Successive overprinting granulite facies metamorphic events in the Anmatjira Range, central Australia

Abstract

The Anmatjira Range and adjacent Reynolds Range, central Australia, comprise early Proterozoic metasediments and othogneisses that were affected by three, and possibly four, temporally distinct metamorphic events, M1–4, and deformation events, D1–4, in the period 1820–1590 Ma. The north‐western portion of the range, around Mt Stafford, preserves the effects of ±1820 Ma M1‐D1, and shows a spectacular lateral transition from muscovite + quartz‐bearing schists to interlayered andalusite‐bearing migmatites and two‐pyroxene granofelses that reflect extremely low‐pressure granulite facies conditions, over a distance of less than 10 km. Orthopyroxene + cordierite + garnet + K‐feldspar + quartz‐bearing gneisses occur at the highest grade, implying peak conditions of ±750°C and 2.5 ± 0.6 kbar. An anticlockwise P–T path for M1 is inferred from syn‐ to late‐D1 sillimanite overprinting andalusite, petrogenetic grid considerations and quantitative estimates of metamorphic conditions for inferred overprinting assemblages. The effects of M1 have been variably overprinted to the south‐east by a c. 1760 Ma M2–D2 event. Much of the central Anmatjira Range, around Ingellina Gap, comprises orthogneiss, deformed during D2, and metapelites that have M1 andalusite and K‐feldspar overprinted by M2 sillimanite and muscovite. The south‐eastern portion of the range, around Mt Weldon, comprises metasediments and orthogneisses that were completely recrystallized during M2–D2, with metapelitic gneisses characterized by spinel + sillimanite + K‐feldspar + quartz‐bearing assemblages that suggest peak M2 conditions of >750°C and 5.5 ± 1 kbar. Overprinting parageneses in metapelitic gneisses imply that D2 occurred during essentially isobaric cooling. A third granulite facies event, M3, affected rocks in the Reynolds Range, immediately to the south of the Anmatjira Range, at c. 1730 Ma. A possible fourth event, M4, with a minimum age of c. 1590 My affected both Ranges, but resulted in only minor overprinting of M1–3 assemblages. The superimposed effects of M1–4, mapped for the entire Anmatjira–Reynolds Range area, indicate that only minor or no dislocation of the regional geology occurred during any of the metamorphic and accompanying folding, events. Although the immediate cause of each of the metamorphic events involved advection, the ultimate causes were external to the metasediments and most probably external to the crust.