EXHUMATION OF LOWER CRUSTAL UHT ROCKS INTO THE UPPER AMPHIBOLITE FACIES MIDDLE CRUST: THE GRUF COMPLEX, EUROPEAN CENTRAL ALPS

Understanding the processes by which lower crustal rocks are exhumed is useful for constraining the thermal and geodynamic evolution of orogens. The Gruf Complex consists predominately of upper amphibolite facies, migmatitic biotite orthogneisses and garnet + silllimanite + biotite paragneisses. Ultra-high temperature (UHT) charnockites and sapphirine-bearing granulites, which occur as elongate m–km scale bodies, are juxtaposed against the migmatites along mylonitic shear zones.

Thermodynamic modeling of whole rocks and microdomains indicates that the paragneisses equilibrated at 650–750°C and ≤7.5 kbar; garnet-replacing, sapphirine + orthopyroxene-bearing symplectites in the granulites formed during decompression at 900–960°C and 8–10 kbar (in agreement with Guevarra et al. 2016 JMG but in contrast to Galli et al. 2011 Lithos). Zr-in-rutile temperatures are strongly coupled with textures and record prograde (500–890°C), peak (840–1000°C), and retrograde (650–850°C) conditions. In the UHT granulites, zircon rims, which overgrow resorbed Permian and Jurassic cores, have a weighted mean ²⁰⁶Pb/²³⁸U age of 32.9 ± 0.5 Ma, but Ti-in-zircon temperatures of any domains do not exceed 800°C. We interpret this to indicate that UHT metamorphism resulted in partial dissolution of inherited zircon and occurred slightly prior to 33 Ma, at which time melt crystallized. Zircon U-Pb ages of felsic dikes, which are deformed within the mylonite zones, constrain the timing of juxtaposition of the UHT and amphibolite facies units to ≤30 Ma. Rutile U-Pb ages in the UHT rocks range from 30 to 19 Ma, indicating that these rocks resided at 650–400°C for ca. 11 m.y.

We interpret that UHT metamorphism occurred between 34 and 33 Ma, when asthenospheric upwelling, resulting from slab breakoff or rollback, provided heat to the lower crust (von Blanckenburg & Davies, 1995 Tectonics; Beltrando et al. 2010 ESR). The lower crustal granulites and charnockites were then exhumed into the middle crust and juxtaposed against the lower-grade migmatites along ductile shear zones by ca. 30 Ma. The amalgamated Gruf Complex subsequently cooled at a rate of ca. 25°C/m.y. until 19 Ma. The Gruf Complex thus provides evidence for mantle heat transfer into the lower crust and tectonic mass transfer into the middle crust within a collisional setting.