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Origin of Migmatites by Deformation-Enhanced Melt Infiltration of Orthogneiss: a New Model Based on Quantitative Microstructural Analysis

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Origin of Migmatites by Deformation-Enhanced Melt Infiltration of Orthogneiss: a New Model Based on Quantitative Microstructural Analysis J. metamorphic Geol., 2008, 26, 29–53 doi:10.1111/j.1525-1314.2007.00743.x Origin of migmatites by deformation-enhanced melt infiltration of orthogneiss: a new model based on quantitative microstructural analysis P. HASALOVA´ , 1,2 K. SCHULMANN,1 O. LEXA,1,2 P. Sˇ TI´PSKA´ , 1 F. HROUDA,2,3 S. ULRICH,2,4 J. HALODA5 AND P. TY´ COVA´ 5 1Universite´ Louis Pasteur, CGS/EOST, UMR 7517, 1 rue Blessig, Strasbourg 67084, France ([email protected]) 2Institute of Petrology and Structural Geology, Charles University, Albertov 6, 12843 Prague, Czech Republic 3AGICO, Jecˇna´ 29a, 621 00 Brno, Czech Republic 4Institute of Geophysics, Czech Academy of Sciences, Bocˇnı´ II/1401, 14131 Praha 4, Czech Republic 5Czech Geological Survey, Kla´rov 3, 118 21 Prague 1, Czech Republic ABSTRACT A detailed field study reveals a gradual transition from high-grade solid-state banded orthogneiss via stromatic migmatite and schlieren migmatite to irregular, foliation-parallel bodies of nebulitic migmatite within the eastern part of the Gfo¨ hl Unit (Moldanubian domain, Bohemian Massif). The orthogneiss to nebulitic migmatite sequence is characterized by progressive destruction of well-equilibrated banded microstructure by crystallization of new interstitial phases (Kfs, Pl and Qtz) along feldspar boundaries and by resorption of relict feldspar and biotite. The grain size of all felsic phases decreases continuously, whereas the population density of new phases increases. The new phases preferentially nucleate along high-energy like–like boundaries causing the development of a regular distribution of individual phases. This evolutionary trend is accompanied by a decrease in grain shape preferred orientation of all felsic phases. To explain these data, a new petrogenetic model is proposed for the origin of felsic migmatites by melt infiltration from an external source into banded orthogneiss during deformation. In this model, infiltrating melt passes pervasively along grain boundaries through the whole-rock volume and changes completely its macro- and microscopic appearance. It is suggested that the individual migmatite types represent different degrees of equilibration between the host rock and migrating melt during exhumation. The melt topology mimicked by feldspar in banded orthogneiss forms elongate pockets oriented at a high angle to the compositional banding, indicating that the melt distribution was controlled by the deformation of the solid framework. The microstructure exhibits features compatible with a combination of dislocation creep and grain boundary sliding deformation mechanisms. The migmatite microstructures developed by granular flow accompanied by melt-enhanced diffusion and/or melt flow. However, an AMS study and quartz microfabrics suggest that the amount of melt present did not exceed a critical threshold during the deformation to allow free movements of grains. Key words: crystal size distribution; melt infiltration; melt topology; migmatites; quantitative textural analysis. fracturing of the host rock and transport of melt INTRODUCTION through narrow dykes (Lister & Kerr, 1991; Petford, Movement of a large volume of granitic melt is an 1995); (iii) and migration of a melt through a network important factor in the compositional differentiation of interconnected pores during deformation or com- of the continental crust (Fyfe, 1973; Collins & Sawyer, paction of solid matrix (McKenzie, 1984; Wickham, 1996; Brown & Rushmer, 2006) and the presence of 1987). melt in rocks profoundly influences their rheology Brown & Solar (1998a) and Weinberg & Searle (Arzi, 1978). The migration of melt through the crust is (1998) proposed that during active deformation melt controlled by melt buoyancy and pressure gradients moves by pervasive flow and it is essentially pumped resulting from the combination of gravity forces and through the system parallel to the principal finite deformation (Wickham, 1987; Sawyer, 1994). There elongation in the form of foliation-parallel veins. are three major mechanisms controlling melt migration Based on a number of field studies, pervasive melt through the continental crust: (i) diapirism resulting in migration at outcrop scale controlled by regional upward motion of low-density magma through higher deformation has been suggested by various authors density rocks (Chandrasekhar, 1961; Ramberg, 1981); (Collins & Sawyer, 1996; Brown & Solar, 1998b; (ii) dyking that describes melt migration by hydro- Vanderhaeghe, 1999; Marchildon & Brown, 2003). Ó 2007 Blackwell Publishing Ltd 29 30 P. HASALOVA´ ET AL. These authors argued that magma intrudes perva- (Lexa et al., 2005); (ii) characterization of dynamic or sively, parallel to the main anisotropy represented by static conditions of melt movement through rocks foliation planes (John & Stu¨ nitz, 1997), fold hinges and using analysis of grain boundaries and shape orienta- interboudin partitions (Brown, 1994; Brown et al., tions (Rosenberg & Riller, 2000; Marchildon & Brown, 1995). It is also commonly observed that vein-like 2002); and (iii) cooling or heating histories of rocks leucosomes are injected into extensional structures using crystal size distribution (CSD) theory (Higgins, provided the magma pressure is high enough (Wick- 1998; Berger & Roselle, 2001). ham, 1987; Lucas & St.Onge, 1995) or parallel to axial In this work, a sequence of deformed felsic rocks is surfaces of folds (Vernon & Paterson, 2001). studied, ranging from high-grade banded orthogneiss Microscopic studies of natural rocks show orienta- to fine-grained isotropic migmatite both at macro- and tions of former melt microstructures that are inter- microscale using structural, petrographic and quanti- preted in terms of grain-scale channel networks tative microstructural analyses. It is shown that a (Sawyer, 2001). Melt migration pathways at the grain sequence of banded orthogneiss, stromatic, schlieren scale are commonly determined from distribution of and nebulitic migmatites results from progressive melt films and pools (now glass) in experimentally deformation in a crustal-scale shear zone in the pres- prepared samples or by distribution of minerals sup- ence of melt. The microstructural and fabric modifi- posed to preserve the original melt topology in natural cations connected with disintegration of parental rocks (Brown et al., 1999; Rosenberg & Riller, 2000; banded orthogneiss and development of random min- Rosenberg, 2001). The melt topology in experiments is eral microstructure are quantified. The relationships of controlled mainly by differential stress, confining the individual rocks types and the possible origin of pressure and the amount of melt in the system this sequence are discussed in terms of deformation (Rosenberg, 2001). At static conditions, the melt and migmatization of different protoliths, melt topology is characterized by equilibrium dihedral infiltration from an external source or in situ melting of (wetting) angles at triple point junctions (Jurewicz & the same protolith during progressive deformation. It Watson, 1984; Laporte & Watson, 1995; Laporte is argued that banded orthogneiss and nebulitic et al., 1997; Cmı´ral et al., 1998; Walte et al., 2003) and migmatites can be interpreted as end-members of a the mobility of the melt remains very low, even if the continuous sequence resulting from melt infiltration melt phase forms an interconnected network along from an external source during deformation. Finally, triple-junction grain edges at dihedral angles lower the role of melt for activity of grain-scale deformation than 60° (Laporte & Watson, 1995; Connolly et al., mechanisms and bulk rheological behaviour of crustal 1997). rocks during melt infiltration is discussed. Experimental studies on rock analogues to investi- gate grain-scale melt flow under laboratory conditions show that during contemporaneous melting and GEOLOGICAL SETTING deformation melt connection allows the nucleation of The Moldanubian zone represents the highest grade shear bands along which a melt is further segregated unit of the Bohemian Massif and is interpreted as an (Rosenberg & Handy, 2000, 2001; Barraud et al., internal zone of the Variscan orogen developed during 2001). Rosenberg (2001) reviewed the experimental the Variscan convergence (Matte et al., 1990). The data and concluded that the melt migration and melt Moldanubian zone is comprised essentially of high- flow direction are controlled by incremental shortening grade gneisses and migmatites containing relicts of and melt pressure gradients between source and areas high-pressure felsic granulites, eclogites and peridotites of melt accumulation. that are intercalated with mid-crustal rocks (Fig. 1a). There have only been a few attempts to quantify Schulmann et al. (2005) described the structural and melt distribution in rocks using methods of quantita- metamorphic evolution of high-grade crustal rocks of tive and computer aided microstructural analysis the so-called Gfo¨ hl Unit and of the adjacent middle (Dallain et al., 1999; Tanner, 1999; Marchildon & crustal units. For the mechanism of exhumation, they Brown, 2003). These studies commonly deal with grain proposed a model of vertical extrusion of orogenic contact frequency distributions, grain size evolution lower crust and its lateral spreading in mid-crustal and orientation of former melt films (Dougan, 1983; levels due to an indentation of the easterly Brunia McLellan, 1983; Rosenberg & Riller, 2000). However, promontory. As a consequence of this process, the modern quantitative microstructural analysis
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