Megacrystic felsic gneisses at Broken Hill: pre to syn-tectonic granites?

J.J. Vassallo and R.H. Vernon, School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia

Felsic gneisses at Broken Hill have been interpreted as metasedimentary (arkoses, volcaniclastic or granitized sediments), volcanic flows, and metagranitoids. Many of them may be metagranitoids, especially those with K-feldspar megacrysts (augen), as suggested by Andrews (1922), Browne (1922), Stillwell (1922), Vernon (1969) and others. They were intruded in the form of sheets, and were emplaced just prior to, or during the early stages of the Olarian orogeny. The sheet-like shapes of many of them may suggest a volcanic or volcaniclastic origin, but many Proterozoic granitoids are of this form (e.g., in the Arunta Block) and deformation could also be a contributing factor.

Three megacrystic felsic gneisses (augen gneisses or granite gneisses) surrounding the Broken Hill Line of Lode are inferred by us to be pre- to syntectonic granitoids that are now represented as deformed orthogneisses. The Feral gneiss is interpreted as a pre-D1 intrusion, as it contains a pre-S1 gneissic foliation that is folded into mesoscopic sinusoidal folds with axial planes approximately parallel to S1. The Alma Gneiss and Rasp Ridge Gneiss, both containing S1 as the earliest foliation, transect and intricately intrude the Feral gneiss, and are interpreted by us as pre- to syn-D1 granitoids.

Evidence suggesting that the megacrystic felsic gneisses were originally granitoids includes: intrusive contacts that cut earlier-formed leucosomes, intrusive contacts between the Feral geniess and the other two gneisses, euhedral to subhedral K-feldspar phenocrysts, microgranitoid enclaves (especially where megacrystic and relatively large), aplite dykes (restricted to plutonic rocks and therefore very reliable indicators), metasedimentary xenoliths, and magmatic foliations overprintedby tectonic foliations. Despite the high mobility of light-ion-lithophile elements during metamorphism, the megacrystic gneisses show chemical affinities with Proterozoic granitoids in the Arunta and Mt Isa Inliers, which is additional evidence for a granitic parent.

K-feldspar megacrysts are much more common in granites than in volcanic rocks, and survive relatively well, even in strongly deformed rocks. Features indicating a magmatic origin for K-feldspar (Vernon, 1986; Vernon & Williams (1988) include: (1) euhedral shape; (2) simple twinning, (3) oscillatory compositional (especially Ba) zoning, (4) zonally arranged inclusions (not inclusion trails), (5) euhedral plagioclase inclusions, (6) alignment of megacrysts in a magmatic foliation or lineation. In contrast, metamorphic K-feldspar is characterized by: (1) irregularly shaped porphyroblasts, (2) general (universal?) absence of simple twinning, (3) rounded inclusions (e.g., Vernon, 1968), (4) lack of oscillatory zoning, and (5) inclusion trails that reflect pre-existing foliations, not zonally (concentrically) arranged.

The full extent of granitoids in the Broken Hill Block is as yet unknown, but felsic gneisses, many of which are megacrystic, appear to be most abundant in the highest-grade metamorphic zone (Brown et al. 1983, fig. 1). A close relationship between high-grade metamorphism and granites is typical of low-pressure/high-temperature (LPHT) regional metamorphic terranes, which are common world-wide and are typical of the Australian Proterozoic. Furthermore, many of the megacrystic granites at Broken Hill show the effects of the first tectonic foliation (Vassallo, 1995), as do markedly transgressive leucogneisses in the north of the Broken Hill and in the Euriowie Block (Stevens et al., 1988, p. 309), indicating that granites were being intruded at the earliest stages of the metamorphic/deformation history. Other evidence also supports the idea of early heating in Australian LPHT terranes (Vernon et al., 1993). Therefore, granites may have contributed to the metamorphic heat, at least locally (e.g., Collins & Vernon, 1991, 1992; Collins et al., 1991; Vernon et al., 1993), although the three felsic gneisses investigated by us appear to have been intruded early, and so may have contributed to the earliest part of the thermal history. If volcanic or subvolcanic activity was involved in the development of the Broken Hill Lode, early granites may have contributed fluids or helped to mobilize external fluids for ore deposition and/or modification. Thus, a plutonic-volcanic origin for the orebody may be appropriate.

Megacrystic felsic gneisses are commonly used as stratigraphic markers in the Broken Hill Block. However, if our inference that at least some of the them are metagranites is correct, they should not be used for stratigraphic correlation, unless they can definitely determined to be of volcanic flow or tuffaceous origin.

Brown, R.E., Stevens, B.P.J., Willis, I.L., Stroud, W.J., Bradley, G.M. & Barnes, R.G. 1983: Rec. Geol. Surv. NSW , 21, 127-226.

Collins, W.J., Vernon, R.H. & Clarke, G.L. 1991:J. Struct. Geol., 13, 1157-1171.

Stevens, B.P.J., Barnes, R.G., Brown, R.E., Stroud, W.J. & Willis, I.L. 1988: Precamb. Res. 40/41, 297-327.

Vernon, R.H., Collins, W.J. & Paterson, S.R. 1993: Tectonophysics, 219, 241-256.

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