The Yarra granite: An intradeformational pluton associated with ductile thrusting, Lachlan Fold Belt, southeastern Australia

The Yarra granite is a deformed S-type pluton on the eastern edge of the Wyangala batholith intruded into deformed early Paleozoic metasedimentary rocks. Detailed structural studies of the pluton include the character and geometry of the foliation and its history of development in both the pluton and wall rock, the relationship of the foliation to the pluton/wall-rock boundary, and microstructural relationships in the contact aureole. Evaluation of the above and regional data indicate that pluton emplacement is broadly synorogenic; in detail, however, it is post-tectonic relative to an early phase and pretectonic relative to later phases of deformation, and it may be more accurately classified as intradeformational.

Structures in the wall rock have a complex history which indicates that at least three cycles of transposition of planar elements have occurred, giving rise to composite structures often exhibiting largely parallel orientation and comparable morphology. These multiple composite structures show a gentle to moderate westerly dip, a down-dip orientation of stretching lineations and rotated fold axes, and consistent west-over-east asymmetry of various kinematic markers. Field and laboratory data indicate that transposition occurred in a gentle to moderately dipping zone of ductile thrusting under upper-greenschist- facies conditions.

The Yarra granite was deformed while it still contained considerable magmatic heat, which promoted and facilitated ductile deformation of the wall rocks and pluton during the thrusting event. We speculate that movement along the zone of ductile thrusting may have attenuated the pluton at depth and possibly detached it from its source. If this is so, it implies that the Yarra granite (and possibly the eastern part and/or all of the Wyangala batholith) may have been brought up from some depth and displaced eastward along the zone of ductile thrusting.