High-pressure Granulite Metamorphism Research Articles

The Multiple Metamorphism of Mafic Granulites From the East Hebei Terrane, North China Craton: Insights Into the Transition of Tectonic Regimes

The East Hebei terrane from the North China Craton preserves the dome–and–keel structures, which was transected by a later linear belt in the north margin. Mafic granulites from the linear belt and domes record two groups of metamorphic ages at Neoarchean and Paleoproterozoic, but their accurate metamorphic peak conditions and paths have not been well addressed. Three samples of mafic granulites, including two-pyroxene granulite (JD15120), garnet-bearing two-pyroxene granulite (YC8-43), and garnet clinopyroxene granulite (JD1546), were documented for detailed metamorphic studies. Two-episode metamorphism can be recognized. The first-episode recovered from JD15120 and YC8-43 is represented by peak assemblage of medium-grained clinopyroxene, orthopyroxene, amphibole, plagioclase, and ilmenite, which yields ultrahigh temperature (UHT) conditions of 940–960°C at 7.5–8.5 kbar and 950–990°C at 8 kbar, respectively, constrained by contours of the maximum anorthite (XAn) in plagioclase cores. The post-peak evolution is dominated by cooling with decompression, constrained mostly from the measured core-to-rim decreasing XAn in plagioclase. By contrast, the second-episode overprinting is recognized in all samples, but exhibits varying textures. In garnet-bearing samples (YC8-43 and JD1546), the overprinting assemblages are characterized by poikilitic garnet that occurs either as coronae around the first-episode pyroxenes, forming “red-eye socket” textures, or as grains in equilibrium with tiny-grained clinopyroxene, plagioclase, amphibole, rutile, and quartz, forming high-pressure (HP) granulite assemblages. These HP granulite assemblages show peak conditions of ∼12 kbar/860°C and ∼12.6 kbar/835°C, constrained by contours of the maximum grossular (XGrs) in garnet cores and the minimum XAn in plagioclase cores. The post-peak evolution is dominated by isothermal decompression, constrained from the outward decreasing XGrs in garnet and increasing XAn in plagioclase. LA-ICP-MS U-Pb zircon dating on JD15120 and JD1546 suggests two metamorphic ages of ∼2.49 Ga and ∼1.78 Ga, being considered to be correlated with the UHT and HP granulite metamorphism, respectively. Tectonically, the late Neoarchean UHT granulite metamorphism may correlate a vertical sagduction regime, whereas the late Paleoproterozoic HP granulite metamorphism is favored to register the continental collision in the northern margin of the North China Craton. This study may have indications for the Neoarchean–Paleoproterozoic tectonic transition of the craton.

Ultrahigh-temperature mafic granulite in the Huai’an Complex, North China Craton: Evidence from phase equilibria modelling and amphibole thermometers

The Huai’an Complex in the north-western Trans-North China Orogen, North China Craton is recognized to have undergone high-pressure metamorphism with controversial ages. Two mafic granulite samples (17HT13 and 16HT22) from Huangtuyao of the Huai’an Complex are documented for metamorphism based on petrography, mineral chemistry, phase equilibria modelling and zircon dating. The rocks comprise garnet, clinopyroxene, orthopyroxene, amphibole, plagioclase and ilmenite. They show clockwise P-T paths involving the pre-Tmax decompressional heating, the Tmax and the post-Tmax cooling stages. The pre-Tmax decompressional heating is revealed by the core-rim/mantle anorthite-ascending zoning in coarse-grained plagioclase. The Tmax stage is constrained to 8.5–10.5 kbar/980–1010 °C (17HT13) and 7.5–9.5 kbar/1000–1030 °C (16HT22) by the high Ti content of amphibole, and the anorthite content of coarse-grained plagioclase rims/mantles and fine-grained plagioclase cores. These temperatures are slightly higher than the results of amphibole thermometers (∼940–980 °C). The post-Tmax cooling is defined by the later growth of amphibole and some conventional geothermobarometers. Because it is dominated by temperature with ilmenite present, the Ti content of amphibole is proposed as a reliable thermometer, and the Ti-rich amphibole with Ti > 0.28 p.f.u. or TiO2 > 2.4–2.6 wt% is indicative of ultrahigh-temperature (>900 °C, UHT) conditions. The anorthite content of plagioclase is also significant in confining peak temperatures for UHT granulite but requires defining proper water content. Zircon dating for 17HT13 yields an upper intercept age of 1910 ± 35 Ma, representing the cooling of UHT granulite. We conclude that the Huai’an Complex may have at least locally undergone UHT metamorphism at ∼1.92–1.91 Ga after high-pressure granulite metamorphism at ∼1.95 Ga, similar to the Jining Complex. Therefore, the Huai’an and Jining Complexes share the same tectonic evolution involving ∼1.95 Ga crustal thickening, ∼1.92–1.91 Ga extension with regional or local UHT metamorphism, and subsequent cooling and uplifting.

High-grade metamorphism during Neoproterozoic to Early Palaeozoic Gondwana assembly, exemplified from the East African Orogen of northeastern Mozambique

Northeastern Mozambique exposes a deeply eroded high-grade segment of the East African Orogen, situated at the critical intersection of the Pan-African orogenic belts, showing widespread granulite facies lithologies. This study documents and characterizes metamorphism attributed to the multistage Neoproterozoic and Early Palaeozoic events, relating to a polyphase Pan-African history of outboard nappe stacking, nappe emplacement onto Mesoproterozoic crust, and compressional and extensional deformation along the Lurio belt: 1) A Neoproterozoic nappe stack (Cabo Delgado Nappe Complex) records high-pressure granulite facies: The highest peak conditions reach P = 1.45 GPa and T > 800 °C are in Grt-Cpx-Pl-bearing mafic granulites of the Xixano Complex. A post-peak isothermal decompressional P-T evolution is supported by garnet chemical zoning pattern, pseudsection modelling and garnet corona textures. 2) The underlying Mesoproterozoic gneiss complexes record amphibolite facies to intermediate-pressure granulite facies, the high-grade metamorphism is documented by Grt-Opx-Cpx-Pl-bearing mafic granulites and charnockitic gneisses in the southern part of the Unango and Marrupa complexes. This metamorphism is attributed to crustal thickening related to overriding of the nappe complex and shortening along the Lurio belt during the early Palaeozoic Kuunga orogeny. 3) The Ocua Complex coring the Lurio belt is a locus of Early Palaeozoic high-pressure metamorphism. A minimum of P = 0.79 ± 0.13 GPa at T = 760 ± 98 °C is constrained by Grt-Opx-Cpx-Pl-Qz assemblage in mafic granulite from the western limb while high-pressure granulite metamorphism was reached in the eastern part of the complex. The high-pressure granulite facies conditions is followed by isothermal decompression.

Neoproterozoic transpression and granite magmatism in the Gavilgarh-Tan Shear Zone, central India: Tectonic significance of U-Pb zircon and U-Th-total Pb monazite ages

The Gavilgarh-Tan Shear Zone (GTSZ) is a crustal-scale shear/fault zone that dissects the unclassified basement gneisses separating two major supracrustal belts, viz. the Paleo- to Mesoproterozoic (≥1.5Ga) Betul Belt and the Neoproterozoic (∼1.0Ga) Sausar Belt, of the Central Indian Tectonic Zone (CITZ). The GTSZ extends for more than 300km strike length, partly covered by the Deccan Trap flows. Granitoid rocks ranging from syenogranite to granodiorite in composition, sheared at temperatures corresponding to the amphibolite facies metamorphic condition, define the GTSZ in the Kanhan River Valley. Earlier geological studies have suggested that the GTSZ underwent a sinistral-sense partitioned transpression in response to an oblique collision between two continental fragments, possibly related to crustal thickening and high-pressure granulite metamorphism (the Ramakona-Katangi granulite: RKG) in the northern part of the Sausar Belt. LA-ICP-MSU-Pb dating of zircon and EPMA U-Th-total Pb dating of monazite grains from four different types of syn-tectonic granitoids of the GTSZ carried out in the present study show that granitoids intruded the basement gneisses between 1.2Ga and 0.95Ga, given the error limit of the calculated ages. The age of transpression and mylonitization is more definitely bracketed between 1.0Ga and 0.95Ga, which correlates well with the published ages of deformation and metamorphism in the Sausar Belt. This age data strongly supports the suggested collisional tectonic model involving the GTSZ and the RKG granulites of the Sausar Belt and underlines a Grenvillian-age tectonic history for the southern part of the Central Indian Tectonic Zone (CITZ), which possibly culminated in the crustal assembly of the Neoproterozoic supercontinent Rodinia.

High-pressure mafic granulites of the South Muya Block (Central Asian Orogenic Belt)

Mineralogical, petrographic, and geochemical studies of mafic granulites of the South Muya Block (Central Asian Orogenic Belt) have been carried out. The granulite protoliths were olivine- and plagioclase- rich cumulates of ultramafic–mafic magmas with geochemical affinities of suprasubduction rocks. The isotope–geochemical characteristics of the granulites indicate the enriched nature of their source, associated with recycling into the mantle of either ancient crust or oceanic sediments, or intracrustal contamination of melts at the basement of the ensialic arc. Formation of garnet-bearing parageneses has occurred during high-pressure granulite metamorphism associated with accretion in the eastern part of the Baikal–Muya composite terrane.

Zircon U–Pb ages of Paleoproterozoic mafic granulites from the Huai’an terrane, North China Craton (NCC): Implications for timing of cratonization and crustal evolution history

The North China Craton (NCC) is considered to have been incorporated into the Columbia supercontinent during late Paleoproterozoic. The Huai’an terrane located in north-central part of the NCC is an important tectonic window to investigate the Paleoproterozoic crustal evolution history of this craton, particularly due to the wide occurrence of high-grade metamorphic rocks including retrograded eclogites and high-pressure granulites. Available data show two controversial peak metamorphic ages of these rocks as 1.95–1.90 or ∼1.85–1.80Ga. Thus, determining the precise timing of metamorphism of the high-pressure granulites is important in reconstructing the collisional history. In this study, we present zircon U–Pb ages, bulk chemistry, and mineral inclusions in zircons and matrix mineral assemblages from the high-pressure granulites to evaluate the timing of metamorphism. Our results reveal ca. 2.05–2.0Ga ages from magmatic zircons in these rocks representing the timing of emplacement of mafic magmas. The metamorphic zircons yield two groups of ages at 1.95–1.90 and ∼1.85∼1.80Ga. Clinopyroxene, plagioclase, titanite, and apatite occur as common mineral inclusions within the ∼1.85–1.80Ga metamorphic zircons. The chemical compositions of clinopyroxene and plagioclase occurring as inclusions are very similar to those of the rim compositions of those minerals in the matrix, suggesting their formation during retrograde granulite metamorphism, and are distinctly different from those of the core domains of the minerals in the matrix which represent high-pressure granulite metamorphism. These results indicate that the metamorphic zircons formed during retrograde granulite metamorphism dated as ∼1.85–1.80Ga and the age range of ∼1.95–1.90Ga corresponds to high-pressure granulite metamorphism. The dual nature of the geological and geochronological features suggest that the Archean gray gneisses in the Huai’an terrane represent the basement rocks detached from the Khondalite Belt of the NCC after collision. Integrating with published data, a double-side collision with scissor-like suturing model is proposed for the Paleoproterozoic cratonization of the NCC.

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.

Charnockitization and enderbitization of mafic granulites in the Porya Bay area, Lapland Granulite Belt, Southern Kola Peninsula: I. Petrology and geothermobarometry

Charnockitization of mafic Opx-Cpx-Grt-Hbl-Bt-Pl ± Qtz granulites and Hbl-Opx-Bt hornblendites was studied in the southeastern part of the Lapland Granulite Belt. The evolutionary trends of the whole-rock compositions and mineral assemblages indicate that the rocks were affected by Na-K-Si-H2O-CO2-Cl brines, which came from outside, alkalinized and debasified the granulites, introduced Na, K, and Si into them, and depleted them in Mg, Fe, and Ca prior to the onset of charnockite melting; the latter began in the granulites only in their most extensively debasified domains. In the course of alkaline metasomatism, pyroxene were replaced by secondary hornblende and biotite with high Ti concentrations, analogous to those in the unaltered granulites. This suggests that the pre-charnockite amphibolization and biotitization were induced not by a temperature decrease but by the effect of Na- and K-bearing fluid during the metamorphic culmination. The metasomatically altered granulites, which were transformed into leucocratic disintegrated amphibolite skialiths, were gradually resorbed and dissolved in the charnockite melt, whose bulk composition corresponded to low-alkaline granites and tonalites. Hence, no contamination took place, and the excess Mg, Fe, and Ca amounts with respect to the eutectic composition were removed from the reaction zone. Variation diagrams indicate that the whole-rock composition of the granulites is gradually shifted toward the composition of charnockitoids. In certain instances, however, melanocratic Hbl-Grt-Opx-Cpx-Pl rims were formed along the granulite-charnockite interface, with the bulk composition of these fringes richer in Mg, Fe, and Ca than that of the ambient granulites. The reason for this was the sporadic redeposition of Mg, Fe, and Ca, which were mobilized from during charnbockitization and redeposited immediately in the reaction zone. In addition, rocks around the charnockite veins bear autonomous melanocratic Grt-Opx-Cpx-Hbl ± Mag ± Ilm ± Scp ± Pl ± Qtz veins whose mineral assemblages and bulk composition are close to those of the melanocratic rims around charnockitoids. The veins were formed via the transportation of Mg, Fe, and Ca for long distances outside the charnockitization zones. TWQ thermobarometric calculations indicate that the pre-charnockite alkaline metasomatism and debasification (amphibolization, biotitization, and feldspathization), anatectic formation of charnockite migma or magma, and the development of the melanocratic veins took place at the peak of the high-pressure granulite metamorphism at the same P-T parameters: approximately 800°C and 9–9.5 kbar. The calculated composition of the charnockitizing fluids suggests that they were homogeneous brines with \( X_{H_2 O} = 0.45 \), \( X_{CO_2 } = 0.10 \), XNaCl = 0.30, and XKCl = 0.15.

Paired metamorphic belts revisited

The modern plate tectonics regime is characterized by a duality of thermal environments, one representing the subduction zone and the other representing the arc–backarc or orogenic hinterland. This duality is the hallmark of one-sided (asymmetric) subduction, and the characteristic imprint of one-sided subduction in the geological record is predicted to be the broadly contemporaneous occurrence of two contrasting types of metamorphic belt, one of high d T/d P type and the other low d T/d P type. The broadly contemporaneous occurrence of granulite and ultrahigh-temperature metamorphism with eclogite–high-pressure granulite metamorphism in the geological record since the Neoarchean Era is evidence of dual thermal environments and indicates that subduction has operated on Earth since that time. Classic ‘paired’ metamorphic belts in which an inboard high d T/d P metamorphic belt is juxtaposed against an outboard low d T/d P metamorphic belt along a tectonic contact—such as the Ryoke and Sanbagawa belts in Japan—are found in Phanerozoic accretionary orogens of the circum-Pacific. Generally, they appear to result from juxtaposition of terranes with different metamorphic facies series that may or may not be exactly contemporaneous and that may or may not be far-traveled. This is a consequence of the difference between globally-continuous subduction, generating a low-to-intermediate d T/d P environment in the subduction zone and a high d T/d P environment in the arc–backarc system, and metamorphic imprints in the geological record that represent discrete ‘events’ due to changes in plate kinematics or subduction boundary dynamics, or as a result of collision of ridges, arcs or continents with the upper plate at the trench. The concept of ‘paired’ metamorphic belts may be generalized and extended more widely than in the original proposition to subduction-to-collision orogenic systems in addition to accretionary orogenic systems. In this wider application, the term “paired metamorphic belts” may be used for “penecontemporaneous belts of contrasting type of metamorphism that record different apparent thermal gradients, one warmer and the other colder, juxtaposed by plate tectonics processes” (Brown, 2009). This extends the original concept of Miyashiro (1961) beyond the simple pairing of high d T/d P and low d T/d P metamorphic belts in circum-Pacific accretionary orogens, and makes it more useful in the context of our better understanding of the relationship between thermal regimes and tectonic settings. This is particularly useful in subduction-to-collision orogenic systems, where the suture and lower plate materials will register the imprint of low-to-intermediate d T/d P and the upper plate will register penecontemporaneous high d T/d P metamorphism commonly manifested at shallow crustal levels by the occurrence of granites in the rock record.

Eclogite–high-pressure granulite metamorphism records early collision in West Gondwana: new data from the Southern Brasília Belt, Brazil

Abstract: Nappes in the southern sector of the Southern Brasília Belt record suturing of the Paranapanema Block and Socorro–Guaxupé Arc with a subducted passive margin on the western side of the São Francisco Craton. We report secondary ion mass spectrometry U–Pb zircon ages that for the first time constrain the age of: (1) retrograded eclogite from a block along the tectonic contact beneath the uppermost nappe in a stack of passive margin-derived nappes; (2) high-pressure granulite-facies metamorphism in the uppermost passive margin-derived nappe; (3) high-pressure granulite-facies metamorphism in the overlying arc-derived nappe. Rare zircons from a retrograded eclogite yield a 206 Pb/ 238 U age of 678 ± 29 Ma, which we interpret as most likely to date close-to-peak- P metamorphism and to provide a minimum age for detachment of the overlying passive margin-derived nappe from the subducting plate. Zircon associated with ilmenite in samples from two structural levels in the passive margin-derived high-pressure granulite nappe yields 206 Pb/ 238 U ages of 648 ± 12 and 647 ± 11 Ma, and Ti-in-zircon crystallization temperatures from c . 860 °C down to c . 785 °C, but skewed toward the lower part of the range. These data indicate zircon formation during cooling from around peak T to the solidus, consistent with the high- T retrograde P – T path deduced from microstructures linked to phase assemblage fields in isochemical phase diagrams. Rb–Sr multi-mineral–whole-rock isochrons from two samples from close to the bottom of this nappe date formation of a retrograde sillimanite-bearing penetrative fabric to c . 590 Ma at temperatures of c . 750 °C (based on Ti-in-quartz thermometry). Rare zircons from leucosome in high-pressure granulite from the overlying arc-derived nappe yield a 206 Pb/ 238 U age of 622 ± 28 Ma and Ti-in-zircon crystallization temperatures from c . 970 °C down to c . 820 °C, which we interpret to record formation of zircon during cooling from peak high-pressure granulite-facies conditions. These ages indicate that the first stage of craton amalgamation in West Gondwana may have occurred earlier than previously inferred. Supplementary material: Geochemical and geochronological data are available at http://www.geolsoc.org.uk/SUP18377 .

Thermodynamic regime of Svecofennian (1.9 Ga) metamorphism of the Umba nappe of the Lapland collisional orogen

Thermodynamic regime of culmination phase of high-grade metamorphism of the Umba nappe (Lapland allochthon) was studied, and peak metamorphic monazite was dated. Based on calculation of end member reactions, the metamorphic assemblages of aluminous gneisses from the upper and lower parts of the nappe are close to equilibrium. The metamorphic conditions of the rocks are estimated to be about 800°C and 7 kbar in its upper part and 9 kbar in its lower part. The formation of orthopyroxene-sillimanite aggregates points to increasing pressure and temperature at the prograde stage of PT path, whereas cordierite rims mark the onset of decompression and cooling. The pressure difference of 2–2.5 kbar identified by thermobarometric methods between aluminous gneisses from the upper and lower parts of the Umba nappe corresponds to a depth gradient about 7.5 km, which agrees with approximate thickness of the Umba nappe. The study of the eclogitelike rocks developed after the rocks of the Paleoproterozoic Kolvitsa gabbronorite massif made it possible to trace the P-T evolution of metamorphsim: the temperature peak of granulite stage (11 kbar, 860°C) was followed by pressure increase (up to 14 kbar and more), and then decompressional cooling due to the exhumation of the Por’ya Guba nappe together with the Kolvitsa Massif. The U-Pb monazite age of 1904 ± 3 Ma obtained for aluminous gneisses from the upper part of the Umba nappe corresponds within error to the timing of metamorphic events determined from metamorphic zircon in the anorthosites of the Kolvitsa Massif (1907 ± 2 Ma) and zircon from aluminous gneisses in the melange zone (1906 ± 3 Ma). These isotopic data confirm the conclusion of simultaneous high-pressure granulite metamorphism in the upper and lower portions of the Umba nappe.

Geothermometry of the ultrahigh-temperature Saxon granulites revisited. Part I: New evidence from key mineral assemblages and reaction textures

The high-pressure (HP) granulite metamorphism in Variscan central Europe has recently been questioned to be ultrahigh temperature (UHT) because ternary feldspars from granulites in the Moldanubian region resembling those considered elsewhere as metamorphic minerals were re-interpreted as relic protolith minerals. Fe-Mg exchange reactions, although indicating similar temperatures as the ternary feldspars, have been discarded as unreliable for determining UHT conditions. Pseudosection modelling of the re-interpreted mineral assemblages has suggested metamorphism at temperatures clearly below those derived in earlier work. To re-examine the metamorphism of the Saxon granulites, we have studied key mineral assemblages and reaction textures, some of which have not yet been introduced into the present debate. The investigated felsic granulites display a close textural association of ternary feldspar with garnet and kyanite, including mutual inclusion relationships, suggesting that these minerals have equilibrated at the metamorphic peak. The UHT-HP conditions of this assemblage are close to P – T data calculated from the plagioclase-bearing inclusion assemblage in garnet and the peak assemblage of a garnet-clinopyroxene granulite. Reaction textures in a feldspathic prismatine granulite showing a transition from garnet-kyanite-corundum to garnet-corundum-sapphirine-sillimanite as well as subsequent overprinting during prismatine-forming H 2 O-deficient albite melting are evidence for continuous UHT conditions during decompression. These assemblages require temperatures above the Mg-staurolite stability, the invariant point [Crd] in the silica-undersaturated FMAS system, and the dravite stability, respectively. Continuous UHT conditions after a strong decrease in pressure emerge equally from multi-equilibrium calculations for a garnet-two-pyroxene granulite, where garnet has re-equilibrated with a two-pyroxene-plagioclase-quartz assemblage in the matrix and the corona assemblage orthopyroxene-plagioclase-quartz. Our findings confirm the earlier proposed UHT-HP decompression path for the Saxon granulites and underpin the metamorphic origin of ternary feldspar. Oxygen-isotope thermometry data for the same mineral assemblages presented in a companion paper agree with the results of chemical equilibrium thermometry and provide an independent confirmation for the UHT metamorphism.

U–Pb geochronology of paragneisses and metabasite in the Xitieshan area, north Qaidam Mountains, western China: Constraints on the exhumation of HP/UHP metamorphic rocks

The Xitieshan HP/UHP metamorphic unit is a polyphase paragneiss–orthogneiss–metabasite complex preserving relics of an early eclogite facies metamorphism (as deduced from the presence of clinopyroxene + plagioclase symplectite after omphacite in metabasite) followed by decompression through the granulite and amphibolite facies. Paragneisses contain garnet, kyanite (partially replaced by sillimanite), feldspar, quartz and biotite, with minor rutile and zircon. The metabasite consists of garnet, amphibole and plagioclase with relict clinopyroxene + plagioclase symplectite. Two main high-temperature metamorphic stages are distinguished: (1) a high-pressure granulite stage, characterized by the assemblage garnet–kyanite–K-feldspar–plagioclase–quartz–rutile in the paragneisses and garnet–clinopyroxene–plagioclase–quartz in the metabasite; (2) a medium–low pressure granulite to upper amphibolite facies stage, leading to the formation of sillimanite in the paragneisses and overgrowth of amphibole on Cpx–Pl symplectite in the metabasite. SHRIMP U–Pb analyses of metamorphic zircons, in conjunction with LA-ICPMS REE data and rare mineral inclusions in zircons, show that the high-pressure granulite metamorphism recorded by these rocks occurred between 451 and 461 Ma and the medium–low pressure granulite–upper amphibolite facies metamorphic overprint between 423 and 427 Ma, suggesting about 30 Ma residence time in medium–lower crustal levels after an earlier eclogite facies metamorphism (ca. 480 Ma). Based on comparison with other HP/UHP metamorphic units in the North Qaidam Mountains (NQD), these results suggest diachronous subduction and exhumation in the different units of the NQD.

Polyphase tectonothermal history recorded in granulitized gneisses from the north Qaidam HP/UHP metamorphic terrane, western China: Evidence from zircon U-Pb geochronology

High-grade gneisses of the north Qaidam high-pressure/ultrahigh-pressure metamorphic terrane enclose minor eclogites and ultramafic rocks. In combination with petrological data, sensitive high-resolution ion microprobe U-Pb geochronology on the granulitized gneisses in the Luliangshan and Xitieshan, western north Qaidam Mountains, reveals a polyphase tectonothermal history including Early Neoproterozoic and Early Paleozoic events. The rocks investigated are two granulite-facies paragneisses and one orthogneiss that both surround garnet peridotite in the Luliangshan, and two paragneisses that enclose eclogite in the Xitieshan. The inherited zircon cores from paragneiss and orthogneiss yield ages between ca. 1000 Ma and ca. 2500 Ma, representing Mesoproterozoic to Archaean source material for these gneisses. A ca. 900 Ma age obtained from one orthogneiss and one paragneiss is interpreted as the age of simultaneous Early Neoproterozoic magmatism and metamorphism. This Early Neoproterozoic tectonothermal event is similar in age to the Jinning orogeny, which is commonly recognized in the metamorphic basement of the south China block, and suggests that the north Qaidam Mountains have an affinity to the south China block. Early Paleozoic metamorphism is recorded in all gneiss samples. In conjunction with cathodoluminescence imagery and mineral inclusions, U-Pb dating of zircons reveals two Early Paleozoic age groups: ca. 450 Ma represents the age of high-pressure granulite metamorphism (Grt + Rt inclusions in zircon; kyanite porphyroblasts), whereas ca. 425 Ma reflects the time of medium-pressure, granulite-facies metamorphism (Pl + Sil inclusions in zircon) and associated anatexis during decompression. The ages obtained suggest that the granulite-facies metamorphism lasted for ~25 m.y. and was related to the Early Paleozoic continental collision between the Qilian and Qaidam blocks, and to subsequent thermal relaxation and exhumation.

Tectonic evolution of the Yaoundé segment of the Neoproterozoic Central African Orogenic Belt in southern Cameroon

The deformation history of the Neoproterozoic Central African Orogenic Belt in southern Cameroon is well recorded in the low- to high-grade rocks outcropping in the area around Yaoundé. The fabrics in these rocks are consistent with two main ductile deformation events D1 and D2. D1 predated emplacement of calc-alkaline dioritic bodies and caused the formation of nappes that resulted in high-pressure granulite metamorphism of soft sediments. A strong overprinting of these nappes during D2 symmetric extension, probably associated with large-scale foliation boudinage and (or) gneissic doming and intense magmatic underplating, gave rise to regional flat-lying fabrics. The latter were further buckled by D3 and D4 folding phases defining a vertical constriction occurring with a major east–west to NW–SE shortening direction. The corresponding F3 and F4 folds trend north–south to NE–SW and east–west to NW–SE, respectively, and represent the main regional strain patterns. Based on the east–west to NW–SE maximum shortening orientation indicated by F3 folds, it is proposed that the nappe-stacking phase D1 occurred in the same direction. The deformation history in the area can thus be described as corresponding principally to alternating east–west to NW–SE contractions and north–south to NE–SW orogenic-parallel extensions. At the regional scale, this could be due to the Transaharan east–west collisional system.

Metamorphism, Plate Tectonics, and the Supercontinent Cycle

Granulite facies ultrahigh temperature metamorphism (G-UHTM) is documented in the rock record predominantly from Neoarchean to Cambrian; G-UHTM facies series rocks may be inferred at depth in younger, particularly Cenozoic orogenic systems. The first occurrence of G-UHTM in the rock record signifies a change in geodynamics that generated transient sites of very high heat flow. Many G-UHTM belts may have developed in settings analogous to modern continental backarcs. On a warmer Earth, the cyclic formation of supercontinents and their breakup, particularly by extroversion, which involved destruction of ocean basins floored by thinner lithosphere, may have generated hotter continental backarcs than those associated with the modern Pacific rim. Medium-temperature eclogite, high-pressure granulite metamorphism (E-HPGM), is also first recognized in the Neoarchean rock record and occurs at intervals throughout the Proterozoic and Paleozoic rock record. E-HPGM belts are complementary to G-UHTM belts and are generally inferred to record subduction-to-collision orogenesis. Blueschists become evident in the Neoproterozoic rock record; they record the low thermal gradients associated with modern subduction. Lawsonite blueschists and eclogites (high-pressure metamorphism, HPM) and ultrahigh pressure metamorphism (UHPM) characterized by coesite (±lawsonite) or diamond are predominantly Phanerozoic phenomena. HPM-UHPM registers the low thermal gradients and deep subduction of continental crust during the early stage of the collision process in Phanerozoic subduction-to-collision orogens. Although perhaps counterintuitive, many HPM-UHPM belts appear to have developed by closure of small ocean basins in the process of accretion of a continental terrane during a period of supercontinent introversion (Wilson cycle ocean basin opening and closing). A duality of metamorphic belts—reflecting a duality of thermal regimes—appears in the record only since the Neoarchean Era. A duality of thermal regimes is the hallmark of modern plate tectonics and the duality of metamorphic belts is the characteristic imprint of plate tectonics in the rock record. The occurrence of both G-UHTM and E-HPGM belts since the Neoarchean manifests the onset of a ‘Proterozoic plate tectonics regime’, although the style of tectonics likely involved differences. The ‘Proterozoic plate tectonics regime’ evolved during a Neoproterozoic transition to the ‘modern plate tectonics regime’ characterized by colder subduction and subduction of continental crust deep into the mantle and its (partial) return from depths of up to 300 km, as chronicled by the appearance of HPM-UHPM in the rock record. The age distribution of metamorphic belts that record extreme conditions of metamorphism is not uniform, and metamorphism occurs in periods that correspond to amalgamation of continental lithosphere into supercratons (e.g. Superia/Sclavia) or supercontinents (e.g. Nuna (Columbia), Rodinia, Gondwana, and Pangea).

Metamorphic evolution and thermobaric structure of the subduction-related Bacariza high-pressure granulite formation (Cabo Ortegal Complex, NW Spain)

The high-pressure Bacariza granulite formation comprises various lithostratigraphic units of granulite orthogneisses, ultramafic, Mg-rich mafic, intermediate and common mafic granulites, as well as of more exotic intercalations. Mineral assemblages in equilibrium in ultramafic- to intermediate rocks contain garnet, clinopyroxene and plagioclase, with different amounts of zoisite/clinozoisite, kyanite, quartz, scapolite, rutile and ilmenite depending on the granulite lithotype, whereas granulite orthogneisses contain garnet, phengite, biotite, K-feldspar, antiperthitic plagioclase, quartz and rutile as primary phases. Thermobarometry of these rocks supports the existence of a high-pressure metamorphism for which near-peak P– T conditions have been estimated at ca. 790 °C and 1.6 GPa. The preserved fabrics and structures enable us to relate the metamorphism to coeval polyphasic deformational processes. Dynamic retrogression began under still high-pressure granulite facies conditions (1.4 GPa and ca. 740 °C) and is postdated by symplectitization (1.3 GPa and ca. 715 °C). Subsequent retrogression under medium pressure amphibolite facies conditions at similar temperature was either widespread and static or localized and dynamic as a result of intense deformation partitioning during uplift. Loading/heating and subsequent decompression/cooling are related to a single cycle in a subduction conduit setting. This study suggests that high-pressure granulite metamorphism might not be as uncommon in the high-pressure metamorphic series as previously thought. Moreover, it might constitute a diagnostic feature of convergent lithospheric settings, whether or not associated with eclogite facies metamorphism in adjacent units.

Age and evolution of a late Archean to Paleoproterozoic upper to lower crustal section in the Wutaishan/Hengshan/Fuping terrain of northern China

The Taihangshan–Wutaishan area forms part of the central North China Craton and consists of three main components: the Hengshan and Fuping complexes, containing predominantly ductilely deformed late Archean to Paleoproterozoic high-grade granitoid orthogneisses, intruded by mafic dykes of gabbroic composition, and the low-grade late Archean Wutai greenstone belt, developed between the high-grade terrains and consisting of bimodal volcanic rocks and metasediments, associated with coeval granitoids. Zircon dating shows that both the Hengshan and Fuping complexes were intruded by major granitoid bodies between 2.52 and 2.48 Ga, with rare occurrences of 2.7 Ga gneisses. Wutai granitoids adjacent to the Fuping complex also contain 2.7 Ga xenocrysts, all possibly reflecting the same older basement. Rare granitic gneisses in the Hengshan have emplacement ages of ∼2.35 Ga and show the same structural features as the older rocks, indicating that the main deformation occurred after ∼2.3 Ga. Deposition of supracrustal rocks and emplacement of mafic dykes occurred in the Paleoproterozoic, together with intrusion of granitic bodies at ∼2.1–2.0 Ga, and all underwent regional metamorphism at 1.88–1.85 Ga. We interpret the Hengshan and Fuping granitoid gneisses as the lower, plutonic, part of a late Archean to early Paleoproterozoic Japan-type magmatic arc, with the upper, volcanic part represented by the Wutai complex. Components of this arc may have evolved at a continental margin as indicated by the 2.7 Ga zircons. Virtually all magmatic events are recorded in all three complexes. However, the 1.85 Ga metamorphic event is only recorded in the higher-grade portions of the Wutai, since low-grade and presumably high crustal level precluded metamorphic zircon growth. Shear zones in the Hengshan are interpreted as major lower crustal discontinuities post-dating the peak of high-pressure granulite metamorphism. We suggest that they formed during orogenic collapse and uplift of the Hengshan complex in the late Paleoproterozoic (>1800 Ma).

The early Proterozoic Pechenga-Varzuga Belt: a case of Precambrian back-arc spreading

The Pechenga-Varzuga Belt (PVB) in the Kola Peninsula, Russia, comprises the western Pechenga and the eastern Imandra-Varzuga portions. These are two major asymmetric synform structures infilled with Proterozoic volcanogenic and sedimentary rocks dated between 2.45 and 1.9 Ga. Two main stages of tectonic-magmatic activity took place: (1) a ∼2.45-2.3 Ga basal siliceous high-Mg volcanic series, typical for the Sumian-Sariolian episodes; and (2) the Jutuli, Ludia and Kalevian episodes, which began ∼2.2 Ga ago. The lower level of the second sequence is made up of subaerial alkali basalts. The upper, 2.1-1.95 Ga old portions of the sequence consist of tholeiitic pillow-lavas, hyaloclastites and ferropicrite basalt flows associated with deep-water sediments such as phyllites, silicites and turbidites. The tholeiites exhibit REE distribution patterns similar to those of MORB, while the ferropicrite basalts resemble intraplate olivine basalts of continental and oceanic environments. Calc-alkaline subaerial volcanism took place along the south-western flank of the Pechenga zone. A final igneous stage is characterized by the emplacement of potassium granitic intrusions. The Main Lapland Thrust (MLT) occurs along the southern border of the Paleoproterozoic Lapland-Umba Granulite Belt, and is a narrow zone of compression, thrusting and folding accompanied by high-pressure granulite metamorphism. It is approximately coeval with the upper part of the Pechenga-Varzuga Belt, and is interpreted as representing a complementary structure, i.e. the zone of subduction of the Karelian-Belomorian craton beneath the Kola block. Thus, the upper part of the Pechenga-Varzuga Belt is considered to have originated as a back-arc basin in the rear of a zone of north-directed subduction and continent collision. It was eventually closed, with the appearance of igneous suites typical of such events. The Pechenga-Varzuga Belt-Main Lapland Thrust system thus provides an instance of an ancient (∼2.0-1.9 Ga ago) collision of lithospheric plates accompanied by back-arc spreading and then closure of the back-arc basin. The geodynamic development is similar that which occurred more recently within the Alpine-Himalayan Belt.