High temperature, high strain rate deformation in the lower crustal Kalka Intrusion, central Australia

Abstract

Mafic and ultramafic gneisses (blastomylonites) characterised by a strongly developed planar lenticular foliation-layering are developed in lower crustal (high pressure) cumulates of the stratiform Kalka Intrusion in central Australia. The gneisses occur in two distinct zones, the Numbunja and West Kalka Gneissic Belts, parallel to or at low angles to igneous layering. They developed during and shortly after the final stages of crystallization of the intrusion at temperatures between about 1000 and 1200 ° C, and represent an incipient thrusting deformation probably with associated high strain rates. The most deformed textures in the centre of the belts typically consist of megacrysts of orthopyroxene, clinopyroxene, olivine and plagioclase showing abundant features of extreme ductile flow within a finegrained syntectonically recrystallized matrix of the same material. Mineral elongation, kink band orientation and matrix petrofabrics indicate that the maximum principal elongation and the maximum principal shortening occurred parallel to the lineation and normal to the foliation respectively.