Quartz microfabrics in ultrahigh-pressure metamorphic rocks as indicators of low stress during uplift

Ultrahigh-pressure (UHP) metamorphic rocks form in convergent tectonic settings at depths > 90 km. In general, UHP conditions are defined by the stability of coesite. Coesite inclusions in various host minerals are a common feature in UHP rocks. We present data of typical quartz microfabrics, which resulted from the transformation of coesite to quartz in inclusions and intergranular regions, and evidence of high temperature quartz microfabrics. Jadeite-quartzites and eclogites from the Dabie Shan - Sulu orogenic belt (central China) were investigated, both of which experienced peak UHP conditions of ~ 660°C / > 2.6 GPa (Liou et al., 1997) and 700-800°C / 3.1-4.1 GPa (Yoshida et al., 2004), respectively, at c. 220-230 Ma due to a N-subduction of the Yangtze craton beneath the Sino-Korean craton, followed by a nearly isothermal decompression to ~ 1 GPa. Both rocks contain coesite and/or quartz inclusions (up to 350 μm in diameter) in clinopyroxene and garnet and in their symplectitic rims with various microstructures: (i) rims of palisade quartz commonly in contact to coesite, (ii) quartz aggregates with sutured grain boundaries and central remnants of coesite, (iii) larger central quartz grains with subgrains, (iv) foam quartz (c. 50 μm sized), and (v) apparently strain-free single quartz crystals. The foam quartz shows a random c-axis distribution but preferred crystallographic orientations of grain boundaries. Intergranular foam quartz is found in quartz-eclogites from the Sulu region, most probably indicating quartz-coesite transformation and, therefore, former UHP conditions. The preservation of foam quartz in intergranular regions and inclusions (especially in symplectites) indicates low stress and nearly no deformation during uplift in the stability field of quartz. This points to strain localization in shear zones (Zhao et al., 2003). In veins in the jadeite-quartzites and quartz-eclogites quartz occurs in larger aggregates with sutured grain boundaries. Fractal analysis of the grain boundaries indicates deformation-T > 700°C for jadeite-quartzite and quartz-eclogite. Furthermore, quartz grains of the Dabie Shan jadeite-quartzites show prism-, basal-, and rhombohedral-parallel subgrain boundaries, evidencing high temperature during formation and nearly no deformation after peak metamorphic conditions. The qualitative and quantitative analyses of quartz microfabrics of UHP rocks give insight into the grade of preservation of peak conditions, potential deformation and/or low-grade overprint, and help to understand uplift processes of deeply subducted crustal slices. Liou, J.G., Zhang, R.Y. and Jahn, B.-m., 1997. Petrology, geochemistry and isotope data on a ultrahigh-pressure jadeite quartzite from Shuanghe, Dabie Mountains, East-central China. Lithos 41, 59-78. Yoshida, D., Hirajima, T. and Ishiwatari, A., 2004. Pressure-temperature path recorded in the Yangkou garnet peridotite, in Su-Lu ultrahigh-pressure metamorphic belt, eastern China. J. Petrol. 45, 1125-1145. Zhao, Z.Y., Fang, A.M. and Yu, L.J., 2003. High- to ultrahigh-pressure (UHP) ductile shear zones in the Sulu UHP metamorphic belt, China: implications for continental subduction and exhumation. Terra Nova 15, 322-329.