Quartz microtextures of the Sambagawa schists and their implications in convergent margin processes

Abstract

Abstract Quartz c‐axis fabrics of the Sambagawa schists produced along a late Mesozoic convergent plate margin were analysed so that their tectono‐metamorphic history could be clarified. It has been noted by many authors that quartz fabrics produced by earlier phase deformation are easily modified by strain increment during later phase deformation. This paper attempts to elucidate the high‐temperature phases of prograde metamorphism (Sim‐Bim phase) and of retrograde metamorphism (Sb 1 phase and Sb 2−1 phase) from quartz grains included in garnet and plagioclase porphyroblasts. Quartz c‐axis fabrics for all these phases are explained in terms of a type I crossed girdle, without (only rarely with) higher concentration in the principal axis of strain Y (X>Y>Z), that must have been produced by the activity of a dominant slip system such as rhomb and basal. As a result, the plastic deformation of quartz, which was responsible for the formation of the type I crossed girdle, occurred even under temperatures greater than 500°C and pressures a little greater than 10–11 kb, which correspond to the physical condition of the Sim‐Bim phase. It has been assumed that a high strain rate (and/or low H 2 O content) caused rhomb and basal to be active as dominant slip systems in the subduction zone related to the formation of the Sambagawa schists even under high temperatures (> 500°C).