Provenance of Late Archaean metasedimentary rocks on the southern Gawler Craton: Implications for its early crustal development

Abstract

Geochemical, whole rock Sm-Nd and detrital zircon U-Pb isotopic data from Archaean metasedimentary successions in the southern Gawler Craton indicate derivation from Late Archaean sources. Detrital zircons from the upper amphibolite-grade Wangary Gneiss have dominant U-Pb age groupings at 2500-2580 Ma and 2600-2720 Ma, with a few analyses ranging up to 2950-3150 Ma. Steep LREE enrichment REE with (La/Yb)N = 46 – 58 and low abundances of compatible trace elements point to a major felsic source component, with REE patterns typical of Na-rich granites. Detrital zircons from the metasedimentary dominated Hall Bay Volcanics (2500-2560 and 2660-2710 Ma) show a similar range of detrital zircon ages to the Wangary Gneiss, suggesting both sequences were derived from a temporally similar Late Archaean source terrain. The Carnot paragneisses and the Hall Bay Volcanics metasedimentary rocks show comparative HREE enrichment ((La/Yb)N = 7.4-12.5 and (La/Yb)N = 10.2-15.7 respectively), suggesting a greater input of mafic or intermediate material relative to the Wangary Gneiss. The correspondence in detrital zircon ages between the Wangary Gneiss and Hall Bay Volcanic metasedimentary rocks suggests both units form part of the same succession. Existing zircon U-Pb data provides no evidence for input of Early Archaean crust into the Wangary Gneiss, Hall Bay Volcanics or Carnot paragneisses, which is further substantiated by Sm-Nd depleted mantle model ages that range from 2900 to 3200 Ma in all metasedimentary rocks. The overlap of detrital zircon and depleted mantle model ages suggests that all three successions in the southern Gawler Craton comprise a single basinal succession. The geochemical compositions of the late Archaean sequences in the southern Gawler Craton are consistent with derivation from a bimodal terrain with variable mixing of three distinct sources: (1) intermediate-felsic volcanics; (2) mafic rocks and (3) highly fractionated Na-rich felsic rocks (tonalites-trondhjemites). These source regions are currently unidentified in the Gawler Craton, however, source terrains for the late Archaean sequences may be preserved in association with the 2500-3000 Ma age crust in rifted off components of the Gawler Craton in Antarctica. Although speculative at this stage, deposition of the Late Archaean sequences in the southern Gawler Craton is likely to have occurred on a rifting margin to a Late Archaean terrain. Sedimentation was terminated during collisional closure of the basin system leading to the 2500-2400 Ma Sleafordian Orogeny.