Quantitative structural analyses and numerical modelling of ...

ClickHereforFullArticleJOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B10406, doi:10.1029/2007JB005508, 2008Evolution of microstructure and melt topology in partially moltengranitic mylonite: Implications for rheology of felsic middle crustKarel Schulmann, 1 Jean-Emmanuel Martelat, 2 Stanislav Ulrich, 3,4 Ondrej Lexa, 4Pavla Štípská, 1 and Jens K. Becker 5Received 19 November 2007; revised 27 April 2008; accepted 30 July 2008; published 24 October 2008.[1] The deformation study of midcrustal porphyritic granite reveals exceptionally highstrain intensities of feldspar aggregates compared to stronger quartz. Three types ofmicrostructures corresponding to evolutionary stages of deformed granite wererecognized: (1) the metagranite marked by viscous flow of plagioclase around strongalkali feldspar and quartz, (2) quartz augen orthogneiss characterized by development ofbanded mylonitic structure of recrystallized plagioclase and K-feldspar surroundingaugens of quartz, and (3) banded mylonite characterized by alternation of quartz ribbonsand mixed aggregates of feldspars and quartz. The original weakening of alkali feldspar isachieved by decomposition into albite chains and K-feldspar resulting from aheterogeneous nucleation process. The subsequent collapse of alkaline feldspar anddevelopment of monomineralic layering is attributed to the onset of syn-deformationaldehydration melting of Mu-Bi layers associated with production of 2% melt. The finaldeformation stage is marked by mixing of feldspars which is explained by higher meltproduction due to introduction of external water. An already small amount of melt isresponsible for extreme weakening of the feldspar because of Melt ConnectivityThreshold effect triggering grain boundary sliding deformation mechanisms. The grainboundary sliding controls diffusion creep at small melt fraction and evolves to particulateflow at high melt fractions. Strong quartz shows a dislocation creep deformationmechanism throughout the whole deformation history marked by variations in the activityof the slip systems, which are attributed to variations in stress and strain rate partitioningwith regard to changing rheological properties of the deforming feldspars.Citation: Schulmann, K., J.-E. Martelat, S. Ulrich, O. Lexa, P. Štípská, and J. K. Becker (2008), Evolution of microstructure and melttopology in partially molten granitic mylonite: Implications for rheology of felsic middle crust, J. Geophys. Res., 113, B10406,doi:10.1029/2007JB005508.1. Introduction[2] Rheology of the continental crust is dominated byquartzo-feldspathic rocks, which are represented mainly bymetagranitoids, orthogneisses and felsic volcanics [Carterand Tsenn, 1987]. To date, the models of crustal rheologyuse laboratory derived laws described by constitutive equationsthat are established for minerals or monomineralicrocks such as quartzites and anorthosites [Ranalli, 1995].Most of laboratory experiments show that the quartz is1 Centre de Géochimie de la Surface, UMR7516, Université LouisPasteur, CNRS, Strasbourg, France.2 Laboratoire de Géodynamique des Chaînes Alpines, UMR5025,Université Joseph Fourier, Observatoire des Sciences de l’Univers deGrenoble, CNRS, Grenoble, France.3 Geophysical Institute, Czech Academy of Sciences, Prague, CzechRepublic.4 Institute of Petrology and Structural Geology, Charles University,Prague, Czech Republic.5 Institut für Geowissenschaften, Universität Tübingen, Tübingen,Germany.Copyright 2008 by the American Geophysical Union.0148-0227/08/2007JB005508$09.00weaker than plagioclase for the same homologous temperatures[Ranalli and Murphy, 1987; Schmid, 1982]. However,the natural quartzo-feldspathic rocks are mixtures withdifferent proportions of strong feldspars and weak quartzwith variable grain shapes and grain size distributions. Thedeformation of such natural rocks leads to strain partitioningbetween the different components and nonuniform deformation[Handy, 1990]. Handy [1994a] defined the loadbearingframework structure and interconnected weak layerstructure and proposed comprehensive empirical equationsthat determine the strength of polyphase composites. Handy[1994a] applied this concept to quartzo-feldspathic rocksand concluded that the proportion of weaker quartz controlsthe bulk rheology. The basis of these models is the coexistenceof two nonlinear viscous phases; the bulk rheologyis a consequence of the rock structure and the relativeproportions of the two mineral phases [Ji and Zhao, 1994].[3] Microstructural studies show that the progressiveorthogneiss deformation is associated with strain partitioningand variations in the deformation mechanisms offeldspars and quartz [Handy et al., 1999; Schulmann etal., 1996; Simpson, 1985]. For instance Gapais [1989] andB104062951of20