Quantitative structural analyses and numerical modelling of ...

TEXTURES OF NATURALLY DEFORMED METAGABBROS 105are absent. The least deformed rock representedby augen mylonite is marked by the presenceof rare 2-3 mm large porphyroclasts of plagioclase(Fig. 6a) and by 1-2ram wide and4-5 mm long porphyroclasts of hornblendewithin the recrystallized matrix (Fig. 6b). Thisrock type is preserved only in the southern partof the metagabbro belt. Plagioclase porphyroclastsrepresent only 5-10% of the total plagioclasevolume and show deformational twinsand undulatory extinction. Recrystallized plagioclasegrains show optical zoning.Modal proportion of amphibole porphyroclastscannot be estimated as they differ fromthe neoblasts only in their chemical composition,making them optically indiscernable. Amphiboleporphyroclasts exhibit patchy or sweeping undulatoryextinction but deformation twins have notbeen observed. The local occurrence of twins isassumed to be growth-related, the twin planesbeing always perfectly straight. Prismatic porphyroclastsattain higher aspect ratios thanmore rounded neoblasts. All rocks in thewestern belt are affected by amphibolite faciesre-equilibration partly destroying the granulitefacies peak metamorphic assemblages.Fig. 6. Drawings and micrographs of the western augenmylonite. (a) Digitized drawing of plagioclase (W1)used for the quantitative textural analysis. Largeporphyroclasts are surrounded by a mantle of finegrainedmatrix grains. (b) Hornblende (W 1) showsvariable grain size and strong SPO. Plagioclase is white,hornblende is light grey, and opaque minerals are black.Banded mylonite (W2). Distinct monomineralbands of 1-10 mm in width are typical of thesehighly deformed mylonites. Small hornblendegrains of irregular shape are common withinthe plagioclase layers, but plagioclase is rarelyincluded within amphibole bands. Both mineralsare completely recrystallized. Hornblende is insome places replaced by later prisrnatic cummingtonite,which grows either parallel orobliquely to the mylonitic foliation and islocally included within the plagioclase layers.Two types of plagioclase microstructuresoccur within one plagioclase band. The firsttype is characterized by grain sizes rangingfrom 0.3 to 1 mm in diameter, subequant orslightly elongate, but always irregular, grainshapes with serrated boundaries (Fig. 7a), andgrowth-related optical zoning. Mechanicaltwins cross-cut the optical zoning in somecases (Fig. 7b), suggesting that at least some ofthem formed later. The second type is characterizedby narrow (c. 0.5 mm) high-strain zonestrending parallel or slightly oblique to the layering(Fig. 7c). Here, the grain size is reduced to0.05-0.2 ram. Plagioclase grains attain higheraspect ratios (up to 2) and exhibit undulatoryextinction and subgrain boundaries elongate parallelto the high-strain zone. The contact betweenthese two deformational domains is either sharpor transitional.Hornblendes of 0.1 - 1 mm in size have straightgrain boundaries locally meeting at high anglessuggesting a high degree of textural equilibration(Vernon 1976) (Fig. 7d). No undulatory extinctionoccurs and twinning is very rare. Wherepresent, the twin planes are always very straight,indicating that twinning is growth-related ratherthan deformational. Aspect ratios vary between1.5 and 2.5, but some grains with aspect ratioclose to 1 occur. Grains with high aspect ratiosare not always parallel to the mylonitic foliation.Bulk rock chemistry, mineral chemistryand zoningBulk rock chemistryTable 1 presents bulk rock chemical analysesstudied by X-ray fluorescence at ClaudeBernard University, Lyon, France. The easternand western metagabbro sheets are regarded tobe a part of the same lower crustal unit prior totheir involvement into the Variscan tectonics(Stfpskfi et al. 2001). However, their chemistryis not identical and bulk rock analyses(Table 1) show that western metagabbros arecharacterized by higher contents in A1 and Caand lower contents in Na and Fe than their257