Conditions and processes during metamorphic and igneous petrogenesis

Abstract

Processes such as nucleation and crystal growth, chemical diffusion, crystal dissolution and deformation fundamentally impact on the mineral content, mineral chemistry and texture of a rock. Studies on the nature and rates of these processes in rocks (e.g. Carlson 2002), their interplay (e.g. Baxter and DePaolo 2004; Selverstone 2005) and their dependence on changes in pressure, temperature and chemical environment with time (e.g. Gaidies et al. 2008) are required to better understand metamorphic and igneous petrogenesis. Recent advances in the development of analytical and experimental techniques allow for the integration of microstructure analyses of rocks and minerals with chemical and isotopic investigations at unprecedented high resolution (Keller et al. 2008). Combined with novel theoretical and computational capabilities, the natural record preserved in geological materials can now be studied through more realistic simulations of mineral reactions in rocks accounting for fundamental thermodynamic, kinetic and mechanic principles (e.g. Schmid et al. 2009). Quantitative knowledge of the conditions and processes that control mineral formation in rocks is essential for a better understanding of the close relationship between the bulk properties of rocks and the way solid Earth responds to changes in external forces. It can be expected that this knowledge will help developing a successively more realistic picture of the dynamics of our planet considering for instance the relevance of the extent of mineral reactions in rocks for catastrophic rock failure such as seismogenic slip (e.g. John et al. 2009). This special issue originates from the session “Controls on metamorphic processes” held at the European Geosciences Union General Assembly 2008 in Vienna. It extends the original focus of this session on metamorphic petrogenesis to conditions and processes also fundamental for igneous rock formation and covers a broad spectrum of topics ranging from fluid-rock interactions on the outcrop scale to intracrystalline chemical diffusion and nucleation. Eight papers are included that address various themes relevant for a better understanding of metamorphic and igneous petrogenesis. Habler et al. (2009) investigate the interplay of deformation, chemical equilibration and mineral reaction during Cretaceous metamorphism in polymetamorphic rocks from the Austroalpine Matsch Unit west of the Tauern Window in the European Eastern Alps. Based on detailed studies of mineral chemistry and rock microstructure, different mechanisms of interrelated deformation and chemical reequilibration could be identified. Habler et al. conclude that understanding the mechanisms of the mutual interrelation between deformation and equilibration is required to deduce thermobarometrical constraints from deformed metamorphic rocks. In order to derive estimates for the pressure and temperature conditions and timescales of polymetamorphism in the Wolz Complex (Eastern European Alps, Austria), Bestel et al. (2009) combine garnet isopleth thermobarometry with chemical diffusion modelling. The authors infer that during the Permian metamorphic event the rocks of the Wolz Complex were exposed to temperatures ranging from 570 to 610°C for about 10 to 20 Ma. F. Gaidies (*) Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada e-mail: fgaidies@earthsci.carleton.ca