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The molybdenum (Mo) isotope composition (defined as δ98Mo measured per mil relative to NIST-3134) of many modern arc systems and the upper continental crust is heavier than the mantle and most subducting slab lithologies. This observation has led to a model whereby fluids leaving the slab transfer isotopically heavy Mo preferentially to the mantle wedge, leaving the residual slab isotopically lighter. We explore this model via an Mo isotope study of the metasedimentary and mélange lithologies of the Catalina Schist in California. These rocks record subduction zone metamorphism over a wide range of high-pressure/low-to-medium temperature conditions. Mo isotope compositions of the metasedimentary rocks decrease with increasing metamorphic grade, from a δ98Mo mean of −0.04 ± 0.13 ‰ (1σ, n = 3) for the lawsonite albite facies to −0.38 ± 0.07 ‰ (1σ, n = 5) for the epidote–amphibolite facies. The highest grade [amphibolite facies] samples show a slight uptick in δ98Mo values, with a mean of −0.19± 0.14 ‰ (1σ, n = 4). When coupled with major and trace element variations, the changes in δ98Mo appear to reflect metamorphic effects rather than sedimentary source rock heterogeneity. The positive relationship between δ98Mo and [Mo] and negative relationship with Ce/Mo argue for progressive loss of Mo with isotope fractionation during increasing P-T conditions; the reversal of this trend, seen in the increase in δ98Mo between the epidote amphibolite and amphibolite facies may reflect a subsequent partial re-fertilization by fluids carrying isotopically heavy Mo. δ98Mo values in the mélange across all grades are highly variable, ranging from −1.75 to −0.19‰, consistent with large scale mobilization of Mo with isotope fractionation. A metasomatized amphibolite block and reaction rind show similar light isotope compositions and signs of Mo depletion and transport. Together these three whole-rock data sets demonstrate pervasive open system behavior and mobility of Mo in the Catalina Schist. LA-ICP-MS Mo measurement of minerals in the differing metasedimentary metamorphic grades indicates that Fe oxides and/or hydroxides, titanite, rutile, and epidote are important reservoirs of Mo but when comparing mineral Mo content and modal abundance to whole-rock Mo concentrations, several samples were found to have "missing Mo", something that has been observed in other Mo inventory studies. We attribute this to sample heterogeneity between the thin section and whole-rock powder scales. [ABSTRACT FROM AUTHOR]

Veins composed of glaucophane + phengite + quartz cross-cut the high pressure-low temperature (HP-LT) Cycladic Blueschist Unit (CBU) of southern Evia, Greece. The veins exhibit a rheology-dependent distribution within layered metamorphic rock comprising cm-scale intercalations of albite-clinopyroxene metabasalt and schistose quartzite. Strain was accommodated by ductile processes in the quartzite, whereas brittle deformation produced four sets of crack-seal syntaxial veins in the coarser-grained metabasalt. All vein sets are subvertical to steeply-dipping and are oriented at high angles to one another. The geometry of the planar vein walls suggests the veins are mode-I (opening mode) fractures, whose sub-vertical orientations indicate formation during extension. Oxygen isotope thermometry using phengite-quartz pairs provides crystallization temperatures of 315–335 °C. Combined 40Ar/39Ar and in-situ87Rb/87Sr geochronology of vein-hosted phengite and glaucophane indicate crystallization and vein sealing at c. 22–23 Ma when the CBU is predicted to be undergoing greenschist facies metamorphism coincident with regional extension. The structural and stable isotope data are likewise consistent with a syn-exhumation extensional setting, and easily reconciled with existing petrological data indicating the CBU sustained prolonged residence near the greenschist-blueschist facies boundary. We propose a model whereby phengite and glaucophane were stabilized at greenschist facies conditions by the elevated αSiO2 and fO2 in the fluid parental to the veins. Our data provide strong new evidence for the sensitivity of nominally blueschist facies minerals to bulk system chemistry, supported by thermodynamic modelling evidence from other orogens that such HP-LT minerals may exhibit stability that spans multiple stages of orogenesis. [ABSTRACT FROM AUTHOR]

The mineralogical and geochemical features of diamond-bearing tourmaline crystals (schorl-uvite series) from garnet-clinopyroxene rocks of the Kumdy-Kol deposit (Northern Kazakhstan) have been studied in detail. The formation of the main rock-forming minerals (garnet + K-bearing clinopyroxene) occurred in the diamond stability field at 4-6 GPa and 950-1000 °C. Crystallization of K-bearing clinopyroxene at these parameters is possible in the presence of an ultra-potassic fluid or melt formed because of crustal material melting in subduction zones. Tourmaline crystals (up to 1 cm) containing diamond inclusions perform veins crosscutting high-pressure associations. The composition of individual zones varies from schorl to uvite within both a single grain and the sample as a whole. The potassium content in this tourmaline does not exceed 0.1 wt.% K2O, and the isotopic composition of boron δ11B varies from –10 to –15.5 ‰, which significantly differs from the previously established isotopic composition of boron in maruyamaite crystals (δ11B 7.7 ‰ in the core and –1.2 ‰ in the rim) of the same deposit. Analysis of the obtained data on δ11B in the tourmalines from the diamond-grade metamorphic rocks within the Kumdy-Kol deposit suggests the existence of two boron sources that resulted in crystallization of K-bearing tourmaline crystals (maruyamaite-dravite series) and potassium-free tourmalines of the schorl-uvite series.

In the Northern Apennines, the Internal Ligurian Units are considered deformed and metamorphosed fragments of the Ligure-Piemontese oceanic basin. In this paper, we report on the temperature and pressure conditions of the metamorphic peak for four Internal Ligurian Units, estimated using different geothermometers and geobarometers based on the white mica and chlorite compositions. These minerals were formed during the D1 deformation phase in the pre-Oligocene. The results indicate that the Portello and Gottero units are both characterized by metamorphic conditions pertaining to low blueschists facies, while the Colli-Tavarone and Bracco-Val Graveglia Units show a lower metamorphic imprint that produces assemblages of prehnite-pumpellyite facies. The estimated geothermal gradient for the metamorphic peak achieved by the analyzed Internal Ligurian Units during the D1 phase is 7–15 °C/Km, which is indicative of deformation in a subduction setting. Under these conditions, the D1 phase developed in these units as a result of underplating at the base of the accretionary wedge during the closure of the Ligure-Piemontese basin. These data indicate a close geodynamic correlation among the Internal Ligurian Units and the ophiolite-bearing units of the Alps. [ABSTRACT FROM AUTHOR]

40Ar/39Ar analyses were made on maruyamaite (potassium-dominant tourmaline) from tourmaline-quartz-feldspar rocks discovered within the Kumdy-Kol microdiamond deposit (Kokchetav massif, North Kazakhstan). Turmaline yielded well-defined 40Ar/39Ar plateau age spectra whose values coincide within the error - ages of 502.3±8.0, 502.2±8.0, 506.0±8.0 Ma. These ages are much younger than the age of 530±2 Ma determined for high-pressure metamorphism by different methods. Thus, the formation of tourmaline-rich rocks of the Kumdy-Kol deposit cannot be associated with the high-pressure metamorphic events, and, therefore, testifies in favor of the low-pressure nature of maruyamaite. Based on the coincidence of age data for tourmaline crystals with different potassium contents, it can be concluded that the K/Ar system in tourmaline can be used for dating metasomatic and metamorphic processes.

In Corsica Island (France), a belt of metamorphic continental units derived from the European margin is exposed at the western rim of the Alpine Corsica. This study provides a complete anatomy of one of these units (Tour de Valletto Unit) consisting of a Palaeozoic basement covered by an Early Jurassic – late Eocene sedimentary succession. This unit records a polyphase tectono-metamorphic history acquired during the late Eocene – lower early Miocene convergence processes involving the Adria and Europe continental plates. Using a multidisciplinary approach, we reconstructed in detail the P-T path of the Tour de Valletto Unit. The path shows a metamorphic climax at blueschist facies conditions suggesting that the unit was accreted at c. 40 km of depth. After its accretion, the Tour de Valletto Unit was exhumed up to the Earth surface following a path characterized by an increase of temperature and a decrease of pressure. The reconstructed tectono-metamorphic history of the Tour de Valletto Unit is compared with those of the other continental units located in western Alpine Corsica to discuss the exhumation processes active during continental subduction. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two types of rodingites recorded ocean-floor and subduction metamorphism. • Type I rodingite originated at <~4kbar/~200–300 °C in a highly reducing environment. • Typy II rodingite formed ~13.5–21 kbar/~450–550 °C in less reducing conditions. • The rodingite provides fingerprints of ocean floor, subduction and exhumation. Abundant rodingite was found within serpentinized peridotite and spatially close to gabbro and diabase dykes in the Xigaze ophiolite of the Yarlung Zangbo suture zone. Two types of rodingite are identified based on field- and textural relationships as well as on mineral- and bulk-rock compositions. Type I rodingite with porphyroblastic texture occurs along extensional fractures within massive serpentinized peridotite and contains two subtypes representing the early stage of rodingitization. Type II rodingite is associated with a foliated serpentinized host rock and contains three subtypes, characterized by prehnite-absent mineral assemblages with fine-grained porphyritic to aphanitic texture. All rodingite II subtypes experienced a higher degree of rodingitization than Type I. Type I rodingite was formed by metasomatism of CaO-rich fluids from serpentinized peridotite during ocean floor metamorphism at ~120–130 Ma. Thermodynamic modeling shows that Type I rodingite originated at low P-T conditions of <~4 kbar and ~200–300 °C during limited desilication and calcium enrichment. within a highly reducing environment (logƒO 2 < −35.3). Type II rodingite originated during subduction starting at ~120 Ma with most intense desilication and calcium increase under peak P-T conditions up to ~13.5–21.5 kbar and ~450–550 °C. Modeled values of logƒO 2 for the formation of rodingite II subtypes vary from highly oxidized conditions of logƒO 2 = −19.7 to −18.6 to relatively reducing conditions of logƒO 2 < ~−26. P-X CO2 pseudosection modelling predicted Type II rodingite to have formed under CO 2 -poor conditions at X CO2 < ~1 × 10-4 during subduction to the maximum depth. Hence the rodingite of the Xigaze ophiolite provides important fingerprints recording events during ocean floor spreading and different stages of subduction as well as late exhumation. [ABSTRACT FROM AUTHOR]

Subduction erosion along active convergent plate margins involves arc subduction, tectonic erosion and sediment subduction. Here we investigate the metasedimentary rocks enclosing blueschists and eclogites in the Changning–Menglian orogenic belt (CMOB) of the southeastern Tibetan Plateau which marks the main suture associated with the closure of the Paleo-Tethyan Ocean between the Baoshan–Tengchong and Lanping–Simao blocks. We present an integrated study of petrology, mineral chemistry, whole-rock geochemistry, zircon U–Pb geochronology, and phengite 40Ar/39Ar isotopes. These rocks consist mainly of Ph-Qz schist, Ab-Qz-Ph schist, Grt-Ph schist, St-Ky-bearing Grt-Ms schist, and Cld-Pg-Ph schist. They display compositions similar to arkose and subarkose with trace element patterns overlapping those of the upper continental crust, and suggest immature and weak to moderate chemical weathering. The detrital zircon age spectra are dominated by two major age groups of 600–450 Ma with a peak at 550 Ma, and 1150–850 Ma with a peak at 950 Ma, suggesting that the metasedimentary rocks were sourced from a mixture of Precambrian sediments in the Lanping–Simao Block and Pan-African Orogeny-related magmatic rocks with minor Early Paleozoic Proto-Tethyan subduction- and volcanic rocks related to the onset of oceanic spreading of Paleo-Tethys. Combined with the maximum depositional age of ~428 Ma based on the youngest detrital age peaks, we infer that the Lancang Group could be the Early Paleozoic sedimentary sequence of the Lanping–Simao Block and was deposited in a foreland setting. The metasedimentary rocks display high-pressure (HP) mineral assemblages involving Ph + Ab + Qz, Grt + Ph + Qz, Grt + Ky + Ph, and Cld + Pg + Ph, and record blueschist facies to eclogite facies peak P – T conditions of 18.8–21.6 kbar at 392–553 °C (Grt-Ph schists) and 20–24.2 kbar at 592–654 °C (St-Ky-bearing Grt-Ms schists). Metamorphic zircon U–Pb and phengite 40Ar/39Ar isotopic analyses from these rocks yield consistent peak HP ages of 238–235 and 237–231 Ma, respectively, whereas 40Ar/39Ar dating for the Grt-Ph schist with low Si contents in phengite yields the post-peak age at 226.2 ± 1.3 Ma. We propose that a significant volume of sediments in Lancang Group at the western margin of the Lanping–Simao Block were off scraped by the downgoing oceanic lithosphere associated with the Paleo Tethyan ocean closure, and subducted to depths of ~60–80 km during Middle Triassic. These were subsequently rapidly exhumated at a rate of ~5.7–6.5 km/myr in the Late Triassic. The subduction erosion model proposed in our study has important implications for reconstructing the evolutionary history of the Paleo-Tethys as well as the architecture variation of the paleosubduction zone. Unlabelled Image • The Lancang Group experienced HP blueschist to eclogite facies metamorphism • Triassic metamorphic event dated as 238–231 Ma • Protoliths correlated to Early Paleozoic sedimentary sequences of the Lanping–Simao Block • Paleo-Tehtyan subduction erosion, sediment subduction and rapid exhumation [ABSTRACT FROM AUTHOR]

The Beloretsk Metamorphic Complex in the SW Urals formed at a convergent eastern margin of Baltica during the Neoproterozoic-Early Cambrian Timanide orogeny. It comprises three major units with lenses of facies-critical metabasites within metasedimentary rocks: A lowermost eclogite unit, an intermediate garnet amphibolite unit and an upper amphibolite-greenschist unit. Pressure (P)-temperature (T)-paths of four rocks from the two lowermost units were determined mainly by PT pseudosection techniques showing similar clockwise loops at different peak metamorphic, water-saturated conditions: A phengite-bearing eclogite shows peak PT conditions of 16.5–18.5 kbar/525–550 °C (stage I) followed by stage II at 11.5–13.0 kbar/585–615 °C. A garnet amphibolite from the intermediate unit yields lower peak conditions of 11.7–14.5 kbar/480–510 °C (stage I) followed by stage II at 9.5–11.0 kbar/535–560 °C. However, a granite gneiss in the eclogite unit shows similar maximum pressures as the eclogite, but higher temperatures at 15.6–16.2 kbar/660–675 °C, whereas a garnet micaschist contains comparable high pressure relicts, but underwent an advanced midcrustal reequilibration at 7.5–9.0 kbar/555–610 °C. We dated the eclogite by a 7-point Rb/Sr mineral isochron (phengite, omphacite, apatite) at 532.2±9.1 Ma interpreted as age of crystallisation of the eclogitic peak PT assemblage. This age is the youngest compared to the known Timanide metamorphic and magmatic ages. [ABSTRACT FROM AUTHOR]

The long-term Pressure-Temperature-time-deformation (P-T-t-d) evolution of the internal zones of orogens results from complex interactions between the subducting lithosphere, the overriding plate and the intervening asthenosphere. 2-D numerical models successfully reproduce natural P-T-t-d paths, but most orogens are non-cylindrical and the situation is far more complex due to 3-D pre-orogenic inheritance and 3-D subduction dynamics. The Mediterranean orogens are intrinsically non-cylindrical. Their 3-D geometry results from the complex shape of the Eurasian and African margins before convergence and from the dynamics of slab retreat and tearing leading to strongly arcuate belts. More than many other segments, the Betic-Rif belt is archetypal of this behavior. A synthesis of the tectonometamorphic evolution of the Internal Zones, also based on recent findings by our group in the framework of the Orogen Project (Alboran domain, including the Alpujárride-Sebtide and Nevado-Filábride complexes) shows the relations in space and time between tectonic and P-T evolutions. The reinterpretation of the contact between peridotite massifs and Mesozoic sediments as an extensional detachment leads to a discussion of the geodynamic setting and timing of mantle exhumation. Based on new 40Ar/39Ar ages in the Alpujárride-Sebtide complex and a discussion of published ages in the Nevado-Filábride complex, we conclude that the age of the HP-LT metamorphism is Eocene in all complexes. A first-order observation is the contrast between the well-preserved Eocene HP-LT blueschists-facies rocks of the eastern Alpujárride-Sebtide Complex and the younger HT-LP conditions reaching partial melting recorded in the Western Alpujárride. We propose a model where the large longitudinal variations in the P-T evolution are mainly due to (i) differences in the timing of subduction and exhumation, (ii) the nature of the subducting lithosphere and (iii) a major change in subduction dynamics at ∼20 Ma associated with a slab-tearing event. The clustering of radiometric ages obtained with different methods around 20 Ma results from a regional exhumation episode coeval with slab tearing, westward migration of the trench, back-arc extension and thrusting of the whole orogen onto the African and Iberian margins.

The Sierra Pie de Palo (W-Argentina) is part of an Ordovician collisional wedge between the Cuyania microcontinent and the Ordovician Famatinian arc along the west Gondwana protomargin. The flatly E -dipping collisional wedge formed by polyphase shearing. The lower plate consists of Neoproterozoic-Cambrian metasedimentary rocks and Mesoproterozoic mafic-ultramafic basement of eastern Cuyania. The Famatinian forearc represents the upper plate composed of Mesoproterozoic metasedimentary rocks intruded by Mesoproterozoic and Ordovician plutons and overlying Neoproterozoic metasedimentary rocks. Pseudosection modeling on 20 samples distributed over the entire wedge yields a range of recorded maximum PT conditions of 7–14 kbar/480–590 °C along metamorphic gradients of 10–20 °C/km. Clockwise PT-paths are common starting from a midcrustal level with gradients of 20–30 °C to maximum depth followed by thermal relaxation. Local anticlockwise PT-paths are observed (1) in high-grade basement klippen of the overriding Famatinian arc (8–10 kbar/700–765 °C) caused by late emplacement onto the nappe stack and (2) within late upper crustal ductile zones with normal shear sense. Isotopic ages of (re)crystallisation of metamorphic minerals and assemblages comprise: 429 ± 2 to 434 ± 7 Ma (Lu-Hf; garnet), 460 ± 6 Ma (U Pb monazite in garnet), 404 ± 7 to 422 ± 8 Ma (Rb-Sr; white mica) and 403 ± 4 to 440 ± 8 Ma (40Ar/39Ar white mica). Including previous data, ages cluster around major peaks at 464 ± 1 Ma, 436 ± 3 Ma, 427 ± 3 Ma, 415 ± 2 Ma and 405 ± 1 Ma related to deformation during three orogenic stages: (1) subduction of the lower plate (eastern Cuyania) to the basis of the wedge at 470–455 Ma and later burial of the upper plate (Famatinian forearc) to similar depths at 440–415 Ma, a characteristic of a "hard" collision; (2) exhumation mainly by erosion and (3) late dissection of the wedge by upper crustal thrusts and some normal faulting at the top of the wedge during arrival of the colliding microcontinent Chilenia (405–360 Ma) at the new Devonian leading margin. • Twenty PTt paths are provided across a coherent continent-arc collisional wedge. • High pressure conditions of 9–13 kbar are observed in lower and upper plates. • The lower plate is earlier subducted than the upper plate ("hard" collision). • Relics of the overriding arc are detected as klippen with high-grade rocks. • The wedge is finally dissected by concomitant reverse and normal fault zones. [ABSTRACT FROM AUTHOR]

The Andrelândia Nappe System consists of three main nappes, from bottom to top: the lower Andrelândia nappe, themiddle Liberdade nappe and the upper Três Pontas-Varginha nappe and associated klippen, Pouso Alto, Aiuruoca, Carvalhosand Serra da Natureza. In the Andrelândia Nappe System, metamorphism increases from north to south and east to west, withthe highest temperatures and pressures recorded in rocks of the Três Pontas-Varginha nappe and associated klippen. Samplesof pelitic and mafic rocks were selected from the three nappes to determine the conditions of metamorphism using the programThermocalc. In this study, peak metamorphic conditions were calculated for the following samples: one sample of theAndrelândia Nappe (688 ± 35 °C and 5.63 ± 0.9 kbar), one sample of the Liberdade Nappe (648 ± 23°C and 7.41 ± 1 kbar), andthree samples of the Carvalhos Klippe (845 ± 53ºC and 15.7 ± 5.2 kbar, and 847 ± 45oC and 13.6 ± 5.8 kbar for two samplesof pelitic granulites respectively, and 854 ± 71ºC and 15 ± 1.4 kbar for one sample of mafic granulite). P-T paths inferred onthe basis of the observed textures are clockwise and typical of collisional belts.

The Shangdan suture zone (SSZ) is the main collisional boundary between the North China Craton and the South China Craton, along which discontinuous Paleozoic ophiolites and subduction–accretion related volcanic arc assemblages occur. Here we report the petrology, geochemistry, geochronology and phase equilibria modeling of garnet amphibolite from the Songshugou ophiolite which is one of the largest ophiolite outcrops in the northern side of the SSZ. From petrological studies, we identify: (1) prograde stage, defined by garnet + clinopyroxene + calcic amphibole + ilmenite + rutile + epidote + plagioclase + quartz; (2) peak stage with garnet + clinopyroxene + ilmenite + rutile + quartz; and (3) retrograde stage with amphibole + plagioclase + titanite + ilmenite. Our pseudosection analysis defines stability of the peak assemblage at 750–850 °C, 15–19 kbar and traces a clockwise P–T path in the system Na 2 O–CaO–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –Fe 2 O 3 (NCFMASHTO), suggesting high pressure (HP) metamorphism. Subsequently, the rocks experienced rapid decompression and cooling. LA-ICP-MS U-Pb analyses of zircons from the garnet amphibolite yield a weighted mean 206 Pb/ 238 U age of 515 ± 12 Ma. This Early Paleozoic metamorphic age represents the emplacement time of the Songshugou ophiolite, and suggests that the HP metamorphism is possibly related to the northward deep subduction of the Shangdan oceanic crust in Early Paleozoic. [ABSTRACT FROM AUTHOR]

Granulite facies rocks that witnessed high pressure and high temperature metamorphism preserve important records of the growth and recrystallizaton of zircons during the multiple stages of tectonic evolution of orogenic belts developed in complex subduction–collision settings. Here we investigate zircons in the Paleoproterozoic high pressure (HP) granulites from Jiaobei Terrane in the eastern North China Craton that experienced a multi-stage metamorphic evolution. We present SIMS zircon U–Pb ages for zircons from garnet amphibolites, mafic granulites and pelitic granulites. Our data show four age peaks at 2.5 Ga, 1.92 Ga, 1.87–1.85 Ga and 1.83 Ga. Zircons belonging to the latter three age groups show similar characteristics of low Th and U contents, low Th/U ratios (< 0.1), sector zoning or unzoned internal structures, corresponding to the features of zircon growth during metamorphism. Using zircon Lu–Hf isotopic compositions combined with a careful analysis of the morphology of the zircons, we evaluate the processes of solid-state recrystallization, intergranular melt growth, coupled dissolution–reprecipitation and subsolidus growth. Zircons formed through solid-state recrystallization during peak metamorphic stage inherited the relatively high 176 Lu/ 177 Hf ratios and low 176 Hf/ 177 Hf ratios of zircons from the protoliths. The youngest U–Pb age of ca. 1899 Ma is interpreted as the maximum estimate of the time of peak metamorphism. Zircons formed through intergranular melt growth during low- to medium-pressure retrogression display homogeneous Lu–Hf isotope compositions, low 176 Lu/ 177 Hf and high 176 Hf/ 177 Hf ratios. The U–Pb age of 1870–1850 Ma yielded by zircons crystallized from intergranular melt is interpreted to represent the time of low- to medium-pressure granulite facies retrogression. Some zircon grains with prismatic morphology and low 176 Lu/ 177 Hf and 176 Hf/ 177 Hf ratios display heterogeneous Lu–Hf isotopic compositions, possibly related to coupled dissolution–reprecipitation. The U–Pb ages of these zircons are identical to those formed from intergranular melt and these two groups of zircons formed during the same metamorphic stage. Zircons formed through subsolidus growth show large variations in Lu–Hf isotopic compositions and their ca.1833 Ma age represents the timing of amphibolite facies retrogression. The HP granulite facies metamorphism in Jiaobei is thus constrained between 1870 and 1899 Ma, with low- to medium-pressure granulite facies retrogression at ca. 1850–1870 Ma followed by amphibolite facies retrogression at ca. 1833 Ma. Our data show that several zircon cores and recrystallized grains were entrained from basement rocks during the emplacement of the mafic magmatic protoliths in the case of mafic granulites. For the pelitic granulites, the older zircons are detrital grains of magmatic origin. The Hf crustal model ages of these zircons that show peaks at ca. 2.7 Ga and 2.9 Ga are consistent with the timing of mafic magmatism in the region as identified in previous studies. The Lu–Hf isotopes of newly grown metamorphic zircons which have equilibrated chemical compositions trace the nature of the protolith. The zircon Hf crustal model age of ca. 2.5–2.7 Ga from the pelitic granulites is consistent with that of the basement rocks in the Jiaobei Terrane and the zircon Hf model ages (T DM ) of ca.2.0 Ga and 2.2–2.3 Ga from the mafic granulites might represent the time of magma generation from the mantle with crustal contamination. The geochronology of the basement rocks as revealed by the old zircon cores and the metamorphic evolution of the Jiaobei Terrane are comparable with the tectonic history of the central and western North China Craton and provide insights into the complex tectonothermal events associated with the Neoarchean–Paleoproterozoic crustal evolution history of the craton. Also, the pervasive ~ 2.5 Ga zircons in a variety of rocks seem to suggest that the Jiaobei Terrane was already part of a coherent tectonic unit at such time. [ABSTRACT FROM AUTHOR]

Abstract: The Zermatt-Saas Fee Zone (ZSZ) was subducted to eclogite-facies conditions, reaching peak pressures and temperatures of 20–28 kbar and 500–630 °C. The rocks were partially overprinted under greenschist-facies conditions during exhumation. Previous Rb–Sr isochron ages obtained on matrix phengites in metasediments of the ZSZ have been interpreted to date early exhumation of the ZSZ. Here we present new Rb–Sr geochronology on phengite inclusions in garnets to date prograde growth of garnets. We show that garnet acted as a shield for the included phengites, limiting Rb and Sr isotope exchange with the bulk rock, upon complete enclosure of the mica, during garnet growth, even if peak metamorphism exceeded the Rb–Sr blocking temperature. Similarly, garnet isolated the micas from the matrix during subsequent recrystallization due to fluid infiltration or deformation during exhumation. Phengite inclusion ages for two metapelitic samples from the same locality (Triftji) are and , and are about 4 m.y. older than the corresponding matrix mica ages of and , respectively. The results confirm previous Sm–Nd and Lu–Hf geochronology on the ZSZ that indicated protracted garnet growth during prograde metamorphism, and confirm that at least parts of the ZSZ underwent peak metamorphic HP conditions less than 43 m.y. ago, followed by rapid exhumation to upper greenschist-facies conditions around 40 Ma ago. [Copyright &y& Elsevier]

Abstract: Protolithic ages of meta-basites are crucial for tectonic reconstructions as they constrain the lifetime of oceans. However, in high-grade metamorphic regions most isotope systems record metamorphic ages due to the metamorphic overprint. The mineral zircon is the most suitable geochronometer for the determination of protolithic ages in such rocks due to its high closure temperature. Zircon from eclogites from the Eastern Erzgebirge metamorphic complex yielded both protolithic (~540Ma; 5 samples) and metamorphic (~340Ma; 4 samples) 207Pb/206Pb ages. Detailed investigations of zircon grains within thin rock sections show that the metamorphic ages are related to new zircon growth caused by water infiltration that triggered decomposition of omphacite into an amphibole–plagioclase symplectite. Rutile and apatite frequently grew together with metamorphic zircons indicating local mobilization of Zr, Nb, LREE and P. Eclogites with omphacite breakdown are very dark in colour. In difference, lighter coloured eclogites contain K-feldspar and quartz indicating minor melt infiltration that caused breakdown of garnet and transformation of amphibole into biotite. This supports additional new growth of zircon grains. In some samples idiomorphic zircon grains with a lot of micro-inclusions indicate a coupled dissolution–reprecipitation process. Additional rounded-resorbed and highly fractured zircon grains together with internal zircon textures imply multiple melt infiltration triggering multiple phases of zircon growth. Therefore, scattering zircon ages may reflect multiple growth stages or inheritance rather than prolonged periods of metamorphic growth. In contrast, protolithic zircon ages were found in eclogites where no water or melt infiltration occurred. This emphasizes the importance of water and/or melt infiltration for the new growth of zircon grains in eclogites as was already shown for granulites in various studies. In the Eastern Erzgebirge, most eclogites and amphibolites were derived from MORB-type or arc-related basalts while the plagioclase-rich amphibolites formed mainly from crustal sources. The protolith ages of meta-basites from the Eastern Erzgebirge (~540Ma) document a bimodal magmatic activity in a Cadomian back-arc basin. [Copyright &y& Elsevier]

Protoliths of highly metamorphosed gneisses from the Erzgebirge are deduced from the morphology, age and chemistry of zircons as well as from whole rock geochemistry and are compared with lower-grade rocks of Lusatia. Gneisses with similar structural appearance and/or geochemical pattern may have quite different protoliths. The oldest rocks in the Erzgebirge are paragneisses representing meta-greywackes and meta-conglomerates. The youngest group of zircon of meta-greywackes that did not undergo Pb loss represents the youngest igneous component for source rocks (about 575 Ma). Similar ages and zircon morphology reflect the subordinate formation of new zircon grains or only zircon rims in the augengneiss from Bärenstein and Wolkenstein, which probably represent metamorphic equivalents to Lower Cambrian two-mica granodiorites from Lusatia. Bulk rock chemistry, intense fracturing and high U and Th concentrations of zircons suggest deformation-induced and fluid-enhanced recrystallisation of zircon grains. Temperatures during tectonic overprinting-too low to reset zircon ages-indicate mid- or upper crustal levels for shearing recorded in these augengneisses. Lower Cambrian (~540 Ma) granodiorites are widespread in Lusatia but are exclusively represented by the Freiberg gneiss dome in the Eastern Erzgebirge. Ordovician protolith ages were recorded by zircons from the augengneisses of the Reitzenhain-Catherine dome and the Schwarzenberg dome (Western Erzgebirge) documenting significant regional differences between the eastern and the western Erzgebirge (~540 vs. ~490 Ma). In the Western Erzgebirge, most meta-volcanic rocks (muscovite gneisses) and meta-granites (mainly red augengneisses) yield Ordovician zircon ages, whereas in the Eastern part, similar rocks mainly recorded Lower Cambrian protolith ages. Zircon overprinting was highest within discrete tectonic zones where the combination of fluid infiltration and deformation induced variable degrees of recrystallisation and formation of a new augengneiss structure. Variable degrees of Pb loss caused age shifts that do not correspond to changes in zircon morphology but may be associated with U and Th enrichments. Major changes in bulk rock composition appear to be restricted to discrete zones and to (U)HP nappes, whereas gneisses with a MP-MT metamorphic overprint basically show no geochemical modifications. [ABSTRACT FROM AUTHOR]

Abstract: Subducting slabs experience deformation and metamorphism as they descend into the upper mantle. The presence of hydrous minerals gained through the interaction with sea water at mid-ocean ridges, transform faults or the outer rise ensures that dehydration reactions will be important at deeper levels. We describe field evidence for brittle hydrofracture in previously subducted rocks from the Western Alps, with a free aqueous fluid phase produced by dehydration reactions in the host blueschists and serpentinites. The protracted history of dehydration reactions, ductile deformation, fluid flow and brittle vein formation in these rocks implies that fluid-filled cracks are continuously produced within the dehydration window. The presence of abundant fluid-filled cracks at these depths has important implications for the seismic anisotropy generated within slabs, which has largely been overlooked. The effects of fluid-filled crack damage on the elastic properties of a blueschist and serpentinite within the slab at depth have been modelled, and show a significant rotation of the fast axes of P and S1 waves to be trench-parallel for receiving stations in the forearc, above the dehydrating portion of the slab. This model provides an alternative explanation for supra-subduction zone seismic anisotropy that does not require high-stress, high-water conditions, or trench-parallel flow in the supra-subduction zone mantle wedge. [Copyright &y& Elsevier]

New occurrences of high-pressure metabasites in the Kutná Hora Unit (KHC) are described. These HP rocks include eclogites (± Ky), Cpx-Grt amphibolites, metagabbros and Cpx-bearing granulites. All the eclogite samples studied are formed of well-preserved HP mineral assemblages with variable degree of subsequent metamorphic overprint. Eclogites from the inner part of the KHC (Roztěž, Bořetice) mostly had a Mg-rich tholeiitic protolith. Chemical, mineralogical and metamorphic data reveal that these eclogites are petrologically similar to the eclogite samples from the better known Běstvina Unit. Eclogites from Bořetice, Poličany and Roztě ž preserve relict mineral assemblage from a pre-eclogite stage, enclosed in garnet with prograde zoning pattern. They bear mineralogical evidence of a very high-pressure history, even substantially higher (maximum 8.4 wt. % of Na2O in Cpx, P~ 4.3 GPa) than the previously described eclogites from Spačice and Úhrov in the Běstvina Unit. A set of three eclogites from the northern margin of the Moldanubian Zone south of Chotěboř was used for comparison (Bída, Borovský Creek, Krátká Ves). They preserve also a peak pressure record that corresponds to minimum pressures above 2.0 GPa. Better understanding of regional relationships amongst the discussed units can, be achieved only using isotopic and geochronological data. [ABSTRACT FROM AUTHOR]

Abstract: Eclogites and blueschists exposed in Saih Hatat, Oman, record the subduction and exhumation of continental crustal material beneath the Cretaceous Semail Ophiolite during ophiolite obduction. The eclogite-bearing lower plate, originally part of Oman''s distal continental margin, is exposed in two tectonic windows through the less metamorphosed upper plate by a previously mapped low angle, high strain, décollement structure. A major tectonic break, currently poorly exposed, records the juxtaposition of the highest pressure eclogites and garnet blueschists against lower pressure epidote-blueschists. The subsequent exhumation of the entire lower plate to mid crustal levels is marked by a pervasive shearing event associated with a regional greenschist facies overprint. The décollement truncates structures and the metamorphic field gradient in the lower plate, but does not significantly truncate structures or stratigraphy in the upper plate. It is not responsible for the exhumation of the high pressure rocks to mid-crustal levels. Most of the displacement across this structure was accommodated during continuing convergence after the subduction system had ceased to be active, and post ophiolite emplacement onto the platform carbonate sequences. A revised tectonic model is presented which accounts for the structural, geochronological and metamorphic observations. [Copyright &y& Elsevier]

Abstract The upper deck of the East Athabasca mylonite triangle (EAmt), northern Saskatchewan, Canada, contains mafic granulites that have undergone high P–T metamorphism at conditions ranging from 1.3 to 1.9 GPa, 890–960 °C. Coronitic textures in these mafic granulites indicate a near-isothermal decompression path to 0.9 GPa, 800 °C. The Godfrey granite occurs to the north adjacent to the upper deck high P–T domain. Well-preserved corona textures in the Godfrey granite constrain igneous crystallization and early metamorphism in the intermediate-pressure granulite field (Opx + Pl) at 1.0 GPa, 775 °C followed by metamorphism in the high pressure granulite field (Grt + Cpx + Pl) at 1.2 GPa, 860 °C. U–Pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c. 2.5 Ga and c. 1.9 Ga. The oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith. A population of 1.9 Ga zircon in one mafic granulite is interpreted to constrain the timing of high P–T metamorphism. Titanite from the mafic granulites yields dates ranging from 1900 to 1894 Ma, and is interpreted to have grown along the decompression path, but still above its closure temperature, indicating cooling following the high P–T metamorphism from c. 960–650 °C in 4–10 Myr. Zircon dates from the Godfrey granite indicate a minimum crystallization age of 2.61 Ga, without any evidence for 1.9 Ga overgrowths. The data indicate that an early granulite facies event occurred at c. 2.55–2.52 Ga in the lower crust (c. 1.0 GPa), but at 1.9 Ga the upper deck underwent high P–T metamorphism, then decompressed to 0.9–1.0 GPa. Juxtaposition of the upper deck and Godfrey granite would have occurred after or been related to this decompression. In this model, the high P–T rocks are exhumed quickly following the high pressure... [ABSTRACT FROM AUTHOR]

• The Egéré/Aleksod Terrane (Tuareg Shield, Algeria) contains high-pressure rocks. • A zircon megacryst occurs in a high-pressure leucosome within migmatitic metapelites. • Leucosome formed by melting of surrounding metapelites (εHf i = −21; high δ18O). • Melting occurred during decompressional heating (c. 800 °C) at 654 ± 5 Ma. • Fluid-driven recrystallisation at 584 Ma (700 °C) pervasively modified δ18O (12 to 7‰). The Tighsi area of the Egéré/Aleksod Terrane (Tuareg Shield) contains mafic eclogites interlayered within anatectic metapelites corresponding to metabasalts coeval with a shallow water marine sedimentation. In this study we present in-situ geochronological (U-Pb) and geochemical (major and trace elements, Hf and O isotopes) analyses from a 2.6 mm zircon megacryst found in a high-pressure kyanite pegmatite enclosed in the metapelites. Oscillatory zoning, HREE-enriched patterns, positive Ce anomalies and the absence of negative Eu anomalies are consistent with crystallization of the megacryst from the anatectic melt. Ti-in-zircon temperatures indicate crystallization at 811 ± 15 °C in an allanite/monazite buffered anatectic melt as evidenced by the low Th/U ratios. Oxygen isotopes yield large intra-grain variations (7.1–12.3‰) with a gradual lowering towards the edge of the grain in contact with secondary feldspathic veinlets. These results indicate fluid-assisted oxygen isotope disturbances, consistent with the low retentivity of O in zircon under wet conditions. Hf isotopes do not display intra-grain variations (mean εHf i = −20.7 ± 1.0) and support production of the leucosome by melting of crustal material. U-Pb analyses of the center of the crystal provide an age of 654 ± 5 Ma (2σ), attributed to post-peak decompression and heating. HP eclogite facies conditions in the Egéré terrane are thus significantly older than HP metamorphism in the western part of the shield (610–625 Ma) in agreement with multiple subduction events along the margins of the Tuareg Shield. The rim of the megacryst, close to feldspathic veinlets, is characterized by a significant decrease of trace elements (U, Y, HFSE), but preserved identical Hf isotope ratios, which is consistent with recrystallization processes. The rim displays a Ti-in-zircon temperature of 717 ± 28 °C and a U-Pb age of 584 ± 6 Ma (2σ) coeval with the climax of batholith intrusion in Central Hoggar. Reheating and softening of the lower/middle crust at that time may have assisted and favored upward viscous flow of basement domes and escape tectonics along lithospheric shear zones. We propose that the final push of the Saharan metacraton in the east was responsible for the observed architecture of the Egéré terrane, where anatectic elongated domes of basement gneisses alternate with HP metasedimentary synforms. [ABSTRACT FROM AUTHOR]