Low-pressure deformation of a small high-level pluton: The syn-tectonic Davys Creek Granite from the southeastern Lachlan Fold Belt, N.S.W.

The Davys Creek Granite (DCG) is a small dyke-assembled granophyre intrusion within greenschist-facies metasediments of the eastern Lachlan Fold Belt, Australia. The granite is a high-level, late syn-tectonic intrusion. Strain in the granite is generally weak but strongly heterogeneous from the microscopic to regional scales. The dominant structure is a crude spaced crack-like cleavage (S) produced by brittle-ductile transitional behaviour involving simultaneous plastic deformation of quartz and groundmass intergrowths and subcritical intergranular crack growth. Strain heterogeneity in the DCG is related to 1. (1) timing of intrusion with respect to waning tectonic forces, 2. (2) strain softness variations due to primary grain-size variations, 3. (3) variable access of aqueous fluids. These factors are typical of epizonal conditions of low confining pressure, abundance of aqueous fluids and rapid and variable cooling rates for small intrusive masses. Subparallel high-temperature solid-state plastic and magmatic foliations were not identified. The weakly-deformed bulbous DCG is contrasted with the nearby, probably older mainly mylonitic gneissic, elongate Wyangala and Wologorong batholiths. Mylonites are confined to initially finest-grained microgranites in the DCG. Deformation style differences relate to lower strain and lower confining pressure associated with DCG deformation. The dyke-like intrusion style of the DCG and above plutons is similar though the DCG is not apparently related to regional-scale ductile shear zones. The absence of complex deformation in the DCG aureole may reflect the lack of a related ductile shear zone.