Introduction
The pressure, temperature and time (P-T-t) path experienced by a metamorphic terrane provides significant information about metamorphic processes and the dynamics of orogenic systems. Building these P-T-t paths however, is not a trivial task and requires a full understanding of the mineralogical and textural relationships preserved within the rocks. In this study we show an example where likely mineral misidentification led to errors in inferences about both P-T-t trajectory and tectonic scenario.
In the central Musgrave Block, central Australia, Maboko et al. (1991) and Ellis and Maboko (1992) reconstructed a P-T-t path that invoked changing physicochemical conditions during one metamorphic episode at ~1200 Ma to account for the textures preserved in granulites, and involved the following sequence of events.
(M1) Granulite-facies metamorphism at ~40 km depth and 850-900°C,
(M2) isobaric cooling and deformation of the granulites to form eclogite,
(M3-M4) rapid tectonically driven exhumation, with little cooling of the lower part of the thickened crust to a depth of 15-20 km at ~1190 Ma, and
(M5) deformation and metamorphism at greenschist- to lower amphibolite-facies during the Petermann Orogeny (~550 Ma) after a residence period of ~640 Ma in the mid-crust with a total exhumation of only 2-7 km in the interval between M4 and M5.
The tectonic mechanism envisaged to produce rapid exhumation in the time interval between M1 to M4 (<20 Ma), involved extensional unroofing, along normal faults that lie outside the immediate area.
Figure 1. Geological map of the Mt Woodroffe region
Geological map of the Mt Woodroffe region showing the distribution of the major faults, shear zones, granulite-facies gneiss and the areas affected by the high-pressure overprint. Modified after Major (1973) and Camacho et al. (1997).
A problem with this P-T-t trajectory is that mafic dykes as young as ~840 Ma (Zhao and McCulloch, 1993) are deformed by the eclogite-facies overprint (Camacho et al., 1997) indicating that the metamorphic and deformational textures developed between M2 to M4 needed re-evaluation.