Compositional zoning of garnet porphyroblasts from the polymetamorphic Wölz Complex, Eastern Alps
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We employ garnet isopleth thermobarometry to derive the P–T conditions of Permian and Cretaceous metamorphism in the Wölz crystalline Complex of the Eastern Alps. The successive growth increments of two distinct growth zones of the garnet porphyroblasts from the Wölz Complex indicate garnet growth in the temperature interval of 540°C to 560°C at pressures of 400 to 500 MPa during the Permian and temperatures ranging from 550°C to 570°C at pressures in the range of 700 to 800 MPa during the Cretaceous Eo-Alpine event. Based on diffusion modelling of secondary compositional zoning within the outermost portion of the first garnet growth zone constraints on the timing of the Permian and the Eo-Alpine metamorphic events are derived. We infer that the rocks remained in a temperature interval between 570°C and 610°C over about 10 to 20 Ma during the Permian, whereas the high temperature stage of the Eo-Alpine event only lasted for about 0.2 Ma. Although peak metamorphic temperatures never exceeded 620°C, the prolonged thermal annealing during the Permian produced several 100 µm wide alteration halos in the garnet porphyroblasts and partially erased their thermobarometric memory. Short diffusion profiles which evolved around late stage cracks within the first garnet growth zone constrain the crack formation to have occurred during cooling below about 450°C after the Eo-Alpine event.
KeywordsCompositional Zoning Garnet Growth Bulk Rock Composition Garnet Porphyroblasts Secondary Zoning
This work was funded by DFG research grant AB 314/2-1 in the framework of the FOR 741 research group. We thank Ralf Milke for his help on the electron microprobe, Ralf Schuster for his lead in the field and Georg Hoinkes, Lukas Keller, and Sha-Wali Faryad for stimulating discussion.
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