High-temperature Granulite-facies Metamorphism Research Articles

Petrogenesis and Metallogenesis of Late Cretaceous Adakites in the Nuri Large Cu-W-Mo Deposit, Tibet, China: Constraints from Geochronology, Geochemistry, and Hf Isotopes

The Gangdese metallogenic belt in Tibet is an important polymetallic metallogenic belt formed during the subduction of the Neo-Tethys Ocean and subsequent India–Asia collision. Adakitic rocks are widely distributed in this belt and are considered to be closely related to porphyry–skarn Cu-Mo polymetallic mineralization. However, the petrogenesis and geodynamic setting of the Late Cretaceous adakites in the Gangdese belt remain controversial. In this study, we focus on the quartz diorite in the Nuri Cu-W-Mo deposit along the southern margin of the eastern Gangdese belt. LA-ICP-MS zircon U-Pb dating yields a Late Cretaceous age of 93.6 ± 0.4 Ma for the quartz diorite. Whole-rock geochemistry shows that the quartz diorite possesses typical adakitic signatures, with high SiO2, Al2O3, and Sr contents, but low Y and Yb contents. The relatively low K2O content and high MgO, Cr, and Ni contents, as well as the positive zircon εHf(t) values (+6.58 to +14.52), suggest that the adakites were derived from the partial melting of the subducted Neo-Tethys oceanic slab, with subsequent interaction with the overlying mantle wedge. The Late Cretaceous magmatic flare-up and coeval high-temperature granulite-facies metamorphism in the Gangdese belt were likely triggered by Neo-Tethys mid-ocean ridge subduction. The widespread occurrence of Late Cretaceous adakitic intrusions and associated Cu mineralization in the Nuri ore district indicate a strong tectono-magmatic-metallogenic event related to the Neo-Tethys subduction during this period. This study provides new insights into the petrogenesis and geodynamic setting of the Late Cretaceous adakites in the Gangdese belt, and has important implications for Cu polymetallic deposit exploration in this region.

Fold interference pattern and crustal decoupling in northern Tanzania

High-temperature granulite facies metamorphism and deformation evolved through Neoproterozoic tectonic processes in large parts of southeastern Africa. The Eastern Granulites of Tanzania, one example for these extreme conditions, show progressive decoupling between lower and middle crust together with progressive cooling and rheology stratification. Coupled deformation during earliest stages of deformation arises from extremely small viscosity contrast of all minerals and rocks at granulite facies metamorphic conditions around 850 °C. With cooling of the orogen rheology stratification increases and deformation decouples into horizontal stretching through gravity driven channel flow in the lower crust and thickening and horizontal west-east shortening in the middle/upper crust. As soon as a stiff and cold indenter, represented by external parts of the Archean Tanzania Craton (Western Granulites) enters the system, the indenter acts as lower crustal ramp along which deep crustal units are exhumed and displaced towards the foreland. During this orogenic period the crust was fully stratified and a second phase of decoupling, detachment over crustal scale ramp, occurs. Superposition with late orogenic north-south shortening structures resulted in type 1 fold interference pattern and formation a of dome-and-basin geometry in northeastern Tanzania. Structural modelling of fold superposition helps to explain the complex structural pattern and to predict sites of decollement horizons. By comparison with fold interference pattern exposed in southern Madagascar we define different types of dome-and-basin formation. Those with neglectable rheology contrast are viscosity determined where mixing of upper and lower crustal material is enabled by vertical flow folding. Those with strong viscosity contrast are mechanically determined where domes and basins are formed by changes in regional principal stress directions.

Garnet: The archive of prograde metamorphic records of granulite-facies terranes

<p indent="0mm">Whether granulite terranes can preserve prograde metamorphic records is a perturbing topic in metamorphic petrology, and related research is rare. This is mainly because the metamorphic records of the granulite terranes have information asymmetry. Generally, the mineral assemblages and compositions of granulite-facies rocks are considered to reflect peak and retrograde metamorphism, rather than prograde metamorphism. However, this assumption must be made with caution. Case studies of granulites from the junction area of Shanxi and Hebei province in the North China Craton suggest that garnet could serve as “temperature and pressure resistant containers” under granulite-facies conditions, and retain the evidence of pre-peak metamorphism. For pelitic granulites, their protoliths are supracrustal rocks, and must have undergone prograde metamorphic processes from the Earth’s surface to the deep crust. Therefore, a confirmatory study seeking prograde metamorphic records for pelitic granulites from the Huai’an-Datong area was conducted. In the studied pelitic granulites, inclusions in garnet display well-preserved microstructural zoning, with radical zonal distribution patterns: Crowded quartz grains in the core, a few K-feldspar-kyanite-bearing assemblages in the mantle, and some sillimanite flakes in the rim. The major element zoning (here, major endmember proportions X_Sps, X_Alm, X_Pyr, and X_Grs are used) of garnet is relatively homogeneous. However, the high rare earth elements (HREEs) exhibit bell-shaped profiles decreasing from the core to the rim. This suggests that even if the major elements of garnet are homogenous due to high-temperature diffusion, microstructural zoning and some trace elements with slow diffusion rates could be protected from peak- and retro-grade high-temperature modification. For garnet mafic granulites, their protoliths are considered as gabbro dykes, and there is no robust evidence of prograde metamorphism from the upper to lower crust; therefore, their tectonic significance is unclear. Here, an exploratory study of prograde metamorphism for garnet mafic granulites from the Huai’an-Hengshan area was conducted based on the study experience of pelitic garnet. In the investigated mafic granulites, garnet also displays a zoned distribution of various mineral assemblages from its center to rim, including quartz- and titanite/ilmenite-bearing assemblages in the core, rutile-bearing assemblages in the outer mantle to the inner rim, and a resorbed rim. Fortunately, garnet of the mafic granulites displays well-preserved compositional growth zoning patterns, with bell-shaped X_Sps, increasing X_Pyr from core to mantle, and a resorbed rim (X_Sps). The HREEs (for example, Yb) show bell-shaped patterns similar to those of the X_Sps. Both textural and compositional zoning patterns in the studied mafic granulites documented multiple generations of garnet growth history from the prograde to peak metamorphic stages. These findings provide robust metamorphic evidence for the Paleoproterozoic subduction and assembly of the Jin-Yu Mobile Belt in the North China Craton. The above two case studies suggest that high-temperature granulite-facies metamorphism may not completely reset the multi-generation growth information retained in the garnet. In contrast, minerals in the matrix only record the history of intensive re-equilibration at the peak or rehydration retrogression metamorphism. Thus, garnet is an important archive of prograde metamorphic information. This study further suggests that caution should be exercised when determining the peak<italic> P-T</italic> conditions of granulites using garnet core compositions.

Polyphase deformation in the Badu complex: Insights into Triassic intraplate orogeny in South China

The occurrence of Triassic high-pressure eclogite- and high-temperature granulite-facies metamorphism in the Badu complex contrasts with regional greenschist-to amphibolite-facies metamorphism in most of South China, hence calling for unraveling related structural and geodynamic processes within a regional intraplate-orogenic setting. We analyzed the geometry, kinematics, and overprinting relationships of structures in the central Badu complex, and dated the structural evolution via geo-thermochronology. We identified deformation episodes D1 to D4: D1 features rootless, tight-isoclinal folds (F1), and an initially likely gentle-dipping foliation (S1) with unclear kinematics, with the majority of structures modified/erased by later deformations; D2 comprises a gentle-dipping foliation (S2), and a ∼N-trending lineation (L2), with dominant top-to-the-south shear subparallel or slightly oblique to the strike of the regional Triassic orogen; D3 includes open to tight, upright folds with subhorizontal axes reflecting orogen-normal, ∼E-W shortening; D4 features ∼ N-trending reverse faults. D1 is bracketed between ∼245 and ∼235 Ma and occurred at temperatures >600 °C, following regional high-pressure metamorphism (251‒245 Ma). D2 and D3 occurred between ∼235 and ∼221 Ma under retrograde, amphibolite-to greenschist-facies metamorphic conditions, with cooling rates of ∼17–21 °C/Myr. By combining our new data with published work on the Badu complex, we trace a >120-km-long, ∼25-km-wide, N- to NNE-striking, high-grade metamorphic and intense deformation zone that represents a localized but distinct anomaly in the vast (∼1300-km-wide) Triassic structural belt in South China. The high-pressure metamorphism, the orogen-subparallel shearing, and the orogen-normal shortening suggest localized (oblique) underthrusting of the Coastal terrane beneath the Wuyishan terrane during the Triassic intraplate orogeny within South China.

Paleozoic post-collisional magmatism and high-temperature granulite-facies metamorphism coupling with lithospheric delamination of the East Kunlun Orogenic Belt, NW China

Lithosphere extension and upwelling of asthenosphere at post-collisional stage of an orogenic cycle generally induce diverse magmatism and/or associated high-temperature metamorphism. Nevertheless, the intimate coexistence of post-collisional magmatic activity and high-temperature metamorphism is rare. In this contribution, a lithological assemblage composing of diverse magmatic rocks deriving from distinct magma sources and coeval high-temperature metamorphism was identified in eastern Kunlun. Petrography, ages, mineral chemistry and whole-rock geochemistry demonstrated that those intimately coexistent diverse rocks were genetically related to post-collisional extension. The garnet-bearing mafic granulites in Jinshuikou area interior of the East Kunlun Orogenic Belt are mainly composed of garnet, orthopyroxene, and plagioclase, with peak metamorphic P–T conditions of ~ 701–756 °C and 5.6–7.0 kbar, representing a granulite-facies metamorphism at 409.7 ± 1.7 Ma. The diverse contemporaneous magmatic rocks including hornblendites, gabbros and granites yield zircon U–Pb ages of 408.6 ± 2.5 Ma, 413.4 ± 4.6 Ma, and 387–407 Ma, respectively. The hornblendites show N-MORB-like REE patterns with (La/Sm)N values of 0.85–0.94. They have positive zircon εHf(t) values of 0.1–4.9 and whole-rock εNd(t) values of 3.9–4.7 but relatively high (87Sr/86Sr)i values of 0.7081 to 0.7088. These features demonstrate that the hornblendites derived from a depleted asthenospheric mantle source with minor continental crustal materials in source. As for the gabbros, they exhibit arc-like elemental signatures, low zircon εHf(t) values (−4.3 to 2.5) and variable whole-rock εNd(t) values (−4.9 to 1.2) as well as high (87Sr/86Sr)i values (0.7068 to 0.7126), arguing for that they were originated from partial melting of heterogeneous lithospheric mantle anteriorly metasomatized by subducted-sediment released melts. Geochemistry of the granites defines their strongly peraluminous S-type signatures. Zircons from the granites yield a large range of εHf(t) values ranging from −30.8 to −5.1, while the whole-rock samples yield consistent (87Sr/86Sr)i values (0.7301 to 0.7342) and negative εNd(t) values (−10.1 to −12.4). These features indicate that the S-type granites could be generated by reworking of an ancient crust. Taken together, the penecontemporaneous magmatism and metamorphic event, demonstrated the early-middle Devonian transition from crustal thickening to extensional collapse. The post-collisional mantle-derived magmas serve as an essential driving force for the high-temperature granulite-facies metamorphism and anataxis of the crust associated with formation of S-type granite. This study not only constructs a more detail Proto-Tethys evolution process of the eastern Kunlun, but also sheds new light on better understanding the intimate relationship between magmatism and metamorphism during post-collisional extensional collapse.

Metamorphism and Zircon Geochronological Studies of Metagabbro Vein in the Yushugou Granulite-Peridotite Complex from South Tianshan, China

Yushugou granulite-peridotite complex, located at the east part of the northern margin of South Tianshan, may represent an ophiolitic slice subducted to 40–50 km depth with high-pressure granulite facies metamorphism. Although a lot of studies have been conducted on rocks in this belt, the rock association and tectonic background of the ophiolitic slice are still in dispute. A detailed study on petrology, phase equilibrium modeling and U-Pb zircon ages have been performed on the metagabbro vein in peridotite unit to constrain the tectonic evolution of the Yushugou granulite-peridotite complex. Three stages of mineral assemblage in the metagabbro were defined as stage I: CpxA+OpxA+PlA, which represents the original minerals of the metagabbro vein; stage II: CpxB+OpxB+PlB+Spl, which represents the mineral assemblage of granulite facies metamorphism with peak P-T conditions of 4.2–6.9 kbar and 940–1 070 °C; stage III is characterized by the existence of prehnite, thomsonite and amphibole in the matrix, indicating that the metagabbro vein may be influenced by fluids during retrograde metamorphism. Combined with the crosscut relationship, it can be deduced that the metagabbro vein, together with the peridotite in Yushugou granulite-peridotite complex has experienced similar high-temperature granulite facies metamorphism. The zircon chronological data shows that the protolith age of the metagabbro vein is 400.5±6.2 Ma, reflecting Devonian magmatism event and the granulite facies metamorphism occurred at ~270 Ma which may be related to the post-collisional magmatism.

Origins and tectonic implications of Late Cretaceous adakite and primitive high-Mg andesite in the Songdo area, southern Lhasa subterrane, Tibet

Late Cretaceous igneous rocks in the southern Lhasa subterrane, Tibet, include primitive high-Mg andesites and adakites, which provide important constraints on the tectonic evolution of the Neo-Tethys Ocean. Here, we present detailed zircon UPb and Hf isotopic and whole-rock geochemical data for granodioritic and dioritic porphyry samples from the Songdo area in the southern Lhasa subterrane. Zircon UPb dating indicates that the granodiorite crystallized at 88 Ma, whereas the diorite yields ages of 68 and 66 Ma. The granodiorite has adakite-like geochemical characteristics, including high Sr (801–1005 ppm) and low Y (6.8–15.2 ppm) and Yb (0.6–1.3 ppm) concentrations, and high Sr/Y (62–145) and La/Yb (39–93) ratios. We infer that the adakitic granodiorites formed through partial melting of subducted oceanic crust. The dioritic porphyry has intermediate moderate SiO2 (53–58 wt%) and high MgO (5.6–8.2 wt%) contents, and high Mg# (66.4–69.5) values, and is therefore classified as a primitive high-Mg andesite that was derived from interaction between subducted sediment and mantle. The presence of coeval adakite and charnockite, as well as high-temperature granulite-facies metamorphism, indicates that mid-ocean ridge subduction occurred at 100–80 Ma, followed by a 10 Myr hiatus in magmatism and subsequent rollback of the Neo-Tethys slab at 68 Ma. These processes resulted in significant crustal growth within the Lhasa terrane.

Late Cretaceous volcanic rocks in the Sangri area, southern Lhasa Terrane, Tibet: Evidence for oceanic ridge subduction

Late Cretaceous magmatism in the southern Lhasa subterrane, Tibet, provides critical constraints on the subduction processes of the Neo-Tethyan oceanic plate. Here, we provide data on the whole-rock geochemistry, zircon UPb ages, trace element contents, and Hf isotopes of early Late Cretaceous volcanic rocks from Sangri County to Sangye Temple (SS) along the southern Lhasa subterrane. The SS volcanic rocks include basalts, andesites, and dacites. SIMS and LA–ICP–MS zircon UPb data indicate that the SS dacites were erupted at ca. 95 Ma, coeval with the early Late Cretaceous “flare-up” event in the southern Lhasa subterrane. The SS volcanic rocks belong to the calc-alkaline to high-K calc-alkaline series and have the geochemical features of subduction-related volcanic rocks with negative Nb, Ta, and Ti anomalies. The basalts have similar rare earth element (REE) patterns to E-MORB and are characterized by high Zr abundances (65–190 ppm) and Zr/Y ratios (3.3–6.5). They were possibly derived by high-degree partial melting of mantle peridotite that had been metasomatized by subduction-related fluids with an input of asthenospheric components. Positive whole-rock εNd(t) (+2.9 to +3.2) values for the andesites, and their variable Mg# values (35–61) and MgO (2.29–7.32 wt%), Cr (12–232 ppm), and Ni (10–127 ppm) contents can be attributed to medium-degree partial melting of mantle peridotite followed by fractional crystallization of clinopyroxene and olivine. The calc-alkaline SS dacites are similar to adakites with high Sr (663–1188 ppm) and low heavy rare earth element (HREE) and Y (10.7–11.5 ppm) contents. These adakitic dacites have positive whole-rock εNd(t) (+2.9 to +3.4) and zircon εHf(t) (+8.4 to +14.9) values with relatively high values of Mg# (36–57) and high contents of compatible elements, indicating that they were derived from the partial melting of subducted oceanic slab, and with the ascending magma interacting with mantle wedge peridotite. Our new data indicate a distinct rock association of coeval asthenosphere-component-involved basalts and slab-derived adakitic dacites. Such an association, together with the coeval high-temperature charnockites and high-temperature granulite-facies metamorphism of the charnockitic country rocks in the southern Lhasa subterrane, indicates an anomalous input of materials and heat during the early Late Cretaceous “flare-up” event. Such an event can probably be attributed to the northwards subduction of a Neo-Tethyan oceanic ridge that allowed for contributions from upwelling asthenosphere that differed from the typical results of normal-angle subduction or low-angle/flat subduction and subsequent slab rollback.

New evidence for an old idea: Geochronological constraints for a paired metamorphic belt in the central European Variscides

New geochronological data reveal a prolonged tectonothermal evolution of the Variscan Odenwald-Spessart basement, being part of the Mid-German Crystalline Zone in central Europe. We report the results from (i) secondary ion mass spectrometry (SIMS) U-Pb dating of zircon, rutile and monazite, (ii) SIMS zircon oxygen isotope analyses, (iii) laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) zircon Lu-Hf isotope analyses and, (iv) LA-ICPMS zircon and rutile trace element data for a suite of metamorphic rocks (five amphibolite- and eclogite-facies mafic meta-igneous rocks and one granulite-facies paragneiss). The protoliths of the mafic rocks formed from juvenile as well as depleted mantle sources in distinct tectonic environments at different times. Magmatism took place at a divergent oceanic margin (possibly in a back-arc setting) at 460 Ma, in an intraoceanic basin at ca. 445 Ma and at a continental margin at 329 Ma. Regardless of lithology, zircon in eclogite, amphibolite and high-temperature paragneiss provide almost identical Carboniferous ages of 333.7 ± 4.1 Ma (eclogite), 329.1 ± 1.8 to 328.4 ± 8.9 Ma (amphibolite), and 334.0 ± 2.0 Ma (paragneiss), respectively. Rutile yielded ages of 328.6 ± 4.7 and 321.4 ± 7.0 Ma in eclogite and amphibolite, and monazite in high-temperature paragneiss grew at 330.1 ± 2.4 Ma (all ages are quoted at the 2σ level). The data constrain coeval high-pressure eclogite- and high-temperature granulite-facies metamorphism of the Odenwald-Spessart basement at ca. 330 Ma. Amphibolite-facies conditions were attained shortly afterwards. The lower plate eclogite formed in a fossil subduction zone and the upper plate high-temperature, low-pressure rocks are the remains of an eroded Carboniferous magmatic arc. The close proximity of tectonically juxtaposed units of such radically different metamorphic conditions and thermal gradients is characteristic for a paired metamorphic belt sensu Miyashiro (1961). Thus, the Odenwald-Spessart basement represents the first recognised paired metamorphic belt in the European Variscides.

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.

Petrogenesis of Cretaceous adakite-like intrusions of the Gangdese Plutonic Belt, southern Tibet: Implications for mid-ocean ridge subduction and crustal growth

We have conducted a whole-rock geochemical, U–Pb zircon geochronological, and in situ zircon Hf–O isotopic compositional study of rocks in southern Tibet from the Langxian igneous suite (including a lamprophyre dyke, mafic enclaves, a granodiorite, and a two-mica granite) and the Nuri igneous suite (a quartz–diorite). U–Pb zircon dating indicates that the timing of crystallization of the mafic enclaves and host granodiorite of the Langxian suite are ca. 105Ma and 102Ma, respectively, that the Langxian lamprophyre dyke and the two-mica granite were emplaced at ca. 96Ma and 80–76Ma, respectively, and that the Nuri quartz–diorite was emplaced at ca. 95Ma. With the exception of the lamprophyre dyke and mafic enclaves in the Langxian area, felsic rocks from the Langxian and Nuri igneous suites all show signs of a geochemical affinity with adakite-like rocks. The high Mg-numbers, high abundance of compatible elements, high εNd(t) (2.7 and 2.8) and δ18O (8.9 and 9.2‰) values, elevated zircon εHf(t) (11.0–17.0) values, and low 87Sr/86Sr(i) ratios (0.7040), collectively indicate that the Nuri adakite-like quartz–diorite was derived from partial melting of the low temperature altered Neo-Tethyan oceanic crust, and that these dioritic magmas subsequently interacted with peridotite as they rose upwards through the overlying mantle wedge. The observation of identical differentiation trends, similar whole-rock Sr–Nd and zircon Hf isotopic compositions, and consistently low (Dy/Yb)N ratios among the Langxian igneous suite rocks, indicates that the adakite-like granodiorite was produced by low-pressure fractional crystallization of precursor magmas now represented by the (relict) mafic enclaves. However, relatively high Al2O3 contents, low MgO, Cr and Ni contents, and low (La/Yb)N and (Dy/Yb)N values indicate that the two-mica granite was derived from partial melting of the southern Tibetan mafic lower crust in the absence of garnet, while isotopic data suggest that at least 70% of the magma source region was juvenile materials. Combined with the presence of HT (high temperature) charnockitic magmatism, HT granulite facies metamorphism, and large volumes of Late Cretaceous batholiths, the oceanic-slab-derived Nuri adakitic rocks indicate a substantial high heat flux in the Gangdese batholith belt during the Late Cretaceous, which may have been related to subduction of a Neo-Tethyan mid-ocean ridge system. According to this model, hot asthenosphere would rise up through the corresponding slab window, and come into direct contact with both the oceanic slab and the base of the overlying plate. This would cause melting of both the oceanic slab and the overlying plate by the addition of heat that was ultimately linked with peak magmatism and the significant growth and chemical differentiation of juvenile crust in southern Tibet during the Late Cretaceous (105–76Ma). In addition, the petrogenesis of the Langxian adakite-like two-mica granite indicates that the southern Tibetan crust was still of normal thickness prior to the emplacement of these intrusions at ca. 76Ma. This probably means that large parts of southern Tibet were not very highly elevated prior to the Indian–Asian collision.

The P–T evolution of ultra high temperature garnet‐bearing ultramafic rocks from the Saxonian Granulitgebirge Core Complex, Bohemian Massif

AbstractGarnet‐bearing ultramafic rocks (GBUR) enclosed in granulite or high‐grade gneiss are rare, yet typical constituents of alpine‐type collisional orogens. The Bohemian Massif of the European Variscides is exceptional for the occurrence of a large variety of mantle‐derived rocks, including GBUR (garnet peridotite and garnet pyroxenite). GBUR occur in several metamorphic units belonging to both the Saxothuringian and the Moldanubian zones of the Bohemian Massif. The northernmost outcrops of GBUR in the Bohemian Massif are situated in the Saxonian Granulitgebirge Core Complex in the Saxothuringian zone and are the subject of this study. Thermobarometric results and exsolution textures imply that the Granulitgebirge GBUR belong to the ultra high temperature group of peridotites. They experienced a decompression‐cooling path being constrained by the following four stages: (i) ∼1300–1400 °C and 32 kbar, (ii) 1000–1050 °C and 26 kbar, (iii) 900–940 °C and 22 kbar, and (iv) 860 °C and 12–13 kbar. Occasional layers of garnet pyroxenite within GBUR lenses are interpreted as high pressure cumulates that crystallized at 32–36 kbar by cooling below 1400 °C. The GBUR were most probably derived from upwelling asthenosphere and came in contact with crustal granulite at ∼60 km depth. Slab break‐off is suggested here as the most probable cause for: (i) asthenosphere upwelling and cooling of the latter as well as (ii) ultra high temperature granulite facies metamorphism of the crustal host rocks. The Granulitgebirge‐type peridotite is very similar to the Mohelno‐type peridotite from the Gföhl unit, Moldanubian zone, in the southern part of the Bohemian Massif. In contrast, peridotite from the adjacent Erzgebirge (also within the Saxothuringian zone) is derived from the subcontinental mantle and much resembles the Nove Dvory‐type peridotite from the Gföhl unit (Moldanubian zone). The fact that the Saxothuringian and Moldanubian zones host the same types of mantle rocks (asthenospheric and lithospheric) of the same metamorphic ages suggests that the classic distinction into the Saxothuringian and Moldanubian zones cannot be supported, at least as far as high‐grade units hosting GBUR are concerned.

Geochemical and isotopic constraints on the tectonic and crustal evolution of the Shackleton Range, East Antarctica, and correlation with other Gondwana crustal segments

Three distinct, spatially separated crustal terranes have been recognised in the Shackleton Range, East Antarctica: the Southern, Eastern and Northern Terranes. First insights into the origin and evolution of critical units from these terranes could be gained from new lithogeochemical and isotope (Rb–Sr, Sm–Nd, Pb–Pb whole rock and Lu–Hf zircon) data. Mafic gneisses from the Southern Terrane provide geochemical evidence for a within-plate, probably back-arc origin of their protoliths. A plume-distal ridge origin in an incipient ocean basin is the favoured interpretation for the emplacement site of these rocks at c. 1850 Ma, which, together with a few ocean island basalts, were subsequently incorporated into an accretionary continental arc/supra-subduction zone tectonic setting. Magmatic underplating resulted in partial melting of the lower crust, which caused high-temperature granulite-facies metamorphism in the Southern Terrane at c. 1710–1680 Ma. Mafic and felsic gneisses there are characterised by isotopically depleted, positive Nd and Hf initials and model ages between 2100 and 2000 Ma. They may be explained as juvenile additions to the crust towards the end of the Palaeoproterozoic. These juvenile rocks occur in a narrow, c. 150 km long E-W trending belt, inferred to trace a suture that is associated with a large Palaeoproterozoic accretionary orogenic system. The Southern Terrane contains many features that are similar to the Australo-Antarctic Mawson Continent and may be its furthermost extension into East Antarctica. The Eastern Terrane is characterised by metagranitoids that formed in a continental volcanic arc setting during a late Mesoproterozoic orogeny at c. 1060 Ma. Subsequently, the rocks experienced high-temperature metamorphism during Pan-African collisional tectonics at 600 Ma. Isotopically depleted zircon grains yielded Hf model ages of 1600–1400 Ma, which are identical to Nd model ages obtained from juvenile metagranitoids. Most likely, these rocks trace the suture related to the amalgamation of the Indo-Antarctic and West Gondwana continental blocks at ∼600 Ma. The Eastern Terrane is interpreted as the southernmost extension of the Pan-African Mozambique/Maud Belt in East Antarctica and, based on Hf isotope data, may also represent a link to the Ellsworth-Whitmore Mountains block in West Antarctica and the Namaqua-Natal Province of southern Africa. Geochemical evidence indicates that the majority of the protoliths of the mafic gneisses in the Northern Terrane formed as oceanic island basalts in a within-plate setting. Subsequently the rocks were incorporated into a subduction zone environment and, finally, accreted to a continental margin during Pan-African collisional tectonics. Felsic gneisses there provide evidence for a within-plate and volcanic arc/collisional origin. Emplacement of granitoids occurred at c. 530 Ma and high-temperature, high-pressure metamorphism took place at 510–500 Ma. Enriched Hf and Nd initials and Palaeoproterozoic model ages for most samples indicate that no juvenile material was added to the crust of the Northern Terrane during the Pan-African Orogeny but recycling of older crust or mixing of crustal components of different age must have occurred. Isotopically depleted mafic gneisses, which are spatially associated with eclogite-facies pyroxenites, yielded late Mesoproterozoic Nd model ages. These rocks occur in a narrow, at least 100 km long, E-W trending belt that separates alkaline ocean island metabasalts and within-plate metagranitoids from volcanic arc metabasalts and volcanic arc/syn-collisional metagranitoids in the Northern Terrane. This belt is interpreted to trace the late Neoproterozoic/early Cambrian Pan-African collisional suture between the Australo-Antarctic and the combined Indo-Antarctic/West Gondwana continental blocks that formed during the final amalgamation of Gondwana.

Late Cretaceous charnockite with adakitic affinities from the Gangdese batholith, southeastern Tibet: Evidence for Neo-Tethyan mid-ocean ridge subduction?

The Gangdese batholith emplaced during the time span of Cretaceous to Neogene in the southern Lhasa terrane of Tibet has been considered as a major constituent of an Andean-type convergent margin derived from the northward subduction of the Neo-Tethyan oceanic lithosphere under Asia. Whereas previous studies assigned the Gangdese granitoids to be comprised predominantly of calc-alkaline rocks, here we report a suite of charnockites from the eastern part of the belt and characterize their petrology, geochemistry and age. These rocks possess an assemblage of andesine, enstatite, diopside, calcic amphibole, Ti-rich biotite, quartz and minor K-feldspar. Geochemically, they are characterized by intermediate SiO 2 (54–63 wt.%), relatively high Al 2O 3 (15.9–18.9 wt.%), REE (55.7–89.4 ppm) and Sr (419.6–619.4 ppm), and low Y (11.3–17.2 ppm) and Yb (1.2–1.8 ppm) concentrations. The rocks display geochemical affinities similar to those of adakites derived from the partial melting of a subducted slab, and also can be compared to magnesian charnockites formed within a continental magmatic arc. The crystallization conditions of the charnockites were estimated at 900 °C and 1.0 GPa. LA-ICP-MS zircon U–Pb analyses of eleven samples yield consistent 206Pb/ 238U weighted mean ages of 86 to 90 Ma, indicating that the charnockites were emplaced in the Late Cretaceous. Considering the coeval calc-alkaline magmatism and high-temperature granulite-facies metamorphism, we propose that such high-temperature and low-H 2O activity charnockites were derived through Neo-Tethyan mid-ocean ridge subduction before the collision of India with the Asian continent.

Incipient eclogite facies metamorphism in the Moldanubian granulites revealed by mineral inclusions in garnet

We investigated several mineral phases and their replacement products which occur as inclusions in garnets from felsic and mafic granulites of the Gföhl Unit in the Moldanubian Zone. The most important mineral inclusions, Ti-rich muscovite and omphacite, were used for the reconstruction of the metamorphic history of granulites. Some inclusions were transformed during high-temperature granulite facies metamorphism, partial melting and decompression to other phases, and so the original mineral can only be deduced from the inclusion morphology and reaction products. These inclusions have columnar shapes and consist of K-feldspar + kaolinite, albite + Fe-oxide, plagioclase + Fe-oxide, or albite + K-feldspar, respectively. The pseudomorphs with albite/plagioclase occur in a Ca-rich garnet that shows prograde zoning. Pressure–temperature ( PT) evolution, derived from mineral assemblages in granulite and based on the inclusions, suggests a prograde metamorphism from amphibolite through eclogite to granulite facies conditions with subsequent amphibolite facies overprint during exhumation. The estimated PT trajectory for the studied granulites, which also host lenses or boudins of eclogites and garnet peridotites, allows reconstruction of the complete clockwise metamorphic path that is consistent with subduction geotherm prior to the tectonic amalgamation within the continental collisional root.

Importance of melt circulation and crust-mantle interaction in the lithospheric evolution beneath the North China Craton: Evidence from Mesozoic basalt-borne clinopyroxene xenocrysts and pyroxenite xenoliths

Mesozoic Fangcheng basalts from the North China Craton contain many clinopyroxene xenocrysts and pyroxenite xenoliths, which provide important information about melt circulation and crust-mantle interaction in the evolution of the sub-continental lithosphere beneath the region. All the xenocrysts show textural and chemical zoning. The zoning is mostly simple (simply-zoned), but some show complex patterns (complexly-zoned). In-situ major and trace element analyses suggest that all the simply-zoned xenocrysts may have been disaggregated from pyroxenite veins at mantle depth. The zoning is interpreted to result from chemical exchange with the host magma after their entrainment. The complexly-zoned xenocrysts record a complicated history of the lithospheric evolution: their cores could preserve information of high-temperature granulite-facies metamorphism in the lower crust and their intermediary zones might record metamorphic overgrowth in the mantle spinel-facies stability field. Therefore, the cores of the complexly-zoned xenocrysts have probably been derived from the Archean lower crust or newly-accreted lower crust and the intermediary zones could be formed through crust–mantle interaction. All the pyroxenite xenoliths are cumulates and were crystallized from the LREE enriched melts at mantle depth or crust–mantle transitional zone. Thus the clinopyroxene xenocrysts and pyroxenite xenoliths provide evidence for the existence of considerable crust–mantle interaction and melt circulation in the lithospheric mantle, which led to rapid lithospheric enrichment.

(Th+U)-Pb monazite ages from Al-Mg-rich metapelites, Rauer Group, east Antarctica

The age of high-temperature granulite-facies metamorphism (>� 800-850 � C) in the Rauer Group, Prydz Bay, east Antarctica, is relevant for establishing the metamorphic and temporal architecture of the Prydz Bay mobile belt. Monazites within Al-Mg-rich granulite- facies metapelites give an overall tanh-estimated Pan- African age of � 511±4 Ma (2r) using in-situ electron microprobe-based (Th+U)-Pb chronology, consistent with existing U-Pb zircon geochronology from the Rauer Group and Prydz Bay. Monazite occurs primarily within cordierite-bearing coronae and symplectic min- eral reaction textures, and also within biotite. Pan- African granulite-facies metamorphism is preferred as responsible for the development of the cordierite-bearing microstructures, and probably (peak) coarse-grained assemblages, constrained using an integrated geologic, geochronologic and metamorphic framework. Thus, Pan-African granulite-facies metamorphism affected the Rauer Group, within the Prydz Bay mobile belt. Moreover, integrated monazite geochronology may be used to decipher the temporal metamorphic histories of potentially complex high-temperature terrains.

Inherent gravitational instability of thickened continental crust with regionally developed low- to medium-pressure granulite facies metamorphism

Petrological arguments show that regionally developed low- to medium-pressure, high-temperature granulite facies metamorphism may critically enhance the lowering of crustal density with depth. This leads to gravitational instability of homogeneously thickened continental crust, mainly due to changes in mineral assemblages and the thermal expansion of minerals in conjunction with the exponential lowering of the effective viscosity of rocks with increasing temperature. It is argued that crustal processes of gravitational redistribution (crustal diapirism) contributing to the exhumation of granulite facies rocks may be activated in this way.

Evolution of the precambrian In-Ouzzal block (Central Sahara, Algeria)

Abstract New structural and geochronological data are used to constrain the evolution of the Precambrian In-Ouzzal block situated on the eastern side of the West African Craton. The results of this study can be summarized as follows: 1. (i) The block can be subdivided into two series; one is mainly composed of Archaean granitic gneisses (3.2–2.5 Ga old), while the other contains metasedimentary and basic to ultrabasic rocks which can be compared with those described in Archaean supracrustal terrains. 2. (ii) The entire block is affected by very high temperature granulite-facies metamorphism. 3. (iii) The metamorphism is associated with two main deformational phases, the first of which is responsible for a regional foliation foliation refolded during the second phase. 4. (iv) The major tectono-metamorphic event in the area is related to a 2.0 Ga-old (Eburnian) orogeny which ended with carbonatite emplacement. Nearly all evidence of the earliest crustal evolution events (i.e. the initial emplacement and deformation of the granitic and supracrustal materials) has been erased by the Eburnian event.

Hercynite-quartz granulites: phase relations, and implications for crustal processes

The assemblage Zn-poor hercynite + quartz, occurring with cordierite, garnet and/or sillimanite, is diagnostic of high-temperature granulite-facies metamorphism at mid-crustal depths. It has been recorded from a handful of terrains worldwide, in which peak temperatures probably exceeded 800°C, and pressures were typically in the range 4-7 kbar, lower than that required for the association sapphirine + quartz. Uncertainty in the absolute P-T conditions of hercynite-quartz rocks results from unresolved inconsistencies in experimental studies of cordierite equilibria, and from the difficulty of reconstructing the peak compositions of minerals, particularly of oxides, which generally show exsolution and corona textures. The coronas record information about the retrograde P-T path, while in some cases larger-scale mineral associations, such as hercynite-quartz aggregates replacing cordierite porphyroblasts, reveal part of the prograde history. Most hercynite-quartz terrains lack evidence for an earlier high-pressure history, and cooled isobarically from a very hot, transient geotherm. The most generally applicable models involve a combination of magmatic advection and lithospheric thinning. -from Author