Journal of the Virtual Explorer

Maboko et al. (1989; 1991) envisaged a metamorphic history characterized by rapid decompression with little cooling from about 12 kbar and 760°C to about 5 kbar and 630°C (Fig. 7). Ilmenite-fibrolite intergrowths in quartzo-feldspathic lithologies, were considered to have resulted from the breakdown of garnet suggesting a reaction of the type:

This reaction texture was inferred to have formed during isothermal decompression between M2 and M4. As a consequence, in the mafic lithologies, garnet-clinopyroxene growth was also regarded to have formed during decompression, although this assemblage does not normally form during a decrease in pressure. In addition, Maboko’s et al. (1989; 1991) P and T estimate of ~6 kbar and 715°C, respectively indicates that the geothermal gradient was ~36°C km^-1, which is considerably higher than current stable cratonic geotherms of about 25°C km^-1 and required the Petermann Orogeny to have been accompanied by an episode of increased heat flux.

Figure 7. Summary of P-T-t trajectories

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Proposed by (a) Maboko et al. (1989; 1991). Red curve: Granulite-facies metamorphism at ~1200 Ma (M1) followed by isobaric cooling and rapid decompression in < 20 Ma (M2-M4). Yellow curve: Cooling history from the Mesoproterozoic to ~550 Ma (M5). (b) This study. Black path: Granulite-facies metamorphism at ~1150 Ma followed by exhumation to mid-crustal level and cooling by about 1100 Ma. The granulites resided at these mid-crustal levels for ~600 Ma before rapid reburial followed by exhumation during the Petermann Orogeny at ~550 Ma (blue path). Numbers in brackets are dates in Ma.

Throughout the Musgrave Block, field and petrographic observations suggest that multiple metamorphic and deformation reaction textures are not present in the eclogite-facies mylonites, which is evidence against multiple overprinting events (Camacho et al., 1997; Scrimgeour et al., 1999; Raimondo et al., 2010). Assemblages containing rutile and kyanite are found only in rocks that have experienced deformation at high-pressure. These minerals have not been found in any of the granulite-facies assemblages (Camacho et al., 1997). In addition to these observations, the fact that the kyanite-titaniferous magnetite mats give compositions that represent an aluminosilicate rather than garnet indicate that reaction 3 did not take place. Thus, we consider that kyanite-bearing assemblages formed at ~12 kbar during peak metamorphic conditions rather than during isothermal decompression. Accordingly, our proposed geodynamic scenario is quite different from that proposed by Maboko et al. (1989; 1991) (Fig. 7) and is summarized as follows.

Magmatic heat input to produce the granulites at ~1200-1150 Ma (Stage 1) was followed by a residence period of > 300 Ma in the mid-crust (Stage 2) before crustal thickening and reburial during the Petermann Orogeny (Stage 3) to depths of ~40 km followed by exhumation to the mid-crust during the same orogeny (Camacho and McDougall, 2000). The estimate for residence in the mid-crust is based on ages of ~1100 Ma for minerals with a closure temperature of ~350°C for the argon system and mineral assemblages in mafic dykes (Camacho et al., 2009). Based on the T and P estimates of Camacho et al., (1997), the calculated geothermal gradient is ~16.5°C km^-1 and implies that the deep crust was not significantly perturbed thermally during the Petermann Orogeny, a finding that is consistent with other studies in the Musgrave Block (Scrimgeour and Close, 1999; Raimondo et al., 2010).