Contrasting deformation of S- and I-type granitoids in the Lachlan Fold Belt, eastern Australia

Abstract

Several examples of contrasting deformation under greenschist facies conditions of closely associated, Silurian-Devonian S-type (peraluminous) and I-type (mainly metaluminous) granitoid plutons of very similar age occur in the Lachlan Fold Belt of eastern New South Wales, Australia. They occur in and/or near the Wologorong Batholith, the Wyangala Batholith and the Kosciusko Batholith. The S-type granitoids generally are strongly foliated and locally mylonitic, whereas most of the I-type granitoids typically show alteration to hydrous minerals (chlorite, epidote and actinolite) but are unfoliated or only locally weakly foliated, commonly fractured, and have spaced ductile shear zones. The more obvious and penetrative deformation in the S-type granitoids probably is due mainly to their larger contents of relatively weak and/or ductile minerals, especially quartz and biotite, and to the common occurrence of quartz in relatively large patches, these features providing more continuous strained volumes and enabling quartz- and mica-rich folia to develop between resistant grains of feldspar. In most of the I-type granitoids, the smaller amounts of quartz and biotite, together with the common occurrence of quartz in smaller, more scattered patches, produce more localized strained volumes, and may give rise to local stress concentrations that lead to fracturing. The latter provides access to fluids responsible for hydrothermal alteration. However, some I-type granitoids have high enough proportions of less ductile minerals to develop foliations, although typically this occurs only adjacent to major faults. Some variation in the deformation of S-types also is to be expected. Thus, the main factor controlling the development of foliations appears to be the ratio of strong to weak (or brittle to ductile) minerals, rather than the S/I subdivision (with which it generally correlates), although other possible factors need further investigation. Such contrasting styles of deformation could lead to the erroneous interpretation that the massive I-types are post-deformation plutons and that the S-types (which commonly are slightly older) are foliated by syn-emplacement deformation, whereas in fact both are pre-deformation granitoids.