PETROLOGICAL EVIDENCE FOR UHT METAMORPHISM DURING OROGENESIS, GRUF COMPLEX, EUROPEAN CENTRAL ALPS

Formation and preservation of UHT rocks is rare in modern orogenic belts. Secular cooling since Proterozoic time has resulted in lower geothermal gradients, and eclogite facies rocks are more abundant than UHT rocks in modern orogenies (Brown, 2007, IGR). However, UHT rocks are present in some young mountain belts. The Gruf Complex contains UHT sapphirine-bearing granulites, making it one of these enigmatic terranes. Mylonitic contacts separate the scarce granulites and associated charnockites from more abundant migmatitic gneisses, which record upper amphibolite to lower granulite facies conditions. UHT metamorphism occurred either during Permian rifting (Galli et al., 2011, CMP) or during Alpine collision (Liati & Gebauer, 2003, SMPM; Schmitz et al., 2009, EJM). We present results from Zr-in-rutile thermometry and equilibrium assemblage diagrams (EADs) calculated with Theriak-Domino to constrain the P-T evolution of the Gruf Complex.

Rutile grains within peak assemblages record temperatures of 918 ± 77°C. UHT rutile also occurs within sapphirine-bearing symplectites around garnet porphyroblasts. Rutile inclusions in garnet record prograde temperatures of 658 ± 94°C, and rutile within retrogressed domains records temperatures of 746 ± 66°C. The EADs reveal peak conditions for residual granulites of 900–960°C at 8–10 kbar; cordierite coronae were formed during UHT decompression to <8 kbar. Monazite grains within garnet breakdown textures constrain the timing of decompression to ca. 31 Ma (Möller et al., 2012, IGC abstract), coeval with slab breakoff in the Central Alps (von Blanckenburg & Davies, 1995, Tect). Muscovite-bearing and muscovite-free migmatitic paragneisses equilibrated at 650–700°C and 700–750°C, respectively, at <7.5 kbar.

The evidence for UHT decompression and the continuous range of Zr-in-rutile temperatures suggests the granulites were metamorphosed to UHT and lower grade conditions along a single P–T path, likely during Alpine orogenesis based on the monazite ages. Similar temperature estimates from retrogressed granulite domains and paragneisses suggest granulite retrogression and paragneiss migmatization occurred coevally during or after juxtaposition along mylonitic shear zones. Heat for UHT metamorphism possibly came from mantle upwelling after slab breakoff.