Results of recent stratigraphic drilling, combined with previous investigations and interpretation of image-processed aeromagnetic data, indicate the presence of three major mafic/bimodal igneous suites within Early Palaeozoic basement lying beneath Cainozoic sediments of the western Murray Basin in South Australia. Suite 1 contains within-plate style basalt (WPB) similar to the Early Cambrian volcanic rocks which are interbedded with metasedimentary rocks along the western margin of the Tasman Fold Belt System (TFBS). It occurs at the north-western termination of the Stavely Belt (central TFBS), is interbedded with limestone and shale, and is separated from the western margin volcanic rocks by Suite 2 mafic rocks having MORB-like geochemistry. Suite 2 consists of metadolerite and metabasalt which closely resemble ?Middle-Late Cambrian igneous dykes, also with MORB characteristics, which have intruded metasediments along the western margins of the TFBS. Suite 3 comprises ?Ordovician, post-orogenic granitoid rocks, porphyritic rhyolite, and a mafic-ultramafic intrusive complex. The occurrence of Suite 1 WPB and limestone in a region traditionally considered to be oceanic or of island arc petrogenetic affinity, has major implications to tectonic models for the central TFBS. It is suggested that either: a) the Suite 1 volcanic rocks were extruded during the onset of Early Cambrian rifting within the Eastern Australian Craton, became separated during rifting, and later were tectonically intercalated within a volcanic arc (Stavely Belt) during the Middle-Late Cambrian; or b) there was contemporaneous development of island-arc calcalkaline volcanic rocks (Mount Stavely Volcanic Complex) and within-plate style Suite 1 volcanic rocks (possibly in a back-arc basin setting), either in the Early or Middle-Late Cambrian. The extensive arcuate zone of Suite 1 mafic rocks may represent a zone of considerable crustal extension that formed during deposition of the Early Cambrian sequences, accompanied by intrusion/extrusion of MORB-like dolerite and basalt in the adjacent continental margin. This arcuate zone of Suite 1 rocks may link up with the Mount Wright Volcanic Arc (NSW), and the Gidgealpa Volcanic Arc in the Warburton Basin (SA).