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Constraining the pressure–temperature evolution and geodynamic setting of UHT granulites and migmatitic paragneisses of the Gruf Complex, Central Alps

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Abstract

Thermodynamic modeling of compositionally mapped microdomains and whole-rock compositions is used to constrain the pressure–temperature (P–T) evolution of sapphirine granulites and migmatitic paragneisses from the Gruf Complex of the Central Alps. The P–T paths and conditions estimated from granulite microdomains and whole-rock compositions are consistent with one another, indicating that the estimates from both types of compositions are accurate. The sapphirine granulites were heated to ultra-high temperature conditions of 900–1000 °C and 7.0–9.5 kbar as they were decompressed from ca. 800 °C and 9–12 kbar, resulting in garnet breakdown. In a subsequent step, nearly isothermal decompression led to the development of cordierite-bearing coronae and symplectites. By ca. 27 Ma, the sapphirine granulites had been exhumed to the midcrustal level of the migmatitic paragneisses, which were undergoing peak metamorphism at ca. 675–750 °C and 5–7 kbar. These results are consistent with a geodynamic model that invokes heat advection to the lower crust closely following the continental-subduction (ultra-high pressure) stage of the Alpine orogeny. The most plausible geodynamic model consistent with the results of this study is breakoff of a southward subducting lithospheric slab, resulting in asthenospheric mantle flow.

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Fig. 1

(modified from Bousquet et al. 2012b). Ages from Bergell and Novate intrusions are from von Blanckenburg (1992) and Liati et al. (2000), respectively

Fig. 2

(modified from Galli 2010). Dashed lines show diffuse contacts

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Acknowledgements

This research is supported by the American National Science Foundation under Grant no. EAR 0911633 to A. Möller. We would like to thank (A) Galli for guiding us in the field area, the Biavaschi family at Rifugio Brasca for their hospitality and logistical help, P.G. Lippert for field assistance, and C. Fischer (University of Potsdam) and W.C. Dickerson (University of Kansas) for preparing thin sections. P. Appel and (B) Mader (University of Kiel) and (C) Gunther and M. Konrad-Schmolke (University of Potsdam) assisted with the EPMA work. We are also grateful to A. Galli, V. Guevara, M. Caddick, and several others for fruitful discussions about the Gruf Complex. Sebastian Weber, Niko Froitzheim, and an anonymous colleague are thanked for their thoughtful and critical reviews.

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  1. Jeffrey Oalmann

    Present address: Earth Observatory of Singapore, Nanyang Technological University, 50 Nanyang Avenue, Block N2-01B-30, Singapore, 639798, Singapore

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  1. Department of Geology, The University of Kansas, 1475 Jayhawk Blvd, Lawrence, KS, 66045, USA

    Jeffrey Oalmann & Andreas Möller

  2. Institute of Geosciences, Christian-Albrechts-Universität zu Kiel, Ludewig-Meyn-Straße 10, 24098, Kiel, Germany

    Erik Duesterhoeft & Romain Bousquet

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Supplementary material 3 (XLSX 34 KB)

531_2019_1686_MOESM4_ESM.eps

Online Resource S4: The H2O content of samples and reaction textures were estimated using temperature versus mole fraction hydrogen (MH) diagrams. Pressure was fixed at 9 kbar for the granulite samples and textures therein (Fig. A1a–d) and 6 kbar for the paragneisses (Fig. A1e–f) based on the previously published P–T estimates of Galli et al. (2011) and Guevara and Caddick (2016). For all samples, MH values ranging from 0 (dry conditions) to 20 (H2O-saturated conditions) were used in the binary plots.

For the garnet breakdown texture from leucogranulite GR11–39 (Fig. A1a), the observed peak assemblage consists of plagioclase + garnet + sapphirine + orthopyroxene + spinel + melt ± K-feldspar (red shaded fields in Fig. A1a). The presence of coexisting plagioclase, sapphirine, orthopyroxene, and spinel in the garnet-breakdown texture limits MH to less than 16. It is unclear whether K-feldspar was stable as a solid phase at peak conditions. Therefore, we used an MH value of 8 to calculate the P–T diagram (Fig. 8a).

For the residual granulite Gruf100 (Fig. A1b), the observed peak assemblage (red-shaded field in Fig. A1b) comprises plagioclase + garnet + orthopyroxene + sapphirine + sillimanite + rutile + melt. The sillimanite-out reaction (purple line in Fig. A1b) is nearly vertical in the calculated diagram, indicating that the presence or absence of sillimanite is strongly affected by H2O content for this bulk composition. In this sample, the coexistence of sapphirine and sillimanite restricts MH to less than 11 (Fig. A1b). Therefore, we used an MH value of 4, which crosses the field of the observed peak assemblage, to calculate the P–T diagram for this sample. The field of the observed peak assemblage (garnet + orthopyroxene + sapphirine + biotite + melt) in residual granulite Br03-56-2 is stable over a wide range of MH values, ranging from 7 to 19 (Fig. A1c). The minerals that were stable at peak conditions (garnet, sapphirine, orthopyroxene) do not tightly constrain the H2O content. Therefore, we chose an MH value of 10, which intersects near the center of field of the observed peak assemblage (Fig. A1c).

The observed assemblage in the monazite bearing microdomain (GRM-37) from residual granulite sample Br03-56-2 is stable over a narrow range of MH values, ranging from 3 to 5 (Fig. A1d). Assuming that neither free H2O nor melt was present at peak conditions in this microdomain, we used an MH value of 5 to calculate the P–T diagram (Fig. 8d).

The stabilities of quartz and feldspars are not used to estimate the P–T conditions experienced by migmatitic paragneiss samples (see “Accuracy of P–T Estimates” above) and, thus, are not used to constrain the H2O contents of these samples. Therefore, the only constraint for H2O content in the muscovite-bearing paragneiss sample GR11–23 is the presence of muscovite and melt, which coexist at MH values ranging from 9 to > 20 (Fig. A1e). The presence of sillimanite and melt and absence of cordierite at peak conditions in the muscovite-free paragneiss sample GR11-25 indicate that MH was > 5.5 at peak conditions (Fig. A1f). We used an MH value of 15 to calculate the P–T diagrams (Fig. 9) for both paragneiss samples because this value intersects the field of the observed peak assemblage in sample GR11–23 and intersects fields that contain and lack free H2O for sample GR11–25 (red shaded fields in Fig. A1f).

Fig. A1 Temperature versus mole fraction hydrogen (MH = 0.5 × MH2O) phase diagrams calculated at 9 kbar pressure for granulite microdomain and whole rock samples (a-d) and at 6 kbar pressure for migmatitic paragneiss samples (e–f). The red-shaded fields correspond to the observed peak assemblages, and the thick, dashed lines correspond to the MH values used to compute the P–T diagrams (Figs. 8, 9) (EPS 1932 KB)

531_2019_1686_MOESM5_ESM.pdf

Overview X-ray composition maps of the modeled microdomains of residual granulite sample GRM-37 and leucogranulite sample GR11-39 (PDF 2317 KB)

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Oalmann, J., Duesterhoeft, E., Möller, A. et al. Constraining the pressure–temperature evolution and geodynamic setting of UHT granulites and migmatitic paragneisses of the Gruf Complex, Central Alps. Int J Earth Sci (Geol Rundsch) 108, 911–930 (2019). https://doi.org/10.1007/s00531-019-01686-x

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