Abstract The extent to which solid‐state volume diffusion modifies rare earth element (REE) abundances in accessory minerals during high‐temperature metamorphism governs our ability to link recorded trace‐element compositions to particular thermal events. We model diffusion of REE in zircon under different temperature–time conditions and show that, for both short‐lived (e.g. 1100°C for 1–5 Ma) and more prolonged (e.g. 1050°C for 10–30 Ma or 1000°C for 200 Ma) episodes of ultra‐high‐temperature (UHT) metamorphism, REE diffusion in igneous zircon is sufficiently rapid for REE in a ~50‐μm grain to equilibrate with the new metamorphic mineral assemblage of the host rock. By contrast, unless diffusion is accelerated by recrystallization, the presence of fluids or other processes at temperatures below 900°C zircon will largely retain its original pre‐metamorphic REE abundance pattern, even when the thermal event is long lived (≥100 Ma). Where volume diffusion is dominant, for instance, in the absence of a fluid phase, the sensitivity of REE mobility to temperature can help constrain the temperature–time path of high‐grade metamorphic rocks. Modelling of well‐characterized natural samples from the regional‐scale aureole surrounding the Rogaland Igneous Complex (RIC) in SW Norway shows that variations in REE concentration patterns in zircon indicate a T–t evolution that is consistent with independent P–T–t estimates for regional metamorphism based on phase equilibrium modelling (850–950°C at 7–8 kbar for ~100 Ma). Greater modification of REE abundance patterns in zircons within 2 km of the RIC contact, however, indicates that UHT conditions persisted for ~150 Ma close to the intrusion, with a temperature of ~1100°C for 1–5 Ma at the RIC contact. Thermal modelling suggests that the inferred T–t histories of samples from different distances from the RIC contact are best explained if the complex was emplaced incrementally over 1–5 Ma.
Oxide mineral phases within high-grade metamorphic rocks are often largely ignored compared to silicate minerals, except for when constraining the redox state of a sample. It is becoming increasingly apparent that unusual concentrations of oxide phases (e.g. magnetite, ilmenite and spinel) are more common in granulite facies metamorphic rocks that previously thought. However, the mechanism of their formation remains poorly constrained. For example, it is currently unclear what process or combination of processes result in high (over 50% oxide concentration in a sample in some cases) concentrations. There is an ongoing debate if a single process can be applied across all protoliths, with the goal that these assemblages could be used to pinpoint particular crustal process(es). A number of mechanisms have been suggested to form such extreme concentrations of oxides within metamorphic rocks. These include melt fluxing in a deformation zone (Ghatak et al., 2022), partial melt loss (Morrissey et al., 2016), deformation related metamorphic reactions and protolith composition or a combination thereof. Within a collection of high grade metapelites from Rogaland, SW Norway, we see variations in mineralogy, including changes in orthopyroxene and cordierite content with oxide concentrations, variations in grain size, variable layering as well as variable signature of the amount of deformation. Using a combination of microstructures, EBSD, EDS, XCT and other data we will assess and illustrate the processes behind the generation of high oxide concentrations within metapelites and what this could mean for crustal processes during high-grade metamorphism. Ghatak, H., Gardner, R. L., Daczko, N. R., Piazolo, S., & Milan, L. (2022). Oxide enrichment by syntectonic melt-rock interaction. Lithos, 414–415, 106617. https://doi.org/10.1016/J.LITHOS.2022.106617Morrissey, L. J., Hand, M., Lane, K., Kelsey, D. E., & Dutch, R. A. (2016). Upgrading iron-ore deposits by melt loss during granulite facies metamorphism. Ore Geology Reviews, 74, 101–121. https://doi.org/http://doi.org/10.1016/j.oregeorev.2015.11.012
Abstract The metamorphic conditions of the Natal Metamorphic Province (NMP) have been the focus of previous studies to assist with Rodinia reconstructions but there are limited constraints on the age of metamorphism. We use a combination of modern techniques to provide new constraints on the conditions and timing of metamorphism in the two southernmost terranes: the Mzumbe and Margate. Metamorphism reached granulite facies, 780–834°C at 3.9–7.8 kbar in the Mzumbe Terrane and 850–892°C at 5.7–6.1 kbar in the Margate Terrane. The new pressure and temperature constraints are supportive of isobaric cooling in the Margate Terrane as previously proposed. Peak metamorphism of the two terranes is shown to have occurred c. 40 myr apart, which contrasts strongly with previous assumptions of coeval metamorphism. While the age of peak metamorphism of the Margate Terrane (1032.7 ± 4.7 Ma) coincides with the tectonism and magmatism associated with the emplacement of the Oribi Gorge Suite ( c. 1050–1030 Ma), the age of metamorphism of the Mzumbe Terrane (987.4 ± 8.1 Ma) occurs c. 30–40 myr after tectonism is previously thought to have finished. We propose that models of advective cooling during transcurrent shearing can explain the metamorphic conditions and timing of the NMP.
Abstract The final assembly of the Mesoproterozoic supercontinent Nuna was marked by the collision of Laurentia and Australia at 1.60 Ga, which is recorded in the Georgetown Inlier of NE Australia. Here, we decipher the metamorphic evolution of this final Nuna collisional event using petrostructural analysis, major and trace element compositions of key minerals, thermodynamic modelling, and multi‐method geochronology. The Georgetown Inlier is characterised by deformed and metamorphosed 1.70–1.62 Ga sedimentary and mafic rocks, which were intruded by c . 1.56 Ga old S‐type granites. Garnet Lu–Hf and monazite U–Pb isotopic analyses distinguish two major metamorphic events (M1 at c . 1.60 Ga and M2 at c . 1.55 Ga), which allows at least two composite fabrics to be identified at the regional scale— c . 1.60 Ga S1 (consisting in fabrics S1a and S1b) and c . 1.55 Ga S2 (including fabrics S2a and S2b). Also, three tectono‐metamorphic domains are distinguished: (a) the western domain, with S1 defined by low‐ P ( LP ) greenschist facies assemblages; (b) the central domain, where S1 fabric is preserved as medium‐ P ( MP ) amphibolite facies relicts, and locally as inclusion trails in garnet wrapped by the regionally dominant low‐ P amphibolite facies S2 fabric; and (c) the eastern domain dominated by upper amphibolite to granulite facies S2 foliation. In the central domain, 1.60 Ga MP– medium ‐T (MT) metamorphism (M1) developed within the staurolite–garnet stability field, with conditions ranging from 530 – 550°C at 6 – 7 kbar (garnet cores) to 620 – 650°C at 8 – 9 kbar (garnet rims), and it is associated with S1 fabric. The onset of 1.55 Ga LP– high‐ T ( HT ) metamorphism (M2) is marked by replacement of staurolite by andalusite (M2a/D2a), which was subsequently pseudomorphed by sillimanite (M2b/D2b) where granite and migmatite are abundant. P–T conditions ranged from 600 to 680°C and 4–6 kbar for the M2b sillimanite stage. 1.60 Ga garnet relicts within the S2 foliation highlight the progressive obliteration of the S1 fabric by regional S2 in the central zone during peak M2 metamorphism. In the eastern migmatitic complex, partial melting of paragneiss and amphibolite occurred syn‐ to post ‐ S2, at 730–770°C and 6–8 kbar, and at 750–790°C and 6 kbar, respectively. The pressure–temperature–deformation–time paths reconstructed for the Georgetown Inlier suggest a c . 1.60 Ga M1/D1 event recorded under greenschist facies conditions in the western domain and under medium‐ P and medium‐ T conditions in the central domain. This event was followed by the regional 1.56–1.54 Ga low‐ P and high‐ T phase (M2/D2), extensively recorded in the central and eastern domains. Decompression between these two metamorphic events is ascribed to an episode of exhumation. The two‐stage evolution supports the previous hypothesis that the Georgetown Inlier preserves continental collisional and subsequent thermal perturbation associated with granite emplacement.
Abstract Here, we present results of the first 40 Ar/ 39 Ar dating of osumilite, a high‐ T mineral that occurs in some volcanic and high‐grade metamorphic rocks. The metamorphic osumilite studied here is from a metapelitic rock within the Rogaland–Vest Agder Sector, Norway, an area that experienced regional granulite facies metamorphism and subsequent contact metamorphism between 1,100 Ma and 850 Ma. The large grain size (~1 cm) of osumilite in the studied rock, which preserves a nominally anhydrous assemblage, increases the potential for large portions of individual grains to have remained essentially unaffected by the effects of diffusive argon loss, potentially preserving prograde ages. Step‐heating diffusion experiments yielded a maximum activation energy of ~461 kJ/mol and a pre‐exponential factor of ~8.34 × 10 8 cm 2 /s for Ar diffusion in osumilite. These parameters correspond to a relatively high closure temperature of ~620°C for a cooling rate of 10°C/Ma in an osumilite crystal with a 175 µm radius. Fragments of osumilite separated from the sample preserve a range of ages between c . 1,070 and 860 Ma. The oldest ages are inferred to date the growth of coarse‐grained osumilite during prograde granulite facies regional metamorphism, which pre‐date contact metamorphism that has historically been ascribed to the growth of osumilite in the region. The majority of fragments record ages between c . 920 and 860 Ma, inferred to reflect the growth of osumilite and/or diffusive argon loss during contact metamorphism. The retention of old 40 Ar/ 39 Ar dates was facilitated by the low diffusivity of Ar in osumilite (i.e. a closed system), large grain sizes, and anhydrous metamorphic conditions. The ability to date osumilite with the 40 Ar/ 39 Ar method provides a valuable new thermochronometer that may constrain the timing and duration of high‐ T magmatic and metamorphic events.
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