DIRECT OBSERVATION OF POROSITY IN QUARTZITE AND PHYLLONITE

Direct observation of porosity by Scanning Electron Microscopy in a qu artz-sericite phyllonite (and its precursor quartzite) from a low meta morphic grade shear zone of Quadrilatero Ferrifero region, southeaster n Brazil, revealed a heterogeneous distribution of fluid-filled pores. In the phyllonite, pores are very common on quartz-quartz boundaries, are rare on quartz-sericite boundaries, and never occur on sericite-s ericite grain boundaries along folia. The precursor quartzite shows a non-wetting porosity (pores on grain boundaries, edges and corners) in the domains adjacent (on a outcrop scale) to the phyllonite layers, b ut is essentially ''dry'' away from them. The regular pore shapes obse rved in both lithologies (triangles and hexagons) are quite similar to the etching figures of quartz, indicating that a strong crystallograp hic control operated on the development of pore geometry. These pore g eometries were used to monitor the crystallographic orientation of the quartz grains on the SEM microscope. Pores are much more conspicuousl y developed on grain boundaries of quartz grains oriented with c-axes at high angles to the lineation, which disappear in the most phyllonit ized domains. Most of the pores, therefore, were interpreted as dissol ution pits rather than as an equilibrated crystal-fluid texture. In th e phyllonite, a dominal c-axis analysis revealed that the thinnest qua rtz ribbons (< 50 mum) show an uncommon concentration of c-axes at low angles to the lineation, attributed to selective dissolution and prec ipitation during solution-transfer. The wide (100 to 500 mum), polycry stalline quartz ribbons show c-axis patterns with maxima at high and l ow angles to the lineation (similar in skeleton to the fabrics found i n the adjacent quartzite), reflecting participation of both crystal-pl astic and solution-transfer processes. These observations and fabrics were interpreted as indicating a process of selective quartz dissoluti on during progressive phyllonitization of the quartzite.