ROLE OF SMALL-SCALE COMPETENCY VARIATION ON THE SYN-TECTONIC NEOBLAST OCCURRENCE AND DISTRIBUTION

Petrographic features of metapelites from North Delhi fold belt (NDFB), India shows remarkable textural variation of neoblastic phases even at thin section-scale. Garnet in these rocks is found to develop as neoblast all across the rock, however, depending on matrix mineral composition (biotite vs. quartz-feldspar proportion), the grain size distribution pattern varies significantly between layers. Depending on matrix composition, the rock layers are broadly divided in two types, mica (biotite) poor QL layer and comparatively mica rich QML layer. Certain parameters like the proportion of garnet (modal amount), number of grains, and range of grain size were measured and statistically analyzed to compare these two layers. The results show that QL not only contains higher proportion and number of garnet but also displays higher variation in grain size. However, within QML the zones that contain only biotite (Fe-Mg phase) show significantly higher modal abundance and grain counts for garnet (Fe-Mg phase). When all the small-scale difference in matrix compositions in considered, the large-sized grains certainly occur in the QL layer. The results suggest nutrient-rich biotite layer may support formation of garnet but the minerals could not grow in these areas. The favored site for growth was rather determined by the competency of layer (mica poor vs. mica-rich layer), and relatively competent QL layer was preferred for the growth of syn-tectonic neoblasts developed during the deformation-metamorphism process.