Crustal-mantle lithosphere decoupling as a control on Variscan metamorphism in the Eastern Alps

Mica schists of the Variscan Austroalpine Otztal basement (Eastern Alps) co ntain synkinematically grown garnet porphyroblasts showing three zones with complex inclusion trails. Staurolite, kyanite and sillimanite grew over th e main schistosity that envelops the garnet porphyroblasts. The mineral-for ming reaction sequences modelled in the MnKFMASH and KFMASH systems show th at garnet of zone 1 grew at approximately 1300 MPa. whereas zone 2 and zone 3 grew during substantial decompression and partial thermal relaxation. Fo r the growth of staurolite. kyanite and sillimanite, this modelling shows t hat thermal relaxation continued after decompression under only slightly ch anging pressure conditions, until the peak temperature conditions of Varisc an metamorphism were reached, The result of this modelling is a pressure-te mperature-time (P-T-I) heating path that is consistent with a tectonic inte rpretation implying thickening of the lithosphere during continental collis ion and subsequent mantle-crustal lithosphere decoupling plus extension. A direct implication of this model is that the heat budget of Variscan metamo rphism was controlled by the convective replacement of the thickened lithos pheric root with asthenospheric material.