Prograde garnet growth along complex P–T–t paths: results from numerical experiments on polyphase garnet from the Wölz Complex (Austroalpine basement)

Abstract

Garnet in metapelites from the Wolz Complex of the Austroalpine crystalline basement east of the Tauern Window characteristically consists of two growth phases, which preserve a comprehensive record of the geothermal history during polymetamorphism. From numerical modelling of garnet formation, detailed information on the pressure–temperature–time (P–T–t) evolution during prograde metamorphism is obtained. In that respect, the combined influences of chemical fractionation associated with garnet growth, modification of the original growth zoning through intragranular diffusion and the nucleation history on the chemical zoning of garnet as P and T change during growth are considered. The concentric chemical zoning observed in garnet and the homogenous rock matrix, which is devoid of chemical segregation, render the simulation of garnet growth through successive equilibrium states reliable. Whereas the first growth phase of garnet was formed at isobaric conditions of ∼3.8 kbar at low heating/cooling rates, the second growth phase grew along a Barrovian P–T path marked with a thermal peak of ∼625°C at ∼10 kbar and a maximum in P of ∼10.4 kbar at ∼610°C. For the heating rate during the growth of the second phase of garnet, average rates faster than 50°C Ma−1 are obtained. From geochronological investigations the first growth phase of garnet from the Wolz Complex pertains to the Permian metamorphic event. The second growth phase grew in the course of Eo-Alpine metamorphism during the Cretaceous.