Quantitative structural analyses and numerical modelling of ...

98 L. BARATOUX ETAL.bearing rocks are rare (e.g. Hacker & Christie1990; Wilks & Carter 1990). However, metabasitesare considered to constitute a major partof the lower continental crust (Rutter & Brodie1992) and the study of deformation microstructuresand mechanisms is therefore crucial forunderstanding the deformation behaviour andrheology of mafic tectonites. Plagioclase rheologyis believed to control the strength of the lowercrust in several models (e.g. Carter & Tsenn1987; Ord& Hobbs 1989). Hornblende isassumed to be a relatively strong phase and tobehave passively during deformation unless itforms a load-supporting framework (Brodie &Rutter 1985).Many studies concerned with deformationmechanisms of plagioclase and hornblendehave been published to date. Experiments suggestthat plagioclase is deformed by dislocation creepunder lower crustal conditions (e.g. Tullis &Yund 1987; Ji & Mainprice 1990; Kruse et al.2001) with commonly active (010)[001] principalslip system (e.g. Olsen & Kohlstedt 1985;Kruhl 1987). Kruhl (1987) also observed a(001) [ 100] slip system in naturally deformed plagioclasesand Stiinitz et al. (2003) have shownthat slip on (001) and {111} in (110) directionare similarly active in experimentally deformedAn60 crystals. Besides these examples, otherless common slip systems were suggested byMarshall & McLaren (1977a, b), Montardi &Mainprice (1987), and Olsen & Kohlstedt(1984, 1985). In addition, grain size sensitiveprocesses in plagioclases have been referred toin some studies (Boullier & Gu6guen 1975;Lapworth et al. 2002). Hornblende generallyundergoes brittle deformation under low temperatureconditions (e.g. Brodie & Rutter 1985;Nyman et al. 1992; Lafrance & Vernon 1993).Many studies document crystal plastic deformationof hornblende with a dominant (100)[001] slip system from experiments and naturalrocks (e.g. Rooney et al. 1970; Cumbest et al.1989a, b; Hacker & Christie 1990). Volume diffusionrates are, however, slow in amphiboles(e.g. Freer 1981) implying that dislocationclimb is limited even at geological strain rates.Syndeformational chemical reactions betweenhornblende and plagioclase are common in metabasicrocks (Brodie 1981; Brodie & Rutter 1985)and involve deformation mechanisms such aschemically induced grain boundary migration(CIGM) (Cumbest et al. 1989a), on nucleationof new plagioclase (Rosenberg & Stfinitz 2003)or hornblende at plagioclase grain boundaries(Kruse & Sttinitz 1999).Metagabbros from the Star6 M6sto belt (easternmargin of the Bohemian Massif) representan example of a two-phase metabasic systemdeformed at different temperatures and straingradients. The aim of this study is to show twotypes of progressive evolution of deformationmicrostructures and textures of dynamicallyrecrystallized plagioclase-hornblende bearingmetagabbros at amphibolite and upper amphibolitefacies conditions with increasing bulk strain.Methods such as quantitative textural analysis,crystallographic preferred orientations (CPO)and study of mineral chemistry were used toconstrain deformation mechanisms for bothminerals. Finally, deformation mechanisms ofrheologically contrasting plagioclase and hornblendeare correlated and the mechanical behaviourof mafic rocks deformed under lowercrustal metamorphic conditions is discussed.Geological settingThe Star6 M6sto (SM) belt in the eastern marginof the Bohemian Massif separates the highgrade gneisses of thickened continental crust ofthe Lugian domain in the west from a Neo-Proterozoic continental margin in the east(Fig. l a). The SM domain was thinned duringCambro-Ordovician rifting and underwent granulitefacies metamorphism (Stfpskfi et al. 2001).Subsequent Variscan tectonics resulted in NE-SW trending structures dipping at relativelyhigh angles to the west (Figs 1 a and b).Strongly deformed 'western' metagabbros ofCambro-Ordovician protolith ages occur at thetop of the SM belt (Fig. 1) (Kr6ner et aL 2000),The metagabbros were pervasively affected bya ductile shear zone along which was emplacedsyntectonically a Carboniferous tonalite silldated at 340 Ma (Stfpskfi et aL 2004). A carboniferousmetamorphism of adjacent metagabbrosreached higher amphibolite facies conditionsbecause of the strong heat input from the tonalitesill. A leptyno-amphibolite complex of Cambro-Ordovician age comprising a sequence of alternatingamphibolites and tonalitic gneissesoccurs in the footwall of the tonalite sill. Thiscomplex is underlined by the 'eastern' metagabbrosheet, which is supposed to form part of thesame lower crust as the western (upper) metagabbrosheet (Stfpskfi et aL 2001). Amphibolitefacies Carboniferous metamorphic conditions ofthe eastern (lower) metagabbro are also attributedto the heat input of a more distant hangingwalltonalite intrusion.Rocks of Cambro-Ordovician age suffered twodeformations: D~ of Cambro-Ordovician age andD2 of Carboniferous age. The latter one prevailsin most lithologies of the SM belt, marked by apenetrative west-dipping $2 foliation bearing a250