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This study presents new petrological and fluid inclusion datasets of migmatites from the Karakorum Shear Zone, Ladakh, India, to know the P-T-fluid evolution of mineral assemblages and the associated tectonic history. The presence of plagioclase, quartz, and biotite inclusions in the coarse-grained poikiloblastic pargasite (amphibole) is indicative of the hydration reaction [Formula omitted], which is consistent with diffusive H.sub.2O-fluxed melting. Phase equilibria calculations are consistent with migmatization at 0.85-1.02 GPa and 640-670 °C in water-saturated conditions (i.e., 0.7 wt% H.sub.2O). The monophase primary and secondary carbonic fluid inclusions present in quartz display eutectic temperatures between - 56.9 and - 56.6 °C, suggesting pure CO.sub.2 composition. The isochores of primary CO.sub.2 inclusions reveal that the post-peak migmatization event took place between 0.59-0.55 GPa and 550-670 °C, which occurred due to density reversal during the re-equilibration. The fluid inclusion microtextures preserved the signature of isothermal decompression, which is well corroborated with mineralogical P-T calculations. Primary inclusions were preserved initially as carbonic-aqueous fluids; however, the H.sub.2O phase diffused out subsequently and dissolved with the melt such that the inclusions became pure carbonic. Fluid infiltration along the Karakorum Shear Zone played a critical role in forming migmatites. The P-T path derived from thermodynamic modeling and fluid inclusion data are consistent with isothermal decompression during exhumation following crustal thickening of the Asian continent (or South Tibaten Crust) between 18 and 15 Ma.

Aluminosilicates (kyanite, sillimanite and andalusite) are useful pressure–temperature (P–T) indicators that can form in a range of rock types through different mineral reactions, including those that involve partial melting. However, the presence of xenocrystic or inherited grains may lead to spurious P–T interpretations. The morphologies, microtextural positions, cathodoluminescence responses and trace element compositions of migmatite-hosted kyanite from Eastern Bhutan were investigated to determine whether sub-solidus kyanite could be distinguished from kyanite that crystallised directly from partial melt, or from kyanite that grew peritectically during muscovite dehydration reactions. Morphology and cathodoluminescence response were found to be the most reliable petrogenetic indicators. Trace element abundances generally support petrographic evidence, but protolith bulk composition exerts a strong control over absolute element abundance in kyanite. Sample-normalised concentrations show distinctive differences between petrogenetic types, particularly for Mg, Ti, V, Cr, Mn, Fe and Ge. LA-ICP-MS element maps, particularly combined to show Cr/V, provide additional information about changing geochemical environments during kyanite growth. Most kyanite in the studied migmatitic leucosomes is of sub-solidus origin, with less widespread evidence for peritectic crystallisation. Where present, grain rims commonly crystallised directly from the melt; however, entire grains crystallised exclusively from melt are rare. The presence of kyanite in leucosomes does not, therefore, necessarily constrain the P–T conditions of melting, and the mechanism of growth should be determined before using kyanite as a P–T indicator. This finding has significant implications for the interpretation of kyanite-bearing migmatites as representing early stages of melting during Himalayan evolution.

Migmatites are common in the hinterland of orogenic belts. The timing and mechanism (in situ vs. external, P-T conditions, reactions, etc.) of melting are important for understanding crustal rheology, tectonic history, and orogenic processes. The Adirondack Highlands has been used as an analog for mid/deep crustal continental collisional tectonism. Migmatites are abundant, and previous workers have interpreted melting during several different events, but questions remain about the timing, tectonic setting, and even the number of melting events. We use multiscale compositional mapping combined with in situ geochronology and geochemistry of monazite to constrain the nature, timing, and character of melting reaction(s) in one locality from the eastern Adirondack Highlands. Three gray migmatitic gneisses, studied here, come from close proximity and are very similar in microscopic and macroscopic (outcrop) appearance. Each of the rocks is interpreted to have undergone biotite dehydration melting (i.e., Bt+Pl+Als+Qz=Grt+Kfs+melt). Full-section compositional maps show the location of reactants and products of the melting reaction, especially prograde and retrograde biotite, peritectic K-feldspar, and leucosome, in addition to all monazite and zircon in context. In addition, the maps provide constraints on kinematics during melting and a context for interpretation of accessory phase composition and geochronology. More so than zircon, monazite serves as a monitor of melting and melt loss. The growth of garnet during melting leaves monazite depleted in Y and HREEs while melt loss from the system leaves monazite depleted in U. Results show that in all three localities, partial melting occurred during at ca. 1160-1150 Ma (Shawinigan orogeny), but the samples show high variability in the location and degree of removal of the melt phase, from near complete to segregated into layers to dispersed. All three localities experienced a second high-T event at ca. 1050 Ma, but only the third (non-segregated) sample experienced further melting. Thus, in addition to bulk composition, the fertility for melting is an important function of the previous history and the degree of mobility of earlier melt and fluids. Monazite is also a sensitive monitor of retrogression; garnet breakdown leads to increased Y and HREE in monazite. Results here suggest that all three samples remained at depth between the two melting events but were rapidly exhumed after the second event.

Subduction of Neo-Tethys oceanic lithosphere beneath the Iranian plate during the Mesozoic formed several igneous bodies of ultramafic to intermediate and felsic composition. Intrusion of these magmas into a regional metamorphic sequence (the Sanandaj-Sirjan Zone) caused partial melting and formation of migmatites with meta-pelitic protoliths. The Alvand complex (west Iran) is a unique area comprising migmatites of both mafic and pelitic protoliths. In this area, the gabbroic rocks contain veins of leucosome at their contact with pyroxenite and olivine gabbro. These leucosomes are geochemically and mineralogically different from leucosomes of the meta-pelitic migmatites and clearly show properties of I-type granites. Microscopic observations and whole rock compositions of the mafic migmatite leucosomes show that migmatization occurred through partial melting of biotite, hornblende and plagioclase. Thermobarometric calculations indicate 800 °C and 3.7 kbar for partial melting, although phase diagram modeling demonstrates that the presence of water could decrease the solidus temperature by about 40 °C. Our results suggest an asthenospheric magma upwelling as the source of heat for partial melting of the gabbroic rock during subduction of Neo-Tethys oceanic crust under the western edge of the Iranian plate. The present study also reveals relationships between migmatization and formation of S- and I -type granites in the area.

Migmatite domes are common in metamorphic core complexes. Dome migmatites deform in the partially molten or magmatic state and commonly record complex form surfaces, folds, and fabrics while units mantling the dome display a simpler geometry, typically formed by transposition during crustal extension. We use field observations and magnetic fabrics in the Naxos dome (Greece) to quantify the complex flow of anatectic crust beneath an extensional detachment system. The internal structure of the Naxos dome is characterized by second‐order domes (subdomes), pinched synforms, and curved lineation trajectories, which suggest that buoyancy‐driven flow participated in dome evolution. Subdomes broadly occur within two compartments that are separated by a steep, N‐S oriented, high‐strain zone. This pattern has been recognized in domes formed by polydiapirism and in models of isostasy‐dominated flow. The preferred model involves a combination of buoyancy‐ and isostasy‐driven processes: the Naxos dome may have been generated by regional N‐S extension that triggered convergent flow of partially molten crust at depth and the upwelling of anatectic migmatites within the dome. This pattern is complicated by gravitational instabilities and/or overturning of the high melt fraction crust leading to the growth of subdomes. As the migmatites within the Naxos dome reached a higher structural level, they were affected by regional top‐to‐the‐NNE kinematics of the detachment system. Dome formation therefore occurred by a combination of coeval and coupled processes: upper crustal extension, deep crust contraction during convergent flow of anatectic crust, diapirism and/or density‐driven crustal convection forming subdomes, and north directed detachment kinematics. Key Points Migmatite fabrics suggest buoyancy‐ and isostasy‐driven flow during doming Subdomes may form by diapirism, crustal convection, or convergent crustal flow Doming occurred by coeval upper crustal extension and flow of anatectic crust

The Barrovian metamorphism of the Lepontine dome is manifested by isograds that cross‐cut tectonic nappe contacts, which is commonly interpreted as metamorphism that occurred after nappe emplacement. However, the pervasive mineral and stretching lineation in amphibolite facies, associated with top‐to‐foreland shearing, suggests that peak Barrovian conditions are coeval with nappe‐overthrusting. Here, we combine mapping and U‐Pb zircon dating to better constrain the relation between metamorphism and overthrusting. Metamorphic zircon rims show two age populations at 31–33 and 22–24 Ma. The younger population is locally observed in post‐foliation dikes (and associated metasomatism) likely sourced from deep‐migmatites exhuming along the Alpine backstop. The older population occurs regionally and is found in syn‐kinematic migmatites which occur along a crustal‐scale shear zone. Below this shear zone, magmatic and detrital zircon cores suggest that the Cima Lunga unit, previously interpreted as a tectonic mélange with Mesozoic fragments, was a pre‐Variscan metasedimentary sequence intruded by Permian granitic sills, now orthogneisses. This unit was strongly sheared along the top of the Simano nappe during overthrusting of a rock pile here‐termed Maggia‐Adula nappe. This large‐scale nappe emplacement imprinted the regional lineation and peak temperatures until 31–33 Ma. Péclet (1–10) and Brinkman (0.002–1.8) numbers, estimated for the overthrusting, suggest an advection‐dominated heat transfer caused by rock exhumation, with some diffusion (conduction) during nappe emplacement. Diffusion contributed to Barrovian isograds discordant to the thrust. Shear heating was important if stress times shearing rate >∼5·10−6 W·m−3 within the nappe. The thermal evolution after overthrusting was spatially heterogeneous until ca. 22 Ma. Plain Language Summary The Lepontine area constitutes the core of the Central European Alps. It has a dome structure and it is internally formed by rock units which register pressure and temperature conditions typical of collisional orogens. The temperatures recorded by minerals are high and the origin of the heat that affected the Lepontine units is still unclear. In this study, we implemented different branches of geology to reveal the age of the Alpine events which juxtaposed the Lepontine units, their provenance and evolution. We performed extensive geological mapping to define the lithologies and structures of rocks. From 13 samples, we extracted 1158 zircon crystals that we analyzed and dated. Fieldwork permitted us to discover new rock units and better characterize the transition between the large‐scale units constituting the Lepontine dome. We propose a geodynamic scenario involving a major large‐scale Alpine unit. The emplacement of this unit generated the main heating event at 31–33 Ma, which is widespread and resulted in peak temperature conditions. The thermal evolution after this event was regionally complex and spatially heterogeneous. Locally in the south we document magmatic/fluid injections at 22–24 Ma, which were sourced from still‐hot regions in the roots of the orogen. Key Points Geological mapping reveals widespread syn‐tectonic migmatitic rocks along a main crustal‐scale shear zone inside the Lepontine dome Alpine nappe emplacement is regionally recorded at 31–33 Ma, locally overprinted in the south by magmatic/fluid pulses at 22–24 Ma Mapping and detrital zircon crystals suggest a structurally coherent Cima Lunga unit of pre‐Variscan age, as part of the Simano nappe

The study presents detailed petrographical, geophysical, structural and geochemical data of the internal nappes zone to establish the deformational history, origin and tectonic setting and constrain the crustal growth and evolution of the active margin of the Dahomeyide belt. Two main lithological units, (i) deformed meta-granitoids (migmatites and gneisses) and (ii) undeformed granitoids, dominate the internal nappes zone. The granitoids are generally I-type, metaluminous to weakly peraluminous, low-K tholeiite to high-K calc-alkaline and of tonalite, granodiorite and granite affinity. The overall trace element patterns of the studied granitoids characterized by the enriched LILE and depleted HFS, with negative peaks of Nb-Ta, Sr, P and Ti, are indications of arc-related magmatism. Structural analysis reveals four deformation phases (D1-D4). D1 represents Northwest-Southeast (NW-SE) Pan African shortening associated with a continent-continent collision, resulting in westward nappe stacking. Progressive NW-SE shortening resulted in D2 and D3 top-to-the-NW dextral and sinistral thrusting events during the Pan-African orogeny. D4 is an extensional event likely associated with the orogenic collapse phase. The gneisses and migmatites, with dominant axial planar foliations, point to their formation in a collisional setting or influence by the Pan-African collisional processes. Continental-arc signatures in these rocks imply continental subduction during their protolith formation. The intrusive granitoid and pegmatite are undeformed, meaning late- to post-orogenic emplacement. These findings suggest that the internal nappes zone archived the subduction-collision and post-collisional phase of the Pan-African orogeny and recorded large-scale migmatization and granitoid emplacement due to partial melting of thickened lower crust between Mid-Cryogenian and late Ediacaran.

The Valpelline Series (Dent-Blanche Tectonic System, Western Italian Alps) is a sector of lower continental crust, which consists of Permian migmatitic metapelite with different mineral assemblages (i.e., garnet-, cordierite- and orthopyroxene-bearing), minor amphibolite and marble, intruded by aplite and pegmatite. Widespread melt production in metapelite and locally in amphibolite occurred during the development of the regional foliation. The P–T conditions during migmatisation, estimated using conventional geothermobarometers, range between 800–900 °C and 0.5–0.8 GPa, with a difference of up to ∼50 °C between cordierite- and orthopyroxene-bearing migmatites, the latter reaching higher temperatures. The Valpelline Series shows rock types, metamorphic assemblages, P–T conditions and published ages of high-temperature regional metamorphism like the archetypal lower crust section of the Ivrea-Verbano Zone in the Southern Western Alps. The latter likely represents an external portion of the same extending lower crust, at the onset of the Tethyan rifting due to lithospheric extension and asthenospheric rising.

The solubility of apatite in anatectic melt plays an important role in controlling the trace-element compositions and isotopic signatures of granites. The compositions of glassy melt inclusions and nanogranitoids in migmatites and granulites are compared with the results of experimental studies of apatite solubility to evaluate the factors that influence apatite behavior during prograde suprasolidus metamorphism and investigate the mechanisms of anatexis in the continental crust. The concentration of phosphorus in glassy melt inclusions and rehomogenized nanogranitoids suggests a strong control of melt aluminosity on apatite solubility in peraluminous granites, which is consistent with existing experimental studies. However, measured concentrations of phosphorus in melt inclusions and nanogranitoids are generally inconsistent with the concentrations expected from apatite solubility expressions based on experimental studies. Using currently available nanogranitoids and glassy melt inclusion compositions, we identify two main groups of inclusions: those trapped at lower temperature and showing the highest measured phosphorus concentrations, and melt inclusions trapped at the highest temperatures having the lowest phosphorus concentrations. The strong inconsistency between measured and experimentally predicted P concentrations in higher temperature samples may relate to apatite exhaustion during the production of large amounts of peraluminous melt at high temperatures. The inconsistency between measured and predicted phosphorus concentrations for the lower-temperature inclusions, however, cannot be explained by problems with the electron microprobe analyses of rehomogenized nanogranitoids and glassy melt inclusions, sequestration of phosphorus in major minerals and/or monazite, shielding or exhaustion of apatite during high-temperature metamorphism, and apatite-melt disequilibrium. The unsuitability of the currently available solubility equations is probably the main cause for the discrepancy between the measured concentrations of phosphorus in nanogranites and those predicted from current apatite solubility expressions. Syn-entrapment processes such as the generation of diffusive boundary layers at the mineral-melt interface may also be responsible for concentrations of P in nanogranitoids and glassy melt inclusions that are higher than those predicted in apatite-saturated melt.

Migmatites are widespread in the North Dabie ultrahigh-pressure metamorphic terrane (NDT) of Dabie orogen, East China. Idiomorphic and poikilitic amphibole grains in both leucosome and melanosome contain inclusions of plagioclase, quartz and biotite, suggesting formation of leucosome by fluid-present melting of biotite + plagioclase + quartz-bearing protoliths at P  = 5–7 kbar, T  = 700–800 °C. Precise SIMS zircon U–Pb dating indicates that migmatization of Dabie orogen initiated at ~140 Ma and lasted for ~10 Ma, coeval with the formation of low-Mg # adakitic intrusions in Dabie orogen. Based on mineralogical, petrographic and geochemical data, leucosomes in NDT can be subdivided into three groups. (1) High La/Yb (N) –Medium Sr/Y group (Group I), whose high Dy/Yb (N) but medium Sr/Y ratios are caused by amphibole and plagioclase residual during partial melting of dioritic to granodioritic gneisses. (2) Low La/Yb (N) –Low Sr/Y group (Group II), whose flat HREE patterns are produced by entrainment of peritectic amphiboles into melts derived from partial melting of dioritic gneiss. (3) High La/Yb (N) –High Sr/Y and Eu # group (Group III), whose extremely high Sr and Eu but low other REE concentrations are caused by accumulation of plagioclase and quartz. Although Group I and III fall in the adakitic fields on La/Yb (N) –Yb (N) and Sr/Y–Y diagrams, they are chemically distinct from contemporary high-pressure adakitic intrusions in Dabie orogen in a series of geochemical indexes, for example, lower Dy/Yb (N) and/or Sr/Y ratios at given La/Yb (N) ratio, lower Sr/CaO ratios, lower Rb concentration but higher K/Rb ratios. Therefore, leucosomes are produced by anatexis of the exhumed ultrahigh-pressure metamorphic rocks at middle crustal level, instead of partial melting of thickened lower crust with garnet-rich and plagioclase-poor residual. The coeval occurrence of migmatites and high-pressure adakitic intrusions in Dabie orogen indicates large-scale partial melting of middle to thickened lower crustal column in the early Cretaceous. The required heat source may be the mantle heat conducting through the lithospheric mantle whose lower parts have been convectively removed.