Quantitative structural analyses and numerical modelling of ...

3. Quantitative analyses of metamorphic microstructures 31Figure 3.1: N 0 -Gt plot showing CSD evolution of plagioclase and hornblende. Thisfigure showing two contrasting positions for metamorphic events with different durations.Long-lived mature microstructures are grouped along so-called fabric attractor.microstructural analysis covered digitization of large number of samples which weresubsequently handled using PolyLX software. As a result of this analysis we discoveredthat the Carboniferous event was associated with grain size distribution whichin CSD space shown dominance of nucleation process with respect to slow growth rate.Increase of ambient temperature correlates with decrease in nucleation density accompaniedwith increase growth (Fig. 3.1). In contrast the Cambro-Ordovician rocks revealedpre-dominance of growth process over the nucleation. Our data plot on continuous curvein the CSD space which clearly indicates absence of Ostwald ripening and dominance oftextural coarsening by DeHoff’s communicating neighbours theory, which governs thechange in shape of CSD curve. The analysis show that the time matters in mineral microstructuresleading to development of grain size distribution which can be consideredas a grain size attractor (Fig. 3.1).This study had shown for the first time the power of grain contact frequency analysis.The rocks deformed by short lived Carboniferous pulse reveal aggregate distributionof both mafic and felsic phases. However, the similar lithologies experiencing long lastingCambro-Ordovician event recorded evolution driving contact frequencies towardsthe field of regular distribution. The process of increase of unlike contacts in rocks is