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The Meelpaeg structure in southwestern Newfoundland comprises allochthonous tectonites formed during the Salinic (D1-2) and Acadian (D3) orogenies. D1-2 occurred between 451 and 417 Ma and culminated in Barrovian metamorphism during terminal collision of the Gander margin with composite Laurentia. Collision was followed by tectonic escape of the deeply buried rocks between 417 and 412 Ma. Rocks of the Victoria arc and Exploits backarc preserved in the Port aux Basques and Grand Bay complexes were emplaced during D1 by the Grandys River shear zone, which is outlined by a narrow band of the ca. 451 Ma Port aux Basques granite, towards the southeast above the Harbour le Cou Group. The contrasting histories displayed across the Grandys River shear zone are typical of the Dog Bay Line further northeast. Salinic structures were overprinted by faults and folds formed during Acadian D3 transpression (≤412 Ma), which produced the bi-vergent Meelpaeg structure and emplacement of amphibolite facies tectonites above greenschist facies rocks along its bounding shear zones. F3 folding progressively steepened the faults, which in turn led to progressive localization of dextral strike–slip in narrow fault zones. The high grade of metamorphism in the Meelpaeg structure is attributed to protracted underthrusting of the Cabot promontory of the Gander margin beneath composite Laurentia. Salinic convergence was sinistral but became dextral during the Early Devonian, diachronous Acadian orogeny. The kinematic switch is proposed as a tool to separate Salinic from Acadian structures in the central part of the northern Appalachians. [ABSTRACT FROM AUTHOR]

Abstract\nKEY POINTSAny model invoking gold fluids coming through crustal rocks at granulite facies grade or coming from the mantle is built on a misunderstanding of the way aqueous fluids interact with rocks at high temperature, above their solidus. It is not currently explained in such models how aqueous fluids pass through rocks at granulite facies grade without being involved in melting. At temperatures above 650–750 °C, crustal rocks melt in the presence of an aqueous fluid with the latter being consumed, the amount of melt generally being proportional to the amount of fluid involved. This means auriferous hydrothermal fluids are unable to infiltrate hot lower crust from the mantle. An alternative model exists that avoids this problem, at least for typical gold-only deposits, for example Kalgoorlie. It involves fluid generation by metamorphic devolatilisation in the mid-crust at the greenschist–amphibolite facies transition, at around 450 °C. This process generates significant volumes of aqueous fluid during prograde metamorphism. In the presence of carbonate and sulfide minerals, the resulting fluid is H2O–CO2–H2S dominated by low salinity, a composition conducive to gold dissolution. A strength of this model is that the fluid will become gold-bearing as it is generated. As more or less all mineral grains in the rock are involved in the devolatilisation, gold present at parts-per-billion level in grains as well as on grain boundaries is accessible to the fluid.Introduction of an aqueous fluid into the granulite facies leads to partial melting.Models sourcing gold-bearing hydrothermal fluids from the lower crust or mantle defy metamorphic principles.Metamorphic fluid evolved from the greenschist-to-amphibolite transition addresses the main characteristics found amongst typical gold-only deposits. [ABSTRACT FROM AUTHOR]

The geologic evolution of the Gondwanide orogen recorded during the late Palaeozoic along the Panthalassan border of Gondwana is related to an active continental margin, though it is still debatable whether it was built by collision or accretion. To disentangle its orogenic processes and provide constraints on the orogen type, we characterize the physicochemical conditions of metamorphism and associated deformation of the Mina Gonzalito Metamorphic Complex from northern Patagonia (41°28'30" S-65°40'30" W). New field mapping and comprehensive petrochronologic analysis constrain the evolution of a thick-skinned fold and thrust belt in a retroarc setting, which is spatially and temporally related to the geometry and kinematics of the right-lateral reverse El Jagüelito ductile shear zone. Metamorphic evolution resulted in a clockwise P‐T‐t-D path reflecting three stages of continuously changing P-T conditions under high-pressure (8.0–9.9 kbar) amphibolite facies (540–680°C) during a single-phase progressive ductile deformation event. Monazite ages ranging from 303 ± 5 to 252 ± 6 Ma account for the single-phase regional tectono-metamorphic event spanning ~ 50 My. Our results, integrated with regional data, led to interpreting the late Palaeozoic Gondwanide orogen developed along the Panthalassan margin of Gondwana as accretionary in advancing-mode. [ABSTRACT FROM AUTHOR]

Chromite in the amphibolites of the Myanmar jadeite deposits has not been well studied. Mineralogical studies on chromite and related kosmochlor and Cr-omphacite in the amphibolite of the Myanmar jadeite deposits were conducted. Compared to the chromite in the adjacent serpentinized peridotite, the chromite had higher Cr2O3 (45.67–54.25 wt.%) and MnO (1.82–1.90 wt.%) but lower MgO (1.00–1.96 wt.%) and Al2O3 (1.05–15.09 wt.%), similar to the published chromite compositions in jadeitite. Serpentinite was derived from a highly depleted mantle peridotite. There were at least two stages of metasomatism during the transformation of serpentinite + chromite to magnesio-katophorite + chromite + thin kosmochlor (and/or Cr-omphacite cortex). The first stage was the Ca-rich metasomatism of serpentinite, resulting in sodic-calcic amphibole (magnesio-katophorite), which preceded the formation of jadeite. The second stage of Na-rich metasomatism was produced by the Na-Al-Si-rich fluids with the magnesio-katophorite + chromite (contemporaneous with the formation of jadeite). The composition of the fluid was altered by a reaction with magnesio-katophorite, increasing the Ca-Mg content and resulting in the formation of kosmochlor rich in Ca-Mg and/or peripheral Cr-omphacite. This kosmochlor–Cr-omphacite belongs to the Jd-Kos-Di ternary join, which differs from the kosmochlor–Cr-jadeite (which belongs to the Jd-Kos join in jadeitite). The formation of jadeitite with chromite + kosmochlor + Cr-jadeite occurs when large amounts of Na-Al-Si-rich fluids have wrapped the pieces of chromite-bearing amphibolite. This also explains the proverbial "moss spray green" given that amphibole (with chromite) brings out the green color in jadeitite. [ABSTRACT FROM AUTHOR]

The Middle Allochthon of the Galicia-Trás-os-Montes Zone (GTMZ) in NW Iberia is an assemblage of oceanic units representing the Variscan suture of one or more peri-Gondwanan oceans. This contribution brings new geochronological and geochemical data from the Middle Allochthon of the Morais Complex, in northern Portugal. There, the oceanic ensemble consists of a stack of five tectonic units which from bottom to top are the Junqueira, Pombais and Izeda oceanic supracrustals and the Remondes and Morais-Talhinhas ophiolitic units. The Junqueira and Pombais units consist of greenschists and metapelites. No age data are available for these two units, but a correlation is established with a Cambro-Ordovician unit in the Órdenes Complex of Galicia (NW Spain) dated at 500 Ma. The Izeda Unit consists of fine-grained, low-grade amphibolites transitional to epidote-amphibolites and greenschists, for which an age of 484 ± 3 Ma has been obtained for a metabasic tuff. In the ophiolitic ensemble, the Remondes and Morais-Talhinhas units consist of fine-grained amphibolites associated with deformed metagabbros, mafic cumulates and serpentinized ultramafics. In this study, two plagiogranite samples from the Remondes Unit yielded a complex spread of age data with Silurian ages representing the dominant population. This suggests that the newly defined Remondes Unit is younger than the Izeda Unit but older than the previously published age for these rocks, which are considered part of the overlying Morais-Talhinhas Unit, dated as Devonian (406–395 Ma) in a previous study. The geochemistry of the basic rocks in the five units is that of NMORB with a subduction-derived component that varies from weak to null. The ages, together with new and previously published whole rock geochemical data obtained from basic igneous samples, are interpreted to date and reflect the formation of igneous protoliths in oceanic ridge settings associated with the Mid-Variscan Ocean, a part of the Rheic oceanic realm that registered magmatic activity between the late Cambrian and the Lower Devonian. [ABSTRACT FROM AUTHOR]

Dating of zircons from eclogites and amphibolite in metamorphic rocks of the Eastern Mugodzhar zone (terminal south of the Urals) is conducted. Of four eclogite samples, close (and the oldest ones of 520 ± 4 Ma) concordant age values are obtained in three samples, and these ages probably correspond to the age of the protolith. Concordant dates of 472 ± 3 and 379 ± 3 Ma correspond to the main stages of metamorphism in the East Mugodzhar Range: the former marks the time of high-pressure metamorphism, whereas the latter corresponds to the metamorphism of amphibolite facies. The presence of ancient and at the same time varied-age zircons (probably carrying rounding signatures) indicates the primary sedimentary nature of the amphibolites studied. The youngest zircons from eclogites of the Mugodzhar Range are of 282 ± 2 Ma in age, corresponding to the stage of collision. The obtained data show that the metamorphic complexes of the Mugodzhar Range are not units of Early Proterozoic or Riphean age (as was previously believed), but represent Lower–Middle Paleozoic complexes from the middle of the Earth's crust. Thus, these metamorphic rocks belong to the deep part of an island-arc system that had been located in the eastern sector of the Urals, which later was drawn to the near-surface level when some regions of the Urals underwent uplift and erosion. [ABSTRACT FROM AUTHOR]

This study assesses heavy metal (HM) concentrations and ecological risks in soils derived from granite (GrS), basalt (BtS), and amphibolite (AmS) rocks in southern Bundelkhand, central India. HM analysis reveals substantial variations, with Cr, Mn, Co, Ni, Zn, and Cd exhibiting higher concentrations in AmS, followed by BtS and GrS. Mn and Co content exceeds the Indian limit for soil (IS) and European Union (EU) guidelines for most samples. The geo-accumulation index shows high pollution levels of Cd in all soil types. Enrichment factor (EF) assessment reveals significant enrichment of Cd in GrS and BtS (mean 12.53, 6.91, respectively), and Cr, Ni, and Cd in AmS. The potential contamination index (Cp) and pollution load index (PLI) confirm Cd as the most significant contaminant in all soil types, with AmS exhibiting the highest pollution levels (29.31 and 650.68, respectively). The potential ecological risk (RI) assessment identifies Cd as the major contributor to ecological risk in all soil groups, with GrS and BtS displaying low to moderate risk, and AmS posing high to very high ecological risk. Our study highlights the influence of parent material on soil heavy metal concentrations, potentially leading to groundwater contamination and posing risks to human health. [ABSTRACT FROM AUTHOR]

A structural analysis of the Gol Mountains of North Tanzania is presented in which we identify and characterize a sheet‐like deformation‐localization zone of regional scale. Syn‐metamorphic rock‐deformation within the localization zone gave rise to widespread tectonite development under high‐T in lower‐mid crustal realms. Regional foliations and mineral/stretching lineations, and associated non‐coaxial mineral petrofabrics can be correlated with ductile flow shear planes. At the macro‐scale these structures are genetically connected with tangential movement zones involving large a‐type and sheath folds, thrusts and ductile shear zones, and hydrothermal quartz megaveins plastically deformed. The dimension of the area affected by this structural style is c. 30 km across the Mozambique Belt and >50 km parallel to it. The associated tectonic displacements likely attained several tens of km in the context of an overall W‐directed tectonic vergence transversal to the Mozambique Belt orogenic trend. It is interpreted as a segment of a mid‐lower crustal orogenic channel active during late Ediacaran amalgamation of the Congo‐Tanzania Archean Craton (and its discontinuous fringe of Paleoproterozoic and Mesoproterozoic orogens) with the amalgamated arcs and terranes of the Arabian‐Nubian Shield. Key Points: Regional syn‐metamorphic high‐T deformation occurred in a regional sheet‐like localization zone interpreted as a crustal orogenic channelLarge a‐type/sheath folds, thrusts and ductile shear zones and hydrothermal megaveins recorded large tangential movement zonesThese structures associated tectonic displacements W‐directed, transversal to the Mozambique Belt orogenic trend [ABSTRACT FROM AUTHOR]

The timing and mechanism of the crustal extension that caused blocks of continental crust to rift and drift northwards from the northern Gondwana margin during the Early Palaeozoic are currently debated. Here, we present new field, geochemical and U-Pb zircon data from peri-Gondwanan Central Sakarya terrane in NW Turkey to shed light on the timing and petrogenesis of extensional magmatism and subsequent passive margin development of the northern Gondwana margin. Our 1/25.000 scale mapping of a selected area (250 km2) has revealed a south-vergent thrust stack comprising three slices. The Middle Slice of the thrust stack consists of a gneiss-amphibolite complex that has a basement-cover type stratigraphy (before regional metamorphism). The basement unit consists of meta-granitoids, intruded by meta-syenite and banded amphibolites, with geochemical characteristics reflecting I&S-type, A-type and within plate alkaline/transitional-type magmatism, respectively. The cover meta-clastic sediments have amphibolite lenses with MORB and back-arc basin-type geochemistry. Meta-granitoid bodies yielded zircon U-Pb ages of 507 ± 16 Ma, 503 ± 12 Ma, and 488 ± 2.6 Ma (Late Cambrian-Early Ordovician), whereas the meta-syenites gave ages of 473.9 ± 5.7 Ma and 471.4 ± 2.5 Ma (late Early Ordovician); also, age of the banded amphibolite was dated as 473.3 ± 4 Ma. The youngest detrital zircon population in the pebbly quartzite sample within cover meta-sediments (458 ± 3 Ma) constrains the maximum depositional age to around early Late Ordovician. The late Early Ordovician bimodal magmatism, a key focus of our study, is interpreted to have formed in response to the Rheic Ocean rifting along the northern margin of Gondwana. The Late Cambrian granitic magmatism, a precursor to bimodal magmatism, can be explained by lithospheric thinning and upwelling of the asthenosphere to form anatectic silicic melts. The unconformity between the basement and cover units may represent the initiation of a new phase of extension at the N Gondwana margin that culminated in the northward drifting of the Central Sakarya and the opening of Palaeotethys, associated with back-arc basic magmatism in Late Ordovician-Silurian. [ABSTRACT FROM AUTHOR]

The Poroshiri ophiolite (Hidaka metamorphic belt, Japan) occurs within a crustal‐scale network of high‐temperature, dextral shear zones that accommodated hundreds of kilometres of displacement due to the opening of the Japan Sea in the Neogene. The opholitic rocks comprise ultramafic, mafic and sedimentary protoliths that have been variably hydrated and metamorphosed. This work investigates the mechanisms and timing of fluid influx relative to viscous deformation in metagabbros and amphibolites deformed during exhumation from granulite‐ to amphibolite‐facies conditions. We consider a range of microstructures, from low strain domains and 1–2 mm thick shear bands to mylonites with a thickness of a few meters. Low strain domains of metagabbros exhibit corona textures with symplectites consisting of pargasitic amphibole (Ed0.7) ‐ anorthitic plagioclase (An80–92) ± orthopyroxene ± clinopyroxene forming around olivine and igneous pyroxene and with similar plagioclase–amphibole‐orthopyroxene ± clinopyroxene granoblastic aggregates in micrometric‐thick fractures. These textures result from hydration under low fluid‐rock ratio, with elevated H2O content only occurring locally (H2O > 1–1.2 wt%). Igneous mineral replacement leads to grain size reduction from 1 mm to ~10 μm. Amphibole exhibits a strong core‐rim zonation primarily controlled by the high diffusivity of Fe, Mg and OH and the low diffusivity of Al. Mineral compositional equilibrium is achieved at the scale of 100–200 μm. In mm‐thick localized shear bands and in metric‐scale mylonitic amphibolites, the heterogeneous mineral composition of amphibole (Ed0.2–0.5) and plagioclase (An40−An80) indicates only partial re‐equilibration (at the scale of 200–500 μm) despite higher fluid‐rock ratios and more pervasive fluid percolation than in metagabbros. Plagioclase–amphibole thermometry and equilibrium phase diagrams indicate that the initial fluid infiltration and corona formation occurred at 800–850°C by fracturing and percolation along grain boundaries. This was followed by the main fluid percolation and mylonitization event, which occurred during exhumation and cooling at conditions of 720–580°C, ~4 kbar. Continuous hydration during retrogression was achieved by the influx of dominantly seawater‐derived fluid, as attested by the high chlorine (Cl) contents (>300–400 ppm) of amphiboles in fractures. The heterogeneous distribution of fractures controls the distribution of fluid from the earliest stages of hydration, creating positive feedback where the growth of hydrous minerals as amphibole (up to +300 vol% of amphibole in high strain areas relative to the low‐strain ones) and the formation of fine‐grained mixed domains that led to further localization of viscous strain and mass transfer (variations of ± 30–40% in major elements). The Poroshiri ophiolite therefore represents a good fossil example of a former transpressional plate boundary where coupled metamorphic and deformation processes were triggered by seawater‐derived fluid that percolated to depths of ~15 km. [ABSTRACT FROM AUTHOR]

The Central lapetus Magmatic Province (CIMP) is a large igneous province (LIP) emplaced in the Baltican and Laurentian paleocontinents that marks the onset of the Caledonian Wilson Cycle. Paleozoic magmatism of the CIMP is preserved in both northeastern America and northern Europe. This study investigates rocks belonging to the hyper-extended margin of Baltica currently found in the Seve Nappe Complex of the Scandinavian Caledonides. Specifically, U-Pb zircon geochronology and whole-rock geochemistry are applied to a migmatitic variety of the Vierrucohkka amphibolite of the Marma Terrane, to the Aurek gabbro, and amphibolite of the Aurek Assemblage exposed in the Seve Nappe Complex in the Kebnekaise region, northern Swedish Caledonides. U-Pb zircon geochronology yields crystallization ages of 626 ±7 Ma for the protolith of the Vierrucohkka amphibolite, and 614 ±2 Ma and 609 ±1 Ma for the emplacement of the Aurek gabbro and amphibolite protolith, respectively. A younger age of 599 ±3 Ma is recorded in the Vierrucohkka amphibolite and is interpreted as the age of partial melting and migmatization. The geochemical signatures of the rocks demonstrate crustal assimilation during the emplacement of their protoliths and modification due to prograde metamorphic processes during Caledonian subduction. The Vierrucohkka amphibolite and the Aurek Assemblage samples display upper and lower crustal assimilation, respectively. Trace elements (Dy, Sm, Lu, and Y) record the growth of metamorphic garnet, while elevated TiO2contents record the crystallization of metamorphic rutile. Nevertheless, high field strength elements (HSFE) and ANb suggesta depleted mantle source forthe magmas of the protoliths of the investigated rocks. Altogether, geochemical data indicate that the igneous activity recorded in the Vierrucohkka amphibolite and the Aurek Assemblage between c. 626-609 Ma is related to continental rifting processes associated with the opening of the lapetus Ocean. [ABSTRACT FROM AUTHOR]

In the study of the Khanmeikhoi Formation of the central zone of the Kharbei amphibolite–gneiss complex, it was shown that the amphibolites, schists, and plagiogneisses common here form two primary igneous series: low-alumina tholeiitic metabasalts of normal alkalinity and a differentiated series of high-alumina rocks from metatrachybasalts to metarhyolites with high alkalinity. According to the zoning of garnets, progressive and regressive branches of metamorphism are recorded, the peak conditions of which, according to calculations using the winTWQ program, correspond to the high-temperature stage of the amphibolite facies at high pressures (T = 690–750°C, P = 8.1–9.4 kbar). For the first time, metamorphic zircon from Grt-Bi plagiogneiss with low Th/U ratios of 0.01–0.04 and weakly expressed positive cerium (Ce/Ce* = 1.4–1.7) and negative europium (Eu/Eu* = 0.7–0.9) has been identified in the anomalies, the age of which, according to the results of the U–Pb LA-ICP-MS isotope method, is estimated to be Early Carboniferous (359–341 Ma). This time apparently characterizes the transition from the peak of metamorphism to the regressive stage associated with the exhumation of rocks. The results obtained indicate the formation of the Kharbei amphibolite–gneiss complex during the formation of the Ural orogen and allow us to consider it as an indicator of Paleozoic collision processes. [ABSTRACT FROM AUTHOR]

Rare amphibolite in the New Jersey Highlands containing plagioclase megacrysts ≤13 cm long forms bodies 0.5 to 2 m thick that preserve a penetrative metamorphic fabric and have sharp, conformable contacts against Mesoproterozoic country rocks. The megacrystic amphibolites were emplaced as thin dikes along extensional faults between 1160 and 1130 Ma. Amphibolites contain weakly zoned, subhedral andesine megacrysts (An29–44) in a matrix of plagioclase (An18–38), magnesio-hastingsite, biotite, diopside, Fe-Ti oxides, and apatite. The whole-rock major oxide composition of the megacrystic amphibolite matrix has high TiO2 (2.07 wt.% ± 2.0%), Al2O3 (17.03 wt.% ± 0.87%), and Fe2O3t (12.84 wt.% ± 3.2%) that represents an Al-Fe-rich mafic magma type that is characteristic of anorthosite associations globally. The whole-rock, rare earth element (REE) composition of the megacrystic amphibolite matrix is characterized by enrichments in light rare earth elements (LREEs) (La/YbN = 1.73–10.69) relative to middle (MREEs) and heavy (HREEs) rare earth elements (Gd/YbN = 1.30–1.85), and most samples have small positive Eu anomalies (Eu/Eu* = 0.95–1.25). The megacrystic amphibolite matrix is also enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs) (e.g., Ba/Nb = 24–22). Megacrystic amphibolites formed through partial melting of subduction-modified lithospheric mantle that fractionated olivine and plagioclase, producing a high-Al-Fe mafic magma. Plagioclase megacrysts formed through extraction of a plagioclase-rich crystal-liquid mush from anorthosite that mixed with mafic magma and collected in the upper part of the mid-crustal magma (depth of ~20 km based on Al-in-hornblende geobarometry) chamber through flotation. Periodic tapping of this mixed source by extensional fractures led to emplacement of the amphibolites as dikes and may have interrupted the extensive fractionation and plagioclase separation necessary to form voluminous anorthosite intrusions. [ABSTRACT FROM AUTHOR]

A three-dimensional nonlinear strength criterion for rocks considering both brittle and ductile domains is proposed through the combination of the segmented meridian function and a generalized deviatoric function with one parameter. The segmented meridian function is composed of the Hoek–Brown criterion, the smoothing transition surface, and the modified Drucker–Prager cap model. Besides, two deviatoric functions characterizing the smoothness and convexity based on the Matsuoka–Nakai criterion are suggested to describe the initial yield stage (brittle domain) and compaction yield cap stage (ductile domain). To verify the accuracy of the proposed strength criterion, six representative sets of true triaxial experimental data were selected from the existing literature, including two types of rocks, namely Dunham dolomite, KTB amphibolite, Mizuho trachyte, and Westerly granite that are independent of hydrostatic pressure in the brittle domain, and Bentheim sandstone and Castlegate sandstone which are dependent on hydrostatic pressure in the brittle–ductile domain. The results show that the experimental data are evenly distributed on the surface of the failure envelope in the three-dimensional principal stress space, demonstrating that the proposed strength criterion can accurately describe and predict the strength change of rocks in both brittle and ductile domains. Highlights: A three-dimensional nonlinear strength criterion for rocks considering both brittle and ductile domains is proposed. The segmented meridian function is composed of the Hoek–Brown criterion, the smoothing transition surface, and the modified Drucker–Prager cap model. Two deviatoric functions characterizing the smoothness and convexity based on the Matsuoka–Nakai criterion are suggested to describe the initial yield stage (brittle domain) and compaction yield cap stage (ductile domain). [ABSTRACT FROM AUTHOR]

The timing of extensional events in northern Gondwana remains elusive. In relations to this event, a new zircon U-Pb age and Hf isotopic compositions, and whole-rock geochemistry provided here for the amphibolites in the basement unit of the Menderes Massif (MM) within the Anatolide-Tauride Block (ATB). LA-ICP-MS U-Pb zircon dating of one sample yielded a crystallization age of 367.0 ± 2.5 Ma, providing the first record of Late Devonian (Famennian) mafic magmatism in the Menderes Massif. This sample contains inherited zircon grains with ages spanning from 588 Ma to 541 Ma, likely sourced from late Neoproterozoic country rocks. Chondrite-normalized rare earth element patterns exhibit enrichment in light rare earth elements and flat patterns for heavy rare earth elements, suggesting affinities with tholeiitic EMORB and ocean island basalt. The geochemical data, coupled with the positive εHf(t) values ranging from 0.5 to 7.8, indicate that the amphibolite likely originated from juvenile melts generated by partial melting of the asthenospheric mantle. This melt was subsequently modified by crustal contamination and fractional crystallization. Contemporaneous EMORB- and OIB-like mafic rocks are identified in the eastern Taurides and Iran, respectively. Both regional geological and geochemical evidence proposes that Late Devonian magmatism in the Anatolide-Tauride Block occurred in a within-plate tectonic setting. This magmatic activity might have been triggered by the northward subduction of the Paleotethys beneath the Pontides during the Devonian, a process that would produce a slab-pull force, creating an extensional tectonic regime along the northern margin of Gondwana. [ABSTRACT FROM AUTHOR]

Titanite-bearing calc-silicates and mafic gneisses, metamorphosed under amphibolite- to granulite-facies conditions, crop out in Val d'Ossola area (Ivrea-Verbano Zone, Italy). The Ivrea-Verbano Zone represents an exhumed section of the pre-Alpine middle to lower continental crust which escaped the Alpine subduction, thus provides a unique opportunity to study continental crustal processes and evolution. Among several samples, three, collected from different locations, were chosen for detailed analyses of titanite. Petrochronology of titanite was performed with Laser ablation split-stream (LASS) technique on petrographic thin sections. Petrochronological results on titanite do not define clear correlations with chemistry except for one sample. Rare earth elements (REE) patterns of titanite from the three samples are apparently different in terms of average concentration (i.e., lower or upper 1000 times CI), shapes and occurrence or absence of Eu negative anomaly. Al/Fe vs ΣLREE and Fe content vs Zr/Y plots show that the studied samples coincide with metamorphic rock field deriving from calc-silicates and mafic protoliths, as previously demonstrated in literature. Any compilations of petrochronological data on titanite from the metamorphic volcano-sedimentary sequence of Val d'Ossola can be found in literature. Therefore, these data represent a new insight on an accessory mineral phase whose significance and scientific interest are rising in the last years. Future studies of the evolution of these kinds of rock, widespread in the high-grade metamorphic basements, will benefit from these data as a term of comparison.

The article examines petrography and mineralogy of rocks of the Berkut quartz-feldspar vein in the South Urals, which is characterized by intense deformation in comparison with numerous quartz veins of the Kyshtym block. These data allowed the sequence of formation and transformation of the Berkut vein. The vein is a typical vein of replacement, which is composed of blocks of quartz and granite rocks, and contains relics of host rocks (amphibolites and gneisses). Our studies revealed the tectonic transformations of rocks and minerals, as well as the deformation mechanisms with leading plastic deformation and processes of dissolution of minerals under pressure together with brittle fracturing. Dissolution and further deposition led to the crystallization of minerals of new generations (quartz, albite, and oligoclase) and often minerals of different composition (biotite after amphibole). Subsequent metasomatosis is responsible for new generations of minerals (quartz, albite, microcline, muscovite, epidote, zoisite, and clinozoisite) with different morphology and crystal optical properties.

Strain localization processes in the continental crust generate faults and ductile shear zones over a broad range of scales affecting the long-term lithosphere deformation and the mechanical response of faults during the seismic cycle. Seismic anisotropy originated within the continental crust can be applied to deduce the kinematics and structures within orogens and is widely attributed to regionally aligned minerals, e. g., hornblende. However, naturally deformed rocks commonly show various structural layers (e.g., strain localization layers). It is necessary to reveal how both varying amphibole contents and fabrics in the structural layers of strain localization impact seismic property and its interpretations in terms of deformation. We present microstructures, petrofabrics, and calculate seismic properties of deformed amphibolite with the microstructures ranging from mylonite to ultramylonite. The transition from mylonite to ultramylonite is accompanied by a slight decrease of amphibole grain size, a disintegration of amphibole and plagioclase aggregates, and amphibole aspect ratio increase (from 1.68 to 2.23), concomitant with the precipitation of feldspar and/or quartz between amphibole grains. The intensities of amphibole crystallographic preferred orientations (CPOs) show a progressively increasing trend from mylonitic layers to homogeneous ultramylonitic layers, as indicated by the JAm index increasing from 1.9–4.0 for the mylonitic layers and 4.0–4.8 for the transition layer, to 5.1–6.9 for the ultramylonitic layers. The CPO patterns are nearly random for plagioclase and quartz. Polycrystalline amphibole aggregates in the amphibolitic mylonite deform by diffusion, mechanical rotation, and weak dislocation creep, and develop CPOs collectively. The polymineralic matrix (such as quartz and plagioclase) of the mylonite and the ultramylonite deform dominantly by dissolution-precipitation, combined with weak dislocation creep. The mean P and S wave velocities are estimated to be 6.3 and 3.5 km/s, respectively, for three layers of the mylonitic amphibolite. The respective maximum P and S anisotropies are 1.5%–6.4% and 1.8%–4.5% for the mylonite layers of the mylonitic amphibolite, and 6.0%–6.9% and 4.5%–5.0% for the transition layers; but for the ultramylonite layers, these values increase significantly to 8.0%–9.1% and 5.1%–6.0%, respectively. Furthermore, increasing strain (strain localization) generates significant variations in the geometry of the seismic anisotropy. This effect, coupled with the geographical orientations of structures in the Hengshan-Wutai-Fuping complex terrains, can generate substantial variations in the orientation and magnitude of seismic anisotropy for the continental crust as measured by the existing North China Geoscience Transect. Thickened amphibolitic layers by extensively folding or thrusting in the middle crust can explain the strong shear wave splitting and the tectonic boundary parallel fast shear wave polarization beneath the Hengshan-Wutai-Fuping complex terrains. Therefore, signals of seismic anisotropy varying with depth in the deforming continent crust need not deduce depth-varying kinematics or/and tectonic decoupling. [ABSTRACT FROM AUTHOR]

We use U–Pb dating of allanite and REE-rich epidote in three polymetamorphosed units from the Eastern Alps to constrain the timing of prograde metamorphism. All three units (Ennstal, Wölz and Rappold Complex) record several metamorphic cycles (Variscan, Permian and Eoalpine) and presently define an Eoalpine (Cretaceous) metamorphic field gradient from lower greenschist to amphibolite facies. For U–Pb data, a method is introduced to test the magnitude of 230Th disequilibrium and potentially approximate the Th/U ratio of the reservoir out of which allanite and REE-rich epidote grew. We also show that the modelled stability of epidote-group minerals in the REE-free MnNCKFMASH and MnNCKFMASHTO systems and REE-bearing systems is nearly identical. By combining the stability fields of (clino-)zoisite and epidote modelled in REE-free systems with known geothermal gradients for the region, REE-rich epidote growth is constrained to 200–450 °C and 0.2–0.8 GPa during prograde metamorphism. In the Rappold Complex, allanite cores yield a Variscan age of ca. 327 Ma. In the Ennstal and Wölz Complex, allanite growth during the Permian event occurred at ca. 279–286 Ma. Importantly, recrystallized allanite laths and REE-rich epidote overgrowths in samples from all three units yield prograde Eoalpine ages of ca. 100 Ma, even though these units subsequently reached different peak conditions, most likely at different times. This suggests that all units were buried roughly at the same time during the onset of Eoalpine continental subduction. This interpretation leaves room for the model proposing that diachronous peak metamorphic conditions reported for the field gradient may be related to the inertia of thermal equilibration rather than tectonic processes. [ABSTRACT FROM AUTHOR]

The Tectono-Magmatic Lull (TML) refers to the period between 2.37 and 2.24 Ga when magmatic activity declined globally, the cause of which remains debated. The North China Craton (NCC), which was formed by assembling several micro-blocks in Neoarchean to Paleoproterozoic, preserves the most records of magmatism across the TML. In this contribution, we report petrological, geochemical, and zircon U-Pb-Hf isotopic data of Paleoproterozoic basement rocks from the Xuhuai (XH) micro-block in the southeastern part of the NCC, and use them to constrain the geodynamic regime at the onset of the TML. Our data reveal that the central part of the XH micro-block is mainly composed of biotite granodioritic gneiss and plagioclase amphibolite, which is divided into a ca. 2510 Ma suite of low- and medium-pressure TTGs and a ca. 2430 Ma bimodal igneous suite including high-pressure TTG and plagioclase amphibolite. Variable magmatic zircon εHf(t) values (1.9 to 7.2), Nb, Ta and Ti anomalies for the 2510 Ma TTGs indicate that the parental magma, which was involved with mantle materials, was generated in an active subduction environment in this pre-TML era. The felsic component of bimodal igneous rocks with high Na/Ta ratios (27 to 30) and stable Mg# (41 to 44) suggests their magma source region was as deep as the rutile stability field, and the delamination-derived asthenosphere mantle ascending. The influx of hot mantle induces thermal weakening and mechanical stretching of the overlying crust, leading to back-arc extension. The results of this study support the hypotheses that the NCC formed from micro-block amalgamation and the trigger of the TML could be the large-scale delamination of the lower crust into the mantle. [ABSTRACT FROM AUTHOR]

Black shale sequences whose metamorphic grade exceeds the regional greenschist facies are considered by most researchers exclusively as promising for alluvial gold. The paper reports the results of a study of the Ykan deposit, located within the epidote–amphibolite facies zone, and its comparison with previously studied deposits localized in the greenschist-facies alteration zone. It was found that the the main factor of localization of the Ykan gold deposit is its lithological and stratigraphical restriction to the contact between carbonaceous phyllite shales and sandstones of the Aunakit Formation in the hanging wall of the anticline. The ore mineralization of the deposit is represented by a subsequent change of early dia-catagenetic pyrite (py-I, py-II) → metamorphogenic pyrrhotite (po, cpy, asp) → metamorphogenic-metasomatic pyrite–base metal (cpy, asp, py-III, gln, sph, pn, mrc) → postore pyrite (py-IV) assemblages. The ore stage is characterized by the highest possible PT parameters in the early ore metamorphic phase and their decrease at the ore-productive phase. Data on variations in the sulfur and lead isotopic composition indicate that they were transported into the ore fluid from the host metasedimentary sequences of the Aunakit Formation. Comparison of the obtained parameters of the Ykan deposit with the previously studied Sukhoi Log, Golets Vysochaishii, Ugakhan, and Krasnoe deposits confirmed that the Ykan deposit can be attributed to the Sukhoi Log-type deposits. [ABSTRACT FROM AUTHOR]

Exhumed (ultra)high-pressure ([U]HP) metamorphic rocks are markers of paleosubduction zones and can provide crucial implications for the geodynamic evolution of subduction-exhumation. A variety of (U)HP metamorphic rocks from many collisional orogens have commonly documented a lowpressure high-temperature overprint, which appears to indicate a pervasive geodynamic process behind (U)HP metamorphic rock exhumation. A good case study of garnet amphibolite and associated garnet-biotite gneiss with an unambiguous low-pressure high-temperature overprint in the Xixia-Xiaguan area of the North Qinling Orogen, central China, provides a significant opportunity to uncover this geodynamic process. A detailed study of petrography, mineral chemistry, and phase equilibria modeling suggests that the investigated garnet amphibolite and garnet-biotite gneiss have jointly experienced multistage exhumation processes with near-isothermal decompression from the (U)HP eclogite facies (26.8-29.0 kbar/740-775 °C) to the mediumpressure (MP) amphibolite facies (8.7-10.0 kbar/740-775 °C), subsequent remarkable near-isobaric heating to the low-pressure (LP) granulite facies (6.8-8.2 kbar/832-835 °C), followed by final decompression and cooling to the LP amphibolite facies (5.8-6.8 kbar/740-757 °C). U-Pb age dating of zircons from the garnet amphibolite and garnetbiotite gneiss yielded metamorphic ages of ca. 500 Ma, 450 Ma, and 400 Ma, which we consider to represent (U)HP eclogite-facies, LP granulite-facies, and late LP amphibolite-facies metamorphism, respectively. According to the regional geology and the metamorphic history we have determined, this multistage exhumation process of twostep exhumation stages delimited by a phase of LP near-isobaric heating was associated with slab rollback. Accordingly, we propose a tectonic model of exhumation driven by slab rollback for the geodynamic evolution of (U)HP metamorphic rock exhumation in the North Qinling Orogen. Additionally, a combination of previous studies and our new results suggests that exhumation driven by slab rollback appears to be a fundamental mechanism for the (U)HP metamorphic rock exhumation within collisional orogens. [ABSTRACT FROM AUTHOR]

The investigated amphibolites from the Slasten lithotectonic unit are found as a strongly elongated body trending north-south at the eastern slope of the Pirin Mountain, near the village of Ilinden. These rocks are hosted by biotite and biotite-amphibole gneisses, some partially migmatized and containing thin intercalations of marble. The mineral composition is amphibole, plagioclase, epidote, quartz, garnet, pyroxene, titanite, and calcite. P-T conditions of metamorphism are in the range 691-724 °C and 0.74-0.86 GPa. LA-ICP-MS analyses of zircons yield a Middle Triassic (246.4±3.2 Ma) age of formation of these amphibolites. [ABSTRACT FROM AUTHOR]

The present study investigates the chemical weathering characteristics of an amphibolite enclave within granite from Bundelkhand region, central India. The mineral composition varies from core to rind with depletion of hornblende and plagioclase and enrichment of clay minerals and oxyhydroxides. The chemical index of weathering (CIA) indicates an incipient stage of weathering, within the enclave (CIA = 52.22 – 54.74). A-CN-K (Al2O3-CaO + Na2O-K2O) plot reveal samples plot along the A-CN line, indicating the removal of Ca and Na, due to plagioclase destruction. The weathering trend progresses towards the formation of smectite, a significant component within the clay composition of the enclave. The results of mass balance calculation reveal a substantial mass decrease of approximately 50% to 82% in the amphibolite. Furthermore, major oxides in the amphibolite enclave, experience significant losses. Trace elements display general losses in the enclave. This study provides valuable insights into the complex dynamics of elemental behavior during weathering by shedding light on the processes shaping the geochemical evolution of these rock types and their response to weathering conditions. [ABSTRACT FROM AUTHOR]

Mineral reactions were studied in metamorphic rocks from the Meyeri tectonic zone, and the P–T path of the development of this structure was calculated. According to the P–T path, the Proterozoic granulite complex of the Svecofennian Belt was thrust onto low-temperature rocks of the Archean Karelian Craton. Relict staurolite and other minerals preserved as inclusions in the garnet porphyroblasts made it possible to identify the pre-peak stage of metamorphism with P–T parameters no higher than the low-temperature amphibolite facies of moderate and low pressure. The peak metamorphic conditions of the tectonic zone are estimated at T > 700°C and P ~ 7 kbar using the composition of relict minerals, while the temperature on the prograde trend of metamorphism was 500–600°C at a pressure of about 5 kbar. The post-peak stage began with a distinct decompressional P–T path at the aforementioned high temperatures, with a change from granulite hypersthene-containing assemblages to lower-temperature hydrous ones. The subsequent metamorphic retrogression was characterized by the development of numerous hydrous minerals as a result of the activation of fluids in the shear zone. The P–T path of the tectonic zone is clockwise and reflects the exhumation of the Svecofennian granulite complex during the orogenic events. [ABSTRACT FROM AUTHOR]

The Kalgyn ophiolite massif is a unit of the ophiolite association of the Chersky collision belt. It is the part of the Verkhoyansk–Kolyma folded system on Northeast Asia. The Kalgyn massif includes a complex of mantle peridotites, a lower crustal complex, represented by ultrabasic cumulates, cumulative gabbro-amphibolites and amphibolites, and an upper crustal complex represented by metabasalts. New geochemical data, obtained by the ICP-MS method and data on the isotopic composition (Sm, Nd), made it possible to significantly clarify the geodynamic conditions for the formation of volcanics and to determine the features of the sources of parent melts. [ABSTRACT FROM AUTHOR]

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The Bundelkhand Craton (BuC) represents a significant Archean terrane within the northern Indian Craton and yet its tectono-metamorphic evolutionary history remains relatively understudied. Our investigation involves detailed petrography, geochemistry, and bulk composition modeling of both garnet-bearing and garnet-absent amphibolites with a two-fold objective: (i) to constrain the protolithic nature and tectonic settings involved in the genesis of these rocks, and (ii) to propose a tectono-metamorphic evolutionary history for the BuC. The BuC amphibolites originate from basalt and andesitic-basalt protoliths. Their trace element compositions reveal negative anomalies in Nb, Ta, and Ti, while their rare earth element (REE) normalized patterns indicate enrichment in light REEs over heavy REEs. The basaltic protolith is interpreted to have formed during orogeny in a compressional tectonic regime at the active margins of island arcs in a subduction-related setting. This interpretation is supported by various discrimination plots for amphibolites, such as Nb/Th vs Zr/Nb, Zr vs Zr/Y, and Th/Nb vs Ce/Nb, as well as high Th/Yb and low Nb/Yb contents— all of which suggest an island arc setting influenced by subduction. These amphibolites have undergone three distinct phases of metamorphism, as evidenced by petrography, mineral chemistry, and bulk composition modeling. This interpretation is further supported by geochemical discrimination diagrams which indicate that a subduction tectonic setting was active during peak metamorphism. During the pre-peak phase, the garnet-bearing amphibolites experienced pressure and temperature conditions ranging from 6.25 to 6.5 kbar and 580 to 590ºC, while the garnet-absent amphibolites underwent conditions from 5.0 to 5.8 kbar and 400 to 450ºC. Peak metamorphism was observed at pressures ranging from 6.8 to 7.4 kbar and temperatures from 760 to 805ºC for the garnet-bearing amphibolites, and at pressures from 7.0 to 7.4 kbar and temperatures from 785 to 810ºC for the garnet-absent amphibolites. The metamorphic retrograde conditions for the garnet-bearing amphibolites are defined by P-T conditions ranging from 4.45 to 4.75 kbar and 585 to 615ºC, while for the garnet-absent amphibolites, it ranges from 3.1 to 4.0 kbar and 620 to 710ºC. The mineral assemblages and P-T conditions delineate a clockwise P-T path for both, garnet-bearing and garnet-absent amphibolites from the Babina and Mauranipur regions. This suggests that the rocks underwent a burial process amid subduction tectonic settings in an arc-related environment, followed by a decompression stage that brought them to the surface.

The Alambay ophiolite zone (AOZ) is located in the axial part of the Early Paleozoic Salair orogen and includes the northern extension of the Alambay-Kaim zone, Salair and Altai Mountains. The Shalap area of the AOZ is predominantly composed of clastic mélange with occasional serpentine mélange. The geological and geochemical studies showed that in the Shalap mélange there are basalt blocks of the Alambay formation whose petrogeochemical features are similar to those of the oceanic island basalts (OIB). Metamorphic rocks of the Angurep complex, represented by garnet and non-garnet amphibolites, form a tectonic slab which is a part of the accretionary complex east of the Shalap mélange area. Metamorphic rocks also form blocks in the Shalap mélange. The amphibolites of the Angurep complex are similar in their petrogeochemical features to the basalts of intraoceanic island arcs. The Shalap mélange is a fragment of the Salairian Cambrian paleosubduction zone. The subduction and exhumation processes in this paleosubduction zone terminated by the 500 Ma time stage.

The Trans‐North China Orogen (TNCO) is a major Palaeoproterozoic collisional orogen in the North China Craton and has experienced complex tectonic evolution during the assembly of the Eastern and Western Blocks. The vestiges of subducted oceanic crust preserved in this orogen can be used to gain insights into the subduction–collision process associated with the Palaeoproterozoic orogeny. In this study, we investigate garnet amphibolites from the Wanzi Group in the Fuping Complex along the TNCO through integrated petrography, mineral chemistry, phase equilibrium modelling and zircon U–Pb geochronology. Three stages of metamorphic mineral assemblages are identified as follows: (i) the prograde assemblage is preserved as plagioclase, amphibole, biotite and quartz inclusions in garnet porphyroblasts; (ii) the peak mineral assemblage includes coarse‐grained amphibole, garnet, plagioclase, ilmenite, biotite and quartz; and (iii) the retrograde assemblage is mainly represented by the corona of amphibole, plagioclase and biotite in the matrix. Petrological features and phase equilibrium modelling define a clockwise pressure–temperature path involving a prograde stage of 7.2–9.6 kbar and <670°C, peak conditions of 10.0–10.2 kbar and 690–810°C, and a retrograde stage with near‐isothermal decompression to ~7.4 kbar and cooling. Zircon U–Pb dating shows weighted mean ages of 1848 ± 22 Ma and 1836 ± 31 Ma, representing the growth of zircon in the retrograde stage during the uplift and cooling process. In combination with available data from the TNCO, the post‐peak decompression and cooling process is inferred to be long‐lived during 1.90–1.76 Ga, following collision along the TNCO that occurred during 1.97–1.90 Ga, ultimately leading to the final assembly of the North China Craton. [ABSTRACT FROM AUTHOR]

High-grade granitoid gneisses (740–710 Ma) and elongate bodies of amphibolite and hornblende schist with cm-scale layers of garnet pyroxenite form an overturned fold in the Gabal Um Gunud area, South Eastern Desert of Egypt. Whole-rock geochemical data combined with zircon U-Pb-Hf isotope data suggest that the metabasites represent relics of oceanic crust with an N-MORB affinity, which has been derived from partial melting (2.0–1.4 GPa; ~65- ~ 45 km depth) of a depleted mantle source beneath an intra-oceanic, spreading forearc basin. Such increment of melting degree resulted in small basaltic melt batches with transitional tholeiitic to boninitic affinities. Progressive slab subduction and partial melting of the tholeiitic amphibolite produced high-Al and low HREE melts for trondhjemite at av. T = 854°C and low-Al and high HREE melts for tonalite at higher temperatures (av. 949°C). The low Mg# of the trondhjemite-tonalite rocks may be attributed to limited contamination of subduction components, as evidenced by the weak lanthanide tetrad effects (TE1,3~1) and near-chondritic Zr/Hf, Nb/Ta, and Y/Ho ratios, while the garnet pyroxenite was the residuum produced in equilibrium with the trondhjemite-tonalite melts. [ABSTRACT FROM AUTHOR]

The Northwest Tibet region is defined by several terranes, magmatic belts, basins and sutures, which were primarily shaped by the tectonic activities associated with Proto-, Palaeo- and Neo-Tethys Oceans. However, the basement nature and Precambrian tectonic evolution of the Northwest Tibet region, particularly within the Tashikuergan-Tianshuihai terrane, remain largely unknown. The Hongliutan area, located in the northeastern part of the Tashikuergan-Tianshuihai terrane, contains a critical sequence of Precambrian metamorphic rock strata. Detailed petrological, geochronological, and geochemical analyses of these metamorphic rocks – including plagioclase schist, quartz schist, amphibolite and nearby leucogranite – reveal the intricate processes of tectonic evolution within the Tianshuihai unit. Combining these findings with previous geochronological results is crucial for re-evaluating the nature of the Tashikuergan-Tianshuihai basement and its Precambrian tectonic evolution of the Tashikuergan-Tianshuihai basement. Our results reveal the following: (1) the leucogranite and amphibolite, identified as Cambrian igneous rocks, display distinct geochemical signatures indicative of a continental arc origin. These include calc-alkaline characteristics, enrichment in Th, U, Pb, Zr and Hf and depletion in Ba, Nb, Sr and Ti. Their εNd(t) values, close to zero, further support this tectonic setting, with the leucogranite and amphibolite formed at 506 and 522 Ma, respectively. (2) The plagioclase schist and quartz schist are interpreted to be Neoproterozoic volcaniclastic rocks that formed in a rifted (passive) continental margin setting. The quartz schist is particularly rich in detrital zircons, displaying a broad spectrum of 207Pb/206Pb ages, ranging from 901 to 3364 Ma. (3) A significant subset of detrital zircons within the quartz schist exhibits oscillatory zoning, high Th/U ratios and sharp-edged, anhedral-to-subhedral crystal forms, suggesting a derivation from proximal or deep-seated terranes. The concordant U–Pb zircon ages of 2468 and 974 Ma from the quartz schist, along with the 978 Ma age from the inherited zircons in the amphibolite, and the 1.2–2.1 Ga T2DM(Nd) from leucogranite and metamorphic rocks, collectively suggest that the Tianshuihai unit is likely underpinned by a Palaeoproterozoic basement that indicates Neoproterozoic reworking. Therefore, our findings suggest the presence of a continuous, northwest-southeast trending Palaeoproterozoic basement underlying the entire Tashikuergan-Tianshuihai terrane. An alternative scenario posits that the ancient basement, currently beneath the Tashikuergan terrane, could extend into the Tianshuihai region, potentially indicating a Cambrian continental margin arc interspersed with remnants of older terranes. [ABSTRACT FROM AUTHOR]

Understanding the origin, assembly, and evolution of voluminous magma that erupts in catastrophic caldera-forming eruptions (CCFEs) is a community imperative. A CCFE of the Aira caldera at 30 ka discharged over 350 km³ of magma, which can be grouped into petrographically and geochemically distinct types: voluminous rhyolite, small amounts of rhyodacite, and andesite magmas. To further understand the magma plumbing system of the Aira CCFE, we examined the geochemical characteristics of whole rock and plagioclase from its eruptive deposits. The trace element and 87Sr/86Sr signatures recorded in the plagioclase phenocrysts of these magmas indicate that the three magmas were originally produced by partially melting an identical source rock, which was estimated to be a mafic amphibolite with an 87Sr/86Sr signature of ~0.7055 that comprised the lower crust. Melting of mafic amphibolite produced both felsic and mafic magmas by low and high degrees of partial melting, respectively. The mafic magma assimilated uppermost crustal materials and crystallized to produce an andesite magma type. The andesitic magma consists of phenocrysts (~39 vol%) and melt with a dacitic (~70 wt% SiO2) composition. The felsic magma mixed with ~10% of the andesite magma and crystallized, forming the rhyolite magma. The mixing between the andesite and rhyolite magmas before the Aira CCFE produced the rhyodacite magma. The 30-ka Aira CCFE magmas were generated only by melting two kinds of crustal materials with different geochemical characteristics and had geochemical variations due to different conditions of partial melting and mixing between various crustal melts. The lack of definitive evidence of the mantle component mixing with the Aira CCFE magmas suggests that the mantle-derived magmas worked only as a heat source for crustal melting. [ABSTRACT FROM AUTHOR]

The Bay of Islands complex, Newfoundland, Canada, represents a fossil subduction system that was obducted onto the Laurentian margin during the Taconic Orogeny. We present whole-rock geochemistry and geochronology for the Bay of Islands metamorphic sole from the Table Mountain and North Arm Mountain massifs. The Bay of Islands metamorphic sole can be subdivided into three units: a high metamorphic-grade Grt-Cpx amphibolite and Grt-Cpx granulite unit, a medium metamorphic-grade common amphibolite unit, and a medium metamorphic- grade metasedimentary-dominated unit. Based on their major and trace element contents, two types of protoliths can be inferred for the Bay of Islands sole metabasites: cumulate gabbroic protoliths with high Mg numbers and depleted trace element contents, and basaltic protoliths with trace element contents defining a normal mid-oceanic-ridge basalt (N-MORB) affinity. Titanite in situ U-Pb analyses of two common amphibolite samples yield dates of 486 ± 7 Ma and 484 ± 4 Ma, which are interpreted as a metamorphic age (ca. 485 Ma). One sample from the metasedimentary unit yields detrital zircon dates that range from Archean to Ordovician, with the youngest dates defining the most dominant peak at ca. 490 Ma. This youngest peak is consistent with an arc-related source that may be linked with arc magmatism of the upper plate during the Taconic Orogeny. These observations suggest that (1) the subduction responsible for the formation of the Bay of Islands complex nucleated in an oceanic domain before ca. 485 Ma; and (2) the sediments that form the metasedimentary unit were partly sourced from the upper plate and were deposited on the lower plate while subduction was active. [ABSTRACT FROM AUTHOR]

The results of detailed petrological studies of mafic granulites from the Yubileinaya (Alakit-Markha field) and Novinka (Upper–Muna field) kimberlite pipes, where retrograde metamorphic P–T paths were reconstructed for the first time using mineral geothermobarometry, are provided. These P–T paths demonstrate subisobaric transition of the rocks from the P–T conditions of low-grade granulite facies to low-grade amphibolite facies at the depth of the middle crust. It was found that the compositions of garnet, clinopyroxene, and orthopyroxene depend on the adjacent mineral, reflecting the different closure temperatures of the exchange mineral reactions. The higher temperatures are determined using a two-pyroxene geothermometer, while the lower ones are found by application of garnet–clinopyroxene and garnet–orthopyroxene geothermometers. Phase equilibria modeling showed the thermodynamic conditions of the low-grade amphibolite facies: 540°C, 0.76 GPa, lgfO2 = QFM + 1.7 (Yubileinaya pipe); 530°C, 0.72 GPa, lgfO2 = QFM + 2.2 (Novinka pipe). The stability of the granulite paragenesis of garnet + clinopyroxene + orthopyroxene + plagioclase under such P–T conditions is a poorly known phenomenon reflecting a deficit of aqueous fluid during the crystallization of gabbroids and their subsequent cooling in the deep segments of cratons. [ABSTRACT FROM AUTHOR]

The Araguaia Orogen, located in Central Brazil, represents an Ediacaran-Cambrian collisional orogen associated with the final assembly of West Gondwana. Two contrasting tectonic models have been proposed for the evolution of this orogen, involving different histories for the Amazonian palaeocontinent drift: (i) subduction of a large ocean and final collision of the Amazonian palaeocontinent with Gondwana between ~ 560 and 500 Ma and (ii) intracontinental orogeny after early amalgamation of the Amazonian palaeocontinent at ca. 620 Ma. Detailed tectono-metamorphic studies in this key locality for the evolution of Gondwana are also still scarce. Hence, in this study we present a detailed metamorphic investigation of two metapelite samples (schists) and one garnet amphibolite aiming to unravel the pressure-temperature-time (P-T-t) paths for the higher grade rocks exposed in this orogen. Pressure-temperature estimates based on thermodynamic modelling using isochemical phase diagrams in the MnNCKFMASHTO system for the schists constrained a single and continuous prograde metamorphic path (burial and heating) from ~ 4–6 kbar and ~ 450–500°C to ~ 7–8 kbar and ~ 580–650°C. The near-peak metamorphic conditions of the garnet amphibolite were estimated from classic geothermobarometry at 7.5 ± 0.8 kbar and 552 ± 40°C. Lutetium-hafnium garnet dating constrained an average age for the prograde metamorphism at around 537–533 Ma. These results show that the Araguaia Orogen records one single metamorphic event during the late Ediacaran-early Cambrian, and likely represents part of a collisional system developed between the Amazonian Craton and Parnaíba Block after the closure of the Clymene Ocean. [ABSTRACT FROM AUTHOR]

The Mabujina Complex exposed in south-central Cuba consists of an elongated body of metamorphosed volcanic and subvolcanic mafic rocks and tonalitic gneisses. The complex is largely composed of greenschist facies rocks (Porvenir Formation) directly below the Cretaceous volcanic arc section and of a deeper sequence of deformed amphibolite facies rocks (Mabujina Amphibolite Complex). The origin of the protolith of the Mabujina Complex has been the subject of intense debate in the past. While some authors consider the Mabujina Complex to represent the deepest and oldest (Early Cretaceous) exposed section (root) of the Cretaceous Caribbean volcanic arc others suggested an exotic (non-Caribbean related) Early Cretaceous volcanic arc section. The later proposal implies the tectonic emplacement underneath the Caribbean arc of a segment of the Pacific-related Early Cretaceous Mexican Guerrero volcanic arc terrane. A new set of whole-rock geochemical analyses presented in this study shows that the protoliths of the Mabujina Amphibolite Complex constitute an island-arc sequence with a) less abundant island-arc tholeiitic mafic rocks similar to the Early Cretaceous tholeiitic island arc of the Caribbean and b) dominant transitional to calc-alkaline mafic and felsic igneous rocks that resemble the Early to Late Cretaceous magmatism of the Caribbean arc. The high initial Nd ratios of the rocks from the Mabujina Amphibolite Complex are indistinguishable from the tholeiitic to calc-alkaline sequences from the Caribbean arc, reflecting a depleted MORB mantle source with limited subduction-related component input. The present data, together with published geochemical and geochronological data of the protoliths, allow identifying a Caribbean origin for the Mabujina Complex instead of an exotic provenance in the Guerrero Arc of western Mexico. However, even if the interpretation of the geochemical data favours a Caribbean origin, the Mabujina Amphibolite Complex cannot represent the root of the volcanic arc. Here, we propose that the Mabujina Amphibolite Complex is a fragment of the Early to Mid-Cretaceous Caribbean arc that was tectonically emplaced below this same arc as a likely consequence of arc segmentation and strike-slip trans-compression triggered by oblique subduction during the mid-Cretaceous. [ABSTRACT FROM AUTHOR]

There are extensive outcrops of regional metamorphic rocks that consist of marble, calcschist, metapelite, and amphibolite in the south Sirjan (southern Sanandaj-Sirjan zone). Garnet, muscovite, biotite, calcite, quartz, and feldspar are the rock-forming minerals of calcschists. Kyanite, garnet, and staurolite constitute the index minerals of the studied metapelites. Amphibole and plagioclase are the main minerals of the massive and foliated amphibolites. The P-T conditions of the regional metamorphic event in the southern SSZ were constrained using conventional (single-reaction) and multi-equilibrium thermobarometry (THERMOCALC software). The average temperature and pressure gained by THERMOCALC software are 8 kb, 643 ˚C, and 9.6 kb, 645 ˚C for Ky-Grt schist and calcschist samples, respectively. Using revised and recalibrated conventional thermobarometry methods result in temperature from 504 to 664 °C and pressure from 6.57 to 7.88 kb for the investigated Ky-Grt schist. Mineralogical paragenesis and thermobarometry results of the investigated metamorphic rocks are in accordance with a medium P/T- type metamorphic gradient. Presumably, Neotethys subduction beneath the Sanandaj-Sirjan zone and resulting crustal thickening event led to such a metamorphic gradient during Eo-cimmerian orogeny. Medium P/T metamorphic rocks of the southern Sanandaj-Sirjan zone and accompanying high P/T metamorphic rocks of the north Hajiabad could be defined as a pair of metamorphic belts that were constructed during Zagros orogeny. [ABSTRACT FROM AUTHOR]

Geochronological studies of amphibole-biotite gneisses of the Verkhnii Tokmak Unit and amphibolites and Banded Iron Formation (BIF) of the Kaiinkulak Unit of the West-Azov Group in the Lypovets area of the Lozuvate antiform in the West Azov block of the Ukrainian Shield were carried out. A multi-stage history of the formation of amphibole-biotite gneisses has been established. In terms of chemical composition they correspond to quartz diorites. Rare earth elements are differentiated — (La/YbN = 7.54), with a significant negative europium anomaly — Eu/Eu* = 0.66. The REE distribution indicates that the initial melt for quartz diorites was produced in a crustal magmatic source. In zircon crystals from amphibole-biotite gneisses, relics (cores) of older zircon and mantles that surround them were distinguished. Relict zircon is coarse-zoned, cracked, and its mantles have a thin-zoned structure. A small amount of transparent non-zonal zircon crystals was also found. Based on the results of zircon dating by the LA-ICP-MS method, it was determined that the age of the relict zircon is 3.16 Ga. The thin-zonal mantles have an age of 2.9-2.8 Ga, which probably corresponds to the time of crystallization of quartz diorites. Non-zonal transparent zircon with an age of 2.1 Ga corresponds to the time of dynamometamorphism (collision). BIF of the Lypovets deposit in the West Azov iron ore province, which belongs to the Kaiinkulak Unit of the West Azov Group, has a high Ni/Fe×10–4 = 0.5-2.1 ratio, typical for the Archean BIFs of the Algoma type. This deposit is confined to an Archean greenstone belt in a synclinal structure composed of metamorphosed basaltic komatiites and sedimentary rocks. Thus, the Verkhnii Tokmak Unit comprises Mesoarchean quartz diorites of crustal genesis, while the Kaiinkulak Unit probably represents heavily metamorphosed rocks of an Archean greenstone belt. [ABSTRACT FROM AUTHOR]

The paper presents results of a study of middle crustal (amphibolite-facies level) desilicated rocks exemplified by corundum-bearing plagioclasites developing after metabasites at the unique Khitostrov corundum deposit in the Belomorian–Lapland orogen of the Fennoscandian shield, with emphasis placed onto newly acquired geological data, documentation and analysis of the metasomatic zoning, determination of the P–T conditions of its formation, and a model of the metasomatic process. [ABSTRACT FROM AUTHOR]

Höhn et al. (2021) proposed that the giant Gamsberg Zn deposit, South Africa, initially formed as a sedimentary exhalative (SEDEX) deposit during the Mesoproterozoic and was subsequently oxidized near surface. The oxidized ore was then supposedly sulfidized by sulfur-rich metamorphic fluids during and after upper amphibolite facies metamorphism. We view this model as untenable for various reasons and suggest that the Gamsberg deposit and others in the Aggeneys-Gamsberg district (Swartberg, Broken Hill-Deeps, Big Syncline) are metamorphosed clastic SEDEX deposits rather than having formed by synmetamorphic sulfidation processes. [ABSTRACT FROM AUTHOR]

The China Central Orogenic System (CCOS), extending in an east-west direction in the middle part of China, is composed of the Early Paleozoic Altyn-North Qilian-North Qaidam-East Kunlun-North Qinling-North Tongbai orogens in the west and the Late Paleozoic to Early Mesozoic South Tongbai-Hong'an-Dabie-Sulu orogens in the east. They were produced by oceanic subduction and continental subduction/collision during the closure of the Proto-Tethys and the Paleo-Tethys oceans, respectively. Different types of metamorphic rocks with various ages are extensively exposed in these orogens, and they were produced at different geothermal gradients in different stages during the tectonic evolution of convergent continental margins, making them ideal targets to reconstruct the spatiotemporal evolution of the Eastern Tethys tectonic domain. In this article, an integrated study of metamorphic temperature (T)-pressure (P)-time (t) records is presented for metamorphic rocks along the CCOS, aiming to ascertain the change of metamorphic T/P ratios in both time and space, and then shed light on the tectonic evolution of the East Tethys tectonic domain in association with the thermal state change of convergent continental margins. The results indicate that despite the difference in metamorphic ages, metamorphic rocks in different orogens show a common trend with clockwise P-T-t paths. With respect to plate convergence for subduction and collision, regional metamorphism is categorized into three stages: (1) an early convergent stage, corresponding to low T/P Alpine-type blueschist- to eclogite-facies high-P to ultrahigh-P metamorphism; (2) a later convergent stage, corresponding to the medium T/P Barrovian-type medium-P amphibolite to high-P granulite-facies metamorphism; and (3) a post-convergent stage, corresponding to the high T/P Buchan-type low-P amphibolite to MP granulite-facies metamorphism. Nonetheless, a few metamorphic rocks only record a two-sage metamorphic evolution, with an early Barrovian-type high-P granulite-facies metamorphism and a late Buchan-type low-P granulite-facies metamorphic overprinting. In modern convergent plate margins, Alpine-type metamorphism mainly occurs in the stages of oceanic subduction and continental collision, Barrovian-type metamorphism takes pace in both stages of crustal thickening during continental hard collision and slab exhumation when continental subduction zones have evolved from compressional to extensional regimes, and Buchan-type metamorphism occurs in intracontinental rifting stage after the plate convergence. Therefore, the tectonic evolution of convergent continental margins can be reconstructed by combining metamorphic T/P ratios with their corresponding metamorphic facies series and metamorphic timing of metamorphic rocks. Based on the reported metamorphic rocks of different types and ages along the CCOS, it appears that the continental subduction/collision occurred at 500–490 Ma in the Altyn-North Qinling-North Tongbai orogens but 450–430 Ma in the North Qaidam-East Kunlun orogens, and the intracontinental rifting occurred at 460–450 Ma in the Altyn-North Qinling-North Tongbai orogens but 410–400 Ma in the North Qaidam-East Kunlun orogens, respectively, in the western Proto-Tethys domain. For the eastern Paleo-Tethys domain, in contrast, the continental subduction/collision occurred at 250–220 Ma and post-collisional intracontinental rifting occurred at 140–120 Ma. Furthermore, metamorphic evolution from low T/P ratios in the subduction/collision stage to high T/P ratios in the intracontinental rifting stage needs 40–60 Myr in the Proto-Tethys domain but about 110 Myr in the Paleo-Tethys domain. For the two different orogenic domains, therefore, the convergent continental margins underwent a common tectonic evolution from warm collision/cold subduction to hot rifting, which starts from continental subduction/collision characterized by the formation of medium-P amphibolite to high-P granulite facies series or high-P to ultrahigh-P eclogite facies series in compressional regimes, through exhumation of the deeply subducted crustal rocks, and terminates with intracontinental rifting featured by high-T to ultrahigh-T granulite facies series in extensional regimes. [ABSTRACT FROM AUTHOR]

The Yeongnam Massif is located in the southern part of the Korean Peninsula and holds key elements in understanding the Palaeoproterozoic tectonic evolution of north‐east Asia during the amalgamation of the Columbia supercontinent. In this study, we have conducted geochronological and geochemical studies on the mafic intrusive along the northern margin of the Yeongnam Massif. Sensitive high‐resolution ion microprobe (SHRIMP) U–Pb zircon dating on two amphibolite and one gabbro samples in the Damyang area show that the amphibolites crystallized at ~1.87 Ga and the gabbro crystallized at ~87.6 Ma. The 1.87 Ga Damyang amphibolites are simultaneous with the Muju and Okbang amphibolites along the northern margin of the Yeongnam Massif. They also show similar geochemical characteristics, including tholeiitic basalt composition, enriched Light Rare Earth Elements (LREE) and large ion lithosphile elements (LILEs), depleted high field strength elements (HFSEs), and slightly enriched but various Hf isotopes, indicating that they originated from an enriched but heterogeneous mantle source with fluid addition. Thus, the 1.87 Ga amphibolites along the northern margin of the Yeongnam Massif were probably emplaced in an arc‐related setting. Combined with the 2.02–1.96 Ga magmatism, it is proposed that the subduction might have lasted during the 2.02–1.85 Ga interval and mantle‐derived magma rose up due to the roll‐back process between 1.88–1.85 Ga. [ABSTRACT FROM AUTHOR]

The Almacık Complex is a tectonic unit of high-grade metamorphic rocks in the Intra-Pontide Suture Zone in northwest Turkey. It consists mainly of amphibolite, metaultramafic rock, and gneiss, which are intruded by numerous pre-, syn-, and posttectonic felsic veins. The Almacık Complex is variously interpreted as a Cretaceous or Neoproterozoic ophiolite, or a Permian mafic-ultramafic complex representing the middle to lower crust of the Sakarya Zone. Herein, new petrological and geochronological data were presented from the Almacık Complex. Two-pyroxene geothermometry in the metawebsterites indicated that the Almacık Complex has undergone upper amphibolite-facies metamorphism at 750 ± 30 °C and 8 ± 4 kbar. U-Pb zircon and Ar-Ar ages from 11 samples indicate the presence of late Neoproterozoic, Permian, and Jurassic thermal events. Most of the Almacık Complex consists of late Neoproterozoic amphibolites and gneisses, representing the basement of the İstanbul Zone. This basement was intruded by voluminous mafic magma during the Late Permian. The basement and the Permian ultramafic-mafic rocks subsequently underwent upper amphibolite-facies metamorphism during the Jurassic, possibly at the base of a magmatic arc. [ABSTRACT FROM AUTHOR]

The evidences of partial melting of minerals and the possible involvement of polymetallic melts in the accumulation and redistribution of gold and other metals are shown for the large Svetlinsk gold deposit (Southern Urals). The finding of Bi and Sb minerals including new Au minerals for the deposit (pampaloite, montbrayite, and aurostibite), specific mineral intergrowths (polymineral Sb–Bi–Pb–Te–Ag–Au drop-like inclusions), enrichment of early sulfides in Low-Melting-Point Chalcophile Elements (LMCE), high formation temperatures of ore assemblages (up to 400°C), as well as metamorphism of amphibolite facies of the deposit indicate the possibility of formation of such melts. The polymetallic melts at the deposit could be formed both by partial melting of early sulfides and directly from hydrothermal fluids. The melting features also include calaverite–native gold symplectites in the marginal parts of a large montbrayite grain. [ABSTRACT FROM AUTHOR]

The subduction–exhumation history of the Grapesvare nappe in the northern Seve Nappe Complex (Scandinavian Caledonides) is recorded by late Cambrian/Early Ordovician ultra-high pressure (UHP) and subsequent amphibolite facies metamorphic events. Records of these events obscured earlier metamorphic episodes that are important for understanding the tectonics of the orogen. To extract the pre–UHP metamorphic records, garnet Lu–Hf geochronology, Titanium-in-Quartz thermobarometry, and Quartz-in-Garnet elastic thermobarometry were applied to garnet porphyroblasts in metasedimentary rocks and eclogite. Metasedimentary rocks contain chemically homogeneous garnet (Grt-M1) with shape-matured quartz inclusions. In some rocks, these garnets are overgrown by garnet with bell-shaped Mn-zoning (Grt-M2) containing irregularly-shaped quartz inclusions. This evolution is interpreted as partial dissolution of Grt-M1 and subsequent growth of Grt-M2. Garnet in the eclogite is volumetrically dominated by eclogite-facies garnet (Grt-E1) that envelope remnants of an older, chemically distinct generation (Grt-E0) with highly irregular and diffuse boundaries. Shape-matured quartz inclusions are present within both garnet generations and define a zoning pattern that is not reflective of the chemical zoning. Collectively, these characteristics are interpreted as replacement of Grt-E0 by Grt-E1 via interface-coupled dissolution–reprecipitation, with the latter inheriting the shape-matured quartz inclusions of the former. Pressure–temperature (P–T) conditions extracted from the quartz inclusions in Grt-M1 and Grt-E0/E1 are 1.08 to 1.21 GPa at 645°C to 695°C and 0.94 to 1.03 GPa at 605°C to 640°C, respectively. These conditions are interpreted as cooling of the rocks from a high temperature metamorphic history, altogether preceding subduction of the Grapesvare nappe. The quartz inclusions in Grt-M2 record 1.04 to 1.21 GPa at 620°C to 675°C, interpreted as prograde metamorphic growth of Grt-M2 during subduction at 495.7 ± 3.2 Ma. Subsequent eclogite-facies metamorphism was responsible for the formation of Grt-E1 at the expense of Grt-E0. The collective results indicate a prolonged polymetamorphic history of the Grapesvare nappe prior to UHP metamorphism that has not been recognized previously. [ABSTRACT FROM AUTHOR]

The crystalline complexes of the Hercynian South Altai Metamorphic Belt (SAMB), which is a part of the Central Asian Foldbelt more than 1500 km long. They compose different-scale tectonic plates, the level of metamorphism in which at the early stages reached the conditions of high-temperature amphibolite subfacies and, in places, granulite facies. In terms of tectonics, the band of their outcrops is confined to the margin of the North Asian Caledonian continent, stretching from southeast to northwest along the southern slope of the Gobi, Mongolian, and Chinese Altai to Eastern Kazakhstan, where they are represented in the Irtysh shear zone. The SAMB includes poly- and monometamorphic complexes. The age of granitoids formed at the late episode of metamorphism was determined for the Tsel tectonic plate of the Gobi Altai in the southeastern part of the SAMB: from 374 ± 2 to 360 ± 5 Ma. These and previously obtained results show that the early stage of low-pressure metamorphism and the late stage of high-pressure metamorphism occurred in the age intervals of ~390–385 and 375–360 Ma, respectively, almost throughout this belt. A short-term stage of stabilization was between these stages. These processes occurred during the closure of the basin with the Tethys-type oceanic crust of the South Mongolian Ocean (paleo-Tethys I). The spatial position of the SAMB is determined by the asymmetric structure of the basin, in which the active continental margin is represented along its northern part, and the passive one is represented along the southern one (in modern coordinates). [ABSTRACT FROM AUTHOR]

Most of the tectonic, metamorphic and geochronological data suggest that the Himalaya is essentially the consequence of a single orogenic cycle associated with the India-Asia collision during the Cenozoic era. Therefore, metamorphic assemblages and tectonic structures across the Himalayan range are systematically considered as post-collisional geological records. However, over the last decades, several observations arguing for geological events predating the continental collision have become increasingly recurrent in the literature. Nevertheless, although some of these arguments are thoroughly documented, they are unduly ignored in the construction of models drawing the tectono-metamorphic evolution of the Himalayan range. Yet, the occurrence of a pre-Himalayan history would have considerable consequences on the classical models for the building of the Himalaya. The recent discovery of inclusions of staurolite crystals in greenschist facies garnets from the Miyar Valley in Upper Lahul region (Himachal Pradesh; NW India) revives the debate on the existence of a pre-Himalayan metamorphism. Indeed, the occurrence of high-temperature staurolites included in greenschist facies garnets suggests that the High Himalayan Crystalline rocks experienced an amphibolite facies metamorphism prior the predominant Himalayan greenschist facies metamorphism observed in this part of the range. In this study, phase petrology, microtectonic investigations combined with preexisting geochronological data infer that the crystallization of the included staurolite predates the growth of Himalayan garnets. These original data bring new arguments to bear on the long lasting debate of the existence of a Pre-Himalayan orogenic cycle. They lead to the conclusion that the growth of staurolite predates the continental collision between India and Asia and reflects a metamorphic event that belongs to a pre-Himalaya orogenic cycle. [ABSTRACT FROM AUTHOR]

The metamorphism and geological significance of amphibolites in the Diebusige and Bayanwulashan Complexes of the eastern Alxa Block, North China Craton, were poorly understood until now. This study presents the results of petrology, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb analysis, phase equilibrium modeling and geothermobarometry for these rocks. The peak mineral assemblage of clinopyroxene + hornblende + plagioclase + K-feldspar + ilmenite + quartz + melt is inferred for amphibolite sample ALS2164 in the Diebusige Complex. Correspondingly, the peak mineral assemblage of clinopyroxene + hornblende + plagioclase ± K-feldspar + ilmenite + quartz + melt is identified for amphibolite sample ALS2191 in the Bayanwulshan Complex. Phase equilibrium modelling constrained the peak metamorphic condition of amphibolite sample ALS2164 in the Diebusige Complex to be 825–910 °C/7.2–10.8 kbar, which is similar to that (800–870 °C/7.0–10.7 kbar) of amphibolite sample ALS2191 in the Bayanwulashan Complex. Hbl–pl–qz thermobarometry yielded the metamorphic PT conditions of 732–810 °C/3.0–6.7 kbar for these amphibolites, which are consistent with the average temperatures of 763 °C, 768 °C and 780 °C calculated by Ti-zircon thermometry. As a result, phase equilibrium modelling yielded wide PT condition ranges of 800–910 °C/7.0–10.8 kbar, the lower limit of which is consistent with the upper limit of estimates by the hbl–pl–qz thermobarometer. In addition, LA-ICP-MS U–Pb analysis on metamorphic zircons yielded weighted mean 207Pb/206Pb ages of 1901 ± 22–1817 ± 21 Ma, which represent the timing of amphibolite-facies metamorphism. As a whole, the PT estimates display a high geothermal gradient, which is consistent with coeval ultrahigh-temperature metamorphism and associated mantle-derived mafic-ultramafic rocks in the Diebusige Complex. Combing this information with the previously published data from the Diebusige Complex, an extensional setting after continental collision is inferred for the eastern Alxa Block during the late Paleoproterozoic. The HREE enrichment patterns of metamorphic zircons from the amphibolites in this study are in agreement with that these amphibolites formed at relatively shallower crust than the garnet-bearing mafic granulites in the Diebusige Complex. [ABSTRACT FROM AUTHOR]