Ultrahigh-temperature Metamorphism Research Articles

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.

Petrology and geochronology of sapphirine‐bearing granulites from the Limpopo Complex in eastern Botswana: Implications for Palaeoproterozoic long‐lived high‐pressure/ultrahigh‐temperature metamorphism and rapid exhumation

The Limpopo Complex in southern Africa is a classic example of Neoarchean collisional orogens with a Palaeoproterozoic thermal overprint. We report new petrological, pressure–temperature (P–T), and geochronological data of sapphirine‐bearing Mg–Al‐rich and pelitic granulites from a metasupracrustal unit (the Beit Bridge Complex [BBC]) in the eastern part of Botswana to constrain its P–T–time evolution. Textural observations and phase equilibria modelling of the peak mineral assemblage of Mg–Al‐rich granulites (K‐feldspar + orthopyroxene [Opx2; Al2O3 up to 8.8 wt.%] + sillimanite [Sil2] + quartz + rutile + biotite [Bt2] + inferred melt) in the NCKFMASHTO system recorded a peak metamorphic condition of >930°C and > 10.3 kbar. A pelitic granulite recorded consistent P–T conditions of 960–980°C and > 10.3 kbar, indicating high‐pressure (HP) and ultrahigh‐temperature (UHT) metamorphism for the BBC in Botswana. Post‐peak retrograde conditions were obtained based on the stability of sapphirine + cordierite symplectite assemblage in the Mg–Al‐rich rocks (820–860°C and 6.4–7.0 kbar) and orthopyroxene [Opx3] + cordierite symplectite assemblage in the pelitic granulite (830–930°C and 4.3–5.8 kbar), indicating near‐isothermal decompression after the peak HP–UHT metamorphism along a clockwise P–T path. Similar HP–UHT metamorphism followed by a near‐isothermal decompression along a clockwise P–T path was reported from different regions of the BBC in South Africa and Zimbabwe (P > 14 kbar and ~1,000°C based on the stability fields of magnesian staurolite and sapphirine + quartz assemblage), although the peak P–T condition is slightly lower in Botswana. Monazite grains with patchy zoning show isochron ages of 2,115 ± 77 Ma for Th‐poor domains, and 2,058 ± 40 Ma and 2,033 ± 34 Ma for Th‐rich domains. Similar ages were obtained from zircon U–Pb geochronology as 2,232–2,135 Ma and 2,010 ± 19 Ma (Th/U = 0.05–0.26). The younger ages (2.0 Ga) probably indicate the timing of near‐peak high‐grade event, while the older ages might indicate a thermal event during the prograde heating stage. The P–T conditions and zircon/monazite ages suggest Palaeoproterozoic long‐lived metamorphism associated with intracontinental transpressional tectonics related to Palaeoproterozoic orogenic events around the united Zimbabwe–Kaapvaal Craton.

Occurrence of sapphirine-bearing granulites from Kothuru, Eastern Ghats Mobile Belt: implications on ultra-high temperature metamorphism

Occurrence of sapphirine-bearing granulites from Kothuru, Eastern Ghats Mobile Belt: implications on ultra-high temperature metamorphism

Multi-thermobarometry and microstructures reveal ultra-high temperature metamorphism in the Grenvillian Oaxacan Complex, Southern Mexico

ABSTRACT The Oaxacan Complex represents the largest outcrop of the Mesoproterozoic microcontinent Oaxaquia. It experienced a major tectonometamorphic event, locally known as Zapotecan Orogeny (ca. 1004–980 Ma), during the latest stages of the amalgamation of the supercontinent Rodinia. It is typically interpreted that the syn-tectonic metamorphism occurred at granulite facies conditions (<800°C and ≈0.7 GPa). However, the mineralogical record and recent classic geothermobarometry suggest that the Oaxacan Complex reached peak ultra-high-temperature metamorphism (UHTM) but no conclusive results have been obtained. In this work, we present a combination of microstructural observations, mineral chemistry, phase equilibria thermobarometry, and trace element thermometry to document the processes associated with the granulite petrogenesis and calculate the peak metamorphic conditions. Petrographic observations show that the peak syn-tectonic metamorphism was contemporaneous to partial melting. This is revealed by the presence of euhedral and subhedral plagioclase and feldspar growing with quartz, melt relics included in garnets, films, or veinlets surrounding peak phases, among others. Exsolved clinopyroxene on orthopyroxene, exsolved rutile needles in garnet, ubiquitous mesoperthites, and F- and Ti-rich biotite grains indicate UHTM. The combination of phase equilibria modelling in two metapelites and one garnet-charnockite, along with Zr-in-Rutile and Ti-in-Quartz thermometry, confirms UHT conditions for the Zapotecan Orogeny (T = 895–916°C and P = 0.83–0.99 GPa). Preconditioning of the crust during the earlier Olmecan tectonothermal event (ca. 1100 Ma) and the ascent of the sub-lithospheric mantle follwed by the Zapotecan crustal thickening might account for the UHTM recorded in the Oaxacan rocks. The recognition of syn-tectonic UHTM in the Oaxacan Complex supports palaeographic reconstructions that place Mexican-Colombian terranes entrapped among Amazonia and Baltica during their collision as part of the amalgamation of Rodinia. Our novel results provide new avenues for petrological research for a better understanding of Oaxaquia and its role in the final configuration of Rodinia.

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.

Late Neoarchean terrane and Paleoproterozoic HT–UHT metamorphism on southern Devon Island, Canadian Arctic

The northernmost exposures of the Laurentian shield in Canada outcrop on Devon and Ellesmere islands, near the tectonic domains of the ≥2.6 Ga Rae Province and ca. 2.0–1.9 Ga Thelon–Inglefield orogenic arc. We report (Th)–U–Pb zircon and monazite ages along with phase equilibrium modelling pressure–temperature estimates for southern Devon Island, the only area on Devon or Ellesmere islands with previously known Archean rocks. Our results demonstrate that southern Devon Island is a late Neoarchean (ca. 2.55–2.47 Ga) terrane composed of lower to middle crustal, granulite facies rocks with evidence of anatexis and pervasive solid-state ductile deformation. Protoliths of clinopyroxene–garnet quartzofeldspathic gneiss, orthopyroxene quartzofeldspathic gneiss, two-pyroxene metabasite and biotite granite gneiss were emplaced between ca. 2.55–2.51 Ga, and a clastic sedimentary protolith for rutile–garnet metasedimentary rocks was deposited at ca. ≥2.47 Ga. Earlier, poorly constrained metamorphic events are recorded at ca. 2.54, 2.47 and 2.30 Ga, possibly associated with pulses of the Arrowsmith orogen (ca. 2.5–2.3 Ga). Most recently, this terrane was reworked by high temperature (HT) to ultrahigh temperature (UHT) metamorphism at ca. 1.91 Ga during with the Thelon–Inglefield orogen; obtaining peak P–T conditions of ca. 9 kbar and >850 °C. The discovery of a sapphirine–quartz metapelite assemblage is a first for the Canadian Arctic and is diagnostic evidence of UHT conditions within the Thelon-Inglefield orogenic zone.

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.

New Insight From the First Application of Ti-in-Quartz (TitaniQ) Thermometry Mapping in the Eastern Khondalite Belt, North China Craton

The thermal regime of the lower crust is a critical factor that controls crustal anatexis, high-grade metamorphism, and granite formation, which finally results in crustal differentiation. However, the large-scale thermal regime in the Precambrian continental crust is generally not well established. In this study, we first applied Ti-in-quartz (TitaniQ) thermometer to map the temperature variation in the lower crust within an area of ∼10,000 km2 in the Paleoproterozoic eastern Khondalite Belt, North China Craton. The studied rocks are aluminous gneisses/granulites, which contain abundant quartz that generally coexists with rutile. The results show that matrix-type quartz with substantial rutile exsolution generally contains the maximum Ti concentration, which is &amp;lt;300 ppm higher than that of inclusion-type quartz. This result suggests that two quartz types probably formed at the prograde and near-peak to early cooling metamorphic stages, respectively. Therefore, the temperature mapping result based on the maximum Ti concentrations of the matrix-type quartz can better represent the thermal regime than inclusion-type quartz. Our regime shows that the hottest Paleoproterozoic lower crust is underneath the Liangcheng-Heling’er-Zhuozi area, where ultrahigh-temperature (UHT) metamorphism is closely associated with abundant charnockite. The hottest region may represent the root of an ancient large hot orogeny. Our study provides a new insight into the formation of UHT metamorphism.

U-Pb age of a late Cenozoic ultra-high temperature metamorphic event under Central Mexico, as inferred from granulite xenoliths from Cerro El Toro, Mexico

ABSTRACT Cerro El Toro, a Pliocene scoria cone that carried abundant lower crust xenoliths to the surface, is located in the Mesa Central (MC), Mexico, where mafic alkalic intra-plate volcanism has occurred since the early Miocene (end of the Sierra Madre Occidental volcanism). Early studies of El Toro xenoliths showed that the feldspathic granulite equilibrated at P = 0.9–1.4 GPa and T = 900−1100°C under anhydrous conditions (ternary feldspars calculations). A young (Oligocene-Quaternary?) pulse of ultra-high temperature (UHT) metamorphism was thus proposed for the region, without a strict age constrain. Zircon crystals recovered from a set of nine xenoliths (7 Grt-Sil bearing metapelites, a Px-bearing meta-quartz diorite, and a Grt-Opx bearing orthogneiss) were selected for LA-(MC)-ICPMS U-Pb geochronology. Palaeozoic to Neoproterozoic zircon crystals are scant, whereas Mesozoic to Cenozoic ages are more abundant. Based on their chondrite-normalized REEs all Neoproterozoic to Mesozoic zircons are interpreted as igneous, whereas those of late Oligocene to late Miocene age (ca. 27–6 Ma) are mostly metamorphic and grew during a protracted pulse of UHT metamorphism in the lower crust. The presence of Cenozoic metasediments in the lower crust under Cerro El Toro is indicative of the action of the subducted Farallon plate, coupled with tectonic erosion of continentally derived sediments, either from a forearc basin and/or an accretionary prism that were relaminated to the lower crust by sediment diapirism. Similarities among the xenolith zircon ages with those from modern sediments belonging to Central Mexico Pacific coast point towards a NW-SE 100 km long coast stretch across Zihuatanejo as a possible sediment source. The paucity of Grenville-age detrital zircon grains in the recovered xenoliths suggests that the El Toro area is not underlain by a Proterozoic basement, thus implying a substantial reduction of Oaxaquia extension beneath Central Mexico and presence of the younger Guerrero terrane.

Ultrahigh temperature metamorphism and isobaric cooling of Neoarchean ultramafic‐mafic granulites in the southern granulite terrain, India: Phase equilibrium modelling and SHRIMP zircon U–Pb dating

AbstractThis study presents the results of petrology, SHRIMP zircon U–Pb analysis, conventional Fe–Mg geothermobarometry, REE (rare earth element)‐based geothermobarometry, and phase equilibrium modelling, for newly discovered ultramafic‐mafic granulites (garnet pyroxenite with minor plagioclase and garnet‐bearing granulite) in the Namakkal Block of the southern granulite terrain (SGT), India. Results from this study shed light on the growth, evolution, and thermal state of lower continental crust during the Neoarchean, as a cross‐section of the Archean lower continental crust is well‐exposed in the SGT. SHRIMP zircon U–Pb dating of a garnet pyroxenite sample (18ID‐41) yielded a weighted mean 207Pb/206Pb age of 2531 ± 6 Ma, which is the same within uncertainty as one weighted mean SHRIMP metamorphic zircon 207Pb/206Pb age of 2519 ± 9 Ma from mafic granulite sample (18ID‐43). In addition, zircon U–Pb analyses also yielded an apparent 207Pb/206Pb age of 2474 ± 3 Ma (1σ) for the garnet pyroxenite, and a weighted mean 207Pb/206Pb age of 2489 ± 8 Ma for the mafic granulite. Thus, the timing of metamorphism of the ultramafic‐mafic granulites was constrained to be ca. 2530–2470 Ma. The peak metamorphic phase assemblage of the garnet pyroxenite is grt–cpx–pl–ilm–liq, which was modelled to be stable at 1058–1172°C/7.8–11.6 kbar, whereas the retrograde mineral assemblage of grt–cpx–hbl–pl–qz–ilm–liq occurred at 877–888°C/9.9–11 kbar. The mafic granulite was demonstrated to record similar peak and retrograde metamorphic P–T conditions to those of the garnet pyroxenite but have a relatively small P–T range of the peak mineral assemblage. The ultrahigh temperature metamorphism of these ultramafic‐mafic granulites was further confirmed by the temperature results (1057–1087°C) from the REE‐in‐two‐pyroxene thermometer. As a result, a retrograde metamorphic P–T path characterized by near‐isobaric cooling from ~1080°C at ~10 kbar to ~900°C at ~10 kbar was constructed for these rocks, which is consistent with an extremely high thermal state that persisted for ca. 60 Ma in the lower crust.

Ultrahigh–temperature metamorphism and melt inclusions from the Sør Rondane Mountains, East Antarctica

This paper reports the first outcrop occurrence of an ultrahigh–temperature (UHT) metamorphic rock from the Sør Rondane Mountains (SRM), East Antarctica. A pelitic gneiss from Balchenfjella, eastern SRM, contains mesoperthite that gave UHT condition (>900 °C) by ternary feldspar thermometry. The UHT mesoperthite is present both in the matrix and as an inclusion in garnet. The garnet also has nanogranitoid inclusions next to the mesoperthite, which are interpreted to be an UHT melt. The re–integrated nanogranitoid composition is plotted in the primary phase region of quartz and classified as granite. Even crystallized nanogranitoids can provide appropriate original melt composition in the An–Ab–Or and Qz–Ab–Or spaces, whereas Mg concentration is enriched due to local retrograde Fe–Mg exchange reaction between the nanogranitoid inclusions and the host garnet. Although metamorphic rocks in the SRM are highly retrogressed, this study revealed that the microstructural evidence of UHT condition is partially preserved. Further investigation of timing and areal extent of UHT metamorphism helps us to understand the tectonic model of the SRM.

The Role of Crustal Radiogenic Heating in Ultra-High Temperature Metamorphism of Greater Himalayan Crystalline Complex: Insights from Numerical Modeling

The Role of Crustal Radiogenic Heating in Ultra-High Temperature Metamorphism of Greater Himalayan Crystalline Complex: Insights from Numerical Modeling

Ultrahigh-temperature granulite-facies metamorphism and exhumation of deep crust in a migmatite dome during late- to post-orogenic collapse and extension in the central Adirondack Highlands (New York, USA)

AbstractThis study combines field observations, mineral and whole-rock geochemistry, phase equilibrium modeling, and U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology to investigate sillimanite-bearing felsic migmatites exposed on Ledge Mountain in the central Adirondack Highlands (New York, USA), part of an extensive belt of mid-crustal rocks comprising the hinterland of the Mesoproterozoic Grenville orogen. Phase equilibrium modeling suggests minimum peak metamorphic conditions of 960–1025 °C and 11–12.5 kbar during the Ottawan orogeny—significantly higher pressure-temperature conditions than previously determined—followed by a period of near-isothermal decompression, then isobaric cooling. Petrography reveals abundant melt-related microstructures, and pseudosection models show the presence of at least ~15%–30% melt during buoyancy-driven exhumation and decompression. New zircon data document late Ottawan (re)crystallization at ca. 1047 ± 5 to 1035 ± 2 Ma following ultrahigh-temperature (UHT) metamorphism and anatexis on the retrograde cooling path. Inherited zircon cores give a mean date of 1136 ± 5 Ma, which suggests derivation of these felsic granulites by partial melting of older igneous rocks. The ferroan, anhydrous character of the granulites is similar to that of the ca. 1050 Ma Lyon Mountain Granite and consistent with origin in a late- to post-Ottawan extensional environment. We present a model for development of a late Ottawan migmatitic gneiss dome in the central Adirondacks that exhumed deep crustal rocks including the Snowy Mountain and Oregon anorthosite massifs with UHT Ledge Mountain migmatites. Recognition of deep crustal meta-plutonic rocks recording UHT metamorphism in a migmatite gneiss dome has significant implications for crustal behavior in this formerly thickened orogen.

Paleoproterozoic tectonic evolution from subduction to collision of the Khondalite Belt in North China: Evidence from multiple magmatism in the Qianlishan Complex

This study presents detailed petrology, major and trace element chemistry, Sr-Nd isotope, and in-situ zircon U-Pb dating and Hf-O isotopes on granites from the Qianlishan Complex of the Khondalite Belt in North China. Zircon SIMS U-Pb data indicate three episodes of Paleoproterozoic magmatism at ca. 2.06 Ga, 1.95 Ga and 1.92 Ga. Of those, the ca. 2.06 Ga hornblende monzogranites and fine-grained potassic granites display typical features of A-type granite, represented by metaluminous affinity, high FeOT/MgO ratios (3.29–5.63), and high Ga/Al and Zr + Nb + Ce + Y values with high zircon saturation temperatures (916–1000 °C). Moreover, they have positive zircon εHf values of +0.29 to +5.01 and homogenous δ18O values of 5.11–5.87 ‰, probably derived from the partial melting of calc-alkaline granitoids without addition of supracrustal rocks. The ca. 1.95 Ga garnet-bearing granites contain peraluminous minerals of garnet and muscovite and have high ASI index (>1.1) and δ18O values (7.14–9.18 ‰), indicating S-type granite. Their negative whole-rock εNd(t) values (−2.87 to −2.81) and zircon εHf(t) values (−4.06 to +1.29) suggest a source of ancient supracrustal rocks. The ca. 1.92 Ga coarse-grained potassic granites present metaluminous and magnesian signatures of I-type granite, including low A/CNK and FeOT/MgO ratios. Their negative εHf(t) values (−5.00 to −1.64) and homogenous δ18O values (5.20–6.00%) suggest that the original magma was derived primarily from enriched lower crust. The results, combined with regional geology and previous studies, establish a prolonged magmatic-metamorphic evolution of the Khondalite Belt from subduction to collision, involving ca. 2.3–2.0 Ga subduction-related arc magmatism, ca. 1.95 Ga syn-collisional high-pressure high-temperature crustal anatexis, and ca. 1.92 Ga post-collisional magmatism and synchronous ultrahigh temperature metamorphism.

Neoarchean ultrahigh-temperature sapphirine granulites from the Karimnagar Granulite Terrane, Southern India: Implications for hot orogen along the Eastern Dharwar Craton margin

We have uncovered evidence of Neoarchean ultrahigh-temperature (UHT) metamorphism in the Karimnagar Granulite Terrane (KGT), southern India, based on new petrological and mineralogical data of sapphirine-bearing and related granulites. We investigated their petrogenesis and implications of a hot orogen along the north-eastern margin of the Eastern Dharwar Craton (EDC). We found various reaction textures, such as spinel mantled by sapphirine and hydration of orthopyroxene, indicating a peak mineral assemblage of cordierite + sapphirine + spinel + orthopyroxene + biotite + rutile + K-feldspar. Phase-equilibria modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 system indicates peak metamorphic conditions of 905 °C–910 °C and 4.5–4.6 kbar. The peak stage was followed by a retrograde metamorphic stage represented by increasing modal abundance of biotite and consumption of rutile and spinel at ~730 °C and ~ 4 kbar, suggesting near-isobaric cooling. Using the chemical Th–U-Pbtotal isochron method, we dated monazites in textural association with sapphirines and cordierites in a sapphirine–spinel–orthopyroxene–cordierite granulite. The resulting Neoarchean age (2638 ± 44 Ma) probably represents the timing of peak UHT metamorphism. We infer that Neoarchean UHT metamorphism and post-peak near-isobaric cooling took place in the terrane during the terminal stage of complex accretion–collision tectonic processes related to amalgamation of microcontinents and/or magmatic arcs that occurred during 2.70–2.62 Ga, one of the major episodic crust-forming events in the EDC. Our findings and regional distribution of lithologies overturn the existing tectonic model of the KGT as a collisional suture zone between the EDC and the Bastar Craton. Moreover, we propose that the KGT is a discrete tectonic block composed of ~2.7–2.6-Ga granulites and younger granite representing the mid-crustal segment of the EDC, which was uplifted as a tilted tectonic block along the southern flank of the Kadam outlier.

Ultrahigh‐temperature Palaeoproterozoic rocks in the Neoproterozoic Borborema Province, implications for São Francisco Craton dispersion in NE Brazil

AbstractThis work reports on the first identification of sapphirine‐bearing ultrahigh‐temperature (UHT) metamorphic rocks within the Borborema Province, in NE Brazil. The investigated UHT diatexites outcrop in the Arapiraca Complex, a high‐grade Palaeoproterozoic block embedded within the Neoproterozoic metavolcano‐sedimentary sequence of the Sergipano fold‐and‐thrust belt. A sapphirine‐bearing diatexite sample preserves two distinct residual mineral associations as inclusions in cordierite poikiloblasts: a volumetrically dominant silica‐saturated, garnet–sillimanite–orthopyroxene–quartz domain, and a restricted silica‐undersaturated, sapphirine–magnetite–spinel–corundum domain. Phase equilibrium modelling for the whole‐rock bulk composition constrains the reactions preserved in the silica‐saturated domain, which, coupled with reintegrated ternary feldspar thermometry and Al‐in‐orthopyroxene thermobarometry, indicates peak UHT metamorphism occurring at ~970°C/8.5 kbar followed by a clockwise near isobaric cooling stage crossing the H2O‐undersaturated solidus at ~910°C/7.7 kbar. The T–MSiO2 and P–T phase equilibrium models reveal that the stabilization of magnetite‐spinel in the silica‐undersaturated domain can be a product of domanial SiO2‐depletion ascribed to inherited compositional banding and progressive melt‐loss episodes. The appearance of sapphirine as thin coronae around previously formed magnetite‐spinel cores requires an increase in the bulk silica content during retrograde metamorphism. This was driven by variable degrees of chemical interaction between the silica‐undersaturated domain and trapped silica‐rich leucosomes. U–Pb dating of zircon coupled with trace element behaviour records the timing of melt crystallization between ca. 2.03 and 1.96 Ga, after peak UHT metamorphism and supporting a protracted, ca. 70 My period of melt presence in the crust. The UHT metamorphism in the Arapiraca Complex is related to a contemporaneous UHT event recorded within the São Francisco Craton and more broadly coincides with high geothermal gradients at ca. 2.0 Ga linked to the formation of the Columbia supercontinent. The integration of new petrological and geochronological data indicates that the Arapiraca Complex is a continental ribbon pulled away from the São Francisco–Congo palaeocontinent possibly during the early Neoproterozoic intermittent rifting events and opening of the Sergipano oceanic basin between ca. 0.98–0.74 Ga. During the late Neoproterozoic at ca. 0.63–0.57 Ga, the Arapiraca Complex continental ribbon was thrusted back onto the northern margin of the São Francisco Craton as an inlier of the Sergipano fold‐and‐thrust belt.

An extensive record of orogenesis recorded in a Madagascar granulite

AbstractWe present a comprehensive petrological and geochronological study of a single granulite sample from the lithosphere‐scale Beraketa shear zone in southern Madagascar to constrain the orogenic history of Gondwana assembly in this region. The studied sample provides a panoply of data constraining the prograde, retrograde, and late metasomatic history of the region via the application of Ti‐in‐quartz, Ti‐in‐zircon, Zr‐in‐rutile, and Al‐in‐orthopyroxene thermobarometry; phase‐equilibrium modelling; U–Pb monazite, zircon, and rutile petrochronology; and trace element diffusion chronometry in rutile. Our results reveal five stages of metamorphism along a narrow clockwise P–T path that may have begun as early as 620–600 Ma and certainly by 580–560 Ma, based on the oldest concordant zircon dates. The rock was heated to &gt;725°C at less than 7.5 kbar (Stage 1) before burial to ~8 kbar (Stage 2). By c. 540 Ma, the rock had heated to ~970°C at ~9 kbar, and lost approximately 12% melt (Stage 3), before decompressing and cooling to the solidus at ~860°C and 6.5 kbar within 10 Ma (Stage 4). The vast majority of monazite and zircon dates record Stage 4 cooling and exhumation. Monazite and zircon rim dates as young as c. 510 Ma record subsolidus cooling (Stage 5) and associated symplectite formation around garnet. U–Pb rutile dates record partial resetting at c. 460 Ma; Zr‐ and Nb‐in‐rutile diffusion chronometry link these dates to a metasomatic event that lasted &lt;1 Ma at ~600°C. In addition to chronicling a near‐complete cycle of metamorphism in southern Madagascar, this study constrains the rates of heating and cooling. We estimate that heating (7–14°C/Ma) outpaced reasonable radiogenic heating rates with modest mantle heat conduction. Therefore, we conclude that elevated mantle heat conduction or injection of mantle‐derived magmas likely contributed to regional ultrahigh‐temperature metamorphism (UHTM). Exhumation and cooling from peak metamorphic conditions to the solidus occurred at rates greater than 0.45 km/Ma and 14°C/Ma.

Petrology and crustal evolution of the Tartarugal Grande Granulitic Complex - Northeastern Amazonian Craton

The Tartarugal Grande Complex (TGC) is an association of high-grade metamorphic rocks with Archean and Paleoproterozoic protoliths, located at the border of the Archean Amapá and Rhyacian Lourenço domains, in southeastern Guiana Shield, northeast of the Amazonian Craton. It consists of felsic granulites, and aluminous leucogneisses and rare mafic granulites. Both granulites and gneisses are affected by thrust and transcurrent shear zones along NW-SE trend. Migmatization with quartz-feldspathic neosomes, with or without orthopyroxene, and garnet-rich neosomes are present in felsic granulites and leucogneisses, respectively. The gneisses have high silica contents and are peraluminous due to the presence of biotite, garnet, and cordierite, indicating the derivation from metasedimentary sequences. The felsic granulites have medium to high silica content and geochemical signature of calc-alkaline magmatic arc series. The mafic granulites are small bodies embedded in the felsic granulites and leucogneisses. They display geochemical affinities with tholeiitic series and represent ancient diabase dykes. The metamorphic paragenesis in the granulite facies is represented by Pl-antiperhtite + Qtz + Mc-mesoperthite + Opx + Cpx + Bt ± Hbl (enderbitic granulite), Pl-antiperhtite + Qtz + Mc-mesoperthite + Opx ± Bt (charnoenderbitic granulite); Mc-mesoperthite + Qtz + Pl + Opx + Bt (charnockitic granulite), Pl (An60)+Opx + Cpx ± Hbl ± Grt (mafic granulite), and Qtz + Mc + Pl ± Bt ± Grt ± Crd (leucogneiss). The presence of orthopyroxene, clinopyroxene and hornblende in equilibrium in the granulites allows estimating temperatures at 800° ± 20 °C. In addition, non-extensive anatexis under anhydrous conditions produced charnockitic neosomes in the granulites and garnet-bearing leucogranite neossomes in the paragneisses. The presence of cordierite in leucogneisses indicates low-medium pressure conditions around 6–7 kbar. The partial or total replacement of pyroxenes by hornblende and/or biotite, garnet by biotite, clinopyroxene by titanite, and myrmekite intergrowths, indicate retrometamorphism and cooling during exhumation to higher crustal levels. The U–Pb LA-ICP-MS dating on zircon from a charnoenderbitic granulite yielded an age of 2082 ± 5 Ma for the crystallization of the igneous protolith of the granulites. Metamorphic zircons from an enderbitic granulite yielded a U–Pb LA-ICP-MS age of 2045 ± 14 Ma, which corresponds to the high-grade metamorphic episode that has affected the region at the end of the Rhyacian. The age of the protoliths is not constrained but 207Pb/206Pb dates at 2.57–2.70 Ga indicated inherited crystals and/or xenocrystals from Archean source. In agreement with previous works, we conclude that the TGC consists of calcium-alkaline magmatic rocks and pelitic sedimentary sequences formed during the development of a Rhyacian magmatic arc at the edge of the Archean Amapá Block. This was followed by a thermo-tectonic event at the end of Rhyacian, around 2.05 Ga, under high-grade conditions reaching the granulite facies, with restricted anatexis. Records of the tectonic exhumation of these terranes were defined by partial disequilibrium of some mineral phases in amphibolite facies. This Late-Rhyacian thermo-tectonic event is contemporaneous to the ultra-high temperature metamorphism (2.09–2.03 Ga) of the Bakhuis Granulite Belt in Suriname. The thermal differences between these two high-grade metamorphic terrenes may be related to the different crustal levels where they settled. Late-Rhyacian asthenospheric upwelling by crustal stretching during oblique continental collision seems to be the more plausible scenario as previously proposed, even if a slab break-off scenario cannot be ruled out.

Validation of clinopyroxene-garnet magnesium isotope geothermometer to constrain the peak metamorphic temperature in ultrahigh-temperature ultramafic-mafic granulites

Abstract Conventional Fe-Mg exchange geothermometers generally are not effective means to measure the peak metamorphic temperatures of granulites because of Fe-Mg diffusion during the re-equilibration during the exhumation and cooling. Therefore, it is essential to find alternative geothermometers. In this study, we investigated the magnitude of Mg isotope fractionation between the coexisting clinopyroxene and garnet in garnet pyroxenites and high-pressure mafic granulites from southern India. The clinopyroxene and garnet from the garnet pyroxenites have δ26Mg values of −0.04 to −0.07‰, and −0.65 to −0.64‰, respectively, with Δ26MgCpx-Grt = δ26MgCpx − δ26MgGrt = 0.62 and 0.57‰. The δ26Mg values of the coexisting clinopyroxene and garnet in the high-pressure mafic granulites are 0.03 to 0.07‰ and −0.54 to −0.55‰, respectively, with Δ26MgCpx-Grt = 0.57 and 0.62‰. The inter-mineral Mg isotope fractionations between the clinopyroxene and garnet of the garnet pyroxenites are similar to those of the high-pressure mafic granulites and more or less fall within the equilibrium fractionation lines at peak metamorphic temperatures. The measured peak temperatures for all four samples indicate equilibrium Mg isotope fractionations between the clinopyroxene and garnet. The clinopyroxene-garnet Mg isotope geothermometer of Li et al. (2016) yields temperatures of 994 ± 60 °C and 1048 ± 89 °C for the garnet pyroxenites, and 1048 ± 89 °C and 994 ± 65 °C for the high-pressure mafic granulites, which are slightly lower than those at both 10 and 12 kbar from the clinopyroxene-garnet Mg isotope thermometer of Huang et al. (2013). Compared with the peak metamorphic conditions from phase equilibrium modeling, the clinopyroxene-garnet Mg isotope thermometry yielded temperatures corresponding to the peak metamorphic conditions of the garnet pyroxenites and the high-pressure mafic granulites, whereas the conventional clinopyroxene-garnet Fe-Mg exchange thermometry yielded lower temperatures corresponding to the retrograde metamorphism. These results underscore the inter-mineral Fe-Mg exchange between clinopyroxene and garnet during the retrograde cooling of the ultrahigh temperature (&amp;gt;900 °C), which, however, did not disturb their Mg isotope fractionation equilibrium attained during the ultrahigh-temperature metamorphic condition. Therefore, we conclude that clinopyroxene-garnet Mg isotope thermometry, together with phase equilibrium modeling, is a valid tool to constrain the peak metamorphic temperature conditions even for ultramafic-mafic granulites that have undergone ultrahigh-temperature metamorphism.

Cenozoic ultrahigh-temperature metamorphism in pelitic granulites from the Mogok metamorphic belt, Myanmar

Cenozoic ultrahigh-temperature metamorphism in pelitic granulites from the Mogok metamorphic belt, Myanmar