Sapphirine-bearing rocks with sedimentary and volcanogenic protoliths from the Arunta Block
AT least 15 separate localities for sapphirine-bearing rocks have been found in the Arunta Block, a Precambrian metamorphic complex in central Australia (Fig. 1), although only two have been documented in any detail1–3. Such rocks are the most spectacular examples of a suite of rocks with a distinctive, characteristic chemistry. This suite includes, inter alia, the better known cordierite–anthophyllite rocks, and is widely distributed both within the Arunta Block and world-wide. From field observations it is proposed here that two different geological processes, one sedimentary or diagenetic and the other volcanic, generated rocks that are now indistinguishable in their whole-rock chemistry and mineral assemblages. It had formerly been proposed that sapphirine-bearing rocks of volcanogenic and sedimentary origins were mineralogically and chemically distinct4.
Metamorphic and tectonic evolution of granulites, Arunta Block, central Australia
Petrographical and chemical evidence presented here indicate a prolonged metamorphic evolution of the granulite terrain in the northern Strangways Range, Arunta Block, central Australia (NSR in Fig. 1), beginning with migmatization, followed by dry granulite metamorphism at high temperatures (T) and moderate pressures (P) (estimated at 850–920 °C, 8±1 kbar1). This occurred at ∼1,800 Myr (refs 2, 3), very shortly after deposition of the protolith4, a bimodal volcanic suite5 and associated sediments. Granulite metamorphism was followed by isobaric cooling, during which there was an episode of pervasive but partial hydration. Later retrogression and hydration, localized along shear zones, occurred at the normal geothermal gradient in the kyanite stability field. A tensional, rift-like regime best describes the geological environment in which the protolith formed, was deeply buried, metamorphosed with high heat flow, and then remained in isostatic equilibrium. Metamorphic parageneses recording uplift may be matched with mid-Palaeozoic tectonic events in central Australia. If this correlation is correct, the rocks of the northern Strangways Range may have remained deeply buried for 1,400 Myr, during which the shear zones were reactivated several times (Fig. 2).
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.