High-pressure Granulite Research Articles

Importance of crustal relamination in origin of the orogenic mantle peridotite–high-pressure granulite association: example from the Náměšť Granulite Massif (Bohemian Massif, Czech Republic)

A characteristic association of crustal and mantle rocks is commonly used to decipher processes at the mantle–crust interface of HP–UHP collisional orogenic systems. Also, in the Variscan orogenic root of the Bohemian Massif (the Moldanubian Zone), high-pressure felsic granulites are often accompanied by spinel or garnet peridotites. This association was investigated using petrography, zircon geochronology and whole-rock geochemical data from the Náměšť Granulite Massif. The geochemical signature of the granulite is the same as for other Moldanubian occurrences, suggesting nearly isochemically metamorphosed felsic metaigneous rocks of Saxothuringian provenance. SHRIMP zircon dating yielded two main age maxima, at 395.2 ± 4.4 and 337.2 ± 1.7 Ma, reflecting an Early Devonian protolith and Visean HP metamorphism. As shown by Sr–Nd isotopic data, the variably refertilized harzburgite or depleted lherzolite was variously contaminated by mature crustal material resembling the studied granulites. To account for the origin of these HT–HP rock associations we suggest a new geotectonic model. An eastward continental subduction of Early Palaeozoic felsic metaigneous material of Saxothuringian origin was followed by its relamination at the bottom of the autochthonous lower crust. Ascending felsic granulites derived from the relaminated lower plate material sampled refertilized harzburgites originally formed in a back-arc. The complete assemblage was subsequently exhumed, forming large, diapir-like bodies. Supplementary material: Sample coordinates from the Náměšť Granulite Massif, analytical techniques, SHRIMP age measurements on zircon grains and whole-rock geochemical data are available at http://www.geolsoc.org.uk/SUP18833 .

Using calculated chemical potential relationships to account for replacement of kyanite by symplectite in high pressure granulites

AbstractCalculated mineral equilibria are used to account for the formation of sapphirine–plagioclase, spinel–plagioclase and corundum–plagioclase symplectites replacing kyanite in quartz–plagioclase–garnet–kyanite granulite facies gneisses from the Southern Domain of the Athabasca granulite terrane, a segment of the Snowbird tectonic zone in northern Saskatchewan, Canada. Metamorphic conditions of >14 kbar and 800 °C are established for the high pressure, garnet–kyanite assemblage using constraints from P–T pseudosections and Zr‐in‐rutile thermometry. Replacement of kyanite by symplectites reflects the reaction of kyanite with the matrix following near‐isothermal decompression to <10 kbar. The chemical potential gradients developed between the kyanite and the matrix led to diffusion that attempted to flatten the gradients, kyanite persisting as a stable phase while it is consumed by symplectite from its edge. In this local equilibrium model, the mineral and mineral compositional spatial relationships are shown to correspond to paths in μ(Na2O)–μ(CaO)–μ(K2O)–μ(FeO)–μ(MgO) in the model chemical system, Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2 (NCKFMAS), with SiO2 and Al2O3 taken to be completely immobile. The values of μ(Na2O) and μ(CaO) are constrained by fixing P–T conditions and choosing appropriate μ(Na2O) and μ(CaO) values that correspond to the observed plagioclase compositions. μ(FeO)–μ(MgO) diagrams show the corresponding spatial relationships with kyanite and the symplectite phases. These results demonstrate that the replacement of kyanite by sapphirine–plagioclase and spinel–plagioclase appears to be metastable with respect to replacement by corundum–plagioclase. Replacement by corundum–plagioclase does also occur, apparently overprinting pre‐existing symplectite and also kyanite. Ignoring corundum, the resulting diagrams account for the spatial relationships and compositions observed in the spinel–plagioclase and sapphirine–plagioclase symplectites. They are predicted to occur over both a wide range of P–T conditions (6–11 kbar, 650–850 °C) and plagioclase compositions (XAn = 0.5–0.9). The wide range of P–T conditions that may result in identical spatial and compositional relationships suggests that such reaction textures may be of limited use in accurately quantifying the P–T conditions of retrograde metamorphism.

Charnockite magmatism during a transitional phase: Implications for late Paleoproterozoic ridge subduction in the North China Craton

The Trans-North China Orogen (TNCO) is a major Paleoproterozoic collisional belt along which the Western and Eastern Blocks of the North China Craton were amalgamated. The Lüliang Complex occurs along the western margin of this belt and incorporates several arc magmatic suites, supracrustal sequences, granitoids and charnockites. Here we investigate the Luyashan charnockite suite, as well as the surrounding granitoids, from this complex. We present results from petrology, fluid inclusions, mineral chemistry, whole rock geochemistry and zircon U–Pb and Lu–Hf isotopic studies. The charnockites are dominantly anhydrous with the typical presence of orthopyroxene. Microthermometric and laser Raman spectroscopic data on fluid inclusions in the charnockite characterize the trapped fluids as dominantly CO2 with traces of N2 and CH4. Geochemically, the charnockite suite shows compositional variation from monzogabbro through gabbroic diorite and diorite to monzodiorite. They are dominantly calc-alkaline and ferroan type and display distinct enrichment in large ion lithophile elements with depletion in HREE. They also show prominent negative anomalies of Nb, Ta, Sr and Ti, and positive anomalies of Rb, Pb, Zr and Hf indicating crustal components in the magma source. The majority of the granitoid suite shows 207Pb/206Pb mean ages in the range of 2172–2032Ma, correlating with similar ages reported from various groups in the Lüliang Complex, the formation of which is correlated with subduction along an active convergent margin. However, the charnockites which intrude the granitoids and their dioritic enclaves are clearly younger and show tightly constrained emplacement ages in the range of 1853–1872Ma. The recrystallized zircons in the older granitoids also show similar ages, suggesting a prominent thermal event at this time. These ages are close to the timing of final collision between the Western and Eastern Blocks of the North China Craton and associated high-grade metamorphism at ca. 1.85Ga. The zircon Lu–Hf isotope data reveal dominantly negative ɛHf(t) values (−8.7 to −0.1) and their TDMC (2477–3042Ma) suggest that the magma sources mainly involved Meso- to Neoarchean reworked crustal components with limited input of Paleoproterozoic juvenile components. Our study reveals that charnockite magmatism and regional metamorphism in the western margin of the TNCO broadly overlap in space and time. The geochemical features, mineral assemblages and fluid inclusion characteristics of the charnockites suggest that the rocks were derived from high temperature (900–1000°C), CO2-enriched and anhydrous (less than 3%H2O) magmas. The magma evolved into more hydrous conditions during cooling marked by the crystallization of amphiboles at 800–850°C. The geochemical features of the charnockite suite also suggest that these rocks were formed during a transitional phase from syn-subduction to collision. Such a scenario is compatible with a tectonic setting involving the subduction of an oceanic ridge when flushing of heat and CO2-rich fluids from asthenosphere into the overriding plate might have occurred through the slab window, generating hot and anhydrous magmas through melting of the basement rocks. The slab window mechanism would also account for broadly coeval high-temperature and high pressure granulites reported from the TNCO. We therefore speculate that the formation of the Luyashan charnockite suite might have been associated with ridge subduction at the terminal stage of the convergence.

Provenance of metasedimentary rocks from the Ceará Central Domain of Borborema Province, NE Brazil: implications for the significance of associated retrograded eclogites

In the Forquilha area (NE Brazil), in NW Borborema Province, high to ultra-high pressure rocks are an important geological key to understanding West Gondwana amalgamation. U–Pb geochronological data for a retrograded eclogite sample yielded an upper intercept age of ca. 1520 Ma and a lower intercept age of ca. 620 Ma. These ages most likely represent the crystallization age of the basaltic protolith and the regional metamorphism, respectively. The retrograded eclogites are enclosed in migmatized quartz-feldspathic gneiss and sillimanite (after kyanite)-garnet-biotite gneiss. Detrital U–Pb zircon data for these paragneisses show only Paleoproterozoic zircon grains with ages clustering from ca. 1800 Ma (the maximum depositional age) to ca. 2480 Ma, and frequency peaks at 2.2–2.0 Ga. Combined with Nd isotopic data from the Forquilha paragneisses, one can assume a single Paleoproterozoic source. Basement rocks of the Ceará Central and the Rio Grande do Norte domains are the most likely candidates. The absence of Meso- and Neoproterozoic zircon grains suggest that the retrograded eclogite bodies possibly do not represent slivers of oceanic rocks captured in active margin sequences during subduction. It was identified that the high-pressure rocks of the Forquilha area are in tectonic contact with high-pressure granulite facies rocks of the Ceará Complex (Independência unit) that present detrital zircon records of an active margin setting, with ages ranging from ca. 660 Ma to 2200 Ma. Metamorphism of this sequence occurred at ca. 650 Ma. Considering previous studies, field relationships, and metamorphic paragenesis, a tectonic scenario is inferred, in which the Forquilha retrograded eclogites represent Mesoproterozoic basaltic rocks of an extensional event that were metamorphosed under eclogite facies conditions during Late Neoproterozoic continental subduction/collision, and juxtaposed to an active margin sequence during the exhumation process.

Mesoarchaean collision of Kapisilik terrane 3070 Ma juvenile arc rocks and >3600 Ma Isukasia terrane continental crust (Greenland)

The Mesoarchaean Kapisilik and Eoarchaean Isukasia terranes in the Nuuk region of southern West Greenland were tectonically juxtaposed in the Archaean. The north of the Isukasia terrane is distal from the Kapisilik terrane and has only rare growth of ∼2690Ma metamorphic zircon and no 2980–2950Ma metamorphic zircon. The southern part of the Isukasia terrane lies between two ∼2690Ma shear zones, and has locally preserved high pressure granulite facies assemblages and widespread growth of 2980–2950Ma metamorphic zircon and also sporadic growth of ∼2690Ma metamorphic zircon. Within this southern part of the Isukasia terrane there is a folded klippe of mylonitised Mesoarchaean detrital meta-sedimentary rocks (carrying >3600 and ∼3070Ma detrital zircons), mafic and ultramafic rocks, with ∼2970Ma metamorphic zircon overgrowths. South of the Isukasia terrane is the Kapisilik terrane, containing ∼3070Ma arc-related volcanic rocks, gabbro-anorthosites and meta-tonalites, intruded by 2970–2960Ma granites. Zircons of an Ivisârtoq supracrustal belt ∼3075Ma intermediate volcanic rock have initial ɛHf values of +2 to +5 thus are juvenile crustal additions. ∼3070Ma tonalites along the northern edge of the Kapisilik terrane have whole rock positive initial ɛNd values and thus are also juvenile crustal additions. In contrast, igneous zircons in 2960Ma granites intruded into juvenile ∼3075Ma supracrustal rocks of the Kapisilik terrane have initial ɛHf values of −5 to −10, and must have involved the partial melting of >3600Ma Isukasia terrane rocks.The integrated structural and zircon U-Th-Pb-Hf isotopic data show that at 2980–2950Ma the Kapisilik terrane juvenile arc components collided with, and over-rid, the Isukasia terrane. The southern edge of the Isukasia terrane came to lie in the deep crust under the Ivisârtoq supracrustal belt and melted at 2970–2960Ma to produce granites. These granites derived from ancient crust rose into the upper crust, where they intruded the overlying allochthonous juvenile ∼3075Ma Ivisârtoq supracrustal belt arc assemblages. The southern edge of the Isukasia terrane is interpreted as an interior nappe of Eoarchaean basement rocks interfolded with a klippe of Mesoarchaean metasedimentary and mafic/ultramafic rocks, both of which are affected by 2980–2950Ma metamorphism. The mixed Eoarchaean–Mesoarchaean detrital provenance suggests that the klippe could be dismembered components of an accretionary prism or forearc crust. The northern part of the Isukasia terrane is interpreted as foreland, free of 2980–2950Ma high-grade metamorphic overprint. This shows that the Isukasia terrane is not a coherent block, but contains ancient rocks that are parautochthonous or allochthonous to each other, with contrasting later metamorphic history.At ∼2690Ma the crustal architecture arisen from Mesoarchaean collision between an older continental block and an island arc was reworked along intra-crustal shear zones, coeval with amphibolite facies metamorphism. This reworking followed on from major terrane assembly at 2710–2700Ma in the southern part of the Nuuk region, when the Eoarchaean Færingehavn terrane was juxtaposed with 2840–2825Ma arc rocks. Thus the 2980–2950Ma assembly of the Isukasia and Kapisilik terranes is distinct from the later 2710–2700Ma terrane assembly further south in the Nuuk region.

Exhumation of an eclogite terrane as a hot migmatitic nappe, Sveconorwegian orogen

We demonstrate a case of eclogite exhumation in a partially molten, low-viscosity fold nappe within high-grade metamorphosed crust in the Eastern Segment of the Sveconorwegian orogen. The nappe formed during tectonic extrusion, melt-weakening assisted exhumation and foreland-directed translation of eclogitized crust, and stalled at 35–40km depth within the collisional belt. The eclogites are structurally restricted to a regional recumbent fold in which stromatic orthogneiss with pods of amphibolitized eclogite make up the core. High-temperature mylonitic gneiss with remnants of kyanite eclogite (P>15kbar) composes a basal shear zone 50km long and <4km wide. Heterogeneously sheared and partly migmatized augen gneiss forms a tectonostratigraphic marker in front of and beneath the nappe, and is in turn structurally enveloped by a composite sequence of orthogneisses and metabasites. The entire tectonostratigraphic pile underwent near-pervasive deformation and recrystallization under high-pressure granulite and upper amphibolite conditions. U–Pb SIMS metamorphic zircon ages of eclogite and stromatic orthogneiss constrain the time of eclogitization at 988±6Ma and 978±7Ma. Migmatization, concomitant deformation, and exhumation are dated at 976±6Ma, and crystallization of post-kinematic melt at 956±7Ma. Orthogneiss protoliths are dated at 1733±11 and 1677±10Ma (stromatic gneiss) and 1388±7Ma (augen gneiss in footwall), demonstrating origins indigenous to the Eastern Segment. Eclogitization and exhumation were coeval with the Rigolet phase of the Grenvillian orogeny, reflecting the late stage of continental collision during construction of the supercontinent Rodinia.

Anatomy of zircon growth in high pressure granulites: SIMS U–Pb geochronology and Lu–Hf isotopes from the Jiaobei Terrane, eastern North China Craton

Granulite facies rocks that witnessed high pressure and high temperature metamorphism preserve important records of the growth and recrystallizaton of zircons during the multiple stages of tectonic evolution of orogenic belts developed in complex subduction–collision settings. Here we investigate zircons in the Paleoproterozoic high pressure (HP) granulites from Jiaobei Terrane in the eastern North China Craton that experienced a multi-stage metamorphic evolution. We present SIMS zircon U–Pb ages for zircons from garnet amphibolites, mafic granulites and pelitic granulites. Our data show four age peaks at 2.5Ga, 1.92Ga, 1.87–1.85Ga and 1.83Ga. Zircons belonging to the latter three age groups show similar characteristics of low Th and U contents, low Th/U ratios (<0.1), sector zoning or unzoned internal structures, corresponding to the features of zircon growth during metamorphism. Using zircon Lu–Hf isotopic compositions combined with a careful analysis of the morphology of the zircons, we evaluate the processes of solid-state recrystallization, intergranular melt growth, coupled dissolution–reprecipitation and subsolidus growth. Zircons formed through solid-state recrystallization during peak metamorphic stage inherited the relatively high 176Lu/177Hf ratios and low 176Hf/177Hf ratios of zircons from the protoliths. The youngest U–Pb age of ca. 1899Ma is interpreted as the maximum estimate of the time of peak metamorphism. Zircons formed through intergranular melt growth during low- to medium-pressure retrogression display homogeneous Lu–Hf isotope compositions, low 176Lu/177Hf and high 176Hf/177Hf ratios. The U–Pb age of 1870–1850Ma yielded by zircons crystallized from intergranular melt is interpreted to represent the time of low- to medium-pressure granulite facies retrogression. Some zircon grains with prismatic morphology and low 176Lu/177Hf and 176Hf/177Hf ratios display heterogeneous Lu–Hf isotopic compositions, possibly related to coupled dissolution–reprecipitation. The U–Pb ages of these zircons are identical to those formed from intergranular melt and these two groups of zircons formed during the same metamorphic stage. Zircons formed through subsolidus growth show large variations in Lu–Hf isotopic compositions and their ca.1833Ma age represents the timing of amphibolite facies retrogression. The HP granulite facies metamorphism in Jiaobei is thus constrained between 1870 and 1899Ma, with low- to medium-pressure granulite facies retrogression at ca. 1850–1870Ma followed by amphibolite facies retrogression at ca. 1833Ma.Our data show that several zircon cores and recrystallized grains were entrained from basement rocks during the emplacement of the mafic magmatic protoliths in the case of mafic granulites. For the pelitic granulites, the older zircons are detrital grains of magmatic origin. The Hf crustal model ages of these zircons that show peaks at ca. 2.7Ga and 2.9Ga are consistent with the timing of mafic magmatism in the region as identified in previous studies. The Lu–Hf isotopes of newly grown metamorphic zircons which have equilibrated chemical compositions trace the nature of the protolith. The zircon Hf crustal model age of ca. 2.5–2.7Ga from the pelitic granulites is consistent with that of the basement rocks in the Jiaobei Terrane and the zircon Hf model ages (TDM) of ca.2.0Ga and 2.2–2.3Ga from the mafic granulites might represent the time of magma generation from the mantle with crustal contamination. The geochronology of the basement rocks as revealed by the old zircon cores and the metamorphic evolution of the Jiaobei Terrane are comparable with the tectonic history of the central and western North China Craton and provide insights into the complex tectonothermal events associated with the Neoarchean–Paleoproterozoic crustal evolution history of the craton. Also, the pervasive ~2.5Ga zircons in a variety of rocks seem to suggest that the Jiaobei Terrane was already part of a coherent tectonic unit at such time.

Archaean Kuru-Vaara eclogites in the northern Belomorian Province, Fennoscandian Shield: crustal architecture, timing, and tectonic implications

This paper addresses the relationships between relic amphibole-eclogite facies (AE) eclogites and their host units, Archaean amphibolites, enveloped by Archaean tonalite–trondhjemite–granodiorite (TTG) gneisses, in the Kuru-Vaara study area in the northern Belomorian Province. According to observational constraints, the crystallization of the relic peak omphacite + Mg-garnet ± kyanite assemblage and the subsequent replacement of omphacite by clinopyroxene–plagioclase symplectite occurred before the earliest deformational, metamorphic, and migmatization events that are recorded in the amphibolites. The amphibolites and their TTG hosts have a shared deformational and metamorphic history that is composed of the Archaean and Palaeoproterozoic periods. This history favours the conclusion that the AE metamorphism recorded in the relic eclogites within the amphibolites occurred during the Mesoarchaean to Neoarchaean periods. The deformation and metamorphism of the amphibolite facies of the second period resulted from the Lapland–Kola collisional orogeny at 1.91–1.93 Ga, which led to eclogite–high-pressure granulite (E–HPG) facies conditions in the lowermost portions of the over-thickened crust in Belomorian Province (the southwestern foreland of the Lapland–Kola collisional orogen). The Palaeoproterozoic E–HPG overprint was reported from the Palaeoproterozoic Gridino mafic dikes. Although the ages of the oldest low Th/U zircons are close to the time of the Lapland–Kola collision, the low Th/U 1.9–1.8 Ga zircons reflect a zircon response to regional fluid infiltration in the eclogites during slow exhumation following the Lapland–Kola orogeny and do not record any metamorphic event. Contrary to the Palaeoproterozoic E–HPG overprint, the areal occurrence of the 2.7–2.8 Ga AE eclogites with mid-ocean ridge basalt-like chemistry and their paragenetic link with the TTG gneisses suggest a tectonic regime that involves subduction. This research favours concepts suggesting that the modern-style plate tectonics has operated in some places, at least since the late Mesoarchaean.

Long-lived high-temperature granulite-facies metamorphism in the Eastern Himalayan orogen, south Tibet

Abstract The Namche Barwa Complex exposed in the Eastern Himalayan Syntaxis, south Tibet, underwent high-pressure (HP) and high-temperature (HT) granulite-facies metamorphism and associated anatexis. The HP pelitic granulites contain garnet, kyanite, sillimanite, cordierite, biotite, quartz, plagioclase, K-feldspar, spinel, ilmenite and graphite. These minerals show composite reaction texture and varying chemical compositions and form four successive mineral assemblages. Phase equilibrium modeling constrains the P–T conditions of 10–12 kbar and 550–700 °C for the prograde stage, 13–16 kbar and 840–880 °C for the peak-metamorphic stage, and 5–6 kbar and 830–870 °C for the late retrograde stage, indicating that the HP granulites recorded a clockwise P–T path involving the early heating burial and anatexis through dehydration melting of both muscovite and biotite, and the late isothermal decompression and gradual melt crystallization under HT granulite-facies conditions. The zircon U–Pb dating reveals that the HT granulite-facies metamorphism probably initiated at ca. 40 Ma, and lasted to ca. 8 Ma. Therefore, the present study provides robust evidence for a long-lived HT metamorphism and associated anatexis in the deeply buried Indian continent and important constraints on the leucogranite generation and tectonic evolution of the Himalayan orogen.

Garnet-forming reactions in felsic orthogneiss: Implications for densification and strengthening of the lower continental crust

Growth of garnet and pyroxene in orthogneiss from the Athabasca granulite terrane (AGT), northern Saskatchewan, provides a model for progressive densification and strengthening of the lower continental crust with implications for the recycling and long-term evolution of continental crust. Two distinct assemblages and textures are preserved in granitic and granodioritic gneiss. Low-strain orthogneiss displays igneous textures and assemblages of Opx+Kfs+Pl+Mag+Qz (± Bt, Hbl, Ilm). High-strain, dynamically recrystallized tectonites have additional garnet, clinopyroxene, and a more Na-rich plagioclase, along with relict orthopyroxene. The reaction (Opx+Ca-rich Pl=Grt+Cpx+Na-rich Pl+Qz) is informally called the “Mary reaction” after documented occurrences in the Mary granitoid batholith. The reaction represents the transition from medium-pressure to high-pressure granulite (Green and Ringwood, 1967), but reaction progress was achieved in these deep crustal rocks along an isobaric cooling path at ca. 1 GPa (35–40 km-depth). Ambient P–T conditions were well within the product (low-T-side) stability field. The abundance of the product assemblage (Grt+Cpx+Na-rich Pl) increases with deformation. Metastable igneous assemblages are widely preserved in low-strain samples. With increasing strain, garnet occurs within recrystallized mantles of plagioclase porphyroclasts, and clinopyroxene occurs in the deformed tails of orthopyroxene crystals. Deformation is interpreted to aid in the breakdown of plagioclase and/or the nucleation of garnet and clinopyroxene. Garnet and pyroxene modes have been observed to exceed 10 vol% in the AGT, but larger amounts are possible because Ca-rich plagioclase and orthopyroxene remnants are widely preserved. Densities increase from ca. 2.6 to ca. 3.0 g/cm3 and modeled P-wave velocities approach 7.0 km/s in felsic rocks. Densities in mafic rocks approach 3.4 cm3. The reaction occurred at least twice in the AGT, 2.6 and 1.9 Ga, and may have occurred at other times during long-term deep crustal residence. Thus, densification can occur incrementally and correspond with the time and intensity of tectonism and concomitant recrystallization. The reaction provides a mechanism for significant densification and strengthening of lower continental crust long after initial stabilization. It may explain the discrepancy between seismic velocities suggestive of mafic/intermediate compositions at lower crustal depths and the observed abundance of felsic rocks in exposed granulite terranes.

The Cora Lake shear zone, Athabasca granulite terrane, an intraplate response to far-field orogenic processes during the amalgamation of Laurentia

The Cora Lake shear zone (CLsz) is a 4–6 km wide localized high-strain zone that bisects the polydeformed Athabasca granulite terrane, northern Saskatchewan. It also coincides with the geophysical trace of the Snowbird tectonic zone. The CLsz represents a major lithotectonic and thermobarometric discontinuity within the exposure of &gt;20 000 km2 of high-pressure granulites. Most rocks have a strong mineral lineation plunging moderately to the southwest. The Northwestern subdomain (hangingwall) is characterized by ca. 2.6 Ga plutonic rocks that contain an early, subhorizontal gneissic layering (ca. 2.57 Ga) that was overprinted by large amplitude folds and a partitioned, but pervasive, axial planar, dextral, shear fabric at ca. 1.9 Ga. Thermobarometry suggests metamorphic conditions of ∼0.9 GPa and ∼750 °C during both of the phases of tectonism. The footwall is predominantly underlain by the ca. 3.3–3.0 Ga Chipman tonalite, layers of intercalated mafic and felsic granulite, and the widespread 1.9 Ga Chipman mafic dyke swarm. Early subhorizontal layering in the footwall was also folded at ca. 1.9 Ga and transposed into a steeply dipping, northeast-striking axial planar shear fabric that corresponds with the metamorphic peak (1.1–1.2 GPa and 800–900 °C). These distinct domains were juxtaposed across the CLsz, which contains a gneissic foliation striking 231° and dipping moderately to steeply to the northwest. Abundant sinistral–normal kinematic indicators are consistent with the distinctly lower pressures to the northwest. The shear zone is characterized by very fine grain sizes, despite its high-temperature assemblages including clinopyroxene and garnet. Thermobarometry from the CLsz displays progressive decompression of reworked footwall rocks with increasing mylonitization. In situ monazite geochronology indicates shearing at 1.89–1.87 Ga shortly after the granulite facies metamorphic peak. The anomalous sinistral kinematics of the CLsz, bracketed in time between periods of dextral shearing, can be explained by changing regional stresses during alternating convergent tectonics to the west and to the southeast of the Athabasca granulite terrane.

Silurian high-pressure granulites from Central Qiangtang, Tibet: Constraints on early Paleozoic collision along the northeastern margin of Gondwana

High-pressure (HP) granulites are commonly regarded as indicators of plate convergence and collision following the subduction of oceanic or continental crust. In this study we report the discovery of Silurian HP basic granulites from Central Qiangtang on the Tibetan Plateau. Detailed petrology and geochronology reveal a three-stage metamorphic history based on inclusions, reaction textures, and garnet zoning patterns. Peak metamorphism at 830–860 °C and 1.15–1.45 GPa (M1) is defined by high-Ca garnet cores, high-Al clinopyroxene, and high-Na plagioclase. Symplectites or coronas of orthopyroxene + plagioclase ± magnetite around garnet porphyroblasts indicate garnet breakdown reactions at ca. 810–830 °C and 0.65–0.85 GPa (M2). Kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 590–650 °C and 0.62–0.82 GPa (M3). These results help define a sequential P–T path containing near-isothermal decompression (ITD) and near-isobaric cooling (IBC) stages. Identification of mineral inclusion assemblages in zircons dated by U–Pb SHRIMP and LA–ICP–MS reveals peak HP metamorphism at ca. 427–422 Ma, subsequent near-isothermal decompression with associated retrograde reactions at ca. 392–389 Ma, and continued cooling at ca. 360 Ma. The P–T–t path of HP basic granulites reflects collision followed by extensional exhumation during early Paleozoic orogenesis. The present results indicate the occurrence of a collisional event along the northern margin of Indo-Australian Gondwana during the Silurian. Renewed Gondwana-directed subduction and subsequent collision probably led to the opening of the Paleo-Tethys Ocean.

Middle Paleozoic metamorphism in the Hongseong area, South Korea, and tectonic significance for Paleozoic orogeny in northeast Asia

The Baekdong and Gwangcheon areas are located within the Hongseong collision belt of South Korea, and the discovery of late Permian–Triassic eclogites has confirmed this belt as an extension of the Qinling–Dabie–Sulu collision belt in China. The Qinling–Dabie–Sulu belt records a middle Paleozoic continental collision in addition to the late Permian–Triassic collision of the North and South China cratons. In this paper, we present evidence for middle Paleozoic igneous and metamorphic events in the Baekdong and Gwangcheon areas. Metabasites and meta-basaltic andesites in the Baekdong area originated in an arc tectonic setting at ca. 470–437Ma, and underwent an intermediate-P/T metamorphism (6.2–8.2kbar, 620–700°C) at ca. 418–405Ma. The meta-basaltic andesite in the Gwangcheon area underwent a high-pressure granulite facies metamorphism (close to the eclogite facies) at ca. 392–381Ma. P–T values of 15.7–19.9kbar and 860–890°C were obtained for the peak metamorphism using conventional geothermobarometry, and conditions of 16–17kbar and 870–920°C were estimated by thermodynamic pseudosection analysis using the Perplex X program. Together with the late Permian–Triassic eclogites previously reported from the Hongseong area, these middle Paleozoic igneous activities that were related to subduction and metamorphism indicate that the Hongseong area underwent collision in the middle Paleozoic (418–381Ma), in addition to the late Permian–Triassic collision, as recorded in the Qinling–Dabie–Sulu belt in China.

Neoarchean to Paleoproterozoic high-pressure mafic granulite from the Jiaodong Terrain, North China Craton: Petrology, zircon age determination and geological implications

The North China Craton is an ideal place for studying the transition of the Earth's thermal structure and tectonics at the Archean–Proterozoic boundary due to its good preservation of the ~2.5Ga tectono-thermal events. We report the discovery of a high-pressure mafic granulite from the Jiaodong Terrain in the North China Craton. The mafic granulite occurs as garnet–clinopyroxene–orthopyroxene–hornblende gneiss enclaves within a late-Archean trondhjemite–tonalite–granodiorite (TTG) gneiss. Typical high-pressure mineral assemblage of garnet–clinopyroxene–plagioclase–quartz±rutile has been identified. Plagioclase+clinopyroxene±orthopyroxene±hornblende symplectite surrounding garnet (“white eye”) is also observed. Using the conventional geothermobarometry and the pseudosection modeling, a clockwise metamorphic P–T path with the peak conditions at ~17kbar and ~880°C was determined. Zircon U–Pb analyses (SHRIMP) on the overgrowth rim of zircon grains of two samples from the same outcrop yielded a metamorphic age of 2473±6Ma (MSWD=0.8). The analyses on magmatic core gave a probable magmatic age of 2527±12Ma (MSWD=1.9). The high-pressure granulite facies metamorphism corresponds to a collisional event between the ~2.5Ga crust and ~2.9Ga crust at the dawn of Paleoproterozoic in the North China Craton. It also represents a new but rare case of a subduction–collision tectonics at the Archean–Proterozoic transition and provides insight into the change of the Earth's thermal structure.

Palaeozoic polymetamorphism in the North Qinling orogenic belt, Central China: Insights from petrology and in situ titanite and zircon U–Pb geochronology

The Qinling orogenic belt experienced multiple phases of orogenesis during the Palaeozoic. Unraveling the timing and P–T conditions of these events is the key to understanding the convergence processes between the South China and the North China Blocks. The Songshugou Complex, located in the southern part of the North Qinling orogenic belt, has registered multistage metamorphism in Palaeozoic, and thus potentially provides insights into the tectonic evolution of the Qinling orogenic belt. In this study, three metabasic rocks (a garnet pyroxenite, a garnet amphibolite and a gneissic amphibolite) from the Songshugou Complex were selected for petrological study and zircon and titanite U–Pb dating. Our results show that the metabasic rocks experienced three metamorphic events during the Palaeozoic. The first metamorphic event (M1) is characterized by high pressure conditions. Two zircon grains in equilibrium with garnet and in absence of plagioclase were recognized from the garnet pyroxenite sample. They yielded Ti-in-zircon temperatures of 660–851°C at ∼12.0kbar and a weighted mean age of 498±15Ma, providing the constraints on the temperature and timing of prograde or peak metamorphism (M1-1). Zircons that are inequilibrium with garnet from the garnet pyroxenite and the garnet amphibolite gave U–Pb ages of 494±9Ma and 484±4Ma, and Ti-in-zircon temperatures of 793±33°C and 738±18°C, respectively. Thus, these zircons were formed on the retrograde amphibolite-facies conditions at ∼8.0kbar (M1-2). Titanite inclusions were found in actinolite cores of zoned amphibole from the garnet amphibolite. They yielded a U–Pb age of ∼470Ma and Zr-in-titanite temperature of 676±23°C at pressure of ∼7.0kbar, suggesting that the amphibolite-facies retrogression perhaps persisted to ∼470Ma.Weakly zoned zircons from the garnet amphibolite and inclusion-free titanites from the garnet pyroxenite gave consistent U–Pb ages of 418±5Ma and 423±10Ma, and Ti-in-zircon temperature of 742±26°C and Zr-in-titanite temperature of 764±18°C at ∼7.0kbar, respectively. It is suggested that a heating event (M2) is registered by a subsequent phase of amphibolite-facies metamorphism. The ilmenite-bearing titanite crystals from the garnet pyroxenite yielded a U–Pb age of 352±4Ma, recording a late thermal event (M3).On the basis of combined petrological and geochronological results, we propose a revised tectonic model for the North Qinling orogeny in Palaeozoic. The high pressure granulites were formed by the northward subduction of the Shangdan oceanic slab and the arc-continent collision at ca. 500Ma. Their exhumation happened at ca. 494–484Ma as a result of slab breakoff. Subsequent amphibolite-facies metamorphism dated at ca. 440–420Ma are coeval with the widespread magmatism in the North Qinling Terrane, which are likely caused by the reinitiation northward-subducted of Shangdan oceanic slab. At ca. 350Ma, the North Qinling Terrane was likely affected by another thermal overprinting event.

Paleoproterozoic granulites from the Xinghe graphite mine, North China Craton: Geology, zircon U–Pb geochronology and implications for the timing of deformation, mineralization and metamorphism

The Xinghe graphite deposit is located in the north-central part of the North China Craton (NCC) where Archean gray gneiss, metasedimentary rocks (khondalites) and mafic (high-pressure) granulites constitute the dominant rock types. In this study, we report results of detailed geological and structural mapping around the Xinghe graphite mine. We identify three stages of deformation (D1–D3) and the graphite mineralization is correlated with D1 or between D1 and D2. The first stage of deformation represented by rootless intrafolial folds whose axial surface cuts across the regional penetrative gneissosity (S2) of D2, was constrained at ca. 1.95–1.90Ga by U–Pb age of zircon grains from a mafic high-pressure granulite. The second stage of deformation is represented by tight folds with formation of regional penetrative foliation and strong ductile shearing, when garnet broke down to plagioclase+pyroxene. This stage is dated as ca 1.85Ga, from U–Pb analysis of zircons in leucosome from a migmatitic gneiss. The third stage of deformation is represented by large scale folding, which probably formed between 1.85 and 1.80Ga, as constrained from the ca. 1.80Ga emplacement age of an undeformed pegmatite that cuts across the host metasediments. Prior to the high-pressure granulite metamorphism, voluminous ca. 2.15–2.0Ga granites were emplaced within the khondalites. The graphitization postdates the emplacement of these granites as deduced from the presence of flaky graphites in the metagranites. The protolith of the mafic high-pressure granulite dike that intruded the khondalite at ca. 2.0Ga was also metamorphosed at 1.95–1.90Ga, and is part of the exploited graphite ore body. We therefore constrain the timing of graphite mineralization to be between ca. 1.95 and 1.85Ga. Our geological and geochronological data indicate that all the rock units in the Xinghe graphite mine were subjected to high-pressure granulite facies metamorphism at ca. 1.95–1.90Ga.

Paleozoic HP granulite-facies metamorphism and anatexis in the Dulan area of the North Qaidam UHP terrane, western China: Constraints from petrology, zircon U–Pb and amphibole Ar–Ar geochronology

HP mafic and felsic adakitic granulite bodies in the Dulan area of the North Qaidam ultrahigh-pressure (UHP) terrane record high-pressure (HP) granulite-facies metamorphism and anatexis and provide temporal and tectonic constraints on deep subduction of continental crust and its subsequent exhumation. Mafic HP granulite components dominate the main outcrop and preserve features diagnostic of anatexis and locally may be described as migmatite. The HP mafic granulites comprise garnet, clinopyroxene, plagioclase and quartz. The felsic granulite (leucosome) is mainly composed of K-feldspar+plagioclase+quartz+kyanite+garnet. Detailed zircon U–Pb and amphibole Ar–Ar geochronology, combined with trace element geochemistry, indicate peak metamorphism for the mafic HP granulite at 434±3 to 435±3Ma and peak metamorphism and partial melting for the felsic HP granulite at 433±5 to 438±4Ma, which overlaps the ages of UHP metamorphism for adjacent eclogite (430–446Ma). 40Ar/39Ar amphibole ages of 423 to 432Ma represent amphibolite-facies retrograde metamorphism and indicate rapid cooling during exhumation of the HP granulite bodies. Our geochronological data, combined with field relationships, petrology and geochemistry suggest that HP granulite-facies metamorphism and the partial melting that produced adakitic melts represent the same tectonic event. In this case, the felsic HP granulites (leucosome) formed from an adakitic melt derived from partial melting of mafic HP granulite in the overriding plate in a relatively higher geothermal gradient (15–18°C/km), leaving garnet-cumulate and/or meta-ultramafic (mainly garnet pyroxenite) as the residual component. In contrast, the nearby UHP eclogite is thought to have formed in the subducted plate in a relatively lower geothermal gradient (6–10°C/km). Penecontemporaneous metamorphic ages but different geothermal gradients between HP granulites and related UHP eclogite define a possible paired metamorphic belt generated in a subduction–collision setting associated with the North Qaidam continental collisional orogeny during the Late Ordovician–Early Silurian.

The contribution of metamorphic petrology to understanding lithosphere evolution and geodynamics

In the early 1980s, evidence that crustal rocks had reached temperatures >1000 °C at normal lower crustal pressures while others had followed low thermal gradients to record pressures characteristic of mantle conditions began to appear in the literature, and the importance of melting in the tectonic evolution of orogens and metamorphic–metasomatic reworking of the lithospheric mantle was realized. In parallel, new developments in instrumentation, the expansion of in situ analysis of geological materials and increases in computing power opened up new fields of investigation. The robust quantification of pressure (P), temperature (T) and time (t) that followed these advances has provided reliable data to benchmark geodynamic models and to investigate secular change in the thermal state of the lithosphere as registered by metamorphism through time. As a result, the last 30 years have seen significant progress in our understanding of lithospheric evolution, particularly as it relates to Precambrian geodynamics.Eoarchean–Mesoarchean crust registers uniformly high T/P metamorphism that may reflect a stagnant lid regime. In contrast, two contrasting types of metamorphism, eclogite–high-pressure granulite metamorphism, with apparent thermal gradients of 350–750 °C/GPa, and granulite–ultrahigh temperature metamorphism, with apparent thermal gradients of 750–1500 °C/GPa, appeared in the Neoarchean rock record. The emergence of paired metamorphism is interpreted to register the onset of one-sided subduction, which introduced an asymmetric thermal structure at these developing convergent plate margins characterized by lower T/P in the subduction channel and higher T/P in the overriding plate. During the Paleoarchean to Paleoproterozoic the ambient mantle temperature was warmer than at present by ∼300–150 °C. Although the thermal history of Earth is only poorly constrained, it is likely that prior to ca. 3.0 Ga heating from radioactive decay would have exceeded surface heat loss, whereas since ca. 2.5 Ga secular cooling has dominated the thermal history of the Earth. The advent of paired metamorphism is consistent with other changes in the geological record during the Neoarchean that are best explained as the result of a transition from a stagnant lid to subduction and a global plate tectonics regime by ca. 2.5 Ga. This interpretation is supported by results from 2-D numerical experiments of oceanic subduction that demonstrate a change to one-sided subduction is plausible as upper mantle temperature declined to <200 °C warmer than at present during the late Neoarchean–Paleoproterozoic. This is the beginning of the Proterozoic plate tectonics regime.At 1.0 Ga the ambient mantle temperature was still ∼150–100 °C warmer than at present. Continued secular cooling caused a transition to cold subduction registered in the crustal record of metamorphism by the first appearance of blueschist and high to ultrahigh pressure metamorphism during the Neoproterozoic. Results of 2-D numerical experiments of continental collision demonstrate a transition from shallow to deep slab breakoff associated with stronger crust–mantle coupling that enabled continental subduction to mantle depths as upper mantle temperature declined to <100 °C warmer than at present during the late Proterozoic. This is the beginning of the modern plate tectonics regime.

Metamorphic P–T–t evolution of mafic HP granulites in the northeastern segment of the Tarim Craton (Dunhuang block): Evidence for early Paleozoic continental subduction

The Dunhuang block, which constitutes the northeastern segment of the Tarim Craton, is located between the Central Asian Orogenic Belt (CAOB) to the north and the Altyn–Qilian Orogenic Belt to the south. The present study reveals that the early Paleozoic HP mafic granulites from the Dunhuang block underwent four stages of metamorphism: prograde amphibolite-facies (M1), peak high-pressure (HP) granulite-facies (M2) and two late stages of amphibolite-facies retrograde metamorphism. Based on phase equilibrium modeling, P–T conditions of the four stages of metamorphism are estimated at 720–750°C and 11–13kbar, 760–800°C and 14–16kbar, 690–720°C and 8–8.5kbar, and at <6kbar and <640°C respectively. Peak granulite-facies metamorphism is characterized by a low geothermal gradient of ca. 16°C/km. Zircon U–Pb dating shows that the HP granulite-facies metamorphism occurred at ca. 431Ma and the early retrograde amphibolite-facies overprint at ca. 403Ma. Thus the investigated mafic rocks here reveal a clockwise P–T–t path involving burial heating before peak granulite-facies metamorphism and subsequent decompression–cooling with an uplift rate of ca. 0.8km/Ma. This, together with a continental affinity of the HP metamorphic rocks, indicates that the Dunhuang block experienced a collisional orogenesis during the early Paleozoic.

Discussion of ‘Metamorphic P–T and retrograde path of high-pressure Barrovian metamorphic zones near Cairn Leuchan, Caledonian orogen, Scotland’

K. Aoki, B. F. Windley, S. Maruyama &amp; S. Omori reply: First, we thank Viete, Oliver &amp; Wilde for their interesting and thought-provoking comments on the timing of the high-pressure granulite facies (HGR) metamorphism recorded in metamorphic rocks at Cairn Leuchan, Scotland, published by Aoki et al. (2013). Based on new metamorphic data of garnetites and garnet-amphibolites at Cairn Leuchan and new zircon U–Pb ages of amphibolitized eclogite at Tomatin, we suggested in our publication that the HGR metamorphism was retrograde after eclogite facies before the c. 470 Ma ‘Barrovian metamorphism’. Viete, Oliver &amp; Wilde however speculate that the HGR metamorphism at Cairn Leuchan may have occurred at c. 1000 Ma, as a result of their new U–Pb zircon age of the Cowhythe Gneiss at Portsoy and from previous studies of the geological structure and geochronology. We are grateful for this opportunity to describe, albeit in a preliminary manner, our new understanding and tectonic model of the Caledonian orogen in Scotland and western Ireland of which the Barrovian metamorphism is a key component. A reply to a comment is not the correct place to propose an entirely new paradigm for such a classic orogen, but we will present our model more fully in a future publication.