Mafic Granulites Research Articles

Recycling of granulitic lower crust into the mantle

Foundering of eclogitic lower crust into the convecting mantle is a well-recognized process invoked to explain the relatively evolved chemical composition of Earth's continental crust. However, it remains unclear the extent to which the foundering could control the newly-made lowermost crust, and whether mafic granulite in a thickened eclogitic-granulitic lower continental section is effectively removable. Here we report a geochemical study of zoned clinopyroxene phenocrysts in Early Cretaceous high-Mg, Eu-depleted andesites (adakite-like intermediate lavas with high La/Yb and Sr/Y ratios and low heavy rare-earth element and Y contents) from the eastern North China craton (NCC), which suggests crust-to-mantle recycling of plagioclase-rich lithologies. Three types of zoned clinopyroxenes are recognized. The type I and II zoned clinopyroxenes show normal- and reverse-zonation, respectively, and both are featured by clear negative Eu anomalies in their cores. The type III zoned clinopyroxene is poikilitic with a high-Mg# core but no Eu anomaly, and enclosing low-Mg#, partially resorbed olivine grain. We therefore propose that these high-Mg andesites were derived from the partial melting of recycled plagioclase- and garnet-bearing granulitic lower crust, which have interacted with refractory peridotites at spinel-facies depth. Owning to mafic plagioclase- and garnet-bearing granulite with density higher and viscosity lower than refractory peridotite at the same temperature, the subcontinental granulitic lower crust could be preferentially recycled into the mantle through delamination. This correlates well with the continental Moho depth, as lower crustal xenolithes and geophysical studies in the eastern NCC revealed the existence of a granulite-facies rather than eclogite-facies lowermost crust residue (present-day ~30-km-thickness) with a pronounced Eu depletion (Eu/Eu* = ~0.8). Consequently, our preferred model has important implications for the compositional evolution of the Earth's continental crust, and offers new constraints on the scale and timing of lithosphere removal beneath the eastern NCC.

Paleoproterozoic multiple magmatic-metamorphic events in the Dunhuang Block, eastern Tarim Craton: Implications for assembly of the Columbia supercontinent

The evolution of the Dunhuang Block associated with the assembly of the Columbia supercontinent is poorly constrained. The geochemical composition and temporal evolution of magmatic and metamorphic rocks may hold key information regarding critical geodynamic processes that occurred during the evolution of the Dunhuang Block. In this paper, we present a systematic petrographic, geochemical, and zircon U-Pb and Hf isotopic investigation on tonalitic gneiss, carbonatite, granitic dyke, mafic granulite and A-type granite in the Dunhuang Block. The results suggest that tonalitic gneiss was derived from partial melting of a thickened oceanic plateau crust or island-arc crust at ~3.67 Ga and then it recorded an important subduction-collision orogenesis in the Paleoproterozoic (~2.02 Ga to ~1.96 Ga). The ~1.94 Ga carbonatite and ~1.85 Ga granitic dyke were formed by partial melting of marble and Meso- to Neoarchean basement rocks, respectively. Both of them were formed in post-collisional setting. The ~1.82 Ga mafic granulite recorded high-pressure granulite facies metamorphism followed by a medium–low pressure granulite facies metamorphic overprint, suggesting a clockwise P-T path and implying an environment of collisional orogenesis. The ~1.76 Ga A-type granite resulted from the reworked Neoarchean crust and was most likely related to a post-collisional episode. Combined with previous studies, we identify two Paleoproterozoic orogenic belts of the Dunhuang Block: the ~2.00–1.96 Ga South Orogen and ~1.85–1.80 Ga North Orogen. According to the coupling of these two orogenies with the Columbia orogeny, we conclude that the Dunhuang Block was associated with the assembly of the Columbia supercontinent.

Localized foundering of Indian lower crust in the India–Tibet collision zone

The deep structure of the continental collision between India and Asia and whether India's lower crust is underplated beneath Tibet or subducted into the mantle remain controversial. It is also unknown whether the active normal faults that facilitate orogen-parallel extension of Tibetan upper crust continue into the lower crust and upper mantle. Our receiver-function images collected parallel to the India-Tibet collision zone show the 20-km-thick Indian lower crust that underplates Tibet at 88.5-92°E beneath the Yarlung-Zangbo suture is essentially absent in the vicinity of the Cona-Sangri and Pumqu-Xainza grabens, demonstrating a clear link between upper-crustal and lower-crustal thinning. Satellite gravity data that covary with the thickness of Indian lower crust are consistent with the lower crust being only ∼30% eclogitized so gravitationally stable. Deep earthquakes coincide with Moho offsets and with lateral thinning of the Indian lower crust near the bottom of the partially eclogitized Indian lower crust, suggesting the Indian lower crust is locally foundering or stoping into the mantle. Loss of Indian lower crust by these means implies gravitational instability that can result from localized rapid eclogitization enabled by dehydration reactions in weakly hydrous mafic granulites or by volatile-rich asthenospheric upwelling directly beneath the two grabens. We propose that two competing processes, plateau formation by underplating and continental loss by foundering or stoping, are simultaneously operating beneath the collision zone.

Origin of orthopyroxene-bearing felsic gneiss from the perspective of ultrahigh-temperature metamorphism: an example from the Chilka Lake migmatite complex, Eastern Ghats Belt, India

AbstractOrthopyroxene-bearing felsic gneiss occurs as foliation-parallel layers and bands together with aluminous granulite, mafic granulite, and quartzofeldspathic granulite in the Chilka Lake migmatite complex of the Proterozoic Eastern Ghats Belt, India. The rock was classified previously as charnockite which underwent granulite-facies metamorphism. Field and textural features of this rock show evidence of the partial melting of a biotite-bearing greywacke protolith. Orthopyroxene with/without garnet and cordierite were produced with K-feldspar as peritectic phases of incongruent melting of presumed metaluminous sediments. Fluid-inclusion data suggest the presence of high-density CO2-rich fluids during peak metamorphism, which are similar to those found in associated aluminous granulite. Whole-rock major and trace element data show wide variability of the source materials whereas REE distributions show enriched LREE and flat HREE patterns. Zircon grains from representative samples show the presence of inherited cores having spot dates (SHRIMP) in the range c. 1790–3270 Ma. The overgrowth on zircon was formed predominantly during c. 780–730 Ma and sporadically during c. 550–520 Ma. Some neoblastic zircons with c. 780–730 Ma ages are also present. U-rich dark zones surrounding cores appear partially metamictised, but spot ages from this zone vary within c. 1000–900 Ma. The <1000 Ma ages represent metamorphism that mirrors the events in associated aluminous granulite. The sources of metaluminous sediments are speculative as the rock compositions are largely modified under granulite-facies metamorphism and partial melting. Considering the accretionary tectonic setting of the Eastern Ghats Belt during the c. 1000–900 Ma time frame, a greywacke-type protolith for the migmatite complex has been proposed.

Deciphering deformation in ultramafic intrusions via magnetic fabric (AMS) and palaeomagnetic studies, Savannah Ni-PGE camp, NW Australia

Knowledge of the internal architecture of mafic-ultramafic intrusions is critical to efficient targeting of magmatic Ni-PGE mineralisation. However, the visual homogeneity of such rocks in drill-core, coupled with poor outcrop in many areas can make their structure difficult to decipher using standard techniques. In this study we use measurements of anisotropy of magnetic susceptibility (AMS) and remanent magnetisation to examine the structural history of four adjacent, temporally equivalent, ultramafic intrusions from the east Kimberley Province, NW Australia. Whilst temporally equivalent, the intrusions have contrasting shape including bladed dyke, sill and stock geometries, and are hosted in rocks of contrasting rheology, including mafic granulite and meta-sedimentary migmatite. Early palaeomagnetic signatures present at Savannah and Savannah North originally had shallow NNE- to ENE-oriented inclinations, consistent with acquisition at low palaeolatitudes at ca 1800 Ma. However, these primary magnetisations were subsequently rotated during the ca. 1810 Ma Mount Stafford Event and/or ca. 1400–1200 Ma Yampi Orogeny, demonstrating that significant deformation occurred post-emplacement. Magmatic fabrics observed in AMS data for Savannah, Savannah North and Dave Hill all show evidence of NE or SW rotation during the same events. However, whilst there are similarities in the implied shortening directions across all four intrusions, they each preserve a unique deformation style, which is attributed to the unique shape of each intrusion coupled with the contrasting rheology of their respective host rocks. Each intrusion preserves a common dominant palaeomagnetic signature which is consistent with a high-grade metamorphic overprint at ca. 1200 Ma during the latter stages of the Yampi Orogeny. The lack of subsequent rotation of this dominant palaeomagnetic signature demonstrates that the majority of deformation pre-dates the Yampi event. This study demonstrates that even where tectonic conditions are similar, that intrusion shape and host rock rheology exert strong control on their deformation style.

HT–LP crustal syntectonic anatexis as a source of the Permian magmatism in the Eastern Southern Alps: evidence from xenoliths in the Euganean trachytes (NE Italy)

Oligocene trachytes from the Euganean Hills include various regionally metamorphosed gneissic and granulitic xenoliths. These xenoliths provide the unique opportunity to investigate South Alpine intermediate to deep crustal levels that are not at present exposed in the Eastern Alps. The estimated P – T conditions are in the range of 780–850°C and 0.45–0.55 GPa for a migmatitic gneiss xenolith. Sensitive high-resolution ion microprobe (SHRIMP II) U–Pb analyses on zircon from this xenolith provide concordant ages around 259.7 ± 3.5 Ma, consistent with a proton-induced X-ray emission (PIXE) U–Th–Pb age on monazite of 262 ± 12 Ma. The Sr–Nd–Pb isotopic compositions, and major and trace element data show distinct origins for the different types of xenoliths. Mafic granulite xenoliths have an isotopic signature close to mantle-derived rocks and to Permian gabbroic rocks from the Western Southern Alps. Metapelite xenoliths have high Sr and low Nd initial ratios like those of acid crustal rocks and could possibly represent the source of the crustal component that is dominant in the acid Permian supervolcanoes. The migmatitic xenolith provides the first documented evidence for a Permian thermal event associated with crustal thinning in the Eastern Southern Alps. Here the South Alpine basement escaped most of the Alpine crustal shortening and still preserves most of the original Permian extension under thick Mesozoic cover. Supplementary material: Microprobe analyses of mineralogical phases and Ti-in-biotite geothermometric calculations are available at https://doi.org/10.6084/m9.figshare.c.5032337

Tectonic setting of the Bipindi greenstone belt, northwest Congo craton, Cameroon: Implications on BIF deposition

The banded iron formations (BIFs) in the Bipindi greenstone belt (GB) are hosted within metavolcano-sedimentary sequence of the Nyong Group at the northwestern edge of the Congo Craton (Ntem Complex) in southern Cameroon. New data on the BIFs and its associated rocks allow us to discuss the tectonic setting of this greenstone belt in a regional perspective. Metavolcanic rocks consist of epidosites, mafic granulites, and garnet-pyroxene gneisses, whereas the metasedimentary rocks comprise chlorite-epidote schists. These rocks have been metamorphosed under greenschist-to granulite-facies conditions. The geochemical features of the metavolcanic rocks such as the trace element ratios and rare earth elements (REE) patterns suggest that their protolith corresponded to tholeiitic to transitional and calc-alkaline basalts. The associated metasedimentary rocks are characterized by low La/Sc and Th/Sc discrimination ratios, and REE abundances, indicating that they were derived from mafic and intermediate igneous rocks. Similar protoliths were recently described for metamorphic rocks interbedded with BIFs at the Nyong Group, which were formed in a shallow basin near the continent. The Bipindi BIFs are further characterized by both weak (1.31–1.34) and slightly high (2.00–2.51) positive (Eu/Eu)SN ratio, that are typical of Superior- and Algoma-type BIFs respectively. However, the geochemical features of the host rocks (e.g., REE patterns, Th/Nb, La/Yb, and La/Sc ratios) suggest that the Bipindi BIFs were deposited at an extensional geodynamic setting likely related to a back-arc, similar to the depositional environment for many Algoma-type BIFs. Archean-Paleoproterozoic arc setting is widely distributed across the Congo Caton and the West African Craton, suggesting integration of the tectonic evolution of the Bipindi GB into a continental-scale paleogeography.

Petrofabrics and Seismic Properties of Himalayan Amphibolites: Implications for a Thick Anisotropic Deep Crust Beneath Southern Tibet

AbstractThe bulk composition of overthickened Tibetan deep crust has been generally believed to be mafic granulite with eclogite at the lowermost crust. However, a granulitic/eclogitic deep crust is in contradiction to geological and geophysical observations in southern Tibet. Here we present petrofabrics and seismic properties of amphibolites from exhumed crustal part of the Indian plate in the eastern Himalayan syntaxis. Our results show strong fabrics of amphibole, nearly random fabrics of plagioclase and strong seismic anisotropies of amphibolites (AVp = 5.6–12.0% and Max. AVs = 5.1–7.7%). Comparing to a deep crust composed of nearly isotropic mafic granulite and weakly anisotropic eclogite, a thick amphibolitic layer in the middle‐lower crust would better account for the strong shear wave splitting (0.2–0.5 s in delay times or 4–15% in S wave anisotropy), the suture boundary parallel faster shear wave polarization, and the widespread postcollisional adakite‐like potassic rocks in southern Tibet.

Magnesium isotopic behaviors between metamorphic rocks and their associated leucogranites, and implications for Himalayan orogenesis

Magnesium isotopic compositions, along with new Sr–Nd–Pb isotopic data and elemental analyses, are reported for 12 Miocene tourmaline-bearing leucogranites, 15 Eocene two-mica granites and 40 metamorphic rocks to investigate magnesium isotopic behaviors during metamorphic processes and associated magmatism and constrain the tectonic-magmatic-metamorphic evolution of the Himalayan orogeny. The gneisses, granulites and amphibolites represent samples of the Indian lower crust and display large range in δ26Mg from −0.44‰ to −0.09‰ in mafic granulites, −0.44‰ to −0.10‰ in amphibolites, and −0.70‰ to −0.03‰ in granitic gneisses. The average Mg isotopic compositions of the granitic gneisses (−0.19 ± 0.34‰), mafic granulites (−0.22 ± 0.17‰) and amphibolites (−0.25 ± 0.24‰) are similar, indicating the limited Mg isotope fractionation during prograde metamorphism from granitic gneisses to mafic granulites and retrograde metamorphism from mafic granulites to amphibolites. The Eocene two-mica granites and Miocene leucogranites are characterized by large variations in elemental and Sr–Nd–Pb isotopic compositions. The leucogranites and two-mica granites have their corresponding (87Sr/86Sr)i varying from 0.7282 to 0.7860 and 0.7163 to 0.7191, (143Nd/144Nd)i from 0.511888 to 0.512040 and 0.511953 to 0.512076, 207Pb/204Pb from 15.7215 to 15.7891 and 15.7031 to 15.7317, 208Pb/204Pb from 38.8521 to 39.5286 and 39.2710 to 39.4035, and 206Pb/204Pb from 18.4748 to 19.0139 and 18.7834 to 18.9339. However, they have similar Mg isotopic compositions (−0.21‰ to +0.06‰ versus −0.24‰ to +0.09‰), which did not originate from fractional crystallization nor source heterogeneity. Based on hornblende/biotite/muscovite dehydration melting reaction and Mg isotopic variations in two-mica granites and leucogranites with the proceeding metamorphism, along with elemental discrimination diagrams, Eocene two-mica granites and Miocene leucogranites could be related to hornblende dehydration melting and muscovite dehydration melting, respectively. Mg isotopic compositions of Eocene two-mica granites become heavier compared to the source because of residues of isotopically light garnet in the source; while those of Miocene leucogranites become lighter because of entrainment of isotopically light garnet from the source region. Thus, a new model for crustal anatexis and Himalayan orogenesis was proposed based on the Mg isotope fractionation in the leucogranites and metamorphic rocks. This model emphasizes a successive process from Indian continental subduction to rapid exhumation of the Higher Himalayan Crystalline Series (HHCS). The former underwent high-temperature (HT) and high-pressure (HP) granulite-facies prograde metamorphism, which resulted in the hornblende dehydration melting and the formation of Eocene two-mica granites; while the latter experienced amphibolite-facies retrogression and decompression, which resulted in the muscovite dehydration melting and the formation of Miocene leucogranites.

Deciphering garnet genesis in meta-igneous rocks: An example from the Jiao-Liao-Ji Belt, North China Craton

Garnet is common in metamorphic rocks and can be used to efficiently constrain pressure–temperature conditions experienced during orogenesis; however, it may also crystallize as a primary phase in many types of magma, which can complicate deciphering the petrogenesis of meta-igneous rocks. In this study, geochronological and geochemical analyses of garnet and zircon were integrated to constrain garnet genesis in plagioclase-absent mafic granulite from the Jiao-Liao-Ji Belt, North China Craton. The rare earth element patterns show an equilibrium relationship between garnet and metamorphic zircon, indicating that garnet has a metamorphic origin. U–Pb dating shows that garnet has a whole-grain age of c. 1.83 Ga, with no difference between core and rim domains, which is slightly younger than metamorphic ages recorded by zircon (c. 1.87 Ga). The age difference between garnet and zircon is likely a result of differences in their closure temperature for the U–Pb isotope system. The mineral assemblage garnet + clinopyroxene + K-feldspar + titanite shows textural equilibration and was used to determine peak pressure–temperature conditions of 0.8–1.2 GPa at 830–880 °C. Given the age of high-pressure metamorphism (c. 1.87 Ga) calculated here being younger than c. 1.95–1.90 Ga proposed by previous studies for the west Liaodong peninsula, we suggest that c. 1.95–1.87 Ga represents the timing of progressive collision between the Longgang Block and the Nangrim Block, with the western part having collided slightly earlier than the eastern part.

Numerical modeling of post-collisional carbonated alkaline magmatism: Variscan style Orogeny (the Ivrea Zone as natural laboratory)

The Ivrea Zone of northwest Italy exposes a section of the lower crust and lithospheric mantle deformed during the Variscan Orogeny, where a series of carbonated alkaline pipes was emplaced over a time span of ca. 40 Ma, following a crustal underplating event at ca. 288 Ma. We use a coupled 2D petrological-thermomechanical approach to geodynamically model the processes that led to the Variscan continental collision and subsequent emplacement of the alkaline pipes with application to craton margin metasomatic events. The following criteria are assessed to develop a generic model: 1) closure of Rheic Ocean through subduction of oceanic crust and subduction-related metasomatism of the lithospheric mantle, 2) high-temperature metamorphism of the lower crust associated with continental collision, preceding the 3) emplacement of a crustal underplating event, and 4) partial melting of metasomatized lithospheric mantle domains during gravitational collapse of the orogen to source the alkaline pipes. Our model investigates the collision process by varying thermal ages of the intervening oceanic crust (40, 60, and 80 Ma) and by changing the rheological strength of the lower crust. We employ two ideal end-member cases, wet quartzite and plagioclase An75, an intermediate dry quartz case to represent a migmatized lower crust, and two polymineralic cases of felsic granulite and mafic granulite as possible representations to the Ivrea Zone specifically. We discuss the results through three scenarios controlled by the age of the oceanic crust and rheology of the lower crust that can reproduce tectonic scale features that are observed in the Ivrea Zone. Scenario 1: the wet quartzite 40 Ma model features widespread H2O and CO2 fluxes within the continental lithospheric mantle. The model displays little melt productivity due to the presence of an overthickened crust and the continental lithospheric mantle decouples and peels off beneath the lower crust of the colliding terrane. Scenario 2: the plagioclase An75 60 Ma model shows high-temperature metamorphism in the lower crust and reproduces the metasomatism of the continental lithospheric mantle that is a precursor to the genesis of the alkaline pipes. The model is permissive for the emplacement of the crustal underplating event, but the relative timings are misplaced. Scenario 3: the mafic granulite 80 Ma model recreates the tectonic relationships between lower crustal high-temperature metamorphism followed by initiation of the magmatic underplating event, and lastly by localized melting of metasomatized domains of the lithospheric mantle. This model can potentially explain two of the prevailing hypotheses for the formation of the alkaline pipes. Whereas all models reproduced different stages of the tectono-magmatic evolution of continental collision, only scenario 3 could sufficiently recreate the relative sequence of events observed in the Ivrea Zone. Scenarios 1 and 2 may provide precious information about the nature and timing of metasomatic processes in the lithospheric mantle, with implications for the enhanced prospectivity for a wide range of ore systems in specific domains along the margins of lithospheric blocks.

U–Pb geochronology and REE geochemistry of zircons in mafic granulites from the Lützow-Holm complex, East Antarctica: Implications for the timing and P–T path of post-peak exhumation and Antarctica–Sri Lanka correlation

We report new petrological and geochronological data, together with zircon REE patterns, of mafic granulites from the Lützow-Holm Complex (LHC), East Antarctica, and investigate the pressure-temperature-time (P-T-t) evolution of granulite-facies metamorphism in order to unravel the timing and tectonics of Neoproterozoic collisional processes related to the amalgamation of Gondwana. The LHC is composed of three tectonic units: a ca. 2.5 Ga continental fragment (Shirase Microcontinent), a ca. 1.0 Ga magmatic arc (Prince Olav–Vijayan Block; PVB), and a latest Neoproterozoic suture zone (Lützow-Holm Bay Suture Zone; LHSZ) separating the two. Mafic granulites from Tenmondai Rock and Sudare Rock, which belong to the PVB and the LHSZ, respectively, contain orthopyroxene + plagioclase symplectites around garnets, probably formed during post-peak decompression. The application of phase equilibrium modeling and geothermobarometry indicates similar peak P-T ranges of 850–860 °C/7.8–8.4 kbar (Tenmondai Rock) and 800–810 °C/8.5–9.0 kbar (Sudare Rock), together with clockwise P-T paths for both localities. LA-ICP-MS zircon U-Pb dating of the studied granulites yielded Mid-Neoproterozoic magmatic ages of >808 Ma (Tenmondai Rock) and >783 Ma (Sudare Rock) from igneous cores, and Late Neoproterozoic to Cambrian metamorphic ages of ca. 630–481 Ma from overgrown rims and/or structureless grains. Zircon REE analyses indicate that younger 560–510 Ma metamorphic zircons from the localities exhibit heavy-REE-enriched patterns and negative Eu anomalies, suggesting that the consumption of garnet and growth of plagioclase took place during zircon growth. This process was probably related to the formation of post-peak decompressional textures (orthopyroxene + plagioclase symplectite around garnet) observed in the mafic granulites. The results of this study therefore indicate that post-peak exhumation of the LHC started at ca. 560 Ma. Available U-Pb and REE data from 560 to 510 Ma metamorphic zircons of the Highland Complex, Sri Lanka, show similar heavy-REE-enriched patterns and Eu negative anomalies, suggesting that post-peak exhumation of both the LHC and the Highland Complex took place almost simultaneously at ca. 560 Ma. The results of this study further support the proposed tectonic model in which the Highland Complex and the LHSZ are parallel collisional sutures formed during the Late Neoproterozoic-Cambrian Gondwana amalgamation.

Major lithologies of the high-grade Zhoutan terrane within the Cathaysia Block and their tectonic implications for the Neoproterozoic - Paleozoic South China

Pre-Devonian crystalline basement of the Cathaysia Block was united through the Wuyi-Yunkai orogeny. The uplifted orogenic core is dominantly composed of Paleozoic high-grade terranes occurring in a northeast-southwest trending belt. The Zhoutan terrane in the middle part of this belt is one of the key terranes to reveal the tectonic architecture of the South China Block. However, how to correlate it with other high-grade terranes, what kind of rock records are preserved in this terrane and their tectonic implications for the Wuyi-Yunkai orogen are still unresolved. This paper carries out an integrated study on the petrology, zircon U--Pb geochronology, geochemistry and zircon Lu--Hf isotopic compositions of the main rock types in the Zhoutan terrane, with a purpose of better understanding the Wuyi-Yunkai orogeny. Mafic granulite (noritic) of high-T and low-P conditions occurs as lenses within the Zhoutan Formation. The zircon U--Pb geochronological data give magmatic and metamorphic ages of 460–450 Ma and ~ 430 Ma, respectively. Neoproterozoic and Paleoproterozoic (1.89 Ga) xenocrysts also occur. Meta-pelites of the Zhoutan Formation experienced high T/P amphibolite facies metamorphism, and ages of their detrital zircons are dominantly Neoproterozoic (810–820 Ma) with some subordinate Paleo- to Mesoproterozoic and Archean signatures. Amphibole gneiss layers of this formation give a Paleozoic metamorphic age of ~450 Ma, consistent with the high-P amphibolite facies metamorphism of the neighboring Longyou terrane. Fine-grained garnet mica-schist of the Wanyuan Formation consists of greenschist to amphibolite facies mineral assemblage and detrital zircons of these meta-sediments display both Neoproterozoic and early Precambrian ages at ~770 Ma, ~820 Ma, ~850 Ma, ~1.84 Ga and 2.48 Ga. Metamorphosed calc-silicates of the Wanyuan Formation show similar detrital zircon age peaks of 624 Ma, 780–790 Ma, ~810 Ma, 840–850 Ma, 1.8–1.98 Ga and ~ 2.48 Ga, and a Paleozoic metamorphic age of ~447 Ma. The Cizhu pluton is a composite granitoid body intruding the Zhoutan and Wanyuan formations, with emplacement ages ranging at least from 485 Ma to 446 Ma. εHf(t) values of zircons from the mafic granulite range from −10.9 to +8.5, indicating its protolith derived from enriched mantle sources with or without crustal contaminations. The Cizhu magmatic event is a crustal reworking event as indicated by the coherently negative zircon εHf(t) values and ancient Hf crustal model ages of 1530–1830 Ma. Zircon εHf(t) values of the metamorphosed calc-silicates show large variations, ranging from −21.2 to +11, consistent with its complex provenances. The deposition of the sedimentary rocks of the Zhoutan and Wanyuan formations is consistent with the development of the Neoproterozoic-Paleozoic Nanhua Rift. The development of continental margin sequences like the Zhoutan and Wanyuan formations, the evolution of the Paleozoic metamorphism from early higher P/T metamorphic stage to later lower P/T stage, as well as the generation of the Paleozoic mafic-ultramafic rocks and the intermediate Cizhu pluton, are best explained by collision to post-collision settings. We, therefore, conclude that the Zhuotan terrane (as well as other high-grade terranes) witnessed a whole geological cycle, from early rifting to the closure of a rifted basin (Nanhua Basin), and the final lithospheric delamination following the rejoining of continental blocks flanking the rift.

Geochronology, geochemistry, origin, and tectonic implications of high‐pressure mafic granulites of the Amdo region, Central Tibet

The Amdo Terrane, Central Tibet, was previously located in the middle of the Bangong–Nujiang Ocean (BNO), and is key to understanding the subduction history of the BNO. However, the high‐pressure (HP) mafic granulites of the Amdo Terrane remain poorly studied. Here, we report the petrology, geochemistry, and zircon U–Pb geochronology of the Amdo HP mafic granulites. The granulites occur mainly as enclaves or deformed dikes in granitic gneisses of the Amdo Terrane. Zircons from the granulites yield two clusters of ages, at 193–192 and 180 Ma. The former represents the best estimate of the age of peak metamorphism, and the latter represents the age of retrograde amphibolite‐facies metamorphism. Although the contents of mobile elements in the granulites, including the large‐ion lithophile elements (Rb, Sr, Cs, Pb, K), were significantly altered during metamorphism, the contents of the high‐field‐strength elements (HFSEs; Th, Nb, Zr, Ti) and rare earth elements (REEs) remained largely unchanged. The granulites belong to the tholeiitic basalt series, with SiO2 contents of 47.05–51.03% and Mg# of 35–50. The REE contents of the granulites are characterised by flat chondrite‐normalised patterns (LaN/YbN = 0.96–1.92; LaN/SmN = 1.0–1.59) and weak Eu anomalies (Eu/Eu* = 0.84–0.99). Most of the granulite samples have high TiO2 contents, clear negative Nb and Ta anomalies, and high Pb enrichment, with positive zircon εHf(t) values (+8.14 to +16.02). The granulites exhibit both normal mid‐ocean ridge basalt (N‐MORB) and island‐arc basalt (IAB) characteristics. Using the data presented here and previous data, we suggest that the protolith of the granulites were back‐arc basin basalts derived from high‐degree partial melting of depleted mantle in the spinel stability field, which records the early evolution of the BNO. Northward subduction of the BNO oceanic plate led to back‐arc spreading and the formation of back‐arc basin basalts on the northern margin of the Amdo Terrane. The back‐arc basin extended further to form an ocean basin, which separated the Amdo Terrane and the Southern Qiangtang–Baoshan (SQB) Terrane, resulting in the Amdo Terrane becoming an isolated microcontinent in the BNO.

Phanerozoic magma underplating associated with remelting of the lower crust beneath the Cathaysia Block: Evidence from zircon U[sbnd]Pb ages and Hf[sbnd]O isotopes of granulite xenoliths from Daoxian, South China

Widespread Mesozoic granitoids in the Cathaysia Block of South China are associated with intensive reworking of the lower crust as a result of magma underplating. This inference is based mainly on studies of mafic igneous rocks, whereas there is little evidence from lower-crustal rocks. Lower-crustal xenoliths in Mesozoic basalts in the Daoxian region within the Cathaysia Block might record information on the relationship between magma underplating and remelting of the pre-existing crust beneath the block. The xenoliths are mainly mafic granulites, with minor felsic granulites. The mafic granulites have low SiO2 contents (47.22–49.46 wt%) and high Mg# values (77.8–82.4), suggesting that their protoliths were derived from a mantle source. The felsic granulite xenoliths have high SiO2 (69.56–70.27 wt%) and low MgO (1.63–2.50 wt%) contents, and zircons in these xenoliths yield negative εHf(t) values (−6.1 to −12.6) and high δ18O values (6.8–7.6‰), consistent with a crustal source. Both mafic and felsic granulite xenoliths record magmatic (226–218 Ma) and metamorphic (218–193 Ma) events, suggesting a genetic link between mafic and felsic rocks in the lower crust. We propose that the magma underplating was responsible for the origin of the mafic granulites and partial melting of pre-existing lower crust, as recorded by the felsic xenoliths. The granulite xenoliths also contain abundant inherited zircons that formed during the Archean–Neoproterozoic (2584–659 Ma), early Paleozoic (peaking at ca. 425 Ma), and late Paleozoic (peaking at ca. 261 Ma). Zircons from the Daoxian granulite xenoliths have distinct HfO isotopic compositions that record the multistage evolution of the lower crust beneath the Cathaysia Block. This evolution involved the accretion of juvenile crust during the late Archean (εHf(t) = +4.2 to +4.6) and late Paleozoic (εHf(t) = +1.3 to +5.3; δ18O = 5.8–6.2‰), crustal reworking during the Neoproterozoic (εHf(t) = −7.5 to −11.8; δ18O = 5.1–9.5‰) and early Paleozoic (εHf(t) = −0.5 to −2.2; δ18O = 7.3–7.5‰), and crustal accretion with significant reworking during the early Mesozoic (εHf(t) = −19.2 to +5.9; δ18O = 6.8–7.6‰). The UPb ages and HfO isotopic data of zircons from the Daoxian granulite xenoliths are consistent with the distribution of Phanerozoic igneous rocks in the Cathaysia Block. These data support the inference that Mesozoic magma underplating triggered the remelting of pre-existing crustal materials and produced extensive granitoid magmatism.

Petrological and Geochronological Constraints on the Evolution of Charnockitic Rocks in the Massifs of Cauvery Shear Zone, Southern Granulite Terrain, India

Abstract Cauvery shear zone of the southern granulite terrane, India, is sandwiched between the amphibolite to granulite transition zone in the north and the Neoproterozoic granulite belt in the south. Two massifs of this shear zone consist of charnockitic rocks of tonalitic to granodioritic composition and include hornblende-mafic granulite enclaves. Petrology and geochemistry clearly indicate dehydration partial melting of mafic rocks as the mode of origin of the charnockitic rocks. However, high MgO, Ni and Cr contents and negative Nb anomalies suggest some interaction with the mantle in a subduction zone environment. U-Pb and Lu-Hf isotopic data of zircons in mafic granulite enclave and charnockite-enderbite host rocks indicate ca. 2.5 Ga granulite-cum-anatectic event. Juvenile mafic magmatism at ca. 2.7 Ga could represent the precursor of the mafic granulites. With the chemical and isotopic evidence of subduction zone environment, the Cauvery shear zone could be appropriately described as a suture along which Archaean-Paleoproterozoic granulite terrain and Neoproterozoic granulite terrain had collided.

Chemical Composition, U–Th–Pb Age, and Geodynamic Setting of Metavolcanic Filla Series (Rauer Islands, East Antarctica)

An assemblage of mafic granulites (schists), plagiogneisses, and metasedimentary rocks, which is referred to as the Filla Series, is exposed in the Rauer Islands (opposite the eastern coast of the Prydz Bay). The chemistry and Nd isotope composition of the schists, which are interpreted as metavolcanics, are characterized; the results of U–Th–Pb dating (SHRIMP) of zircons from these rocks are presented. The data obtained indicate that the protolith of these rocks crystallized ca. 1500 Ma ago and that the rocks later underwent thermal events at ca. 1000 Ma and 545–515 Ma. The relatively high eNd(t) values, ranging from 2 to 4.5 for most samples, indicate the primitive composition of the mantle source and the limited extent of crustal contamination. The specific chemical composition of these rocks suggests that the Filla Series was formed in a convergent (back-arc(?)) setting with contributions from both plume and lithospheric sources. The apparently heterogeneous rock assemblage may be a product of tectonic interaction between the active continental margin and oceanic plateau. The obtained Early Mesoproterozoic age of the mafic rocks of the Filla Series argues for correlation of the Rauer Islands area with the long-lived Musgrave–Albany–Fraser–Wilkes Proterozoic (super) province in Australia and Antarctica, on the one hand, and the Eastern Ghats Province in India, on the other. The formation of the Early Mesoproterozoic Filla Series suggests that the development of the hypothetical paleocean (its convergent margins) did not terminate during the Paleoproterozoic but continued into the Mesoproterozoic. Correlation with the Musgrave–Albany–Fraser–Wilkes (super) province suggests a shared geological evolution of large crustal blocks, represented by the Prydz Bay coast and the Australian–Antarctic block starting from the Early Mesoproterozoic.

The effect of host magma infiltration on the Pb isotopic systematics of lower crustal xenolith: An in-situ study from Hannuoba, North China

The Pb isotopic compositions of granulite xenoliths are crucial to constrain the formation and evolution of the lower continental crust (LCC) and are potentially the key to resolve the first Pb paradox of the Earth. However, it remains unclear how host magma infiltration affects the Pb isotopic compositions of LCC xenoliths and how different this effect is for different xenoliths in composition (from mafic to felsic). To address these issues, here we provide an in-situ chemical and PbSr isotopic study on feldspars and clinopyroxenes for a suite of fresh mafic and felsic granulite xenoliths from the Hannuoba region in the northern margin of the North China Craton (NCC), which have been previously well characterized for bulk-rock chemical and Sr–Nd–Pb isotopic compositions. Our study shows that the in-situ plagioclase and bulk-rock Pb isotopic compositions of the Hannuoba mafic granulite xenoliths are significantly decoupled. Such decoupling is demonstrated to have been caused by host magma infiltration through contributing the main Pb budget into the grain boundaries and cracks. However, this process appears to have relatively less or negligible effect on the Pb isotopic systematics of feldspar-rich intermediate-felsic granulite xenoliths, which is reflected by their consistent in-situ (feldspar) and bulk-rock Pb isotopic compositions. Such different effects on Pb isotopic systematics for different LCC xenoliths in composition are highly related to the Pb contents or abundances of feldspar in LCC xenoliths. Furthermore, our data suggest that the effect of host magma infiltration on the Pb isotopic systematics of LCC xenoliths may be not easily eliminated by the leaching technique and thus it should be prudent to use the bulk-rock Pb isotopic data of low-Pb LCC and potentially upper mantle xenoliths, such as peridotite, pyroxenite and garnet−/pyroxene-rich mafic granulite. Instead, in-situ analyses of minerals (e.g., feldspar, clinopyroxene) could provide a more convenient and reliable method to obtain the pristine Pb isotopic compositions of these low-Pb xenoliths.

Rapid fluid infiltration and permeability enhancement during middle–lower crustal fracturing: Evidence from amphibolite–granulite-facies fluid–rock reaction zones, Sør Rondane Mountains, East Antarctica

Permeability is a key control on fluid infiltration in the crust. However, quantitative geological constraints on crustal permeability are limited, particularly with regards to its temporal evolution. Here we constrain the permeability evolution in the middle–lower crust, based on metamorphic processes associated with fluid infiltration and crustal fracturing. We investigated mafic granulite and orthopyroxene–hornblende schist (opx–hbl schist) samples from Mefjell, Sør Rondane Mountains, East Antarctica. Millimetre-scale amphibolite-facies reaction zones occur along fractures in these rocks. In the mafic granulite, two zones were identified: (i) reaction zones (1–2 mm thick); and (ii) mafic granulite host rock. The opx–hbl schist sample can be divided into the following three zones: (i) actinolite–cummingtonite zones (1.4 mm thick); (ii) actinolite–orthopyroxene zones (1.6 mm thick); and (iii) host rock. These zones are evident from the modal mineralogy, reaction textures, and trace element profiles. The P–T conditions of fluid infiltration are estimated to be 0.55 GPa and 620 °C for the mafic granulite, and 0.3 GPa and 450 °C for the opx–hbl schist, respectively. Chlorine concentrations in apatite grains show a gradual decrease from the fractures towards the host rocks. Chlorine concentration profiles suggest that the dominant processes of chlorine transport were advection with minor diffusion for both samples. Based on these results, the timescales of fluid infiltration are constrained to be ~8 h for the mafic granulite and ~10 h for the opx–hbl schist. The pressure gradient across the reaction zones was estimated from the H2O activity in the reaction zones to be 2–15 MPa/mm. The permeability of the host rock and fractures were estimated to be 10−21–10−23 and 10−8–10−9 m2, respectively. Our results show that rapid infiltration of Cl-bearing fluids (~10 h) occurred due to a limited fluid flux from low-permeability (10−21–10−23 m2) host rocks. The low-permeability media led to fluid accumulation and further fracturing. The spatio-averaged permeability then increased by more than several orders of magnitude (10−10–10−16 m2) and the fluid pressure decreased. The contrasting permeability between the host rocks and fractures reveals permeability enhancements associated with crustal fracturing on timescales comparable to geophysical observations. Compared with average long-term (Myr) permeability estimates from previous studies (e.g., 10−18 m2), the permeability obtained from the fracture–reaction zone systems shows large fluctuations in the middle–lower crust (10−21–10−23 to 10−10–10−16 m2).

Field, petrographic and geochemical characteristics of Sullya alkaline complex in the Cauvery Shear Zone (CSZ), southern India: Implications for petrogenesis

Significant, but volumetrically smaller, unmetamorphosed and largely undeformed alkaline magmatic suites have been reported from the Southern Granulite Terrain in southern India. These Neoproterozoic alkaline magmatic rocks occur as lenses, dykes and plugs that are mostly within, or proximal to, major shear zones or transcrustal faults. In this contribution, field, petrographic and whole-rock geochemical data of Sullya syenites and associated mafic granulites from the Mercara Shear Zone (MSZ), which separates low-grade (greenschist to upper amphibolite facies) Dharwar Craton and high-grade (granulite facies) Southern Granulite Terrain is presented. The isolated body of the Sullya syenite, similar to other alkaline plutons of the Southern Granulite Terrain, shows an intrusive relationship with the host hornblende-biotite gneisses and mafic granulites. The Sullya syenites lack macroscopic foliations and unlike, other plutons, they are not associated with carbonatites and ultrapotassic granites. Potash feldspar and plagioclase dominates the felsic phases in the Sullya syenite and there is negligible quartz. The studied syenites show evidence of melt supported deformation, but show no evidence of recrystallization. Geochemically, they most resemble the Angadimogar syenites (situated 3 km west of the Sullya syenites) with similar major oxide and trace element concentrations. The petrogenetic studies of the Sullya syenite have indicated that they were generated by mixing of two different sources derived from the partial melting of metasomatized continental mantle lithosphere and lower crustal mafic granulites. This melt source could have been emplaced in a rift-related tectonic setting. The emplacement is considered to be controlled by shears.