Quantitative structural analyses and numerical modelling of ...

ClickHereforFullArticleJOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B10210, doi:10.1029/2006JB004820, 2007Extreme ductility of feldspar aggregates—Melt-enhanced grainboundary sliding and creep failure: Rheological implications forfelsic lower crustProkop Závada, 1,4 Karel Schulmann, 2 Jiří Konopásek, 3 Stanislav Ulrich, 1,4and Ondrej Lexa 1,2Received 25 October 2006; revised 17 July 2007; accepted 13 August 2007; published 27 October 2007.[1] High-grade orthogneisses from granulite-bearing lower crustal unit show extremefinite strains of both K-feldspar and plagioclase with respect to weakly deformedquartz aggregates. K-feldspar aggregate in the most intensely deformed sampleshows interstitial grains of quartz and albite, which also mark some intragranular fractureswithin K-feldspar grains. Both interstitial grains and fractures are oriented mostlyperpendicular to the sample stretching lineation. Quartz and albite grains withinK-feldspar bands are interpreted as crystallized from interstitial melt and the petrologystudy shows that the melt was produced by a metamorphic reaction inplagioclase-mica bands. Thermodynamic Perple_X modeling shows that melt volumeincrease was negligible and melt amount was too small to generate considerablemelt overpressure for calculated PT conditions. It is therefore suggested that dilation ofK-feldspar aggregates and fracturing of its grains represent a final creep failure state,which resulted from the cavitation process accompanying grain boundary slidingcontrolled diffusion creep. The consequence of cavitation-driven dilation of K-feldsparaggregates is the local underpressure resulting in infiltration of melt from plagioclasebands. Analogy with metallurgy experiments shows that the cavitation process,exclusively developed in cryptoperthitic K-feldspar, can be attributed to its lower puritycompared to more pure plagioclase. Contrasting rheological behavior of feldsparswith respect to quartz prior to fracturing is attributed to different deformationmechanisms. Feldspars appear weaker due to grain boundary sliding accommodated bycoupled melt-enhanced diffusion creep along grain boundaries and dislocation creepwithin grains, in contrast to quartz deforming via grain boundary migrationaccommodated dislocation creep.Citation: Závada, P., K. Schulmann, J. Konopásek, S. Ulrich, and O. Lexa (2007), Extreme ductility of feldspar aggregates—Meltenhancedgrain boundary sliding and creep failure: Rheological implications for felsic lower crust, J. Geophys. Res., 112, B10210,doi:10.1029/2006JB004820.1. Introduction[2] It has been experimentally demonstrated that smallamount of interstitial melt increases creep rates of deformingrocks and can induce switch of deformation mechanisms[Cooper and Kohlstedt, 1984; Dell’Angelo et al., 1987;Dell’Angelo and Tullis, 1988]. Dell’Angelo et al. [1987]have shown transition from dislocation creep to meltenhanceddiffusion creep in fine-grained granitic aggregates1 Institute of Petrology and Structural Geology, Charles University,Prague, Czech Republic.2 Centre de Geochimie de la Surface, UMR 7516, Université LouisPasteur, Strasbourg, France.3 Czech Geological Survey, Prague, Czech Republic.4 Geophysical Institute, Czech Academy of Sciences, Prague, CzechRepublic.Copyright 2007 by the American Geophysical Union.0148-0227/07/2006JB004820$09.00at small volume fractions of melt F = 0.01–0.03 contemporaneouslywith strength drop below the limit of detection.Rosenberg and Handy [2005] have shown that the ‘‘meltconnectivity transition’’ marked by melt fraction F = 0.07 iscritical in mineral aggregate strength drop at experimentalconditions. These results imply that small amount of meltcan be responsible for considerable weakening of crustalrocks, which can explain, e.g., the development of largescaleshear zones bounding regions of rapid uplift [Hollisterand Crawford, 1986].[3] Melt bearing microstructures in deformed rocks canbe also used for reconstruction of grain-scale melt migrationpathways, because deformation and melt extraction arecoupled [Brown and Rushmer, 1997; Rosenberg and Handy,2000; Rosenberg, 2001]. A typical feature of deformationexperiments at low melt volumes is the preferential distributionof melt along grain boundaries oriented at low angleto principal compressive stress direction [Dell’Angelo et al.,B102102791of15