Quartz deformation across interlayered monomineralic and polymineralic rocks: A comparative analysis

Abstract

Abstract Given the increased availability of EBSD and fabric analyzer facilities, quartz microstructural and CPO analysis have become ‘tools of-the-trade’ for studies investigating crustal tectonics. Therefore, improved understanding of the complexities of quartz deformation is needed to ensure that such analyses distinguish primarily between tectonic outcomes and artefacts of the quartz deformation behaviours in different deformed rock volumes. We contribute to this understanding by comparing quartz deformation between monomineralic rocks, which have simple compositions; and polymineralic rocks, which are more complex and representative of the crust. Mylonite quartzites and gneisses of the Main Central Thrust (NW Himalaya) are used for comparison. Under similar deformation conditions, quartz inside monomineralic rocks forms well-developed CPOs and dynamically recrystallized microstructures, typical of grain size insensitive (GSI) creep mechanisms. Quartz inside polymineralic rocks, by contrast, exhibits weak CPOs and fine-grained microstructures commonly interpreted as grain size sensitive creep; but also exhibits lattice distortions (e.g. undulose extinction, sub-grain boundaries), associated with GSI creep. We postulate that intrinsic factors, such as interphase boundaries, cause these ambiguous results. The lack of clear evidence for deformation mechanisms in polymineralic rocks casts doubt on the reliability of quartz microstructures and CPOs for rapid diagnoses of their rheology and broader tectonic significance.