Complex and imitation tectonites from the Kosciusko Pluton, Snowy hiountains, New South Wales

Abstract

Comparative fabric studies of the Kosciusko granodiorite and its country rocks reveal that apparent similarities in petrology and megascopic fabric can be very deceptive if no proper integration with microfabric investigations is attempted. Thus it may be shown that the granodiorite is locally an “imitationtectonite”, simulating the tectonic megafabric of the country rocks where frictional drag, due to viscous flow along its walls, was considerable. The isoclinally folded metasediments of the country rock have been subject to various crossed strains with B ⊥ B′ = B″. Their characteristic cross-girdle microfabric (due to penecontemporaneous folding and flattening with stretching mainly in b = A′ along two sets of equivalent (O k l) slip planes but also in a = A along the strain-slip cleavage of the fold fabric and a complementary set of (h O l) planes) is preserved in the internal fabric of a completely granitized xenolith, but no trace of it is encountered in the granitic host-rocks. Bodily rotation of the xenolith over 60° or more is further evidence of the emplacement of the granitic rocks by viscous flow. Explanations for the occurrence of flat quartzite rods in a: by differential stretching of competent beds in b and a, or by crystallization in a semi-solid medium subject to tension in a, arise from the multiple-scale fabric comparison of the country rocks and the granodiorites.