In zircon, transgressive textures cutting oscillatory zoning and associated chemical modifications are usually attributed to fluid-mediated coupled dissolution-precipitation (CDP). Here, we show an example of melt-mediated CDP in zircon from metagranite, with consequences on calculated ages and chemical composition. Zircon oscillatory zoning is commonly blurred and truncated by embayments and channels of replaced zircon with irregular and sharp boundaries. These are interpreted as replacement fronts and may be spatially associated with micro-porosity. Some superimposed replacement fronts indicate repeated CDP and porosity healing. Micro-porosity and inclusions are associated with replaced domains and thus are epigenetic. We highlight the necessity of combined high-resolution back-scattered electron imaging, otherwise the CDP textures may be overlooked in cathodoluminescence images. Inclusions of Ph–Grt–Ttn–Ap–Qz–Bt are compatible with the matrix assemblage Grt−Ph−Bt−Ttn−Kfs−Pl−Qz ± Rt ± Ilm, which equilibrated at c. 15−17 kbar, 690–740 °C with partial re-equilibration to c. 12 kbar, 680 °C. Since the conditions were above the wet solidus, the zircon changes happened through melt-mediated CDP. The replaced domains show an overall decrease of REE, Y, Th and Th/U, P, a large variation in Yb/Gd, shallower Eu anomaly, and increased U, and Hf. The chemical changes indicate a tendency of zircon purging trace elements during reequilibration with migrating melt. Spots with high LREE are probably due to the effect of micro-inclusions. Based on textures, inherited zircons are c. 540–600 Ma, protolith granite is c. 507–496 Ma, and CDP replacement occurred down to c. 335 Ma. But, the melt-mediated CDP resulted in a smear of mostly concordant, but spurious spot dates between inherited, protolith and modified domains, spanning > 200 Myr.

The Paleozoic Altai orogenic belt results from a suprasubduction polycyclic metamorphism of a sedimentary accretionary wedge. In the Mongolian-Altai, a Barrovian-type metamorphic sequence marks the transition between exhumed lower-and upper-crustal domains. It is studied by field and petrological observations, phase equilibria modeling, U-Pb zircon LA-ICP-MS, and in-context monazite LASS geochronology. Prograde micaschist and retrogressed migmatite are juxtaposed by an extensional shear zone hosting variably deformed Permian dikes (ca. 295-290 Ma, U-Pb zircon) and a sharp metamorphic gradient (< 5 km from biotite to sillimanite zone). In the hangingwall micaschist, garnet and staurolite size increase in the neck zone of boudinaged quartz veins suggests syntectonic growth during the activity of the shear zone. Sequential growth of garnet 1-chlorite (M1), garnet 2-staurolite or garnet 2-staurolite-kyanite (M2) and sillimanite (M3) indicate M1 P-T increase (525-550 degrees C, 3-4 kbar to 550-575 degrees C, 5-6 kbar); M2 isobaric heating (similar to 7 kbar, from 550-575 degrees C to 625-680 degrees C) and M3 decompression (<6 kbar). Microtextures indicate garnet 2, staurolite, and sillimanite crystallisation in the extension-related foliation. Monazite in garnet 1 is Carboniferous (ca. 350-340 Ma) while monazite related to garnet 2, staurolite and sillimanite is Permian (ca. 290-285 Ma). Migmatite contains Devonian zircon (ca. 375 Ma) and Permian monazite (ca. 285 Ma). Anhedral garnet core in a garnet-kyanite-sillimanite migmatite documents an undated early subsolidus metamorphism. Resorbed kyanite, euhedral sillimanite, and garnet rim document a suprasolidus isothermal decompression (9 to 4-7 kbar at similar to 750 degrees C). Melt crystallisation indicates subsequent cooling under the solidus (<700 degrees C). Formerly shallow-dipping migmatitic foliation is related to the decompression. Monazite crystals associated with either kyanite or sillimanite systematically indicate a Permian age, related to the decompression. Consequently, lower crustal rocks decompression was coeval with magmatism, shearing, and heating higher in the crust. The shear zone accommodated the exhumation of a migmatite dome and focused an intense magmatism, inducing the isobaric heating and related Barrovian metamorphism in the hangingwall micaschists. As such, the envelope of extensional domes appears a favourable site for the development of the Barrovian metamorphic sequence.

Findings of coesite and diamond in quartzo-feldspathic rocks confirmed that continental crust, despite its buoyancy, can be subducted to ultra-high pressure (UHP) conditions. In addition to these index minerals, UHP conditions can be revealed by specific minor elements incorporated in common minerals, as it is well known from the Earth's mantle but poorly explored in continental crust. Here, we investigate garnet with coesite inclusions from subducted metagranites of the Eger Crystalline Complex, Bohemian Massif (Czech Republic). The garnet shows chemically distinct concentric domains with minor amounts of P, Na, and Li. From the correlation of these elements, we infer (Na,Li)1P1M2+−1Si−1 substitution, where Li compensates for the Na deficiency in a 2∶3 ratio. This is the first time that such coupled substitution in garnet has been defined and clearly connected to UHP conditions in natural samples, proving itself as a new tool indicative of UHP conditions. Moreover, garnet in subducting slabs needs to be considered as an important Li carrier, capable of transporting significant amounts of Li into the Earth’s mantle.

The Bohemian Massif, located at the north-eastern termination of the Variscan Orogenic Belt, preserves a polyphase record of high-pressure (HP) metamorphism spanning approximately 60 million years (400-340 Ma). The earliest phase of this evolution, termed the Eo-Variscan, is recognized in three principal but geographically and lithologically disparate metamorphic complexes: MarianskeLazne, Munchberg, and Gory Sowie. Despite their contrasting lithologies and structural relationships, all three exhibit a strikingly similar tectono-metamorphic evolution characterized by HP metamorphism followed by high-temperature (HT) overprinting between c.400 and 380 Ma. This study reinforces that tectono-metamorphic framework through new petrological and zircon geochronological data from previously underexplored retro-eclogites in the Gory Sowie Metamorphic Complex, situated in the north-eastern Bohemian Massif. An HP eclogite-facies stage is constrained by garnet compositions, rare omphacite inclusions, and Zr-in-rutile thermometry of rutile inclusions in garnet, indicating conditions of 1.9-2.4 GPa and > 650 degrees C. A subsequent HT stage is recorded by clinopyroxene-plagioclase-amphibole symplectite assemblages after omphacite and by Zr-in-rutile thermometry of associated rutiles, indicating conditions of c. 1.5 GPa and > 800 degrees C. U-Pb zircon geochronology constrains both metamorphic stages to the interval between 400 and 380 Ma. In addition, new whole-rock major and trace element geochemical and Sm-Nd isotopic data combined with zircon U-Pb and Lu-Hf isotopic analyses suggest that the protoliths of these rocks were emplaced in a supra-subduction zone setting during the late Cambrian (c.500-480 Ma). These findings are broadly consistent with data from other Eo-Variscan complexes within the Bohemian Massif and additionally support correlations with similar units in the NW Iberian Massif. Taken together, these correlations imply that the Eo-Variscan complexes share common late Cambrian volcanic arc-related protoliths, including fore-arc assemblages. This has important implications for geodynamic models aiming to reconstruct the pre-orogenic architecture of the Variscan Belt.

Ultrapotassic rocks are widely considered to originate from a metasomatized mantle source enriched by crustal components; however, mechanisms controlling the crustal inputs remain poorly understood. To address this issue, the petrogenesis of Miocene ultrapotassic volcanics, a vital lithoprobe, from the Lhasa terrane, southern Tibet, is investigated. Their geochemistry shows (1) ultrahigh Th/La ratios, indicating recycling of crustal material at shallow depths (<= 80 km), and (2) evolved Sr-Nd isotopic signatures, consistent with contributions from Himalayan sediments and Yarlung Zangbo ophiolite. Phase equilibria modeling of such mixtures at 25 kbar and 900-1100 degrees C-consistent with xenolith pressure-temperature constraints- produces 10-20 vol% melts resembling the natural ultrapotassic rocks. Integrating geological and geophysical evidence, we propose a tectonic model in which subducted sediments and oceanic crust form a m & eacute;lange, are relaminated beneath the Tibetan lithosphere, and subsequently melt to generate ultrapotassic magmas. This model provides new insights into crustal recycling in subduction systems in general.

This study presents results of provenance analysis based on U-Pb ages and Hf isotopic composition of detrital zircons from (meta-)sedimentary rocks of two sections of the Zavkhan Block in W Mongolia. Detrital zircon age populations for most of the studied samples show a dominant Tonian-Cryogenian peak (c. 750 Ma), considered as detritus derived from the local rift-related magmatic sources. Some samples also have significant Neoarchean-Paleoproterozoic (c. 2.7-1.7 Ga) zircons sourced from cratonic basement. The large range of epsilon Hf(t) values (-30 to +15) of the Tonian-Cryogenian zircons indicates significant mixing of mantle-derived magmas with mature crustal material, interpreted as a result of crustal melting during rifting. The Hf isotopic composition of the older zircons suggests contributions from both juvenile magmas and crustal recycling. Maximum depositional ages indicate that although dominantly Tonian-Cryogenian, the studied sequences also include subordinate older (pre-rift) stratigraphic members. The zircon age and Hf isotopic systematics of the younger part of studied rocks are interpreted to reflect sedimentation in Tonian-Cryogenian post-rift sequences evolving towards continental passive margin during the break-up of Rodinia. On the other hand, the pre-Tonian detritus from post-rift strata were dominantly recycled directly from the older pre-rift strata or indirectly from the Tonian anatectic magmatic rocks. These data are correlated with corresponding datasets from potential cratonic sources, which indicate close similarity and possible paleogeographic connection of the Zavkhan Block to the western Siberian Craton. Thus, it is proposed that the Zavkhan Block together with other Mongolian continental fragments were rifted from western Siberia in the Tonian-Cryogenian. The whole archipelago was subsequently dextrally translated along the Siberian margin into its Ediacaran-early Paleozoic position south of the Siberia Craton.

Despite numerous studies of metamorphic and magmatic rocks from the Dunhuang block, its protracted Cambrian to Permian geodynamic evolution as well as its role in the final amalgamation of the Tarim-North- China Craton Collage during the Pangea assembly, remain controversial. In order to understand the evolution of the Dunhuang block in the frame of Paleozoic plate tectonics, we review and synthetize recently published P-T data along with geochronological and geochemical data for its northern, central and southern mountain ranges, which we placed in the context of structural observations. Zircon and monazite U-Pb ages combined with P-T constraints determined for Ordovician to Devonian metamorphic rocks reveal that M1 and M2 metamorphic events define a clockwise P-T evolution and distinctly hot metamorphic gradients. Ordovician to Devonian protracted garnet growth evidenced by low (Yb/Gd)N ratios in metamorphic zircon and monazite, together with low Th/U ratios, negative epsilon Hf(t) and Eu anomalies in magmatic and metamorphic zircon, mark a period of continuous metamorphism accompanied by crustal reworking during thickening of a previously thermally softened crust. Zircon and monazite U-Pb ages suggest that the early Paleozoic D1-M1 and D2-M2 events started ca. 10 m.y. earlier and lasted longer in the northern and central ranges compared to the southern range consisting of significantly older crustal basement components. In all three mountain ranges, the formation of an E-W trending steep cleavage related to D3-M3 event was concomitant with emplacement of numerous late Devonian to Carboniferous diorites and I-type granitoids. The negative epsilon Hf(t) of magmatic zircon and high (Yb/Gd)N ratios in metamorphic zircon and monazite show that a magma-assisted N-S-directed shortening event responsible for further crustal reworking started in the late Devonian and lasted over 60 m.y. The latest Permian M4 metamorphic event, restricted mainly to the southern range, defines anticlockwise P-T paths associated with the emplacement of high-K calc-alkaline granitoids. This event is characterized by the increasing from negative to positive zircon epsilon Hf(t) values, suggesting input of juvenile crustal and mantle-derived material. High metamorphic gradients and the petrogenesis of magmatic arc-related rocks intruding a Precambrian basement suggest that the Dunhuang block developed as a supra-subduction continental hot orogenic system formed above subducting oceanic plates similar to the northern and southern margins of the American Cordillera. Our data show that its evolution was related to two kinematically independent early to middle and late Paleozoic orogenic cycles. In the context of plate tectonic reconstructions, the revised chronology of events points to the kinematic interplay between the continental blocks moving within the Proto-Tethys oceanic domain in association with the south- dipping Panthalassa, Paleo-Asian, and north-dipping Paleo-Tethys oceanic subduction systems.

Augen to banded metagranite from the Snieznik dome have been modified locally to have stromatic, schlieren, nebulitic and granite-looking textures typical of migmatites. Former presence, and increasing role of melt in transformation towards nebulite is inferred from interstitial phases along grain boundaries in the dynamically recrystallized monomineralic feldspar and quartz aggregates, and from textures of fine-grained plagioclase and quartz replacing K-feldspar. These features are interpreted as resulting from dissolution-reprecipitation along grain boundaries due to grain-scale melt migration, being pervasive at the grain-scale, but localized at hand-specimen to outcrop scales. The new minerals crystallized from melt are in textural equilibrium with phengite. All the rock types have the same mineral assemblage of Grt−Ph−Bt−Ttn−Kfs−Pl−Qz±Rt±Ilm, with similar garnet, phengite and biotite composition, leading to modelled equilibration conditions of 15−17 kbar and 690–740 °C. Because the mineral compositions in the assemblage of interest are independent of the amount of melt, the modelling did not allow to estimate melt quantities in individual rock types. However, migmatite textures suggest that increasing degree of melt-rock interaction occurred from the banded to the schlieren and nebulitic types. The initiation of melt migration is related to gently dipping structures related to continental subduction to eclogite-facies conditions, and more pronounced melt migration is related with vertical fabrics leading to exhumation of the continental subduction wedge from eclogite-facies to mid-crustal conditions. The effects of melt migration had impact on partial recrystallization of zircon. Zircon in augen to banded types shows oscillatory zoning and gives Cambro-Ordovician age of the protolith. In schlieren to nebulite types, zircon shows domains of blurred oscillatory zoning to structure-less textures. These metamorphic domains are located along grain boundaries, form embayments, form straight or curved linear structures cutting through the oscillatory zoned domains, or are affecting the whole grains. The domains with sharp oscillatory zoning tend to give Cambro-Oridovician ages, while the metamorphic domains tend to give Carboniferous age. Zircon shows numerous apparent “inclusions” of phengite, K-feldspar, quartz, plagioclase, rare garnet, rutile and biotite. However, the “inclusions” of phengite, garnet and rutile are located in the metamorphic domains of the zircon grains. In places, the inclusions are aligned, and these structures are interpreted as former cracks, along which the metamorphic phases crystallized and zircon (re)crystallized. As the assemblage of phengite-garnet-rutile is compatible with previously inferred eclogite-facies conditions, we interpret the Carboniferous zircon (re)crystallization as dating the eclogite-facies grain-scale melt migration process.

In the Mongolian Collage, metamorphic pressure-temperature (P-T) and timing reveal a one-stage evolution defined by a duality of late Neoproterozoic-Ordovician subduction-related low T/P metamorphism and suprasubduction high T/P metamorphism recorded in the Mongolia-Manchuria and Baikal-Sayan belts. This was followed by gradual prevalence of suprasubduction high T/P metamorphism towards the late Paleozoic corresponding to the Altai and South Altai cycles. In the Tarim-North China Collage, metamorphic P-T and timing reveal a two-stage evolution, from dominant intermediate T/P metamorphism possibly resulting from Ordovician-Devonian amalgamation and Andean-type evolution of the collage, to dual low and high T/P metamorphism in the Carboniferous-Permian reflecting subduction-collision processes along the South Tianshan suture in the west and a suprasubduction evolution along the Solonker suture in the east. Altogether, the Paleozoic tectonometamorphic evolution of the two collages in the Central Asian Orogenic Belt shows remarkable differences, with the Mongolian Collage displaying features typical of peripheral accretionary style reflecting recurrent tectonic switches that can be regarded as a single orogenic system, and a two-stage evolution of the Tarim-North China Collage with features of both peripheral-accretionary and interior-collisional orogenic cycles, but mostly related to recurrent subductions of interior oceans.

We use a wide database of pressure (P), temperature (T) and petrochronological data from late Neoproterozoic to early Mesozoic metamorphic rocks together with a review of compressive and extensional tectonic cycles to evaluate and correlate the tectonothermal and temporal evolutions of the Mongolian and the Tarim–North China collages forming the Central Asian Orogenic Belt. In the Mongolian Collage, metamorphic pressure–temperature (P–T) and timing reveal a one-stage evolution defined by a duality of late Neoproterozoic–Ordovician subduction-related low T/P metamorphism and suprasubduction high T/P metamorphism recorded in the Mongolia–Manchuria and Baikal–Sayan belts. This was followed by gradual prevalence of suprasubduction high T/P metamorphism towards the late Paleozoic corresponding to the Altai and South Altai cycles. In the Tarim–North China Collage, metamorphic P–T and timing reveal a two-stage evolution, from dominant intermediate T/P metamorphism possibly resulting from Ordovician–Devonian amalgamation and Andean-type evolution of the collage, to dual low and high T/P metamorphism in the Carboniferous–Permian reflecting subduction–collision processes along the South Tianshan suture in the west and a suprasubduction evolution along the Solonker suture in the east. Altogether, the Paleozoic tectonometamorphic evolution of the two collages shows remarkable differences, with the Mongolian Collage displaying features typical of peripheral accretionary cycle reflecting recurrent tectonic switches that can be regarded as a single orogenic system, and a two-stage evolution of the Tarim–North China Collage with features of both peripheral–accretionary and interior–collisional orogenic cycles, but mostly related to recurrent subductions of interior oceans. Furthermore, the Paleozoic tectonic cycles recognized in the Mongolian and Tarim–North China collages are tentatively correlated to distinct retreating and advancing subduction dynamics of Paleozoic oceanic domains. Funding: This research was funded in part by the Polish National Science Centre (Grant DEC-2023/51/D/ST10/02611/R). K.S. and P.S. acknowledge the support of the Czech Science Foundation (grant number 19-27682X to K.S.) and of an internal grant of the Czech Geological Survey (number 329805 to K.S.). J.S. acknowledges the support of project No. 2021/43/P/ST10/02996 co-funded by the National Science Centre and the EU H2020 research and innovation program under MSCA GA No. 945339.

In southern Madagascar two main tectono-metamorphic events, corresponding to the East African Orogeny (ca. 630–610 Ma) and the Kuunga Orogeny (ca. 580–515 Ma) were recognized. An early structures include recumbent folds and sub-horizontal foliation S1 that were transposed during a regional east–west shortening resulting into north–south-oriented upright folds, with horizontal axes and new vertical axial planar foliations S2. This second deformation event is coeval with the development of a network of vertical ductile shear zones such as the Ejeda SZ, Ampanihy SZ, Beraketa SZ, Ihosy SZ, Zazafotsy SZ and Tranomaro SZ. These megascale 15–25 km-wide intracrustal near-vertical N-S or NW-SE trending strike-slip ductile shear zones crosscut the entire high-grade metamorphic basement of southern Madagascar and separate lower-strain domains where complex fold interference patterns are visible. These high-strain zones developed between 580 and 530 Ma with a slight diachronism from the west to east and south to north and are coeval with melting and UHT/HT granulite facies conditions. The granulite facies metamorphism is widespread throughout the whole basement in all lithologies, in the southern part reaching peak conditions of 900–1000°C at 6–10 kbar and slightly lower temperature conditions (≤800°C) to the west, north, and in the Ikalamavony fold-thrust belt to the north–northeast. Importantly, the shear zones reveal large fluid/melt transfer from the depth that caused extensive fluid/melt rock interaction (in the shear zone as well as in the surrounding basement) that resulted in localized charnockitisation of the granulites. The melt/fluid flux here was structurally controlled and seems to be penetrative throughout the basement rocks. This offers a unique opportunity to study fluid-assisted pervasive melt migration controlled by localized deformation across the whole crust. In this context, our objective is to investigate the tectonometamorphic changes occurring in the intensely deformed high-grade lower to middle crust located in southern Madagascar. Additionally, we seek to delineate the chronological connection between the deformations and high-temperature metamorphism (U)HT, along with the associated processes of crustal anatexis and magmatism.

Structural analysis, U-Pb monazite and xenotime dating, Ar-Ar dating of biotite and amphibole and thermodynamic modeling of peak metamorphic assemblages allow constraining the tectono-thermal evolution of the migmatite-magmatite domain on the southern periphery of Precambrian Zavkhan Block. The main subvertical metamorphic fabric resulted from upright folding and almost complete transposition of early sub-horizontal foliation of uncertain age. P-T conditions of 760-790 degrees C at 0.7-0.8 GPa and in situ U-Pb metamorphic monazite 505-495 Ma ages characterize this tectono-metamorphic event. It was associated with syn-deformational partial melting and intrusion of axial planar syntectonic leucogranite veins. Monazite and xenotime from these veins give U-Pb age of c. 500 Ma, confirming that the migmatization and formation of subvertical fabric were coeval. Such time constraints are contemporaneous with ages of gneissified Ikh-Mongol Arc granite sheets intruding horizontally shortened partially molten crust thereby confirming syn-compressional nature of arc emplacement. Thermal modeling of cooling history of the whole migmatite-magmatite domain constrained by Ar-Ar ages is compatible with the shortening of the hot system followed by moderate erosion. The P-T and geochronological data of the Zavkhan block margin are almost identical to those of short lived late Cambrian magmatism and metamorphism described in the Khondalite belt in Far East China and in other places of Mongolian Collage suggesting that these two domains may have formed a continuous belt.

The Mongolian micro-continental fragments play an important geodynamic role in the Palaeozoic evolution of the Central Asian Orogenic Belt. However, the original provenance of individual blocks within the Nuna/Columbia and Rodinia supercontinents is not well constrained. In this work, we present zircon U-Pb and Lu-Hf isotopic data from the Mesoproterozoic to Neoproterozoic metamorphosed cover of the Baidrag Block in central Mongolia. These data reveal important maxima in the Neoarchean, Palaeoproterozoic and locally Neoproterozoic. Although the majority of the new data can be attributed to local sources within the basement of the Mongolian micro-continental fragments, an important contribution of juvenile 2.0 Ga zircons is linked to the western margin of the Siberian Craton, and 1.5 Ga zircons are either directly derived from North Australia or indirectly recycled from sediments in NW Laurentia.

Archean to Palaeoproterozoic basement rocks exposed in the Dunhuang block in NW China were affected by Palaeozoic crustal reworking, as constrained by previous zircon U-Pb geochronological investigations. However, relationships between the Palaeozoic metamorphic ages, P-T evolution and deformational history of the region remain ambiguous. In order to address this issue, P-T-t-D paths of paragneisses from the basement of the Hongliuxia belt in the southern Dunhuang block were investigated. Inclusions in garnet and kyanite from the paragneisses are considered as vestiges of Palaeozoic M1 metamorphism corresponding to initiation of the prograde evolution. The earliest continuous metamorphic fabric is an originally steep N-S striking foliation S2. This fabric was reworked by vertical folds F3 associated with the development of a ubiquitous steep, mainly south-dipping, E-W striking axial planar foliation S3. The S2 foliation in paragneisses is mainly associated with Grt-St-Ky-Sil-Bt-Ms-Pl-Qz-Rt assemblages in samples from the western domain and with Grt-Ky-Sil-Bt-Kfs-Pl-Qz-Rt assemblages in samples from the northeastern domain of the Hongliuxia belt. The S3 foliation is associated with Grt-Sil-St-Bt-Ms-Pl-Qz-Ilm assemblages in the western domain and with Grt-Sil-Bt-Ms-Pl-Qz-Kfs-Ilm assemblages in the northeastern domain, followed by growth of chlorite in both domains. Early prograde stage (M1) from 4.0-6.5 kbar and 540-560 degrees C to metamorphic peak (M2a) at 9-10 kbar and similar to 650-675 degrees C is mainly recorded by paragneisses from the western domain. Subsequent decompression is initially accompanied by heating (M2b) constrained to 6.5-7 kbar and 675-710 degrees C in the western domain, and to 6-6.5 kbar and similar to 730 degrees C in the northeastern domain, followed by cooling (M3) through 4-6.5 kbar and 550-650 degrees C till late chloritization (late M3). In situ U-Pb dating of monazite combined with monazite trace-element compositions suggests that prograde evolution (M1) most likely started at c. 406 Ma, peak-P conditions (M2a) were reached at 400-394 Ma, decompression associated with heating (M2b) took place at 393-391 Ma, and cooling (M3) during exhumation probably lasted from 380 to 354 Ma. The prograde metamorphism probably reflects burial during underthrusting of neighbouring continental basement (the Alxa block or an equivalent) below the Dunhuang block. This event culminated in pure shear thickening (D2a) of the whole supra-subduction margin followed by minor heating and exhumation (D2b). The D3-M3 event is interpreted as reflecting exhumation during orthogonal shortening of the system, possibly in response to an independent orogenic cycle. Combined with the available regional data, this study reveals the existence of a complex tectono-metamorphic evolution for the Dunhuang block characterized by two distinct orogenic phases with (i) the thickening of a previously thinned arc-back-arc crust recorded in the northern and central belts at 420-410 Ma in the pro-wedge side of the active margin (Sanweishan phase), followed by (ii) the 410-390 Ma thickening in the retro-wedge side (Hongliuxia phase). Such a tectonic evolution of the whole Dunhuang block resembles Andean-type migration of crustal thickening from the convergent front to hinterlands. The D3-M3 event, potentially responsible for the juxtaposition of rocks from different geological occurrences and depths, is seemingly independent from this Andean-type orogenic cycle.

In-situ monazite geochronology with P-T modelling of Barrovian-type metasediments was carried in the Burd Gol zone, NE Baidrag block. The P-T peak recorded by Grt-Sil migmatite is similar to 780 degrees C, 7.5 kbar; by Grt-St-Ky gneiss similar to 690 degrees C, 10 kbar and by Grt schist similar to 590 degrees C, 8 kbar. Monazite ages related to metamorphic peaks indicate diachronous burial events, c. 570-565 Ma for the Grt-Sil migmatite, c. 565-560 Ma for the Grt-St-Ky gneiss, and at c. 538 Ma for the Grt schist. Tonian Grt-St schist recorded new monazite crystallization at c. 546-543 Ma. By c. 545-542 Ma, the Grt-Sil migmatite and Grt-St-Ky gneiss experienced exhumation to similar to 5 kbar, while the Grt schist underwent burial. Further exhumation is dated by xenotime, rutile and monazite alteration down to c. 515 Ma. The data suggest a prolonged orogenic cycle initiated by burial of distal orogenic core to the depths of similar to 35 km at c. 570-565 Ma while the proximal continental margin was buried at c. 545-538 Ma. The higher-grade core was subsequently exhumed to the middle crust at c. 550-540 Ma and the orogenic activity ceased at c. 515 Ma. The difference in metamorphic gradients around 570-560 Ma, with similar to 27-28 degrees C/km for the Grt-Sil migmatite and similar to 18-19 degrees C/km for the Grt-St-Ky gneiss suggests lateral variations as a result of localized juxtaposition of migmatite-magmatite lower-middle crust with colder regions. A new model shows the Burd Gol zone as a progressively thickened and exhumed continental back-arc system beneath the supracrustal 590-570 Ma continental back-arc represented by the adjacent Ulziit Gol unit. The data provide important constraint on the timing and mechanisms of orogenic Baikal cycle at the margin of the Siberian Craton.

The St. Leonhard granulite massif in Lower Austria, dominantly formed by kyanite-bearing felsic granulite, encloses countless up to 5 cm sized mantle xenoliths of garnet clinopyroxenite and peridotite. The mineralogical, textural and chemical consequences of a mutual metasomatic interaction at the contact between these xenoliths and the host orthopyroxene-bearing felsic granulite are described. Movement of Mg, Al and, especially, Ca from the garnet clinopyroxenite to the granulite and migration of K and Na in the opposite direction, caused the breakdown of clinopyroxene and formation of orthopyroxene–plagioclase symplectite coronae at the expense of the garnet clinopyroxenite xenoliths. Around the peridotite xenoliths, monomineralic orthopyroxene coronae have developed due to the supply of Si from the host granulite. The P–T conditions of this interaction were established to 900–1000 °C and 1.0–1.2 GPa by thermodynamic modelling. The duration of coronae growth was constrained to 13–532 ka based on modelling of Fe–Mg interdiffusion underlying secondary compositional zoning of garnet from the garnet clinopyroxenite xenolith extending to the coronae. The most significant change in the host granulite was caused by the supply of Ca from the garnet clinopyroxenite xenolith, which led to the breakdown of the Al2SiO5 phase – probably kyanite – and stabilization of orthopyroxene. K-feldspar-poor haloes surrounding mantle xenoliths formed due to the depletion of K in the granulite adjacent to the garnet clinopyroxenite. The observed origin of felsic–intermediate orthopyroxene-bearing granulite by transformation of felsic kyanite-bearing granulite through the metasomatic interaction with mantle xenoliths implies that the deep crustal chemical exchange between mantle- and crust-derived lithologies may have an important consequences on composition, thermal structure and geodynamic evolution of orogenic lower crust especially in hot collisional orogens, such as the European Variscides.

In the Altai Accretionary Wedge, several periods of Barrovian- and Buchan-type metamorphic cycles were dated from Ordovician to Permian. However, the timing and link between these cycles are not clear, and their causes are debated. In order to contribute to the understanding of Barrovian- to Buchan-type evolution of the accretionary wedges, we studied an area composed of three parallel belts in the easternmost extremity of the Hovd domain located in Mongolian Altai Zone: garnet gneiss in the north, garnet-staurolite-kyanite schist overprinted by +/- sillimanite +/- cordierite +/- andalusite-bearing assemblages in the centre and garnet-sillimanite gneiss in the south. Petrography, garnet zoning and thermodynamic modelling indicate that the garnet gneiss from the northern belt records burial from similar to 510 degrees C and similar to 3-4 kbar to similar to 600 degrees C and similar to 5 kbar, followed by heating to similar to 660 degrees C and decompression to similar to 4.5 kbar. The garnet-staurolite-kyanite schist from the central belt records burial from similar to 550 degrees C and similar to 3-4.5 kbar to similar to 640-680 degrees C and similar to 7 kbar, followed by decompression to the sillimanite stability field at similar to 650 degrees C and similar to 6 kbar. Crystallization of cordierite, andalusite, late muscovite and chlorite in some samples indicates cooling on decompression to similar to 540 degrees C and similar to 3.5 kbar. In the southern gneiss belt, the garnet-sillimanite gneiss with almost unzoned garnet suggests re-equilibration at similar to 6 kbar and similar to 710 degrees C. In situ U-Pb monazite and xenotime dating carried out inclusions in porphyroblasts and matrix grains revealed Carboniferous and Permian ages. The monazite and xenotime from gneisses of the northern and southern belts record Carboniferous and Permian ages, which are interpreted as Carboniferous crystallization at c. 347 Ma associated with metamorphic peak, followed by Permian (re)crystallization at c. 300 and 283 Ma. In the central belt, rare Carboniferous xenotime grains in a garnet-staurolite-kyanite-andalusite-muscovite schist indicate a possible Carboniferous age of the prograde metamorphism. Predominant ages between c. 280 and 260 Ma recorded by monazite are interpreted as a result of complete recrystallization during an LP metamorphic overprint. The Carboniferous ages from the gneisses can be interpreted as constraining the timing of the exhumation of deep crustal rocks to shallow crustal levels. This event corresponds to the formation of crustal-scale migmatite-magmatite domes in the Mongolian Altai Zone. The prograde Barrovian assemblages in the central schist belt are interpreted as having formed contemporaneously during burial in a synform between the migmatite-magmatite domes. The Permian ages reflect LP-HT metamorphism, best recorded by the Buchan-type assemblages in the central schist belt, and are related to massive heat flux from tectonically mobile deep partially molten crust. Correlation with similar Barrovian- and Buchan-type episodes from the Chinese Altai Zone indicates multiple compressional and extensional events in the upper plate accretionary wedge, probably related to retreating and advancing modes of the subduction zone.

The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known. In contrast, accretionary orogenic processes occurring at the supercontinent periphery following Rodinia assembly are poorly understood. To fill this gap, we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt, where numerous data testify for Meso- to Neoproterozoic magmatic reworking. The tectono-metamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic–early Cambrian (Baikalian) cycle. However, we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block, previously considered as a typical example of the Baikalian metamorphic belt. This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist. Grt-Sil-Ky gneiss records initial burial to the sillimanite stability field at ∼720 °C and 6.0 kbar followed by further burial to the kyanite stability field at ∼750 °C and ∼9 kbar and decompression to ∼650 °C and ∼8 kbar. The Grt-St schist records initial burial to the staurolite stability field at ∼620 °C and 6 kbar, followed by further burial to ∼590 °C and 8.5 kbar. The monazite data yield a continuum of 207Pb-corrected 238U/206Pb dates of ca. 926–768 Ma in the Grt-Sil-Ky gneiss, and ca. 937–754 Ma in the Grt-St schist. Based on monazite textural positon, internal zoning, and REE patterns, the time of prograde burial to 6.0 kbar under a thermal gradient of 27–32 °C/km is estimated at ca. 890–853 Ma. It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18–22 °C/km dated at ca. 835–815 Ma. The late monazite recrystallization at ca. 790 Ma is related to decompression. Additionally, monazite with dates of ca. 568–515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint. Metamorphic zircon rims with Th/U ratios of ∼ 0.01–0.06 in Grt-Sil-Ky gneiss with 877 ± 7 Ma age, together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism. The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice – probably of back-arc or arc type. This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri-Rodinian subduction of the Mirovoi Ocean. The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly, followed by strong mechanical coupling potentially related to onset of Rodinia dispersal.

In continental crust, rapid melt flow through macroscopic conduits is usually envisaged as the most efficient form of melt transport. In contrast, there is growing evidence that in hot continental crust, grain‐scale to meso‐scale porous melt flow may operate over long distances and over millions of years. Here, we investigate the dynamics of such porous melt flow by means of two‐dimensional thermo‐mechanical numerical models using the code ASPECT. Our models are crustal‐scale and describe the network of pores through which the melt flows by permeability that depends on the spacing of the pores. Our results suggest that assuming realistic material properties, melt can slowly migrate in the hot and thick continental crust through pores with a characteristic spacing of 1 mm or larger. Despite its low velocity (millimeters to centimeters per year), over millions of years, such flow can create large partially molten zones in the middle‐lower crust and significantly affect its thermal state, deformation, and composition. We examined the role of the permeability, melt and solid viscosities, the slope of the melting curve and temperature conditions. We obtained contrasting styles of melt distribution, melt flow, and solid deformation, which can be categorized as melt‐enhanced convection, growth of partially molten diapirs and melt percolation in porosity waves. Our numerical experiments further indicate that grain‐scale porous flow is more likely in rocks where the melt productivity increases slowly with temperature, such as in metaigneous rocks.

High-pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra-subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure-temperature-time-deformation (P-T-t-D) paths. In the current study, P-T-t-D paths of Early Palaeozoic HP granulite facies rocks, in the form of metabasic lenses enclosed in migmatitic metapelite, from the Dunhuang block, NW China, are investigated in order to constrain the nature of the HP rocks and shed light on the geodynamic evolution of a modern hot orogenic system in an active margin setting. The rocks show a polyphase evolution characterized by (1) relics of horizontal or gently dipping fabric (S1) preserved in cores of granulite lenses and in garnet porphyroblasts, (2) a N-S trending sub-vertical fabric (S2) preserved in low-strain domains and (3) upright folds (F3) associated with a ubiquitous steep E-W striking axial planar foliation (S3). Garnet in the granulites preserves relics of a prograde mineral assemblage Mia equilibrated at similar to 11.5 kbar and similar to 770-780 degrees C, whereas the matrix granulite assemblage (M1b) from the S1 fabric attained peak pressure at similar to 13.5 kbar and similar to 850 degrees C. The granulites were overprinted at similar to 8-11 kbar and similar to 850-900 degrees C during crustal melting (M2) followed by partial re-equilibration (M3) at similar to 8 kbar and similar to 625 degrees C. A garnet Lu-Hf age of 421.6 +/- 1.2 Ma dates metamorphism M1, while a garnet Sm-Nd age of 385.3 +/- 4.0 Ma reflects M3 cooling of the granulites. The mineral assemblage, M1, of the host migmatitic metapelite formed at similar to 9-12.5 kbar and similar to 760-810 degrees C, partial melting and migmatization (M2) occurred at similar to 7 kbar and similar to 760 degrees C and re-equilibration (M3) at similar to 5-6 kbar and similar to 675 degrees C. A garnet Lu-Hf age of 409.7 +/- 2.3 Ma dates thermal climax (M2) and a garnet Sm-Nd age of 356 +/- 11 Ma constrains M3 for the migmatitic metapelites. The timing of this late phase is also bracketed by an emplacement age of syntectonic granite dated at c. 360 Ma. Decoupling of M1 and M2 P-T evolutions between the mafic granulites and migmatitic metapelites indicates their different positions in the crustal column, while the shared pressure-temperature (P-T) evolution M3 suggests formation of a melange-like association during the late stages of orogeny. The high-pressure event D1-M1 is interpreted as a result of Late Silurian-Early Devonian moderate crustal thickening of a thermally softened and thinned pre-orogenic crust. The high-temperature (HT) reequilibration D2-M2 is interpreted as a result of Mid-Devonian shortening of the previously thickened crust, possibly due to 'Andean-type' underthrusting. The D3-M3 event reflects Late Devonian supra-subduction shortening and continuous erosion of the sub-crustal lithosphere. This tectono-metamorphic sequence of events is explained by polyphased Andean-type deformation of a 'Cascadia-type' active margin, which corresponds to a supra-subduction tectonic switching paradigm.