Geologic Setting

The east-west trending Musgrave Block is a Meso-Neoproterozoic mobile zone consisting of high-grade metamorphic and intrusive rocks covering an area of about 120,000 km2 in the centre of the Australian continent. Felsic to mafic volcanic rocks in the central Musgraves were metamorphosed at granulite-facies during the ~1200-1150 Ma Musgravian Orogeny (Camacho and Fanning, 1995). Felsic magmatism, partly coeval and partly after this high-temperature event was followed by two episodes of extension and emplacement of dolerite dyke swarms at ~1070 Ma and ~830 Ma (Zhao and McCulloch, 1993). These rocks were heterogeneously overprinted by high-strain zones at ~550 Ma (Petermann Orogeny) under eclogite to greenschist-facies conditions (Maboko et al., 1992; Camacho et al., 1997). These zones trend east-west with a combined strike-slip and reverse movement, and occurred in an intraplate setting (Camacho and McDougall, 2000).

High-strain deformation and metamorphism at eclogite-facies conditions (T ≈ 660°C and P ≈ 12 kbar) in the Musgrave Block appears to be confined to the granulite-facies gneisses (Fig. 1). In shear zones, this deformation event obliterates all pre-existing fabrics by development of a strong, subhorizontal, rodding lineation on high-strain planes, and quartz and feldspar ribbons in the quartzo-feldspathic layers (Camacho et al., 1997). Mylonitized gneiss preserves some of the granulite-facies minerals such as garnet and plagioclase (Fig. 2). Detailed descriptions of the eclogite-facies rocks are published elsewhere (Camacho et al., 1997; Ellis & Maboko, 1992; Camacho et al., 2009) but are overviewed below.

The zone affected by eclogite-facies deformation lies between the eclogite-facies north Davenport shear zone and the Mann Fault (Fig. 1).

Figure 2. Mesoproterozoic granulite felsic gneiss overprinted at ~550 Ma by high-strain deformation at ~12 kbar.

Mesoproterozoic granulite felsic gneiss overprinted at ~550 Ma by high-strain deformation at ~12 kbar.

(a) Plane-polarized light. Mylonitic foliation deflected around relict granulite-facies garnets (Grt1) with fringes of new garnet growth. (b) Plane-polarized light, (c) Reflected light, same field, both rotated 45° relative to (a). New garnet (Grt2), rutile (Rt), magnetite (Mag) formed in pressure shadows and in the foliation. Magnetite (with ilmenite exsolution lamellae) and rutile, in textural equilibrium (at the margin of Grt1). Note how magnetite infills the fracture in Grt1. Section parallel to the stretching lineation and normal to the foliation. Abbreviations after Kretz (1983). Rectangles outlined in 2(a) correspond to areas shown later in Figs 3 and 4(a).