Strathbogie batholith: Field-based subdivision of a large granitic intrusion in central Victoria,Australia

Abstract

The Late Devonian Strathbogie batholith is a 1500 km², semi-contiguous mass of undeformed, peraluminous monzogranite and syenogranite in SE Australia. The rocks are S-type and contain abundant igneous cordierite. Internal variations within the batholith have been investigated here by systematic fieldwork guided by modern concepts of granite batholith structure, combined with geophysical and geomorphological data. Important field parameters include the outcrop character and abundance of tors and pavements, feldspar phenocryst size and abundance, groundmass grain size, the textures, grain sizes and abundances of quartz, cordierite, garnet and biotite, and the presence or absence of tourmaline, vugs and enclaves. Mapping has been supplemented with aeromagnetic data to define the external boundaries of this essentially non-magnetic batholith, but these data cannot be used to delineate internal structures. The K-Th-U distribution, from airborne radiometric surveys, was found to assist in sub-division of the batholith, and the digital elevation model formed the basis of an interpretation of batholith structure, including lineaments. Some parts of the batholith have quite distinctive field characteristics and are interpreted as discrete plutons (e.g. the Kerrisdale and Lightning Ridge plutons). Broad trends in feldspar phenocryst abundance and the presence of tourmaline and vugs (miarolitic cavities) indicate an upper contact (roof) of the batholith in the northwest. Cordierite is ubiquitous, most rocks contain a few percent of biotite, there is widespread accessory garnet, and tourmaline is present within the inferred near-roof zones. The field-based approach described here offers a method for mapping large batholiths without resorting to grid sampling and the accumulation of vast sets of whole-rock analyses. This field-based method is especially applicable in cases where there are no obvious pluton boundaries, such as internal wall-rock screens or contacts exposed on glacial pavements. The field approach records outcrop- and district-scale variations, and many textural and mineralogical features that would be omitted in any map based only on geochemistry.