Can the size of K-feldspar phenocrysts be used to map the occurrence of mylonitic shear zones? Results from the Wyangala Granite, Eastern Lachlan Fold Belt, New South Wales

Abstract

We report the results of mapping the high-strain (shear) zones in the Wyangala Granite. The high-strain zones are part of the Wyangala Fault System and primarily occur along the eastern margin of the Wyangala Granite, Eastern Lachlan Fold Belt in New South Wales. The aim of this study was to determine if the size of K-feldspar phenocrysts could be used to map variations in strain intensity in the Wyangala Granite and map the location of these high-strain shear zones. The high-strain zones were classified into components of relative strain intensity on the basis of K-feldspar phenocryst grainsize, the presence of S and C fabrics (and the angle between the foliations) and the width of mylonite and ultramylonite outcrops. Our results suggest that there is no correlation between the size of K-feldspar phenocrysts and recognised indicators of high strain, such as the acute angle between S and C fabrics, and thickness of bands of mylonite/ultramylonite. This may indicate that the size of K-feldspar in the Wyangala Granite was not homogeneous after crystallisation and prior to deformation. We therefore suggest that the size of K-feldspar phenocrysts is not a good guide to the strain intensity. However, the high-strain shear zones in the Wyangala Granite can be mapped using the acute angle between S and C fabrics and the intensity and thickness of mylonites/ultramylonites. This work suggests that the high-strain zones are not continuous along strike.