the orogen-foreland transition of the Variscan belt. Results of our study show that it contains the youngest eclogite in the orogen, similar in age to migmatite and granite that crystallized under low-pressure conditions. P–T conditions for an exceptionally unaltered eclogite from the central Montagne Noire were estimated using a pseudosection supplemented by garnet–clinopyroxene and Zr-in-rutile thermometry. Results indicate peak P∼1.4 GPa and T∼725°C for Mg-rich garnet rim (50 mol% pyrope) + omphacite (36 mol% jadeite) + rutile + quartz. U–Pb geochronology (LA-ICP-MS) of 16 zoned zircon grains yielded ∼360 Ma (4 cores) and ∼315 Ma (12 rims and cores). Rare earth element abundances determined by LA-ICP-MS for dated zircon are consistent with crystallization of ∼315 Ma zircon under garnet-stable, plagioclase-unstable conditions that we interpret to indicate high pressure; in contrast, the ∼360 Ma zircon core corresponds to crystallization under lower pressure plagioclase-stable conditions. Based on garnet zoning and inclusion suites, rutile textures and Zr zoning, P–T results, and zircon petrochronology, we interpret the ∼315 Ma date as the age of eclogite-facies metamorphism that only slightly preceded dome formation and crystallization at 315–300 Ma. This age relation indicates that eclogite formation at high pressure and migmatite dome emplacement at low pressure were closely spaced in time. We propose that collapse-driven material transfer from the hot orogen to the cool foreland resulted in thickening of the orogen edge, leading to eclogite facies metamorphism of the deep crust. Soon after, the low-viscosity partially molten crust flowed from the plateau toward the foreland, incorporating and exhuming eclogite. The P–T–t history of the Montagne Noire eclogite shows that some dome material ascended from >40 km depth to shallow crustal levels, likely in a single decompression event, and that migmatite domes are therefore very efficient at exhuming the deep crust.

The role of radiogenic heat in prolonged intraplate reworking: The Capricorn Orogen explained?

Links between intraplate crustal deformation and the thermal regime of the lithosphere have been investigated using numerical models, but these conceptual models must be tested using well-constrained natural systems. The Proterozoic Capricorn Orogen of Western Australia is ideally suited for such a study; after arc accretion, it records a protracted history of intracrustal differentiation and over one billion years of subsequent tectonic reworking. The early addition of juvenile arc material initially reduced the heat production of the crust, whereas subsequent recycling of the deep crust ultimately produced radiogenic granites in the shallow crust. Using whole rock and magmatic zircon compositions, we show that the shallow crust of the Capricorn Orogen has been progressively enriched in heat-producing elements with time. The high heat production is mainly controlled by thorium, concentrated in LREE-enriched phosphates that grew during the late stages of crystallization of crustally-derived granites. Using a numerical model, we investigate the thermal effects of this evolved crustal stratification and compare them with quantitative data on the pressure–temperature–time evolution of a non-magmatic Mesoproterozoic reworking event. The results provide new information concerning the process by which the thermal conditions necessary for the reworking of orogenic crust are generated and sustained for prolonged timescales (>110 Ma). Furthermore, we demonstrate that perturbations to the radiogenic crust lead to warmer geotherms and consequent lithospheric weakening, which has a fundamental control on intraplate reworking of long-lived orogens.

LASS U–Th–Pb monazite and rutile geochronology of felsic high-pressure granulites (Rhodope, N Greece): Effects of fluid, deformation and metamorphic reactions in local subsystems

The specific chemical composition of monazite in shear zones is controlled by the syndeformation dissolution–precipitation reactions of the rock-forming minerals. This relation can be used for dating deformation, even when microfabric characteristics like shape preferred orientation or intracrystalline deformation of monazite itself are missing. Monazite contemporaneously formed in and around the shear zones may have different compositions. These depend on the local chemical context rather than reflecting successive crystallization episodes of monazite.This is demonstrated in polymetamorphic, mylonitic high-pressure (HP) garnet–kyanite granulites of the Alpine Sidironero Complex (Rhodope UHP terrain, Northern Greece). The studied mylonitic rocks escaped from regional migmatization at 40–36Ma and from subsequent shearing through cooling until 36Ma. In-situ laser-ablation split-stream inductively-coupled plasma mass spectrometry (LASS) analyses have been carried out on monazite from micro-scale shear zones, from pre-mylonitic microlithons as well as of monazite inclusions in relictic minerals complimented by U–Pb data on rutile and Rb–Sr data of biotite.Two major metamorphic episodes, Mesozoic and Cenozoic, are constrained. Chemical compositions, isotopic characteristics and apparent ages systematically vary among monazite of four different microfabric domains (I–IV). Within three pre-mylonitic domains (inclusions in (I) pre-mylonitic kyanite and (II) garnet porphyroclasts, and (III) in pre-mylonitic microlithons) monazite yields ages of ca. 130–150Ma for HP-granulite metamorphism, in line with previous geochronological results in the area. Patchy alteration of the pre-mylonitic monazite by intra-grain dissolution–precipitation processes variably increased negative Eu anomaly and reduced the HREE contents. The apparent age of this altered monazite is reduced.Monazite in the syn-mylonitic shear bands (IV) differs in chemical composition from unaltered and altered monazite of the three pre-mylonitic domains by having a significantly more pronounced negative Eu anomaly, a flatter HREE pattern, and high Th content. These compositional characteristics are linked with syn-mylonitic formation of plagioclase and resorption of garnet in the shear bands under amphibolite facies conditions. The absence of pre-mylonitic monazite in the shear zones, in contrast to the other domains, suggests complete dissolution of old and formation of new monazite. This probably results from an increased alkalinity and reactivity of the fluid that again is controlled by syn-mylonitic interaction with feldspar and apatite in the shear zones. There, the deformation was accommodated by dissolution precipitation creep at ca. 690±50°C and 6–7.5kbar. Growth of monazite at 55±1Ma dates this deformation, which precedes the regional migmatization of the Sidironero Complex, whereas rutile and biotite ages reflect these later stages. This new pressure–temperature–time constraint for a relictic deformation structure provides insight into the still missing parts of the overall metamorphic, deformation and exhumation processes of the UHP units in the Rhodope.

Pressure–temperature–time–deformation path of kyanite-bearing migmatitic paragneiss in the Kali Gandaki valley (Central Nepal): Investigation of Late Eocene–Early Oligocene melting processes

Kyanite-bearing migmatitic paragneiss of the lower Greater Himalayan Sequence (GHS) in the Kali Gandaki transect (Central Himalaya) was investigated. In spite of the intense shearing, it was still possible to obtain many fundamental information for understanding the processes active during orogenesis. Using a multidisciplinary approach, including careful meso- and microstructural observations, pseudosection modelling (with PERPLE_X), trace element thermobarometry and in situ monazite U–Th–Pb geochronology, we constrained the pressure–temperature–time–deformation path of the studied rock, located in a structural key position. The migmatitic gneiss has experienced protracted prograde metamorphism after the India–Asia collision (50–55Ma) from ~43Ma to 28Ma. During the late phase (36–28Ma) of this metamorphism, the gneiss underwent high-pressure melting at “near peak” conditions (710–720°C/1.0–1.1GPa) leading to kyanite-bearing leucosome formation. In the time span of 25–18Ma, the rock experienced decompression and cooling associated with pervasive shearing reaching P–T conditions of 650–670°C and 0.7–0.8GPa, near the sillimanite–kyanite transition. This time span is somewhat older than previously reported for this event in the study area. During this stage, additional, but very little melt was produced. Taking the migmatitic gneiss as representative of the GHS, these data demonstrate that this unit underwent crustal melting at about 1GPa in the Eocene–Early Oligocene, well before the widely accepted Miocene decompressional melting related to its extrusion. In general, kyanite-bearing migmatite, as reported here, could be linked to the production of the high-Ca granitic melts found along the Himalayan belt.

Behaviour of geochronometers and timing of metamorphic reactions during deformation at lower crustal conditions: phase equilibrium modelling and U–Pb dating of zircon, monazite, rutile and titanite from the Kalak Nappe Complex, northern Norway

AbstractThis study investigates the behaviour of the geochronometers zircon, monazite, rutile and titanite in polyphase lower crustal rocks of the Kalak Nappe Complex, northern Norway. A pressure–temperature–time–deformation path is constructed by combining microstructural observations with P–T conditions derived from phase equilibrium modelling and U–Pb dating. The following tectonometamorphic evolution is deduced: A subvertical S1 fabric formed at ~730–775 °C and ~6.3–9.8 kbar, above the wet solidus in the sillimanite and kyanite stability fields. The event is dated at 702 ± 5 Ma by high‐U zircon in a leucosome. Monazite grains that grew in the S1 fabric show surprisingly little variation in chemical composition compared to a large spread in (concordant) U–Pb dates from c. 800 to 600 Ma. This age spread could either represent protracted growth of monazite during high‐grade metamorphism, or represent partially reset ages due to high‐T diffusion. Both cases imply that elevated temperatures of >600 °C persisted for over c. 200 Ma, indicating relatively static conditions at lower crustal levels for most of the Neoproterozoic. The S1 fabric was overprinted by a subhorizontal S2 fabric, which formed at ~600–660 °C and ~10–12 kbar. Rutile that originally grew during the S1‐forming event lost its Zr‐in‐rutile and U–Pb signatures during the S2‐forming event. It records Zr‐in‐rutile temperatures of 550–660 °C and Caledonian ages of 440–420 Ma. Titanite grew at the expense of rutile at slightly lower temperatures of ~550 °C during ongoing S2 deformation; U–Pb ages of c. 440–430 Ma date its crystallization, giving a minimum estimate for the age of Caledonian metamorphism and the duration of Caledonian shearing. This study shows that (i) monazite can have a large spread in U–Pb dates despite a homogeneous composition; (ii) rutile may lose its Zr‐in‐rutile and U–Pb signature during an amphibolite facies overprint; and (iii) titanite may record crystallization ages during retrograde shearing. Therefore, in order to correctly interpret U–Pb ages from different geochronometers in a polyphase deformation and reaction history, they are ideally combined with microstructural observations and phase equilibrium modelling to derive a complete P–T–t–d path.

Petrological and geochronological constraints on lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton

AbstractNew petrological and geochronological data are presented on high‐grade ortho‐ and paragneisses from northwestern Ghana, forming part of the Paleoproterozoic (2.25–2.00 Ga) West African Craton. The study area is located in the interference zone between N–S and NE–SW‐trending craton‐scale shear zones, formed during the Eburnean orogeny (2.15–2.00 Ga). High‐grade metamorphic domains are separated from low‐grade greenstone belts by high‐strain zones, including early thrusts, extensional detachments and late‐stage strike‐slip shear zones. Paragneisses sporadically preserve high‐pressure, low‐temperature (HP–LT) relicts, formed at the transition between the blueschist facies and the epidote–amphibolite sub‐facies (10.0–14.0 kbar, 520–600 °C), and represent a low (~15 °C km−1) apparent geothermal gradient. Migmatites record metamorphic conditions at the amphibolite–granulite facies transition. They reveal a clockwise pressure–temperature–time (P–T–t) path characterized by melting at pressures over 10.0 kbar, followed by decompression and heating to peak temperatures of 750 °C at 5.0–8.0 kbar, which fit a 30 °C km−1 apparent geotherm. A regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise P–T–t path, characterized by peak metamorphic conditions of 7.0–10.0 kbar at 550–680 °C, which match a 20–25 °C km−1 apparent geotherm. These P–T conditions were reached after prograde burial and heating for some rock units, and after decompression and heating for others. The timing of anatexis and of the amphibolite facies metamorphic overprint is constrained by in‐situ U–Pb dating of monazite crystallization at 2138 ± 7 and 2130 ± 7 Ma respectively. The new data set challenges the interpretation that metamorphic breaks in the West African Craton are due to diachronous Birimian ‘basins’ overlying a gneissic basement. It suggests that the lower crust was exhumed along reverse, normal and transcurrent shear zones and juxtaposed against shallow crustal slices during the Eburnean orogeny. The craton in NW Ghana is made of distinct fragments with contrasting tectono‐metamorphic histories. The range of metamorphic conditions and the sharp lateral metamorphic gradients are inconsistent with ‘hot orogeny’ models proposed for many Precambrian provinces. These findings shed new light on the geodynamic setting of craton assembly and stabilization in the Paleoproterozoic. It is suggested that the metamorphic record of the West African Craton is characteristic of Paleoproterozoic plate tectonics and illustrates a transition between Archean and Phanerozoic orogens.

Evolution of the Corsica–Sardinia Batholith and late-orogenic shearing of the Variscides

The Corsica–Sardinia Batholith formed in the late Carboniferous–Permian along the northern Gondwana margin. One or more of the following processes may have raised the Variscan geotherm enhancing melting of the crust and uppermost mantle, such as: i) break-off and detachment of the Rheic oceanic slab, ii) mantle delamination due to gravitational re-equilibration of thickened crust, iii) shear heating, iv) advection of mantle-derived melts, and v) concentration of heat-producing elements. In this paper, we present a simple one-dimensional thermal model to explain the origin of the batholith in a geodynamic setting consistent with intraplate shearing between Gondwana and Cadomian–Avalonian blocks. Input parameters, and boundary and initial conditions of the model crust are derived from a careful re-examination of large petrological, geochronological, and structural datasets. All parameters are varied within a range of geologically realistic values to reproduce thermal histories related to different tectonic processes, and the reliability of models is quantitatively evaluated by comparing the simulated geotherms with a large dataset of pressure–temperature–time (P–T–t) constraints. The best fit to P–T–t constraints is obtained for: i) break-off in the Late Devonian–Early Carboniferous, ii) decreasing exhumation rates from 340 to 280Ma, and iii) transpression at high average strain rates, mostly coeval with the peak of high-T–low-P metamorphism.

Prograde and retrograde growth of monazite in migmatites: An example from the Nagercoil Block, southern India

Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure–temperature–time (P–T–t) evolution of the Nagercoil Block at the southernmost tip of peninsular India. An inferred peak metamorphic assemblage of garnet, K-feldspar, sillimanite, plagioclase, magnetite, ilmenite, spinel and melt is consistent with peak metamorphic pressures of 6–8 kbar and temperatures in excess of 900 °C. Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 °C. SHRIMP U–Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palaeoproterozoic crystallisation ages. New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation, and the minimum age of the metamorphic peak, to around 560 Ma. The data suggest two stages of monazite growth. The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite. Relatively REE-depleted rims, which have a pronounced negative europium anomaly, grew during melt crystallisation along the retrograde path at around 535 Ma. Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer, supporting a long-lived high-grade metamorphic history. The metamorphic conditions, timing and duration of the implied clockwise P–T–t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.

Petrogenesis and chemogenesis of oceanic and continental orogens in Asia: Current topics, Part I

Petrogenesis and chemogenesis of oceanic and continental orogens in <scp>A</scp>sia: Current topics, <scp>Part I</scp>

Coupled phengite40Ar–39Ar geochronology and thermobarometry:P-T-tevolution of Andros Island (Cyclades, Greece)

AbstractAndros is a key island for understanding both the timing of high-pressure–low-temperature (HP-LT) metamorphism and the dynamics of crustal-scale detachment systems exhuming high-grade units in the Cyclades (Greece). Using phengite40Ar–39Ar geochronology coupled with thermobarometry, as well as data from literature, we constrain the pressure–temperature–time (P-T-t) paths of the Makrotantalon and Attic–Cycladic Blueschist units on Andros. Peak conditions of the HP-LT episode in the Makrotantalon unit are 550°C and 18.5 kbar, dated at 116 Ma. We correlate this episode with Early Cretaceous blueschist facies metamorphism recognized in the Pelagonian zone of continental Greece. This is a new argument favouring a Pelagonian origin for the Makrotantalon unit. In the Attic–Cycladic Blueschist unit, theP-T-tpath is characterized by: (1) exhumation after peak conditions in HP-LT conditions between 55 and 35 Ma; (2) isobaric heating at 7 kbar until 30 Ma; and (3) isothermal decompression until 21 Ma. This thermal evolution and timing are similar to those of the neighbouring Tinos Island, emphasizing major thermal re-equilibration at the transition between stable and retreating subduction. Modifications of the crustal thermal state played a major role in the evolution of the North Cycladic Detachment System, below which Andros HP-LT units were exhumed.

In search of transient subduction interfaces in the Dent Blanche–Sesia Tectonic System (W. Alps)

Abstract In this paper we study the Alpine metamorphic history of a major tectonic zone which formed during Alpine orogeny, the Dent Blanche Thrust (DBT). This contact, located in the Northern Western Alps, juxtaposes some ophiolitic metasediment-rich remnants of the Liguro-Piemontese ocean (Tsate Complex) with a composite continental, km-sized complex (Dent Blanche Tectonic System, DBTS) of Adriatic affinity thrusted over the ophiolite. In order to better understand the geodynamic meaning of the DBT region and adjacent units, we have reconstructed the pressure–temperature–time–deformation (P–T–t–d) history of these two units using modern thermobarometric tools, Rb/Sr geochronology, and field relationships. We show that the Tsate Complex is formed by a stack of km-thick calcschists-bearing tectonic slices having experienced variable maximum burial temperatures between 360 °C and 490 °C at depths of ca. 25–40 km. Associated deformation ages span a range between 37 Ma and 41 Ma. The Arolla gneissic mylonites at the base of the DBTS experienced high-pressure (12–14 kbar), top-to-NW deformation at ca. 450 °C between 43 and 48 Ma. A first age of ca. 58 Ma has been obtained for high-pressure ductile deformation in the Valpelline shear zone, atop Arolla gneisses. Some of the primary, peak metamorphic fabrics have been reworked and later backfolded during exhumation and collisional overprint (ca. 20 km depth, 37–40 Ma) leading to the regional greenschist-facies retrogression which is particularly prominent within Tsate metasediments. We interpret the Dent Blanche Thrust, at the base of the Arolla unit, as a fossilized subduction interface active between 43 and 48 Ma. Our geochronological results on the shear zone lining the top of the Arolla unit, together with previous P–T–t estimates on equivalent blueschist-facies shear zones cutting the Sesia unit, indicate an older tectonic activity between 58 and 65 Ma. We demonstrate here that observed younger ages towards lowermost structural levels are witness of the transient, downwards migration of the Alpine early Cenozoic blueschist-facies subduction interface. This down-stepping is interpreted to reflect the progressive underplating acting between 30 and 40 km depth in the Alpine subduction zone between late Cretaceous and late Eocene. Underplating involved first continental material derived from the stretched Adriatic margin followed by underplating of ocean-derived rocks in the Eocene. These results shed light on subduction-zone accretion processes and therefore provide a new perspective for the understanding of geophysical results imaging the plate-interface region in active subduction zones.

Two-stage granulite formation in a Proterozoic magmatic arc (Ongole domain of the Eastern Ghats Belt, India): Part 2. LA-ICP-MS zircon dating and texturally controlled in-situ monazite dating

To quantify the Mesoproterozoic pressure–temperature–time evolution of the granulite-facies Ongole domain, a deep crustal Proterozoic magmatic arc in the Eastern Ghats Belt, LA-ICP-MS U–Pb zircon dating and in-situ electron microprobe U–Th–total Pb monazite dating are integrated with the petrologic observations. Zircon grains in the metapelites often preserve oscillatory-zoned, sometimes partly resorbed cores yielding a spread of 207Pb/206Pb ages between ca. 2700 and ca. 1750Ma, interpreted to reflect the age of detrital grains from the surrounding crustal blocks. Metamorphic zircon overgrowths and rims yield Palaeoproterozoic concordia ages from about 1625 to 1600Ma, indicating the timing and duration of an ultrahigh-temperature (UHT) metamorphic event.Texturally controlled in-situ monazite dating however reveals two metamorphic events separated by 60 to 80Ma. In metapelites, the monazite inclusions in garnet and cores of some matrix monazite yield weighted mean ages of ca. 1610Ma, similar to the concordia ages from zircon and indicate the timing of the UHT metamorphism. The monazite grains in the matrix, growing along with a second generation of garnet and in late-stage symplectites are highly recrystallized and yield weighted mean ages of ca. 1540Ma, indicating the timing of a second metamorphism at higher pressures but lower temperatures. The weighted mean ages obtained from metapelites in the adjoining Vinjamuru domain are the same as those obtained from the Ongole domain. The chemical differences between the two generations of monazite (Th, Y and HREE contents) support the above interpretation of monazite growth under different conditions and at different times. Monazite in the charnoenderbites records the second metamorphic event but only rarely the earlier UHT event. In addition, they show evidence for later imprints of ductile to brittle deformation between ca. 1450 and 1360Ma, which may correlate with Mesoproterozoic crustal extension and alkaline plutonism along the western boundary of the Eastern Ghats Belt. Though the Ongole domain was not involved in major crustal reworking during the Neoproterozoic, monazites do record minor disturbances at ca. 730Ma and ca. 510Ma. Hence the combination of zircon and monazite dating unveils the sequence of tectonothermal events in the Ongole domain in a hitherto unknown exactness: UHT metamorphism at ca. 1610Ma due to magmatic heat advection, subsequently, 60 to 80Ma later, a second metamorphic event at medium-pressure and lower-temperature due to the collision of the arc with the Indian continent, and finally minor disturbances due to rifting between ca. 1450 and 1360Ma.

Migmatization and large-scale folding in the Orlica–Śnieżnik Dome, NE Bohemian Massif: Pressure–temperature–time–deformation constraints on Variscan terrane assembly

New research indicates that there is stratigraphic and structural continuity between the Młynowiec and Stronie Formations of the Orlica–Śnieżnik Dome (OSD), Bohemian Massif, and reveals the primary importance of the collision of Armorican Terrane Assemblage and Brunovistulian terrane in Variscan development of the OSD. The Młynowiec Formation is a locally migmatized metasedimentary unit and understanding its tectonometamorphic history is crucial for making regional-scale Variscan reconstructions. Structural studies, pseudosection modelling in MnNCKFMASH system and microprobe monazite dating revealed that this Variscan collision led to crustal thickening of the OSD, tectonic burial of the Młynowiec Formation to depths corresponding to 10–11kbar and progression of regional metamorphism at 361±5Ma. Continuous underthrusting of the Brunovistulian terrane led to exhumation of the OSD metasediments and the formation of subhorizontal, N–S-trending tight folds and a peak-metamorphic axial planar foliation. This episode took place at 650°C and 6–7kbar in the non-migmatized rocks of structurally upper parts of the Młynowiec Formation, and at 700°C and 6–7kbar in its structurally lower parts, where metamorphism was accompanied by localised in-situ migmatisation. During final consolidation of a mosaic of terranes, top-to-the-NE tectonic movements reactivated former fabrics, produced further uplift and cooled the rocks to greenschist facies. This pervasive shearing could be also responsible for the metasomatic event at ~330Ma. The continuation of the NE-transpression of the OSD involved its interactions with adjacent crustal units in the east and in the north, which led to large-scale folding represented now by synforms and associated antiforms. The metamorphic structure of the rocks subjected to the Barrovian-type metamorphism was rebuilt so that the migmatitic rocks outcrop in the cores of large-scale antiforms, and the lithological contacts, penetrative foliation and metamorphic isograds dip locally at different angles (shallow, moderate to high) and at different azimuths (E-, NE-, N, NW and W).

Burial and exhumation during Archean sagduction in the East Pilbara Granite-Greenstone Terrane

Archean granitic domes and intervening volcano-sedimentary basins are commonly interpreted as the product of “sagduction”, a process involving the gravitational sinking of surficial greenstone cover sequences into narrow belts and the coeval exhumation of deeper granitic crust into broad domes. Alternatives to the sagduction model that can account for the regional dome and basin pattern include fold interferences and extensional metamorphic core complexes. In order to provide quantitative constraints on the pressure–temperature–time (P–T–t) evolution experienced by greenstone–granite pairs we investigate the Warrawoona greenstone belt and adjacent Mount Edgar granitoid dome the East Pilbara craton (Western Australia). We adopt a multidisciplinary approach that includes structural, metamorphic, geochronological and numerical investigation of the 3.5–3.2 Ga Mount Edgar high-grade metamorphic rocks. Garnet-bearing metasediments and metabasalts collected along the SW and SE rims of the Mount Edgar Dome show higher pressure but lower temperature of equilibration (9–11 kbar and 450–550 °C) than enclaves collected in the core of the dome (6–7 kbar and 650–750 °C). In situ oxygen isotope analysis and U–Pb dating of zircons from the enclave indicate a metasedimentary origin (∂18O∼+13‰) for the protoliths and a metamorphic age of 3311.9±4.9 Ma. In addition, monazites included in garnet from the SW dome margin yield an age of 3443.4±4.5 Ma. These monazites suggest the existence of an older metamorphic cycle and imply a polymetamorphic evolution of the unit. The P–T–t data support fast, gravity-driven tectonics, wherein sedimentary rocks were buried to lower crustal conditions, metamorphosed and exhumed back to the surface during a cycle lasting only a few million years. Forward thermo-mechanical modeling confirms the P–T–t evolutions deduced from thermobarometry and geochronology. Our model shows a large range of possible apparent geothermal gradients during sagduction, including low apparent geothermal gradients that are similar to those proposed for Archean and modern subduction.

Garnet oxygen analysis by SHRIMP-SI: Matrix corrections and application to high-pressure metasomatic rocks from Alpine Corsica

Garnet is a key mineral used to constrain pressure, temperature and age of metamorphic rocks. This contribution reports oxygen isotope measurements in garnet using the SHRIMP-SI ion microprobe. The reproducibility of oxygen isotope analyses on garnet standard UWG2 is ~0.3–0.4‰ (2σ) within and across sessions. The correlation between oxygen isotope measurements and the grossular and andradite components in garnet fits a second-degree polynomial with a maximum bias in δ18O of 2.4 and 8.3‰, respectively. This bias is similar to that determined for other large ion microprobes. Analysis of two additional Mn-rich garnet crystals allowed identification of a separate bias caused by the spessartine component, which can reach a maximum of 2.3‰.The standardisation and correction scheme proposed in this study are applied to garnet crystals from two samples from Alpine Corsica in order to link fluid evolution with the pressure–temperature–time path. The samples have experienced a polyphase metamorphic history, which includes Permian high temperature metamorphism, followed by late However, 92-W2 was re-analysed because of the variability observed in the previously published δ18O values (-0.29 and 0.53‰, Kohn and Valley, 1998). The δ18O value found in this study (0.81 ± 0.44‰, 2s, Table 1) agrees within error with the highest value previously published for this garnet. Eocene high pressure–low temperature metamorphism. Permian Ca-poor garnet cores have high δ18O values (9.9±0.6‰ and 11.1±0.5‰ 2σ, in two samples) with respect to garnet mantles (7.2±0.4‰) and rims (5.4±0.5‰ and 2.2±0.4‰). The dramatic decrease in δ18O from Permian garnet cores to Alpine rims in both samples reflects a combination of external fluid influx and change in sample mineralogy. The low δ18O of the garnet rims that formed at eclogite facies conditions indicates that the metasomatic fluid equilibrated with mafic or ultramafic rocks. This study illustrates that fluid-mediated mass transfer during subduction occurred at lithological contacts between felsic and ultramafic rocks.

Monazite ages and pressure–temperature–time paths from anatectites in the southern Ruby Range, Montana, USA: evidence for delamination, ultramafic magmatism, and rapid uplift at ca. 1780 Ma

The Wolf Creek ultramafic bodies of the southern Ruby Range, southwest Montana, intruded as high-temperature magma during the late stages of an Early Proterozoic orogeny affecting the Great Falls Tectonic Zone of the western Wyoming craton. Field and petrologic evidence indicate high-temperature metamorphism and deformation uniquely affected metapelitic anatectites adjacent to the ultramafic bodies. Leucosomes are mesoperthite and quartz with garnet. Restite contains sillimanite, garnet, quartz, microcline, and biotite that was partially resorbed by the melt-producing reaction. Late cordierite and sillimanite replace garnet in the migmatites and other pelitic gneiss in the area. Petrogenic grids for the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) system indicate that conditions during anatexis were 750–800 °C, 600–900 MPa. Anatexis occurred during deformation of metapelites and associated rocks adjacent to the larger ultramafic bodies, but crystallization and cooling occurred after deformation, as indicated by the preservation of igneous textures and mesoperthite with undeformed lamellae in the leucosome. Element mapping of monazite using electron-probe micro-analysis (EPMA) identifies four distinct chemical domains of monazite growth during the orogenic event in addition to relict, Archaean to earliest Proterozoic cores. U–Th–Pb dating of the domains establishes two periods of growth of matrix monazite in the paleosome that indicate fabric-forming prograde metamorphism at 1829 ± 17 and 1805 ± 4 Ma (2σ). Coarser, igneous monazites from the leucosome also contain two distinct chemical zones visible in yttrium maps, indicating growth with garnet and then during garnet resorption as decompression led to replacement of garnet by cordierite. The domain ages cannot be statistically distinguished. Together they yield an age of 1782 ± 9 Ma (2σ). The data imply that regional thickening leading to upper amphibolite facies metamorphism occurred prior to 1800 Ma. Thickening was followed after ∼25 Ma by the intrusion of ultramafic magmas, contact anatexis, and then decompression. This sequence is consistent with mantle delamination after orogenic thickening.

A P–T–t–D discontinuity in east-central Nepal: Implications for the evolution of the Himalayan mid-crust

Rock specimens collected from lower, middle, and upper portions of the exhumed Himalayan mid-crust in the upper Tama Kosi region of east-central Nepal yield pressure–temperature–time–deformation (P–T–t–D) paths that demonstrate the presence of a structural, metamorphic, and geochronologic discontinuity. A P–T–t–D path from below the discontinuity defines a loading path with deformation at staurolite-grade metamorphic conditions occurring at ~10–8Ma. P–T–t–D paths from above the discontinuity, in contrast, record an earlier and protracted metamorphic history that includes decompressional heating. Monazite ages from specimens collected above the discontinuity constrain prograde garnet growth to be >21Ma with decompression-related garnet breakdown initiating at c.19Ma and likely continuing until c.15Ma. These contrasting P–T–t–D histories separated by a tectonometamorphic discontinuity are consistent with an evolutionary model for the Himalaya in which rocks above the discontinuity were metamorphosed in the deep hinterland of the orogen and deformed during lateral extrusion from beneath Tibet while rocks below the discontinuity were deformed and metamorphosed later within the shallower foreland of the orogen. These results demonstrate the spatial and temporal relationships and compatibility between lateral midcrustal extrusive flow and wedge taper processes within an evolving orogen.

The Heart of China revisited: II Early Paleozoic (ultra)high‐pressure and (ultra)high‐temperature metamorphic Qinling orogenic collage

Orogens with multiple (ultra)high‐pressure ((U)HP) and (ultra)high‐temperature ((U)HT) metamorphic events provide a complex but telling record of oceanic and continental interaction. The Early Paleozoic history of the “Heart of China,” the Qinling orogenic collage, offers snapshots of at least three (U)HP and two (U)HT metamorphic events. The preservation of remnants of both oceanic and continental domains together with a ≥110 Myr record of magmatism allows the reconstruction of the processes that resulted in this disparate metamorphism. Herein, we first illuminate the pressure‐temperature‐time (P‐T‐t) evolution of the Early Paleozoic (U)HP and (U)HT events by refining the petrographic descriptions and P‐T estimates, assess published, and employ new U/Th‐Pb zircon, monazite, and titanite, and40Ar‐39Ar phengite geochronology to date the magmatic and metamorphic events. Then we explore how the metamorphic and magmatic events are related tectonically and how they elucidate the affinities among the various complexes in the Qinling orogenic collage. We argue that a Meso‐Neoproterozoic crustal fragment—the Qinling complex—localized subduction‐accretion events that involved subduction, oceanic‐arc formation, and back‐arc spreading along its northern margin, and mtantle‐wedge exhumation and spreading‐ridge subduction along its southern margin.

Geologic evolution of the Sør Rondane Mountains, East Antarctica: Collision tectonics proposed based on metamorphic processes and magnetic anomalies

The Sør Rondane Mountains of eastern Dronning Maud Land, East Antarctica can be subdivided into two different crustal terranes: the NE-terrane and the SW-terrane. The former is underlain by basement rocks of amphibolite-facies (unit A) and granulite-facies (unit B), and the latter by granulite-facies (unit C) to greenschist-facies (units D and D′) rocks. The metamorphic evolution of the NE-terrane exhibits a clockwise pressure–temperature–time (P–T–t) path and the SW-terrane exhibits a counter-clockwise P–T–t path, and detrital zircon U–Pb ages are also different between the two terranes. The differences in the metamorphic evolution processes and detrital zircon provenance in these two regions can be explained by the collision of the NE-terrane and the SW-terrane which is constrained to have occurred at 600–650Ma. The collision is interpreted to reflect convergence between the East Maud-East African orogen terrane and the Maud-Nampula terrane of the Kalahari Craton, which are bounded by the Main Tectonic Boundary and passes through the Sør Rondane Mountains. The RTP magnetic anomalies suggest this suture is cut by the subsequent collision boundary between the Kalahari Craton and the Rayner Belt, which is exposed in the Lützow–Holm region. This collisional belt is considered to have formed at approximately 520–580Ma.

Tectonic evolution of high-grade metamorphic terranes in central Vietnam: Constraints from large-scale monazite geochronology

Several metamorphic complexes in Southeast Asia have been interpreted as Precambrian basement, characterized by amphibolite to granulite facies metamorphism. In this paper, we re-evaluate the timing of this thermal event based on the large-scale geochronology and compositional variation of monazites from amphibolite to granulite facies metamorphic terranes in central Vietnam. Most of the samples in this study are from metamorphic rocks (n=38) and granitoids (n=11) in the Kontum Massif. Gneisses (n=6) and granitoids (n=5) from the Hai Van Migmatite Complex and the Truong Son Belt, located to the north of the massif, were also studied. Two distinct thermal episodes (245–230Ma and 460–430Ma) affected Kontum Massif gneisses, while a single dominant event at 240–220Ma is recorded in the gneisses from the Hai Van Complex and the Truong Son Belt. Monazites from granitoids commonly yield an age of 240–220Ma. Mesoproterozoic ages (1530–1340Ma) were obtained only from monazite cores that are surrounded by c. 440Ma overgrowths. Thermobarometric results, combined with concentrations of Y2O3, Ce2O3, and heavy rare earth elements in monazite, and recently reported pressure–temperature paths suggest that Triassic ages correspond to retrograde metamorphism following decompression from high- to medium-pressure/temperature conditions. Ordovician–Silurian ages reflect low-pressure/temperature metamorphism accompanied by isobaric heating during prograde metamorphism. Some samples were affected by both metamorphic events. We conclude that high-grade metamorphism observed in so-called Precambrian basement terranes in central Vietnam occurred during both the Permian–Triassic and the Ordovician–Silurian, while peraluminous granitoid magmatism is Triassic. Additionally, our preliminary analyses for U–Pb zircon age and whole-rock chemistry of granitic gneisses from the Truong Song Belt suggests the presence of the Ordovician–Silurian volcanic arc magmatism in the region. Based on the pressure–temperature–time–protolith evolutions, metamorphic rocks from central Vietnam provide a continuous record of subduction–accretion–collision tectonics between the South China and Indochina blocks: in the Ordovician–Silurian, the region was characterized by active continental margin tectonics, followed by continental collision during the Late Permian to Early Triassic and subsequent exhumation during the Late Triassic. The results also suggest that the timing of metamorphism and protolith formation as well as the geochemical features in other Southeast Asian terranes should be verified to achieve a better understanding of the Precambrian to Early Mesozoic tectonic history in Asia.