Journal of Metamorphic Geology最新文献

Fluid–rock interactions play a key role in the formation, evolution and recycling of the Earth's crust. For fluids to infiltrate rocks and enable and sustain fluid-mediated mineral transformations, fluid pathways are required. In this study, we examined the potential mechanisms of formation of such pathways via detailed mineralogical, petrophysical and thermodynamic analysis of a dry, essentially ‘non-porous’ gabbro that was hydrated and transformed into an amphibolite under amphibolite-facies conditions. During a previous regional HP eclogite-facies metamorphism, the gabbro did not equilibrate and preserved almost entirely its igneous textures and magmatic minerals. Rock transformation during amphibolitization was triggered by fluid infiltration through a newly opened N–S striking fracture network. An equally spaced fracture network formed by mode I opening related to the formation of an E–W striking shear zone at the northern and southern borders of the gabbro body. The amphibolitization process allowed the fluid to pervasively infiltrate the rock from the fracture into the pristine gabbro. The essentially fully amphibolitized sample exhibits some unaffected gabbroic mineral relicts. Even though the amphibolitization process led to the formation of ~70 vol.% hydrous phases, it was accompanied by densification and related porosity formation. The modes and compositions of minerals within partly amphibolitized rocks indicate that besides the uptake of H₂O, no significant mass exchanges were necessary for this transformation, at least on the thin section scale. Thermodynamic modelling and petrological data show that the transition from gabbro to amphibolite favours porosity formation. In the model, the reaction front proceeded as soon as the gabbro at the reactive interfaces of the affected minerals was sufficiently transformed. At this point, fluid was not consumed further but remained as a free fluid phase, which progressed through the newly formed pore space and advanced amphibolitization. Once the gabbro was almost entirely amphibolitized, its mineral content and mineral chemistry no longer changed, so the progress of amphibolitization progress was controlled by fluid availability. This case study shows that fluid–rock interaction leading to hydration of a rock can be efficiently maintained in almost non-permeable, dry and mafic crust and, therefore, strongly affects the petrophysical properties of the Earth's crust.

The southern Dora-Maira Massif, where coesite was first discovered 40 years ago, is among the most studied and better known example of high/ultra-high-pressure (HP/UHP) terranes. Previous to this study, the Polymetamorphic Basement Complex of the southern Dora-Maira Massif has been defined as a nappe stack consisting of three juxtaposed tectono-metamorphic units: the HP San Chiaffredo Unit at the bottom, the UHP Brossasco-Isasca Unit in the middle and the HP Rocca Solei Unit at the top. The origin of UHP metamorphism in the Brossasco-Isasca Unit is still controversial, due to the difficulties in reconciling the abrupt difference between the UHP conditions recorded by the Brossasco-Isasca Unit (i.e., 700°C–730°C, 4.0–4.3 GPa) and the HP conditions (i.e., ~500°C–520°C, 2.0–2.2 GPa) registered by the adjacent units. Here, we report new petrologic and microstructural evidence supporting the existence of a previously unrecognised UHP unit in the southern Dora-Maira Massif. Our data demonstrate that the tectonic unit overlying the Brossasco-Isasca Unit (i.e., the former Rocca Solei Unit), so far considered a HP unit, is actually divided in two units, one of which (the lowermost Rocca Solei Unit sensu stricto) experienced UHP conditions and the other (the uppermost Grimbassa Unit) reached HP conditions. The newly defined Rocca Solei Unit experienced UHP metamorphism at significantly different P–T conditions (520°C–550°C, 2.7–2.9 GPa) compared to the underlying Brossasco-Isasca Unit, but along a similar ‘cold’ T/P ratio (< 200°C/GPa), markedly lower than that defined in the neighbouring Grimbassa Unit and San Chiaffredo Unit (> 230°C/GPa). After more than 30 years of petrologic investigations, the tectono-metamorphic architecture of the southern Dora-Maira Massif is thus redefined, bridging the gap between the UHP Brossasco-Isasca Unit and the adjacent HP units and opening to new scenarios on its HP–UHP architecture. The results of this study have both regional and petrologic implications: (i) Similarities emerge in the structural position, thickness and metamorphic evolution of the new UHP Rocca Solei Unit in the southern Dora-Maira Massif and those of the Chasterain Unit recently discovered in the northern Dora-Maira Massif, suggesting a common architecture throughout the whole Dora-Maira Massif; (ii) the peculiar quartz microstructure in the metagranites described below represents an exceptional documentation of a ‘frozen’ quartz-to-coesite polymorphic reaction caught in the act and suggests that the availability of fluids was the most crucial factor controlling the progress of the reaction. The metastable persistence of quartz in H₂O-undersaturated lithologies makes even more challenging the identification of UHP units that have only slightly exceeded the quartz–coesite transition and justifies why the newly defined UHP Rocca Solei Unit has remained ‘hidden’ for more than 30 years.

The reactive flow of melt through the mantle or crust triggers chemical disequilibrium, driving reactions that significantly alter the mineral assemblages and physical properties of host rocks. However, the degrees of chemical difference required to initiate these reactions and their timescale remain poorly understood. In this study, we present piston–cylinder reaction experiments simulating lower crustal conditions, where largely anhydrous lower crustal granoblastic dioritic gneiss interacts with a hydrous mafic melt, created from the same gneiss but modified by the addition of ~6-wt.% H₂O. Remarkably, reactions occurred within just 12 h, producing microstructures that closely resemble those observed in natural, melt-fluxed rocks from the lower arc crust in Fiordland, New Zealand. Melt–rock interactions led to the formation of epitaxial, multilayer symplectic coronae of pargasite + plagioclase or quartz partially replacing pre-existing pyroxene grains. The protolith plagioclase and amphibole are either completely dissolved into the melt or replaced by a modified composition of the same mineral. The melt exhibits compositional variations that correlate with distance from the melt–rock reaction front. Quenched melt chemistry data demonstrate the potential for melt compositions to continuously evolve in response to both crystallisation and melt–rock interactions during reactive flow. Importantly, our findings reveal that melt–rock reactions, initiated by melt not drastically different from the solid rock (protolith), can induce significant changes in rock composition and thus physical properties in a short time. Our findings have broad implications for understanding the compositional evolution of migrating melts and the chemical and mechanical evolution of the Earth's mantle and lower crust in general.

Trace element zoning in kyanite can retain information about its growth history, particularly in anatectic metapelites. There, kyanite can grow (i) at sub-solidus conditions through metamorphic reactions involving other aluminous phases as reactants, (ii) through muscovite dehydration melting reactions, and (iii) during cooling and melt crystallisation either through back-reactions between melt and solid phases (e.g., garnet) or crystallising directly from the melt. Thermodynamic modelling successfully reproduces these reactions, allowing a more robust interpretation of the observed features based on predicted reactants and products. In this study, we interpret the kyanite trace element zoning (particularly of Cr, V, and partly of Fe) observed through cathodoluminescence and quantified through LA-ICP-MS maps, using the forward thermodynamic modelling approach. The studied samples are biotite + kyanite + garnet migmatites from the Lower-Greater Himalayan Sequence of eastern Nepal, which experienced muscovite and incipient biotite dehydration melting. Three main generations of kyanite revealed by trace element zoning have been identified (i.e., Ky1, Ky2, and Ky3), consistent with the three main kyanite-producing reactions predicted by forward thermodynamic modelling, also applying a melt reintegration approach. Ky1 (i.e., sub-solidus kyanite) integrated only minimum amounts of Cr, V and Fe. Ky2 (i.e., peritectic kyanite) incorporates Cr and V released from muscovite during its dehydration melting reaction. Ky3 (i.e., back-reaction overgrowth or magmatic kyanite) is particularly developed in samples where melt segregation has been absent or limited and incorporates lower amounts of Cr and V than Ky2, but is enriched in Fe. The major implications of this study concern the interpretation of the melt segregation processes in anatectic rocks and our understanding of the Cr and V partitioning between minerals and melt. Further methodological considerations are also provided, which could help guide similar studies in the future.

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.