Mafic dykes of the southeastern Gawler Craton: ca 1564 Ma magmatism with an enriched mantle source

Abstract

This study investigates the age and composition of a suite of mafic dykes in the southeastern Gawler Craton. Mafic dykes intrude the gneissic fabric of the ca 1850 Ma Donington Suite and were previously interpreted to have been emplaced at ca 1845 Ma. However, sensitive high-resolution ion microprobe U–Pb dating of zircon within one of these mafic dykes shows it was emplaced at 1564 ± 4 Ma. The sampled dyke also contains a population of inherited zircon cores with an age of 1770 ± 6 Ma, and one grain with an age of ca 1860 Ma, likely derived from country rock. Whole-rock geochemistry from this dyke and nearby dykes with similar structural and mineralogical features show that there is a suite of dykes in the region with gabbroic, low SiO2 and TiO2 contents and tholeiitic characteristics. The dykes are enriched in Ba, K and Rb, with low to moderate MgO, Ni and Cr contents, and moderate light rare earth element enrichment. Negative Nb and Ta anomalies with subtle negative Ti anomalies suggest some interaction with continental crust. Sm–Nd isotopic data have εNd(1560 Ma) between −6.8 and −2.9, and depleted mantle model ages >2.5 Ga. Th/Nb ratios are 0.21–0.73 consistent with a metasomatised, subduction modified lithospheric mantle source. The relatively primitive nature of the tholeiites suggests the crustal-like signatures are inherited from the source region, which likely represents an enriched continental lithospheric mantle in the eastern Gawler Craton. This ca 1564 Ma mafic magmatism is newly named the Daly Head Metadolerite. In addition, narrow high-strain zones overprint the dated mafic dyke and locally the Donington Suite wall rocks. A syn-kinematic granitic dyke intrusion within one high-strain zone has been dated via laser ablation-inductively coupled plasma mass spectrometry U–Pb zircon methods and was emplaced at ca 1555 Ma. This localised ca 1555 Ma deformation is a previously unrecognised tectonic event in the southern Gawler Craton. This study adds to the growing database showing magmatism and deformation in the Gawler Craton continued after the voluminous Gawler Range Volcanics and Hiltaba Suite magmatism (ca 1596–1575 Ma). These younger tectono-magmatic events typically occur around the margins of the Gawler Craton, suggesting the internal ‘core’ of the craton may have been substantially less fertile for melting during these younger thermal/structural events, likely because of the high-temperature nature of the early Mesoproterozoic Gawler Range Volcanics and Hiltaba Suite magmatism. KEY POINTS Geochronology shows mafic dykes in southeastern Gawler Craton emplaced at 1564 ± 4 Ma. Mafic dykes are tholeiitic, derived from a metasomatised, subduction modified lithospheric mantle source. Narrow high-strain zones overprint the dated mafic dyke and contain syn-kinematic granitic dyke dated at ca 1555 Ma. Localisation of post-ca 1575 Ma tectono-magmatic events occur around margins of the craton, reflecting reduced melt fertility in the centre owing to previous high-temperature events.