Delamerian Glenelg tectonic zone, western Victoria: geology and metamorphism of stratiform rocks

Abstract

The Cambro-Ordovician Glenelg tectonic zone of western Victoria is a distinctive metamorphic-igneous segment of the Delamerian Orogenic Belt comprising two northwest-striking regional metamorphic segments of andalusite-sillimanite type prograding towards an axial granitic batholith. The second of five deformations (D2) was most significant, producing isoclinal folds, transposition and a pervasive regional foliation (S2). Southwest of the central batholith, biotite to migmatite zones contain mainly quartzo-feldspathic rock (turbiditic metagreywacke, quartzo-feldspathic schist and migmatite), plus less common metaquartzite and calc-silicate rocks and minor metapelite. Metagabbro, metadolerite and amphibolite typically have the chemistry of mid-ocean ridge basalts. Serpentinite pods and sheets were tectonically introduced to low-grade areas. Northeast of the central batholith, quartzo-feldspathic rock occupies the sillimanite and migmatite zones exclusively, with a regional concentration of pegmatites adjacent to the zone boundary. Gross interleaving of quartzo-feldspathic schist, migmatite, pegmatite and muscovite-bearing granitic rock is characteristic. Peak metamorphic conditions of 550 MPa at 640°C leading to migmatite formation were established by D2 time and accompanied by tonalite-granodiorite and pegmatite emplacement. Subsequently, the thermal high contracted to the northeast culminating in the more extensive syn-, post-D4 to pre-D5 granitic magmatism.