The structural setting and deformation associated with pseudotachylite occurrences in the Palala Shear Belt and Sand River Gneiss, Northern Transvaal

Abstract

Two occurrences of shear-related pseudotachylite from the Limpopo Metamorphic Province are described. In particular, the structural setting as well as deformation textures in pseudotachylite inclusions and host rocks are presented for these occurrences in gabbroic anorthosite from the Palala Shear Belt and dioritic to granodioritic rocks from the Sand River Gneiss locality. The Palala Shear Belt is a 13 km wide zone of intensely mylonitized rocks of the Beit Bridge and Bushveld Complexes, situated at the southern boundary of the Central Zone of the Limpopo Metamorphic Province. The Sand River Gneiss comprises a bimodal suite of high-grade dioritic and granodioritic rocks which were subjected to at least three periods of deformation. In the context of comparisons between pseudotachylites from different geological settings (i.e. cryptoexplosion structures, meteorite craters and tectonic settings) the major results are as follows: 1. (1) The deformation phenomena associated with pseudotachylite formation as recorded in quartz and feldspar, occurring as inclusions or at the host rock contact, are undulatory extinction in quartz, brecciation, mechanical twinning and kinking of plagioclase, and characteristic formation of irregular and planar (Vredefort-type) fractures, the latter occurring in quartz. 2. (2) Frequency distributions of crystallographic orientations of planar fractures and their annealed manifestation (planar fluid inclusion trails) are very similar for the Vredefort structure, the Sudbury structure, the Johannesburg dome, the Witwatersrand basin and the Limpopo pseudotachylites. It is therefore concluded that planar fractures in quartz are the typical microdeformation associated with pseudotachylite, irrespective of the geological setting of the latter. Planar fractures alone do not provide unambiguous information on the genesis of the structure in which they are found.