Ultrahigh-temperature Granulites Research Articles

Multi-stage ultrahigh temperature metamorphism in the lower crust of the Kaapvaal craton recorded by U–Pb ages of garnet

U–Pb ages were determined by split-stream LA-SF/MC-ICPMS in garnets from UHT granulite xenoliths (Star mine, South Africa; 124 Ma). They give a considerable age range of 400 million years with well-defined maximas at 3.09, 3.01 and 2.75 Ga. The oldest peak overlaps with the changeover from tonalites to K-granites at 3.14–3.04 Ga and with zircon ages of the mid-crustal granulites of the Vredefort dome (3.1 Ga) in the wake of the 3.2 Ga collision of three terrains that compose the Witwatersrand block. Subduction (or sagduction) of the uppermost crust in an ultrahot orogen setting brought shales and greenstones to the lower crust. Ultrahigh temperature (UHT) conditions are the result of high mantle potential temperatures and self- heating by the radioactive inventory of the subducted lithologies. Metamorphism, anatexis to very high degrees and melt extraction left UHT granulites as residue. Rejuvenation of UHT conditions was brought about by Dominion Group magmatism between 3.0 and 2.95 Ga. Magmatic uprise caused intense shearing in the lower crust followed by recrystallisation of the shear zones to generate the younger garnet age group. Ventersdorp flood basalt volcanism caused similar processes at around 2.72 Ga and generated the third garnet age group. Zircon gives U–Pb ages mainly around 2.72 Ga (both literature and our own data) i.e. zircon adjusted or newly crystallized at the youngest UHT event. Only few zircon grains retained older ages up to 2.94 Ga. Still unconstrained, but very high closure temperatures (≥ 1100 °C) for the U–Pb system in garnet keep the memory of the oldest ages in UHT granulites. Such ages can only be reset by recrystallization. This way, garnet records a prolonged high-temperature history of the lower crust of the Kaapvaal craton.

Neoarchean clockwise P-T trajectories with evolution sequence from HP to UHT metamorphism during decompression: Evidence from petrology, mineralogy, and phase equilibria modeling of mafic granulites in the Sittampundi Complex, southern India

The dispute of tectonic environments such as subduction-collision setting versus extension tectonic setting, is a common phenomenon in the Precambrian. Despite the ongoing controversy, the pressure–temperature (P–T) paths recorded by metamorphic rocks are often used as an indicator for distinguishing between subduction-collision and extension tectonic settings in the early Precambrian. It is widely acknowledged that high-pressure (HP) granulites may have formed in crustal-thickening-dominated collision/subduction environment. Conversely, ultra-high temperature (UHT) granulites may have formed in the elevated mantle heat flux environment. The Sittampundi Complex (SC), where HP granulites and UHT granulites coexist, plays a crucial role in unraveling the late-Neoarchean to early-Paleoproterozoic tectonic evolution of northern South Granulite-facies Terrane (NSGT) of India. In this study, we present new P–T paths from three HP mafic granulites overprinted by UHT granulite facies metamorphism in the SC and provide more data to unravel metamorphic evolution of NSGT. Phase equilibria modeling and traditional thermobarometry calculations illustrated that clockwise metamorphic pressure–temperature (P–T) paths were yielded, passing from 3.3 to 9.3 kbar/631–669 °C (prograde stage) through 11.9–18.5 kbar/828–986 °C (pressure-peak stage), and through 6.6–11.1 kbar/1019–1085 °C (temperature-peak stage) to 4.4–8.1 kbar/845–930 °C (retrograde stage). The HP mafic granulites exhibit clockwise P–T paths with a considerable heating process (ca. 150–180 °C) from the pressure-peak stage (HP-granulite sub-facies) to the temperature-peak stage (UHT-granulite sub-facies). Zircon U-Pb dating yields an age of 2513 ± 29 indicates that the metamorphism took place in the Neoarchean. The uncommon P–T paths from Neoarchean HP granulite in the NSGT suggest that during late Neoarchean, the NSGT had experienced the crustal-thickening dominated collision/subduction (assembly of micro-blocks of Dharwar Craton) and subsequently mantle diapirism process.

Metamorphic evolution of high-pressure and ultrahigh-temperature granulites from the Alxa Block, North China Craton: Implications for the collision and exhumation of Paleoproterozoic orogenic belts

High-pressure (HP) and ultrahigh-temperature (UHT) granulites with a high geothermal gradient (greater than 500 °C/GPa) are prominent features of Paleoproterozoic orogenic belts and may represent paired metamorphic belts present during the early stages of plate tectonics. Understanding their pressure−temperature−time (P-T-t) paths and metamorphic evolutionary relationships could provide valuable constraints on the tectonic processes of Paleoproterozoic orogenic belts. Here, we describe garnet mafic and clinopyroxene-orthopyroxene (Cpx-Opx) granulites from the Diebusige area of the Alxa Block in the western part of the Khondalite Belt, North China Craton. Through detailed petrographic, phase equilibrium modeling, and Ti-in-amphibole thermometric studies, we obtained the preserved peak mineral assemblages of two types of mafic granulites: garnet + clinopyroxene + amphibole + plagioclase + quartz + ilmenite, and clinopyroxene + orthopyroxene + plagioclase + amphibole + garnet (rare) + ilmenite. The preserved peak P-T conditions were determined to be 850−890 °C/11.4−13.2 kbar (HP granulite-facies) and 950−970 °C/8.2−9.2 kbar (UHT conditions), with thermal gradients of ∼70 °C/kbar (moderate differential temperature/differential pressure, dT/dP) and ∼110 °C/kbar (high dT/dP), respectively. Using sensitive high-resolution ion microprobe U-Pb dating and rare earth element analysis of zircons, we found that the garnet mafic granulite recorded an HP granulite-facies metamorphic age of ca. 1.95 Ga and a retrograde cooling age of ca. 1.8 Ga, while the Cpx-Opx granulite recorded a consistent retrograde cooling age of ca. 1.8 Ga. By combining these results with the metamorphic evolution and timing (ca. 1.92−1.91 Ga) of UHT rocks from the Khondalite Belt, we suggest that the garnet (HP) mafic and Cpx-Opx (UHT) granulites may represent different stages of the same metamorphic event, shedding light on the processes involved in the collision and subsequent exhumation of Paleoproterozoic orogenic belts.

Geochemical and geochronological evolution of the UHT granulites from the Eastern Ghats Mobile Belt, India: implications from F-OH substitution in biotite, thermal events and cooling age

ABSTRACT The thin lenses and bands of ultra-high temperature (UHT) granulites with diagnostic sapphirine and quartz in contact are occasionally found on the western flank of the Eastern Ghats Mobile Belt (EGMB), which inherits the suturing and over-thrusting signatures of the EGMB with the Greater Indian Landmass (GIL). The mineral biotite contains an abnormally high fluorine concentration, as revealed by the geochemical examination of these granulites from the research area (Kothuru), which was caused by the F-OH substitution to withstand extreme thermal metamorphic conditions. The geochemical results from the multi-element spider diagram and the Harker’s variation diagram with primitive-mantle normalization also show an enrichment of light rare earth elements and large-ion lithophile elements over depleted heavy rare earth elements and high-field strength elements. An evolutionary history has been established for the high-grade rocks that were subjected to ultra-high temperature metamorphism, based on the geochemical signatures of the examined granulites. The use of isotopic K-Ar cooling ages and electron microprobe monazite chemical ages of the thermal episodes, which act as useful tools to explain the time and span of the polycyclic metamorphic terrane, is another significant component of the current work. Numerous researchers have attempted to estimate the thermal episodes in various areas of the Eastern Ghats Mobile Belt, but their cooling ages are still poorly understood. Consequently, two thermal episodes that happened at 842 Ma and 637 Ma ago, as well as a cooling age of about 448 Ma that marked the quick exhumation of the high-grade rocks under study, have been recorded in the current investigation. The association between the Greater Indian Landmass and Eastern Antarctica during the existence of the Rodinian supercontinent has been established by recorded geochronological events.

Late Paleoproterozoic ultrahigh-temperature mafic granulite from the eastern Alxa Block, North China Craton

The timing and mechanism of ultrahigh-temperature (UHT) granulite facies metamorphism is critical to understanding the tectono-thermal evolution of granulite terranes and their relationships to supercontinent assembly. Here we document late Paleoproterozoic UHT mafic granulites from the Diebusige Complex in the eastern Alxa Block, North China Craton. These rocks occur as lens within felsic gneiss, marble, and garnet-bearing pyroxenite and contain representative granulite-facies minerals such as garnet, clinopyroxene, plagioclase, and Ti-rich amphibole. Based on phase equilibria modelling, Ti-in-amphibole (Amp) thermometer, and rare earth element (REE)-based thermometers, UHT metamorphic condition of 948 – 1048 ℃/11 – 14 kbar was obtained for the peak metamorphic stage, which was followed by retrograde metamorphism under lower amphibolite – greenschist facies (401 – 565 ℃/5.2 – 7.1 kbar). Zircon U-Pb dating and trace element systematics suggest that the protolith of the studied UHT mafic granulites emplaced at ∼ 1.97 Ga. This is followed by rapid metamorphism to UHT conditions and slow cooling in the lower crust during 1.96 – 1.86 Ga, suggesting a long-lived hot orogen in the eastern Alxa Block, possibly resulting from Andean-type subduction – accretion in the eastern Alxa Block. Combining chronological and metamorphic P-T data on UHT metamorphism from adjacent areas, our results support a long-lived late Paleoproterozoic (ca. 2.0 – 1.8 Ga) hot orogen that extends at least from the Khondalite Belt to the eastern Alxa Block.

Different Cooling Histories of Ultrahigh-Temperature Granulites Revealed by Ti-in-Quartz: An Electron Microprobe Approach

The cooling history of granulite is crucial to understanding tectonic scenarios of the continental crust. Ti-in-quartz, a useful indicator of temperature, can decipher the thermal evolution of crustal rocks. Here we apply the Ti-in-quartz (TitaniQ) thermometer to ancient ultrahigh-temperature (UHT) granulites from the Khondalite Belt (KB) in the North China Craton (NCC) and young UHT granulites from the Mogok Metamorphic Belt (MMB), Myanmar. Ti content in quartz was analyzed using a highly precise method constructed in a CAMECA SXFive electron probe microanalyzer (EPMA). The granulites from the two localities show different quartz Ti contents with a constant deforced beam of 10 μm. Matrix quartz and quartz inclusions from the NCC granulites have 57–241 ppm and 65–229 ppm, respectively, corresponding to the TitaniQ temperatures of 653–810 °C and 666–807 °C. The calculated temperatures are significantly lower than the peak temperatures (850–1096 °C) obtained by other methods, due to the formation of abundant rutile exsolution rods in quartz during cooling. Thus, the low calculated temperatures for the NCC granulites reflect a cooling state near or after the exsolution of rutile from quartz, most likely caused by a slow cooling process. However, the matrix quartz from the MMB granulites is exsolution-free and records higher Ti contents of 207–260 ppm and higher metamorphic temperatures of 894–926 °C, close to the peak UHT conditions. This feature indicates that the MMB granulites underwent rapid cooling to overcome Ti loss from quartz. Therefore, determining the amount of Ti loss from quartz by diffusion can provide new insight into the cooling behavior of UHT granulites. When a large deforced beam of 50 μm was used to cover the rutile rods, the matrix quartz in the KB granulites could also yield the TitaniQ temperatures above 900 °C. Thus, our new data suggest that the TitaniQ thermometer could be useful for revealing UHT conditions.

Revealing the link between A-type granites and hottest melts from residual metasedimentary crust

Abstract Among S-, I-, and A-type granites, the latter are the most debated in terms of origin, and metasedimentary crust is usually discarded as a potential source. Here we tackle this issue by adopting an in-source perspective, rather than focusing on the final product (granite), documenting the occurrence of pristine melt inclusions (MIs) in garnet from residual metapelitic ultrahigh-temperature (UHT) granulite from East Antarctica. Coexistence of sapphirine + quartz, phase equilibria calculations, and Zr-in-rutile thermometry indicate that MIs trapped UHT melts formed at peak conditions (930–1000 °C) from a residual metapelitic source. MIs are granitic with weakly peraluminous to weakly peralkaline affinity, ferroan character, high alkali contents, high K/Na and Ga/Al, and low Ca, Ba, Sr, and H2O concentrations. These features and geochemical modeling indicate that MIs represent primary melts for high-SiO2 A-type granites. Therefore, MIs reveal the missing link between A-type granites and the hottest metasedimentary crust. Voluminous amounts of slightly peraluminous, high-SiO2 A-type granites can be produced in large, residual UHT terranes such as those of eastern Gondwana. Our results provide a wider view of processes responsible for granite formation and show that a larger variety of granites must be considered in models of the effects of UHT anatexis on crustal differentiation.

Archean ultrahigh temperature event in the northwestern Superior Province, Canada: Metamorphic evolution and tectonic implications of the Pikwitonei Granulite Domain

AbstractThe Pikwitonei Granulite Domain (PGD), located in the northwestern Superior Province, is one of the largest Neoarchean high‐grade metamorphic domains in the world, and is a key to understanding the Neoarchean crustal evolution of the Superior Province. Here we report results of a study on ultrahigh temperature (UHT) granulites with a sapphirine + quartz‐bearing peak assemblage from the Sipiwesk Lake area in the PGD. Three stages of metamorphic assemblage development are recognized based on petrographic observations: Pre‐peak stage is marked by garnet, sillimanite, K‐feldspar, and biotite inclusions within sapphirine and orthopyroxene. Peak stage is charactered by the typical UHT associations of sapphirine + quartz and sapphirine + orthopyroxene. The retrograde stage is represented by the retrograde formation of cordierite, biotite and sillimanite in the matrix. Phase equilibrium modelling based on the bulk compositions of sapphirine‐bearing granulites suggest that the rocks have experienced extensional UHT metamorphism in excess of 1035°C at pressures of 7.9–9.0 kbar, followed by isobaric cooling process. SHRIMP U–Pb dating of metamorphic zircons in sapphirine‐bearing granulite record the timing of the UHT event in the PGD with a weighted mean 207Pb/206Pb age of 2681 ± 13 Ma. This study in combination with other metamorphic P–T paths and age information reveals that this UHT metamorphism at 2.68 Ga in PGD was generated by upwelling asthenosphere due to slab break‐off in collision during the amalgamation of the Superior Province.

Metamorphic evolution of the UHT granulites from Mandikota, Eastern Ghats Mobile Belt, India: Constraints from phase equilibria modelling, fluid inclusions and monazite U-Th-total Pb geochronology

The Eastern Ghats Mobile Belt (EGMB) had been subjected to extreme thermal metamorphic history at various sections of its terrane. One such terrane (Mandikota) located near the western part of the EGMB has been speculated based on asserted reaction textures, conventional geothermobarometry, geochronology, fluid inclusions, and pseudosection modelling. The present integrated study aims to understand the intricacies of the diversified and controversial tectono-metamorphic evolution of these polycyclic granulite terranes. The stable occurrence of sapphirine + quartz in contact is rare in its own and is a diagnostic feature of Ultra-High Temperature (UHT) metamorphism. Conventional geo-thermobarometeric tools applied, suggests an ordered series of reactions for the different stages of metamorphism experienced by the investigated granulites. Also, the P-T estimated by conventional methods lies in sound correlation with the peak P-T condition (in excess of 950 °C at nearly 9 kbar) attained in the constrained pseudosection model. The exhumation trajectory of the examined UHT terrane reflects P-T path responsible for the thrusting mechanism with isothermal decompression of 3 kbar followed by late-stage isobaric cooling. This study also presents the primary report for the systematic fluid inclusion study of the UHT granulites and its correlation with the evolved P-T trajectory. In addition, monazite dating using Electron Micro Probe Analysis (EPMA) is procured to assess the time frame in understanding the P-T-t evolution of the high-grade terrane. The geochronological study further revealed a new age (∼600 Ma) associated with the Visakhapatnam Domain which adheres to the Pan-African orogeny and marks the reworking phase of the widely reported the Grenvillian event in the EGMB province. As such, the results from the studied fluid inclusions displaying monophase inclusions of CO2-rich carbonic fluid, and its corroboration with the P-T pseudosection model interpreting the phase equilibrium conditions prevailing during the Neoproterozoic time (∼900 Ma).

Age and petrogenesis of mafic granulites from central Madurai block, south India: implications on regional tectonics

AbstractThe Precambrian Southern Granulite Terrane (SGT) of south India is well-known for the preservation of high- to ultrahigh-temperature (HT-UHT) granulites, prominently exposed in its central part forming a linear belt referred to as the Kambam UHT belt. This belt also hosts widespread occurrences of mafic granulites that are observed in close spatial association with the HT-UHT granulites. This study presents detailed petrology, geochemistry and geochronology of representative mafic granulites from the area to understand their petrogenesis and tectonic setting. The results demonstrate that mafic granulites are low- to medium-K tholeiites, with continental arc affinity, formed by the partial melting of a subduction-modified enriched mantle source. The composition of the parent mantle source is modelled with a spinel/garnet lherzolite contribution ratio between 100/0 and 70/30, suggesting the mixing of spinel and garnet bearing melts during asthenosphere upwelling. Zircon U–Pb geochronology of mafic granulites constrains their emplacement between 612 Ma and 625 Ma, that subsequently underwent metamorphism between 581 Ma and 531 Ma. This overlaps with the timing of HT-UHT metamorphism in the Kambam UHT belt bracketed between 593 and 532 Ma. Zircon Hf isotopic studies reveal parent magma generation from reworked melting sources involving Archean and Proterozoic components. These results propose an alternative heat source for the formation of HT-UHT granulites in the Kambam UHT belt which can be designated as a major terrane boundary within the SGT.

Nb and Ta intracrustal differentiation during granulite-facies metamorphism: Evidence from geochemical data of natural rocks and thermodynamic modeling

Abstract Both continental crust and depleted mantle are characterized by subchondritic Nb/Ta, leading to a mass imbalance when compared to the bulk Earth. Even though several potential high-Nb/Ta reservoirs in Earth’s core and undepleted mantle have been proposed, little attention has been given to those in the crust. Here we present bulk-rock and rutile geochemical data for samples from a lower crustal pelitic granulite, North China Craton, which exhibit systematic variation in their Nb and Ta contents. High-temperature (HT) and ultrahigh-temperature (UHT) granulite residues exhibit Nb/Ta ratios that are close to chondritic and subchondritic, respectively, whereas leucosomes from UHT granulite mostly have suprachondritic Nb/Ta. These variations are best explained via competition for Nb and Ta between biotite and rutile during metamorphism, although initial bulk-rock Nb/Ta values also have an effect. As biotite preferentially incorporates Nb over Ta, the early stages of biotite dehydration melting produce a high-Nb/Ta residue and a low-Nb/Ta melt; however, geochemical modeling suggests that once biotite is depleted, the Nb/Ta ratio of the system is instead controlled by rutile growth, which promotes the formation of a lower Nb/Ta residue and a higher Nb/Ta melt, even though the volume of melt produced at this stage may be small. We propose that in situ and in-source leucosomes and leucocractic veins in UHT terranes may retain a high-Nb/Ta geochemical signature. However, residual crustal-derived A2-type granites that experience significant fractionation of Nb- or Ta-bearing minerals during crystallization or contamination from other low-Nb/Ta sources cannot retain this high-Nb/Ta ratio, even though these ratios are generally higher than that of S-type granites. Anhydrous partial melting of metapelite can generate Nb-rich melts, such that high-temperature leucosomes, in addition to related A2-type granites, may represent significant Nb deposits.

Tibetan Plateau insights into >1100 °C crustal melting in the Quaternary

Abstract Partial melting during high-temperature to ultrahigh-temperature (UHT) metamorphism facilitates crustal differentiation, element transfer, and the evolution of topography in orogens, however the mechanisms that drive heating of Earth’s crust remain controversial. We provide new evidence from ca. 2.3 Ma dacites in the Tibetan Plateau, representing the youngest known UHT metamorphic event. Our results show that these dacites were mainly generated by fluid-absent melting of metasedimentary rocks and minor mafic rocks at peak temperatures of 1100–1150 °C and pressures of 0.8–0.9 GPa. The dacites represent mixtures of UHT melts and granulite residues and are geochemically similar to A-type granites with extremely high heat-production values (5.33–5.99 μW m–3). Compared with the geological and geophysical observations, numerical modeling indicates that the key factor determining the thermal evolution of Tibet is the thickness of the radioactive layer. Orogens dominated by rocks of felsic composition, like Tibet, could easily reach UHT conditions within a short period of time (20–40 m.y.) after crustal thickening by radioactive heating, without the need for an additional tectonic mechanism.

Response of trace elements to partial melting of felsic crust at high to ultrahigh temperatures: Implications for granite geochemistry

It is known that partial melting and melt extraction of crustal rocks result in chemical differentiation of the continental crust. But it is unknown how these two processes have affected the composition of granites due to a limited knowledge of trace element behaviors during crustal anatexis. In order to quantify this issue, a combined study of whole-rock and mineral trace elements and mineral modal proportions was conducted for high-temperature (HT) to ultrahigh-temperature (UHT) felsic granulites from the Tongbai orogen in central China. Reconstruction of LREE budgets suggests that LREE mainly reside in monazite in both HT and UHT granulites. As monazite is predicted to be not stable at UHT conditions in the modelling of previous studies, the presence of monazite in the Tongbai UHT granulites suggest that it was not sufficiently dissolved into anatectic melts. Considering monazite mainly occurs as interstitial grains, fast melt-residue separation may be the dominant factor for impeding its dissolution. Calculation of mineral/mineral trace element ratios indicates that K-feldspar has a higher capacity to accommodate Ba and Sr than biotite and plagioclase, with DBaK-feldspar/biotite of 2.5–3.0 and DSrK-feldspar/plagioclase values of 1.4–1.9, and ilmenite can preserve more Nb and Ta than biotite with DNbbiotite/ilmenite values of 0.07–0.1 and DTabiotite/ilmenite values of 0.04–0.05. In addition, partitioning of first row transition elements (FRTE) was constrained in the felsic granulites, with preference of Sc into garnet, and Co, V, Ni, and Cr into biotite. Modelling based on the presently constrained partition coefficients indicates that trace elements show consistent differentiation trends during anatexis of various felsic rocks, with enrichment of Sr, Ba, Nb, Ta and FRTE but depletion of Rb in anatectic restites. The high DNb/Ta values of biotite and ilmenite cause the residues produced by biotite dehydration melting to be commonly of higher Nb/Ta ratios than protoliths, indicating that the residues are potentially high Nb/Ta reservoirs. In contrast, the change of peritectic minerals due to various protolith compositions and melting conditions results in differential responses of Sr, Ba, and FRTE to partial melting, suggesting that these elements may be useful indicators to trace the petrogenetic process of granites. The correlation between Ba and Sc contents or between Sr/Ba and Sc/Co ratios is demonstrated to be a valid index in discriminating melts from metagreywackes and metapelites due to different modes of garnet, orthopyroxene, and K-feldspar in their anatectic residues. Moreover, as the proportions of garnet and orthopyroxene in the residues are significantly influenced by melting pressures, the Sc/Co ratio and Sc content of granites can be used to trace their anatectic depth.

Multi‐stage metamorphism of ultrahigh‐temperature Mg‐Al granulites during Gondwana assembly: Evidence from southern India

The Southern Granulite Terrane (SGT) in India hosts granulite facies rocks metamorphosed at ultra‐high temperature (UHT) conditions in the various crustal blocks, as well as within the Palghat‐Cauvery Suture Zone (PCSZ), that is considered as a trace of the Late Neoproterozoic—Cambrian Gondwana suture. Here we investigate UHT granulites from the northern margin of the Madurai Block adjacent to the PCSZ where Mg‐Al‐rich granulites are exposed. We identify sodic gedrite + kyanite in these rocks as the high‐pressure prograde stage assemblage, followed by sillimanite‐garnet‐orthopyroxene that formed during pressure decrease and temperature increase. The rare remnant gedrite is also stable at the near‐peak UHT metamorphism until it was replaced by sapphirine. The rocks subsequently underwent decompression that formed sapphirine + cordierite and sapphirine + plagioclase symplectite around sillimanite. Dehydration during decompression generated orthopyroxene‐sillimanite‐quartz assemblage with the appearance of sapphirine, defining the diagnostic mineral assemblage indicative of peak UHT metamorphism (T > 900°C) at relatively high‐pressure (P > 9 kbar). The UHT peak metamorphism in this region is consistent with the results of P–T calculations using conventional geothermometers and phase equilibrium modelling (T up to 1,050°C, P over12 kbar). Zircon and monazite geochronology on the UHT metapelites indicate distinct stages. Detrital zircon grains in the metasediments indicate protolith from ca. 2.5 Ga igneous source and the metamorphic overgrowths yield 206Pb/238U mean ages concentrated at ca. 550–520 Ma. Monazite ages define another younger group 206Pb/238U mean ages at ca. 450 Ma. The prograde high‐pressure granulite‐facies metamorphism and following UHT event correlate with the subduction‐collision tectonics at 550–500 Ma associated with the final stage of amalgamation of the Gondwana supercontinent, while the 420–460 Ma monazite age records a later hydration at the post‐orogenic stage, possibly associated with deep shearing and fluid influx.

Thermal regime of the lower crust in the eastern Khondalite Belt, North China Craton, constrained by Zr-in-rutile thermometry mapping

Ultrahigh temperature (UHT) metamorphism represents the thermally extreme metamorphic conditions in the mid- to lower crust, and its generation is significant for our understanding of crustal differentiation and crust-mantle interaction. Several Paleoproterozoic UHT metamorphic localities have been reported from the eastern Khondalite Belt, North China Craton. However, the precise spatial scale that was affected by this UHT metamorphism is still unknown, and therefore the mechanism to generate UHT metamorphism is broadly debated. Zr-in-rutile thermometry is regarded as a valid tool to estimate the near-peak metamorphic temperature of UHT granulites. Here we establish the thermal regime of the lower crust in the eastern Khondalite Belt using large-scale (∼10,000 km2) Zr-in-rutile thermometry mapping. Thirty-eight rutile-bearing garnet-sillimanite gneisses were collected, and rutile that is far from zircon and lacks ilmenite reaction rims was selected for electron probe microanalysis. Results show that the size of UHT metamorphism in the eastern Khondalite Belt is at least 3000 km2. UHT rocks closely correspond with large-scale charnockites, and small exposed mafic rocks imply that UHT metamorphism is not caused by intrusion of mafic magma. UHT metamorphism likely developed during post-collisional extension in which anomalous heat reflects both the radiogenic heating of an overthickened orogen and additionally elevated mantle flux due to subsequent lithosphere extension and asthenosphere upwelling. Nearly all large-scale UHT metamorphic terranes formed prior to ∼550 Ma, but their drivers are probably not simply the same.

Identification of UHT Granulites in the Pan-African Dahomeyide Suture Zone in SE Ghana: Implications for Evolution of Collisional Orogens

AbstractThis study presents the petrology, geochemistry, U–Pb ages, Lu–Hf and oxygen isotope compositions of Adaklu mafic granulites (ADMGs), from the Pan-African Dahomeyide suture zone in southeastern Ghana. The ADMGs show mafic precursor with low-K tholeiitic affinity. They display convex rare earth and trace elements characteristics without any obvious anomalies of Eu, Ti, Nb and Ta. The geochemical characteristics of ADMGs mimic those of N-MORB. Zircon U–Pb dating on the ADMGs reveals granulite facies metamorphic ages of ca. 595–602 Ma. However, a few zircons yield relatively older apparent 206Pb/238U ages of ca. 620 Ma, representing prograde metamorphic age. ADMGs preserve mean zircon εHf (t) values of +7.0 to +9.7 and δ18O values of 6.1–8.0‰. Based on petrographic observations, geothermobarometric calculations using conventional thermobarometry, mineral equilibria modeling, Ti-in-zircon and Zr-in-rutile thermometers reveal peak granulite facies P–T conditions of 0.95–1.2 GPa/940–1000°C, and retrograde amphibolite facies conditions of 0.83–0.93 GPa/575–710°C. The prograde metamorphic stage is inferred to be amphibolite or eclogite facies metamorphism. Hence, a clockwise a P–T–t path is proposed for the ADMGs. The overall results indicate that ADMGs are ultra-high temperature (UHT) granulites, and the protolith is altered oceanic crust consumed during the Pan-African collisional events. Asthenospheric upwelling induced by lithospheric delamination in the earliest extension of the thickened orogen, or by slab break-off in the background of collision may have provided the additional heat for UHT granulite facies metamorphism.

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.

Newly-discovered ultra-high temperature granulites from the East Hebei terrane, North China Craton

Newly-discovered ultra-high temperature granulites from the East Hebei terrane, North China Craton

Spatial distribution of ultrahigh-temperature granulites of the Highland Complex of Sri Lanka: Lowermost continental crust above an ultrahot palaeo-Moho

The spatial distribution of isolated ultra-high-temperature (UHT) granulites in Sri Lanka corresponds to the exposure level of the deepest granulites of the Highland Complex (HC). Their spatial distribution, and that of ultramafic bodies in the HC, defines a relatively thin (several km) lowermost crustal layer that has been folded into a N-plunging asymmetric fold (F3 in our scheme), Z-shaped looking N. We postulate that the UHT granulites represent suitable lithologies within this lowermost crustal layer above an ultrahot palaeo-Moho. Prior to D3, this layer had already been imbricated and intensely folded with the HT granulites and with ultramafic units after peak metamorphism, leading to a thick package of HT granulites with isolated UHT assemblages. This concept has some important implications for granulite terrains in general: (i) UHT granulites elsewhere may also represent a thin zone above an ultrahot palaeo-Moho and overlain by HT granulite terrains, and their spatial relationships may have been complicated by ductile deformation during early cooling; (ii) many other HT granulite terrains may be underlain by a similar zone of UHT granulites, and, if so, new discoveries of UHT terrains may be expected.

Paleoproterozoic emplacement and Cambrian ultrahigh-temperature metamorphism of a layered magmatic intrusion from the Central Madurai Block, southern India: From Columbia to Gondwana

The Madurai Block in the Southern Granulite Terrane (SGT) of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly. This block is composed of three sub-blocks: the Neoarchean Northern Madurai block, Paleoproterozoic Central Madurai block and the dominantly Neoproterozoic Southern Madurai Block. The margins of these blocks are well-known for the occurrence of ultrahigh-temperature (UHT) granulite facies rocks mostly represented by Mg-Al metasediments. Here we report a dismembered layered mafic–ultramafic intrusion occurring in association with Mg-Al granulites from the classic locality of Ganguvarpatti in the Central Madurai Block. The major rock types of the layered intrusion include spinel orthopyroxenite, garnet-bearing gabbro, gabbro and gabbroic anorthosite showing rhythmic stratification and cumulate texture. The orthopyroxene-cordierite granulite from the associated Mg-Al layer is composed of spinel, cordierite and orthopyroxene. The pyroxene in both rock units is high-Al orthopyroxene formed under UHT metamorphic conditions. Conventional thermobarometry yields near-peak metamorphic conditions of 9.5–10 kbar pressure and a minimum temperature of 980 °C. We computed P–T pseudosections and contoured for the compositional as well as modal isopleths of the major mineral phases, which yield temperature above 1000 °C. FMAS petrogenetic grid, Al-in-orthopyroxene isopleth, conventional thermobarometry and calculated pseudosection reveal a clockwise pressure–temperature (P–T) path and near isothermal decompression. The U–Pb data on zircon grains from the layered magmatic suite indicate emplacement of the protolith at ca. 2.0 Ga and the metamorphic overgrowths yield weighted 206Pb/238U mean ages ca. 520 Ma. Monazite from the garnet-bearing gabbro and Opx-Crd granulite yielded 206Pb/238U weighted mean ages of ca. 532 Ma and 523 Ma marking the timing of metamorphism. We correlate the layered intrusion to a Paleoproterozoic suprasubduction zone setting, defining the Ganguvarpatti area as part of a collisional suture assembling the Northern and Central Madurai Blocks. The Paleoproterozoic magmatism and late Neoproterozoic-Cambrian UHT metamorphism can be linked to the tectonics of the Columbia and Gondwana supercontinents.