Honours Thesis Abstract
Deformation of the Entia Gneiss Complex, Harts Range, central Australia; implications for the dynamics of the Alice Springs Orogen
Stacey Curtis
University of Adelaide, SA
2005 PIRSA - AIG Honours Bursary Winner
The Entia Gneiss Complex (EGC) is located in the Harts Range, central Australia, and is a key tectonic element of the Alice Springs Orogeny (450-320Ma). It is defined by a sub-horizontal fabric cropping out in a domal structure through a cover sequence. In the southeast of the domal structure it is intruded by a voluminous suite of pegmatite dykes. Mapping of the structural relationships between the sub-horizontal gneissic fabric and pegmatites suggests that the gneissic fabric formed by coaxial flattening within a mid-crustal pure shear-dominated ductile transpressional shear zone, which is both kinematically and temporally connected with the Arltunga Nappe Complex, a southwest-directed piggy-back thrust system at the southeast margin of the Arunta Complex. Deformation of the EGC occurred in two stages, and involved both spatial and temporal partitioning of strain and fabric reworking. The metamorphic conditions and timing of these events have been determined by PT calculations of mineral assemblages defining the fabric and electron microprobe analysis monazite dating. The first phase consists of fabric development and metamorphism at ~730°C and 10kbar, and occurred at 390±11Ma. The second phase consists of fabric reworking and decompression and cooling to ~630°C and 7kbar at 334±8Ma during thrusting of the EGC and HRMC over the Illogwa Shear Zone and underlying Paradise Nappe system. Transport-parallel localised extension occurred by gravity-induced thinning in the thrust sheet, accommodating voluminous pegmatite dykes. Amphibolite dykes cross-cutting the compositional layering of the EGC but deformed by the gneissic fabric may represent a previously unrecognised major phase of extension during the Alice Springs Orogeny. The dykes represent the first evidence of potential mantle involvement in the Alice Springs Orogeny, and may account for the heat required to generate multiple generations of felsic melt in the Harts Range, of which the pegmatite dykes of this study are a part, and explain why the Arunta Complex remained reactive to deformation for such an extended period.