DYNAMIC RECRYSTALLIZATION AND PARTIAL OVERPRINTING OF BLUESCHIST FACIES MINERALS AND TEXTURES

Interlayered quartzite, marble, and metabasalt in the southern Sivrihisar Massif (SSM), Turkey, experienced subduction (blueschist/eclogite facies) metamorphism that was partially to completely overprinted by later regional (Barrovian) metamorphism at greenschist to amphibolite facies conditions. The range of rock types and the progressive nature of the transition zone between metamorphic domains allows analysis of the interaction of heat and deformation in the overprinting of subduction metamorphism minerals and textures during later tectonic/thermal events. In the SSM, the record of subduction metamorphism was obliterated in metabasalt and marble at the low-grade end of the Barrovian sequence, but is preserved up to lower amphibolite facies (staurolite-in isograd) in white mica-rich quartzite. Quartzites in the Barrovian sequence are L-tectonites in which quartz microstructures record a strong constrictional fabric that developed at least in part during Barrovian metamorphism. In some quartzites, white mica is very elongate and boudinaged. Despite the intensity of the fabric, white mica in quartzite that equilibrated below the staurolite-in isograd typically consists of phengitic cores rimmed by muscovite. Phengite cores have similar composition to phengite in pristine blueschist in the SSM and likely represent relict high-P mica. The presence of zoned white mica is consistent with the complexity of Ar spectra obtained for SSM rocks equilibrated at temperatures below the staurolite-in isograd.

Understanding the conditions and principal mechanisms of recrystallization of fabric-forming white mica is critical to thermochronology, stable isotope, and metamorphic studies in a variety of tectonic settings. The persistence of zoned white mica up to moderate temperature (500 C), even in intensely deformed rocks, may indicate that a thermally controlled process such as diffusion is the main factor in recrystallization. Alternatively, the shape of isolated mica in L-tectonites, in contrast to mica arranged in planar folia, may have limited mica-fluid exchange and therefore mica dissolution-reprecipitation, facilitating preservation of zoned mica grains.