Deciphering polymetamorphism by garnet growth modelling : an application to the Austroalpine crystalline basement east of the Tauern Window

Abstract

This thesis is based upon studies conducted between June 2003 and June 2007 at the Institute of Mineralogy and Petrography at Basel University. It was developed under the supervision of Rainer Abart (FU Berlin) and Christian de Capitani (Basel University) and essentially benefited from the guidance of Ralf Schuster (Austrian Geological Survey) in terms of the regional geological framework. The thesis is divided into six chapters, four of which (Chapter 2 to 5) can be regarded as individual manuscripts: Chapter 1 (‘Introduction’) briefly presents the general background and the aim of the thesis. Chapter 2 (‘Characterization of polymetamorphism in the Austroalpine basement east of the Tauern Window using garnet isopleth thermobarometry’) focuses on the description of the method ‘garnet isopleth thermobarometry’ and its application to polymetamorphic garnet porphyroblasts from the Austroalpine basement. This method allows to estimate P-T conditions for the formation of pre-existing equilibrium assemblages, employing bulkrock chemistries and garnet as the only relict mineral phase. This study is published in the Journal of Metamorphic Geology. Chapter 3 (‘THERIA G: A software program to numerically model prograde garnet growth’) contains the description of the software program THERIA G, which was developed throughout the course of the preparation of this thesis. THERIA G extends earlier work to simulate the formation of garnet in a given volume of rock to garnet populations. Chapter 3 clearly demonstrates that the chemical composition of garnet porphyroblasts may be influenced by chemical fractionation between garnet and rock matrix during growth, intracrystalline diffusion in garnet, and the nucleation history. This study is under review in Contributions to Mineralogy and Petrology. Chapter 4 [‘Prograde garnet growth along complex P-T-t paths: Results from numerical experiments on polyphase garnet from the W¨olz Complex (Austroalpine basement)’] presents an application of THERIA G. Due to the application of THERIA G, essential parts of the P-T-t paths of two different metamorphic events can be obtained from a single polyphase garnet porphyroblast of the Austroalpine basement. This study is under review in Contributions to Mineralogy and Petrology. Chapter 5 [‘Coupling garnet growth forward modelling with monazite geochronology: An application to the Rappold Complex (Austroalpine crystalline basement)’] links thermobarometrical information obtained from garnet growth modelling with microprobe monazite ages. As a result, a possible geothermal history is suggested, which indicates the complex metamorphic evolution of the Austroalpine crystalline basement. This study is to be submitted for publication in the Journal of Metamorphic Geology. Chapter 6 (‘Overall conclusion’) presents the main summery and conclusion of the entire thesis. The Appendix, which contains supplementary material to all the chapters, and the enclosed CD, at which the remaining data are stored, complete the thesis.