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The paper reports comprehensive petrological, geochemical and mineralogical studies of osumilite-bearing andesite-dacitic lavas of Kordieritoviy Volcano (Keli Highland, Greater Caucasus) erupted at the end of the Pleistocene (about 200 ka). The petrographic and microprobe analysis showed that the rocks contain three paragenetic mineral associations: (1) "xenogenic" (metamorphic) association consisting of garnet (XPrp = 0.42, XAlm = 0.51–0.53, XGrs = 0.04–0.05) + hercynite + sapphire + bronzite + pargasite + ilmenite; (2) early magmatic association represented by hypersthene + hercynite + garnet (XPrp = 0.21–0.31, XAlm = 0.52–0.71, XGrs = 0.04–0.13) + ferro-kaersutite + ilmenite; (3) late magmatic association including hypersthene-ferrohypersthene + labradorite + garnet (XPrp = 0.04–0.14, XAlm = 0.65–0.81, XGrs = 0.06–0.18) + osumilite-(Mg) + phlogopite + tridymite + ilmenite + apatite. The osumilite-(Mg) (phenocrysts, xenomorphic aggregates in the rock matrix, and crystals in miarolic cavities), the average formula for dacites of Kordieritoviy Volcano can be written as (K0.73Na0.06Ca0.02 )1.00(Mg1.06 Mn0.04)2.00(Al2.75 Ti0.01)3.00(Si10.34Al1.66)12O30, formed mainly at late magmatic stages – in intermediate chambers immediately prior to the rise of the melt to the surface or after its eruption. Accordingly, this mineral in the studied lavas has a purely magmatic origin. Thermobarometric calculations and petrological modeling showed that the deep magma chamber of Kordieritoviy Volcano was located at a depth of 45–53 km near the Moho discontinuity. The temperature of the melt at the early magmatic stage was no less than 1100°C at 17–23 kbar. Crystallization of osumilite-(Mg) in intermediate magmatic chambers (at depths of 30–40 km) and during the lava ejection occurred at 1030–870°C and pressure progressively decreasing from 14–9 to 1 kbar. A petrogenetic model has been proposed to explain the genesis of exotic osumilite-bearing lavas of Kordieritoviy Volcano. It includes several stages: (1) formation of an enriched upper-mantle source (lithospheric mantle metasomatized by permanent interaction at the Moho discontinuity with the overlying lower crust composed of metamorphosed terrigenous-volcanogenic complexes); (2) generation of "dry" basaltic magmas in the source; (3) crystallization differentiation in the source (fractionation of olivine and chromium spinels) with the formation of a "dry" overheated andesitic melt; (4) limited-scale assimilation of the lower crustal material made up of leucocratic granulites by highly differentiated andesitic melts rising to the surface, with simultaneous fractionation of garnet, orthopyroxene and ilmenite from the melt. [ABSTRACT FROM AUTHOR]

Granulites exposed in high-grade regional metamorphic belts and exhumed as xenoliths in basaltic pipes are considered as window into the deep crust thus play a key role in constraining models of crustal processes and evolution. Here we present a detailed investigation of the tectono-metamorphic history of the two-pyroxene mafic granulite located in the southern region of the Sonapahar area. This involves conducting monazite chemical dating, analyzing petrological and geochemical characteristics, applying geothermobarometry, performing phase equilibria modeling, and tracing a pressure-temperature (P-T) path. Metamorphic P-T conditions estimated for the mafic granulite using conventional thermobarometer and winTWQ shows temperature in excess of 800°C and pressure of about 8.6 kbar, stand for high temperature granulite facies metamorphism. The metamorphic evolution path obtained from P-T pseudosection suggest a clockwise P-T evolution path, thus signify isothermal decompression and indicate rapid upliftment. Geochemical study of trace and rare earth elements (REE), suggest protolith is of tholeiite basalt in nature that is derived from back arc basin setting near to subduction zone. Additionally, the analyzed rock was examined using primitive mantle-normalized trace element spider diagram. The results indicate an enrichment in large-ion lithophile elements (Th, U, K, Pb) and a depletion in high field-strength elements (Nb, Ta, Ti). The presence of negative anomalies in Nb and Ti, coupled with elevated values of Th, K, and Pb, suggests the possibility of crustal contamination. Monazite chemical data from the studied rock reveals a peak metamorphism age of 521.3 ± 4.20 Ma, which corresponds to the Kuunga Orogeny in the later phase of global Pan-African collision. [ABSTRACT FROM AUTHOR]

We present a reconstruction of the physicochemical conditions of melts in the Pleistocene storage and plumbing system of the Doña Juana Volcanic Complex (SW Colombia): a poorly known, potentially active polygenetic volcano of dacitic composition comprising four major edifices and showing periods of long quiescence. Compositional data for plagioclase, amphibole, pyroxene, and Fe-Ti oxides were combined with new and existing whole-rock data from representative eruptive products, allowing for the implementation of equilibrium tests and geothermobarometry calculations within an established stratigraphic, petrographic, and geochronological framework. Textural and geochemical variabilities of all mineral phases suggest the existence of a trans-crustal magmatic system feeding the Pleistocene eruptions of Doña Juana, and cyclic rejuvenation of a crystal mush following each volcano edifice collapse. The assemblage of different crystal cargos before magma recharge and final eruption is attested by (i) the coexistence of equilibrium and disequilibrium textures and variable compositions in crystals of all studied species, (ii) felsic cores in antecrysts, (iii) mafic overgrowth rims, and (iv) significantly less differentiated microcrysts relative to the composition of meso- and macrocrysts. By integrating multiple mineral-only and mineral-liquid geothermobarometers, after careful textural analyses, we estimate the intensive parameters of the mush–melt interaction zone of the plumbing system in the middle crust, providing a preliminary view of the architecture of a trans-crustal magmatic system in a complex tectonic setting at a previously understudied area of the north-Andean volcanic zone. [ABSTRACT FROM AUTHOR]

Based on the proposed numerical model of the stress-strain state of polymineral rocks, which describes the formation of blastomylonites in the Yenisei Regional Shear Zone (YRSZ) in the Yenisei Ridge, the possibility of local tectonic overpressure exceeding the lithostatic pressure in rocks subjected to shear deformations is shown. For tectonites of the southern (Angara–Kan block) and northern (Isakovka terrane and Garevka complex) segments of the YRSZ, estimates of the maximum overpressure were obtained from 2–3 to 4–5 kbar, which range from 25 to 50% of the lithostatic pressure. It is shown that the excess pressures can be preserved in a local volume on a geological time scale sufficient for their fixation in metamorphic minerals. Model values of overlithostatic pressure in the garnet–amphibole tectonites and geobarometric estimates of peak values during stress metamorphism allow us to offer new evidence for pressure inhomogeneity in natural mineral associations. Using the results of numerical modeling for the evolution of metabasite blastomylonites, it was established that the overpressure at the stage of syn-deformation metamorphism in the shear zone is possible at temperatures up to 600–650°C and not reaching 800°C; the presence of fluid or partial melt prevents the occurrence of overpressure. Magnitudes of excess pressure caused by shear stresses depend on the mineral composition and structure of the rock. [ABSTRACT FROM AUTHOR]

A detailed mineral-scale study was conducted on pumices of the latest, dominantly explosive eruption epoch (56–30 ka) of Ciomadul, the youngest, long-dormant volcano in eastern-central Europe for characterizing the magma storage system and for understanding better the changes in eruption style from effusive to explosive. The mineral cargo of dacitic pumices enables us to constrain the conditions of the pre-recharge crystal mush, the recharge magmas and the post-recharge magma prior to eruptions. A careful evaluation of the results yielded by various thermometers, barometers, oxybarometers, chemometers and hygrometers as well as direct comparison with experimental data were necessary to select the appropriate techniques and therefore to constrain the conditions for the Ciomadul magmatic system. Beneath the volcano, a felsic crystal mush body is inferred at 8–12 km depth comprising slightly oxidized (0.5–1.6 ∆NNO), low-temperature (680–750 °C), highly crystalline magma. This zone is underlain by a deep magma storage zone with less evolved, hot (> 900 °C) magma at 16–40 km depth. The dominantly explosive volcanism after the effusive eruptions (160–90 ka) can be explained by the ascent of distinct recharge magmas. They contained high-Mg (MgO > 18 wt%) amphibole, which could have crystallized from ultrahydrous (H2O > 8 wt%) magma at near-liquidus conditions. The rates of amphibole overgrowth and microphenocryst formation require weeks to months for the magma mixing and the eruption events. The hybridized melt became more oxidized and contained dissolved water in around 5.5 wt% at temperature of 790–830 °C calculated from the re-equilibrated Fe-Ti oxides. These magma properties along with the degree of crystallinity (27–38 vol% crystals) favored rapid magma ascent and an explosive style eruption. Thus, the strongly hydrous nature of the recharge magma in addition to the crystallinity and H2O content of the pre-eruption magma plays an important role in controlling the eruption style. [ABSTRACT FROM AUTHOR]

The southern Sanandaj-Sirjan Zone (SSZ) forms the core of the Zagros orogen and consists of a stack of four major tectonic units, all of which were thrust southwestward over the Neyriz ophiolites. Precambrian and early Paleozoic rocks were affected by high-grade metamorphism, whilst the late Paleozoic (Carboniferous and Permian) and Mesozoic rocks are of low grade to unmetamorphosed. Geothermobarometric results indicate that the investigated area experienced peak temperature–pressure conditions of ~ 650 °C and 8–9 kbar, before being partly overprinted by greenschist facies conditions. We identified two main deformational phases (D2, D3) and some relics of an older one (D1) in the southern SSZ. D1 is inferred based on local evidence of tight D2 folds that fold a pre-existing schistosity (S1), associated with prograde- and peak metamorphism. D2 shows large-scale, tight-to isoclinal F2 folds and a penetrative S2 foliation, forming the dominant foliation in our study area. The P–T path is clockwise and associated with a geothermal gradient of ~ 20 °C/km, suggesting a collision-related geodynamic setting, preceding D2, and possibly linked to an early stage of D1. D3 is characterized by open folds (F3) and thrusts developed after greenschist facies metamorphism. Based on the metamorphic grade and stratigraphic age of folded units, D2 is inferred to be Eo-Cimmerian, and D3 post-Cretaceous, coeval to the Zagros orogeny. Concerning the oldest phase (D1), for which only circumstantial structural evidence exists, we discuss whether its age is pre-Cimmerian, possibly Variscan, or Eo-Cimmerian as suggested in previous literature. This study uses metamorphic assemblages and structures of the southern SSZ, to retrace its geodynamic evolution, which started with burial associated with high-temperature metamorphism, followed by Eo-Cimmerian shortening, and finally terminated with additional shortening related to the Zagros collisional event. [ABSTRACT FROM AUTHOR]

A recent petrological study on the lesser-known Sholayar alkaline syenite complex (10°17′59.9′′N: 076°47′26.3′′E) accommodated in the high-grade rocks of the Southern Granulite Terrane reveals the presence of three distinct petrographic types namely leuco syenite, melanofelsic syenite and mela syenite. The syenites, in general, have the following constituent minerals, namely alkali feldspar (Or76.8Ab23.2–Or93.4Ab6.6), plagioclase (An4.1–An17.3), calcic amphibole (edenitic hornblende/ferropargasitic hornblende), quad pyroxene (diopside–hedenbergite), biotite and opaque minerals (magnetite and ilmenite). Mode-based statistical studies indicate that a highly significant correlation exists among certain mineralogical parameters including quartz%, total feldspar%, colour index, alteration index and hydration index. Isopleth plots of these parameters strongly suggest maintenance of a magmatic regime throughout, with increasing water content towards the end stage of crystallization. Several geothermobarometric methods point to shallow to moderate level emplacement (~9–23 km) of the alkaline/sub-alkaline syenite magma which was facilitated by a 'hydration event'. The steep change in water gradients (in localized pockets) of the syenite intrusives has been corroborated by textural evidences. Research highlights: First systematic petro-mineralogical report of Sholayar alkaline syenite complex from SGT. Involvement of single magmatic lineage with variable water content. Geothermobarometry suggests a moderate to shallow level of syenite emplacement. [ABSTRACT FROM AUTHOR]

The reliability of eight Fe–Mg exchange geothermobarometers for garnet-bearing peridotites, pyroxenites and eclogites has been examined using a database comprised of more than 300 published peridotite, pyroxenite and eclogite experiments conducted from 10 to 70 kbar and 850 to > 1650 ∘ C . We have tested Fe–Mg exchange geothermometers suitable for a range of mantle lithologies, including websterite, harzburgite, wehrlite and eclogite. All geothermometers maintained an average difference in experimental and calculated temperature (T) Δ T = T exp - T calc of less than ± 50 °C with a standard deviation of Δ T between ± 50 to 150 °C. Most geothermometers performed well across a narrow range in ln Kd Fe - Mg A - B (where Kd Fe - Mg A - B = Fe A × Mg B (Fe B × Mg A) ), however, systematic overestimation and underestimation of T were observed outside of the optimal range of lnKd Fe - Mg A - B . Increases in experimental pressure (P) adversely affected several geothermometers, particularly those calibrated empirically using natural samples. All previously published calibrations of the garnet-clinopyroxene geothermometer were unable to reliably reproduce the experimental T for both peridotite and eclogite experimental compositions, which hinders their confident application to natural datasets. To improve the state of mantle geothermobarometry we have used our experimental database to recalibrate the (1) garnet-clinopyroxene Fe–Mg exchange geothermometer, and (2) garnet-orthopyroxene Fe–Mg exchange geothermometer. Each geothermometer has been recalibrated across an extended P, T, and compositional range. The inclusion of eclogitic experiments in the calibration for the garnet-clinopyroxene geothermometer permits application to both eclogitic and peridotitic/pyroxenitic assemblages equilibrated under a wide range of PT conditions in the upper mantle. Using multiple linear regression to solve for lnKd, we found the following expressions best reproduced the experimental T (℃) of our dataset: T Fe - Mg grt - cpx (∘ C) = 3356.34 - 0.008 × P kbar + 0.259 × X Ca grt + 0.914 × X Mg grt + - 0.159 × Jd cpx + ln Kd Fe - Mg grt - cpx + 1.265 - 273 T Fe - Mg grt - opx (∘ C) = 1851.85 - 0.007 × P kbar + - 1.83 × X Ca grt + ln Kd Fe - Mg grt - cpx + 1.08 - 273 . where, X Ca grt = Ca Ca + Fe + Mg , Kd Fe - Mg grt - opx = Fe grt × Mg opx (Fe opx × Mg grt) , X Mg grt = Mg Ca + Fe + Mg , Jd cpx = Na - Cr - 2 × Ti , Kd Fe - Mg grt - cpx = Fe grt × Mg cpx (Fe cpx × Mg grt) , with all elements calculated on the basis of 12 oxygen anions in garnet and 6 oxygen anions in clino- and orthopyroxene. Fe2+ = total Fe. Our updated calibrations resolve several issues with earlier calibrations, including a poor performance at elevated P and compositional limitations. An improvement in precision and accuracy has been demonstrated through application to the experimental calibration dataset, a second independent set of published experimental data, and to natural peridotites, pyroxenites and eclogites from on and off craton settings. Iterative PT estimates on natural datasets calculated using our updated calibrations compare well with estimates from widely used calibrations such as the Taylor (1998) two-pyroxene solvus geothermometer. We anticipate that this contribution will provide an important reference for the reliability of mantle geothermometers and that our updated calibrations will be used in future studies on peridotite, pyroxenite and eclogite inclusions in diamond and mantle-derived xenoliths. [ABSTRACT FROM AUTHOR]

Characterising equilibrium and disequilibrium crystal-melt processes is critical in determining the extent of magma mixing and crystallization conditions in the roots of volcanoes. However, these processes remain poorly investigated in most Pacific intraplate ocean settings that are difficult to access and study. To help address this issue, we investigated crystallization conditions of clinopyroxene phenocrysts in an accreted Palaeogene oceanic island in Panama. Petrographic and geochemical observations, petrological modelling of major and trace elements, and liquid-mineral multicomponent equilibrium tests were carried out using basalts, picrites, and hawaiites of the transitional tholeiitic shield to alkaline post-shield volcanic stages of the island. Five types of clinopyroxene crystals were identified, including (1) microphenocrysts with micron-scale oscillatory zoning, (2) primitive, yet resorbed picrite-hosted phenocrysts, (3) chemically homogeneous, anhedral crystals found in the remaining basalts, (4) Ti–rich euhedral hawaiite-hosted phenocrysts, and (5) evolved sector-zoned phenocrysts. Liquid-clinopyroxene multicomponent equilibrium tests in combination with textural analysis show that ~ 74% of the studied clinopyroxenes are in possible major element equilibrium with one of the available whole rock magma compositions, of which only 21% are equilibrated with their carrier liquid. To deconvolute clinopyroxene-melt pairings and determine plumbing system conditions, we combine rhyolite-MELTS modelling, geothermobarometry, and major- and trace-element equilibrium evaluations, limiting crystallization conditions to crustal levels (< 23 km depth). No migration of magmatic reservoirs to deeper levels is observed during the shield- to post-shield transition. These results suggest the occurrence of an extensive crystal mush system during the late shield to post-shield volcanic stages of this intraplate volcanic system, with both primitive and evolved crystallization domains sampled during eruptions. [ABSTRACT FROM AUTHOR]

Although there is a consensus about Pangea assemblage in northwestern Gondwana spanning from the Late Carboniferous to the Early Permian, the tectonics of the Permo–Triassic period, including the onset of Pangea breakup, is still controversial. In this context, three regional tectonic features need to be considered: (1) The Ouachita-Marathon-Sonora suture, (2) east-dipping subduction of the Panthalassa oceanic crust beneath Gondwana, and (3) an extensional setting documented in Colombia, Ecuador, and México suggesting Pangea breakup at ca. 240–220 Ma. A chemical, isotopic, and geochronological dataset is presented in this contribution to constrain the effects of these tectonic processes in the metamorphic basement of the Mérida Andes in western Venezuela. U–Pb Secondary Ion Mass Spectrometry analyses on unpolished surfaces of zircons from orthogneisses yielded an average age of 251 ± 4 Ma. The corresponding δ18O values suggest metamorphic recrystallization of zircon instead of high-T fluid interaction. Rb–Sr and Sm–Nd geochronology in white mica and garnet yielded ages of 234 ± 3 Ma and 249 ± 2 Ma, respectively, whereas a younger Rb–Sr date of 197 ± 1 Ma was obtained from biotite. U–Pb and Sm–Nd ages constrain the latest Permian–earliest Triassic metamorphism in the Mérida Andes at 251 ± 4 Ma. Geothermobarometry data suggest amphibolite-facies peak metamorphic conditions at ~ 685 °C and ~ 6.0 kbar. Metamorphism might be related to post-orogenic collapse, after the collision of Gondwana and Laurentia to form Pangea. Rb–Sr ages suggest retrogression and cooling, possibly caused by thermal relaxation of a tectonically overthickened crust and onset of extensional setting, followed by Pangea breakup during the Late Triassic–Early Jurassic. [ABSTRACT FROM AUTHOR]

In our study we characterize the thermodynamic regimes in the thrust zone of the newly formed Paleoproterozoic continental crust of the Svecofennian mobile belt on the outskirts of the Archean Karelian continent along the sublatitudinal, in modern coordinates, powerful strike-slip zone of the Northern Ladoga area. The Meyeri thrust zone is divided into northern and southern domains, considered allochthonous and autochthonous blocks according to material composition and structural features. The autochthonous block of the northern domain is different from the allochthonous block of the southern one by the presence of gneiss domes with Archean rocks in the cores. The allochthonous block is composed only of Paleoproterozoic rocks. During the formation of the thrust zone 1.86–1.85 Ga ago, the low-temperature amphibolite facies of biotite–muscovite parageneses were developed after the early mineral parageneses of the high-temperature amphibolite and granulite facies. [ABSTRACT FROM AUTHOR]

Like the Moldanubian parts of the Bohemian Massif and the French Massif Central, the Vosges Mts (NE France) comprise an uppermost tectonic unit that consists of retrograded (U)HP/UHT granulites, eclogites and garnet peridotites, originally metamorphosed at pressures of ≥ 4 GPa and temperatures of ≥ 950 °C. Two amphibole–garnet–spinel-bearing peridotite bodies located near to Col des Bagenelles (CB) and Crébimont (CR) show a pronounced retrograde evolution. Primary garnet was completely transformed to a symplectite of orthopyroxene + clinopyroxene + Cr-poor spinel ± amphibole, partially surrounded by a discontinuous outer zone of coarser grains of pyroxenes ± amphibole. Such symplectites are typical for the sliding reaction garnet + olivine ± Na2O ± K2O ± H2O = orthopyroxene + clinopyroxene + spinel ± amphibole. Both peridotites contain ‘primary’ Cr–Al–spinel. In the sample from CB, such grains occur only within the symplectites after garnet, but never in the matrix, while in the sample from CR, Cr–Al–spinel is present as inclusions within the symplectites and as isolated grains within the surrounding matrix. In the peridotite from CR, that is more depleted than that from CB, these Cr–Al–spinel grains show intense chemical zoning with Cr and Fe2+ decreasing, but Al and Mg increasing from core to rim. This pattern is interpreted as retrograde zoning during decompression. Primary P–T conditions deduced from compositions of matrix pyroxenes under the assumption of equilibration with earlier garnet are ≥ 1.6–1.7 GPa/ ≥ 900 °C for sample CB, but ≥ 2.3 GPa/ ≥ 860 °C for sample CR. Temperature conditions during the breakdown of garnet are indicated by pyroxenes from the symplectites and are 833 ± 18 (CB) and 812 ± 11 °C (CR) using the Ca-in-Opx thermometer at a chosen pressure of 1.3 GPa. [ABSTRACT FROM AUTHOR]

The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule lnD grt / olv Ni ; Ni grt Ni olv , the Ca mole fraction in garnet ( X grt Ca ; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet ( X grt Cr ; Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as: T ∘ C = - 8254.568 X grt Ca × 3.023 + X grt Cr × 2.307 + lnD grt olv Ni - 2.639 - 273 ± 55 where D grt / olv Ni = Ni grt Ni olv , Ni is in ppm, X grt Ca = Ca/(Ca + Fe + Mg) in garnet, and X grt Cr = Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer. [ABSTRACT FROM AUTHOR]

The pressure dependence of the exchange of Cr between clinopyroxene and garnet in peridotite is applicable as a geobarometer for mantle-derived Cr-diopside xenocrysts and xenoliths. The most widely used calibration (Nimis and Taylor Contrib Miner Petrol 139: 541–554, 2000; herein NT00) performs well at pressures below 4.5 GPa, but has been shown to consistently underestimate pressures above 4.5 GPa. We have experimentally re-examined this exchange reaction over an extended pressure, temperature, and compositional range using multi-anvil, belt, and piston cylinder apparatuses. Twenty-nine experiments were completed between 3–7 GPa, and 1100–1400 °C in a variety of compositionally complex lherzolitic systems. These experiments are used in conjunction with several published experimental datasets to present a modified calibration of the widely-used NT00 Cr-in-clinopyroxene (Cr-in-cpx) single crystal geobarometer. Our updated calibration calculates P (GPa) as a function of T (K), CaCr Tschermak activity in clinopyroxene a CaCrTs cpx , and Cr/(Cr + Al) (Cr#) in clinopyroxene. Rearranging experimental results into a 2n polynomial using multiple linear regression found the following expression for pressure: P GPa = 11.03 + - T K ln (a CaCrTs cpx) × 0.001088 + 1.526 × ln Cr # cpx T K where Cr # cpx = Cr Cr + Al , a CaCrTs cpx = Cr - 0.81 · Cr # cpx · Na + K , with all mineral components calculated assuming six oxygen anions per formula unit in clinopyroxene. Temperature (K) may be calculated through a variety of geothermometers, however, we recommend the NT00 single crystal, enstatite-in-clinopyroxene (en-in-cpx) geothermometer. The pressure uncertainty of our updated calibration has been propagated by incorporating all analytical and experimental uncertainties. We have found that pressure estimates below 4 GPa, between 4–6 GPa and above 6 GPa have associated uncertainties of 0.31, 0.35, and 0.41 GPa, respectively. Pressures calculated using our calibration of the Cr-in-cpx geobarometer are in good agreement between 2–7 GPa, and 900–1400 °C with those estimated from widely-used two-phase geobarometers based on the solubility of alumina in orthopyroxene coexisting with garnet. Application of our updated calibration to suites of well-equilibrated garnet lherzolite and garnet pyroxenite xenoliths and xenocrysts from the Diavik-Ekati kimberlite and the Argyle lamproite pipes confirm the accuracy and precision of our modified geobarometer, and show that PT estimates using our revised geobarometer result in systematically steeper paleogeotherms and higher estimates of the lithosphere‒asthenosphere boundary compared with the original NT00 calibration. [ABSTRACT FROM AUTHOR]

An effective method for identifying the tectonic-magmatic causes of metamorphism is the reconstruction and analysis of P–T–t paths, which can be used to develop a model of rock evolution with time in the "pressure–temperature–time" coordinates. This approach is most widely used in interpreting the development of orogens with a polycyclic history, where different types of metamorphism are combined in contrasting geodynamic settings. The progress in this direction in recent years is due to the improvement of the instrumental base, precision methods for the study of matter – in situ local microanalysis and dating of zonal metamorphic minerals, and the computing apparatus of geothermobarometric procedures using multi-equilibria. The article discusses these issues on the example of geological complexes of various geodynamic nature, which are typical for different types of metamorphism. Reaction microtextures, chemical zoning in minerals, geochronological dating as well as the trajectory of the Р-Т paths provide evidence for the polymetamorphic history. A generalized P-T diagram was built using data on well-studied metamorphic complexes, which are characterized by the simultaneous presence of prograde and retrograde segments of a P-T path. Diagnostic P-T-t paths characteristic of rocks formed in zones of active tectogenesis—under collision, extension and in shear zones of the earth's crust—have been established. This paper discusses some of the ambiguous interpretations of P-T-t paths in areas with simultaneous manifestation of different metamorphic processes, which requires further research. [ABSTRACT FROM AUTHOR]

Monazite dating in metapelites is an emerging method to investigate polymetamorphic areas. A protocol for Th–U–Pb dating of monazite by electron microprobe was adopted for a JEOL JXA-8530F. It was applied to the Variscan and Early-Alpine metamorphic Austroalpine Oetztal-Stubai Complex (OSC). In the Alpeiner Valley in the Stubai region, the Schrankogel complex is the eastern succession of the Central Metabasite Zone. In this part, metabasites are alternating with metapelites. In 4 samples from micaschist lenses, dominantly Carboniferous monazite isochrone ages at 335 ± 4 Ma, 320 ± 4 Ma; 319 ± 4 Ma and 319 ± 4 Ma were obtained. The micaschist samples with diverse modal compositions and variable bulk rock Ca contents of calculated assay, display distinct monazite microstructures, as quantified by automated SEM-MLA (mineral liberation analysis) routines. Clusters of small monazite could indicate new crystallization and yielded isochrones at 313 and 304 Ma. In contrast, corona structures of apatite and allanite around large monazites with isochrones between 350 and 315 Ma suggest a decomposition during decreasing temperature. Garnets in metapelitic assemblages display growth zonations with low pyrope contents in the cores and pyrope-rich rims. A prograde metamorphism with high pressure amphibolite-facies peak conditions at ~ 12 kbar and ~ 680 °C, and a post Pmax path with decompression to 4 kbar and 640–600 °C was estimated from the micaschists and from zoned Ca-amphiboles in retrogressed amphibolitized eclogites. The P–T path entered the monazite stability field during the decompression. This signals a Carboniferous age of the metamorphism. A minor population in one sample is composed of sporadic Permian single monazite ages. A Cretaceous monazite population is lacking. In the wide parts of the Austroalpine basement with Carboniferous-to-Cretaceous mica mixing ages, monazite age populations allow to discriminate a distinct Permian metamorphic event. [ABSTRACT FROM AUTHOR]

Trace-element concentrations in olivine and coexisting garnets included in diamonds from the Akwatia Mine (Ghana, West African Craton) were measured to show that olivine can provide similar information about equilibration temperature, diamond paragenesis and mantle processes as garnet. Trace-element systematics can be used to distinguish harzburgitic olivines from lherzolite ones: if Ca/Al ratios of olivine are below the mantle lherzolite trend (Ca/Al < 2.2), they are derived from a harzburgitic mantle source, and syngenetic garnets are without exception subcalcic G10 garnets. For harzburgitic olivines that cannot be identified this way, Na and Ca contents can be used: olivine inclusions with < 60 µg/g Na and Na/Al < 0.7 are all harzburgitic, whereas those with > 300 µg/g Ca or > 60 µg/g Na are lherzolitic. Conventional geothermobarometry indicates that Akwatia diamonds formed and resided close to a 39 mW/m2 conductive geotherm. A similar value can be derived from Al in olivine geothermometry, with TAl-ol ranging from 1020 to 1325 °C. Ni in garnet temperatures is on average somewhat higher (TNi-grt = 1115–1335 °C) and the correlation between the two thermometers is weak, which may be not only due to the large uncertainties in the calibrations, but also due to disequilibrium between inclusions from the same diamond. Calcium in olivine should not be used as a geothermobarometer for harzburgitic olivines, and often gives unrealistic P–T estimates for lherzolitic olivine as well. Diamond-hosted olivine inclusions indicate growth in an extremely depleted (low Ti, Ca, Na, high Cr#) environment with no residual clinopyroxene. They are distinct from olivines from mantle xenoliths which show higher, more variable Ti contents and lower Cr#. Hence, most olivine inclusions in Akwatia diamonds escaped the refertilisation processes that have affected most mantle xenoliths. Lherzolitic inclusions are probably the result of refertilisation after undergoing high-degree melting first. Trivalent cations appear to behave differently in harzburgitic diamond-hosted olivine inclusions than lherzolitic inclusions and olivine from mantle xenoliths. Some divalent chromium is predicted to be present in most olivine inclusions, which may explain high concentrations up to 0.16 wt% Cr2O3 observed in some diamond inclusions. Strong heterogeneity of Cr, V and Al in several inclusions may also result in apparent high Cr contents, and is probably due to late-stage processes during exhumation. However, in general, diamond-hosted olivine inclusions have lower Cr and V than expected compared to mantle xenoliths. Reduced Na activity in depleted harzburgites limits the uptake of Cr, V and Sc via Na–M3+ exchange. In contrast, Al partitioning in harzburgites is not significantly reduced compared to lherzolites, presumably due to uptake of Al in olivine by Al–Al exchange. [ABSTRACT FROM AUTHOR]

The Eastern Nigerian terrane forms the eastern flank of the Trans-Saharan Orogenic Belt within the Benin-Nigerian Shield. We report new petrological and P–T data of metamorphic rocks (migmatitic gneisses with subordinate granite gneiss, meta-quartz diorite, and meta-ultramafic rock) from Obudu in the southeastern flank of the Benin-Nigerian Shield for unraveling the tectono-metamorphic signatures of the eastern margin of the Trans-Saharan Orogenic Belt. The migmatized pelitic gneiss contains a peak mineral assemblage of plagioclase + rutile + garnet + biotite + K-feldspar + quartz ± liquid, based on which the peak metamorphic conditions are constrained as 850–890 °C and 9–10 kbar using phase equilibria modeling, optimal thermobarometry, and garnet-biotite geothermometry. Retrograde conditions were also estimated for a plagioclase + quartz + garnet + biotite + cordierite + rutile + sillimanite assemblage as 3.8 kbar/590 °C to 6.8 kbar/780 °C. The peak condition under granulite facies confirmed a marked increase of metamorphic grade from the greenschist facies in the western Benin-Nigerian Shield through the middle-upper amphibolite facies in the north-central Nigerian terranes to the granulite facies in the eastern part of the Benin-Nigerian Block. The results presented in this study are the first evidence of granulite-facies metamorphism from the easternmost parts of the Benin-Nigerian Shield. The clockwise path obtained in this study might suggest a continent-continent collisional setting for the evolution of this region related to the closure of the Goiás-Pharusian Ocean and the formation of the Trans-Saharan Orogenic Belt. [ABSTRACT FROM AUTHOR]

The paper presents detailed data on the mineralogy, chemical composition, geothermobarometry, and U–Pb zircon isotopic dating of peraluminous sapphirine-bearing crystalline schists, which occur in association with biotite–garnet–sillimanite gneisses, hypersthene–plagioclase–potassic feldspar and high-K hypersthene orthogneisses, and Ti-rich and subalkaline metabasites of the Kilegir Formation of the Daldyn Group at the Anabar Shield. In contrast to known granulites of the Daldyn and Upper Ananbar groups, the rocks of this association are enriched in K, Rb, Ba, Th, and LREE and have elevated (La/Yb)n. Thermobarometric studies of these rocks yielded the P–T parameters of the origin and evolution of the sapphirine-bearing granulites in the Anabar Shield, with peak values of UHT metamorphism in the ranges of T = 920–1000°C at P = 9–11 kbar. Isotope-geochronological data indicate that the rocks underwent a polymetamorphic evolution. The detrital zircon cores yielded ages of 3.36, 2.75, 2.6, and 2.5 Ga. Later overprinted metamorphic transformations of the detrital zircon formed rims, which are dated at 2.4, 2.3, 2.2, and 1.8 Ga. The timing of formation of the aluminous metasedimentary and associated metamagmatic rocks is estimated in the range of 2.5 to 2.4 Ga. A potential eroded source of the detrital zircons could be hypersthene plagiogneisses and metabasites of the Daldyn Group (whose premetamorphic age was no younger than 3.3 Ga) and products of their metamorphism (dated at about 2.7 Ga), and perhaps, also Na–K granites with an age of about 2.6–2.5 Ga, which still have not been found in the area but known at other shields and are enriched in radioactive (K, Th, and U) and trace elements. An additional source of clastic material for the aluminous sediments could be the two-feldspar magmatic rocks of rhyolite composition found in association with the gneisses and having the same model age. [ABSTRACT FROM AUTHOR]

The Central Mackenzie Valley (CMV) area of Northwest Territories is underlain by Precambrian basement belonging to the North American Craton. The potential of this area to host kimberlitic diamond deposits is relatively high judging from the seismologically-defined lithospheric thickness, age of basement rocks (2.2-1.7 Ga) and presence of kimberlite indicator minerals (KIMs) in Quaternary sediments. This study presents data for a large collection of KIMs recovered from stream sediments and till samples from two study areas in the CMV, the Horn Plateau and Trout Lake. In the processed samples, peridotitic garnets dominate the KIM grain count for both regions (> 25% each) while eclogitic garnet is almost absent in both regions (< 1% each). KIM chemistry for the Horn Plateau indicates significant diamond potential, with a strong similarity to KIM systematics from the Central and Western Slave Craton. The most significant issue to resolve in assessing the local diamond potential is the degree to which KIM chemistry reflects local and/or distal kimberlite bodies. Radiogenic isotope analysis of detrital kimberlite-related CMV ilmenite and rutile grains requires at least two broad age groups for eroded source kimberlites. Statistical analysis of the data suggests that it is probable that some of these KIMs were derived from primary and/or secondary sources within the CMV area, while others may have been transported to the area from the east-northeast by Pleistocene glacial and/or glaciofluvial systems. At this stage, KIM chemistry does not allow the exact location of the kimberlitic source(s) to be constrained. [ABSTRACT FROM AUTHOR]

The Diavik Diamond Mine in the NWT of Canada has produced in excess of 100 million carats from 3 kimberlite pipes since mining commenced in 2002. Here, we present new findings from deep (>400 m below surface) mining, sampling and drilling work in the A154N kimberlite volcano that require a revision of previous geological and emplacement models and provide a window into how the sub-continental lithospheric mantle (SCLM) below Diavik was sampled by kimberlite magmas through time. Updated internal geological models feature two volcanic packages interpreted to represent two successive cycles of explosive eruption followed by active and passive sedimentation from a presumed crater-rim, both preceded and followed by intrusions of coherent kimberlite. Contact relationships apparent among the geological units allow for a sequential organization of as many as five temporally-discrete emplacement events. Representative populations of mantle minerals extracted from geological units corresponding to four of the emplacement events at A154N are analyzed for major and trace elements, and provide insights into the whether or not kimberlites randomly sample from the mantle. Two independent geothermometers using clinopyroxene and garnet data indicate similar source depths for clinopyroxenes and G9 garnets (130-160 km), and suggest deeper sampling with time for both clinopyroxene and garnets. Harzburgite is limited to 110-160 km, and appears more prevalent in early, low-volume events. Variable ratios of garnet parageneses from the same depth horizons suggest random sampling by passing magmas, but deeper garnet sampling through time suggests early preferential sampling of shallow/depleted SCLM. Evaluations of Ti, Zr, Y and Ga over the range of estimated depths support models of the SCLM underlying the central Slave terrane. [ABSTRACT FROM AUTHOR]

The Jurassic Victor kimberlite (Attawapiskat Field) was emplaced into an area of the central Superior Craton that was affected by a lithosphere-scale thermal event at ~1.1 Ga. Victor diamonds formed ca. 400 million years after this event, in a lithospheric mantle characterized by an unusually cool model geotherm (37-38 mW/m2; Hasterok and Chapman 2011). The bulk of Victor diamonds derives from a thin (<10 km thick) layer that is located at about 180 km depth and represents lherzolitic substrates (for 85% of diamonds). Geothermobarometric calculations (average pressure and temperature at the 1 sigma level are 57 ± 2 kbar and 1129 ± 16 °C) coupled with typical fluid metasomatism-associated trace element patterns for garnet inclusions indicate diamond precipitation under sub-solidus (lherzolite + H2O) conditions. This conclusion links the presence of a diamond-rich lherzolitic layer in the lithospheric mantle, just above the depth where ascending melts would freeze, to the unusually low paleogeotherm beneath Attawapiskat, because along an average cratonic geotherm (40 mW/m2) lherzolite in the presence of hydrous fluid would melt at depths >140 km. [ABSTRACT FROM AUTHOR]

The garnet-muscovite geothermometer and garnet-muscovite-Al2SiO5-quartz (GMAQ) geobarometer have been empirically calibrated under P-T conditions of 1-12 kbar and 460-760 ºC using natural metapelitic rocks. The chemical compositions of the calibrant muscovite are in the ranges of Fe=0.03-0.21 atoms, Mg=0.02-0.32 atoms and AlVI=1.62-1.96 atoms, respectively, on the 11-oxygen basis per formula unit. The garnet-muscovite thermometer yields similar temperature estimates to the well calibrated garnet-biotite thermometer within error of ±55 °C, and successfully discriminates the systematic temperature change of the different zones of either the prograde or inverted metamorphic terranes or thermal contact aureoles. The six formulations of GMAQ barometry yield similar pressure estimates to the well calibrated GASP barometer within error of ±1.2 kbar, and plot the Al2SiO5-bearing metapelite into the correct stability field of the Al2SiO5 polymorphs. Moreover, the GMAQ thermobarometers show that the pressure is almost constant for every thermal contact aureole within limited geographic region, which reflects geological condition. The random errors are estimated to be of ca. ±60 °C and ±1.4 kbar for the geothermometer and geobarometer, respectively. A spreadsheet for applying GMAQ geothermobarometry is supplied in the Electronic Supplementary Materials. [ABSTRACT FROM AUTHOR]

To reveal the petrogenesis of rhyolite from the southwestern Okinawa Trough, the mineral chemistry of plagioclase, orthopyroxene, amphibole, quartz and Fe-Ti oxide phenocrysts were analyzed using an electron microprobe, and in suit Sr and Ba contents of plagioclase analysed by LA-ICPMS were chosen for fingerprinting plagioclases of different provenances. Results indicate an overall homogeneous composition for each of the mineral phases except for plagioclase phenocrysts which have a wide range of composition (An=39~88). Plagioclase crystals characterized by An contents of >70 are not in equilibrium with their whole-rock compositions, and coarse-sieved plagioclase phenocryst interiors record high An contents (>70) and Sr/Ba ratios (>7), which are similar to the those of plagioclase crystals in basalt. Therefore, these crystals must have been introduced to the rhyolitic magma from a more mafic source. Equilibrium temperatures estimated using orthopyroxene-liquid, iron-titanium oxide, titanium-in-quartz and amphibole geothermometers show consistent values ranging from 792 to 869°C. The equilibrium pressure calculated using amphibole compositions is close to 121 MPa which corresponds to an approximate depth of 4 km. The fO conditions estimated from Fe-Ti oxides and amphiboles plot slightly above the NNO buffer, which indicates that the rock formed under more oxidized conditions. Our results suggests that petrogenesis of the rhyolite due to basaltic magma ascend with the high An and Sr/Ba plaigoclases from deep magma chamber into the shallow chamber where the fractional crystallization and crustal assimilation happened. It also indicates that a two-layer magma chamber structure may occur under the southwestern Okinawa Trough. [ABSTRACT FROM AUTHOR]

In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and electron probe microanalysis (EPMA) are used to characterize magnetite and ilmenite of the phoscorite-carbonatite association at Phalaborwa. We trace the behavior of the compatible elements for two different generations of magnetite related to (1) a magmatic stage, with variable Ti-V content, which pre-dates the copper mineralization, and (2) a late hydrothermal, low-Ti, low-temperature event, mostly post-dating sulfide formation. Magnetite is shown to be a robust petrogenetic indicator; no influence on its chemical composition is detected from the intergrowth with the accompanying phases, including the interaction with coexisting sulfides. High spatial resolution EPMA characterize the tiny late-stage hydrothermal magnetite veins, as well as the ilmenite granular and lamellar exsolutions mostly developed in the magnetite from the phoscorite. By combining geochemical data with geothermo-oxybarometry calculations for magnetite-ilmenite pairs, we infer that the most primitive magnetite probably formed at oxygen fugacity above the nickel nickel oxide (NNO) buffer, revealing an evolutionary trend of decreasing temperature and oxygen fugacity. Geochemical similarity exists between magnetite from phoscorite and carbonatite, thus supporting a common mantle source for the phoscorite-carbonatite association. [ABSTRACT FROM AUTHOR]

The metamorphic evolution of the contact aureole around the Late Paleoproterozoic Jhirgadandi pluton in the eastern part of Parsoi Formation of Mahakoshal terrain, central India represents three distinct metamorphic zones, characterized by definite mineral assemblages. The contact-metamorphic event produced the peak-metamorphic mineral assemblages Bt + Qtz + Alb + Sil ± Cd ± Grt ± Mus ± Kfs in the metapelites of inner aureole, Bt + Qtz + And + Mus + Kfs + Plag ± Cd ± Chl in middle aureole and Chl + Mus + Bt ± And + Alb + Qtz ± Ep + Mt ± tourmaline in the outer aureole. The estimated P-T conditions based on detailed geothermobarometric calculations in the thermal metamorphosed rocks are 690°C/3.4 kbar, 580 ± 15°C and 487 ± 30°C in inner aureole, middle aureole and outer aureole, respectively. The variation in metamorphic condition suggests that the shallow crustal level emplacement of Jhirgadandi pluton is responsible for the overprinting of contact metamorphic assemblages (M) in the low grade metapelites (regional metamorphism M) of Mahakoshal Group. [ABSTRACT FROM AUTHOR]

Phenocryst assemblages of lavas from the long-lived Aucanquilcha Volcanic Cluster (AVC) have been probed to assess pressure and temperature conditions of pre-eruptive arc magmas. Andesite to dacite lavas of the AVC erupted throughout an 11-million-year, arc magmatic cycle in the central Andes in northern Chile. Phases targeted for thermobarometry include amphibole, plagioclase, pyroxenes, and Fe-Ti oxides. Overall, crystallization is documented over 1-7.5 kbar (~25 km) of pressure and ~680-1,110 °C of temperature. Pressure estimates range from ~1 to 5 kbar for amphiboles and from ~3 to 7.5 kbar for pyroxenes. Pyroxene temperatures are tightly clustered from ~1,000-1,100 °C, Fe-Ti oxide temperatures range from ~750-1,000 °C, and amphibole temperatures range from ~780-1,050 °C. Although slightly higher, these temperatures correspond well with previously published zircon temperatures ranging from ~670-900 °C. Two different Fe-Ti oxide thermometers (Andersen and Lindsley ; Ghiorso and Evans ) are compared and agree well. We also compare amphibole and amphibole-plagioclase thermobarometers (Ridolfi et al. ; Holland and Blundy ; Anderson and Smith ), the solutions from which do not agree well. In samples where we employ multiple thermometers, pyroxene temperature estimates are always highest, zircon temperature estimates are lowest, and Fe-Ti oxide and amphibole temperature estimates fall in between. Maximum Fe-Ti oxide and zircon temperatures are observed during the middle stage of AVC activity (~5-3 Ma), a time associated with increased eruption rates. Amphibole temperatures during this time are relatively restricted (~850-1,000 °C). The crystal record presented here offers a time-transgressive view of an evolving, multi-tiered subvolcanic reservoir. Some crystals in AVC lavas are likely to be true phenocrysts, but the diversity of crystallization temperatures and pressures recorded by phases in individual AVC lavas suggests erupting magma extensively reams and accumulates crystals from disparate levels of the middle to upper crust. [ABSTRACT FROM AUTHOR]

The Bracco-Gabbro Complex (Internal Liguride ophiolite), that intruded subcontinental mantle peridotite, contains layers of chromitite that are associated with ultramafic differentiates. The chromitites and disseminated chromites in the ultramafics have Al contents similar to the Al-rich podiform chromitites [0.40 < Cr# = Cr/(Cr + Al) < 0.55]. TiO contents of the chromitites are unusually high and range up to 0.82 wt%. The calculated AlO and TiO content of the parental melt suggest that the melt was a MORB type. Geothermobarometrical calculations on few preserved silicate inclusions revealed formation temperatures between 970 and 820 °C under a relatively high oxygen fugacity (Δlog fO at +2.0-2.4). Chromitites were altered during the post-magmatic tectono-metamorphic uplift and the final exposure at the seafloor, as evidenced by the formation of ferrian chromite. The PGE contents of the chromitites and associated ultramafics are unusually low (PGEmax 83 ppb). The chondrite-normalized PGE spidergrams show positive PGE patterns and to some extent similarities with the typical trend of stratiform chromitites. No specific PGM have been found but low concentrations of PPGE (Rh, Pt, and Pd) have been detected in the sulphides that occur interstitially to or enclosed in chromite. Recently, it has been shown that the Internal Liguride gabbroic intrusions have formed by relatively low degrees of partial melting of the asthenospheric mantle. We conclude that the low degree of partial melting might be the main factor to control the unusual low PGE contents and the rather unique PGE distribution in the Bracco chromitites. [ABSTRACT FROM AUTHOR]

Three monazite generations were observed in garnet-bearing micaschists from the Schobergruppe in the basement to the south of the Tauern Window, Eastern Alps. Low-Y monazite of Variscan age (321 ± 14 Ma) and high-Y monazite of Permian age (261 ± 18 Ma) are abundant in the mica-rich rock matrix and in the outer domains of large garnet crystals. Pre-Alpine monazite commonly occurs as polyphase grains with low-Y Variscan cores and high-Y Permian rims. Monazite of Eo-Alpine age (112 ± 22 Ma) is rarer and was observed as small, partly Y-enriched grains (3 wt. % YO) in the rock matrix and within garnet. Based on monazite-xenotime thermometry, Y + HREE values in monazite indicate minimum crystallization conditions of 500 °C during the Variscan and 650 °C for the Permian and Alpine events, respectively. Garnet zoning and thermobarometric calculations with THERMOCALC 3.21 record an amphibolite facies, high-pressure stage of ~600 °C/13-16 kbar, followed by a thermal maximum at 650-700 °C and 6-9 kbar. The Eo-Alpine age for these two events is supported by inclusions of Cretaceous monazite in the garnet domains used for thermobarometric constraints and through the high growth temperatures of Eo-Alpine monazite, which is consistent with that of the thermal maximum (~700 °C). The age and growth conditions of a few Mn-rich garnet cores, sporadically present within Eo-Alpine garnet, are unclear because inclusions of monazite, plagioclase and biotite necessary for thermobarometric- and age constraints are absent. However, based on monazite thermometry, Permian and Variscan metamorphic conditions were high enough for the growth of pre-Alpine garnet. The formation of Variscan garnet and its later resorption, plus Y-release, would also explain the high Y in Permian monazite, which cannot originate from preexisting Variscan monazite only. Monazite of Variscan, Permian and/or Eo-Alpine ages were also observed in other garnet-bearing micaschists from the Schobergruppe. This suggests that the basement of the Schobergruppe was overprinted by three discrete metamorphic events at conditions of at least lower amphibolite facies. While the Variscan event affected all parts of this basement, the younger events are more pronounced in its structurally lower units. [ABSTRACT FROM AUTHOR]

The Guarguaraz Complex in West Argentina formed during collision between the microplate Chilenia and South America. It is composed of neritic clastic metasediments with intercalations of metabasic and ultrabasic rocks of oceanic origin. Prograde garnet growth in metapelite and metabasite occurred between 1.2 GPa, 470°C and 1.4 GPa, 530°C, when the penetrative s-foliation was formed. The average age of garnet crystallization of 390 ± 2 Ma (2σ) was determined from three four-point Lu-Hf mineral isochrones from metapelite and metabasite samples and represents the time of collision. Peak pressure conditions are followed by a decompression path with slight heating at 0.5 GPa, 560°C. Fluid release during decompression caused equilibration of mineral compositions at the rims and also aided Ar diffusion. An Ar/Ar plateau age of white mica at 353 ± 1 Ma (1σ) indicates the time of cooling below 350-400°C. These temperatures were attained at pressures of 0.2-0.3 GPa, indicative of an average exhumation rate of ≥1 mm/a for the period 390-353 Ma. Late hydrous influx at 0.1-0.3 GPa caused pervasive growth of sericite and chlorite and reset the Ar/Ar ages of earlier coarse-grained white mica. At 284-295 Ma, the entire basement cooled below 280°C (fission track ages of zircon) after abundant post-collisional granitoid intrusion. The deeply buried epicontinental sedimentary rocks, the high peak pressure referring to a low metamorphic geotherm of 10-12°C/km, and the decompression/heating path are characteristics of material buried and exhumed within a (micro) continent-continent collisional setting. [ABSTRACT FROM AUTHOR]

Pseudosections, geothermobarometric estimates and careful petrographic observations of gneissic migmatites and granulites from Neoproterozoic central Ribeira Fold Belt (SE Brazil) were performed in order to quantify the metamorphic P-T conditions during prograde and retrograde evolution of the Brasiliano Orogeny. Results establish a prograde metamorphic trajectory from amphibolite facies conditions to metamorphic peak (T = 850 ± 50 °C; P = 8 ± 1 kbar) that promoted widespread dehydrationmelting of 30 to 40% of the gneisses and high-grade granitization. After the metamorphic peak, migmatites evolved with cooling and decompression to T ≈ 500 °C and P ≈ 5 kbar coupled with aH2O increase, replacing the high-grade paragenesis plagioclase-quartz-K-feldspar-garnet by quartz-biotite-sillimanite-(muscovite). Cordierite absence, microtextural observations and P-T results constrain the migmatite metamorphic evolution in the pseudosections as a clockwise P-T path with retrograde cooling and decompression. High-temperature conditions further dehydrated the lower crust with biotite and amphibole-dehydration melting and granulite formation coupled with 10% melt generation. Granulites can thus be envisaged as middle to lower crust dehydrated restites. Granulites were slowly (nearly isobarically) cooled, followed by late exhumation/retrograde rapid decompression and cooling, reflecting a two step P-T path. This retrograde evolution, coupled with water influx, chemically reequilibrated the rocks from granulite to amphibolite/greenschist facies, promoting the replacement of the plagioclase-quartz-garnet-hypersthene peak assemblage by quartz-biotite- K-feldspar symplectites. [ABSTRACT FROM AUTHOR]

Field studies supplemented by petrographic analyses clearly reveal complete preservation of ophiolite suite from Port Blair (11°39′N: 92°45′E) to Chiriyatapu (11°30′24″N: 92°42′30″E) stretch of South Andaman. The ophiolite suite reveals serpentinite at the base which is overlain unconformably by cumulate ultramafic-mafic members with discernible cumulus texture and igneous layering. Basaltic dykes are found to cut across the cumulate ultramafic-mafic members. The succession is capped by well exposed pillow basalts interlayered with arkosic sediments. Olivine from the basal serpentinite unit are highly magnesian (Fo). All clinopyroxene analyses from cumulate pyroxenite, cumulate gabbro and basaltic dyke are discriminated to be ‘Quad’ and are uniformly restricted to the diopside field. Composition of plagioclase in different lithomembers is systematically varying from calcic to sodic endmembers progressively from cumulate pyroxenite to pillow basalt through cumulate gabbro and basaltic dyke. Plagioclase phenocrysts from basaltic dyke are found to be distinctly zoned (An-An) whereas groundmass plagioclase are relatively sodic (An-An). Deduced thermobarometric data from different lithomembers clearly correspond to the observed preservation of complete ophiolite suite. [ABSTRACT FROM AUTHOR]

We describe texture, mineralogy and whole-rock composition of cognate monzonite sub-volcanic clasts within debris flow deposits related to the 5000 years catastrophic phreatomagmatic eruption probably linked to the Sciara del Fuoco sector collapse. The debris flows are at the top of accretionary lapilli-rich ash deposits overlying potassic (KS, shoshonites) lavas of the Neostromboli period. The monzonites are inferred to be crystallized in situ, at low P, at the side walls and/or roof margins of a shallow magma chamber and to be cogenetic with the KS Neostromboli extrusives. They can be considered "ideal orthocumulates" since they approximately retain a bulk liquid composition and possibly represent "slowly cooled equivalents" of their KS shoshonite host rock. The "closure temperature" of final solidification of the monzonite lithic suite was estimated through ternary-feldspar geothermobarometry, plagioclase–K-feldspar and K-feldspar–biotite equilibria and is in the range of 750–790 °C with a maximum –logfO2 around 15.1–15.3. The estimated pressure of crystallization is <0.5 kbar. Potassic lavas and dikes, previously emplaced during the Neostromboli period, also resemble the monzonites in both major trace elements and mineral chemistry. The cogenetic relationship between KS Neostromboli extrusives and the monzonite host-rock magma from which the sub-volcanic clasts were derived is clear evidence that a shallow magma chamber existed between the caldera collapse of the Vancori period and the Sciara del Fuoco sector collapse (i.e. between 13 000 and 5000 years). The monzonite clasts were derived from crystallization at very shallow depth (ca. 1 km) and strongly support the hypothesis of violent decompression of the shallow magmatic plumbing system during the Sciara del Fuoco sector collapse. Climax of the regressive landslide event, with maximum disruption of the chamber walls, took place during emplacement of the debris flows, i.e. during the late stage of the Neostromboli phreatomagmatic eruption. [ABSTRACT FROM AUTHOR]

Calbuco volcano is a Late Pleistocene composite stratovolcano and member of the Southern Volcanic Zone of the Chilean Andes (41°19′S, 72°36′W). It lies ~ 20 km west of the Liquiñe–Ofqui Fault Zone, but is not located directly upon any major regional structures. During April 2015, a sub-Plinian eruption occurred, with a bulk erupted volume of ~ 0.3–0.6 km3 (~ 0.1–0.2 km3 DRE). The eruption was a rapid-onset event that produced highly crystalline products (from 40 to 60 vol.%) including the mineral phases: plagioclase, clinopyroxene, orthopyroxene, amphibole, olivine, apatite, ilmenite, titanomagnetite and chalcopyrite. An upper-crustal reservoir is inferred using available geophysical data combined with amphibole geobarometry. Consideration of textural features, including high crystallinity, complex mineral zonation, crystal clots and interstitial glass between crystals from clots, suggests the presence of a mush zone within this reservoir. From the nine collected samples, whole-rock chemistry and an array of geothermometers (amphibole, amphibole-plagioclase, two-pyroxenes and Fe–Ti oxides) gave similar results for all samples possessing ~ 40 vol.% of crystals, with the exception of the sample Cal-160 (~ 60 vol.% crystallinity), which is slightly more evolved and yields lower temperatures for all geothermometers. By comparing temperatures calculated in sample Cal-160 using pairs of ilmenite-titanomagnetite core compositions with those calculated using rim compositions, we observe a late-stage temperature increase of between 70 and 200 °C. We suggest that this local-scale heating event was at least partly responsible for triggering the eruption. Our data suggest that the bulk of the erupted magma was derived from a relatively uniform (970–1000 °C), crystal-rich magma mass. Sample Cal-160 was derived from a cooler environment (910–970 °C), where it was subjected to pre-eruptive heating to temperatures considerably higher than those observed in associated, erupted magmas (up to 1070 °C). This requires the involvement of a hot, presumably mafic magma injection at the base of a shallow, crystal-rich reservoir, though the mafic magma was not itself erupted. The localised nature of interaction and rapidity of eruption onset have implications for potential future hazards at Calbuco volcano. [ABSTRACT FROM AUTHOR]