Granulite Metamorphism Research Articles

ARCHEAN AND PALEOPROTEROZOIC HISTORY OF ROCK METAMORPHISM IN THE URALS CRUSTAL SEGMENT

In the Urals, Archean and Paleoproterozoic formations were found in several polymetamorphic complexes which crop out as relatively small tectonic blocks (within the first thousands sq. km in area). It is most likely that the rocks forming the polymetamorphic complexes located within the paleocontinental Ural area extending to the west of the Main Uralian Fault belong to the Archean-Paleoproterozoic section. These complexes are interpreted as fragments of the crystalline basement of the Ural part of the East European Craton, which are included in the structure of the Uralides. Two stages of the granulite facies metamorphism - earlier Neoarchean and later Paleoproterozoic - were established in the rocks of the polymetamorphic complexes in the paleocontinental (and partially paleooceanic) Ural region. High-pressure mineral parageneses are the products of the earliest endogenous rock transformational processes in eclogite-bearing complexes, where high-pressure metamorphism is apparently complementary to the Paleoproterozoic granulite metamorphism.

P-T-X parameters of metamorphogene and hydrothermal fluids, isotopy and age of the Bogunai gold deposit, southern Yenisei Ridge (Russia)

Fluid inclusions in quartz, sulfides from quartz veins, and quartz, garnet, plagioclase, and orthoclase from granulites of the Bogunai gold deposit located in the granulites of the Angara-Kan block of the Yenisei Ridge were studied by thermobarometry, gas chromatography, chromato-mass-spectrometry, Raman spectroscopy, and mass spectrometry with inductively coupled plasma. The formation temperatures (850-950 °C) and pressures (8.5-9.0 kbar) of minerals of the granulite metamorphic facies are much higher than the crystallization temperatures (220-420 °C) and pressures (0.1-1.6 kbar) of gold-quartz veins of the Bogunai deposit. These veins formed with the participation of H2O-CO2-hydrocarbon fluids with a salt (predominantly MgCl2) concentration of 2-19 wt.% NaCl equiv. The gas phase of fluid inclusions from quartz, pyrite, chalcopyrite, galena, and sphalerite contains not only H2O, CO2, CH4, and N2 but also the first found compounds of sulfur (CS2, O2S, COS, C2H6S2) and nitrogen (C3H7N, C3H7NO, C4H8N2O) and numerous hydrocarbons of different classes (paraffins, arenes, naphthenes, alcohols, aldehydes, ketones, carbonic acids, and furans). The age of the Krasnoyarsk mineralized zone, one of the sites of the Bogunai deposit, is 466 ± 3.2-461.6 ± 3.1 Ma, which is almost 1400 Ma younger than the age of granulite metamorphism and 255 Ma younger than the age of diaphthoresis but is close to the age of the Lower Kan granitoid pluton (455.7 ± 3.4 Ma). The sulfur isotope ratios (534S) of sulfides (pyrite, chalcopyrite, sphalerite, and galena) are close to the mantle values, 0.8 to 3.5%c, and are in the range of the granitoid values, which indicates the crustal source of the fluid sulfur. Gold of the Bogunai deposit accumulated with the participation of H2O-CO2-hydrocarbon fluids generated both in deep-fault zones and in granitoid intrusions.

Early precambrian ferruginous-siliceous rocks (eulysites) of Eurasia

The paper presents geological and petrogeochemical data on the intensely metamorphosed Early Precambrian ferruginous-silicate rocks (eulysites) in Eurasia. Normative-mineral compositions of primary eulysites are given for the first time. These rocks were likely formed from ferruginous-silicate sediments rather than ultramafic metaigneous rocks. The sedimentary origin of eulysites is suggested by their formational affiliation, banded-laminated structures, low contents of protocrystallization trace elements (Cr, Ni, Co, V) in the major rock-forming minerals (fayalite, ortho- and clinopyroxenes), and the presence of biogenic carbonaceous matter. Probably, eulysites are products of the ferruginous-silicate sediments formed under reductive conditions and subjected to granulite metamorphism.

Sub-surface structure of a craton–mobile belt interface: Evidence from geological and gravity studies across the Rengali Province–Eastern Ghats Belt boundary, eastern India

Bouguer gravity anomalies characteristically vary from negative to positive values across craton–mobile belt boundaries in Precambrian shields. This transition is also documented in eastern India, where Proterozoic granulites of the Eastern Ghats Belt (EGB) form part of a mobile belt to the south of the late Archaean, amphibolite facies Rengali Province. The northern margin of the EGB with the Rengali Province is a sub-vertical Cambro-Ordovician strike-slip shear zone, on which the Talchir Gondwana sedimentary basin was deposited during late Palaeozoic extension. This extension also led to partial uplift of the lower crust below the terrane boundary. Closely coordinated geological and gravity studies were conducted through the basin, across the craton–mobile belt contact. Modeling of the Bouguer anomaly using 2-D and 3-D compact inversion schemes, along with 2-D forward modeling, indicates significant differences in the density configurations of the uppermost crusts of the EGB and the Rengali Province, as indicated by the surface geology. However, both inverse and forward models consistently predict that below ~7km, density configurations across the contact are similar and have low upper crustal values, suggesting that typically cratonic crust lies below both terranes. Since it is highly unlikely that lower crust can remain unaffected during granulite facies metamorphism, it is inferred that the present sub-surface EGB crust could not have experienced the high grade event. Rather, the EGB rocks were probably overthrust onto the craton significantly after granulite metamorphism. The top of the thrust sheet may have been eroded prior to or post-dating the overthrusting, leaving the present EGB granulites stranded on low density cratonic crust below.

The early Precambrian history of rock metamorphism in the Urals segment of crust

Early Precambrian rock units in the Urals are present in several polymetamorphic complexes, which are exposed in the Urals in the form of small (<1500 km2) tectonic blocks. Their ages are Archaean (as old as 3.5 Ga) and Palaeoproterozoic. During the formation of these complexes in the early Precambrian, two stages of ultra-high-temperature (granulite) metamorphism occurred. The maximum age of the early Neoarchaean stage of metamorphism is 2.79 Ga. Evidence of this metamorphic event includes the dating of the Taratash gneiss-granulite complex of the South Urals. Gneiss-migmatite complexes, which dominate the lower Precambrian section of the Urals, were formed in the Palaeoproterozoic during the sequential appearance of granulite facies metamorphism followed by amphibolite facies metamorphism and accompanying granitization. The maximum age of the Palaeoproterozoic stage of granulite metamorphism in the Alexandrov gneiss-migmatite complex, the most well-studied complex in the South Urals, is 2.08 Ga.

Evolução metamórfica e estrutural do Complexo Várzea do Capivarita, Cinturão Dom Feliciano, Encruzilhada do Sul, RS

O Complexo Várzea do Capivarita está representado por paragnaisses dispostos como megaxenólitos nos granitos da Suíte Encruzilhada do Sul e como fragmentos menores nos granitos Quitéria e Cordilheira. Essas unidades graníticas afloram no extremo norte do Batólito Pelotas, porção leste do Cinturão Dom Feliciano, próximo a Encruzilhada do Sul. O complexo é constituído por gnaisses pelíticos, com ocorrência subordinada de gnaisses calci-silicáticos, mármores, gnaisses quartzo-feldspáticos e raros quartzitos. Os gnaisses pelíticos possuem injeções de corpos tabulares de muscovita leucogranitos peraluminosos com espessuras centimétricas a métricas. Neste trabalho foram feitas análises petrográficas e microestruturais de seções delgadas representativas das litologias do complexo e a caracterização estrutural da área de estudo. A integração desses dados permitiu avaliar a migmatização dos paragnaisses e a geração das injeções leucograníticas. As condições metamórficas estabelecidas com base nas paragêneses identificadas indicam temperaturas da ordem de 850 a 1000°C e pressões entre 6 e 10 kbar, caracterizando um metamorfismo orogênico de fácies granulito, da série de pressão intermediária e de ultra-alta temperatura (UAT). A concordância estrutural entre o bandamento (S2) dos paragnaisses e a foliação ígnea (S0 ) do leucogranito indica que ambas foram formadas no mesmo evento, implicando que a migmatização dos paragnaisses do Complexo Várzea do Capivarita gerou os leucogranitos peraluminosos durante o ápice do metamorfismo orogênico colisional.

The timing of successive fluid events in the Southern Marginal Zone of the Limpopo Complex, South Africa: Constraints from 40Ar–39Ar geochronology

We have carried out a geochronological study in the high grade metamorphic Southern Marginal Zone of the Limpopo Complex, South Africa, aimed at providing time constraints on episodes of fluid activity. This work chiefly comprises 40Ar/39Ar dating, with accompanying petrography, of amphibole and mica, and further incudes zircon U–Pb geochronology on a post peak metamorphic trondhjemite intrusion. The so-called Zone of Rehydration within 30km N of the Hout River Shear Zone (where the Southern Marginal Zone is thrust over the Kaapvaal Craton) was studied in particular, but sampling also included areas of patchy metasomatism in the Granulite Zone further north, where no post-peak metamorphic rehydration on a regional scale occurred.Amphibole in a mafic granulite and an older generation of zircons in the Petronella trondhjemite yielded dates in the 2700–2730Ma range, corresponding to the published age of peak granulite metamorphism in the Southern Marginal Zone. A cluster of 40Ar/39Ar dates on amphiboles from the Zone of Rehydration indicate an age between 2660 and 2670Ma for this event, in accord with results from previous work on the Hout River Shear Zone. The main population of U–Pb zircon dates from the trondhjemite, dating its intrusion, also falls in this age range, as does the published age of the Matok pluton. These data suggest that rehydration at 2660–2670Ma was a discrete episode. A further 40Ar/39Ar age cluster, defined by dates on both amphiboles and micas, is seen at 2010–2040Ma, which is the age of a major tectonometamorphic event in the Central Zone of the Limpopo Complex, and previously reported from the Southern Marginal Zone as Rb–Sr biotite dates. Events we found in the 2010–2040Ma bracket are strike-slip shearing (dated by fabric forming amphibole), as well as localized potassic metasomatism (dated by both amphiboles and micas). The latter was not confined to the Zone of Rehydration, and appears in one case to be associated with gold mineralization. A number of 40Ar/39Ar dates intermediate between 2600 and 2000Ma were obtained, but these are diverse and can be accounted for by mixing, incomplete resetting and/or excess 40Ar. They do not indicate events in this intermediate age bracket.

Paleoproterozoic S-type granites in the Helanshan Complex, Khondalite Belt, North China Craton: Implications for rapid sediment recycling during slab break-off

S-type granites, typically derived from the rapid recycling of sedimentary rocks, are sometimes accompanied by contemporary mafic magmatism and granulite metamorphism. However, the geodynamic context for such rock suites is often highly disputed, with various model proposed, including back-arc basin opening, lithospheric delamination, mantle plume and continental rifting. The Paleoproterozoic Khondalite Belt in the North China Craton provides an example of synchronous mafic and felsic magmatism that was accompanied by granulite-facies metamorphic events for which the tectonic affinities of these rocks remains unclear. This study integrates in situ zircon Hf–O isotope analyses, whole-rock geochemistry and Nd isotope results for the earliest two-mica granites (ca. 1.95Ga) in order to provide constraints on the above issues. The granites are strongly peraluminous (A/CNK value >1.1), and characterized by high zircon δ18O values of 7.3–10.6‰, corresponding to calculated magmatic δ18O values of 9.1–12.3‰, similar to those of typical S-type granites. They have relatively high and homogeneous ɛNd(t) values of −1.1 to +0.9 and highly variable zircon ɛHf(t) values ranging from −1.0 to +8.3. In situ zircon Hf–O isotopic compositions indicate that the S-type granites may contain some mantle or juvenile crustal components in addition to a sediment component. Based on the new results and published data, a slab break-off model is proposed to explain the rapid recycling of sedimentary precursors and the generation of the ca. 1.95Ga S-type granites.

Earth’s youngest known ultrahigh-temperature granulites discovered on Seram, eastern Indonesia

Episodes of ultrahigh-temperature (UHT, ≥900 °C) granulite metamorphism have been recorded in mountain belts since the Neoarchean. However, evidence for the tectonic mechanisms responsible for the generation of such extreme thermal conditions is rarely preserved. Here we report the discovery of 16 Ma UHT granulites—the youngest identified at the Earth’s surface—from the Kobipoto Mountains of Seram in eastern Indonesia. UHT conditions were produced by a modern tectonic system in which slab rollback–driven lithospheric extension caused core complex–style exhumation of hot subcontinental lithospheric mantle. Overlying continental crust, heated and metamorphosed by exhumed lherzolites, developed spinel + quartz and sapphirine-bearing residual assemblages, shown by phase equilibria modeling to have required temperatures of ∼950 °C at ∼8 kbar pressure. Seram is therefore a possible modern analogue for ancient orogens that incorporate UHT granulites.

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.

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.

Multi-stage crustal growth and cratonization of the North China Craton

The North China Craton (NCC) has a complicated evolutionary history with multi-stage crustal growth, recording nearly all important geological events in the early geotectonic history of the Earth. Our studies propose that the NCC can be divided into six micro-blocks with >∼3.0–3.8 Ga old continental nuclei that are surrounded by Neoarchean greenstone belts (GRB). The micro-blocks are also termed as high-grade regions (HGR) and are mainly composed of orthogneisses with minor gabbros and BIF-bearing supracrustal beds or lenses, all of which underwent strong deformation and metamorphism of granulite- to high-grade amphibolite-facies. The micro-blocks are, in turn, from east to west, the Jiaoliao (JL), Qianhuai (QH), Ordos (ODS), Ji'ning (JN) and Alashan (ALS) blocks, and Xuchang (XCH) in the south. Recent studies led to a consensus that the basement of the NCC was composed of different blocks/terranes that were finally amalgamated to form a coherent craton at the end of Neoarchean.Zircon U-Pb data show that TTG gneisses in the HGRs have two prominent age peaks at ca. 2.9–2.7 and 2.6–2.5 Ga which may correspond to the earliest events of major crustal growth in the NCC. Hafnium isotopic model ages range from ca. 3.8 to 2.5 Ga and mostly are in the range of 3.0–2.6 Ga with a peak at 2.82 Ga. Recent studies revealed a much larger volume of TTG gneisses in the NCC than previously considered, with a dominant ca. 2.7 Ga magmatic zircon ages. Most of the ca. 2.7 Ga TTG gneisses underwent metamorphism in 2.6–2.5 Ga as indicated by ubiquitous metamorphic rims around the cores of magmatic zircon in these rocks. Abundant ca. 2.6–2.5 Ga orthogneisses have Hf-in-zircon and Nd whole-rock model ages mostly around 2.9–2.7 Ga and some around 2.6–2.5 Ga, indicating the timing of protolith formation or extraction of the protolith magma was from the mantle. Therefore, it is suggested that the 2.6–2.5 Ga TTGs probably represent a coherent event of continental accretion and major reworking (crustal melting).As a distinct characteristic, nearly all GRBs in the NCC underwent amphibolite-facies metamorphism. Zircon U-Pb ages of metamorphosed GRB mafic rocks mainly show two peak ranges at ∼2.6–2.5 and 2.8–2.7 Ga. The mafic rocks are commonly believed to be derived from metabasalts, it is therefore possible that the ages represent the time of metamorphism. The tectonic settings of the GRBs are still a problem. Their geochemical characteristics are, respectively, similar to back-arc basins, rifts, island arcs or suggest imprints of mantle plumes. BIFs occur in all GRBs but also in the HGRs. This metallogenic specificity is quite different from all Phanerozoic geotectonic settings.The ∼2.5 Ga metamorphic-magmatic event is stronger than in most other cratons in the world. How to understand the geological significance of the 2.5 Ga event? The following points are emphasized: (1) nearly all old rocks >2.5 Ga underwent metamorphism at ∼2.52–2.5 Ga; (2) Archean basement rocks in the NCC experienced strong partial melting and migmatization; (3) granitoid rocks derived from partial melting include potassium granites, TTG granites and monzonites. These granitoids rocks intruded both the Archean greenstone belts and micro-blocks; (4) ∼2.5 Ga mafic dikes (amphibolites), granitic dikes (veins) and syenitic-ultramafic dykes are also developed. Therefore, we suggest an assembly model that all micro-blocks in the NCC were welded together by late Archean greenstone belts at the end of the late Neoarchean. We also propose that the various micro-blocks were surrounded by small ocean basins, and the old continental crust and the oceanic crust were hotter than today. Subduction and collision were on much smaller scales as compared to the Phanerozoic plate tectonic regime, although the tectonic style and mechanisms were more or less similar. The formation of crustal melt granites is one of the processes of cratonization, inducing generation of stable upper and lower crustal layers. This process also generated an upper crust of more felsic composition and a lower crust of more mafic composition, due to molten residual materials and some underplated gabbros.

Temporal links between pluton emplacement, garnet granulite metamorphism, partial melting and extensional collapse in the lower crust of a Cretaceous magmatic arc, Fiordland, New Zealand

AbstractGarnet granulite facies mid‐to lower crust in Fiordland, New Zealand, provides evidence for pulsed intrusion and deformation occurring in the mid‐to lower crust of magmatic arcs. 238U‐206Pb zircon ages constrain emplacement of the ∼595 km2 Malaspina Pluton to 116–114 Ma. Nine Sm‐Nd garnet ages (multi‐point garnet‐rock isochrons) ranging from 115.6 ± 2.6 to 110.6 ± 2.0 Ma indicate that garnet granulite facies metamorphism was synchronous or near synchronous throughout the pluton. Hence, partial melting and garnet granulite facies metamorphism lasted &lt;5 Ma and began within 5 Ma of pluton emplacement. Garnet granulite facies L‐S tectonites in the eastern part of the Malaspina Pluton record the onset of extensional strain and arc collapse. An Sm‐Nd garnet age and thermobarometric results for these rocks directly below the amphibolite facies Doubtful Sound shear zone provide the oldest known age for extension in Fiordland at ≥112.8 ± 2.2 Ma at ∼920 °C and 14–15 kbar. Narrow high Ca rims in garnet from some of these suprasolidus rocks could reflect a ≤ 1.5 kbar pressure increase, but may be largely a result of temperature decrease based on the Ca content of garnet predicted from pseudosections. At peak metamorphic conditions &gt;900 °C, garnet contained ∼4000 ppm Ti; subsequently, rutile inclusions grew during declining temperature with limited pressure change. Garnet granulite metamorphism of the Malaspina Pluton is c. 10 Ma younger than similar metamorphism of the Pembroke Granulite in northern Fiordland; therefore, high‐P metamorphism and partial melting must have been diachronous for this &gt;3000 km² area of mid‐to‐lower crust. Thus, two or more pulses of intrusion shortly followed by garnet granulite metamorphism and extensional strain occurred from north to south along the axis of the lower crustal root of the Cretaceous Gondwana arc.

Metamorphism and partial melting of high-pressure pelitic granulites from the Qianlishan Complex: Constraints on the tectonic evolution of the Khondalite Belt in the North China Craton

High-pressure pelitic granulites with a peak assemblage of garnet+kyanite+K-feldspar+plagioclase+biotite+quartz have been found from the Qianlishan Complex in the westernmost part of the Khondalite Belt, an east–west-trending Paleoproterozoic orogen between the Ordos and Yinshan Blocks in the North China Craton. Petrographic evidence indicates that high-pressure pelitic granulites underwent the prograde, peak and post-peak decompression stages. The early prograde metamorphic stage (M1) is represented by an inclusion-type mineral assemblage of quartz+plagioclase+muscovite+biotite+sillimanite, which occurs within the core of garnet porphyroblasts. The peak metamorphic stage (M2) is marked by the mantle growth of garnet porphyroblasts and matrix minerals kyanite, K-feldspar, biotite, plagioclase and quartz. The decompression stage (M3) is indicated by sillimanite replacing kyanite, the formation of the cordierite+sillimanite symplectite in the matrix (M3–1), and the cordierite corona replacing garnet (M3–2). These mineral assemblages and their P–T estimates based on the assemblage stability fields of P–T pseudosection constructed in NCKFMASHTO define a clockwise P–T path involving near-isothermal decompression. Combined with available metamorphic age data, the clockwise P–T paths reconstructed for high-pressure pelitic granulites from the Qianlishan Complex and medium-pressure pelitic granulites from other complexes in the Khondalite Belt are in accord with collision between the Yinshan and Ordos Blocks to form the Western Block at ∼1.95Ga, followed by the exhumation and associated decompressional partial melting at ∼1.88Ga.

Fluid inclusion studies on the Koraput Alkaline Complex, Eastern Ghats Province, India: Implications for mid-Neoproterozoic granulite facies metamorphism and exhumation

Following ultrahigh temperature granulite metamorphism at ∼1Ga, the Eastern Ghats Province of India was intruded by the Koraput Alkaline Complex, and was subsequently re-metamorphosed in the granulite facies in the mid-Neoproterozoic time. Fluid inclusion studies were conducted on silica undersaturated alkali gabbro and syenites in the complex, and a pre-metamorphic pegmatitic granite dyke that intrudes it. High density (1.02–1.05g/cc), pseudo-secondary pure CO2 inclusions are restricted to metamorphic garnets within the gabbro and quartz within the granite, whereas moderate (∼0.92–0.95g/cc) and low density (∼0.75g/cc) secondary inclusions occur in garnet, magmatic clinopyroxene, plagioclase, hornblende and quartz. The isochores calculated for high density pseudo-secondary inclusions pass very close to the peak metamorphic window (∼8kbar, 750°C), and are interpreted to represent the fluid present during peak metamorphism that was entrapped by the growing garnet. Microscopic round inclusions of undigested, relict calcite in garnet suggest that the CO2 present during metamorphism of the complex was internally derived through carbonate breakdown. Pure to low salinity (0.00–10.1wt% NaCl equivalent) aqueous intra-/intergranular inclusions showing unimodal normal distribution of final ice-melting temperature (Tm) and temperature of homogenization (Th) are present only in quartz within the granite. These represent re-equilibrated inclusions within the quartz host that were entrapped at the metamorphic peak. Rare, chemically precipitated graphite along the walls of carbonic inclusions is interpreted as a post-entrapment reaction product formed during decompression. The fluid inclusion evidence is consistent with rapid exhumation of a thickened lower crust following the mid-Neoproterozoic granulite facies metamorphic event. The study suggests that mantle CO2, transported by alkaline magma into the crust, was locked up within carbonates and released during granulite metamorphism.

Late Cretaceous (~81Ma) high-temperature metamorphism in the southeastern Lhasa terrane: Implication for the Neo-Tethys ocean ridge subduction

An integrated study of UPb zircon dating, geochemical and SrNdHf isotopic compositions of garnet-bearing granulite and marble from the granulite-facies domain of the Nyingchi Complex (eastern Himalayan syntaxis) has provided insights into the tectonic evolution of the southern Lhasa terrane. The peak metamorphism of the garnet-bearing granulite is marked by a mineral assemblage of garnet + orthopyroxene + high-Ti amphibole + plagioclase + quartz + rutile. Abundant exsolved rutile needles are observed within amphibole, garnet and quartz. The peak metamorphic temperatures are estimated at 803–924 °C. Geochemical data from the garnet-bearing granulites provide evidence for a basaltic protolith that formed in a continental-margin arc setting. SrNdHf isotopic compositions indicate that this protolith was sourced from partial melting of a depleted mantle. LA-ICP MS UPb zircon dating shows that the protolith age and metamorphic age of the garnet-bearing granulites are 89.3 ± 0.6 Ma and 81.1 ± 0.8 Ma, respectively. The detrital magmatic zircons from the marble yield ages from 86.3 to 167 Ma. The age distribution and Hf isotopic composition (εHf(t) = + 5.9 to + 17.5) of the detrital magmatic zircon in the marble are consistent with the isotopic data of zircons from the Jurassic–Cretaceous Gangdese batholiths, suggesting that the clastic sediments were partially derived from these intrusives or associated volcanic rocks, and deposited in the fore-arc basin of the Gangdese arc. The metamorphic zircons in the marble yield a metamorphic age of 81.4 ± 0.5 Ma. These results show that both the arc magmatic rocks and forearc sedimentary rocks underwent high-temperature (HT) granulite-facies metamorphism at ~ 81 Ma, indicating anomalously high heat input in the forearc region. A range of tectonic observations, including a coeval hiatus in arc magmatism and a period of regional uplift, indicate that HT metamorphism resulted from the subduction of the Neo-Tethys ocean ridge beneath the southern Lhasa terrane during the Late Cretaceous.

Detrital provenance of the Grenvillian Oaxacan Complex, southern Mexico: a zircon perspective

The Oaxacan Complex is the largest exposure of Grenvillian-age rocks in Mexico, constituting the backbone of the Oaxaquia microcontinent. Whereas the main rock-forming events were previously established at 1,150–1,200 Ma (charnockite–syenite–gabbros), 1,020 Ma (AMCG suite), 990 Ma (granulite-facies metamorphism), and ca. 970 Ma post-tectonic pegmatites, no data are yet available to establish provenance links with other Grenville-age terranes. In this work, we studied detrital zircons belonging to 12 samples, all metamorphosed under granulite facies but variably affected by retrogression. Laser ablation inductively coupled plasma mass spectrometry U–Pb geochronology was employed on selected zircons to determine their crystallization age and geochemistry. The results of the analysis of about 100 crystals per sample show that the studied zircons range between ca. 940 and 1,400 Ma, with only three samples having zircons between 1,400 and 1,600 Ma, and only one showing older zircons up to ca. 1,775 Ma. Whereas some of the slightly discordant (1–5 %) zircons in several samples show ages younger than the granulite metamorphism (probably as a result of Pb loss), and thus a disturbed geochemical pattern (abnormal enrichment in LREE, decreasing HREE), a few metamorphic zircons show flat and depleted HREE patterns, contrasting with the igneous pattern of older zircons (positive Ce anomaly, negative Eu anomaly, enriched HREE pattern). The main distributions observed using the kernel density estimator diagrams fall in the range 975–995 Ma (six samples), 1,100 Ma (four samples) and 1,120–1,170 Ma (six samples). Only the southernmost sample shows a marked peak at ca. 1,400 Ma. The application of the Kolmogorov–Smirnov (K–S) statistical test to the studied samples and particularly the comparison of obtained P values yield interesting similarities. Overall, two sample groups show internal similarities, i.e., they may belong to the same source area, whereas only one sample is dissimilar, failing to pass the K–S test. Comparison of these data with the timing of comparable events in the Sveconorwegian orogens, the Sunsas and Rondonia-San Ignacio belts of Amazonia, and some of the Precambrian massifs cropping out in the Andes help to constrain possible Mesoproterozoic conjugate margins of Oaxaquia.

Petrogenesis, P–T–t path, and tectonic significance of high-pressure mafic granulites from the Jiaobei terrane, North China Craton

High-pressure (HP) mafic granulites in the Jiaobei terrane are composed predominantly of garnet mafic granulites, garnet–hypersthene granulites, and garnet amphibolites, and they are found as irregular lenses or deformed dike swarms within tonalitic–trondhjemitic–granodioritic gneisses and granitic gneisses. The HP mafic granulites contain four distinct metamorphic assemblages, of which the early prograde assemblage (M1) is represented by the cores of garnets, together with mineral inclusions of clinopyxene+plagioclase±quartz, and it formed at 740–770°C and 0.90–1.00GPa. In contrast, the peak assemblage (M2) consists of high-Ca cores in garnet, high-Al cores in clinopyroxene, and high-Na cores in plagioclase in the matrix, which formed under P–T conditions of 850–880°C and 1.45–1.65GPa. The peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of orthopyroxene+clinopyxene+plagioclase±quartz±amphibole±magnetitie symplectites or coronas surrounding some garnet grains, with P–T conditions of 780–830°C and 0.65–0.85GPa. Surrounding some garnet grains are symplectites of amphibole+plagioclase+quartz±magnetitie, which formed during a cooling retrograde stage (M4) with P–T conditions of 590–650°C and 0.62–0.82GPa. An integrated study involving laser Raman analysis of mineral inclusions, cathodoluminescence imaging, and in situ U–Pb dating of zircons shows that the protolith ages of the HP mafic granulites are mainly 2550–2500Ma, and that the timing of the peak metamorphism of the HP mafic granulites ranges from 1900 to 1860 Ma, as recorded by the cores of metamorphic zircons. On the other hand, the medium- to low-pressure granulite–amphibolite facies retrogression occurred mainly at 1860–1820Ma, as recorded by the rims of some single zircon grains and the zircon grains that contain inclusions of garnet+orthopyroxene+plagioclase+sphene. The combination of petrography, mineral compositions, metamorphic reaction history, thermobarometry, and geochronology defines a near-isothermal decompressional clockwise P–T–t path for the Jiaobei HP mafic granulites, suggesting that the Jiaobei terrane underwent initial crustal thickening during 1950–1860Ma, followed by relatively rapid exhumation, cooling, and retrogression in the period 1860–1820Ma. This tectonothermal path was probably generated by subduction and collision-related tectonic processes.

U–Cr-rich high Mg-Al granulites from Karimnagar Granulite Belt, India: implications for Neoarchean-Paleoproterozoic events in southern India

High Mg-Al granulite occurs as enclave within granite gneisses at Karimnagar, southern India, and it contains coarse granoblastic aggregates of orthopyroxene and sapphirine with minor amount of cordierite, spinel and phlogopite. An important chemical characteristic of these minerals is their extremely high MgO content and the high Cr2O3 in sapphirine and spinel. Textural analysis shows sapphirine + orthopyroxene + cordierite as the peak-metamorphic assemblage that possibly evolved though the breakdown of a spinel-bearing assemblage. Cation exchange geothermometers involving orthopyroxene, sapphirine and spinel yield temperatures of 600–800 °C with a maximum of 860 °C implying an event of high temperature (HT) metamorphism. Pseudosection analysis in the FeO–MgO–Al2O3–SiO2 chemical system shows the stability of the peak- assemblage below 6.2 kbar. Subsequently, the rock underwent hydration and cooling with the appearance of phlogopite in the assemblage. Chromium enrichment is possibly inherited from the protolith and its presence presumably stabilized sapphirine and spinel below their high-temperature stability field. The recorded Rb–Sr age of ca. 2,500 Ma in host granite gneiss marks the upper age limit of HT metamorphism. Presence of patchy, lobate grains as well as veinlets of uraninite and brannerite is also a characteristic feature of the rock. Uranium mineralization took place during the post peak metamorphic stage, sulfide mineralization represented by tiny grains and veinlets of pyrite, millerite and pentlandite coincided with, and outlasted the uranium mineralization. The U–Th–Pb chemical ages of uraninite grains suggest ca. 2,200 ± 12 Ma for the age of uranium mineralization in the granulite. Based on the field relations, it is surmised that the granulite metamorphism in the study area is older than ca. 2,500 Ma and is comparable with an event in the other parts of Eastern Dharwar Craton. It can be conceived as a widespread event in southern India.

The Belomorian eclogite province: sequence of events and age of the igneous and metamorphic rocks of the Gridino association

Within the Belomorian eclogite province, near Gridino Village, rocks of different compositions (tonalite-trondhjemite-granodioritic gneisses, granites, mafic and ultramafic rocks) were metamorphosed. The metamorphism included subsidence with increasing pressure and temperature, an eclogite stage, decompression in the granulitic facies, and a retrograde stage in the amphibolitic facies. We attempted to characterize the succession and to date igneous and metamorphic events in the evolution of the Gridino eclogite association. For this purpose, we conducted the following studies: U–Pb isotope dating of zircon (conventional and SHRIMP II methods) from gneisses, a mafic dike, and a high-pressure granitic leucosome; U–Pb dating of rutile from mafic dikes; 40Ar/39Ar dating of amphibole and mica; and Sm–Nd studies of rocks and minerals. The Sm–Nd model ages of felsic (2.9–3.1 Ga) and mafic (3.0–3.4 Ga) rocks from the Gridino eclogite association and individual magmatic zircon grains with an age of ca. 3.0 Ga indicate the Mesoarchean age of the metamorphic-rock protoliths. The most reliable result is the upper age bound of eclogitic metamorphism (2.71 Ga), which reflects the time of the posteclogitic decompression melting of eclogitized rocks under high-pressure retrograde granulitic metamorphism. The mafic dikes formed from 2.82 Ga to 2.72 Ga, most probably, at 2.82 Ga, in accordance with the crystallization age of magmatic zircon from metagabbro. Superimposed amphibolitic metamorphism and the “final” exhumation of metamorphic complexes at 2.0–1.9 Ga are associated with the later Svecofennian tectonometamorphic stage. Successive cooling of the metamorphic associations to 300 °C at 1.9–1.7 Ga is shown by U–Pb rutile dating and 40Ar/39Ar mica dating.