Deformational features of intraformational para-conglomerate of the Paleoproterozoic Sanvordem Formation, Goa Group indicates a brittle- ductile dextral shear

Abstract

Structures at outcrop and microscale have been studied of an intraformational para-conglomerate that belongs to the basal section of Sanvordem Formation of the Goa Group of rocks. The mylonitic foliation plane is sub vertical striking NNW-SSE with steep easterly dips. High viscosity contrast between the clasts and the chlorite-biotite rich matrix have rendered a unique texture to the rocks. The elliptical rounded to subrounded microscopic to boulder size clasts have been flattened and subjected to subsequent shearing. Outcrop scale structures abound in mantled clasts with "bearded" and recrystallized "tails" along with detached/stretched pebbles parallel to foliation. Chlorite-biotite rich matrix has suffered maximum deformation and recrystallization. Quartz grains show high degree of internal strain as evidenced by undulatory extinction and subgrain formation. Quartz fish and ribbons also occur abundantly. The quartzofelspathic clasts are rimmed by dynamically recrystallized finer grained mineral aggregates. Mantled lithoclasts indicate ductile deformation in response to flow in the matrix. The porphyroclasts of quartz, alkali feldspar and calcite are round to sigmoid in outline with beards of neocrystallized fibrous minerals i.e. muscovite, chlorite, quartz and calcite. However, these minerals also exhibit effects of brittle deformation such as displaced twin lamellae in plagioclase and microfaulting of quartz and calcite porphyroclasts. The structures indicate Top-to-E (down) dextral shear. Microscopic structural features have been used as temperature gauges. Width of twinned lamellae in calcite, ductile behavior of biotite, brittle fracturing of plagioclase and alkali feldspar, dynamic recrystallization and BLG in quartz, indicate that the rock has undergone mylonitization occurring at a transition zone of brittle-ductile regime within a temperature range of 200-450 degrees C.