Contrasting sources of Palaeozoic mafic dykes during intracratonic rifting in Central Australia.

Abstract

The rift-related Cambro-Ordovician Harts Range Metamorphic Complex, in the eastern Arunta Block, central Australia, contains a suite of early Ordovician meta-basaltic rocks (the Harts Range Dykes) that were emplaced syn-kinematically into low-angle regional-scale ductile extensional detachment zones that accommodated N-S extension. The early Ordovician extension was coeval with the deposition of the Larapinta Group in the Amadeus Basin, and was associated with development of the Larapintine Seaway, which linked Australia’s eastern margin to the Canning Basin. Major element variation within the dykes is consistent with an evolutionary path dominated by olivine, and to a lesser extent clinopyroxene and plagioclase fractionation. Based on trace element and isotopic criteria, the dykes have been subdivided into two suites: Group 1 and 2 dykes. This subdivision is evident in discrimination diagrams which assign them to a variety of petrological settings, despite the knowledge of their emplacement within an intracratonic rift. These diagrams suggest that the Group 1 dykes have N-type MORB affinities, while Group 2 dykes were derived from E-type MORB or volcanic/island arc sources. The Group 1 dykes are characterised by higher Zr and Y but lower Nb, Ba, La, Ni, Cr and Ce values than those of the Group 2 dykes. High Zr/Nb ratio values of 62 for the Group 1 dykes suggest they were derived from a depleted source, as does the near flat REE pattern, (La/Yb)N = 1.21, which is N-type MORB like. The Group 1 dykes have positive Nd(480) values associated with less contaminated, depleted signatures. The Group 2 dykes have lower Zr and Y, but higher Nb, Ba, La, Ni, Cr and Ce compositions than the Group 1 dykes, and have Zr/Nb ratio values of 14, indicating an arc-like source. They also display a slightly LREE enriched pattern (La/Yb)N = 5.12, similar to that of E-type MORB. The low and variable Nd(480) values –11.5, -2.1 and 0 and extremely high (87Sr/86Sr)i values of the Group 2 dykes suggests varying degrees of enrichment. The Nd(480) values and variations in Nd and Sr concentrations in the Harts Range Dykes provide evidence for multiple magma sources. Geochemical and isotopic data suggest that the Group 1 dykes have upper asthenospheric, depleted mantle characteristics similar to other meta-tholeiites in the Harts Range region. The enriched geochemical signature of the Group 2 dykes may reflect contamination by a metasomatised mafic crust. However, a subduction-modified and chemically isolated continental lithospheric mantle source is preferred because of the close-fit comparison in trace element data with a similarly interpreted adjacent suite of dykes. The bulk average continental lithospheric mantle (CLM) is ruled out as a dominant source for the enriched chemical signature of the Group 2 dykes because it does not exhibit the HFS-element depleted and LIL-element enriched character of a subduction-modified continental lithospheric mantle. Additionally, the Group 2 dykes have higher incompatible element compositions than the bulk average CLM