Petrogenesis of orthogneisses in the Dry Valleys region, South Victoria Land

Abstract

Granitoid gneisses intercalated with Koettlitz Group metasediments in the upper Ferrar, Taylor and Wright valleys of South Victoria Land comprise various hornblende+biotite orthogneisses and biotite orthogneisses, including the km-scale Dun and Calkin plutons. K-feldspar megacryst inclusion textures and discordant cross-cutting relationships with enclosing metasediments are interpreted as firm evidence of an intrusive origin for hornblende+biotite and biotite orthogneiss. The scale of several concordant orthogneiss bodies (including the Dun and Calkin plutons), the presence of mafic enclaves, and relict flow differentiation in hornblende+biotite orthogneiss are also compatible with a plutonic origin. Orthogneisses were emplaced prior to deformation that produced macroscopic upright, tight, folds about NW-trending axes. Petrography and geochemistry indicate I-type affinities for hornblende+biotite orthogneisses and the Dun Pluton. Hornblende+biotite and biotite orthogneisses (with the exception of the Dun Pluton) are part of a single petrogenetic suite, together with younger Bonney, Valhalla, and Hedley plutons. Emplacement of a continuum of I-type intrusives is envisaged which spanned Koettlitz Group deformation, and possibly caused much of the deformation. Hornblende+biotite and biotite orthogneisses are deformed precursors to the younger Bonney, Valhalla, and Hedley plutons. The Dun Pluton contains Fe-rich salitic clinopyroxene relicts and exhibits a unique geochemistry. It is rich in Sr, Al₂O₃, Na₂O, and poor in FeO, K₂O, Rb, Y, V. Chemical and petrographic features indicate an evolved body, possibly derived from a primitive source distinct from other orthogneisses and granitoids.