High-grade reworking of central Australian granulites. Part 1: Structural evolution

Abstract

Four distinct deformational events (D 2–D 5) accompanying a granulite facies metamorphic cycle (M 2–M 5), are shown to structurally overprint (rework) pre-existing granulites (M 1) of the northeast Strangways Range in the central Arunta Block. The first metamorphic cycle (M 1) at 1800 Ma, involved an anticlockwise P- T path peaking at 850–950°C at 8–9 kbar. M 1 metamorphism involved widespread partial melting that produced stromitic migmatites and map-scale concordant granitic gneisses. Isobaric cooling after the peak, and accompanying hydration, gave rise to a wide variety of coronitic and symplectitic reaction textures that enclose and replace M 1, mineral parageneses. No kinematic structuring is associated with M 1, and the period encompassing the thermal peak and immediately subsequent are thought to have been absent of deviatoric stress. The second metamorphic cycle (M 2–M 5), possibly at 1400–1500 Ma, involved a clockwise P- T path with a maximum P of > 9–10 kbar and a thermal peak of approximately 800°C. This metamorphic cycle accompanied the major ductile deformations recognized (D 2–D 5), and has been labelled the “Proterozoic Reworking”. The Proterozoic Reworking has been divided into two thermo-barometric and structurally distinct periods. D 2–D 3 involved regionally inclined, ENE-WSW non-coaxial shear of high bulk shear strains, giving rise to an intense (often mylonitic) pervasive fabric ( S 2- L 2) and isoclinal and sheath folds on all scales ( F 2, F 3). S 2- L 2 is the first recognized kinematic fabric and does enclose and partially recrystallize M 1 mineral parageneses, including late-M 1 metamorphic reaction textures. D 2–D 3 deformation was due to crustal shortening and gave rise to crustal over-thickening (loading) accompanying prograde metamorphism (M 2). D 4–D 5 involved upright, open and asymmetrical folding ( F 4) and E-W-trending shear zone development ( S 5) within a regionally extensive system of inclined, oblique, sinistral transpression. Both D 2–D 3 and D 4–D 5 episodes occurred under the same approximately E–W-directed compressive stress, and are considered sequential episodes in the one tectonothermal cycle. F 4 folding accompanied significant melt formation and the peak of metamorphism of the M 2–M 5 metamorphic cycle. S 5 shear zones accompanied 3–4 kbar of decompression with cooling, presumably during uplift in isostatic response to crustal over-thickening in D 2–D 3. Progression from inclined non-coaxial shear to transpression occurred in response to an increase in the relative buoyancy forces limiting crustal thickening, as is typical of many compressional orogens. However, the Proterozoic Reworking is atypical of simple linear or arcuate mountain belts because late-stage shearing ( S 5) is aligned sub-parallel, not at a high angle, to the transport vector of the earlier ductile deformations (D 2–D 3). Consequently, this region is modelled in terms of crustal shortening directed along the length of an E–W-trending orogen confined to the north and south by relatively stable crustal blocks. When E–W shortening could no longer be accommodated by crustal thickening, strain was partitioned by sinistral transpression between the north and south bounding crustal blocks.