Epidote Amphibolite Research Articles

A New Scandium-Bearing Variety of Tusionite from the Eastern Pamirs (Tajikistan)

A new Sc-bearing variety of tusionite has been found in the Eastern Pamirs, in the near-miarolitic Dorozhny granite pegmatite complex at the Kukurt pegmatite field (left bank of the Kukurt River, 45 km east of the settlement of Murghab, Gorno-Badakhshan Autonomous Oblast, Tajikistan). Miarolitic pegmatites are related to the alpine leucocratic granites of the Shatput intrusive complex and occur in rocks of the Sarydzhilga Formation (PR3?) metamorphosed under amphibolite and epidote–amphibolite facies. Tusionite occurs in association with quartz, K-feldspar, albite, elbaite, Sc-bearing spessartine (up to 0.2 wt % Sc2O3) and accessory fluorapatite, Y-bearing fluorite, Sn-bearing titanite, magnetite, polycrase-(Y), B-rich gadolinite-(Y), ixiolite, manganocolumbite, pyrochlore, chernovite-(Y), cassiterite, varlamoffite, lepidolite, and Hf-rich zircon (up to 17.0 wt % HfO2). Tusionite forms thin lamellae (0.05–0.25 mm) and fanlike aggregates. It is yellow and transparent with a vitreous luster. It has weak pleochroism, No (orange–yellow) > Ne (light yellow). It is a uniaxial negative mineral, no = 1.870(5), ne = 1.760(3). The Raman spectrum has been reported for tusionite for the first time (laser excitation 532 nm); main bands: 228, 305, 379, 466, 661, 733, 750, 942, 1218, and 1458 cm–1. Its X-ray diffraction pattern is similar to tusionite from the Southwestern Pamirs. Hexagonal unit cell parameters: a = 4.772(3), c = 15.28(3) A. Chemical composition (electron microprobe, wt %, average of 14 analyses): 0.06 (0.00–0.31) Ta2O5; 50.28 (49.43–51.03) SnO2; 0.02 (0.00–0.19) TiO2; 1.02 (0.62–1.53) Sc2O3; 0.03 (0.00–0.19) CaO; 0.21 (0.16–0.46) FeO; 24.06 (23.59–24.34) MnO; 24.51 (calc. 23.90) B2O3; total 99.62 (99.08–100.79). The substitution mechanism involving Sc is not clear. Substitution of Sc+3 for Sn+4 with partial replacement of Mn+2 by Mn+3 is possible; however, Sc negatively correlates not only with Sn, but also with (Fe + Mn), which may result from the location of Sc in both octahedral positions with Sn+4 and Mn+2.

Episodic Fluid Action in Chinese Southwestern Tianshan HP/UHP Metamorphic Belt: Evidence from U–Pb Dating of Zircon in Vein and Host Eclogite

Fluid plays a key role in metamorphism and magmatism in subduction zones. Veins in high-pressure (HP) to ultrahigh-pressure (UHP) rocks are the products of fluid–rock interactions and can thus provide important constraints on fluid processes in subduction zones. In this study, we present an integrated study of zircon in situ U–Pb dating, trace element and mineral inclusion analysis for a complex vein and its host eclogite in the southwestern Tianshan UHP terrane, aiming to decipher the episodic fluid action during slab subduction and exhumation. Both zircon in eclogite and vein have euhedral, prismatic morphology similar to those crystallized from metamorphic fluid. Zircon in eclogite shows core–rim structures with distinct bounds and mineral inclusions. Zircon in the vein shows sector zoning or weak zoning, with bright rims around most zircon grains, which suggests recrystallization of the zircon crystals after their formation and multiple evolution of the vein. Eclogite zircon rims yield a weighted mean of 311 ± 3 Ma and cores yield a range from 413 ± 4 to 2326 ± 18 Ma, respectively. Vein zircon yields four groups of age (~355 Ma, ~337 Ma, ~315 Ma, and ~283 Ma), which date four episodes of fluid flow involving zircon growth. The first two groups of age may represent prograde epidote–amphibolite facies and amphibolite/blueschist facies metamorphism stage, respectively. The third group is similar to that of eclogite zircon rims, which is thought to date the eclogitic facie metamorphism (320–305 Ma), and the fourth group dates a later retrograde metamorphism after greenschist facies. The vein-forming fluid system was supposed to be an open system indicated by trace element of vein zircon and mineral assemblage of the vein. The coexistence of rutile, zircon, and garnet in prograde vein and the heavy rare earth elements (HREE) enrichment characteristic of vein zircon suggest that the vein-forming fluid are enriched in high field strength elements (HFSE) and HREE, and such fluid could be formed under low P–T conditions.

Metamorphism of Jhyallaphay-Barpak-Bhachchek area of Gorkha District, Lesser Himalaya and Higher Himalaya, Central Nepal

The study is focused on geological mapping, petrography and metamorphism of Jhyallaphat–Barpak–Bhachchek area, a part of Gorkha District, Central Nepal using a base of 1:25000 scale covering an area of 139.80 sq. km. The rocks of the study area can be broadly divided into two tectonic zones; the Lesser Himalaya consisting of fie lithological units, and the Higher Himalaya consisting of Formation I of the Tibetan Slab. Three metamorphic zones can be distinguished in the study area; biotite zone, garnet zone and kyanite zone. The biotite zone of the mineral assemblage in pelitic rocks consists of biotite+muscovite+chlorite+quartz, in psammitic rocks comprises of biotite+muscovite+chlorite+feldspar+quartz and in carbonate rocks comprises of biotite+muscovite+calcite/dolomite+feldspar+quartz, respectively. These mineral assemblages show that the area belongs to the greenschist facies. The mineral assemblage of the garnet zone in pelitic rocks constitutes garnet+biotite+muscovite+chlorite+quartz, and in psammitic rocks constitutes of garnet+biotite+muscovite+feldspar+quartz. The minerals assemblages found within the biotite and garnet zones represent the well-known inverted metamorphism in the Lesser Himalaya. Mineral assemblage of the kyanite zones constitutes of kyanite+garnet+biotite+muscovite+feldspar+quartz. The mineral assemblages of the both garnet and kyanite zones show that the area belongs to the epidote amphibolite facies.
 The bedding and foliation planes are almost parallel, showing that isograds also cut across the foliation. Therefore, the main metamorphic event should have followed development of foliation in the area. The rocks of the area show at least two metamorphic events: syntectonic prograde and post-tectonic retrograde. Syn-tectonic prograde metamorphism (M1), which has grown during a single phase of deformation and most frequently encountered garnet prophyroblast. Metamorphic deformation is represented by the presence of metamorphic foliation, stretching lineation, and S-C fabric. Post-tectonic retrograde metamorphism (M2), which is followed by retrograde mineral formation changing its P-T condition from high to low grade minerals, such as the formation of the biotite and chlorite minerals around the rims of the garnet porphyroblasts.

Ultrahigh Pressure Metamorphism and Tectonic Evolution of Southwestern Tianshan Orogenic Belt, China: A Comprehensive Review

AbstractRecently, a huge ultrahigh‐pressure (UHP) metamorphic belt of oceanic‐type has been recognized in southwestern (SW) Tianshan, China. Petrological studies show that the UHP metamorphic rocks of SW Tianshan orogenic belt include mafic eclogites and blueschists, felsic garnet phengite schists, marbles and serpentinites. The well‐preserved coesite inclusions were commonly found in eclogites, garnet phengite schists and marbles. Ti‐clinohumite and Ti‐chondrodite have been identified in UHP metamorphic serpentinites. Based on the PT pseudosection calculation and combined U‐Pb zircon dating, the P‐T‐t path has been outlined as four stages: cold subduction to UHP conditions before ∼320 Ma whose peak ultrahigh pressure is about 30 kbar at 500oC, heating decompression from the Pmax to the Tmax stage before 305 Ma whose peak temperature is about 600oC at 22kbar, then the early cold exhumation from amphibolite eclogite facies to epidote‐amphibolite facies metamorphism characterized by ITD PT path before 220 Ma and the last tectonic exhumation from epidote amphibolite facies to greenschist facies metamorphism. Combining with the syn‐subduction arc‐like 333‐326 Ma granitic rocks and 280‐260 Ma S‐type granites in the coeval low‐pressure and high‐temperature (LP‐HT) metamorphic belt, the tectonic evolution of Tianshan UHP metamorphic belt during late Cambrian to early Triassic has been proposed.

Gold-Bearing Pyrrhotite Ores in East Sayan: Composition and Formation Conditions (by the Example of the Ol’ginskoe Ore Occurrence)

Abstract —We present results of study of the chemical composition, formation conditions, and genesis of gold-bearing pyrite–pyrrhotite ores widespread within the ophiolite belts in the southeast of East Sayan. The study was performed at the Ol’ginskoe ore occurrence localized in the Ol’gino gold ore zone. Sulfide mineralization zones are composed of lenticular interbeds and bodies subconfoimable with the bedding of the enclosing schist strata. The ores are carbonaceous siliceous garnet–biotite schists variably enriched in sulfide minerals, mainly pyrrhotite, with impurities of other silicate minerals: tremolite, chlorite, albite, plagioclase, diopside, and epidote. In addition, sphalerite, chalcopyrite, pyrite, and arsenopyrite are present in small amounts in the ores. The established Р–Т conditions of ore metamorphism (T = 430–540 °C, P ~ 5 kbar) correspond to the lower boundary of the epidote–amphibolite facies. Thermobarogeochemical studies of fluid inclusions in vein quartz showed close temperatures (536–340 °C) but lower pressures, 200–800 bars, which indicates that the quartz vein formation was related to a pressure drop. The predominant salts of the fluid inclusions are Fe and Mg chlorides with Na and K impurities. The geologic location, structures, textures, and mineral, geochemical, and isotope compositions of the studied sulfide ores indicate their formation in submarine deep-water environments as a result of the activity of hydrothermal systems, analogues of “black smokers”, later subjected to metamorphism. During metamorphism, primary ores underwent mineral and chemical transformations: pyrrhotitization of pyrite, appearance and coarsening of native gold particles, and redistribution of components (Mn, As, etc). The low contents of gold and some ore-forming elements (Zn, Cu, and Pb) in pyrrhotite ores testify to the removal of these elements with a metamorphogenic fluid. The removed ore-forming components might have been a source of material for later gold–sulfide–quartz deposits of the Urik–Kitoi zone in East Sayan.

Age constraints on high-pressure/low-temperature metamorphism and sedimentation in the Luk Ulo Complex (Java, Indonesia)

Pre-Cenozoic blueschist- and eclogite-facies rocks occur widely scattered in the central Indonesian region, possibly representing lateral equivalents of the same subduction zone. High-pressure/low-temperature (HP/LT) metamorphism is thought to have occurred during the Early Cretaceous, which however is only constrained by K–Ar ages. This study is part of a systematic re-investigation of the regional geochronological record of various metamorphic rocks from this belt and focuses on the Luk Ulo Complex in central Java. Rubidium–Sr multipoint isochrons of one eclogite, two epidote-glaucophane schists and one epidote amphibolite yielded Early Cretaceous (Aptian) ages of 119–117 Ma (± 0.6–0.8 Ma). These ages are similar to previously reported K–Ar results, indicating that the existing data had not significantly been compromised by extraneous Ar contamination. Relatively low peak metamorphic temperatures (ca. 400 °C) during the HP/LT stage suggest that the Rb–Sr isotopic system was not significantly affected by diffusive loss during cooling. The Rb–Sr ages most likely constrain the time of mica growth or recrystallization during high-pressure metamorphism. The matching ages of high-pressure and lower pressure rocks are attributed to inheritance, but mixing of rocks that originally had formed at the same time at different depths of the subduction complex cannot completely be ruled out yet. Rubidium–Sr white mica ages of eclogite- and blueschist-facies rocks from central Java are younger than those of similar rocks from the Western Sulawesi tectonic province (130–120 Ma), revealing a previously unknown age difference that had been hidden by the poorer precision of the K–Ar dataset. At this point, it remains unclear whether this age difference is related to processes operating within the same or in different subduction zones.The zircon populations of clastic metasedimentary rocks from the Luk Ulo Complex are dominated by a prominent Jurassic to Carboniferous population (ca. 320–180 Ma) with a distinct Permo-Triassic peak (ca. 270–200 Ma). Another less pronounced Paleozoic age signal shows up at 450–380 Ma. Proterozoic and Archean zircon grains are very rare. At least some data points at the lower end of the age spectrum (ca.135–117 Ma) most likely are related to in-situ metamorphic processes. The youngest zircon grains of unambiguous detrital origin show up at ca. 160 Ma, indicating a Late Jurassic maximum depositional age. Additional detrital zircon data for the Barru Complex (Western Sulawesi) further substantiate previous findings documenting a complex age spectrum with contributions of Mesozoic (Triassic), Paleozoic (Permian, Carboniferous, Devonian to Ordovician), Meso- to Paleoproterozoic subpopulations and rare Archean zircons. The youngest zircons indicate a maximum depositional age of ca. 255–250 Ma. New and existing U–Pb zircon data indicate that the pre-metamorphic basin histories recorded in the Luk Ulo and Barru complexes are linked to different source terrains and depositional systems.

Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

AbstractGarnet grains in Sanbagawa quartz eclogites from the Besshi region, central Shikoku commonly show a zoning pattern consisting of core and mantle/rim that formed during two prograde stages of eclogite and subsequent epidote–amphibolite facies metamorphism, respectively. Garnet grains in the quartz eclogites are grouped into four types (I, II, III, and IV) according to the compositional trends of their cores. Type I garnet is most common and sometimes coexists with other types of garnet in a thin section. Type I core formed with epidote and kyanite during the prograde eclogite facies stage. The inner cores of types II and III crystallized within different whole‐rock compositions of epidote‐free and kyanite‐bearing eclogite and epidote‐ and kyanite‐free eclogite at the earlier prograde stage, respectively. The inner core of type IV probably formed during the pre‐eclogite facies stage. The inner cores of types II, III, and IV, which formed under different P–T conditions of prograde metamorphism and/or whole‐rock compositions, were juxtaposed with the core of type I, probably due to tectonic mixing of rocks at various points during the prograde eclogite facies stage. After these processes, they have shared the following same growth history: (i) successive crystal growth during the later stage of prograde eclogite facies metamorphism that formed the margin of the type I core and the outer cores of types II, III, and IV; (ii) partial resorption of the core during exhumation and hydration stage; and (iii) subsequent formation of mantle zones during prograde metamorphism of the epidote–amphibolite facies. The prograde metamorphic reactions may not have progressed under an isochemical condition in some Sanbagawa metamorphic rocks, at least at the hand specimen scale. This interpretation suggests that, in some cases, material interaction promoted by mechanical mixing and fluid‐assisted diffusive mass transfer probably influences mineral reactions and paragenesis of high‐pressure metamorphic rocks.

Basement Characteristic Western Part of Java, Indonesia ; Case Study in Bayah Area, Banten Province

Recent study reveals that in Bayah Complex, 20 km west of Ciletuh Melange Complex, discovered a metamorphic rock that interpreted as the basement of Java. The aim of this research is to know the characteristic of metamorphic in Bayah areas. The result shows that the metamorphic rocks of Bayah Geological Complex are dominated by mica schist group i.e. muscovite schist, muscovite - biotite schist, garnet biotite schist and chlorite schist associated with Pelitic - Psammitic protolith. The amphibolite, epidote amphibolite and actinolite schist found were metamorphosed of mafic rock protolith. All of them have been deformed and altered. Based on mineral assemblage, mica schist group included lower greenschist - epidote amphibolite facies, whereas actinolite schist, epidote amphibolite schist and hornblende schist included greenschist facies, epidote amphibolite facies and amphibolite facies respectively. Based on the data, these metamorphic rocks are associated with orogenic style. The metamorphic rocks exposed to the surface through a complex process since Late Cretaceous. Metamorphic rocks have been deformed, folded and faulted since its formation. Its possible this rock was uplifted to the surface due to intrusion of Cihara Granodiorite.

Physicochemical Conditions of Metamorphism of Rocks of the Ilyinovskii Complex (Trans-Uralian Uplift): First Data

This work presents a short description of the geological structure of the Ilyinovskii metamorphic complex. Based on the results of studying the garnet–biotite paragenesis, widespread among paragneisses and diaphthorites, two stages of metamorphic transformation of rocks were distinguished. During the Early Proterozoic, sedimentary rocks were transformed under amphibolite (cummingtonite–amphibolite) facies metamorphism (T = 530–580°C; P = 3.0–3.1 kbar); during the Late Riphean, in connection with the structural arrangement in the Southern Urals, they were metamorphosed under epidote–amphibolite facies conditions (T = 500–560°C; P = 5.4–6.0 kbar).

Ultrahigh pressure metamorphism and tectonic evolution of southwestern Tianshan orogenic belt, China: a comprehensive review

Abstract Recently, a huge ultrahigh-pressure (UHP) metamorphic belt of oceanic type has been recognized in SW Tianshan, China. Petrological studies show that the UHP metamorphic rocks of SW Tianshan orogenic belt include mafic eclogites and blueschists, pelitic garnet phengite schists, marbles and serpentinites. The well-preserved coesite inclusions are commonly found in eclogites, garnet phengite schists and marbles. Ti-clinohumite and Ti-chondrodite have been identified in UHP metamorphic serpentinites. Based on the P–T pseudosection calculation and combined U–Pb zircon dating, the P–T–t path has been outlined as four stages: cold subduction to UHP conditions before c. 320 Ma whose peak ultrahigh pressure is about 30 kbar at 500°C; heating decompression from the P max to the T max stage before 305 Ma whose peak temperature is about 600°C at 22 kbar; then the early cold exhumation from amphibolite eclogite facies to epidote-amphibolite facies metamorphism before 220 Ma; and the last tectonic exhumation from epidote amphibolite facies to greenschist facies metamorphism. Combining with the syn-subduction arc-like 333–326 Ma granitic rocks and 280–260 Ma S-type granites in the coeval low-pressure and high-temperature metamorphic belt, the tectonic evolution of Tianshan UHP metamorphic belt during late Cambrian to early Triassic has been proposed in this paper.

Replacement reactions and deformation by dissolution and precipitation processes in amphibolites

AbstractThe deformation of the middle to lower crust in collisional settings occurs via deformation mechanisms that vary with rock composition, fluid content, pressure, and temperature. These mechanisms are responsible for the accommodation of large tectonic transport distances during nappe stacking and exhumation. Here, we show that fracturing and fluid flow triggered coupled dissolution–precipitation and dissolution–precipitation creep processes, which were responsible for the formation of a mylonitic microstructure in amphibolites. This fabric is developed over a crustal thickness >500 m in the Lower Seve Nappe (Scandinavian Caledonides). Amphibolites display a mylonitic foliation that wraps around albite porphyroclasts appearing dark in panchromatic cathodoluminescence (CL). The albite porphyroclasts were dissected and fragmented by fractures preferentially developed along the (001) cleavage planes and display lobate edges with embayments and peninsular features. Two albite/oligoclase generations, bright in CL, resorbed and overgrew the porphyroclasts, sealing the fractures. Electron backscattered diffraction shows that the two albite/oligoclase generations grew both pseudomorphically and topotaxially at the expense of the albite porphyroclasts and epitaxially around them. These two albite/oligoclase generations also grew as neoblasts elongated parallel to the mylonitic foliation. The amphibole crystals experienced a similar microstructural evolution, as evidenced by corroded ferrohornblende cores surrounded by ferrotschermakite rims that preserve the same crystallographic orientation of the cores. Misorientation maps highlight how misorientations in amphibole are related to displacement along fractures perpendicular to its c‐axis. No crystal plasticity is observed in either mineral species. Plagioclase and amphibole display a crystallographic preferred orientation that is the result of topotaxial growth on parental grains and nucleation of new grains with a similar crystallographic orientation. Amphibole and plagioclase thermobarometry constrains the mylonitic foliation development to the epidote amphibolite facies (˜600°C, 0.75–0.97 GPa). Our results demonstrate that at middle to lower crustal levels, the presence of H2O‐rich fluid at grain boundaries facilitates replacement reactions by coupled dissolution–precipitation and favours deformation by dissolution–precipitation creep over dislocation creep in plagioclase and amphibole.

Early Precambrian tectono-thermal events: coupled U–Pb–Hf of detrital zircons from Jiao–Liao–Ji Belt, North China Craton

The Dashiqiao Formation on the Liaodong Peninsula constitutes an important component within the Jiao–Liao–Ji Belt, North China Craton. It is composed dominantly of dolomitic marbles intercalated with minor carbonaceous slates and mica schists, hosting one of the largest magnesite deposits on Earth. This study presents zircon cathodoluminescence (CL) images and U–Pb–Hf isotope data, as well as single-mineral geochemical data for the staurolite–garnet–mica schist from the Dashiqiao Formation, in order to constrain its protolith age and provenance, and further to discuss the early Precambrian tectono-thermal events of the North China Craton. U–Pb isotopic dating using the LA–ICP–MS method on detrital zircons from the schist preserves at least three age populations ranging in age from 2.99 to 2.02 Ga, and grains as old as ca 4087 Ma. The dominant Neoarchean detrital zircons were most probably sourced from the basement within the Longgang and Nangrim blocks, while the minor Mesoarchean zircons were only sourced from the Longgang Block. The subordinate middle Paleoproterozoic zircons are consistent with ages of the regionally distributed coeval Liaoji granites and volcanics within the Jiao–Liao–Ji Belt. Zircon U–Pb dating yields a metamorphic age of 1930 Ma for the sample, interpreted to represent the peak stage of epidote amphibolite facies metamorphism. Thus, the depositional age for the protolith of the schist was proposed in the period between 2.01 and 1.93 Ga. LA–MC–ICP–MS Lu–Hf isotopic data show that all Archean (2.45–2.55) detrital zircons possess positive eHf(t) values from + 0.7 to + 7.5 with juvenile depleted mantle model ages, suggesting a significant crustal growth event during the Neoarchean in the North China Craton. The Paleoproterozoic detrital zircons possess variable eHf(t) values (− 5.5–+ 8.3) and depleted mantle model ages from Mesoarchean to Paleoproterozoic. The zircons with negative eHf(t) values implies the Mesoarchean to Neoarchean crust undergoing a recycling event in the period 2.40–2.01 Ga, while those with positive eHf(t) value suggest some indication of juvenile addition to the crust during the Paleoproterozoic. Using regional geological and new detrital zricon U–Pb–Hf isotopic data, the early Precambrian tectono-thermal events can be subdivided into the following episodes: Mesoarchean, late Neoarchean, middle Paleoproterozoic, and late Paleoproterozoic times.

Early-middle Triassic basic magmatism and metamorphism of ultramafic-mafic complexes of the Ust’-Belaya terrane (central Chukotka, NE Russia): 40Ar/39Ar ages, petrological and geochemical data, geodynamic interpretations

ABSTRACTThe Ust’-Belaya terrane (Chukotka, NE Russia), belonging to the West Koryak fold belt, is made of mantle and crustal ultramafic-mafic complexes which originated during several discrete episodes of magmatic activity in the Neoproterozoic, the late Neoproterozoic-Cambrian and the early-middle Triassic and accreted to the Asian continental margin in the early Cretaceous. This paper focuses on the latest magmatic episode expressed in intrusion of microgabbro dikes and nearly coeval metamorphism superimposed on both ancient complexes and the dikes. We present original 40Ar/39Ar ages, mineral and bulk-rock chemistry of the microgabbro dikes, metamorphosed dike and vein cutting ultramafic-mafic complexes. Dike microgabbros resemble differentiated arc-tholeiitic magmas originated in a subduction setting. Differentiated magmas intruded into much older spinel peridotites and dunites located at mid-crustal levels. Intrusion and crystallization of these magmas was followed by down-going movement of spinel peridotites and rocks of the mantle-crust transition zone towards a mantle wedge where they were metamorphosed at high-P conditions. This metamorphism resulted in transformation of microgabbro to garnet amphibolite, diorite to albite-zoisite-paragonite-pargasite rock and spinel peridotites to metaperidotites. P-T parameters of this metamorphism reconstructed based on mineral assemblage of garnet amphibolite correspond to those of epidote amphibolite – amphibolite – amphibole eclogite facies transition. The peculiar zoisite (clinozoisite)-paragonite mineral assemblage typical of metamorphosed vein rock indicates high-P metamorphic conditions of epidote-amphibolite facies.

The Early Precambrian metamorphic events in Eastern Sarmatia

Four metamorphic events were identified in the Early Precambrian history of the Eastern Sarmatia. The oldest Mesoarchaean (?) ultrahigh temperature (UHT) metamorphism at 900–1000 °C and 10–12 kbar occurred in the Kursk and Azov domains. Although attempts to constrain accurately the age of metamorphism failed, they provided evidence for the existence of the early Mesoarchaean ultra-hot orogen in the central part of the Archaean core of the Eastern Sarmatia. The 2.82 Ga metamorphism, widespread within the Eastern Sarmatia, finished the formation of the ancient Mesoarchaean basement. The granulite facies metamorphism at ∼750 °C and ∼5 kbar is confined to zones composed of gneisses and migmatites, while the epidote–amphibolite and amphibolite facies metamorphism occurred within preserved fragments of greenstone belts. The Palaeoproterozoic low-pressure metamorphism occured at 2.07 Ga in all Palaeoproterozoic structures under P-T conditions ranging from 350 °C at 2 kbar to 750 °C at 5 kbar. The metamorphic event was caused by a collision between the Volga-Uralia segment and Sarmatia at ca. 2.1 Ga. The youngest high-T granulite metamorphism at 850 °C and 5.5 kbar (2.04 Ga) recorded in the Bryansk domain likely reflects a thermal effect related to magmatic event that preceeded the main episode of the Osnitsk-Mikashevichi volcano-plutonic belt formation at the active margin of Sarmatia at 1.96–2.00 Ga.

The relationship between systematic metamorphic patterns and collisional processes along the Qinling–Sulu–Odesan collisional belt between the North and South China Cratons

Abstract Along the Qinling–Sulu–Odesan collisional belt, the peak metamorphic conditions reflect systematically changing metamorphic regimes from east to west: ultra-high temperature (UHT) granulite-facies metamorphism in the Odesan area; high pressure (HP) eclogite-facies metamorphism in the Hongseong area; ultra-high pressure (UHP) eclogite-facies metamorphism in the Sulu, Dabie and Hong'an areas; HP eclogite-facies metamorphism in the Tonbai area; epidote–amphibolite- and blueschist facies metamorphism in the Wudang area; and HP granulite-facies metamorphism in the western Qinling area. Additionally, the Triassic post-collisional igneous rocks regionally intruded at both ends of the collision belt, in the west Qinling and Hongseong–Odesan areas. These observations indicate that the following collisional processes occurred along the Qinling–Dabie–Odesan collisional belt. The North China Craton and South China Craton first collided at their eastern margins (the Odesan area) with an angle of approximately 60° between their long axes. Consequently, the amount of oceanic slab subducted before the collision increased from the Odesan area to the Sulu area, resulting in an increase in the depth of slab break-off. After slab break-off occurred in the Sulu area, the lateral tearing force strengthened, decreasing the depth of slab break-off from the Sulu area to the western Qinling area as collision propagated westward.

Polyorogenic reworking of ore‐controlling shear zones at the South Range of the Sudbury impact structure: A telltale story from in situ U–Pb titanite geochronology

AbstractThe post‐impact orogenic evolution of the world class Ni–Cu–PGE Sudbury mining camp in Ontario remains poorly understood. New temporal constraints from ore‐controlling, epidote–amphibolite facies shear zones in the heavily mineralised Creighton Mine (Sudbury, South Range) illuminate the complex orogenic history of the Sudbury structure. In situ U–Pb dating of shear‐hosted titanite grains by LA‐ICP‐MS reveals new evidence for shear zone reworking during the Yavapai (ca. 1.77–1.7 Ga), Mazatzalian–Labradorian (1.7–1.6 Ga) and Chieflakian–Pinwarian (1.5–1.4 Ga) accretionary events. The new age data show that the effects of the Penokean orogeny (1.9–1.8 Ga) on the structural architecture of the Sudbury structure have been overestimated. At a regional scale, the new titanite age populations corroborate that the Southern Province of the Canadian Shield documents the same tectonothermal episodes that are recorded along orogenic strike within the accretionary provinces of the Southwestern United States.

Progressive evolution of whole‐rock composition during metamorphism revealed by multivariate statistical analyses

AbstractThe geochemical evolution of metamorphic rocks during subduction‐related metamorphism is described on the basis of multivariate statistical analyses. The studied data set comprises a series of mapped metamorphic rocks collected from the Sanbagawa metamorphic belt in central Shikoku, Japan, where metamorphic conditions range from the pumpellyite–actinolite to epidote–amphibolite facies. Recent progress in computational and information science provides a number of algorithms capable of revealing structures in large data sets. This study applies k‐means cluster analysis (KCA) and non‐negative matrix factorization (NMF) to a series of metapelites, which is the main lithotype of the Sanbagawa metamorphic belt. KCA describes the structures of the high‐dimensional data, while NMF provides end‐member decomposition which can be useful for evaluating the spatial distribution of continuous compositional trends. The analysed data set, derived from previously published work, contains 296 samples for which 14 elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Rb, Sr, Zr and Ba) have been analysed. The KCA and NMF analyses indicate five clusters and four end‐members, respectively, successfully explaining compositional variations within the data set. KCA indicates that the chemical compositions of metapelite samples from the western (Besshi) part of the sampled area differ significantly from those in the east (Asemigawa). In the west, clusters show a good correlation with the metamorphic grade. With increasing metamorphic grade, there are decreases in SiO2 and Na2O and increases in other components. However, the compositional change with metamorphic grade is less obvious in the eastern area. End‐member decomposition using NMF revealed that the evolutional change of whole‐rock composition, as correlated with metamorphic grade, approximates a stoichiometric increase of a garnet‐like component in the whole‐rock composition, possibly due to the precipitation of garnet and effusion of other components during progressive dehydration. Thermodynamic modelling of the evolution of the whole‐rock composition yielded the following results: (1) the whole‐rock composition at lower metamorphic grade favours the preferential crystallization of garnet under the conditions of the garnet zone, with biotite becoming stable together with garnet in higher‐grade rock compositions under the same P–T conditions; (2) with higher‐grade whole‐rock compositions, more H2O is retained. These results provide insight into the mechanism suppressing dehydration under high‐P metamorphic conditions. This mechanism should be considered in forward modelling of the fluid cycle in subduction zones, although such a quantitative model has yet to be developed.

The composite North American Craton, Superior Province: Deep crustal structure and mantle-plume model of Neoarchaean evolution

This paper aims to synthesize geological and geophysical data on Superior Province and, in general, Archean crust of the composite North America Craton, relying on experience with deep geological and geophysical studies of composite East European Craton. By onset of the Neoarchean, Archean provinces of the North American Craton were parts of the single stable continent. Intensely reworked in the Neoarchean, crustal rocks were combined into a round-oval domain, wherein distribution of geophysical, structural, and metamorphic parameters displays concentric zoning. The Central zone occupies Hudson Bay basin located immediately above the lithospheric keel. The Internal zone is mainly composed of granulite facies plutonic, volcanic and sedimentary rocks. The External zone encompasses southern part of the Superior Province together with Hearne and Rae Provinces. This paper presents 3D crustal models of southern Superior Province obtained through synthesis of geological maps with a new geological interpretation of seismic sections along LITHOPROBE Geotransects and of the eastern Abitibi Belt based on higher resolution reflection seismic images. Neoarchean evolution of the North American craton represents plate-tectonic processes initiated by a superplume. It is only one of series of similar phenomena recorded ∼2.75Ga ago in a number of regions. The most important features, repeated to a certain degree in tectonic units of this type, are: (1) synchronous formation between 2.79 and 2.58Ga; (2) frequently repeated combination of high-grade (granulite and high-temperature amphibolite facies) and low- or moderate-grade (greenschist and epidote–amphibolite facies) metamorphic rocks; (3) lower crustal granulite-basaltic layer (reflectivity zone displayed in seismic images of crust) formed or transformed at the final stage of endogenic activity.

Age and P–T conditions of the Gridino‐type eclogite in the Belomorian Province, Russia

ABSTRACTAlthough eclogites in the Belomorian Province have been regarded as Archean in age and among the oldest in the world, there are also multiple studies that have proposed a Paleoproterozoic age. Here, we present new data for the Gridino‐type eclogites, which occur as boudins and metamorphosed dykes within tonalite–trondhjemite–granodiorite gneisses. Zircon from these eclogites has core and rim structures. The cores display high Th/U ratios (0.18–0.45), negative Eu anomalies and strong enrichment in HREE, and have Neoarchean U–Pb ages of c. 2.70 Ga; they are interpreted to be magmatic in origin. Zircon cores have δ18O of 5.64–6.07‰ suggesting the possibility of crystallization from evolved mantle‐derived magmas. In contrast, the rims, which include the eclogite facies minerals omphacite and garnet, are characterized by low Th/U ratios (<0.035) and flat HREE patterns, and yield U–Pb ages of c. 1.90 Ga; they are interpreted to be metamorphic in origin. Zircon rims have elevated δ18O of 6.23–6.80‰, which was acquired during eclogite facies metamorphism. Based on petrography and phase equilibria modelling, we recognize a prograde epidote amphibolite facies mineral assemblage, the peak eclogite facies mineral assemblage and a retrograde high‐P amphibolite facies mineral assemblage. The peak metamorphic conditions of 695–755°C at >18 kbar for the Gridino‐type eclogites suggest an apparent thermal gradient of <39–42°C/kbar for the Lapland–Kola collisional orogeny.

Cretaceous high-pressure metamorphism and low pressure overprint in the Sistan Suture Zone, eastern Iran: Additional temperature estimates for eclogites, geological significance of U-Pb zircon ages and Rb-Sr constraints on the timing of exhumation

The Sistan Suture Zone, eastern Iran, includes blocks and lenses of eclogite-, blueschist- and/or epidote amphibolite-facies rocks that provide an excellent opportunity to examine the exhumation history of oceanic HP/LT rocks and their retrograde derivatives. Zr-in-rutile thermometry of eclogites corroborates previous interpretations suggesting metamorphic temperatures of ca. 550–600°C during the HP stage in the Sistan area. Flat HREE distribution patterns and Ti-in-zircon temperatures of ca. 500–600°C document that zircon in eclogite is of metamorphic origin. REE patterns of zircon from felsic meta-igneous rocks do not allow to distinguish between a magmatic or metamorphic origin, but relatively low temperatures indicated by Ti-in-zircon thermometry (ca. 500–600°C) and the close similarity of zircon (U–Pb) and white mica (Rb–Sr, Ar–Ar) ages favor a metamorphic zircon origin. Previously published isotopic ages of the felsic rocks cannot unambiguously be linked to the eclogite- and/or blueschist-facies P–T conditions due to the absence of unequivocal mineralogical and petrological evidence. Instead, these rocks may record contemporaneous metamorphic processes that took place at a different depth within the subduction complex, or may indicate active ridge subduction and/or melt formation in the subduction zone at relatively low pressures. Biotite-based internal Rb–Sr isochrons of newly dated epidote amphibolite and biotite-albite gneisses indicate ages of ca. 74–80Ma, either dating fluid-infiltration-induced formation of biotite during relatively fast uplift, or the time of final passage through the effective biotite closure temperature. Rb–Sr ages of phengite from both an epidote amphibolite and a biotite-albite gneiss yield ages that correspond to the HP/LT stage. This outcome, combined with textural evidence for derivation from eclogitic precursors documents that white mica ages of some strongly overprinted Sistan rocks are compromised by inheritance and do not record later exhumation.