10.07.2015 Views

Quantitative structural analyses and numerical modelling of ...

Quantitative structural analyses and numerical modelling of ...

Quantitative structural analyses and numerical modelling of ...

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

J. metamorphic Geol., 2008, 26, 29–53 doi:10.1111/j.1525-1314.2007.00743.xOrigin <strong>of</strong> migmatites by deformation-enhanced melt infiltration<strong>of</strong> orthogneiss: a new model based on quantitativemicro<strong>structural</strong> analysisP. HASALOVÁ, 1,2 K. SCHULMANN, 1 O. LEXA, 1,2 P. ŠTÍPSKÁ, 1 F. HROUDA, 2,3 S. ULRICH, 2,4J. HALODA 5 AND P. TÝCOVÁ 51 Université Louis Pasteur, CGS/EOST, UMR 7517, 1 rue Blessig, Strasbourg 67084, France (hasalovap@seznam.cz)2 Institute <strong>of</strong> Petrology <strong>and</strong> Structural Geology, Charles University, Albertov 6, 12843 Prague, Czech Republic3 AGICO, Ječná 29a, 621 00 Brno, Czech Republic4 Institute <strong>of</strong> Geophysics, Czech Academy <strong>of</strong> Sciences, Boční II/1401, 14131 Praha 4, Czech Republic5 Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech RepublicABSTRACTA detailed field study reveals a gradual transition from high-grade solid-state b<strong>and</strong>ed orthogneiss viastromatic migmatite <strong>and</strong> schlieren migmatite to irregular, foliation-parallel bodies <strong>of</strong> nebulitic migmatitewithin the eastern part <strong>of</strong> the Gfo¨ hl Unit (Moldanubian domain, Bohemian Massif). The orthogneiss tonebulitic migmatite sequence is characterized by progressive destruction <strong>of</strong> well-equilibrated b<strong>and</strong>edmicrostructure by crystallization <strong>of</strong> new interstitial phases (Kfs, Pl <strong>and</strong> Qtz) along feldspar boundaries<strong>and</strong> by resorption <strong>of</strong> relict feldspar <strong>and</strong> biotite. The grain size <strong>of</strong> all felsic phases decreases continuously,whereas the population density <strong>of</strong> new phases increases. The new phases preferentially nucleate alonghigh-energy like–like boundaries causing the development <strong>of</strong> a regular distribution <strong>of</strong> individual phases.This evolutionary trend is accompanied by a decrease in grain shape preferred orientation <strong>of</strong> all felsicphases. To explain these data, a new petrogenetic model is proposed for the origin <strong>of</strong> felsic migmatites bymelt infiltration from an external source into b<strong>and</strong>ed orthogneiss during deformation. In this model,infiltrating melt passes pervasively along grain boundaries through the whole-rock volume <strong>and</strong> changescompletely its macro- <strong>and</strong> microscopic appearance. It is suggested that the individual migmatite typesrepresent different degrees <strong>of</strong> equilibration between the host rock <strong>and</strong> migrating melt duringexhumation. The melt topology mimicked by feldspar in b<strong>and</strong>ed orthogneiss forms elongate pocketsoriented at a high angle to the compositional b<strong>and</strong>ing, indicating that the melt distribution wascontrolled by the deformation <strong>of</strong> the solid framework. The microstructure exhibits features compatiblewith a combination <strong>of</strong> dislocation creep <strong>and</strong> grain boundary sliding deformation mechanisms. Themigmatite microstructures developed by granular flow accompanied by melt-enhanced diffusion <strong>and</strong>/ormelt flow. However, an AMS study <strong>and</strong> quartz micr<strong>of</strong>abrics suggest that the amount <strong>of</strong> melt present didnot exceed a critical threshold during the deformation to allow free movements <strong>of</strong> grains.Key words: crystal size distribution; melt infiltration; melt topology; migmatites; quantitative texturalanalysis.INTRODUCTIONMovement <strong>of</strong> a large volume <strong>of</strong> granitic melt is animportant factor in the compositional differentiation<strong>of</strong> the continental crust (Fyfe, 1973; Collins & Sawyer,1996; Brown & Rushmer, 2006) <strong>and</strong> the presence <strong>of</strong>melt in rocks pr<strong>of</strong>oundly influences their rheology(Arzi, 1978). The migration <strong>of</strong> melt through the crust iscontrolled by melt buoyancy <strong>and</strong> pressure gradientsresulting from the combination <strong>of</strong> gravity forces <strong>and</strong>deformation (Wickham, 1987; Sawyer, 1994). Thereare three major mechanisms controlling melt migrationthrough the continental crust: (i) diapirism resulting inupward motion <strong>of</strong> low-density magma through higherdensity rocks (Ch<strong>and</strong>rasekhar, 1961; Ramberg, 1981);(ii) dyking that describes melt migration by hydr<strong>of</strong>racturing<strong>of</strong> the host rock <strong>and</strong> transport <strong>of</strong> meltthrough narrow dykes (Lister & Kerr, 1991; Petford,1995); (iii) <strong>and</strong> migration <strong>of</strong> a melt through a network<strong>of</strong> interconnected pores during deformation or compaction<strong>of</strong> solid matrix (McKenzie, 1984; Wickham,1987).Brown & Solar (1998a) <strong>and</strong> Weinberg & Searle(1998) proposed that during active deformation meltmoves by pervasive flow <strong>and</strong> it is essentially pumpedthrough the system parallel to the principal finiteelongation in the form <strong>of</strong> foliation-parallel veins.Based on a number <strong>of</strong> field studies, pervasive meltmigration at outcrop scale controlled by regionaldeformation has been suggested by various authors(Collins & Sawyer, 1996; Brown & Solar, 1998b;V<strong>and</strong>erhaeghe, 1999; Marchildon & Brown, 2003).Ó 2007 Blackwell Publishing Ltd 29315

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!