The tabular Strathbogie batholith in central Victoria: Response to comments by C. D. Branch

We thank Colin Branch for his astute comments regarding cauldron subsidence as a possible mechanism for granite magma emplacement. We demonstrated that the Strathbogie batholith is thin and sheet-like (or pizza-like) but did not specifically address its detailed emplacement dynamics, other than to suggest overlapping or stacked sheets (Phillips et al., 2022). Our paper primarily concerned the geometry of the pluton as a flat and thin sheet, thereby challenging the paradigm of steep-sided pluton geometries, as is commonly advocated in the Australian literature (e.g. Sambrooks et al., 2022). We were able to determine the geometry through detailed field mapping, in which both pluton floor and roof contacts could be identified, thereby placing firm limits on pluton thickness near the southern margin. Branch does not question this geometry but offers a more holistic model linking subsurface structure with broader dynamic implications, which automatically renders it more speculative. In contrast, we interpreted only as far as the detailed mapping of the southern boundary of the batholith would allow, so Branch’s comment provides a broader genetic perspective, and is therefore welcome. In our figure 1, we provided generic, cartoon sections of granitic intrusions to illustrate the gross differences between what we termed the ‘pipe’ and ‘pizza’ models for the shapes of large intrusive bodies. These are not meant to be valid, alternative interpretations for the Strathbogie batholith. As we show in the paper, the pizza model is the only one that is consistent with our detailed mapping, as well as volumetric considerations. In our paper, we note that many plutons pass downward into narrow feeder structures. However, as shown by geophysics, e.g. for the Cobaw batholith (Cayley et al., 2010) and the Mount Disappointment nested laccolith (Clemens & Benn, 2010), these structures are not pipe-like. In plan, they are linear features, and we would suggest that they correspond to the dykes that Clemens and Mawer (1992) and Petford et al. (1993) advocated as the main physical products of the hydrofracturing by which most granitic magmas must ascend through the crust. We only mention the term ‘pipe’ in the context of the untenable model of steep-sided and vertically extensive batholiths. In agreement with the work of Branch (1966), and also shown for some intrusions in the Andean region (e.g. Myers, 1975), the subsurface cauldron subsidence mechanism of granite emplacement is certainly viable, consistent with the structural features of some plutons, and should be considered as a possibility in studies of high-level granitic bodies. However, for the Strathbogie batholith, the overall shape and the absence of bounding faults are features that suggest that, for this batholith, cauldron subsidence would not be a preferred model. The example that Branch uses from North Queensland involves both intrusive and extrusive rocks. In the Strathbogie Igneous Complex, the analogy would be the slightly older Violet Town Volcanics (VTV), which are contact metamorphosed near the granite, as noted by White (1954). We have no field evidence that the granite intruded extensively beneath the volcanic rocks. Partly as an aside, it could be suggested that the Violet Town Volcanics represent fractionated liquids withdrawn from a deeper crystal mush that later became the Strathbogie batholith. Such a model for relations between volcanic and plutonic rocks has become popular among the volcanological community (e.g. Bachmann & Bergantz, 2004, 2008). However, for the proposed examples (e.g. the Ivrea-Verbano Zone in Italy, Searchlight pluton in Nevada, Yellowstone caldera in the northwestern USA and Altiplano-Puna Volcanic Complex in the central Andes) the geological, geophysical, geochronological and petrological relations suggest that this apparently attractive model is, at best, not proven. For the Strathbogie batholith and the Violet Town Volcanics, Clemens and Phillips (2014) specifically examined the idea of a close petrogenetic relationship between these two S-type units. To quote from that study (p. 15 of 22), ‘The VTV mostly have subtly lower K2O and ASI . than the Strathbogie rocks. Likewise, at the same SiO2 content, although the VTV have similar Sr concentrations . they define a separate trend on the Ba variation diagram . The VTV have generally lower SiO2 than the Strathbogie S-type rocks and their initial Sr/Sr ratios extend to somewhat less radiogenic values. Also, the VTV and the Strathbogie batholith display contrasting trends in their Hf and O isotope covariations suggesting that the two are not co-magmatic (Kemp et al., 2008)’. The Ba mismatch rules out any model in which the Violet Town magmas were extracted as eruptible liquids segregated from a Strathbogie cumulate crystal mush, or that they represent a remobilisation of that mush (Figure 1).