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In this paper, we report a new metamorphic age for kyanite-bearing garnet-biotite gneiss collected from Akebono Rock, which is located on Prince Olav Coast, in the Lutzow-Holm Complex (LHC), East Antarctica. The metamorphic P–T path estimated from pseudosection modeling, together with garnet compositional zoning and experimentally calibrated geothermobarometry, suggests that the gneiss underwent a clockwise amphibolite-facies P–T history. Two populations of U–Pb zircon ages of 1121–1014 Ma and 972–904 Ma (n = 65) were identified, with the latter population having a weighted mean age of 937 ± 6 Ma. The younger population was dated from overgrowth rims and single zircon grains interpreted as metamorphic in origin. Ti-in-zircon thermometry supports crystallization of these zircon domains and grains at a temperature close to the metamorphic peak of 642 °C at 937 Ma. Monazite EMP dating from the other four samples show an age between 977 and 917 Ma. These lines of evidence indicate that prograde metamorphism of the Akebono Rock gneiss occurred during the Tonian, that is, substantially earlier than the LHC regional metamorphic age of late Neoproterozoic to Cambrian. Our results indicate that a fundamental revision of the sequence of metamorphic events in the Prince Olav Coast area of the LHC is required. We also identified three possible relationships between Akebono Rock and other geological units that record Tonian metamorphism, although we cannot determine which relationship is most likely. In particular, linear magnetic anomalies were identified near the boundary between the LHC and the Western Rayner Complex/inland nunataks of western Enderby Land, suggesting the presence of geological discontinuities.

The Qinling Complex is the only continental nucleus that outcrops in the North Qinling Orogenic Belt (NQOB) and preserves valuable evolutionary information of NQOB during the early Paleozoic. There are many high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic rocks distributed in the northern and southern parts of the Qinling Complex. Whether the central part (namely the Qihe tectonic slab) underwent a similar HP–UHP metamorphism and what the relationship among different tectonic slabs of Qinling Complex is, are the key to revealing the formation and exhumation process of these HP–UHP rocks. Four episodes of deformation (D1–4) have been recognized in the Qinling Complex. The metamorphic pressure‐temperature conditions of syn-D3 amphibolite in the Qihe tectonic slab are limited to 3.3–6.4 Kbar and 479–538 ℃ based on equilibrium modeling, corresponding to an extensional tectonic setting. The U-Pb dating of the amphibolite yielded a protolith age of 714 ± 46 Ma, an eclogite-facies metamorphic age of 514 ± 4 Ma, and two stages of retrograde metamorphism at 462 ± 3 Ma and 418 ± 5 Ma. The parametamorphic rocks of different tectonic slabs have similar detrital zircon age spectra, which is a very different pattern from adjacent areas of the Qinling Complex. The consistency in the protolith ages, peak metamorphic ages, two stages of retrograde metamorphism, and detrital zircon age from different tectonic slabs in the Qinling Complex indicate that HP–UHP metamorphic rocks from Qinling Complex formed during the same tectonic event. Therefore, the Qinling Complex evolved in deep subduction and underwent eclogite facies metamorphism, followed by superimposed deformation structures and retrograde metamorphism during the two stages of exhumation.

Mechanisms of syn-collision magma generation are unclear due to the uncertainty in defining the term collision. We address this issue by presenting detailed petrographic, chronological, and geochemical data for the Tanggula Batholith located immediately to the north of the Longmu Tso–Shuanghu suture zone (LSSZ) in northern Tibet. Zircon LA–ICPMS U–Pb dating suggests that the Tanggula Batholith was emplaced at 235–226 Ma. Samples from the batholith show variable SiO2 of 66.6–80.6 wt% and are characterized by high normative corundum (1.09–4.35 wt%) and high aluminum saturation index (A/CNK = 1.05–1.40). These samples exhibit high initial 87Sr/86Sr (0.7165–0.7214), negative whole-rock eNd(t) (−10.1 to −8.5), eHf(t) (−8.7 to −6.3), enriched Pb isotope compositions, and variable zircon eHf(t) values (−29.9 to +5.6), as well as variable zircon δ18O value (6.90–9.22‰). These signatures are characteristic of S-type granites, and were derived from anataxis of metasedimentary rocks from the Qiangtang Terrane with input from mantle-derived components. We propose that the Tanggula Batholith was emplaced in a syn-collisional setting following closure of the Longmu Tso–Shuanghu Ocean along a north dipping subduction zone based on: the timing of high-pressure, low-temperature metamorphism at 244–221 Ma marking collision between Southern and Northern Qiangtang; the timing of exhumation of metamorphic rocks at ~223–204 Ma; and, bimodal volcanism (225–202 Ma) associated with slab breakoff at 220–215 Ma.

South Altyn Tagh contains ultrahigh-pressure (UHP) terranes that have been exhumed from ~300 km mantle depth. Previous zircon U–Pb geochronology has yielded an eclogite-facies age of ca. 500 Ma and a high-pressure (HP) granulite-facies retrograde age of ca. 450 Ma in the Jianggalesayi area in the western segment of South Altyn Tagh. However, in the eastern segment (Yinggelisayi and Danshuiquan localities), an age range of 500–480 Ma has been determined, and it remains uncertain as to whether this age range represents the timing of the peak metamorphic stage or the retrograde overprint. Our study of newly discovered retrograde eclogite in the Danshuiquan locality shows that it underwent three stages of metamorphism, under eclogite-facies, HP granulite-facies and amphibolite-facies P–T conditions of 2.5–4.0 GPa and 870–1050 °C, 2.0–1.4 GPa and 830–940 °C, and 0.7–1.3 GPa and 704–880 °C, respectively. The decompression-dominated P–T path evolved mainly after crossing the solidus, indicating marked retrograde modification under melt-bearing conditions. LA–ICP–MS and SIMS zircon U–Pb dating yielded ca. 500 Ma eclogite-facies and ca. 484 Ma granulite-facies retrograde ages and a later retrograde age of ca. 452 Ma. The clockwise P–T–t path indicates rapid exhumation from eclogite-facies to granulite-facies within around 16 Myr, which is faster than that of the UHP rocks in the western segment. Thus, the HP–UHP rocks in South Altyn Tagh suggest a differential exhumation process for the eastern and western segments. The distinct HP–UHT metamorphism and rapid exhumation of (U)HP rocks in the eastern segment likely resulted from local mantle heating. The continuing P–T evolution of the (U)HP rocks under UHT conditions during exhumation led to a pervasive granulite-facies overprint in the eastern segment of the South Altyn Tagh. The rapid exhumation recorded in the eastern segment provides valuable insights into the exhumation mechanism of ultra-deep subducted UHP terranes.

The evolution of the Mozambique Ocean spans the Rodinia and Gondwana supercontinent cycles playing a key role in Earth history. Integrated petrological, geochronological, geochemical, and isotopic data on the Neoproterozoic units from the Lhasa terrane, Tibet, constrain the magmatic and metamorphic evolution of the ocean. Our results reveal two magmatic (ca. 695 Ma and ca. 671 Ma) and two metamorphic (ca. 700 Ma and ca. 650 Ma) pulses. Gabbros define the first magmatic suite. They have within-plate affinities and experienced crustal contamination. Their positive zircon eHf(t) (+1.6 to +9.6) and whole-rock eNd(t) (+2.7 to +6.1) values are indicative of a depleted mantle source. Gabbros from the second magmatic event belong to the tholeiitic series, lack evidence for crustal contamination, and are geochemically similar to E-MORB. They have negative to positive zircon eHf(t) (−3.2 to +1.5) and low positive whole-rock eNd(t) (+1.4 to +2.0) values, and were probably derived from an E-MORB-type enriched mantle source, slightly modified by subduction components. Evidence for the metamorphic events (ca. 700 and ca. 650 Ma) was obtained from U Pb dating of zircon rims of marbles and gabbros, which are characterized by low Th/U ratios (0.01–0.07) indicative of a metamorphic origin. Integrating previous studies with the data presented in this contribution, we suggest that the history of the Lhasa terrane was linked to the evolution of the Mozambique Ocean. The older mafic magmatism (ca. 695 Ma) and metamorphism (ca. 700 Ma) formed during terrane accretion processes along an active continental margin to the ocean. The relatively younger mafic magmatism (ca. 671 Ma) provides a record from an oceanic fragment that was metamorphosed (ca. 650 Ma) during closure of the Mozambique Ocean.

The North Qinling Orogen in central China contains a typical continental high- and ultrahigh-pressure (HP-UHP) metamorphic belt, but its metamorphic evolution remains controversial. We report a combined investigation on petrology, geochronology and phase modelling for felsic granulite (samples SN1504 & SN1517) and garnet clinopyroxenite (sample SN1505) from the Songshugou area. Four stages of metamorphic evolution are constrained: (i) A possible prograde from HP amphibolite facies to peak UHP eclogite facies stage (~3–8 GPa/700–1080 °C) recorded by the chemical zoning of garnet core and mantle in sample SN1504 and also the presence of supersilicic titanite in sample SN1505. (ii) A post-peak decompression evolution is recorded by the formation of coronary garnet and plagioclase around kyanite in sample SN1504 and high-Ti amphibole and plagioclase in sample SN1505, including the evolution from eclogite facies to HP–ultrahigh temperature (UHT) conditions of 1.3–2.2 GPa/900–980 °C for sample SN1504 and 1.0–2.1 GPa/960–1010 °C for SN1505, respectively. (iii) Further decompression is revealed from the growth of corundum + spinel in local domains in sample SN1504, suggesting a low-pressure (LP) condition of ~0.8 GPa/950 °C. (iv) A subsequent cooling evolution is recorded by thin films of K-feldspar + quartz + biotite in sample SN1504 and low-Ti amphibole in sample SN1505, indicative of melt crystallization towards the solidus with conditions of 830–770 °C/0.55–0.8 GPa and 880–820 °C/0.8–0.9 GPa, respectively. Zircon and monazite U-Pb dating yields three metamorphic ages of c. 521 Ma, c. 503 Ma and c. 500–480 Ma, which are interpreted to represent the prograde HP amphibolite facies stage, the peak eclogite facies stage and the late cooling stage, respectively. The metamorphic P–T–t path suggests a complete history of deep continental subduction to depths of >90–250 km, followed by rapid exhumation to crustal depths for the Qinling micro-continent during the early Proto-Tethys evolution.

As one of the oldest crustal blocks in Southern Peninsular India, the Coorg Block has been the focus of investigations related to crustal evolution in the early history of the Earth with implications on Mesoarchean plate tectonic processes. The Coorg Block is dominantly composed of arc magmatic rocks and bordered on the north by the Mercara Suture Zone, a Mesoproterozoic subduction-collision zone hosting extruded high-pressure and ultra-high temperature metapelitic and meta-mafic rocks. Here, we report the occurrence of a dismembered gabbro-anorthosite complex corresponding to a layered intrusion from the northern margin of the Coorg Block. We present results from an integrated study on the petrology, mineral chemistry, P-T phase equilibria, whole-rock geochemistry, and zircon and monazite U–Pb geochronology to understand the magmatic and metamorphic evolution of the layered intrusion and associated rocks. The geochemical data suggest that the parental magma was generated within a suprasubduction zone setting. Phase equilibrium modelling of garnet-bearing metagabbro from the Coorg Block suggests metamorphic P–T range of 10.5-11 kbar at 1000-1100°C corresponding to ultra-high temperature conditions. The zircon U-Pb data from the various rock types of the mafic-ultramafic suite yield weighted mean ages at 3176 Ma, 3174 Ma, 3143 Ma, and 3124 Ma whereas the associated charnockite shows a slightly older age of 3319±12 Ma. Metamorphic zircon from charnockite yield an age of 3101 Ma, close to the monazite age of 3110±24 Ma from the same rock, constraining the timing of metamorphism as Mesoarchean, and marking the collisional event between the Coorg Block and the Dharwar Craton along the Mercara Suture Zone. We propose a tectonic model involving southward subduction of the Western Dharwar Block beneath the northern margin of the Coorg Block that can explain the extensive arc magmatism and suprasubduction zone rock suites, followed by ocean closure and collision along the Mercara Suture Zone, accompanied by high P-T metamorphism.

Mashan Complex in the southeastern margin of the Jiamusi Massif has traditionally been interpreted to be the oldest stratigraphic sequence in eastern NE China. Here we investigated the Mashan Complex in the Ximashan area and present lots of new LA–ICP–MS zircon U–Pb, petrological and mineralogical chemical data, in addition with phase equilibria modeling, to constrain the timing of metamorphism, P–T conditions and the P–T path of the Mashan Complex, and furthermore to deduce the crustal evolution of the Jiamusi Massif. The protoliths of the metapelites from the Mashan Complex in the Ximashan area are clay rocks and sandstones, whose provenance is dominantly felsic rocks which formed from a continental island arc. Three metamorphic generation assemblages (M2–M4) are recognized in the studied samples: a peak granulite facies stage, a post-peak near-isothermal decompression stage, and a late near-isobaric cooling stage, which together define a clockwise P–T path. It reveals important information related to the collision orogeny, subsequent uplift, and post-collision process. Zircon U–Pb dating of two metapelitic samples yielded metamorphic ages of the older group varying from 535 ± 6 to 480 ± 5 Ma, which can be perfectly correlated with the age range (501 ± 6 to 498 ± 6 Ma) of magmatic activities recorded by the gneissic granite sample. The data presented here, together with those published, allow us to preliminarily infer that the peak of the granulite-facies metamorphism in the Ximashan area of the Jiamusi Massif might have occurred at 530–500 Ma. The younger of the metamorphic ages (476–453 Ma) should be regarded as the age of late stage. Therefore, a collision orogeny regime is favored to interpret the metamorphic evolution of metapelites of the Mashan Group in the Mashan area.

Records of the thermal-age path of orogenic rocks are crucial in understanding subduction and collisional processes between a continent and an arc. Tectonic events and thermochronological data of the Taiwan Orogen have been extensively investigated. We use the zircon U Pb youngest ages as the (initial) subduction age, the Raman Spectroscopy of Carbonaceous Material, 40Ar/39Ar age of mica porphyroblasts, whole-rock Lu Hf isochron age and thermal results as prograde peak metamorphic condition during subduction and basal accretion, and the apatite fission-track data for cooling age of the Taiwan Orogen to illuminate the subduction and collisional processes between the Eurasian Plate and the Luzon Arc. The Hsuehshan Range Nappe in the west of the Taiwan Orogen, whose subduction was initiated at 12 Ma, reached peak metamorphism with a temperature of ~250 °C to 490 °C at a depth of 8 km during the interval 6–2.5 Ma, mainly by basal superimposed accretion. The isothermal dwelling during peak metamorphism condition of the Hsuehshan Range suggests that tectonic underplating through basal superimposed accretion contribute significantly to the formation of the Taiwan Orogen, and led to a sudden increase of subduction and decollement depths, and slow downward thickening of the orogenic wedge and mountain root growth. Located in the east of the Taiwan Orogen, the Yuli Beltsubduction was initiated at 15.4–16 Ma, and attained peak metamorphism at a depth of 40 km, with a temperature of 550–560 °C and a pressure of 10.5–13 kbar during the interval 12.2–4.4 Ma due to basal superimposed accretion. The exhumation of the Hsuehshan Range commenced at ~2.5 Ma, which is later than that of the Yuli Belt at ~3.4 Ma. Such distinct and different thermal-age paths of the Hsuehshan Range and the Tananao Complex indicate that different tectonic wedges in the Taiwan Orogen are formed through different subduction and accretional processes.

Felsic rocks of the Erzgebirge in the Central European Variscides experienced ultra-high pressure metamorphism at ∼340 Ma, followed by nearly isothermal decompression during channel like rapid exhumation within ∼10 Ma. Despite the general time frame of exhumation and emplacement within mid-crustal levels is known, available geochronological data do not provide a detailed timescale of individual stages of the entire process. Addressing this problem we combined white mica 40Ar/39Ar ages obtained from multi-grain step heating and multiple single grain total fusion experiments with mineral chemical, structural and tectono-metamorphic constraints. Gneisses of the channel center are characterized by E-W elongated white mica exclusively aligned in the continuous foliation. The rocks of the channel contact zone and in the hanging and footwall contain NW-SE stretched white mica parallel to the foliation, but also components of ductile to brittle-ductile shear bands truncating the main foliation. Well defined weighted plateau ages of all samples range between 340.0 ± 1.1 Ma and 327.9 ± 1.3 Ma, confirming previously published data, but represent presumably mixed ages due to multi-phase deformation. Single grain age distributions, in contrast, exhibit two statistically significant age peaks at 338.6 ± 0.2 Ma and at 332.8 ± 0.3 Ma. Rocks without shear bands exclusively contain white mica belonging to the older age fraction, whereas rocks with shear bands show a broad age scatter including both age fractions. We interpret these age distributions as being independent of closure temperature, but reflecting different dynamic recrystallization events. The older age fraction reflects the formation of the main foliation during west-directed emplacement of the ultra-high pressure rocks in mid-crustal levels at ∼339 Ma, whereas the shear band related neo- or recrystallization is responsible for the younger age fraction dating the final transport in the upper crust at ∼333 Ma.

Timing of the major Baiyun gold deposit (~ 70 t Au) in the northeastern North China Craton is controversial. A textural and geochronological study of high-grade gold ores identifies three generations of xenotime (Xtm-1, Xtm-2 and Xtm-3), even in single grains. The oldest Xtm-1, in grain cores, is of metamorphic origin, with very high U and U/Th ratios, and hump-shaped, HREE-enriched patterns with maximum normalized values at Yb and large negative Eu anomalies. It is dated at 1856 ± 6 Ma, consistent with its crystallization during regional metamorphism related to cratonization. In contrast, 244 ± 2 Ma Xtm-2 overgrowths are of hydrothermal origin, having low U and U/Th ratios and hump-shaped, MREE-enriched patterns with maximum normalized values at Ho and negligible Eu anomalies. Xtm-2 records a previously unrecognized Middle Triassic hydrothermal episode. Xtm-3, in equilibrium with native gold-hosting pyrite and rutile, is also hydrothermal. The Xtm-3 domains have similar U, U/Th ratios, and MREE-enriched patterns to Xtm-2, but a younger crystallization age of 231 ± 1 Ma. Associated rutile grains that show close textural associations with auriferous pyrite and native gold have an in situ SIMS U–Pb an intercept age of 229 ± 4 Ma, consistent with that of Xtm-3. Collectively, these reliable ages indicate that gold mineralization at Baiyun occurred at ca. 230 Ma, predating the emplacement of granite intrusions by more than 3 million years. Both the Middle Triassic hydrothermal event (~244 Ma) and the Late Triassic gold mineralization event (~230 Ma) lie within the temporal window of collisional tectonics in the host orogen. In combination, the new geochronology and fluid-inclusion and geological data support Bayun’s classification as an orogenic gold deposit.

Fluid is a key agent for the mass transfer between the subducting slab and the mantle wedge, which greatly affects the evolution of the crust-mantle system at convergent plate boundaries. A geochemical study was carried out for coesite-bearing jadeite quartzite and its country rock granitic gneiss from the Dora Maira Massif in the Western Alps. The results provide new insights into the composition of metamorphic fluids and fluid-rock interaction processes in the continental subduction zone. Coesite inclusions are found for the first time in metamorphic zircons of the granitic gneiss. The ultrahigh-pressure (UHP) metamorphism was dated to occur at 34.7 ± 0.3 Ma, confirming that the country rock experienced the UHP metamorphism synchronously with the whiteschist and jadeite quartzite. The jadeite quartzite occurs as layers and boudins within the coesite-bearing whiteschist, and shows similar whole-rock REE distribution patterns to both granitic gneiss and whiteschist. In addition, relict domains of magmatic zircon in the three types of UHP metamorphic rocks exhibit similar U-Pb ages and δ18O values, indicating that they have the same protolith of granites. However, metamorphic zircons in the jadeite quartzite show significantly lower δ18O values of 6.1–7.3‰ than the relict magmatic domains of 9.4–10.6‰. Furthermore, the jadeite quartzite has whole-rock δ18O values of 7.9–8.7‰, significantly lower than those of 9.5–10.9‰ for the granitic gneiss but higher than 6.5–7.9‰ for the whiteschist. These differences suggest that the jadeite quartzite was probably formed through metasomatism of metagranite by external fluids with relatively low δ18O values. The jadeite quartzite shows high δ56Fe values of 0.69–0.87‰, considerably higher than those of 0.18–0.38‰ for the granitic gneiss, but falling within the range of 0.32–1.03‰ for the whiteschist from the same outcrop. The Fe isotope modeling results suggest that the metasomatic fluids responsible for the formation of jadeite quartzite were possibly derived from the host whiteschist under UHP metamorphic conditions. Such fluids are enriched in Si, Al and Mg, and have high δ56Fe but low δ18O values. Therefore, the jadeite quartzite records the action of metamorphic supercritical fluids at the subarc depth. If the metasomatized rocks in the continental subduction zone would dehydrate to generate fluids with variable geochemical compositions due to multistage fluid-rock interactions, the fluids could further infiltrate the surrounding rock and possibly the mantle wedge to result in further geochemical transfer from the subducting slab to the mantle wedge.

The Bundelkhand craton hosts Banded Iron Formation (BIF) bodies within the east–west trending Bundelkhand Tectonic Zone (BTZ). The BIFs near Mauranipur, BTZ, are massif and are composed of amphibole, quartz, magnetite, and garnet. P–T pseudosection analysis implies that the protolith of these BIFs were carbonate-bearing sediments that underwent metamorphism at 400–450 °C at 0.1–0.2 GPa to stabilize amphibole and garnet via a series of dehydration and decarbonation reactions forming 10–50 µm sized monazite grains in the process. The U-Th-Pb (n = 74) analyses by the electron probe microanalyzer yield three age clusters. The oldest mean age, 3137 ± 96 Ma, is retrieved from low-ThO2 monazites occurring as discrete grains within amphibole bands and is interpreted as the timing of monazite growth caused by fluid-rock interaction. The second age cluster yields an average age of 2567 ± 48 Ma from high-ThO2 domains that laced the low-ThO2 monazite core. Anhedral monazites occurring as inclusions within garnet yield an average age of 2538 ± 120 Ma. These 2500 Ma dates reflect monazite growth during prograde metamorphism. An age of 2168 ± 83 Ma is retrieved from monazite overgrowths in amphibole bands. The ~ 3200 Ma metamorphic event in the Bundelkhand craton is similar to those reported from the Singhbhum and the Dharwar cratons. The ~ 2500 Ma age, when compared with the available geochronological data, constrains the timing of Neoarchean-Paleoproterozoic tectonism and stabilization. The minor Meso-Paleoproterozoic (2100–1900 Ma) age may be related to extension and dyke emplacement.

Introduction: Basite intrusions are important in the process of decoding the history of the formation of folded areas. Within our study we studied in detail the peculiarities of the geological structure as well as mineralogical and petrological characteristics of the Keleinogubsky massif, one of the key objects to be studied when decoding the history of the development of the Belomorian mobile belt of the Fennoscandian shield. Methodology: The parameters of metamorphism expressed in the formation of corona structures in the centre of the Keleinogubsky massif were assessed using the TWEEQU multi-equilibrium thermobarometry method in the TWQ program. The conditions of metamorphic transformations that led to the formation of new mineral parageneses on the edge of the intrusion were calculated in the NCTiFMMnASHO system using the chemical composition of the rocks (the method of pseudosections) in Perple_X. Results and discussion: Mineralogical and petrological study of the Keleinogubsky massif of the Belomorian mobile belt showed that the rocks of the intrusion had various level of preservation, from intensively metamorphised rocks of the edges to those with the preserved relicts of the primary-magmatic minerals and corona structures of the rocks in the centre of the intrusion. One of the rock-forming minerals of the massif is olivine: magmatic olivine was preserved in the rocks in the centre of the massif and along its edges and metamorphic olivine was present only in the edges of the intrusion. Both types of olivine have similar magnesium content but metamorphic olivine is different as it has higher concentrations of Mn and contains inclusions of metamorphic minerals, such as diopside, amphibole, and orthopyroxene. The assessment of the conditions of the formation of metamorphic olivine showed that its crystallisation could occur at Т~900°С and Р ≥ 8 kbar. The assessment of the conditions of metamorphic transformations in the rocks in the centre of the massif demonstrated that relict early metamorphic orthopyroxene-clinopyroxene rims around the magmatic olivine were formed at T ~850–950°С and P = 3–4 kbar. Conclusions: As a result of the study of metabasites of the Keleinogubsky massif, two stages of granulite metamorphism were identified. The early metamorphic stage corresponding to the conditions of the granulite facies of low pressure (Т=850–950oС, Р=3–4 kbar) is expressed by the formation of corona structures preserved in the central part of the massif. The late granulite metamorphism of high pressure was recorded along the garnet rims of the central part of the intrusion and intensive metamorphic processing (Т=900 oС, Р ≥ 8 kbar) of the rocks of the massif edge with the formation of secondary olivine.

We present Early Cretaceous partial melting recorded by pelitic gneiss from the Nagasaki Metamorphic Complex, western Kyushu, Japan. The existence of the partially melted rock in the complex may im...

It is controversial about the tempo–spatial distribution and tectonic attributes of the Paleoproterozoic orogenic belts in the North China Craton (NCC). The Hongqiyingzi Complex in the northern margin of the NCC, comprising orthogneiss, supracrustal sequence and an ophiolitic melange, was investigated to have experienced multiple tectonic evolution from Paleoproterozoic to Mesozoic. However, the depositional age and metamorphic P–T path of the supracrustal sequence have not been well addressed. Garnet mica schists are investigated on the basis of petrography, mineral chemistry, phase equilibria modelling and zircon and monazite dating, which record an extreme case with four phases of metamorphism. The first-phase is characterized by high pressure (HP) granulite facies with the peak conditions of 10–11 kbar/>780 °C, which occurred at ~1.95 Ga. It is considered to correspond to a collisional orogeny with crust thickening widespread in the Paleoproterozoic orogenic belts of the NCC, followed by an exhumation with heating at ~1.92 Ga. The second-phase is marked by the overprinting of staurolite-bearing assemblages with the peak conditions of 6–7 kbar/610–630 °C, which occurred at ~1.85 Ga. It may correlate a separate orogeny occurred along the northern margin of the NCC, following a ~1.90 Ga oceanic subduction. Phase modelling suggests this overprinting over the first-phase HP granulites is mainly controlled by water infiltration. The third-phase andalusite-type and the fourth-phase greenschist facies metamorphism are confined to local equilibrium domains and their ages have not been well constrained. With a comparison of regional geology, the third-phase metamorphism is inferred to be related with the 320–290 Ma extension in the northern part of the NCC during Late Paleozoic, and the fourth-phase metamorphism is correlated with the 250–230 Ma collision occurred along the Solonker Suture Zone in the Central Asia Orogenic Belt during Early Mesozoic. Moreover, detrital magmatic zircons yield ages of 2.0–2.1 Ga, providing a definite upper limit on the deposition of the Hongqiyingzi supracrustal sequence.

Unravelling the genesis of metamorphosed mineral deposits can be complicated due to difficulties in separating between primary features and features that formed during the metamorphic overprint. Such uncertainty exists for stratabound and dolomite- and skarn-hosted Zn-Pb-Ag sulfide deposits in 1.89 Ga rocks in the Bergslagen lithotectonic unit (BLU) of Sweden, where a metasomatic vs. regional metamorphic origin for skarns has long been discussed. By integrating geological mapping with new lithogeochemical, mineralogical, and stable isotope data (C, O, S), we show that complexly zoned garnet and clinopyroxene skarns in the Sala area of the central BLU predate mineralization. Sphalerite-galena mineralization formed after the deposition of a younger, more Mn-rich ferroan diopside and andradite-grossular garnet, and is associated with phlogopite, tremolite-actinolite, chlorite, serpentine, and calcite. Mineralization in conjunction with a transition from high-T metasomatism to hydrolytic alteration is inferred. An average δ34SV-CDT of 1.6 ± 1.9‰ in sulfides is consistent with a primordial sulfur source. Trends defined by negative shifts in δ18OV-SMOW and δ13CV-PDB in dolomite and calcite are consistent with fluid infiltration at 300–500 °C. The alteration system is sharply truncated by unaltered, c. 1.89 Ga calc-alkaline granite and porphyritic intrusions, which along with F1 folding of the alteration zones and mineralization suggest that mineralization predate regional metamorphism. The Sala deposits are interpreted as Zn skarn deposits formed in conjunction with the emplacement of intrusions into penecontemporaneous marine volcanic and dolomitized limestone strata. The unusually Mg-rich mineralogy in relation to Zn skarns worldwide most likely reflects the dolomitic precursor.

A large number of gases are releasing from the medium-high temperature geothermal fields distributed along the large-scale strike-slip fault zones in the southeastern margin of the Tibetan Plateau. In this study, 11 hot spring water and the associated bubbling gas samples were collected along the Xianshuihe-Anninghe fault zones (XSH-ANHFZ) and analyzed for chemical and isotopic compositions. The $${\delta ^{18}}{{\rm{O}}_{{{\rm{H}}_2}{\rm{O}}}}$$ and $$\delta {{\rm{D}}_{{{\rm{H}}_2}{\rm{O}}}}$$ values indicate that hot spring waters are predominantly meteoric origin recharged from different altitudes. Most water samples are significantly enriched in Na+ and HCO3− due to the dissolution of regional evaporites, carbonates and Na-silicates. 3He/4He ratios of the gas samples are 0.025–2.73 times the atmospheric value. The 3He/4He ratios are high in the Kangding region where the dense faults are distributed, and gradually decrease with increasing distance from Kangding towards both sides along the Xianshuihe fault zones (XSHFZ). Hydrothermal fluids have dissolved inorganic carbon (DIC) concentrations from 2 to 42 mmol L−1, δ13CDIC from −6.9‰ to 1.3‰, $${\delta ^{13}}{{\rm{C}}_{{\rm{C}}{{\rm{O}}_2}}}$$ from −7.2‰ to −3.6‰ and Δ14C from −997‰ to −909‰. Combining regional geochemical and geological information, the CO2 sources can be attributed to deep-sourced CO2 from mantle and metamorphism of marine carbonate, and shallow-sourced CO2 from the dissolution of marine carbonate and biogenic CO2. The mass balance model shows that 11±6% of the DIC is sourced from the dissolution of shallow carbonate minerals, 9±8% formed by pyrolysis of sedimentary organic matter, 80±9% derived from deep metamorphic origin and mantle-derived CO2. Among them, the deep-sourced CO2 in Anninghe fault zones (ANHFZ) is merely metamorphic carbon, whereas ca. 12% and ca. 88% of the deep-sourced CO2 in the XSHFZ are derived from the mantle and metamorphic carbon, respectively. The average deep-sourced CO2 flux in the Kangding geothermal field is estimated to be 160 t a−1. If all the hot springs in various fault zones in the southeastern margin of the Tibetan Plateau are taken into account, the regional deep-sourced CO2 flux would reach ca. 105 t a−1. These results show that the deep-sourced CO2 released from non-volcanic areas might account for a considerable proportion of the total amount of global deep-sourced carbon degassing, which should be paid more attention to.

Supracrustal rocks and garnet granite within the high-grade metamorphic complex in Daqingshan, at the northern margin of the Palaeoproterozoic Khondalite Belt, North China Craton (NCC), preserve various mineral assemblages that record three distinct stages of their metamorphic evolution. The mineral assemblages representative of prograde metamorphism (M1) comprise plagioclase (Pl) + quartz (Q) ± biotite (Bio) inclusions preserved in garnet porphyroblasts in gneiss and granite members. The metamorphic peak mineral assemblages (M2) are represented by garnet (Gt) + Pl + Bio + Q + sillimanite (Sil) + K-feldspar (Kfs) in each rock’s matrix. Retrograde mineral assemblages (M3) that formed during post-peak, near-isothermal decompression (ITD) occur between relict garnet porphyroblasts and matrix minerals. These M3 assemblages consist of Bio + cordierite (Crd) + Pl + Gt + Q + Sil + Kfs. Phase equilibria modelling shows that the supracrustal rocks record clockwise pressure–temperature (P–T) trajectories from 650 to 750 °C/5.3–8.9 kbar (M1) up to 800–830 °C/9.8–11.2 kbar (M2), to 760–830 °C/4.8–6.2 kbar (M3). Geochemical similarities between the Daqingshan supracrustal rocks (i.e. garnet-biotite gneisses) and garnet granites, such as relative enrichment in K, Rb, and Ba, and depletion in Th, U, Ta, Nb, P, and Ti, imply a petrological affinity. Sensitive high-resolution ion microprobe analysis (SHRIMP) U–Pb dating of zircons reveals that the protolith of the Daqingshan supracrustal rocks formed at ~2.5 Ga and subsequently metamorphosed at ~2.45–2.37 Ga. Anatexis likely occurred at ~2.43–2.40 Ga during the retrograde near-ITD stage. Combined with previous studies, the timing of metamorphism and anatexis in the Daqingshan area may be connected with the regional mafic magmatism.

The Banchetta-Rognosa tectonic unit (BRU), covering an area of 10 km2 in the upper Chisone valley, consists of two successions referred to a continental margin (Monte Banchetta succession) and a proximal oceanic domain (Punta Rognosa succession) respectively. In both successions, Mesozoic meta-sedimentary covers discordantly lie on their basement. This paper presents new data on the lithostratigraphy and the metamorphic evolution of the continental basement of the Monte Banchetta succession. It comprises two meta-sedimentary sequences with minor meta-intrusive bodies preserving their original lithostratigraphic configuration, despite the intense Alpine deformation and metamorphic re-equilibration. Phase equilibrium modeling points to a metamorphic eclogitic peak (D1 event) of 20–23 kbar and 440–500 °C, consistent among three different samples, analyzed from suitable lithologies. The exhumation P–T path is characterized by a first decompression of at least 10 kbar, leading to the development of the main regional foliation (i.e. tectono-metamorphic event D2). The subsequent exhumation stage (D3 event) is marked by a further decompression of almost 7–8 kbar associated with a significant temperature decrease (cooling down to 350–400 °C), implying a geothermal gradient compatible with a continental collision regime. These data infer for this unit higher peak P–T conditions than previously estimated with conventional thermobarometry. The comparison of our results with the peak P–T conditions registered by other neighboring tectonic units allows to interpret the BRU as one of the westernmost eclogite-facies unit in the Alps.

The Sambagawa Belt is a subduction-related high-pressure intermediate type metamorphic complex with its protoliths characterized by the ocean plate stratigraphy. The Iyo-nada Sea area of this belt at western Shikoku, Southwest Japan exposes lenticular bodies of metagabbro. Besides, low P/T type metamorphosed Karasaki mylonites occur in this area as an upper structural unit of the Sambagawa Belt. LA-ICP-MS zircon U-Pb dating of the metagabbros in the Sambagawa Belt revealed their crystallization ages of 139–135 Ma as a magmatic event. Trace element composition of zircon suggests that the metagabbros were formed under the arc tectonic setting. Metagabbros do not show any evidence of high P/T metamorphism, instead, they indicate low P/T metamorphism based on the chemical compositional analysis of the hornblende. From the present study, it is constrained that the 139–135 Ma metagabbros were possibly originated as the hanging-wall arc crust. Zircon U-Pb dating of the Karasaki mylonites revealed imprints of multiple tectono-thermal events of 114.3 ± 1.5 Ma (magmatic event), ∼108–105 Ma (metamorphic with or without magmatic event), and 102.4 ± 0.9 Ma (metamorphic event). The first two tectono-thermal events can be correlated with those of the Higo Belt, central Kyushu. However, the last metamorphic event is younger than the tectono-thermal events of the Higo Belt, instead, it coincides with the initial stage tectono-thermal events of Ryoke-San’yo igneous province. Detrital zircon age of the pelitic schist in Karasaki mylonites revealed their origin as Jurassic to earliest Cretaceous sedimentary rock and its age distribution pattern is likely comparable to the previously reported age spectra of Higo metasedimentary rocks and their equivalent rock units. The newly obtained dataset indicates that the Karasaki mylonites are the eastern extension of the late Early Cretaceous Higo Belt.

Ultrahigh-temperature (UHT) metamorphism is critical for understanding the most extreme thermal evolution of continental crust. However, UHT metamorphism predominantly occurred in the Precambrian and is rarely observed in the modern Earth. Here, we report the discovery of ∼25 Ma UHT granulites from the Mogok metamorphic belt (MMB) in Myanmar via a combined study of petrology and geochronology. The studied pelitic granulites well preserve a peak mineral assemblage of garnet + sillimanite + plagioclase (antiperthite) + K-feldspar + quartz + Ti-rich biotite + rutile + ilmenite. Pressure (P)-temperature (T) pseudosections and conventional geothermobarometry data only constrain the P-T conditions of the peak stage to 900°C. In situ SIMS and LA-ICP-MS U-Pb dating and trace element analyses show that both metamorphic zircon cores and rims have flat heavy rare earth element (HREE) patterns with negative Eu anomalies. The metamorphic zircon rims show the lowest HREE contents and yield 206Pb/238U ages of 24.9±0.5 and 25.4±0.6 Ma, respectively, representing the timing of UHT metamorphism. Our results indicate that the central MMB underwent ∼25 Ma UHT metamorphism, which is possibly induced by continental rifting along the thinned orogenic lithosphere. Our data, as well as reported Cenozoic UHT events, further suggest that UHT metamorphism can be produced in the modern plate tectonic regime by lithospheric extension.

The Chavanon metamorphic sequence in the Variscan French Massif Central contains marble lenses that have been exploited since at least the 18th century to produce lime. They provided an opportunity to some major pioneers among nineteenth century French geologists to understand how metamorphic rocks are formed. The intimate association between marbles and gneisses led them to propose that the initial pile of rocks was deposited as a single unit under water. They thoroughly described the attitude, folding and mineralogy of the marbles, making their works of great historical value. Originally written in French, their translation into English and the reproduction of original figures from the works they published will be very useful to the international community of geologists interested in the history of their science.

Zircon separates from the contact aureole of the syn-tectonic Dawros–Currywongaun–Doughruagh Complex, western Ireland, are studied to constrain the nature and timing of magmatism associated with the early stages of the Grampian Orogeny. The samples analysed come from the uppermost part of the Dalradian Supergroup in northern Connemara (the Ben Levy Grit Formation), where a laterally extensive (>10 km) package of metamorphosed siliciclastic sedimentary rocks containing heavy mineral seams crops out. The seams mainly comprise magnetite, but zircon is also present in greater than accessory quantities. The seams have been locally reworked at granulite-facies metamorphic conditions during intrusion of the Dawros–Currywongaun–Doughruagh Complex magmas. Here we combine in situ mineral chemical and U–Pb geochronological analyses of zircons from samples of these heavy mineral seams collected at different locations in the Dawros–Currywongaun–Doughruagh Complex thermal aureole. An important finding is that the zircons studied have magmatic trace-element compositions, interpreted here as a function of their growth during contact metamorphic-induced partial melting. The zircons yield a range of U–Pb spot ages whose uncertainties suggest a maximum duration of zircon growth of ˜11 Ma, between 477.1 and 466.1 Ma, though it is likely that zircon growth occurred much more quickly than this. The age constraints revealed here match well with the range of 475 to 463 Ma previously proposed for the Grampian Orogeny overall in Connemara and lend useful support to models that argue for high-intensity, relatively short-lived Grampian orogenesis in the Connemara Caledonides.

The thermal effect caused by deep magma intrusion can not only accelerate the metamorphism of coal body, but also bring additional thermal field that changes the mechanical environment of coal seams, thereby affecting the permeability of coal seams. Different from shallow coal resources, deep coal resources are in a mechanical environment characterized by limited stress and strain. Thus, the thermal effect has a more significant influence on the distribution and permeability characteristics of deep coal seams. In this study, the evolution history of highly metamorphic coal seams in Yangquan mining area was analyzed, and the main effect of magmatic activity on coal seams was obtained. Based on the determined vitrinite reflectance data of typical mines in Yangquan mining area, the maximum paleotemperature was calculated by adopting the Barker’s method. Furthermore, the paleotemperature distribution in Yangquan mining area was summarized, and its relationship with the metamorphic degree was acquired. Then, a new permeability model considering the thermal strain was proposed to analyze the permeability evolution in deep coal seams at different ground temperatures. Finally, through a combination of the results of gas pressure and outburst number in Sijiazhuang Mine, Yangquan No. 5 Mine and Xinjing Mine, the influence of ground temperature on the gas outburst risk in Yangquan mining area was explored. The following conclusions were drawn: The maximum paleotemperature in Yangquan area can be 303 °C. In addition, the paleotemperature in the south is higher than that in the north of Yangquan mining area. The various temperatures at different depths bring about different degrees of thermal stress to different coal seams, leading to different strains. Under the fixed displacement boundary conditions in the deep, the coal seam folds and bends to varying degrees. Moreover, the difference in the ground temperature raises the a value of coal seams and lowers the permeability, which promotes the formation of gas-rich zones and increases the risk of coal seam outburst. The research results can help mines to make proper gas disaster prevention plan for different zones.

International audience; the Alpine belt that exhibits peridotite massifs associated with high-grade crustal metamorphic units. The timing and mechanisms of exhumation of these crust–mantle associations are still debated in the Rif in Morocco as well as in the Betic Cordilleras in Spain. A structural, petrographic, and geochronological study (40Ar–39Ar method) is performed in the Beni Bousera region (Internal Rif) on the Upper Sebtides units that consist of Paleozoic basement and Permo-Triassic metasedimentary rocks. We point out that, in the metagreywackes, an early assemblage of K-feldspar, plagioclase, quartz, muscovite, biotite, and garnet, which is diagnostic of amphibolite facies metamorphism. By dating white mica porphyroclasts from this metamorphic assemblage, we provide the first evidence of internal Rif metamorphism of Triassic age. We assume that these crustal units belong to the ancient continental margin of the Western Tethys, and thus, we interpret this metamorphic event in the framework of a hyperextended margin that is coeval with the exhumation of the subcontinental mantle during the Upper Triassic. Afterward, these units underwent greenschist facies metamorphism during the Lower Miocene due to a thermal event related to the back-arc opening of the Alboran Basin.

High-pressure mafic granulites (retrograded eclogites?) were discovered as minor lenses enclosed in garnet-kyanite-cordierite gneiss from the Badu Complex of the East Cathaysia terrane in South China. These rocks consist mainly of garnet, clinopyroxene, hornblende, quartz, and rutile/ilmenite with or without omphacite pseudomorphs that are indicated by clinopyroxene + sodic plagioclase symplectic intergrowths. Mineral textures and reaction relationships suggest three metamorphic stages: (1) an eclogite-facies stage (M1) characterized by the mineral assemblage of garnet + clinopyroxene (omphacite) + hornblende + rutile + quartz; (2) a high-pressure granulite-facies (M2) stage mainly represented by garnet + clinopyroxene + plagioclase + hornblende + rutile + quartz in the matrix; and (3) an amphibolite retrograde stage (M3) defined by hornblende + plagioclase + ilmenite + quartz symplectites surrounding garnet porphyroblasts. Conventional geothermometers and geobarometers in combination with phase equilibria modeling constrain metamorphic P–T conditions of 15.8–18.2 kbar/625–690 °C (M1), 11.8–14.5 kbar/788–806 °C (M2), and 5.4–6.4 kbar/613–668 °C (M3), respectively. Two-staged decompression processes are defined after the peak pressure, which suggests a two-staged exhumation of these deeply buried rocks. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating and trace element analysis show that the high-pressure metamorphism occurred at 240–244 Ma. Complete early Mesozoic orogenic processes characterized by initial subduction and/or crustal thickening and subsequent exhumation followed by rapid uplift are reconstructed for this part of the East Cathaysia terrane, South China.

The paper presents data acquired on epigenetic processes in rocks of the Meso- to Neoarchean greenstone complex of the Bundelkhand Craton of the Indian Shield. The study was focused on felsic and mafic metavolcanic and mafic–ultramafic rocks of the Central Bundelkhand Greenstone Complex (CBGC). The rocks contain metamorphic mineral assemblages produced during an earlier high-temperature [Grt–Prg–Pl; Cpx–Prg–Pl; Cpx–Grt–Pl; Cpx–Prg (± Ksp, Qz, Mag)] and later retrograde [Grt–Pl–Hbl; Grt–Chl–(Hbl–Act); Grt–(Zo–Czo)–(Hbl–Act); Grt–Chl–Pl (± Ksp, Qz, Mag, Ms)] episodes of metamorphism. The retrograde metamorphism was associated with the origin of zoned Ep and Qz–Ep (±Grt, Act) veins, patches, stripes, and small bodies of epidosites and actinolites. The mineral assemblages of the final episodes of the rock transformations contain prehnite and pumpellyite (Prh–Pmp–Alb; Prh–Pmp–Grs; Pmp–Grs–Alb; Prh–Cc–Act; and Prh–Grs–Cc). The composition of the garnet and amphibole was determined to broadly vary (Alm–Sps to Grs and Prg–Ts to Act and Cum, Gru, respectively). The garnet is contrastingly zoned (comprises three zones), with the boundaries of the zones marking drastic changes in the crystallization parameters (episodes of transformations). The identified sequence of the epigenetic events was as follows: amphibolite-facies metamorphism under elevated pressures (580–680°C at 7.2–10 kbar), with a clockwise P-T-t path and retrogression to the greenschist facies → Ca(Mn) metasomatism → metamorphism to the prehnite–pumpellyite facies. The metamorphic evolution of the CBGC rocks in the Bundelkhand Craton is in good agreement with the geodynamic model of cratonic crustal growth, with Archean subduction–accretion processes and Paleoproterozoic intraplate rifting.

The Sangilen and Khan-Khuhei blocks in the southwestern part of the Tuva-Mongolian terrane (TMT) combine into a composite structure, formed in the course of the Early Paleozoic (ca. 505–495 Ma) regional low- to moderate-pressure metamorphism. Manifestations of an earlier higher pressure metamorphism are known in both blocks. The upper age limit for the Sangilen block is defined by granites with an age of 536 ± 6 Ma. The early metamorphic rocks in both blocks are intruded by the granitoids of the Orto-Adyr complex with ages of 516 ± 5 and 513 ± 4 Ma, respectively. The emplacement of this complex preceded the Early Paleozoic (505–495 Ma) low- to moderate-pressure metamorphism. The high-temperature metamorphic rocks of the Sangilen and Khan-Khuhei blocks of TMT can be considered as the fragments of the Late Ediacaran higher pressure metamorphic belt, accreted to the TMT margin at ca. 510–505 Ma and reworked in the interval of 505–495 Ma under conditions of a regional low- to moderate-pressure metamorphism. The emplacement of the granitoids of the Orto-Adyr complex marks the conversion of the passive margin of the Neoproterozoic TMT block, covered by the Ediacaran carbonate platform, into an active margin. The geochronological studies of detrital zircons from the metaterrigenous rocks of the Sangilen block demonstrated that the source rocks of these zircons were mostly the Early Neoproterozoic magmatic rocks.

Введение: На основе полученных ранее результатов петро лого-минералогических исследований ксенолитов гранатсодержащих трондьемитов центральной части Лапландского гранулитового комплекса из района р. Лотта проведено моделирование их минеральных ассоциаций с помощью метода псевдосечений. Методика: Расчеты Р-Т параметров и флюидного режима метаморфизма проведены с использованием хорошо зарекомендовавших себя программных комплексов GEOPATH, TWQ, PERPLEX, основанных на взаимосогласованных базах термодинамических данных. Составы минералов определялись на электронном микроскопе CamScan MV2300 (VEGA TS 5130MM) с энерго дисперсио нным микроанализатором Link INCA Energy. Флюидные включения в метаморфических породах были изучены в пластинках толщиной 200-300 мкм с двусторонней полировкой на установке Linkam с рабочим температурным интервалом от -196°С до 600°С (THMSG 600) и автоматическим режимом нагревания и охлаждения образца со скоростью от 0.1 до 90°/мин. Результаты и обсуждения: 1. Моделирование минеральных ассоциаций ксенолитов методом псевдосечений и термобарометрия: Рассчитанные Р-Т псевдосечения в интервале 5-9 кбар и 600-800°С представляют простые диаграммы, в которых резко доминирующим является поле ассоциации Opx+Cpx+Pl+Qz+Ilm. С помощью программного комплекса winTWQ_2.32 рассчитаны температуры по равновесию Cpx+Opx для 59 пар минералов. Для моделирования процесса амфиболизации использовалось амфибол-плагиоклазовое равновесие, которое зафиксировало температуры амфиболизации ксенолитов 740-780°С при давлении 5.0-5.5 кбар. 2. Флюидные включения: Флюидные включения были изучены в кварце и представлены углекислотными и водно-солевыми разностями. Выводы: Амфиболизация ксенолитов связана с проникновением в них существенно водного флюида из остывающей трондьемитовой магмы, что подтверждается резким преобладанием водно-солевых (17-20 мас. % NaCl эквивалента) и подчиненным количеством углекислых включений в минералах гранитоидов. Несмотря на активное флюидное взаимодействие расплава и ксенолитов, признаков термального воздействия не установлено.

The eastern part of the European Variscan belt is exposed in the Bohemian Massif (BM). During the Variscan Orogeny, the Saxothuringian and Moldanubian BM units collided with the Brunovistulicum microcontinent. The western part of the Brunovistulicum is composed of Neoproterozoic crystalline rocks, occurring in four units: the Thaya Dome, the Svratka Dome, the Silesian Domain, and the Strzelin Massif, the northernmost exposed part of the Brunovistulicum. Another newly recognized Brunovistulicum subunit is the Drosendorf Unit in Austria. During the Variscan orogeny, the rocks of these units have been reworked to variable degree. The article presents new SHRIMP U–Pb ages which were obtained for three samples: N24—the sillimanite-bearing Nowolesie gneiss, affected by migmatization I; SK 1P—a pegmatite produced during migmatization II, and SK3—a dyke cross-cutting migmatized gneisses. These new data supplement already published zircon ages of the Neoproterozoic gneisses and the Variscan granitoids. Based on these data, this article documents the thermal-tectonic history of the Strzelin Massif during the 600–280 Ma timespan. The Neoproterozoic (Ediacaran) stage lasted about 30 Ma (600–570 Ma). There is no evidence for those thermal events which might have occurred between the Late Neoproterozoic and the Mid Devonian. The Variscan thermal history occurred from 350 to 280 Ma. Metamorphic changes started after the collision of the Bohemian Massif and the Brunovistulicum. The main thermal events are documented by two stages of migmatization, at 335 Ma and 305 Ma, respectively, and three stages of granitoid magmatism: ca. 324 Ma, ca. 306–303 Ma, and ca. 295–283 Ma. Compared with other structural units of the western part of the Brunovistulicum, the major peculiarities of the geology of the Strzelin Massif are: (1) klippen of the Saxothuringian rocks on top of rocks of the Brunovistulicum; (2) medium and low pressure metamorphism; (3) two stages of migmatization; (4) presence of a thermal dome; (5) long-lasting granite magmatism; (6) repeated intrusions of small volume, geochemically diverse magmas, controlled by faults and cracks.

Transformation of the oceanic crust into the continental one in orogenic belts is an important problem in petrological studies. In the paleocontinental sector of the Urals, a key object for tracing the stages of metamorphism and investigating the origin of anatectic granites is the Murzinka-Adui metamorphic complex. We have analyzed trace elements in zircons and established their genesis, sources, crystallization conditions, and stages of metamorphic events and granite generation in this complex. Zircons compositions were determined by the LA-ICP-MS method. Temperatures were calculated from Ti contents in the zircons. We distinguish three geochemical types of zircons, which differ in the ratios of light and heavy REE, U, Th, Ti, Y and show different values of Ce- and Eu-anomalies and Zr/Hf ratios, which are indicative of different crystallization conditions, as follows. Type I: minimal total LREE content; clear negative Eu- and Ce- anomalies; features of magmatic genesis; crystallization temperatures from 629 to 782 °C. Type II: higher contents of Ti, La, and LREE; low Ce-anomaly; assumed crystallization from highly fluidized melts or solutions. Type III: low positive Eu-anomaly; high REE content; low Th/U-ratio; zircons are assumed to originate from a specific fluidized melt with a high Eu-concentration. Ancient relict zircons (2300–330 Ma) in gneisses and granites show features of magma genesis and belong to types I and II. Such grains were possibly inherited from granitoid sources with different SiO2 contents and different degrees of metamorphism. Based on the geological and petrogeochemical features and zircon geochemistry of the Murzinka-Adui complex, there are grounds to conclude that the material composing this complex was generated from the sialic crust. The main stages of metamorphism and/or granite generation, which are traceable from the changes in types and compositions of the zircons, are dated at 1639, 380–370, 330, and 276–246 Ma. Thus, transformation of the oceanic crust into the continental one was a long-term and complicated process, and, as a result, the thickness of the sialic crust is increased in the study area.

The mineral assemblage Pl + Bt + Qtz + Grt + Sil + Ilm + Rt ± St (without cordierite) found in metapelites at the Kitela garnet deposit in the North Ladoga Metamorphic Complex, southeastern Fennoscandian Shield, was produced by Barrovian-type (elevated-pressure) medium- to high-temperature metamorphism. The P–T parameters of the metamorphic processes were evaluated using the technique of multiequilibrium thermobarometry (TWEEQU) at 610–700°C and 6–8 kbar. The metamorphism was associated with deformations and anatexis (partial melting) in a water-saturated system. The U–Pb monazite age (ID‑TIMS) of the Kitela schists is 1800 Ma (the final stage of the Svecofennian orogeny). Evidence for the identified metamorphic event is found locally in high-strain zones among schists with low- to medium-pressure mineral assemblages that are common to the North Ladoga Metamorphic Complex and the Svecofennian Orogen as whole (Buchan-type metamorphism). The Kitela schists were retrogressed at temperatures lower than 300°C. This retrogression occurred not during the final stage of the Late Svecofennian metamorphic event but was related to a younger low-temperature fluid-assisted overprinting.

The pressure-temperature-time (P-T-t) evolution of metamorphic rocks is directly related to geodynamics as different tectonic settings vary in their thermal architecture. The shapes of P-T paths and thermobaric ratios (T/P) of metamorphic rocks have been extensively used to distinguish different tectonic domains. However, the role of metamorphic timescales in constraining tectonic settings remains underutilized. This is because of the poorly understood relationship between them, and the difficulty in accurately constraining the onset and end of a particular metamorphic event. Here, we show why and how the intrinsic relationship between thermal regime, rheology and rate of motion controlled by the heat, mass and momentum conservation laws translate to differences in heating, cooling, burial, exhumation rates of metamorphic rocks and thereby, to the duration of metamorphism. We compare the P-T-t paths of the orogenic metamorphic rocks of different ages and in particular, analyse their retrograde cooling rates and durations. The results show that cooling rates of the metamorphic rocks are variable but are dominantly 100 °C/Ma) during the late Neoproterozoic to Phanerozoic. To seek what controlled this secular change in metamorphic cooling rates, we use thermomechanical modelling to calculate the P-T-t paths of crustal rocks in different types of continental orogenic settings and compare them with the rock record. The modelled P-T-t paths show that lithospheric peel-back driven orogenic settings, which are postulated as an orogenic mode operating under the hotter mantle conditions of late Archean to early Proterozoic, are characterised by longer durations of metamorphism and slower cooling rates (a few 10s of °C/Ma) as compared to the modern orogenic settings (a few 100s of °C/Ma) operating under relatively colder mantle conditions. This is because peel-back orogens feature: (1) hot lithospheres with very high crustal geotherms being sustained by high mantle heat-flow and profuse magmatism, and (2) distributed deformation patterns that limit vertical extrusion (exhumation) of the metamorphic rocks along localized deformation zones and instead, trap them in the orogenic core for a long time. In contrast, modern orogens mostly involve colder lithospheres and allow rapid exhumation through localized deformation, which facilitates faster cooling of hot, exhumed metamorphic rocks in a colder ambience. Thus, we propose that the secular change in metamorphic cooling rates indicates a changing regime of orogenesis and thereby, of plate tectonics through time. Predominance of the slower metamorphic cooling rates before the Neoproterozoic indicate the occurrences of peel-back orogenesis and truncated hot (collisional) orogenesis during that time, while the appearance of faster cooling rates since the late Neoproterozoic indicates the transition to modern style of orogenesis. A transition between these orogenic styles also accounts for the prolonged longevity (>100 million years) of many Precambrian orogenic belts as compared to the Phanerozoic ones. This study underscores the strength of timescales in combination with P-T paths to distinguish tectonic settings of different styles and ages. ISSN:1342-937X ISSN:1878-0571

The western part of the Central Asian Orogenic Belt, which comprises folded areas in Kazakhstan, Kyrgyzstan, and northwestern China, includes a number of large Precambrian sialic massifs that are framed by deformed and dismembered Paleozoic ophiolites and by island arc and flysch formations. The basements of the massifs are commonly made up of diverse metamorphic complexes, some of which were metamorphosed under high and ultrahigh pressures in the Early Paleozoic at ~480–530 Ma. These metamorphic formations are the Zerendy Group of the Kokchetav massif in northern Kazakhstan; Akdzhon Group of the Issyk-Kul massif in the northern Tien Shan); Aktyuz, Kemin, and Koyandy complexes of the Chu-Kendyktas and Zheltau massifs in southern Kazakhstan and the northern Tien Shan; and the Kassan Group of the Ishim–Naryn massif in the central Tien Shan. The paper reviews data on the structures, compositions, and metamorphic evolutions of the high- and ultrahigh-, and medium-pressure metamorphic rocks of these massifs. Numerous P–T assessments have been made for the near-peak and/or post-peak retrograde metamorphism, and some prograde P–T paths have been calculated for the key rock types over the past three decades of the studies. Near-peak and/or post-peak metamorphic ages and some ages of retrograde metamorphism are estimated for most of the high- and ultrahigh-pressure rocks. The paper discusses problems faced by the researcher when building geodynamic models for the high- and ultrahigh-pressure complexes in various massifs of the western part of the Central Asian Orogenic Belt. It is shown that any reliable model shall be underlain by detailed information on the compositions, ages, and formation environments of the protoliths for the ultrahigh-, high-, and medium-pressure rocks and complexes. Moreover, the structures and compositions of Paleozoic complexes surrounding the Precambrian massifs shall also be taken into consideration.

Eoarchean rocks in the Napier Complex of East Antarctica are largely known from a few localities in the western Tula Mountains of Enderby Land. Zircon from trondhjemitic and mafic gneisses from Aker Peaks in Kemp Land, 200 km further east, were analysed by secondary ion mass spectrometry (SIMS), yielding concordant U Pb dates between 3860 and 3700 Ma, which can be attributed to magmatic and possibly metamorphic activity. Concurrent analysis of 207Pb/206Pb ratios and Lu Hf isotopes in the trondhjemitic sample by laser ablation ICPMS provide initial eHf(t) estimates for this age range that are slightly sub-chondritic (ca 0 to −2). This can be attributed to the incorporation of older crust into the magmatic protoliths of the gneisses, although there is no requirement that this crustal source be older than Eoarchean. Much scatter in the U Pb dataset is attributable to isotopic disturbance of Pb during high-temperature metamorphism at 2.5 Ga, and if not corrected for, can lead to overestimation of model crust formation ages, a critical problem in the search for evidence of Hadean crust in Eoarchean rocks, and for estimating the timing and rate of ancient continental growth.

Monazite is an important accessory mineral in the pelitic granulites (Weihai area, Sulu orogen), and is also a powerful monitor for understanding the metamorphic evolution of the granulites. The pelitic granulites incongruously occur as lenses in granitic gneisses. The lithologies of the lenses gradually change from core to margin: The undeformed coarse-grained granulite, the foliated fine-grained granulite, the garnet-biotite-gneiss, and the migmatized granulite. The lens cores mostly preserve a peak granulite-facies metamorphic mineral assemblage of garnet + plagioclase (antiperthite) + quartz + sillimanite + biotite with accessory minerals of rutile, zircon, and monazite. The lens margins display a fluid-induced retrogression. In order to decipher the metamorphic processes of the pelitic granulites, a combined study of BSE imaging, U-Pb dating, and trace element composition for the monazites from the metapelitic lens were conducted. Monazites from the undeformed coarse-grained granulite only record a Paleoproterozoic age (1832±7 Ma, n=40). Monazites from the other lithologies yield the inherited Paleoproterozoic age and Triassic overgrowth age. For example, monazites from the migmatite yield intercept ages of 1818±10 and 211±22 Ma (n=56) with Triassic concordant age of 223.8±2.9 Ma. The Paleoproterozoic monazites are characterized by remarkable depletion in HREE and Y with obviously negative Eu anomalies, indicating their formation equilibrated with garnet and feldspar under granulite-facies conditions. During Triassic fluid modification, the monazite bright rims assimilated Th and Si but released U, HREE, Y, and P. This process resulted in that the Triassic overgrowth monazites have higher HREE and Y contents, and lower Th and U contents with relatively low Th/U ratios. Thus, the monazites in the pelitic granulites recorded a Paleoproterozoic metamorphic event and Triassic fluid modification. The Weihai pelitic granulites might have a tectonic affinity with the North China Craton. Therefore, the Paleoproterozoic pelitic granulites were mechanically drawn into the orogen during the Triassic continental collision, and subsequently were remoulded by the fluids during its exhumation.

In the Variscan Schwarzwald metabasic rocks form small bodies included within anatectic plagioclase-biotite gneisses. Many metabasites first underwent an eclogite-facies metamorphism at about 2.0 GPa and 670–700 °C, resulting in the assemblage garnet + omphacite + rutile + quartz ± epidote ± amphibole ± kyanite. Since these eclogites are nearly free of an OH-bearing phase, they underwent almost complete dehydration during subduction, suggesting formation along an average to warm top-of-the-slab geotherm of 10–13 °C/km. The age of the Variscan high-P/high-T metamorphism is > 333 Ma. After partial exhumation from ~ 65 to ~ 15 km depth, the eclogites were overprinted under increasing activity of H2O by a number of retrograde reactions. The degree of this overprint under amphibolite-facies conditions (0.4–0.5 GPa/675–690 °C) was very different. Up to now, only retrograde eclogites have been found, but some samples still contain omphacite. Kyanite is at least partially transformed to aggregates of plagioclase + spinel ± corundum ± sapphirine. On the other hand, there are amphibolites that are extensively recrystallized and show the assemblage amphibole + plagioclase + ilmenite/titanite ± biotite ± quartz ± sulphides. The last relic phase that can be found in such otherwise completely recrystallized amphibolites is rutile. After the amphibolite-facies metamorphism at ~ 333 Ma, the metabasites underwent a number of low-temperature transformations, such as sericitization of plagioclase, chloritization of amphibole, and formation of prehnite. The intimate association of metabasite bodies with gneisses of dominantly meta-greywacke compositions suggests derivation from an active plate margin. This view is corroborated by bulk-rock geochemical data. Excluding elements that were mobile during metamorphism (Cs, Rb, Ba, K, Pb, Sr, U), the concentrations of the remaining elements in most of the metabasites are compatible with a derivation from island-arc tholeiites, back-arc basin basalts or calc-alkaline basalts. Only some samples have MORB precursor rocks.

The paper presents data on amphibolites dredged during Cruise 249 of the German R/V Sonne at three sites: one at the northwestern termination of the Stalemate Ridge (ocean-side slope of the Aleutian Trench) and two at the western slope of the north Shirshov Rise (in the western sector of the Bering Sea). Data on the petrography and geochemistry of the amphibolites and their estimated P–T metamorphic parameters provide an insight into the probable geodynamic environments in which their protoliths were formed and into the geodynamic regime of their metamorphism. The magmatic protolith of amphibolites in the Shirshov Rise was produced by the melting of a strongly depleted mantle source and, perhaps, was a product of suprasubductional magmatism. The isotope composition and distribution of HFSE in amphibolites from the Stalemate Ridge suggest that the protolith of these rocks was a derivative of tholeiitic melt parental for N-MORB. The typical mineral assemblage and P–T metamorphic parameters of glaucophane-bearing amphibolites from the Shirshov Rise indicate that the protolith of these rocks was metamorphosed within a subduction zone related to the Cretaceous–Paleogene Olyutorsky Arc. Interpretations of the age and geodynamic nature of metamorphism of amphibolites in the Stalemate Ridge based on currently available data cannot be regarded as absolutely realistic, and the most likely scenario for the origin of the amphibolites is that they were produced within a subduction zone that was no younger than 55–50 Ma and preceded the modern Aleutian Arc. The extinction of this ancient subduction zone should have been associated with the development of the Aleutian Arc north of it. According to this geodynamic scenario, an ophiolite sequence was produced where the ancient subduction zone had occurred. The ophiolite consisted of rocks of an mafic–ultramafic association and amphibolites. This scenario may, however, be a matter of further discussion.

Electron back scattered diffraction data of garnet crystals from the Nelson Aureole, British Columbia and the Mosher’s Island formation, Nova Scotia, reveals that 22 garnet crystals are all oriented with one of three crystal directions parallel to the trace of the foliation plane in thin section. Structural models suggest that these relationships are due to preferential garnet nucleation onto muscovite, with the alignment of repeating rows of Al octahedra and Si tetrahedra in each leading to inheritance of garnet orientation from the muscovite. These results highlight that epitaxial nucleation may be a prevalent process by which porphyroblast minerals nucleate during metamorphism and carry implications for the role that non-classic nucleation pathways play in the crystallization of metamorphic minerals, the distribution of porphyroblasts in metamorphic rocks, and, in cases in which nucleation is the rate limiting step for crystallization, the energetics of metamorphic reactions. This work was funded by two GSA Graduate Student Research Grants, an AAPG Grant in Aid, and a research scholarship from the Virginia Tech Department of Geosciences to Nagurney. Nagurney also acknowledges support from the Virginia Tech ICTAS Doctoral Scholars Program. Published version

We constrain the multistage tectonic evolution of the Palaeoproterozoic UHT metamorphic (P ​= ​0.9–1.0 ​GPa, T >1000 ​°C, t ​= ​2088–2031 ​Ma) Bakhuis Granulite Belt (BGB) in Surinam on the Guiana Shield, using large- to small-scale structures, Al-in-hornblende thermobarometry and published fluid inclusion and zircon geochronological data. The BGB forms a narrow, NE–SW striking belt between two formerly connected, ~E–W oriented granite-greenstone belts, formed between converging Amazonian and West African continental masses prior to collision and Transamazonian orogeny. Inherited detrital zircon in BGB metasediments conforms agewise to Birimian zircon of West Africa and suggests derivation from the subsequently subducted African passive margin. Ultrahigh-temperature metamorphism may have followed slab break-off and asthenospheric heat advection. Peak metamorphic structures result from layer-parallel shearing and folding, reflecting initial transtensional exhumation of the subducted African margin after slab break-off. A second HT event involves intrusion, at ca. 0.49 ​GPa, of charnockites and metagabbros at 1993–1984 ​Ma and a layered anorthosite at 1980 ​Ma, after the BGB had already cooled to

The results of geochronological studies (LA-ICP-MS) of detrital zircons of the Tatsaingol block in the southeastern part of the Neoproterozoic Southern Khangay metamorphic belt are presented. Two sequences, metaclastic rocks and hornblende schists (metavolcanics), are distinguished in the Tatsaingol complex of this block. The metaclastic rocks are found to contain manifestations of the processes of polymetamorphism. The end of the early metamorphic episode in these rocks is defined by pegmatoid granites with the ages of 561 ± 12 and 562 ± 2 Ma and granite dikes with the age of 571 ± 9 Ma; the age of the late metamorphic episode is estimated in the range of 550–540 Ma. The lower age limit of metamorphism in the metavolcanics of the island arc assemblage is defined by gabbroids with the age of 603 ± 3 Ma, but the early episode of high pressure metamorphism is not established. The late episode (550–540 Ma) is manifested in both sequences. The results of the isotopic dating of detrital zircons from biotite gneisses indicate the predominance of Paleoproterozoic and Neoarchean rocks (1.8–2.1 and 2.4–2.9 Ga, respectively) in their sources. The rocks of the island arc assemblage of the Tatsaingol complex can be viewed as a fragment of the Bayan-Khongor zone, which was accreted to the Southern Khangay metamorphic belt in the course of the accretion–collision process. The late metamorphic episode was not manifested in the central and northwestern parts of the Southern Khangay metamorphic belt, which indicates the separateness of this part of the belt from the Tatsaingol block during this period.

The Central Asian Orogenic Belt, as one of world-class gold economic belts, preserves a number of giant, large black shale-hosted gold deposits, while it is still debated for origin of sulfides and gold mainly due to lack of identification for multiple stages of sulfides. The Haoyaoerhudong gold deposit is hosted in a sequence of Mesoproterozoic carbonaceous and pyritic slate, phyllite, and schist that form a tight syncline along the north margin of the North China Craton. Detailed petrography of the host rocks and mineralization have defined five stages of pyrites. The earliest form of pyrite (Py1) occurs as fine-grained dispersed pyrite in black carbonaceous slate and medium- to coarse-grained disseminated pyrite in pyrite-rich layers, contains relative low gold and high arsenic content, indicating a syn-sedimentary or diagenetic in origin. Stage II pyrite (Py2) occurs with garnet and quartz inclusions and Py3 occurs as pyrite veins, contains higher gold and lower As content, and are interpreted to have formed from the dissolution-reprecipitation of Py1 during the peak metamorphism or post-peak metamorphism. Stage IV pyrite (Py4) from the pyrite-quartz veins crosscut the metamorphic garnet, contains the highest gold concentrations and other trace elements, and is considered to have formed post-peak metamorphism. Abundant native gold, electrum, and maldonite occur as inclusions within Py4 and monazite and in fractures that crosscut garnet. While, Py5 with typical remobilized feature is thought to be a product of melting of former pyrites (Py1 to Py4) triggered by the large-scale Hercynian magmatism. The sedimentary/diagenetic Py1 have δ34S values that range from +12.4‰ to +16.2‰. Later generations of sulfides, including Py2 to Py5, and Ccp2 to Ccp3, have δ34S values from +9.5‰ to +12.7‰. Monazite with maldonite inclusions from quartz-pyrite veins yielded an intercept age of 341.3 ​± ​6.6 ​Ma, while coarse grained monazite associated biotite along fractures in the reefs yielded an intercept age of 254.6 ​± ​8.2 ​Ma. The paragenetic, textural, chemical, and isotopic data suggest three distinct gold producing episodes at Haoyaoerhudong gold deposit. Gold and arsenic were clearly initially concentrated in organic muds, and enriched along the structures of diagenetic arsenic-rich pyrite. Subsequently, accompanying metamorphism and deformation, gold was liberated from the dissolution of diagenetic pyrites to form the pyrite veins. Finally, accompanying transformation of pyrite into pyrrhotite, gold was released into the metamorphic fluids to become concentrated as native gold, electrum, and maldonite in pyrite-quart veins. Monazite with age of 341 ​Ma from quartz-pyrite veins suggests that the third major gold mineralizing event in Haoyaoerhudong occurred before the Hercynian magmatism, suggesting that the Haoyaoerhudong deposit is a typical orogenic gold deposit rather than intrusion-related deposit.

The peak temperature and timescale of ultrahigh-temperature (UHT) metamorphism are significant for understanding its thermal budget and geodynamic evolution. The spinel-bearing (sapphirine-absent) UHT granulite at the Xuwujia area in the Jining complex, North China Craton was revealed to have undergone a metamorphic evolution that involves the pre-Tmax (maximum temperature) heating decompression to the Tmax stage with an extreme temperature of ~1125 °C, the post-Tmax cooling to the fluid-absent solidus (~860 °C) at 0.8–0.9 GPa, and sub-solidus decompression. The Tmax condition was recorded by the inferred feldspar-absent peak assemblage of spinel ± garnet. The post-Tmax cooling evolution was indicated by the sequential appearances of plagioclase, K-feldspar, sillimanite and biotite, as well as the core-to-rim ascending grossular component in garnet. Moreover, some texturally zoned spinel has exsolved lamellae of magnetite in the core and shows rim-ward increase of Mg and Al, which suggests a temperature decrease from >1100 °C in local domains isolated from quartz. Spinel with exsolved magnetite lamellae or low Al/(Al + Fe3+) is therefore proposed as an indicator for UHT conditions in metapelites. Probability simulation on the collected zircon ages yields a timescale of ~40 Myr (95% confidence; during 1.90–1.94 Ga) for the supra-solidus cooling stage of the UHT metamorphism in the Jining complex. This extreme UHT metamorphism has reached the rock’s dry solidus (~1125 °C) and undergone slow cooling, which is interpreted to result from a post-orogenic plume activity with sufficient advective heating from hyperthermal mafic intrusions.