Ultrahigh-temperature Metamorphism Research Articles

The Sveconorwegian orogeny – Reamalgamation of the fragmented southwestern margin of Fennoscandia

The Sveconorwegian orogeny encompasses magmatic, metamorphic and deformational events between ca. 1140 and 920 Ma at the southwestern margin of Fennoscandia. In recent years, the tectonic setting of this nearly 200 Myr-long evolution has been debated, with some workers arguing for collision with an unknown continent off the present-day southwest coast of Norway, and others advocating accretionary processes inboard of an active margin. Recently, it has been suggested that orogeny may have been gravity-driven by delamination and foundering of heavy subcontinental lithospheric mantle in an intraplate setting, in some ways similar to proposed sagduction processes in the Archaean. Resolving the tectonic setting of the Sveconorwegian orogen has implications for correlation with other orogens and Rodinia supercontinent reconstructions and for assessments of the evolution of plate tectonics on Earth, from the Archaean to the present. Here, we present new mapping and geochronological data from the Bamble and Telemark lithotectonic units in the central and western Sveconorwegian orogen – the former representing a critical region separating western parts of the orogen that underwent long-lived high- to ultrahigh-temperature metamorphism and magmatism from parts closer to the orogenic foreland that underwent episodic high-pressure events. The data show that the units constituting the Sveconorwegian orogen most likely formed at the southwestern margin of Fennoscandia between ca. 1800 and 1480 Ma, followed by fragmentation during widespread extension between ca. 1340 and 1100 Ma marked by bimodal magmatism and sedimentation. A summary of Sveconorwegian magmatic, metamorphic and depositional events in the different units shows disparate histories prior to their assembly with adjacent units. The most likely interpretation of this record seems to be that episodic, Sveconorwegian metamorphic and deformational events in the central and eastern parts of the orogen represent accretion and assembly of these units. This process most likely took place behind an active margin to the southwest that sustained mafic underplating in the proximal back-arc, resulting in high- to ultrahigh-temperature metamorphism in the western parts. In this interpretation, all features of the Sveconorwegian orogen are readily explained by modern-style plate tectonic processes and hypotheses involving some form of vertical, intraplate tectonics are not supported.

AusLAMP 3D MT imaging of an intracontinental deformation zone, Musgrave Province, Central Australia

The central Australian Musgrave Province at the junction of the South, North and West Australian cratons has undergone and continues to retain evidence of significant whole-of-crust, and most likely ‘whole-of-lithosphere’ tectono-magmatic processes. The area is known for some of the largest geophysical anomalies related to significant Moho offsets of up to 15 km, which resulted from repeated intracratonic reworking since the Neoproterozoic. New magnetotelluric (MT) data have been collected across the Musgrave Province in Western Australia and South Australia as part of the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP). Station spacing was sim 50,hbox {km} between 96 sites over an area of 500 × 700 km. Long-period MT impedance and tipper data over a bandwidth of 8 s to 10,000 s period have been inverted using a smooth 3D inverse algorithm. The 3D model shows two predominant resistivity trends. There are deep (>65,hbox {km}) north–south mantle conductors that we infer to be related to the Palaeo- to Mesoproterozoic north-trending arc-related rocks that experienced ultra-high temperature metamorphism and widespread magmatism during the Mesoproterozoic Musgravian Orogeny. These conductors are preserved in the crust south of the Musgrave Province. The upper mantle also contains a localised resistive zone that possibly represents generation of mafic- to ultramafic magmas during the c. 1090–1040 Ma Giles Event. The crust (<65,hbox {km} depth) contains strong east–west crustal conductors interpreted to reflect the east–west structural grain that initiated during the c. 1090–1040 Ma Giles Event and overprinted the older N–S-oriented mantle anomalies. These E–W crustal conductors coincide with magnetic anomalies that represent crustal-scale structures, and high gravity anomalies associated with significant Moho offsets resulting from further reactivation during the c. 630–520 Ma Petermann and c. 450–300 Ma Alice Springs orogenies.

UHT Metamorphism Peaking Above 1100 °C with Slow Cooling: Insights from Pelitic Granulites in the Jining Complex, North China Craton

Abstract The peak temperature and duration of ultrahigh-temperature (UHT) metamorphism are critical to identify and understand its tectonic environment. The UHT metamorphism of the Jining complex in the Khondalite Belt, North China Craton is controversial on the peak temperature, time and tectonic setting. A representative sapphirine-bearing granulite sample is selected from the classic Tianpishan outcrop for addressing the metamorphic evolution and timing. The rock is markedly heterogeneous on centimetre scale and can be divided into melanocratic domains rich in sillimanite (MD-s) or rich in orthopyroxene (MD-o), and leucocratic domains (LD). On the basis of detailed petrographic analyses and phase equilibria modelling using THERMOCALC, all three types of domains record peak temperatures of 1120–1140 °C and a series of post-peak cooling stages at 0·8–0·9 GPa to the fluid-absent solidus (∼890 °C), followed by sub-solidus decompression. The peak temperature for MD-s is constrained by the coexistence of sillimanite-I + sapphirine + spinel + quartz, where sillimanite-I contains densely exsolved aciculae of hematite, yielding reintegrated Fe2O3 contents up to 2·1–2·3 wt %. The post-peak cooling evolution involves the sequential appearance of K-feldspar, sillimanite-II + garnet, orthopyroxene and biotite, where sillimanite-II is exsolution-free and contains variable Fe2O3 contents of 1·3–1·8 wt %. The peak temperature for MD-o is constrained by the sapphirine + orthopyroxene assemblage, where orthopyroxene has a maximum AlIV of 0·22 (Al2O3 = 9·5 wt %) in the core. The cooling evolution involves the sequential appearance of K-feldspar, garnet and biotite, and the decreasing AlIV (0·22→0·17) from core to rim in orthopyroxene. The peak temperature for LD is constrained by the inferred K-feldspar-absent assemblage and the maximum anorthite content of 0·11 in K-feldspar. The cooling evolution involves the crystallization of segregated melts, exsolution of supra-solvus ternary feldspars and growth of biotite. The Al in orthopyroxene, Fe2O3 in sillimanite and anorthite in K-feldspar are good indicators for constraining extreme UHT conditions although they depend differently on bulk-rock compositions. In-situ SHRIMP U–Pb dating of metamorphic zircon indicates that the UHT metamorphism may have occurred at &amp;gt;1·94 Ga and the cooling under UHT conditions lasted over 40 Ma. The extreme UHT metamorphism in the Jining complex is interpreted to be triggered by the advective heating of intraplate hyperthermal mafic magmas together with a plume-related hot mantle upwelling, following an orogenic crustal thickening event.

The timing and duration of high-temperature to ultrahigh-temperature metamorphism constrained by zircon U–Pb–Hf and trace element signatures in the Khondalite Belt, North China Craton

Understanding the timing and duration of metamorphism is fundamental to correctly interpreting the geodynamic evolution of orogenic events. Metamorphic constraints are still ambiguous in the Khondalite Belt, North China Craton, so Paleoproterozoic orogenic processes remain unclear. Here, we investigate a suite of high-grade rocks from the Dongpo locality in the Daqingshan Terrane by integrating zircon trace elements and Hf isotopic signatures with U–Pb ages to track zircon formation events and confidently constrain the timing and duration of metamorphism. The application of the Ti-in-zircon thermometer confirms the occurrence of regional HT-UHT metamorphism. Linking zircon HREE patterns and U–Pb ages, in combination with HREE partitioning between zircon and garnet, reveals zircon formation during partial melting to thermal peak stage, and during the post-peak cooling stage. Local reaction control by garnet breakdown has affected some zircons, which show a more radiogenic 176Hf/177Hf ratio (0.28164–0.28174) relative to unaffected grains (0.28128–0.28156). Metamorphic zircon U–Pb ages, combined with results from previous studies, constrain partial melting to UHT conditions with decompression-heating at 1.94–1.90 Ga, post-peak near isobaric cooling at 1.90–1.86 Ga, and the thermal peak very close to ca. 1.90 Ga. Local garnet breakdown occurred during another decompression-heating stage to HT-UHT conditions at 1.86–1.84 Ga, and further cooling occurred until ca. 1.80 Ga. Our work reveals relatively short-lived partial melting (less than 40 million years) in comparison to long-lived cooling stages (nearly 100 million years) in the eastern part of the Khondalite Belt. Two pulses of decompression-heating to HT-UHT conditions in the deep crust could result from repeated asthenospheric upwelling driven by lithospheric extension after subduction-collision. Our study sheds new light on zircon behaviour during HT-UHT metamorphism and elucidates the underlying driver of UHT metamorphism in this part of the Khondalite Belt.

Large-Scale Fluid Transfer between Mantle and Crust during Supercontinent Amalgamation and Disruption

Abstract —Supercontinents are a unique feature of the planet Earth. A brief review of supercontinents formed since the Archean shows that before the Eocambrian, supercontinents, notably Gondwana and Rodinia, amalgamated through high-temperature mobile belts, all of them containing ultrahigh-temperature granulite occurrences. During the final stage of the amalgamation, the lower continental crust was brought to magmatic temperature (from ~900 to more than 1000 °C) during a variable time span, from less than 10 Ma in the recent shortlived orogens to more than 150 Ma in the Eocambrian (Gondwana) or Neoproterozoic (Rodinia) long-lived orogens. Ultrahigh-temperature granulites worldwide contain the same types of fluid inclusions, namely, dense CO2 and highly saline aqueous brines. The fluid amount in the peak metamorphic conditions is indicated by the amount of preserved fluid inclusions (especially CO2) and by the secondary effects caused by the fluids when they left the lower crust, including regional feldspathization, albitization or scapolitization, and formation of megashear zones, either oxidized (quartz–carbonate) or reduced (graphite veins). While some fluids may be locally derived either from mineral reactions or from inherited sediment waters, carbon isotope signature and petrographical arguments suggest that most fluids, both CO2 and high-salinity brines, are derived from carbonatite melts resulting from partial melting of metasomatized mantle. Ultrahigh-temperature metamorphism is critical for supercontinent amalgamation, but the associated fluid causes instability and disruption shortly after amalgamation.

Petrological, geochronological, and geochemical potential accounting for continental subduction and exhumation: A case study of felsic granulites from South Altyn Tagh, northwestern China

Abstract Deciphering the formation and geodynamic evolution of high-pressure (HP) granulites in a collisional orogeny can provide crucial constraints on the geodynamic evolution of subduction-exhumation. To fully exploit the geodynamic potential of metamorphic rocks, it is necessary to constrain the metamorphic ages, although it is difficult to link zircon and monazite ages to metamorphic evolution. A good case study for understanding these geodynamic processes is felsic granulites in the Bashiwake area, South Altyn Tagh. Petrographic observations suggest that the studied felsic granulites have suffered multi-stage metamorphism, and the distinct metamorphic events were documented by compositional zoning and high Y + heavy rare earth element (HREE) concentrations in the large garnet porphyroblast. Zircon U-Pb dating yielded two major age clusters: one age cluster at ca. 900 Ma represents the age of the protolith for the felsic granulite, and another age cluster at ca. 500 Ma represents the post-UHT (ultrahigh temperature) stage based on the rare earth element distribution coefficients between zircon and garnet. Meanwhile, in situ monazites U-Pb dating yielded a weighted mean 206Pb/238U age of 482 ± 3.5 Ma, and the monazite U-Pb age was interpreted to be in agreement with the metamorphic zircon rims data, which together with zircon recorded the cooling time after the UHT stage. Whole-rock major and trace elements as well as Sr-Nd isotopes suggest that the protolith of the felsic granulite derived from partial melting of ancient crustal materials with the addition of mantle materials. Integrating these results along with previous studies, we propose that the felsic granulites metamorphosed from the Neoproterozoic granitic rocks, and the granitic rocks with associated mafic-ultramafic rocks suffered a common high-pressure–ultrahigh temperature (HP-UHT) metamorphism and subsequent granulite-facies metamorphism. A tentative model of subduction-relamination was proposed for the geodynamic evolution of the Bashiwake unit, South Altyn Tagh.

Decoupling between Ti‐in‐zircon and Zr‐in‐rutile thermometry during ultrahigh temperature metamorphism of the Dabie Orogen, China

The Ti‐in‐zircon (TIZ) and Zr‐in‐rutile (ZIR) geothermometers record metamorphic temperatures. In theory, their results should be consistent when zircon and rutile equilibrate as they form. However, we show that these geothermometers were decoupled during ultrahigh temperature (UHT) metamorphism of the Dabie Orogen, China. The TIZ geothermometer records a peak temperature of 927 ± 38°C for the granulite‐facies metamorphism. Rutile occurs as inclusions in UHT metamorphic zircon domains, and they were syn‐crystallised. Rutile records ZIR temperatures of 783 to 798°C. We infer that diffusion of Ti and Zr between zircon and rutile is the main cause of decoupling between the TIZ and ZIR geothermometers. Diffusion of Zr in rutiles is closely related to the distribution of elements, the duration of UHT metamorphism, and rutile grain size.

Isotopic systematics of zircon indicate an African affinity for the rocks of southernmost India

Southern India lies in an area of Gondwana where multiple blocks are juxtaposed along Moho-penetrating structures, the significance of which are not well understood. Adequate geochronological data that can be used to differentiate the various blocks are also lacking. We present a newly acquired SIMS U–Pb, Lu–Hf, O isotopic and trace element geochemical dataset from zircon and garnet from the protoliths of the Nagercoil Block at the very tip of southern India. The data indicate that the magmatic protoliths of the rocks in this block formed at c. 2040 Ma with Lu–Hf, O-isotope and trace element data consistent with formation in a magmatic arc environment. The zircon data from Nagercoil Block are isotopically and temporally distinct from those in all the other blocks in southern India, but remarkably correspond to rocks in East Africa that are exposed on the southern margin of the Tanzania–Bangweulu Block. The new data suggest that the tip of southern India has an African affinity and a major suture zone must lie along its northern margin. All of these blocks were finally brought together during the Ediacaran-Cambrian amalgamation of Gondwana where they underwent high to ultrahigh temperature metamorphism.

Paleoproterozoic UHT metamorphism with isobaric cooling (IBC) followed by decompression–heating in the Khondalite Belt (North China Craton): New evidence from two sapphirine formation processes

AbstractInterpretation of reaction microstructures may provide constraints on the P–T path followed by rocks, with implications for the geodynamic evolution. Sapphirine generally occurs in diverse microstructures in ultrahigh‐temperature (UHT) Mg–Al‐rich granulites. Understanding multi‐stage sapphirine formation processes and the resultant P–T path may provide insights into the cause of UHT metamorphism, which is otherwise under broad debate. Here, we investigate samples of UHT granulite containing two distinct types of sapphirine from the Dongpo locality in the Khondalite Belt, North China Craton, with the aim of understanding the processes of sapphirine formation and the metamorphic evolution of the host rocks. Petrographic observations show that early sapphirine, which occurs as coronas on spinel and as single porphyroblasts, formed together with biotite, sillimanite, and inclusion‐rich garnet. Late symplectitic sapphirine along with fine‐grained plagioclase and spinel plus plagioclase symplectites, formed by consumption of sillimanite, biotite, and garnet. Three pseudosections based on the bulk compositions of microdomains inferred to reflect spatially restricted equilibrium suggest that the rocks record near isobaric cooling (IBC) from ~980 to 830ºC at ~0.9 GPa for early sapphirine formation, and decompression and heating to ≤0.7 GPa and ~900ºC for late sapphirine formation. Our study in combination with other metamorphic P–T and age information reveals the common occurrence of IBC paths and long duration (c. 1.93 to 1.86 Ga) regional UHT metamorphism in the Khondalite Belt, North China Craton. Locally, this is followed by decompression–heating paths at c. 1.86 Ga. The Palaeoproterozoic UHT metamorphism with long‐lived IBC path in the Khondalite Belt, North China Craton supports large hot orogen model in the amalgamation of this part in the supercontinent Nuna.

Polyphase Zircon Growth during Slow Cooling from Ultrahigh Temperature: an Example from the Archean Pikwitonei Granulite Domain

AbstractQuantifying the timescales of Archean ultrahigh temperature (UHT) metamorphism is essential for constraining the style of plate tectonics on the early Earth. However, such timescales can be difficult to quantify, due to the antiquity of Archean rocks and the extreme thermal conditions of UHT metamorphism. We constrain the timescales of Archean UHT metamorphic processes recorded by a single rock sample from the Pikwitonei granulite domain (northwestern Superior Province), through the integration of two U–Pb zircon petrochronologic techniques. In this study we combine: (1) high-spatial resolution laser ablation split-stream inductively coupled mass spectrometry (LASS) on in situ zircon (in thin section) and hand-picked zircon; and (2) high-precision isotope dilution thermal ionization mass spectrometry (ID-TIMS) analyses on microsampled fragments from the same hand-picked zircon analysed by LASS. Phase equilibria modelling and Zr-in-rutile thermometry suggest the rock followed a P–T path characterized by decompression at &amp;gt; 960 °C, followed by near-isobaric cooling at ∼0·8 GPa. In situ LASS zircon analyses could be interpreted to record zircon growth at broadly ∼2665 Ma, though the large uncertainties on isotopic dates make potentially distinct growth episodes difficult to distinguish. ID-TIMS U–Pb dates of zircon fragments reveal a polyphase zircon growth history over a 24 Ma duration, from 2673 to 2649 Ma. Zircon trace element compositions, textures, and microstructural relationships, as well as evaluation of zircon-garnet equilibrium, suggest zircon grew during melt crystallization, after UHT decompression and garnet resorption. Variable Ti concentrations within zircon domains indicate: (1) zircon crystallized through the temperature interval of ∼875 °C to ∼730 °C, potentially in isolated rock domains with variable zircon saturation temperature; and/or (2) zircon crystallized over a narrower temperature interval in isolated rock domains with variable aTiO2 and/or aSiO2. Collectively, the data suggest the west-central Pikwitonei granulite domain reached peak UHT conditions prior to 2673 Ma, after which suprasolidus conditions in the lower crust persisted for at least 24 Ma. Such an interpretation would be impossible if based on either the LASS or ID-TIMS zircon data alone, which highlights the utility of applying both techniques in tandem to constrain metamorphic timescales in ancient UHT terranes.

Granulites record the tectonic evolution from collisional thickening to extensional thinning of the Tongbai orogen in central China

AbstractA combined study of petrology and geochemistry was carried out for granulites from the Tongbai orogen in central China. The results reveal the tectonic evolution from collisional thickening to extensional thinning of the lithosphere at the convergent plate boundary. Petrographic observations, zircon U–Pb dating, and pseudosection calculations indicate that the granulites underwent four metamorphic stages, which are categorized into two cycles. The first cycle occurred at 490–450 Ma and involves high‐P (HP) metamorphism (M1) at 785–815°C and 10–14 kbar followed by decompressional heating to 840–880°C and 8–9 kbar for medium‐pressure granulite facies metamorphism (M2), defining a clockwise P–T path. The high pressure is indicated by the occurrence of inclusions of rutile+kyanite+K‐feldspar in the garnet mantle. The second cycle occurred at c. 440 Ma and shows an anticlockwise P–T path with continuous heating to ultrahigh‐temperature (UHT) metamorphism (M3) at 890–980°C and 9–11 kbar, followed by decompressional cooling to 740–880°C and 7–9 kbar (M4) till 405 Ma. The HP metamorphism is synchronous with the ultrahigh‐pressure eclogite facies metamorphism in the Qinling orogen, indicating its relevance to the continental collision in the Cambrian. The UHT metamorphism took place at reduced pressures, indicating thinning of the collision‐thickened orogenic lithosphere. Therefore, the Tongbai orogen was initially thickened by the collisional orogeny and then thinned, possibly as a result of foundering of the orogenic root. Such tectonic evolution may be common in collisional orogens where compression during continental collision switched to extension during continental rifting.

Ultrahigh temperature metamorphism in collisional orogen and its tectonic significance: An example from the Pan-African orogens

Ultrahigh temperature metamorphism in collisional orogen and its tectonic significance: An example from the Pan-African orogens

Ultra-high temperature metamorphism in the Guaxupé Complex: a lower crust segment

The area is located at the northern boundary of the Guaxupé Complex, which corresponds to the northern domain of the Socorro-Guaxupé Nappe, in the Southern Brasília Orogen, close to the Varginha Shear Zone. This study integrates petrography, mineral chemistry, optimized geothermobarometry, and trace elements thermometry data, aiming to define and characterize the metamorphic conditions and the P-T path of the granulitic rocks from the northern portion of the Guaxupé Complex. In the area, the Guaxupé Complex consists predominantly of ortho-derived rocks represented by granulitic gneisses (orthopyroxene gneisses) of felsic to mafic composition with clinopyroxene, garnet, amphibole, biotite, and rare rutile. The calculated pressure and temperature conditions record metamorphic peak around 1,078°C and 14.8 kbar, compatible with ultra-high temperature metamorphism conditions, marked by grt + cpx + opx + pl ± qtz ± rt mineral assemblages, and granulite facies re-equilibration in the sillimanite stability field at P-T conditions of 930 to 800°C and 13.5 to 11.0 kbar. The results are compatible with deeper portions of the lower crust involved in a continental collision and suggest a clockwise P-T path, typical of crustal thickening zones, followed by near-isothermal decompression during tectonic exhumation.

First comprehensive characterization of osumilite from India ( Eastern Ghats Province): Physicochemical characteristics, stability of the mineral and its breakdown products

A rare occurrence of osumilite has been reported from a metapelite from parts of the Eastern Ghats Province, India. Incidentally, this is the first report of osumilite along with its composition and textural description from any Indian rock. The presence of osumilite has been confirmed by electron microscopy (BSE images), electron microprobe analyses (and X-ray intensity maps) as well as Raman spectroscopy. In the studied sample, osumilite occurs as rare anhedral relicts (~300–350 μm) that are surrounded by a variety of extremely fine-grained symplectitic intergrowths namely cordierite + K-feldspar, cordierite + K-feldspar + orthopyroxene and cordierite + K-feldspar + orthopyroxene + quartz. The symplectites partially to completely pseudomorph the early formed osumilite grains. An isochemical pressure-temperature phase diagram has been calculated in the Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O system in order to model the breakdown of osumilite and formation of such symplectitic mineral assemblages, using the average micro bulk compositions calculated with the XMapTools software. The calculated P-T phase diagram suggests that osumilite was formed during ultra-high temperature metamorphism (at P~8.5 kbar and T~1110°C) in an Mg-Al-rich pelitic assemblage and subsequently broke down to produce the symplectitic mineral assemblages during isobaric cooling.

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.

The Archean granulite-enderbite complex of the northern Carajás province, Amazonian craton (Brazil): Origin and implications for crustal growth and cratonization

The central portion of the Canaã dos Carajás domain (Ouro Verde area) exhibits a peculiar scenario involving two units of orthopyroxene-bearing sodic rocks: (i) the Mesoarchean Ouro Verde felsic granulite with protolith crystallization and metamorphism ages of 3.05–2.93 Ga and 2.89–2.84 Ga, respectively, and (ii) the Neoarchean Café enderbite, composed of tonalites, trondhjemites and scarce quartz diorites that crystallized at 2.75–2.73 Ga and show syntectonic character. While the orthopyroxene in the felsic granulites was formed by breakdown of primary biotite during metamorphism, in the enderbitic rocks, this mineral was formed by crystallization of a hot water-poor magma at deep crustal levels. This terrane evolved in three important geodynamic stages: (1) at 3.05–2.93 Ga, TTG crust was generated by partial melting of LILE-enriched basalts in a N–S subduction setting; (2) from 2.89 to 2.84 Ga, in a collisional setting, large volumes of anatectic granites formed and contributed to crustal thickening, which triggered granulite-facies ultrahigh-temperature metamorphism in the TTG crust, forming the Ouro Verde felsic granulite; and (3) in the Neoarchean, at 2.75–2.73 Ga, the lowermost mafic crust was delaminated during crustal thickening, causing crustal underplating followed by partial melting of Mesoarchean mafic granulites, which was responsible for the origin of the enderbitic magma. This area was subjected to a transpressional tectonic regime, leading to the development of plutons (E–W-trending) amalgamated linearly and emplaced along ancient suture zones (ductile-brittle transition). Lu-Hf isotope data of magmatic zircon cores from the Mesoarchean felsic granulite show Hf-TDM2 values of 3.44–3.15 Ga and εHf(t) values between −1.7 and 3.0, which suggest juvenile sources. The Neoarchean enderbites present Hf-TDM2 ages of 3.46–3.29 Ga and lower εHf(t) values (between −4.8 and −1.9), which indicate a longer crustal residence time for these rocks. All these findings lead us to consider the central portion of the Canaã dos Carajás domain as exhumed lower crust, and the tectonic juxtaposition of anatectic granites, enderbites and high-grade metamorphic rocks suggests that the Ouro Verde area represents middle and deep exposure levels of the same crust.

PM-08 Precipitation of Pigeonite and Enstatite in Augite of Ultrahigh Temperature Metamorphic Rock from Antarctica

PM-08 Precipitation of Pigeonite and Enstatite in Augite of Ultrahigh Temperature Metamorphic Rock from Antarctica

What Drives the Continental Crust To Be Extremely Hot So Quickly?

AbstractThe widespread spatio‐temporal occurrence of ultrahigh‐temperature (UHT) metamorphism in continental crust has been widely documented, but the heat source responsible for generating these extreme conditions over a short period of time remains enigmatic. Here, garnet orthogneiss, spinel‐bearing granulite, and gabbronorite from the east Khondalite belt, China, have been analyzed via integrated geochemistry, U‐Pb zircon isotope geochronology, and phase equilibria modeling to shed light on the timing and nature of rapid UHT metamorphism in this area. Peak metamorphism is characterized in garnet orthogneiss by the assemblage garnet + orthopyroxene + plagioclase + K‐feldspar + quartz, which is calculated to represent pressure–temperature conditions of 0.83–0.88 GPa at 930–990 °C. Zircon ages from this sample yield an igneous (protolith) emplacement age of approximately 1.93–1.95 Ga, which matches the timing of metamorphism in adjacent (pre‐UHT) metapelites. These garnet orthogneiss were then metamorphosed to UHT conditions at approximately 1.92 Ga, coincident with emplacement of mantle‐derived gabbronorites, as supported by REE patterns in zircon, and evidence for a rapid thermal pulse in the east Khondalite belt lower crust at that time. Heat‐source and heat‐transfer calculations indicate that advected heat from mafic magmas is the most likely cause of this regional‐scale event, although ductile shearing might have contributed to a lesser degree.

Timing of magmatism and ultrahigh- to high-grade metamorphism in the Kannak Complex, Kon Tum Massif, Vietnam: Magmatic activity and its tectonic implications

Detailed geochronological and petrological investigations of metamorphic and igneous rocks can significantly improve our understanding of the tectonic processes that occur in orogenic belts. The Kon Tum Massif, central Vietnam, includes various types of magmatic and metamorphic rocks, the latter including ultrahigh- to high-temperature units. The Late Permian–Early Triassic metamorphic rocks in the massif correspond to the metamorphic characteristics of a collision between the Indochina and South China blocks in terms of their metamorphic evolution and timescales of deformation. U–Pb geochronological analyses of zircon obtained from two samples of garnet granite in the massif yielded magmatic ages of 239.6 ± 3.9 Ma and 243.5 ± 3.4 Ma. This permitted the reevaluation of previous Th–U–Pb electron microprobe analyses of monazite from the garnet granite and garnet-orthopyroxene gneiss and previously published U–Pb dating. These data reveal the metamorphic and magmatic history of the western part of the massif. The metamorphic rocks show a thermal evolution from 260 Ma to 240 Ma. Magmatic activity started at 260 Ma and ceased at 240 Ma. Considering this in terms of the regional-scale igneous activities in the Kon Tum Massif, the base of the massif, already thickened by the collision, was situated above underplating plume-related magma. Such magmatic activity resulted in high- to ultrahigh-temperature metamorphism and crustal melting during the Late Permian to Early Triassic, between 260 and 240 Ma.

Timing of two separate granulite-facies metamorphic events in the Helanshan complex, North China Craton: Constraints from monazite and zircon U–Pb dating of pelitic granulites

This study presents the results of monazite and zircon U–Pb dating of garnet–biotite gneiss and recently discovered spinel-bearing ultrahigh-temperature (UHT) pelitic granulites from the Paleoproterozoic Helanshan complex of the North China Craton, and considers implications for the formation of the complex and the tectonic history of this region. SHRIMP monazite U–Pb dating of the garnet–biotite gneiss yielded a weighted mean 207Pb/206Pb age of 1944.4 ± 4.2 Ma, which is the same within error as a weighted mean laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) 207Pb/206Pb age of 1959 ± 18 Ma obtained from metamorphic zircons within the same unit. The age of 1959 ± 18 Ma is similar to the published metamorphic ages of ca. 1.96–1.95 Ga from high-temperature (Tmax < 900 °C) pelitic granulites in the Helanshan, which represent the timing of continental collision between the Yinshan and Ordos terranes as suggested by previous published works. Metamorphic monazites from the spinel-bearing UHT pelitic granulite unit in this area yielded weighted mean 207Pb/206Pb ages of 1930.8 ± 2.6 and 1933.6 ± 3.1 Ma, which are consistent with the ages of metamorphic zircons. These ages record the timing of UHT metamorphism within the Helanshan complex and are similar to 1.93–1.92 Ga ages previously reported from typical UHT pelitic granulites within the Jining and Daqingshan complexes of the Khondalite Belt, where ca. 1.92 Ga metamorphic zircons are thought to record the cooling of the UHT rocks to the solidus. The new data presented here indicate that the ca. 1.93 Ga UHT metamorphic event occurred across a wider area than previously thought and extended into the Helanshan complex. Combining these new data with the results of previous research suggests that the entire Khondalite Belt was influenced by a ca. 1.93 Ga UHT metamorphic event that was preceded by metamorphism associated with continental collision between the Yinshan and Ordos terranes at ca. 1.95 Ga. This suggests that the Khondalite Belt underwent a tectonic transition from compression to extension between 1.95 and 1.93 Ga, a process likely controlled by a late-stage shallow slab-breakoff event.