Journal of Metamorphic Geology最新文献

Integrated field mapping, phase equilibria modelling and in situ U–Pb monazite geochronology from the northern margin of the Rae craton on Baffin Island document three metamorphic events during the Neoarchean to the middle Paleoproterozoic. The Qimivvik area comprises Neoarchean tonalitic gneiss structurally juxtaposed over Neoarchean metasedimentary rocks along the Paleoproterozoic Qimivvik thrust and associated shear zone. High-grade metamorphism at ca. 2.56–2.50 Ga supports a footprint for cryptic late Neoarchean metamorphism over a distance of ∼600 km along the northwestern Rae margin from southern Boothia Peninsula to northern Baffin Island. Thermal peak mineral assemblages in the Qimivvik area equilibrated at ca. 1.9 Ga at conditions of ~710°C–790°C and 4.3–5.5 kbar. The dominant Paleoproterozoic foliation is defined by peak metamorphic phases and is reoriented by folds related to the Qimivvik thrust. Peak metamorphism and associated deformation, including the Qimivvik thrust, are interpreted as a manifestation of the Ellesmere-Inglefield belt of Ellesmere Island and West Greenland, which links with the ca. 1.9 Ga Thelon orogen of western Canada. Partial melting also occurred at ca. 1.8 Ga, possibly resulting from decompression of the Churchill domain following the collisional-accretionary events related to the late stages of amalgamation of Laurentia and supercontinent Nuna. Quantitative trace element maps (acquired using LA-ICP-MS) of monazite reveal distinct trace element signatures associated with each of three growth stages. Ca. 2.5 Ga monazite exhibits complex intragrain compositional zoning, has elevated Y and heavy rare earth elements (HREEs) relative to ca. 1.9 Ga monazite and has higher Th/U overall than both ca. 1.9 Ga and ca. 1.8 Ga monazite. These signatures suggest that ca. 2.5 Ga monazite growth was concomitant with partial melting and preceded the majority of garnet growth. The ca. 1.9 Ga monazite grains are comparatively less zoned and have lower Y + HREE contents than both ca. 2.5 Ga and 1.8 Ga monazite, consistent with the ca. 1.9 Ga monazite forming after most garnet growth. Elevated Y + HREE in the ca. 1.8 Ga monazite imply that it formed after retrograde resorption of garnet rims. In our samples, Y + HREE generally exhibit stronger correlations with monazite age and/or petrographic context than Eu/Eu* and Th/U. As some compositional overlap exists between monazite of different ages and petrographic contexts, quantitative limits (‘cut-offs’) based on trace element concentrations or ratios (e.g., Th/U, Eu/Eu*, La_CN/Yb_CN) are unreliable for distinguishing between monazite populations. In addition to providing important constraints on the early tectonic evolution of northeastern Laurentia, our study offers new insights into trace element behaviour in a key accessory mineral during three metamorphic events occurring over a ~700 Ma time period.

Exhumed high-P/low-T complexes are of paramount importance to directly access rocks that experienced subduction zone processes. However, the original tectono-metamorphic fabrics are often partially obliterated by exhumation and later deformation, hindering our understanding of the processes occurring at depth. We show an example of how multiple field- and lab-based analytical techniques may be used to extract information of the pristine fabrics of polydeformed metamorphic rocks. We investigated a natural cross section through the chlorite, garnet and albite-biotite zones of the exhumed Shirataki Unit in the Sanbagawa metamorphic belt, exposed in the Sarutagawa (Saruta River) area of the Central Shikoku, coupling structural-petrographic analysis with Raman Spectroscopy on Carbonaceous Material (RSCM) thermometry, phase equilibrium modelling and U–Pb zircon dating. RSCM thermometry reveals a progressive temperature increase from 350°C–400°C to 500°C–550°C over an ~400 m distance, characterized by condensed metamorphic isograds in the garnet zone. Phase equilibrium modelling indicates slightly decreasing metamorphic pressures through the transect from 0.6–0.9 GPa at low-T to 0.4–0.7 GPa at high-T with preserved blueschist-facies parageneses, documented for the first time in the area, restricted to the ~400°C–450°C range. Hence, rocks developed close to the Sanbagawa subduction gradient are juxtaposed with rocks that experienced significant retrograde heating during exhumation. Moreover, we found that competent lithologies such as quartzite and basic schist along the transect preserve trenchward-directed deformation structures that are obliterated by orogen-parallel stretching in the surrounding, incompetent pelitic schists. U–Pb dating shows progressively older youngest detrital zircon ages and syn-depositional peaks from 79–76 and ~88–80 Ma in the chlorite zone to ~92 and ~100 Ma in the garnet zone, respectively, indicating that the lower grade units were subducted at a later stage and that, hence, different metamorphic grades in the area correspond to different protolith ages. The data discussed above is consistent with the former presence of a regional shear zone that, although partially obliterated by younger deformation, contributed to the exhumation of higher-T rocks of the albite-biotite and oligoclase-biotite zones over subducting rocks of the chlorite and garnet zones, likely exploiting the subduction interface. These results offer a framework to investigate the geological record of subduction in the polyphase metamorphic rocks of the Sanbagawa belt.

Uncertainty surrounds the cause and interpretation of anticlockwise pressure-temperature ($�P$–$�T$) paths in metamorphic terranes. Here, we focus on the viability of a commonly proposed mechanism—magmatic heat transfer during thickening—using a case study of the Roineabhal terrane in Scotland, where such an anticlockwise $�P$–$�T$ path has been proposed. Phase equilibria modelling of new samples, combined with previous $�P$–$�T$ estimates, provides evidence for regional kyanite-grade granulite-facies metamorphism, with an additional spatially localised region of ultrahigh temperature conditions adjacent to an anorthosite intrusion. The spatial geometry of the ultrahigh temperature samples, combined with scaling arguments and thermal modelling of these results, shows that the ultrahigh temperature metamorphism is contact in nature and should not be joined to the regional metamorphism to infer an anticlockwise $�P$–$�T$ path. Rather, the regional metamorphism features hairpin $�P$–$�T$ loops, overlain adjacent to the anorthosite by a short-lived, high-temperature excursion. Because metamorphic rocks typically yield a fragmentary record during fluctuating thermal conditions, due to requiring hydration to maintain equilibrium during down-temperature evolution, it is critical to assess in this manner the thermal viability of the range of $�P$–$�T$ paths that could connect the preserved assemblages. In general, intrusion radii of tens of kilometres, or repeated intrusions of smaller bodies in quick succession (e.g., < 10 kyr for a 1-km radius), would be required for true magmatically driven anticlockwise $�P$–$�T$ paths. Given the unlikely nature of these requirements in most tectonic settings, such anticlockwise $�P$–$�T$ paths are likely to be rarer than reported. For many scenarios, $�P$–$�T$ paths commonly interpreted as anticlockwise $�P$–$�T$ paths are instead likely to take the form described in this study.

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.