Quantitative structural analyses and numerical modelling of ...

' ~'-----2~--~----TEXTURES OF NATURALLY DEFORMED METAGABBROS 113b)Amphibole~.. ~ Amphibole in plagioclase domainsAmphibole,~ Amphibole in plagioclase domains\ A0 50 100 150Grain size - Ferret diameter (pm)9 Protomylonite- E1"~ A Mylonite- E2-pllLU z~ Mylonite- E2-pl2/x Ultramylonite- E39 ~ 9 Augen mylonite- Wlr o Banded mylonite- W2-pllo Banded mylonite- W2-p12Plagioclaseo50200 181614~8'Plagioclase ~ East ' "2Em Protomylonite - E1I. ~ Mylonit~- E2ttl [~ Ultramylonite- E36420fill L.. Plagioclase - West~21]il-grained plagioclasedlllllllllllllllilLlUL J~nn n~l][]nlIJ]n~Amphibole - Eastt 3~I ~ Mylonite- E2b9 11~ I I Ultramylonite- E3llilJLAmphibole - Westm Augen mylonite W1- [~ Banded mylonite- W250 100 150 200 250 3000 50 100 150 200 250 300 350 400Grain size - Ferret diameter (~m)Grain size - Ferret diameter (p.m)Fig. 11. Grain size evolution in the eastern (triangles) and western (circles) metagabbros. Degree of defomlationincreases from dark to white colour. (a) Calculated medians and standard deviations. (b) Histograms showing thecharacteristic frequencies of grain size for each deformational stage.representing the size of the recrystallized matrixgrains. Plagioclase grain size is substantiallyhigher in the coarse-grained domain (W2-pll)of the banded mylonite (92 ~zm). The grain sizeis reduced to 72 ~m on average in the highstraindomain (W2-p12).The grain size distributions (Fig. 1 lb) in protomylonitesof the eastern belt (El) show slightlybimodal distribution due to presence of largeporphyroclasts surrounded by small recrystallizedgrains. A higher amount of large grainscharacterizes the western banded mylonite(W2) compared to the augen mylonite (W1)and eastern mylonites (El-E3). Lognormal distributionscorrelate with intensity of deformationin both belts.Amphibole. The amphibole grain size is 49 p~min the eastern metagabbro mylonite (E2) and41 ~m in the ultramylonite (E3). However, thegrain size spread is always lower than that ofplagioclase in the same rock.The size of amphibole grains included withinplagioclase domains decreases systematicallywith increasing deformation, from 49 ~m in theleast deformed sample (El) to 39 ~m and43 ~m in the mylonite (E2-pll and E2-p12,respectively) and 33 p~m in the ultramylonite(E3). The grain sizes are always slightly lowerthan those of amphibole matrix grains in thesame thin section.Amphibole grain size in the western metagabbrobelt is highest in the sample including porphyroclasts(W1), reaching 119 Fm on average.Very high variance is typical of this mylonite,which is due to a relatively low number of newgrains and a high number of large porphyroclasts.In the banded mylonites (W2), grain sizedecreases to 94 Fm on average.In the eastern belt, amphibole grain size distributions(Fig. 11 b) are similar to those of plagioclase.They are characterized by well-developedlognormal distribution in both mylonite andultramylonite stages. In contrast, the distributionof amphibole grain size in the western belt exhibitsweaker fits of lognormal distribution withincreasing deformation, which is attributed to ahigher number of large grains of variable size(Table 4).Shape preferred orientationPIagioclase. Recrystallized plagioclase grainsin the protomylonite of the eastern belt (El)and mylonite (E2-pll) have aspect ratio (AR)of 1.59 and 1.56, respectively (Fig. 12a). Bothaspect ratio (AR = 1.82) and SPO increase inthe analysed high-strain plagioclase aggregateadjacent to the rigid amphibole porphyroclastin mylonite (E2-pl2). The ultramylonite (E3)exhibits further strengthening of the SPO andsimilar high aspect ratios (AR = 1.84).265