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

Abstract

The Late Devonian Strathbogie batholith is a 1500 km 2 , 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 nonmagnetic 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.

Inferring a deep-crustal source terrane from a high-level granitic pluton: the Strathbogie Batholith, Australia

“…EA1.1) are relatively widespread, in low abundance, in thin sections, especially of samples from the Mount Wombat and Lima plutons. Cordierite is very widespread in outcrop and its size and abundance were systematically recorded by Phillips and Clemens (2013). As pointed out by Phillips et al (1981), cordierite is the mafic mineral that typifies this batholith.…”

“…Thus, since these rocks were regionally metamorphosed and folded in the Middle Devonian, there was a period of uplift and erosion of several kilometres before the Strathbogie Igneous Complex was emplaced. The Strathbogie batholith is overwhelmingly S-type in character, apart from two tiny hornblende-bearing Itype plutons (Phillips et al 1981, Phillips andClemens 2013). The granitic rocks contain no obvious foliations or lineations, apart from some magmatic flow structures delineated by alignment of the K-feldspar phenocrysts.…”

“…From west to east these are -Kerrisdale, Dabyminga, Mt Wombat, Lima and Lightning Ridge plutons. The two tiny plutons of I-type Tallangallook Hornblende Microgranite have not been individually dated, though they appear to intrude their immediately surrounding S-type granitic rocks (Phillips and Clemens 2013).…”

“…The digital terrain model, the airborne radiometric (K, Th, U) surveys and the Bouger gravity anomalies provide data important in defining the map areas occupied by the various plutons (Phillips and Clemens 2013). The following is a very brief summary of the important points, in the petrogenetic context.…”

“…The gallery of photomicrographs in Electronic Appendix 1 illustrates some of the mineralogical and textural features of the plutonic rocks and their ME. Further descriptions, particularly of the garnet and cordierite varieties can be found in Phillips et al (1981), whereas Phillips and Clemens (2013) presented photographs of and data on the mesoscopic textures. Many of the S-type Strathbogie rocks are porphyritic with potassic alkali feldspar phenocrysts, and the groundmasses of the rocks are equigranular to seriate.…”