Provenance and tectonics of the late Archaean Mulgathing Complex, central Gawler Craton: geochronological, geochemical and isotopic evidence for plume-arc interaction

Abstract

Geochemical and Sm-Nd and U-Pb zircon isotopic data from late Archaean metasedimentary rocks of the Mulgathing Complex in the central Gawler Craton, South Australia, point to a dominance of late Archaean source terrains. Detrital zircon age data from the Kenella Gneiss are dominated by ca. 2700, 2600 and 2560 Ma age groups and the bulk rock composition shows uniform HREE depletion ((La/Yb)n = 76-17), suggesting a felsic volcanic and/or granitic source. Nd depleted mantle model ages (TDM) between ca. 3000-2800 Ma are older than the bulk of the detrital zircon ages, suggesting that the protoliths to the Kenella Gneiss were derived from sources dominated by crustal reworking processes as opposed to first cycle sources dominated by juvenile material. In contrast to the Kenella Gneiss, the Christie Gneiss metapelites have flatter REE patterns ((La/Yb)n = 29-3), but a similar range of TDM ages, suggesting a greater mafic component in their provenance. Metasediments and volcaniclastics associated with the komatiite-bearing Lake Harris Greenstone Belt (LHGB) are dominated by flat REE patterns ((La/Yb)n = 15-10) pointing to a significant mafic input, consistent with the presence of interlayered mafic and ultramafic rocks. Detrital zircons in the LHGB metasedimentary and volcaniclastic rocks range down to ca. 2505 Ma. These data, combined with existing U/Pb age constraints for cross cutting felsic rocks within the greenstones, indicate that the LHGB sequence was deposited at ca. 2505 Ma. This age is analogous to the youngest zircons in the Devil’s Playground Volcanics, which are a sequence of Nb- and Ti-depleted felsic to intermediate volcanics in the NE Gawler Craton. This suggests that the LHGB sequence may be been deposited synchronously with eruption of the Devil’s Playground Volcanics. Conceivably, the late Archaean sequences now preserved in the central Gawler Craton were deposited in an evolving back-arc style setting in which the Devil’s Playground Volcanics represent felsic arc volcanism, and the LHGB localised mantle-plume associated rift development within the broader basin that contained the protolithic sequences to the Kenella and Christie gneisses, deposited between ca. 2535-2510 Ma. Collisional closure of the back-arc system is recorded by the ca. 2500-2420 Ma Sleafordian Orogeny, which was associated with regional high geothermal gradient amphibolite-granulite grade metamorphism, an event recorded by extensive metamorphic zircon growth in the Kenella Gneiss and limited zircon growth in LHGB metasedimentary rocks.