Journal of Metamorphic Geology最新文献

Eclogite-facies rocks provide important constraints on the behaviour of convergent plate boundaries and the geometries of tectonic reconstructions due to the high to ultrahigh pressure conditions at which they form. Many eclogite occurrences are documented near the suture zone of active collisional settings where they are interpreted to mark the approximate location of former ocean basin subduction. Such observations influence tectonic interpretations for older eclogites within more deeply eroded and/or less well-exposed terranes. The eclogitic Glenelg inlier in northwest Scotland is one such example, with c. 1 Ga eclogites having previously been interpreted as marking the trace of a Grenville-aged collisional suture zone that defines a third ‘arm’ to the Grenville orogen alongside well-defined sutures in North America and Scandinavia. Here we use a combination of geochronology, phase equilibrium modelling and accessory-phase thermometry to show that the eclogite-facies assemblages were produced at $��\sim �$18–19 kbar and 700°C–750°C from c. 1.1 to 1.0 Ga. Accounting for the foreland basin setting of equivalent-aged sedimentary rocks in the region and demonstrating the thermal viability of this setting, we show that eclogite formation occurred in deforming foreland crust adjacent to the Grenville orogen, in a setting broadly analogous to fault-bounded basement uplifts in the forelands of active deformation belts, such as the Himalayas and Andes. Our results demonstrate that eclogite-facies rocks can form in a greater range of tectonic settings than are sometimes considered, with implications for tectonic reconstructions of collisional zones. In this instance, our results remove the need for a third ‘arm’ of the Grenville orogen by placing Glenelg in a foreland setting, reconciling the absence of plentiful Grenville-aged metamorphic rocks in northwest Scotland, the sedimentological record and paleomagnetic data in the wider region.

Re di Castello is the southernmost unit of the composite Adamello batholith (central Alps, Italy). It was emplaced between 43 and 40 Ma, yet its depth of emplacement is not well constrained. We have studied the contact metamorphism of pelitic country rocks in two localities: the upper Caffaro Valley and the Borzago Valley in the southern and the northern sub-units of the Re di Castello, respectively. In the Caffaro Valley, the country rocks comprise the Triassic sedimentary Lozio Shale, a carbon-rich slate–siltstone, which was intruded by quartz-dioritic magma. Near the contact, the hornfelses display the mineralogical assemblage Kfs-Crd-Bt-Ms-Pl-Qz-Gr, with rare fibrolitic sillimanite in a single sample. Andalusite was never observed throughout the aureole at this locality. In the Borzago Valley, contact metamorphism developed on schists of the pre-Permian basement with a Variscan, regional metamorphic overprint. The sequence of contact metamorphic mineral assemblages progresses from And-Bt-bearing parageneses to the Sil-Crd-Bt-Kfs-Ms-Pl-Qz that characterizes peak conditions, at which incipient melting is also observed. K-feldspar forms slightly upgrade the first appearance of sillimanite. For samples at both localities, thermodynamic modelling in the NCKFMASHT system failed at predicting a stability field for the sequences of mineral assemblages developed during contact metamorphism. Also the Ti-in biotite thermometer did not constrain temperatures adequately. The P–T conditions at the thermal peak were thus evaluated by an alternative bathograd-like approach, considering phase relationships in the simplified NKASH-C system. To form sillimanite only as product of the incomplete Ms-Qz breakdown—divariant for the presence of Na in Ms and Kfs and shifted to lower temperature due to the presence of graphite (in the Lozio Shale)—an isobaric path typical of contact metamophism must have crossed above the Ms_ss-Qz-Kfs_ss-Sil-And-fluid invariant point. This constrains an emplacement pressure >3.3 kbar in the Caffaro Valley, and >3.2 kbar in the Borzago Valley. Concerning temperature, the same univariant point also constrains the minimum temperature in the Caffaro Valley as >615°C–620°C, consistent with results of RSCM thermometry on graphite from the Lozio Shale. The evidence of incipient melting in some samples from the Borzago Valley indicates higher temperatures, probably approaching 670°C, near the contact to the intrusion. Assuming an average crust density of 2.7 g cm⁻³, the estimated pressures correspond to a minimum paleo-depth of emplacement of about 12 km. These depths are somewhat greater than normally considered and should be regarded as revised constraints on models of the emplacement dynamics of the Adamello batholith and on paleogeographic reconstructions of this part of the Southalpine domain.

Phase equilibrium modelling is a powerful method for obtaining pressure–temperature (P–T) estimates in metapelites; however, the advantages, disadvantages, and limitations of applying this technique to greenschist-facies pelites have received little attention. This study uses two greenschist-facies metapelites from the Nappe Zone in Sardinia (Italy) to examine differences between currently available thermobarometric techniques. First, this work compares P–T estimates from chlorite–K-white mica–quartz–water and K-white mica–quartz–water multiequilibrium approaches, the latter integrated with previously published T_max results from Raman spectroscopy on carbonaceous material. Consistent P–T conditions are retrieved from these methods for both samples, with P–T estimates suggesting a difference in peak metamorphic conditions between the Internal and External Nappe Zones of Sardinia (peak P–T conditions of 0.6–0.8 GPa, 420°C–450°C and 0.4–0.5 GPa, 370°C–400°C for the Internal and External Nappes, respectively). Second, phase equilibrium modelling was used to assess the ability of six different sets of solution models, alongside their associated thermodynamic datasets, to constrain P–T conditions in the same samples, which are characterized by high-variance mineral assemblages. Significant differences were found between the six thermodynamic databases, and important discrepancies were recognized between the observed and predicted mineral assemblages and compositions (e.g., Si in K-white mica). In only one sample, three databases were able to predict the observed natural assemblage. The compositional isopleths for K-white mica, from the same databases, do not intersect, offering no assistance in constraining P–T conditions. However, one database predicted chlorite's natural composition, yielding a P–T estimate consistent with multiequilibrium methods: ~0.9 GPa, ~350°C–400°C for the Internal Nappe Zone and ~0.5 GPa, ~300°C–350°C for the External Nappe Zone. Although P–T conditions constrained using calculated isopleths from greenschist-facies metapelites and databases optimized for modelling low-grade to medium-grade pelitic systems may be reliable, they should be applied cautiously and compared with estimates from inverse and independent approaches.

Melt migration through Earth's crust drives well-documented melt–rock reactions, locally changing rock assemblage and geochemistry. However, melt–zircon interaction remains understudied. We report on three zircon-melt interaction events from the Pembroke Granulite, New Zealand. Primary zircon from gabbroic gneiss which was subject to minor post-emplacement melt migration and primary zircon from younger dykes exhibit straightforward microstructures, microchemistry, and age data. In contrast, zircon from melt-mediated reaction halos adjacent to the dykes and from melt-fluxed high-strain zones display dissolution modification of grains, micro-porosity and blurred or truncated internal zoning typical of replacement by coupled dissolution-precipitation. Replaced zircon domains show changed rare earth element patterns and redistributed or lost radiogenic Pb that generates ambiguous apparent spot date arrays, smeared over tens of millions of years. We conclude that the metamorphism and three melt–rock interaction events were brief, and the arrays misrepresent the true age and duration of the metamorphism. Pb-loss persisted beyond the metamorphism, with porosity and inclusions formed during coupled dissolution-precipitation making replaced zircon domains more susceptible to subsequent Pb-loss compared to the structurally intact, primary magmatic zircon in the host gabbroic gneiss or dykes. We recommend conducting high-resolution microstructural investigations upon recognition of spot date arrays observed in single samples to rule out the possibility of spurious arrays resulting from coupled dissolution-precipitation.

A quartz-rich paragneiss from the Variscan Erzgebirge Crystalline Complex (ECC) was studied in detail because of abundant millimetre-sized and clearly oriented pseudomorphs after a sodic mineral interpreted to have been jadeite. This mineral, or pseudomorphs after it, is rarely found in extensive high-pressure (HP)–ultrahigh-pressure (UHP) terranes worldwide despite reported pressure–temperature (P–T) conditions suitable for the formation of jadeite in common paragneisses and orthogneisses. In the studied rock, which contains abundant large and oriented potassic white mica flakes and minor millimetre-sized garnet grains, the pseudomorphs consist of clusters of small albite grains with thin phengitic muscovite flakes in between. X-ray maps for Ca and Mg in garnet demonstrate that an early generation of this mineral (Gt1) was corroded and subsequently overgrown by a Ca-richer generation (Gt2). White mica is phengite with maximum Si contents of 3.42 atoms per formula unit. P–T conditions of 0.85 GPa and 650°C and 1.7 GPa and 660°C were derived for the formation of Gt1 and Gt2 rim + Si-rich phengite, respectively, using pseudosection modelling. The latter conditions representing the pressure peak experienced by the paragneiss are compatible with the original presence of jadeite and possibly paragonite as well. This metamorphic peak occurred at 338.4 ± 2.3 (2σ) Ma based on in situ dating of monazite grains with the electron microprobe. A single monazite age of 386.4 ± 10.5 (2σ) Ma is related to the formation of Gt1. Thus, a Late Devonian metamorphism is suggested here for the first time to have occurred in ECC gneisses before the major HP event in the Early Carboniferous. Furthermore, the study demonstrates that the eclogite-facies gneisses of the Gneiss-Eclogite Unit of the ECC experienced peak pressures of not more than 2 GPa in contrast to recent proposals of an extensive UHP area in this unit. In addition, it is suggested that the localized occurrence of UHP rocks surrounded by other lithologies otherwise lacking evidence for UHP conditions should be interpreted with caution with respect to their regional extent and significance.