Three generations of monazite in Austroalpine basement rocks to the south of the Tauern Window: evidence for Variscan, Permian and Eo-Alpine metamorphic events

Abstract

Three monazite generations were observed in garnet-bearing micaschists from the Schobergruppe in the basement to the south of the Tauern Window, Eastern Alps. Low-Y monazite of Variscan age (321 ± 14 Ma) and high-Y monazite of Permian age (261 ± 18 Ma) are abundant in the mica-rich rock matrix and in the outer domains of large garnet crystals. Pre-Alpine monazite commonly occurs as polyphase grains with low-Y Variscan cores and high-Y Permian rims. Monazite of Eo-Alpine age (112 ± 22 Ma) is rarer and was observed as small, partly Y-enriched grains (3 wt. % Y2O3) in the rock matrix and within garnet. Based on monazite-xenotime thermometry, Y + HREE values in monazite indicate minimum crystallization conditions of 500 °C during the Variscan and 650 °C for the Permian and Alpine events, respectively. Garnet zoning and thermobarometric calculations with THERMOCALC 3.21 record an amphibolite facies, high-pressure stage of ~600 °C/13–16 kbar, followed by a thermal maximum at 650–700 °C and 6–9 kbar. The Eo-Alpine age for these two events is supported by inclusions of Cretaceous monazite in the garnet domains used for thermobarometric constraints and through the high growth temperatures of Eo-Alpine monazite, which is consistent with that of the thermal maximum (~700 °C). The age and growth conditions of a few Mn-rich garnet cores, sporadically present within Eo-Alpine garnet, are unclear because inclusions of monazite, plagioclase and biotite necessary for thermobarometric- and age constraints are absent. However, based on monazite thermometry, Permian and Variscan metamorphic conditions were high enough for the growth of pre-Alpine garnet. The formation of Variscan garnet and its later resorption, plus Y-release, would also explain the high Y in Permian monazite, which cannot originate from preexisting Variscan monazite only. Monazite of Variscan, Permian and/or Eo-Alpine ages were also observed in other garnet-bearing micaschists from the Schobergruppe. This suggests that the basement of the Schobergruppe was overprinted by three discrete metamorphic events at conditions of at least lower amphibolite facies. While the Variscan event affected all parts of this basement, the younger events are more pronounced in its structurally lower units.