Ultrahigh-temperature Granulite-facies Research Articles

Fluid–Melt–Rock Interaction during the Transition from Magmatism to HT-UHT-Granulite- and Amphibolite-Facies Metamorphism in the Ediacaran Adrar–Suttuf Metamafic Complex, NW Margin of the West African Craton (Southern Morocco)

Abstract Underplated mafic intrusions ponded at the base of the lower continental crust in extensional settings can experience ultra-high-temperature (UHT) granulite-facies metamorphism during tens of My due to slow cooling rates. These intrusions are also the source of heat and carbonic fluids for regional high-temperature (HT) granulite-facies metamorphism in the continental crust. This work analyses the fluid–melt–rock interaction processes that occurred during the magmatic to HT-UHT-granulite- and amphibolite-facies metamorphic evolution of high-grade mafic rocks from the Eastern Ediacaran Adrar–Suttuf Metamafic Complex (EASMC) of the Oulad Dlim Massif (West African Craton Margin, Southern Morocco). P–T conditions were determined using Ti-in-amphibole thermometry, two-pyroxene and amphibole–plagioclase thermobarometry, and phase diagram calculations. The thermobarometric study reveals the presence of tectonically juxtaposed lower- and mid-crustal blocks in EASMC that experienced decompression-cooling paths from, respectively, UHT and HT granulite-facies conditions at ca. 1.2 ± 0.28 GPa and 975 ± 50°C, and ca. 0.82 ± 0.15 GPa and 894 ± 50°C, to amphibole-facies conditions at ca. 0.28 ± 0.28 GPa and 787 ± 45°C (precision reported for the calibrations at 1 s level). An age for the magmatic to UHT granulite-facies metamorphic transition of 604 Ma was constrained from published SHRIMP Th–U–Pb zircon ages of the igneous protoliths. An amphibole 40Ar–39Ar cooling age of 499 ± 8 Ma (precision at 2 s level) was obtained for the lower-crustal blocks. Amphibole 40Ar–39Ar closure temperatures of 520–555°C were obtained for an age range of 604–499 Ma and an average constant cooling rate of 4.2°C/My, suggesting that the lower-crustal blocks cooled down to the greenschist–amphibolite facies transition in ca. 100 My. During the high-temperature stage, interstitial hydrous melts assisted textural maturation of the rock matrix and caused incongruent dissolution melting of olivine and pyroxenes, and, probably, development of An-rich spikes at the grain rims of plagioclase, and local segregation of pargasite into veins. Subsequent infiltration of reactive hydrous metamorphic fluids along mineral grain boundaries during cooling down to amphibolite-facies conditions promoted mineral replacements by coupled dissolution-precipitation mechanisms and metasomatism. Ubiquitous dolomite grains, with, in some cases, evidence for significant textural maturation, appear in the granoblastic aggregates of the high-grade mafic rocks. However, calculated phase relationships reveal that dolomite could not coexist with H2O–CO2 fluids at HT-UHT granulite- and low-medium P amphibolite-facies conditions. Therefore, it is proposed that it may have been generated from another CO2-bearing phase, such as an immiscible carbonatitic melt exsolved from the parental mafic magma, and preserved during cooling due to the prevalence of fluid-absent conditions in the granoblastic matrix containing dolomite. The lower-crustal mafic intrusions from EASMC can represent an example of a source of heat for granulitisation of the mid crust, but a sink for carbon due to the apparent stability of dolomite under fluid-absent conditions.

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.

Titanium in calcium amphibole: Behavior and thermometry

Abstract Thermometry of high-grade metamorphic rocks is difficult due to fast cationic diffusion during slow cooling, but the titanium content of calcium amphibole (Ti-Amp) can retrieve amphibole-forming temperature as high as ~1000 °C. Based on pseudosection modeling and past research survey, we find that Ti-Amp is controlled by temperature under the conditions of oxygen fugacity <2 log units above the Ni-NiO oxygen buffer (ΔNNO+2), in subalkaline systems, and with the presence of Ti-phases (rutile, ilmenite, or titanite). We apply available experimental data to calibrate a new Ti-Amp thermometer for such conditions: T ( C ° ) = 2400 1.52 − log ⁡ Ti Amp − 273 where T is temperature and log TiAmp is the Ti content of amphibole in atom per formula unit (apfu) expressed in the logarithm to base 10. The standard error of the calibration experiments is ±35 °C. This thermometer can be applied only if the aforementioned conditions are fulfilled. Besides, caution should be taken when applying the thermometer to rocks under subsolidus water-unsaturated conditions, which can be observed as non-equilibrium textures or low bulk-rock water content (<1–1.5 wt%). The results of this thermometer may be underestimated if applied to rocks that are equilibrated above 850 °C and contain rutile and a significant amount of water (>3.5–4 wt%). The new thermometer can be successfully applied to amphibole-bearing natural igneous and meta-morphic rocks. In addition, we propose the Si vs. Ti in amphibole diagram that roughly demarcates the boundaries among the high amphibolite, high-temperature granulite, and ultrahigh-temperature granulite facies. Although pseudosection modeling provides details of the P-T evolution of studied rocks, the Ti-Amp thermometer is quick and easy to apply. However, further research is needed to improve our knowledge of the behavior and stability of amphibole and to improve the accuracy of both pseudosection modeling and conventional thermometry.

A unique Paleoproterozoic HP–UHT metamorphic event recorded by the Bengbu mafic granulites in the southwestern Jiao–Liao–Ji Belt, North China Craton

The Wuhe Complex in the Bengbu area of the Jiao–Liao–Ji Belt, southeast North China Craton, contains garnet-bearing mafic granulites that have undergone high-pressure (HP) and ultrahigh-temperature (UHT) metamorphism. These granulites also experienced partial melting and occur as lenses within marbles. Petrographic observations and quantitative phase equilibria modeling reveal clockwise P–T paths, involving an inferred HP stage followed by decompressional, medium-pressure, granulite-facies metamorphism and subsequent cooling. The HP assemblage of garnet + clinopyroxene + plagioclase + K-feldspar ± amphibole ± quartz ± rutile indicates P–T conditions of 840–980 °C and 12–17 kbar. This was followed by post-peak, near-isothermal decompression with the development of orthopyroxene + clinopyroxene + plagioclase + K-feldspar + garnet + amphibole + ilmenite at 850–960 °C and 7–10 kbar, resulting in the development of orthopyroxene rims on resorbed garnet. Pyroxene and ternary feldspar thermometry yielded high temperatures of ~1150 °C and 1055–1087 °C at 10 kbar, respectively, which constrain the minimum crystallization temperatures of the igneous protoliths. The host and lamellae of the pyroxene and ternary feldspar are relict magmatic minerals/textures that survived metamorphism due to the silica-undersaturated bulk-rock conditions. Zr-in-rutile thermometry yielded temperatures of ~935 °C and 800 °C, with the former being consistent with the predicted peak metamorphic temperatures. Small amounts of melts (up to 5%) were generated during decompression of the Bengbu mafic granulites. The generated partial melts were mainly (quartz) monzonite at 900–920 °C, and the silica contents of the melts were controlled by the quartz stability field in P–T pseudosections. The partial melts were enriched in Na and strongly depleted in Fe–Mg at the peak pressure of ~14 kbar and 920 °C, and later evolved to Fe–Mg-rich and high-K compositions during decompression. The melt compositions in the studied rocks are similar when the pressures reached ~9 kbar. The modal proportion of amphibole increased as the melt H2O content decreased at lower pressures, indicating that the limited H2O remaining in the host rocks was consumed to produce amphibole. U–Pb geochronology of zircon containing inclusions of clinopyroxene, plagioclase, and apatite constrains the timing of metamorphism to 1930–1840 Ma, as is the case for HP granulites from Shandong, Liaoning, and southern Jilin in the central and northeastern Jiao–Liao–Ji Belt. The Wuhe HP–UHT mafic granulites were ultimately sourced from upwelling asthenosphere-derived magma at ~2.1 Ga, which intruded and crystallized at shallower depths. The igneous protoliths were then buried to middle–lower crustal levels and experienced HP–UHT granulite-facies metamorphism and partial melting at 1.95–1.90 Ga related to continental subduction and overthickening. The HP–UHT mafic granulites were rapidly exhumed at ~1.85 Ga and generated small volumes of (quartz) monzonite during decompression. The newly discovered Paleoproterozoic HP–UHT mafic granulites associated with partial melting suggest that the continent materials were deeply subducted to the lower crustal levels and that additional heating was not involved. The finding of the HP–UHT granulites, together with the widespread distributions of the granulite-facies metamorphic rocks and the determination of the clockwise P–T–t paths, reveal that the Paleoproterozoic Jiao–Liao–Ji orogenic belt extends at least 1000 km, starting from southern Jilin, passing through the southeastern Liaoning and Jiaobei terranes, and elongating to the Bengbu area in Anhui.

Paleoproterozoic ultrahigh-temperature granulite-facies metamorphism in the Sulu orogen, eastern China: Evidence from zircon and monazite in the pelitic granulite

Because of their essential information for constraining the timescales of high-temperature processes, zircon and monazite are the two most important minerals for understanding crustal processes. Zircon and monazite are common accessory minerals in pelitic granulites (Sulu orogen, eastern China). The pelitic granulites have a peak mineral assemblage of garnet + sillimanite + antiperthite + plagioclase + quartz + rutile ± biotite. Zircons from the granulite have low Th/U ratios (<0.1 mostly), flat HREE patterns with negative Eu anomalies, suggesting a typical of granulite-facies metamorphic zircon. Ti-in-zircon thermometers give temperature estimates ranging from 911 to 954 °C with a weighted mean of 928 ± 10 °C, suggesting a ultrahigh temperature (UHT) metamorphism. These metamorphic zircons yield a concordant 206Pb/238U age of 1843 ± 17 Ma (n = 26) and an upper intercept age of 1842 ± 15 Ma (n = 28), suggesting a Paleoproterozoic UHT metamorphism. The monazites in the pelitic granulite record the same Paleoproterozoic age (ca. 1840 Ma) as the zircons. The monazites generally exhibit a zoned structure in BSE images: gray core and bright rim. Both the cores and rims display significant depletion in HREEs and Y, and negative Eu anomalies, indicating their formation under granulite-facies metamorphic conditions. Notably, the bright rims are more depleted in HREEs and Y, and more negative Eu anomalies than the gray cores. Compared with the gray cores (Th = 45,429–165,236 ppm; U = 2227–6044 ppm; and Th/U = 18–28), the bright rims have high Th (194,616–285,224 ppm) but low U (1145–3209 ppm) contents, resulting in very high Th/U ratios (69–217). These results establish zircon-monazite-garnet- feldspar REE equilibrium and their U–Th–Pb system under UHT metamorphic processes. Paleoproterozoic UHT granulite-facies metamorphism in the Sulu orogen provides a crutial clue to understand its early geodynamic setting and tectonic evolution.

High-temperature S-type granitoids (charnockites) in the Jining complex, North China Craton: Restite entrainment and hybridization with mafic magma

The Liangcheng garnet granitoids are parautochthonous S-type granites that occur within granulite-facies metasediments in the ultrahigh-temperature (UHT) Jining Complex, eastern part of the Khondalite Belt, North China Craton. These garnet granitoids contain mafic microgranular enclaves and are intimately associated with numerous small intrusions of the Xuwujia gabbronorites, showing magma mingling and mixing textures. Typical mineralogy in the garnet granitoids is plagioclase + K-feldspar + quartz + garnet + biotite ± orthopyroxene. UPb ion microprobe dating of zircon suggests that garnet granitoids and associated gabbronorites were coeval with UHT metamorphism at ~1.93–1.92 Ga, and also recorded ~1.92 Ga metamorphism during slow cooling. Garnet granitoids are characterized by the presence of orthopyroxene, high modal proportion of garnet (up to 25%) and strongly peraluminous (ASI = 1.11–1.55) along with lower SiO2 (57.8–70.1 wt%) and higher FeOt + MgO (5.6–14.4 wt%) contents than typical S-type granites. Mineral chemistry (high Ti biotite and high Al orthopyroxene), high Al-in-orthopyroxene temperature (910–989 °C) and Zr saturation temperature (825–901 °C), and low whole-rock Al2O3/TiO2 ratios (11–28), indicate that garnet granitoids were derived from biotite dehydration melting of metasedimentary rocks under UHT granulite-facies conditions. Textural and compositional evidence suggests that a significant proportion of garnets in the Liangcheng garnet granitoids are potentially a peritectic phase entrained from the source. Garnet aggregates may also represent entrained garnet-rich restites. The large HREE variations of garnet granitoids also support the entrainment of garnet-rich restites. Garnet granitoids display nearly linear trends with the mafic enclaves and gabbronorite for most major elements that demonstrates the involvement of mafic magma in the granitoid petrogenesis. The εHf(t) values (−1.0 − +5.6) and δ18O values (9.2–11.6‰) of zircons in garnet granitoids partly overlap with those in gabbronorites (εHf(t) = −2.9 − +3.5, δ18O = 7.7–10.9‰), suggesting the strong interaction and mixing process between them. Major element modelling suggests that the addition of 20–40% restites (garnetites) and gabbronorites to the anatectic melts was responsible for the large chemical variations. Therefore, the Liangcheng garnet granitoids represent high-temperature (> 900 °C), H2O-undersaturated, strongly peraluminous granitoids, and are produced by large-scale melting of metasedimentary rocks and subsequent mixing between restite-rich anatectic melt and mafic magma. The voluminous high-temperature granitoids, coupled with UHT granulites in the Jining Complex, manifest the thermal response of a granulitic lower crust to the mafic underplating.

Paleoproterozoic ultrahigh-temperature pelitic granulites in the northern Sulu orogen: Constraints from petrology and geochronology

Ultrahigh-temperature (UHT) pelitic granulites are reported firstly in Weihai area, the northern segment of Sulu ultrahigh-pressure (UHP) metamorphic belt. The pelitic granulites, together with other metamorphic supracrustal rocks, including gneisses, marbles, quarzites and calc-silicates, occur as blocks within the orthogneisses which formed at Neoproterozoic but underwent Triassic UHP metamorphism. The pelitic granulites have a peak mineral assemblage of garnet+sillimanite+K-feldspar+plagioclase (antiperthite)+quartz+rutile+ilmenite±biotite. The P–T pseudosection and mineral isopleths thermobarometry modeled in the MnNCFKMASHTO system constrain P–T conditions of 11–13kbar and >900°C for the peak stage. In situ LA–ICPMS U–Pb dating and trace element analyses show that zircons from the two granulites have low Th/U ratios, flat HREE patterns with negative Eu anomalies, typical of granulite-facies metamorphic zircon. These zircons yielded 207Pb/206Pb ages of 1818±10Ma to 1854±9Ma, representing the time of the UHT granulite-facies metamorphism. Our result, together with previous investigations, indicates that the Mid-Paleoproterozoic tectonothermal event was widespread in the northern margin of the South China Craton. The Paleoproterozoic UHT granulite-facies metamorphism and associated anatexis, and coeval magmatism possibly derived from accretionary orogeny during the assembly of the supercontinent Columbia.

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

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

Prismatine and ferrohögbomite-2N2S in granulite-facies Fe-oxide lenses in the Eastern Ghats Belt at Venugopalapuram, Vizianagaram district, Andhra Pradesh, India: do such lenses have a tourmaline-enriched lateritic precursor?

AbstractFluorine-rich prismatine, (□,Fe,Mg)(Mg,Al,Fe)5Al4(Si,B,Al)5O21(OH,F), with F/(OH+F) = 0.36–0.40 and hercynite are major constituents of a Fe-Al-B-rich lens in ultrahigh-temperature granulite-facies quartz-sillimanite-hypersthene-cordierite gneisses of the Eastern Ghats belt, Andhra Pradesh, India. Hemo-ilmenite, sapphirine, magnetite, biotite and sillimanite are subordinate. Lithium, Be and B are concentrated in prismatine (140 ppm Li, 170 ppm Be, and 2.8 –3.0 wt.% B2O3). Another Fe-rich lens is dominantly magnetite, which encloses fine-grained zincian ferrohögbomite-2N2S, (Fe2+,Mg,Zn,Al)6 (Al,Fe3+,Ti)16O30(OH)2, containing minor Ga2O3 (0.30 –0.92 wt.%). Fe-Al-B-rich lenses with prismatine (or kornerupine) constitute a distinctive type of B-enrichment in granulite-facies rocks and have been reported from four other localities worldwide. A scenario involving a tourmalineenriched lateritic precursor affected by dehydration melting is our preferred explanation for the origin of the Fe-Al-B-rich lenses at the five localities. Whole-rock analyses and field relationships at another of these localities, Bok se Puts, Namaqualand, South Africa, are consistent with this scenario. Under granulite-facies conditions, tourmaline would have broken down to give kornerupine-prismatine (±other borosilicates) plus a sodic melt containing H2O and B. Removal of this melt depleted the rock in Na and B, but the formation of ferromagnesian borosilicate phases in the restite prevented total loss of B.