Quantitative structural analyses and numerical modelling of ...

122 L. BARATOUX ET AL.recrystallized plagioclase in middle or deepcrustal metabasic rocks are rare and restrictedto rocks with very high plagioclase modal abundancessuch as anorthosites (Ji et al. 1988).However, as mentioned by other authors (e.g.Brodie & Rutter 1985), banded metabasic mylonitesare common, which is confirmed by thiswork. We show that plagioclase may easilyform interconnected weak layer networks inhornblende gabbros under upper amphibolitefacies conditions.Our microstructural study of an amphibolitefacies metagabbro (650 _+ 50 ~ shows thatthe load-bearing framework structure (Handy1990, 1994) is restricted to the lowest deformationintensities. The non-deformed metagabbroshows coarse-grained ophitic structure composedof randomly distributed hornblende and plagioclase,where amphibole grains are only locallyin contact. With ongoing strain, the deformationis mostly concentrated in the plagioclase, whichis at the transient region between brittle andplastic behaviour, leading to development of afine-grained matrix (Tullis & Yund 1987).Amphibole grains showing high internal strainand local fracturing behave as rigid bodies surroundedby interconnected layers of plagioclasegrains. Such microstructures may be interpretedas an interconnected weak layer structure(IWL) with a high viscosity contrast betweenrigid clasts of amphibole and weaker, finegrainedplagioclase layers (Handy et al. 1999).In the amphibolite facies metagabbroic mylohires,the monomineralic hornblende layers areobserved, while plagioclase-rich layers showalmost perfect mixing with hornblende. Thegrain size of plagioclase and amphibole in theplagioclase-rich matrix areas is fairly similar,the latter showing slightly higher elongation. Thephase distribution of plagioclase-hornblendemixture, the absence of CPO in plagioclase,and its aspect ratio suggest that the dominantmechanism is granular flow (grain boundarysliding). Based on this interpretation, wesuggest that the phase mixing is probably amechanical process. The microstructures andCPO of amphibole forming monomineraliclayers indicate either dislocation creep or cataclasticflow. The absence of boudinage and progressivemixing of plagioclase and amphibolesuggest that the diffusion-dominated flowprocess operating in plagioclase aggregates ismechanically as efficient as dislocation or cataclasticflow in the hornblende layers. The finalstructure resembles the interconnected weaklayer structure with low viscosity contrast(Handy 1994). A switch in deformation mechanismfrom dislocation creep towards a grain sizesensitive process is thought to be responsiblefor the convergence of mechanical properties ofamphibole and plagioclase in the mylonite,resulting in a drop of bulk rock strength(Etheridge & Wilkie 1979; Kirby 1985; Rutter &Brodie 1988).Two deformation stages were observed in theupper amphibolite facies (750 -t- 50 ~ metagabbros:(1) augen mylonite with locally preservedporphyroclasts of both plagioclase and hornblende;and (2) banded mylonites with completelyrecrystallized amphibole and plagioclase,each arranged in monomineralic layers. Theinitial stages of deformation are characterizedby tectonic grain size reduction of plagioclasewhile hornblendes represent strong objects floatingin the weak plagioclase matrix. The deformationof the metagabbro is interpreted to haveoccurred via dislocation creep accompanied bydiffusion mass transfer mechanisms, responsiblefor moderate mixing of plagioclase and amphibole.The banded fabric, only developed at highstrains, can be defined as an interconnectedweak layer structure with low viscosity contrast(Handy et al. 1999). The layered structureshows that the strengths of amphibole monornineralicaggregates and plagioclase-rich bandsare similar, suggesting convergence of rheologiesof both minerals at high strains (Jordan1988; Handy 1994).In conclusion, amphibolite and upper amphibolitefacies metagabbroic mylonites are characterizedby layered low-viscosity IWL structures.This indicates that at high strains this bandedstructure in metagabbros forms a so-calledsteady-state foliation (Means 1990). Mechanicalmixing of phases is more important in lowertemperature (eastern belt) than in higher temperature(western belt) amphibolite facies mylonites.The bulk strength of amphibolite andupper amphibolite facies mylonitic metagabbrosis controlled by an equal contribution of bothrock-forming minerals showing contrasting butequally efficient deformation mechanisms.We are indebted to D. Mainprice for his help with theEBSD analyses. Fruitful discussions with F. Holub, D. J.Prior, H. Stiinitz and J. Wheeler are gratefully acknowledged.We thank P. T~)cov~, J. Haloda, P. Grandjean andP. Capiez for the help with microprobe and bulk rock analyses.K. Brodie, H. Van Roermund and D. Gapais arethanked for thorough reviews, which improved significantlythe original manuscript. The project was fundedby grants of Czech National Grant Agency No. 42-201-204 to K.S. and 42-201-318 to P. Stipskfi, by Czech GeologicalService assignment No. 6327 to P. Mixa, and by aPhD financial support attributed by the French governmentto L.B.274