Discussion on detachment faulting and bimodal magmatism in the Palaeoproterozoic Willyama Supergroup, south-central Australia: keys to recognition of a multiply deformed Precambrian metamorphic core complex

Journal of the Geological Society, Mar 2005 by Connor, C H H, Pries, W V, Page, R W, Stevens, B P J, Et al

C. H. H. Connor, W. V. Pries, R. W. Page, B. P. J. Stevens, I. R. Plimer and P. M. Ashley write: Gibson & Nutman (2004) postulate that the Willyama Supergroup in the southern Curnamona Province contains a 1690-1670 Ma metamorphic core complex, and imply that this is relevant to the genesis of the Broken Hill Pb-Zn-Ag deposit. We contend that this is a model-driven, speculative interpretation predicated upon unsupported assertions that conflict with substantial stratigraphic and geochronological data. Not only does the paper fail to demonstrate an early high-grade event and formation of a metamorphic core complex at 1690-1670 Ma, but it is also factually incorrect in several critical geological and geochronological aspects. The detachment concepts of Gibson & Nutman may appear persuasive, but only by omitting reference to a huge body of published and recent mapping and laboratory research, by companies, universities, and geological surveys, some of which are cited here. Surprisingly, Gibson & Nutman ignore much recent work (e.g. Page et al. 2004) to which one of them has contributed.

The essence of the model is that a low-angle, extensional 'detachment' formed along the boundary between the Broken Hill and Sundown Groups in the Broken Hill Domain, and between the Bimba and Plumbago Formations in the Olary Domain (Gibson & Nutman, fig. 1). The 'detachment' was alleged to have occurred as part of a sillimanite-grade D^sub 1^ deformation between 1690 and 1670 Ma, and to have provided a channel for hydrothermal fluids that could have been responsible for regional scale Na (-Fe) alteration and formation of Pb-Zn mineral deposits, including the Broken Hill orebody. As pointed out below, most of the evidence is misinterpreted, misleading and/or ambiguous, and the geochronological case advanced by Gibson & Nutman for such an early (1690-1670 Ma) high-grade event has no foundation.

Ambiguities in the model. Gibson & Nutman have difficulty in defining both the timing and position of their 'detachment'. In Gibson & Nutman (fig. 3) the 'detachment' developed anywhere between 1720 and 1670 Ma. In their figure 1, the proposed 'detachment' is illustrated at the base of the Broken Hill Group, but in figure 2 the 'detachment' postdates the topmost unit of Broken Hill Group, the 1685 ± 3 Ma metavolcanic Hores Gneiss (Page et al. 2000; Stevens & Barren 2002). Similarly, Gibson & Nutman (p. 60) describe the location of their sample 96-BH-13 as 'within or just below the detachment surface', but the amphibolite was emplaced immediately below the Broken Hill Group, much lower than the proposed 'detachment'.

Gibson & Nutman (p. 65) infer that the c. 1675 Ma Pb model age (Carr & Sun 1996) for the Broken Hill orebody 'is more closely aligned with the inferred age of peak MI metamorphism (1670-1690 Ma) than the age of sedimentary deposition in the lower plate (≥ 705 Ma by Page et al. 2000).' But it is the 1685 ± 3 Ma Hores Gneiss (Page et al. 2000), host to the Broken Hill orebody, that constrains the age of the Gibson & Nutman 'detachment", not the ≥ 1705 Ma age derived from lower in the succession. Additionally the c. 1675 Ma Pb model age is now recalculated at c. 1690Ma (Page et al. 2005), and this closely approximates the depositional age of Hores Gneiss.

Importantly, if the 'lower plate' has moved upwards as a core complex, the 'lower and upper plates' should be unrelated, having originated from different crustal depths. The presence of the thin, precisely dated, 1693 ± 3 Ma Plumbago Formation (Page et al. 2000, 2005; Conor 2004) in both the proposed 'upper plate' and Mower plate', indicates that there has been no such displacement, and that both 'plates' are part of a single stratigraphie succession. Willis et al. (1983) indicated this to be the case, a conclusion in accord with recent geochronoiogy (Page et al. 2000).

Erroneous map detail. Gibson & Nutman utilize their only detailed map (fig. 5, c.45 km^sup 2^ Tommie Wattie Bore-Ameroo Hill area) as evidence for their extensional detachment, which they claim juxtaposes the Wiperaminga Subgroup in their 'lower plate' against Saltbush Subgroup in their 'upper plate'. In disregarding much previous mapping (e.g. Laing 1996), figure 5 illustrates the weakness of the detachment model, because Gibson & Nutman have mistakenly placed their 'detachment' between the lower psammitic and upper pelitic parts of a single, c. 2 km thick, upwards-fining package, i.e. Tommie Wattie Formation within the Wiperaminga Subgroup, i.e. 'lower plate'.

Bimodal magmatism. Gibson & Nutman claim abundant bimodal magmatism in the 'lower plate' over the period 1710-1670 Ma, but the only bimodal magmatism was the mafic-dominated 1690-1680 Ma event of the Broken Hill Domain (Gibson & Nutman; Page et al. 2005), and the equivalent c. 1685 Ma Lady Louise Suite in the Olary Domain (Conor & Fanning 2001). Neither the A-type Basso Suite of the Olary Domain (Ashley et al. 1996; Conor 2000), comprising c. 1715-1710 Ma felsic volcanic and intrusive rocks (Page et al. 2000), nor the 1704 ±3 Ma granitic Alma Gneiss (Page et al. 2000) include confirmed mafic components.