Original paper
Direct observation of porosity in quartzite and phyllonite
Hippertt, J. F.
Neues Jahrbuch für Mineralogie - Abhandlungen Band 166 Heft 3 (1994), p. 239 - 259
35 references
published: May 4, 1994
DOI: 10.1127/njma/166/1994/239
ArtNo. ESP154016603001, Price: 29.00 €
Abstract
Direct observation of porosity by Scanning Electron Microscopy in a quartz-sericite phyllonite (and its precursor quartzite) from a low metamorphic grade shear zone of Quadrilátero Ferrífero region, southeastern 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-sericite 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, but is essentially “dry” away from them. The regular pore shapes observed in both lithologies (triangles and hexagons) are quite similar to the etching figures of quartz, indicating that a strong crystallographic control operated on the development of pore geometry. These pore geometries were used to monitor the crystallographic orientation of the quartz grains on the SEM microscope. Pores are much more conspicuously developed on grain boundaries of quartz grains oriented with c-axes at high angles to the lineation, which disappear in the most phyllonitized domains. Most of the pores, therefore, were interpreted as dissolution pits rather than as an equilibrated crystal-fluid texture. In the phyllonite, a dominal c-axis analysis revealed that the thinnest quartz ribbons (< 50 μm) show an uncommon concentration of c-axes at low angles to the lineation, attributed to selective dissolution and precipitation during solution-transfer. The wide (100 to 500 μm), polycrystalline quartz ribbons show c-axis patterns with maxima at high and low angles to the lineation (similar in skeleton to the fabrics found in the adjacent quartzite), reflecting participation of both crystal-plastic and solution-transfer processes. These observations and fabrics were interpreted as indicating a process of selective quartz dissolution during progressive phyllonitization of the quartzite.
Keywords
fluids • metamorphic porosity • dissolution • phyllonitization • c-axis fabrics