Prograde Metamorphic Stage Research Articles

Metamorphism and chronology of Paleoproterozoic mafic granulite from the Kuluketage area, northern Tarim and its tectonic applications

Mafic granulites provide key insights for understanding the composition and evolution of the lower crust but are rarely reported in northern Tarim. In the present study, a systematic petrographic, phase equilibrium modeling and zircon U-Pb geochronological study of a Paleoproterozoic mafic granulite (garnet two-pyroxene granulite) from the Kuluketage area, northern Tarim were conducted. Phase equilibrium modeling demonstrates a decompression–heating clockwise P-T path involving the prograde metamorphic stage (M1) at ∼ 950 °C / 12 kbar, the peak metamorphic stage (M2) at ∼ 1020 °C / 10 kbar, and the retrograde stage (M3) at ∼ 880 °C / 7 kbar. The decompression heating P-T path generally occurs in the post-collisional lithospheric extension setting which may contribute the UHT metamorphism of the M2 stage. Zircon U-Pb geochronological study reveals four age peaks at ∼ 2.45, ∼ 2.3, ∼ 1.91, and ∼ 1.85–1.78 Ga. We interpret the ∼ 2.45 Ga magmatic event as an older captured zircon age corresponding to a widely reported magmatic event of ∼ 2.4–2.5 Ga in Kuluketage. The slightly younger ∼ 2.3 Ga magmatic event may correspond to the crystallization age of the mafic granulite’s protolith, however, rarely reported in northern Tarim. The metamorphic age of ∼ 1.91 Ga represents a collisional-related high-pressure metamorphic age corresponding to the M1 stage and the age peaks of ∼ 1.85–1.78 Ga may represent post-peak cooling events corresponding to the M2-M3 stage.

Chrysoberyl and associated beryllium minerals resulting from metamorphic overprinting of the Maršíkov–Schinderhübel III pegmatite, Czech Republic

AbstractThe Maršíkov–Schinderhübel III pegmatite in the Hrubý Jeseník Mountains, Silesian Domain, Czech Republic, is a classic example of chrysoberyl-bearing LCT granitic pegmatite of beryl–columbite subtype. This thin pegmatite dyke, (up to 1 m in thickness in biotite–amphibole gneiss is characterised by symmetrical internal zoning. Tabular and prismatic chrysoberyl crystals (≤3 cm) occur typically in the intermediate albite-rich unit and rarely in the quartz core. Chrysoberyl microtextures are quite complex; their crystals are irregularly patchy, concentric or fine oscillatory zoned with large variations in Fe content (1.1–5.3 wt.% Fe2O3; ≤0.09 apfu). Chrysoberyl compositions reveal dominant Fe3+ = Al3+ and minor Fe2+ + Ti4+ = 2(Al, Fe)3+ substitution mechanisms in the octahedral sites. Tin, Ga, and V (determined by LA-ICP-MS) are characteristic trace elements incorporated in the chrysoberyl structure, whereas anomalously high Ta and Nb concentrations (thousands ppm) in chrysoberyl are probably caused by nano- to micro-inclusions of Nb–Ta oxide minerals; especially columbite–tantalite. Textural relationships between associated minerals, distinct schistosity of the pegmatite parallel to the host gneiss foliation and fragmentation of the pegmatite body into blocks as a result of superimposed stress are clear evidence for deformation and metamorphic overprinting of the pegmatite. Primary magmatic beryl, albite and muscovite were transformed to chrysoberyl, fibrolitic sillimanite, secondary quartz and muscovite during a high-temperature (~600°C) and medium-pressure (~250–500 MPa) prograde metamorphic stage under amphibolite-facies conditions. A subsequent retrograde, low-temperature (~200–500°C) and pressure (≤250 MPa) metamorphic stage resulted in the local alteration of chrysoberyl to secondary Fe,Na-rich beryl, euclase, bertrandite and late muscovite.

Contrasting geological background based on the geochemistry of the mafic metamorphic rocks in central Dronning Maud Land

AbstractThis paper reports geochemical characteristics of mafic gneisses and granulites collected from four localities in central Dronning Maud Land to evaluate the tectonic setting of their precursor rocks. Precursor rocks for the mafic gneisses and granulites in central Dronning Maud Land were formed in different geological backgrounds and tectonic settings. The mafic gneisses and granulites in the Schirmacher Hills were derived from basaltic rocks in a back-arc setting. Published U–Pb zircon ages and the geochemical variation of the basement rocks indicate their emplacement between 800 and 650 Ma, close to peak metamorphism. While similar protoliths were recognized in the inland nunataks of Hochlinfjellet, in Filchnerfjella the mafic gneisses/granulites were derived from basaltic rocks formed in marginal continental arcs or island arcs. Highly disturbed trace-element patterns indicate that the metamorphic process influenced the geochemical composition during the prograde metamorphic stage. Our results imply that the outcrops in central Dronning Maud Land with different metamorphic ages contain mafic gneisses/granulites from precursor rocks formed under different tectonic settings.

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

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

The key role of \xb5H2O gradients in deciphering microstructures and mineral assemblages of mylonites: examples from the Calabria polymetamorphic terrane

A careful petrologic analysis of mylonites’ mineral assemblages is crucial for a thorough comprehension of the rheologic behaviour of ductile shear zones active during an orogenesis. In this view, understanding the way new minerals form in rocks sheared in a ductile manner and why relict porphyroblasts are preserved in zones where mineral reactions are generally supposed to be deformation-assisted, is essential. To this goal, the role of chemical potential gradients, particularly that of H2O (µH2O), was examined here through phase equilibrium modelling of syn-kinematic mineral assemblages developed in three distinct mylonites from the Calabria polymetamorphic terrane. Results revealed that gradients in chemical potentials have effects on the mineral assemblages of the studied mylonites, and that new syn-kinematic minerals formed in higher-µH2O conditions than the surroundings. In each case study, the banded fabric of the mylonites is related to the fluid availability in the system, with the fluid that was internally generated by the breakdown of OH-bearing minerals. The gradients in µH2O favoured the origin of bands enriched in hydrated minerals alternated with bands where anhydrous minerals were preserved even during exhumation. Thermodynamic modelling highlights that during the prograde stage of metamorphism, high-µH2O was necessary to form new minerals while relict, anhydrous porphyroblasts remained stable in condition of low-µH2O even during exhumation. Hence, the approach used in this contribution is an in-depth investigation of the fluid-present/-deficient conditions that affected mylonites during their activity, and provides a more robust interpretation of their microstructures, finally helping to explain the rheologic behaviour of ductile shear zones.

Pressure–temperature–time evolution of subducted crust revealed by complex garnet zoning (Theodul Glacier Unit, Switzerland)

AbstractCollisional orogens commonly include mono‐metamorphic and poly‐metamorphic units, and their different evolution can be difficult to recognize and reconcile. The Theodul Glacier Unit (TGU) in the Western Alps consists of an association of metasedimentary and metamafic rocks embedded within the Zermatt‐Saas tectonic unit. In spite of recent petrological studies, it remains unclear whether these rocks underwent one or multiple metamorphic cycles. In this study, different lithologies from the TGU unit (mafic schist, mafic granofels, and chloritoid schist) were investigated for petrography, quantitative compositional mapping of garnet, thermodynamic modelling, and Lu–Hf garnet dating. The data reveal a coherent mono‐metamorphic history with a β‐shaped pressure–temperature (P–T) path characteristic of oceanic subduction. Garnet Lu–Hf ages yield a restricted garnet crystallization time window between 50.3 and 48.8 Ma (±0.5%, 2SD). A prograde metamorphic stage recorded in garnet cores yields conditions of 490 ± 15℃ and 1.75 ± 0.05 GPa. Maximum pressure conditions of 2.65 ± 0.10 GPa and 580 ± 15℃ were reached at 50.3 ± 0.3 Ma. Initial exhumation was rapid and led to isothermal decompression to 1.50 ± 0.10 GPa within 1 Ma. This decompression was associated with lawsonite breakdown in mafic schist and in mafic granofels, causing intense fluid–rock interaction within and between different lithologies. This process is recorded in garnet textures and trace element patterns, and in the major element composition of K‐white mica. Initial exhumation was followed by re‐heating of ~30℃ at a pressure of 1.50 ± 0.10 GPa. Perturbation of the subduction‐zone thermal structure may be related to upwelling of hot asthenospheric mantle material and transient storage of the unit at the crust–mantle boundary.

Vein topology, structures, and distribution during the prograde formation of an Archean gold stockwork

Abstract The Archean Cheechoo stockwork gold deposit is hosted by a felsic intrusion of tonalitic-granodioritic composition and crosscutting pegmatite dikes in the Eeyou Istchee James Bay area of Quebec, Canada (Archean Superior craton). The evolution of the stockwork is characterized herein using field relationships, vein density, and connectivity measurements on drill core and outcrop zones. The statistical distribution of gold is used to highlight mechanisms of stockwork emplacement and gold mineralization and remobilization. Two statistical populations of gold concentration are present. Population A is represented by gold grades below 1 g/t with a lognormal cumulative frequency. It is widespread in the hydrothermally altered (albite and quartz) and mineralized facies of the pluton. It is controlled by the development of quartz-feldspar-diopside veins as shown by the similar lognormal distribution of grades and vein density and by the correspondence of grades with network connectivity. Diopside and actinolite porphyroblasts in deformed veins within sodic and calcsilicate alteration zones are evidence for auriferous vein emplacement prior to the amphibolite facies peak of metamorphism. Population B (>1 g/t) is erratic and exhibits a strong nugget effect. It is present throughout the mineralized portion of the pluton and in pegmatites. This population is interpreted as the result of gold remobilization during prograde metamorphism and pegmatite emplacement following the metamorphic peak. The pegmatites are interpreted to have scavenged gold emplaced prior to peak metamorphism. These results show the isotropic behavior of the investigated stockwork during regional deformation and its development during the early stages of regional prograde metamorphism.

The P-T-t path of pelitic gneisses in the Zanhuang Complex: Further constraints on the Palaeoproterozoic tectonic evolution of the Trans-North China Orogen, North China Craton

The Zanhuang Complex, situated on the eastern margin of the central section of the Trans-North China Orogen (TNCO), North China Craton, mainly comprises Archean-Palaeoproterozoic tonalite-trondhjemite-granodiorite (TTG) gneisses, migmatites, granites, metavolcanic rocks, and metasedimentary rocks. Petrographic observations of garnet-bearing pelitic gneisses from the south-eastern Zanhuang Complex show three distinct mineral assemblages (M1-M3) that correspond to three different metamorphic stages. The interpreted prograde metamorphic stage (M1) is mainly represented by the cores of garnet porphyroblasts with fine-grained inclusions of biotite + plagioclase + staurolite + kyanite + quartz + ilmenite. The peak metamorphic stage (M2) is mainly represented by garnet mantles and a matrix-type mineral assemblage of kyanite + biotite + plagioclase + quartz + ilmenite + melt. The retrograde metamorphic stage (M3) is represented by the presence of sillimanite with garnet rims + biotite + plagioclase + quartz + ilmenite + magnetite. Quantitative phase equilibria modelling constrains the pressure–temperature (P–T) conditions of the M1 and M2 stages to 640–670 °C/7.3–7.5 kbar and 705–760 °C/7.9–9.1 kbar, respectively. The zircons and monazites in the Zanhuang pelitic gneisses all record consistent ages of 1839–1831 Ma, which most likely represent the near-peak metamorphic age. Thus, a clockwise P-T-time (P-T-t) path is determined for the garnet-bearing pelitic gneisses from the Zanhuang Complex. The petrological and geochronological evidence from the pelitic gneisses in the Zanhuang Complex in this paper suggests that the TNCO might have experienced continuous continental collision and rapid tectonic uplift at ca. 1839–1831 Ma, thereby providing a metamorphic constraint on the Palaeoproterozoic tectonic evolution of the TNCO.

Polyphase deformation and ultrahigh temperature metamorphism of the deep continental crust: Implications for tectonic evolution of the northern Eastern Ghats Belt, India

We present the structural history of a suite of high-grade rocks of the Phulbani area of the Eastern Ghats Belt, India, based on structural study at meso- and micro-scales combined with published metamorphic and the geochronological data. Structural data suggest multiple generations of fabric (S1–S5) development during a punctuated deformational history (D1−D5). The S1 fabric is present only in grain-scale and developed during the prograde metamorphic stage (M1). The regionally recognizable composite S2/S3 gneissic fabric developed in the realm of ultra-high temperature metamorphism (D2/D3, M2) during ca. 987–970 Ma as a result of the contraction and closure of the continental back-arc basin. The D4 deformation folded the earlier formed composite S2/S3 gneissic fabric and developed a shear-related mylonitic foliation (S4), possibly after ca. 970 Ma. Deformation in the Phulbani area culminated through the formation of the S5 mylonitic fabric (D5, M3) along the Ranipathar Shear Zone, possibly at ca. 500 Ma, which exposed the anomalously hot deep continental crustal rocks to shallower crustal level. Development of the Ranipathar Shear Zone is interpreted to be kinematically linked with several other such crustal-scale shear zones in the area which together played a pivotal role in the final tectonic evolution of the northern Eastern Ghats Belt.

Zircon from diamondiferous kyanite gneisses of the Kokchetav massif: Revealing growth stages using an integrated cathodoluminescence, Raman spectroscopy and electron microprobe approach

AbstractZircon crystals from diamondiferous kyanite gneisses of the Barchi-Kol area (Kokchetav massif, Northern Kazakhstan) have been investigated by a combined application of cathodoluminescence (CL), Raman spectroscopy and electron probe microanalysis (EPMA). The zircon crystals exhibit up to four distinct domains characterised by significantly different CL signatures and parameters of the ν3(SiO4) (1008 cm–1) Raman band (i.e. full width at half maximum, position and intensity). Extremely metamict zircon cores (Domain I) host inclusions of low-pressure minerals (quartz and graphite) and the outer mantles (Domain III) are populated by ultrahigh-pressure relicts (diamond and coesite), whereas inner mantles (Domain II) and overgrowth rim zones (Domain IV) are inclusion free. Both the zircon cores and rims have very low Ti concentrations, implying formation temperatures below 760°C. The Ti content in the inner mantles (up to 40 ppm) is indicative of temperatures in the 760–880°C range. The temperature estimates for the outer mantles are 900–940°C, indicating a pronounced overlap with the peak metamorphic values yielded by the Zr-in-rutile geothermometer for the same rocks (910–950°C). The internal textures of the zircons and the occurrence of index minerals within the distinct domains allow us to unravel the stages of the complex metamorphic history recorded in the zircon. Our data show that the zircon cores are inherited seeds of pre-metamorphic (magmatic?) origin, the inner mantles were formed on the prograde metamorphic stage, the outer mantles record ultrahigh-pressure metamorphism and the outermost rims mark the retrograde metamorphic stage. The observed zircon internal textures are thus clearly correlated with distinct growth events, and in some examples reflect a major part of the metamorphic history. It is concluded that the combined application of the CL, Raman spectroscopy and EPMA techniques to zircon offers significant potential for deciphering the metamorphic evolution of deeply-subducted rocks.

Late Paleozoic geodynamic evolution of the western North Patagonian Massif and its tectonic context along the southwestern Gondwana margin

In this study, we propose a geodynamic model covering sedimentation, metamorphism, magmatism, and exhumation processes for the western North Patagonian Massif basement. The youngest detrital zircon population ages (ca. 369 ± 8 Ma) obtained in a schist sample constrains the sedimentation stage to the Carboniferous Period. A first prograde metamorphic stage (M1-D1) produced the main foliation (S1) under greenschist conditions (~ 500 °C; < 4.5 Kbars). This stage was possibly linked to the regional Carboniferous magmatism event (330–300 Ma). The Permian magmatism (ca. 290 Ma) likely induced partial melting and migmatization of the deepest metasedimentary suite. This event corresponds to the second prograde metamorphic stage (M2) that reached amphibolite conditions (690 °C and 6.5 Kbars). The beginning of the basement uplift corresponds to the first retrograde metamorphic stage possibly developed during Permian – Triassic times (265–235 Ma). This event was triggered by NE-SW compression (σ1) and developed folds (D2-F2), second foliation (S2), micro-textural quartz deformation, and a retrograde evolution path for the garnet-bearing lithofacies. The final stage of the basement exhumation corresponds to the second retrograde metamorphic stage (D3) developed by NNW-SSE compression and linked to open folds (F3) in the Cushamen Formation. The characteristics of the western North Patagonian Massif geodynamic evolution and the adjacent basement regions suggest a paleotectonic subduction setting for the southwestern Gondwana margin during the late Paleozoic times.

Petrogenetic re-examination of spinel + quartz assemblage in the Larsemann Hills, East Antarctica

We present textural evidence of spinel (low-Zn) and quartz from two samples of quartzofeldspathic rocks from the Larsemann Hills, East Antarctica. The present occurrence is unusual for the fact that the direct grain contact is preserved unlike most other occurrences where these two phases are separated by reaction products during retrograde and/or prograde metamorphic stages. Textural features further indicate that Ti-biotite was part of the prograde assemblage that underwent partial melting to produce spinel. The maximum temperature estimated from the rock is 870 °C. Monazite grains are abundant in the rock and these are mostly homogeneous in terms of U, Th and Pb distribution. In situ U–Th-total Pb EPMA data of these monazite yield a group age of 527 ± 8 Ma. Textural and geochronological data match with the second (M2) stage of metamorphism of the Larsemann Hills region as evident from the existing geological record. Although spinel and quartz was considered to be a part of peak M2 assemblage in earlier studies, preservation of their grain contact is enigmatic. We consider the grain contact metastable and its preservation possibly resulted from significant overstepping of cooling reaction due to fast reaction kinetics.

Early Carboniferous HP metamorphism in the Hida Gaien Belt, Japan: Implications for the Palaeozoic tectonic history of proto‐Japan

AbstractWe report two new eclogite localities (at Kanayamadani and Shinadani) in the high‐P(HP) metamorphic rocks of the Omi area in the western most region of Niigata Prefecture, Japan, which form part of the Hida Gaien Belt, and determine metamorphic conditions and pressure–temperature (P–T) paths. The metamorphic evolution of the eclogites is characterized by a tight hairpin‐shapedP–Tpath from prograde epidote–blueschist facies to peak eclogite facies and then retrograde blueschist facies. The prograde metamorphic stage is characterized by various amphibole (winchite, barroisite, glaucophane) inclusions in garnet, whereas the peak eclogite facies assemblage is characterized by omphacite, garnet, phengite and rutile. PeakP–Tconditions of the eclogites were estimated to be ~600°C and up to 2.0 GPa by conventional cation‐exchange thermobarometry, Ti‐in‐zircon thermometry and quartz inclusion Raman barometry respectively. However, the Raman spectra of carbonaceous material thermometry of metapelites associated with the eclogites gave lower peak temperatures, possibly due to metamorphism at different conditions before being brought together during exhumation. The blueschist facies overprint following the peak of metamorphism is recognized by the abundance of glaucophane in the matrix. Zircon grains in blueschist facies metasedimentary samples from two localities adjacent to the eclogites have distinct oscillatory‐zoned cores and overgrowth rims. Laser ablation inductively coupled plasma mass spectrometry U–Pb ages of the detrital cores yield a wide range between 3,200 and 400 Ma, with a peak at 600–400 Ma. In the early Palaeozoic, proto‐Japan was located along the continental margin of the South China craton, providing the source of the older population of detrital zircon grains (3,200–600 Ma) deposited in the trench‐fill sediments. In addition, subduction‐related magmatismc. 500–400 Ma is recorded in the crust below proto‐Japan, which might have been the source for the younger detrital zircon grains. The peak metamorphic age was constrained by SHRIMP dating of the overgrowth rims, yielding Tournaisian ages of 347 ± 4 Ma, suggesting subduction in the early Carboniferous. Our results provide clear constraints on the initiation of subduction, accretion and the development of an arc‐trench system along the active continental margin of the South China craton and help to unravel the Palaeozoic tectonic history of proto‐Japan.

The Main Central Thrust zone along the Alaknanda and Dhauli Ganga valleys (Garhwal Himalaya, NW India): Insights into an inverted metamorphic sequence

New tectono-metamorphic data along the Alaknanda-Dhauli Ganga valleys in the Garhwal Himalaya (NW India) are presented focusing on sheared rocks of the Main Central Thrust zone (MCTz), one of the first order regional structures in the Himalaya. In regard of large hot orogens (LHOs), the MCTz corresponds to the base of the low-viscosity infrastructure. Our new findings support that the MCTz is a c. 4 km thick mylonitic zone, affecting different lithologies and encompassing a possible (previously defined) protolith boundary. Four selected mylonitic samples, at different structural position along the MCTz, were studied with a multi-disciplinary approach in detail, supporting that an inverted metamorphic field gradient, with temperature (T) and pressure (P) ranging from c. 500 up to c. 700 °C and c. 0.50–0.60 GPa up to >1.0 GPa, was present. This inverted P-T array is also discernable by the distribution of Al-rich minerals (e.g. chloritoid, staurolite, kyanite) in the intercalation of sheared quartzite. In situ U-(Th)-Pb monazite geochronology indicates that the high-T shearing in the upper part of the MCTz mylonites developed from c. 20 Ma on after a stage of prograde metamorphism that lasted until c. 23 Ma.The integration of our data with available structural, metamorphic and geochronological information along the MCTz profile, in the study area, sheds light on the development of the associated inverted metamorphic sequence (IMS). These data carry also empirical finite patterns to be tested against theoretical models investigating the possible roles of viscous heating and “tectonic overpressure” in the development of the IMS in the Himalaya, and possibly in LHOs in general. Both phenomena, if have been active at all, are not obvious in regard of the current state of geothermobarometric methods. On the contrary, a model for the tectonic assembly of the IMS is favored. The anomalous apparent P and T inverted field gradients are interpreted to be the result of the differential and diachronous transport along a shear zone experiencing heterogeneous vertical thinning linked to the non-coaxial flow.

Zircon U–Pb geochronology and clockwise P–T evolution of garnet-bearing migmatites from the Qinling complex in the Weiziping area of the Qinling Orogen, Central China: Implications for thermal relaxation after crustal thickening

Zircon U–Pb dating and phase equilibrium modeling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 system were carried out to garnet-bearing migmatites from the Qinling Complex in the Weiziping area of the Qinling Orogen, Central China, to constrain their metamorphic P–T path and elucidate the tectonic setting and geodynamic mechanism of Early to Middle Devonian high-temperature (HT) metamorphism in the North Qinling Belt. The mesosomes of the migmatites are composed of garnet-bearing biotite–hornblende–plagioclase and biotite–plagioclase gneisses, and the leucosomes can be divided into two types: wide leucosomes that were deformed, and narrow penetrative leucosomes. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb analyses yielded weighted mean 206Pb/238U ages of 398.8 ± 4.7, 394.6 ± 2.6, 399.5 ± 7.3, 372.5 ± 4.2, 370.8 ± 4.3, and 368.5 ± 4.2 Ma for metamorphic zircons within the sampled mesosomes, and a weighted mean age of 408.7 ± 5.7 Ma for igneous zircons within a deformed leucosome. Based on the published data and the results in this study, the timing of granulite-facies metamorphism is constrained to be ca. 409–395 Ma. The mesosome sample (garnet-bearing biotite–hornblende–plagioclase gneiss) has a peak mineral assemblage of garnet, biotite, plagioclase, hornblende, quartz, ilmenite, and melt, which equilibrated at P–T condition of 8.5–9.7 kbar at 745–820 °C as constrained by isopleths of XAn and XPy in a P–T pseudosection, suggesting that these gneisses underwent medium-pressure granulite-facies metamorphism. Conventional garnet–biotite–plagioclase–quartz (GBPQ) geothermobarometry yielded P–T conditions of 703 °C at 10 kbar and 685–748 °C at 7.1–8.9 kbar, which correspond to prograde and retrograde metamorphic stages, respectively. On the basis of these P–T results, the studied samples document a clockwise P–T path characterized by temperature increase and pressure decrease during the prograde stage, and temperature decrease and pressure decrease during the retrograde stage. Ti-in-zircon thermometry yielded a temperature range of 781–793 °C for metamorphic zircons from the mesosomes, further demonstrating that they underwent granulite-facies metamorphism at ca. 409–395 Ma. The prograde metamorphic stage characterized by temperature increase and pressure decrease suggests that thermal relaxation after crustal thickening has induced the partial melting and migmatization. Thus, the crustal thickening that may be caused by continental collision between the North China Block and the South China Block should occur no later than ca. 409 Ma.

Petrology, Metamorphic P-T Paths and Zircon U-Pb Ages for Paleoproterozoic Mafic Granulites from Xuanhua, North China Craton

The studied mafic granulites are located at Xiwangshan, Xuanhua region in the north of the Trans-North China Orogen (TNCO), occurring as lens within tonalite-trondhjemite-granodiorite (TTG) gneisses in the eastern part of the Xiwangshan area. The rocks contain the representative granulite-facies minerals such as garnet, clinopyroxene, orthopyroxene, plagioclase, amphibolite, rutile and quartz, and also well-developed melt pseudomorph and antiperthite. Although the prograde metamorphic stage (M1) cannot be retrieved due to rare preservation of pre-peak-stage mineral associations, three distinct mineral assemblages that formed in different metamorphic stages can be identified, based on petrography and mineralogy characteristics. The peak stage (M2) is characterized by Grt2+Cpx2+Amp2+Pl2+Rt+melt pseudomorphs, and a post-peak decompression stage (M3) contains a mineral assemblage of Grt3+Opx3+ Cpx3+Amp3+Pl3, while a later-retrogression stage (M4) is featured by coronas of Amp4+Pl4 surrounding garnet. By calculating metamorphic P-T conditions using THERMOCALC, stage M2 was constrained to be 13.2–14.8 kbar and 1 050–1 080 °C, and M3 recorded P-T conditions of 5.7–7.3 kbar and 825–875 °C, while M4 yielded P of ~5 kbar and T of ~660 °C, consistent with amphibolite facies metamorphism. Taking into account of all these petrological data, we propose that the mafic granulite experienced a high-pressure (HP) and ultra-high temperature (UHT) granulite-facies metamorphism during the peak metamorphism, which was accompanied with a clockwise P-T path. U-Pb dating of metamorphic zircons in the granulites yields two groups of ages at 1 853±14 and 1 744±44 Ma, respectively. We suggest that the older age corresponded to the HP-UHT metamorphism, while the younger age represented an retrograde metamorphic event during cooling.

Metamorphic evolution and tectonic significance of ultrahigh‐temperature mafic granulites in the southern part of the Shiguai area, Daqingshan, China

Ultrahigh‐temperature (UHT) mafic granulites in the southern part of the Shiguai area in Daqingshan, China, crop out within the Shanheyuan Gneiss of the Khondalite Belt (KB), a Paleoproterozoic tectonic belt in the Western Block of the North China Craton (NCC). The UHT mafic granulites occur within hypersthene‐bearing dioritic–granitic gneiss as irregular lenses and deformed dike swarms. Four metamorphic generation assemblages (M1–M4) are recognized in the studied samples. M1 is defined by the enclosed orthopyroxene and clinopyroxene within the “red eye” corona of the garnet, representing the prograde metamorphic stage. M2 (UHT granulite‐facies) is marked by the appearance of “red eye” corona of the garnet, with mineral assemblages of grt–cpx–opx–pl–ilm–q. M3 is defined by spinel‐rich symplectites between garnet and orthopyroxene, including grt–opx–pl–spl–mt–ilm. M4 (amphibolite facies) is featured by the development of amphibole. Phase equilibrium modelling and some critical thermobarometers are combined to constrain the P–T evolution of the studied samples. The P–T conditions for the M1–M4 are respectively estimated at ~8 kbar/~771–852°C, ~9.4–10.0 kbar/~930–965°C, ~5.0–5.4 kbar/~865–910°C, ~5.0–5.2 kbar/~675–682°C, which define a clockwise P–T path, involving near‐isothermal decompression following the peak metamorphism. The P–T path indicates that the rocks underwent a sequence of tectonothermal processes that started with crustal thickening (M1–M2), followed by rapid crustal uplift (M3) and cooling (M4). This process is inferred that the metamorphism of the UHT mafic granulites was associated with rapid exhumation and uplift that followed the continent–continent collision and amalgamation of the Yinshan and Ordos blocks. This amalgamation formed the Paleoproterozoic KB in the Western Block of the NCC.

P–T evolution and episodic zircon growth in barroisite eclogites of the Lanterman Range, northern Victoria Land, Antarctica

AbstractPrograde P–T–t paths of eclogites are often ambiguous owing to high variance of mineral assemblages, large uncertainty in isotopic age determinations and/or variable degree of retrograde equilibration. We investigated these issues using the barroisite eclogites from the Lanterman Range, northern Victoria Land, Antarctica, which are relatively uncommon but free of retrogression. These eclogites revealed three stages of prograde metamorphism, defining two distinctive P–T trajectories, M1–2 and M3. Inclusion minerals in garnet porphyroblasts suggest that initial prograde assemblages (M1) consist of garnet+omphacite+barroisite/Mg‐pargasite+epidote+phengite+paragonite+rutile/titanite+quartz, and subsequent M2 assemblages of garnet+omphacite+barroisite+phengite+rutile±quartz. The inclusion‐rich inner part of garnet porphyroblasts preserves a bell‐shaped Mn profile of the M1, whereas the inclusion‐poor outer part (M2) is typified by the outward decrease in Ca/Mg and XFe (=Fe2+/(Fe2++Mg)) values. A pseudosection modelling employing fractionated bulk‐rock composition suggests that the eclogites have initially evolved from ~15 to 20 kbar and 520–570°C (M1) to ~22–25 kbar and 630–650°C (M2). The latter is in accordance with P–T conditions estimated from two independent geothermobarometers: the garnet–clinopyroxene–phengite (~25 ± 3 kbar and 660 ± 100°C) and Zr‐in‐rutile (~650–700°C at 22–27 kbar). The second segment (M3A–B) of prograde P–T path is recorded in the grossular‐rich overgrowth rim of garnet. Apart from disequilibrium growth of the M3A garnet, ubiquitous overgrowth of the M3B garnet permits us to estimate the P–T conditions at ~26 ± 3 kbar and 720 ± 80°C. The cathodoluminescence (CL) imaging of zircon grains separated from a barroisite eclogite revealed three distinct zones with bright rim, dark mantle and moderately dark core. Eclogitic phases such as garnet, omphacite, epidote and rutile are present as fine‐grained inclusions in the mantle and rim of zircon, in contrast to their absence in the core. The sensitive high‐resolution ion microprobe U–Pb dating on metamorphic mantle domains and neoblasts yielded a weighted mean 206Pb/238U age of 515 ± 4 Ma (tσ), representing the time of the M2 stage. On the other hand, overgrowth rims as well as bright‐CL neoblasts of zircon were dated at 498 ± 11 Ma (tσ), corresponding to the M3. Average burial rates estimated from the M2 and M3 ages are too low (&lt;2 mm/year) for cold subduction regime (~5–10°C/km), suggesting that an exhumation stage intervened between two prograde segments of P–T path. Thus, the P–T–t evolution of barroisite eclogites is typified by two discrete episodes with an c. 15 Ma gap during the middle Cambrian subduction of the Antarctic Ross Orogeny.

Tracking the prograde P\u2013T path of Precambrian eclogite using Ti-in-quartz and Zr-in-rutile geothermobarometry

A Fe–Ti-rich garnet, clinopyroxene, and quartz eclogite sample from the 1.0 Ga Sveconorwegian orogen, SW Sweden, contains abundant quartz, rutile, and zircon in distinct micro-textural sites: garnet core, garnet rim, and matrix, constituting an ideal case for investigation of the behavior of Zr-in-rutile and Ti-in-quartz at high-pressure and temperature. A P–T path, peaking at 16.5–19 kbar and 850–900 °C, has been constrained independently for the same rock by pseudosection modelling; input pressures from this model were used for trace element geothermometry of each garnet micro-textural domain. Trace element thermo(baro)metry, based on in situ Secondary Ion Mass Spectrometry analyses of Ti contents in quartz and Zr contents in rutile, yields P–T estimates of progressive crystallization of quartz and rutile along the prograde metamorphic path. For inclusions in the garnet cores, Zr-in-rutile geothermometry yields 700–715 °C and Ti-in-quartz 620–640 °C at 7 kbar. For inclusions in the garnet rims, temperature estimates are 760–790 °C (Zr-in-rutile) and 740–920 °C (Ti-in-quartz) at 12–18 kbar. Finally, matrix rutile records 775–800 °C and locally ~ 900 °C, and quartz records temperatures up to 900 °C at 18 kbar. Ti-in-quartz estimates for the metamorphic peak (inclusions in the garnet rims and matrix) conform to the pseudosection, but appear too low for the early prograde stage (garnet cores), possibly due to lack of equilibrium at T < 700 °C. The pseudosection shows that rutile was produced by continuous ilmenite breakdown during the early stages of prograde metamorphism, a reaction that was completed at ~ 730 °C. Rutile grains in the garnet rims and the matrix grew subsequently larger by recrystallization of previously produced rutile. However, recrystallized rutile does not predominantly record peak temperatures, but instead yield 745–840 °C between 12 and 18 kbar. In the pseudosection, this temperature range broadly coincides with a stage during which (Ti-bearing) hornblende was consumed and clinopyroxene produced (i.e., dehydration); the Zr contents thus appear to reflect the last stage of efficient rutile recrystallization, catalysed by fluids released by the dehydration of hornblende preceding the metamorphic peak. Concurrently, combination of the isopleths for Ti content in quartz and Zr content in rutile (i.e. independent from pseudosection modelling) yields pressure and temperature conditions in almost perfect agreement with the P–T path as deduced from the pseudosection. The variation in Ti concentration in quartz is small regardless of crystal size, and the Ti-in-quartz geothermometer provides both precise and accurate peak temperatures of 875–920 °C, without a significant diffusional reequilibration. The lack of significant Ti diffusion in quartz is consistent with an inferred short residence time at high temperature. This study illustrates that Zr-in-rutile and Ti-in-quartz geothermobarometry can robustly constrain prograde P–T conditions and yield further insights into recrystallization processes at high temperature. The combination of these methods and integration of the results with pseudosection modelling is a versatile tool for investigating the petrologic history of high-grade rocks.

Metamorphic P–T evolution of the Gotsu blueschists from the Suo metamorphic belt in SW Japan: Implications for tectonic correlation with the Heilongjiang Complex, NE China

In the Gotsu area of the c. 200 Ma high-P/T Suo metamorphic belt in the Inner Zone of southwest Japan, blueschists occur as lenses or layers within pelitic schists. Prograde, peak, and retrograde stages are distinguished in the blueschists, and the prograde and the peak metamorphic conditions are determined using pseudosection modelling in the NCKFMASHO system. The prograde metamorphic stage is defined by inclusions in porphyroblastic epidote and glaucophane, such as phengite, chlorite, albite, epidote and glaucophane/winchite, and the estimated metamorphic conditions are <325 °C and < 4–5 kbar at the boundary between the glaucophane schist facies and the greenschist facies. The peak metamorphic stage is well-defined by the schistosity-forming minerals, i.e. epidote, glaucophanic amphibole, phengite, and chlorite, suggesting the glaucophane schist facies conditions of 475–500 °C and 14–16 kbar. Actinolite/magnesiohornblende, chlorite, and albite replacing the peak stage minerals suggest the retrograde metamorphism into the greenschist facies. The metamorphic facies series of the Suo belt is defined by pumpellyite–actinolite facies to epidote–blueschist facies, and it has a relatively lower-P/T compared with the c. 300 Ma Renge belt in the Inner Zone of southwest Japan, which is defined by a sequence of lawsonite–blueschist facies to glaucophane–eclogite facies. The P– $$ {M}_{{\mathrm{H}}_2\mathrm{O}} $$ pseudosection and water isopleth show that the rocks were dehydrated during the initial stage of the exhumation and remained in water-saturated conditions. Similarities of the detrital zircon and peak metamorphic ages of the blueschists from the Suo metamorphic belt in southwest Japan and the Heilongjiang Complex in northeast China suggest that both metamorphic belts were probably formed in the same Paleo-Pacific subduction system in the Late Triassic to Jurassic period.