Pluton emplacement in a strike-slip fault zone: the Doctors Flat Pluton, Victoria, Australia

Abstract

The Siluro-Devonian Doctors Flat Pluton, Lachlan Fold Belt, Australia, is a hornblende-biotite granite (I-type) that was emplaced into the Early Silurian Omeo Metamorphic Complex. It lies along the Ensay Fault, a major dextral strike-slip fault. A solid-state foliation is developed in much of the pluton, with the most intense development at the outer parts, grading into massive granite at the centre. This foliation is approximately parallel to trace of the Ensay Fault and to the borders of the pluton. It grades into a mylonitic fabric along the Ensay Fault. In the eastern part of the pluton a second fabric is developed, giving rise to S- C fabrics indicating dextral shear. Some microfabrics in the pluton are typical of relatively high-temperature deformation, consistent with syntectonic emplacement, but lower temperature microstructures overprint them and are dominant. A possible mechanism of intrusion involves the pluton being emplaced into a dilational section of the Ensay Fault when a jog in the fault opened up into a pull-apart structure. As intrusion continued, the northeastern branch of the pull-apart took up all the movement, with later intrusive material being less deformed. Along the Ensay Fault, lower grade assemblages of chlorite + epidote + albite + sphene and cataclastic textures developed at lower temperatures as fault displacement continued during and after cooling of the pluton.