Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U–Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P–T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U–Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure

Gradational development of slaty cleavage to schistosity -an example from the Iberian Pyrite Belt, Spain-

The Iberian Pyrite Belt is an Upper Paleozoic succession which has been divided into the Phyllite-Quartzite Group, the Volcanic-Sliceous Complex and the Culm Group in ascending order from base to the top. The contacts among them are conformable. These Groups are tightly folded and metamorphosed un-der low-grade conditions during Hercynian times. The first stage folds of the Volcanic-Siliceous Complex and the Culm Group are associate with slaty cleavages (S1), while the Phyllite-Quartzite Group is asso-ciate with schistosities (S1). Fold analysis using cleavages and minoe folds has been utilized in the analysis of the stratigraphy. This led to systematic horizontal collection of specimens (pelitic rocks). S1 texture is defined by paralled alignment of secondary platy minerals which tend to grow larger and together, forming zonal domains toward the Phyllite-Quartzite Group. The clay matrices change to recrystallized quartz toward the Phyllite Quartzite Group. Textural modifications are evidenced by gradational changes from slaty cleavages to schistosities. This shows that recrystallization is the important deformation mecha-nism responsible for S1 development. It also reflects that the deformation was contemporaneous with metamorphism

What can we learn from melt inclusions in migmatites and granulites?

With less than two decades of activity, research on melt inclusions (MI) in crystals from rocks that have undergone crustal anatexis \u2013 migmatites and granulites \u2013 is a recent addition to crustal petrology and geochemistry. Studies on this subject started with glassy inclusions in anatectic crustal enclaves in lavas, and then progressed to regionally metamorphosed and partially melted crustal rocks, where melt inclusions are normally crystallized into a cryptocrystalline aggregate (nanogranitoid). Since the first paper on melt inclusions in the granulites of the Kerala Khondalite Belt in 2009, reported and studied occurrences are already a few tens. Melt inclusions in migmatites and granulites show many analogieswith theirmore common and long studied counterparts in igneous rocks, but also display very important differences and peculiarities,which are the subject of this review. Microstructurally, melt inclusions in anatectic rocks are small, commonly 10 \u3bcm in diameter, and their main mineral host is peritectic garnet, although several other hosts have been observed. Inclusion contents vary from glass in enclaves that were cooled very rapidly from supersolidus temperatures, to completely crystallized material in slowly cooled regional migmatites. The chemical composition of the inclusions can be analyzed combining several techniques (SEM, EMP, NanoSIMS, LA\u2013ICP\u2013MS), but in the case of crystallized inclusions the experimental remelting under confining pressure in a piston cylinder is a prerequisite. The melt is generally granitic and peraluminous, although granodioritic to trondhjemitic compositions have also been found. Being mostly primary in origin, inclusions attest for the growth of their peritectic host in the presence of melt. As a consequence, the inclusions have the unique ability of preserving information on the composition of primary anatectic crustal melts, before they undergo any of the common following changes in their way to produce crustal magmas. For these peculiar features, melt inclusions in migmatites and granulites, largely overlooked so far, have the potential to become a fundamental tool for the study of crustal melting, crustal differentiation, and even the generation of the continental crust

On the Petrochemical Character of the Pelitic Gneiss from the Southwestern Part in the Hida Metamorphic Belt, Central Japan

In the Hida Metamorphic Belt, metapelites develop in a small amount, in contrast to wide development of basic and calcareous metamorphites. The metapelites in the district have been clarified to be higher in CaO, FeO and MgO contents while lower in Al-excess as compared to the usual metapelite. Judged from the petrochemical characteristics and the mode of occurrence, it is probable that the metapelites in question should not be derived from rocks of "miogeosynclinal" character of higher maturity, but from those intermingled with basic volcanic materials. Also have been discussed the similarity and the difference between the metapelites from other high T-low P type metamorphic belts in Japan along with the Precambrian metapelites from the Kamiaso conglomerate

Metamorphic and metasomatic evolution of the Western Domain of the Karagwe-Ankole Belt (Central Africa)

The tectonometamorphic evolution of the Western Domain of the Karagwe-Ankole Belt, containing widespread granite-related W-Nb-Ta-Sn mineralization in pegmatites and hydrothermal quartz veins of Early Neoproterozoic age, is largely unknown. This study aims to characterize the Meso- and Neoproterozoic metamorphism and metasomatism in the Karagwe-Ankole Belt, to reconstruct the temperature evolution and to investigate its temporal relation to deformation and the widespread granite magmatism and mineralization. A quantitative geothermometric study was conducted on metasiltstones and amphibolites, and applies thin section petrography, garnet-biotite and chlorite geothermometry on samples collected in the representative KibuyeGitarama-Gatumba area in West Rwanda. The presence of garnet, staurolite and kyanite in metasiltstones, and hornblende and andesine-labradorite feldspar in amphibolites indicates prograde Barrovian metamorphism up to syn-to post-deformational (D1 or D2) lower amphibolite facies (up to 630 degrees C). This peak metamorphism was followed by post-D2 greenschist facies metamorphism (c. 525 degrees C-440 degrees C; garnet, biotite, chlorite, muscovite in metasiltstone), probably related to the East African Orogeny as part of the Gondwana assembly. A geothermometric evolution with high temperature conditions ( > 500 degrees C) at least from the flare-up of Early Neoproterozoic tin granites and their metasomatic haloes onwards for most of the Neoproterozoic is proposed, in close correspondence with the geodynamic evolution of the neighboring terranes. Additionally, this high temperature regime is an important factor to be taken into account when interpreting thermal diffusion-sensitive geochronological data

Garnet–monazite rare earth element relationships in sub-solidus metapelites: a case study from Bhutan

A key aim of modern metamorphic geochronology is to constrain precise and accurate rates and timescales of tectonic processes. One promising approach in amphibolite and granulite-facies rocks links the geochronological information recorded in zoned accessory phases such as monazite to the pressure–temperature information recorded in zoned major rock-forming minerals such as garnet. Both phases incorporate rare earth elements (REE) as they crystallize and their equilibrium partitioning behaviour potentially provides a useful way of linking time to temperature. We report REE data from sub-solidus amphibolite-facies metapelites from Bhutan, where overlapping ages, inclusion relationships and Gd/Lu ratios suggest that garnet and monazite co-crystallized. The garnet–monazite REE relationships in these samples show a steeper pattern across the heavy (H)REE than previously reported. The difference between our dataset and the previously reported data may be due to a temperature-dependence on the partition coefficients, disequilibrium in either dataset, differences in monazite chemistry or the presence or absence of a third phase that competed for the available REE during growth. We urge caution against using empirically-derived partition coefficients from natural samples as evidence for, or against, equilibrium of REE-bearing phases until monazite–garnet partitioning behaviour is better constrained

Modelling phase-assemblage diagrams for magnesian metapelites in the system K2O-FeO-MgO-Al2O3-SiO2-H2O: geodynamic consequences for the Monte Rosa nappe, Western Alps

Magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain P-T conditions in terranes where metamorphism is not easily visible. Phase-assemblage diagrams for natural and model magnesian metapelites in the system KFMASH are presented to document how phase relationships respond to water activity, bulk composition, pressure and temperature. The phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites. It is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a(H2O). Specifically, the appearance of the characteristic HP assemblage chloritoid-talc-phengite-quartz (with excess H2O) in the magnesian metapelites of the Monte Rosa nappe (Western Alps) is due to the reduction of a(H2O). Furthermore, the mineral assemblages are determined by the whole-rock FeO/(FeO+MgO) ratio and effective Al content X A as well as P and T. The predicted mineral associations for the low- and high-X A model bulk compositions of magnesian metapelites at high pressure are not dependent on the X A variations as they show a similar sequence of mineral assemblages. Above 20kbar, the prograde sequence of assemblages associated with phengite (with excess SiO2 and H2O) for low- and high-X A bulk compositions of magnesian metapelites is: carpholite-chlorite→chlorite-chloritoid→chloritoid-talc→chloritoid-talc-kyanite→ talc-garnet-kyanite→garnet-kyanite±biotite. At low to medium P-T conditions, a low-X A stabilises the phengite-bearing assemblages associated with chlorite, chlorite+K-feldspar and chlorite+biotite while a high-X A results in the chlorite-phengite bearing assemblages associated with pyrophyllite, andalusite, kyanite and carpholite. A high-X A magnesian metapelite with nearly iron-free content stabilises the talc-kyanite-phengite assemblage at moderate to high P-T conditions. Taking into account the effective bulk composition and a(H2O) involved in the metamorphic history, the phase-assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system KFMASH and therefore may contribute to gaining insights into the metamorphic evolution of terranes. As an example, the magnesian metapelites of the Monte Rosa nappe have been investigated, and an exhumation path with P-T conditions for the western roof of the Monte Rosa nappe has been derived for the first time. The exhumation shows first a near-isothermal decompression from the Alpine eclogite peak conditions around 24kbar and 505°C down to approximately 8kbar and 475°C followed by a second decompression with concomitant coolin

Tracing the cryptic Sardic (Ordovician) metamorphism across Alpine Europe: the Krndija region in the Slavonian Mountains, Croatia

Results of a combined petrological, geochemical and geochronological study suggest that metasedimentary rock units in the Krndija region of the Slavonian Mountains, Croatia, were affected by at least three major tectonometamorphic imprints: during the Middle Ordovician (Sardic event), the early Carboniferous (Variscan event), and the Cretaceous (Alpine event). All three metamorphic phases are established by electron microprobe-based in-situ U–Th–Pb dating of monazite grains. The Sardic metamorphic event is additionally confirmed by a precise Lu–Hf garnet-whole-rock isochron age of 466.0 ± 2.3 Ma. Taken together, the data unveil a relatively large and well-preserved piece of the cryptic Sardic orogen in central Krndija, that we name the Kutjevo Zone. A Sardic subduction-related metamorphic event (ca. 540-580 ℃, 8–11 kbar) at ca. 466 Ma is manifested in the mineral paragenesis Ca-rich garnet plus rutile. A low degree of retrograde reequilibration suggests a subsequent fast exhumation. Low-Ca cores in some garnets and staurolite relics record a pre-HP metamorphic event that involves isobaric heating from 570 to 610 ℃ at ~ 7 kbar. We attribute this (so far undated) event to mid-crustal contact metamorphism caused by early Sardic magmatism. Southern parts of Krndija (the Gradište Zone) experienced an (additional?) clockwise PT evolution in Variscan times at ca. 350 Ma. Garnet formed with ilmenite during a PT increase from 580 ℃/5 kbar to 600 ℃/6 kbar and underwent later strong retrograde resorption. Slow Variscan exhumation resulted in andalusite formation at < 550 ℃/ < 3.8 kbar. Penetrative Alpine metamorphism was observed in low-grade phyllites in the north. The lithology and metamorphic history of the Kutjevo Zone is similar to what has been reported from the Sardic Strona-Ceneri Zone in the western Alps. Both areas expose metapelitic (metagreywacke) rocks with a pre-middle Ordovician formation age. These metasedimentary rocks are inter-layered with numerous small amphibolitic units as well as metagranitoids and were likely deposited along the active Gondwana margin, perhaps in a fore-arc position, prior to their subduction during the middle Ordovician. According to recent palaeogeographic reconstructions, both the Kutjevo Zone and the Strona-Ceneri Zone have once resided in an eastern sector of the northern Gondwana margin (i.e., in E-Armorica). We conclude that in the Middle Ordovician, important subduction activities took place in this E-Armorican segment of north Gondwana, which is today exposed in the Alps. The W-Armorican segment of north Gondwana (now exposed in the French, German, and Czech Variscides) had probably already mutated from a (Cadomian) subduction setting to an extensional (transtensional–transpressional) setting by the late Cambrian

Hydrogeological model of Mijas mountain aquifers under different climate conditions (Málaga, Spain)

Carbonate aquifers represent an important source of freshwater, both for urban and agricultural uses. This is particularly true in semiarid regions, where intensive pumping has often led to aquifer overexploitation. One example is the Mijas mountain carbonate diffuse flow system (80 km2), located to the SW of the city of Malaga, Spain. From a geolo-gical standpoint, this area consists of Triassic dolomitic and calcareous rocks, which overlay Palaeozoic metapelites. The geological structure is formed by ESE-WNW folds and the me-tapelites anticlinal cores have divided the study area into four aquifer systems. The recharge of Mijas mountain aquifers comes from direct infiltration of rainfall, while pumping is the main discharge. To improve the knowledge of geological and hydrodynamic parameters, and therefore to improve water resources management, a hydrogeological model has been developed with Processing Modflow 8.0.42. Piezometric level and spring flows have been modelled, under steady and transient-flow conditions for a 35-year period. Five future scenarios were simulated for different rainfall and pumping conditions. Outcomes confirm that the water level evolution is determined by the quantity and distribution of rainfall during the hydrological year, with the same pumping rate. The results also suggest that current trends are likely to raise sustainability issues in the future.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Geothermobarometry of Cordierite-Bearing Metapelites of the Hudson Highlands, Southeastern NY

Various P-T determinations have been reported on metasedimentary rock suites in the New Jersey and Hudson Highlands using different geothermobarometers, but all have yielded relatively large P-T ranges and none have used cordierite in a quantitative way. The presence of variable cordierite and its narrow stability field in metapelitic gneisses provides an excellent opportunity to better constrain the peak P-T conditions. Calcium depleted, clay-rich, arkosic sedimentary protolith was deposited during the Mesoproterozoic in a continentaloceanic arc basin during the Elzevirian orogeny (~1300-1200 Ma). The Ottawan metamorphic event (~1090-1020 Ma) transformed this protolith into the metasedimentary unit exposed in the Hudson Highlands, southeastern NY (Gates et al., 2001). Twenty samples from outcrops with inlerlayered metapelites and metapsammitic gneisses were sampled from four localities within Harriman State Park, NY in the western Hudson Highlands. The determination of the metamorphic conditions based on the prograde phases of the biotite-cordierite, biotite-cordieritegarnet, and biotite-garnet metapelitic gneisses is the focal point of interest in the current study. Whole-rock major elements analysis of the composite suite of samples defines the differentiation of the metapsammitic and metapelitic gneisses as: metapelites = weight % K2O/ CaO \u3e 1; metapsammitic gneiss = weight % K2O/ CaO \u3c1. Chemical composition and weight % oxide data was collected from five samples that displayed co-existing garnet, biotite, and locally occurring plagioclase and cordierite, to perform geothermobarometry. Garnet-cordierite and garnet-biotite thermometers and grt-sil-crd-qtz, garnet-aluminasilicate-plagioclase (GASP), and garnet-biotite-plagioclase-quartz (GBPQ) barometers were used. The P-T determinations for five samples with appropriate mineralogy are as follows: QB-7: 588 °C (±38) at 4.4 kilobars (± 2) SB-5: 593 °C (± 19) at 5.3 kilobars (± 0.4) QB-1: 654 °C (±18) at 5.1 kilobars (± 0.1) TB-3: 745 °C (± 18) at 6.0 kilobars (± 0.2) SB-7: 730 °C (± 19) at 6.1 kilobars (± 0.1) The P-T reported is indicative of a transition from middle-amphibolite to lower-granulite facies metamorphism. The appearance and disappearance of cordierite with increasing metamorphic grade is expressed as biotite-cordierite gneiss-\u3ebiotite-cordierite-garnet gneiss-\u3egarnet-biotite gneiss. The presence of cordierite in the metapelites of Hudson Highlands is not as imperative as previously thought in regards to determining peak metamorphism conditions. However, its appearance and disappearance provides a unique opportunity to determine the rates of metamorphic processes for the metasedimentary unit exposed in the Hudson Highlands