Geochemistry of Early–Middle Palaeozoic basalts in the Hodgkinson Province: a key to tectono-magmatic evolution of the Tasman Fold Belt System in northeastern Queensland, Australia

Abstract

The Palaeozoic Hodgkinson Province in northeastern Queensland, Australia, is host to Late Ordovician to Devonian rock assemblages that contain tholeiitic to calc-alkaline basalts. These basalts occur as massive fault-bounded units interspersed with marine sedimentary rocks and limestones that are metamorphosed to lower greenschist facies in the Ordovician Mulgrave, Silurian Chillagoe and Devonian Hodgkinson formations, respectively. The petrogenetic and Sm–Nd isotope characteristics of these mafic volcanic rocks were investigated to constrain the tectonic setting in which they erupted. Major, trace and rare earth element analyses were carried out on samples from these formations and intrusive dolerites. The mafic rocks can be classified as basalts and basaltic andesites with distinct MORB characteristics. Furthermore, the basalts are characterized by a slight to moderate enrichment in Th and concomitant depletion in Nb, both of which become less pronounced with basalt evolution through time. These features are consistent with decreasing volcanic arc affinity of Silurian and Devonian MORB-type basalts in the Hodgkinson Province. Sm–Nd isotope characteristics of these basalts indicate a change in source region from dominantly sub-continental lithospheric mantle in the Silurian to asthenospheric input in the Devonian. Collectively, the geochemical and isotopic characteristics of the Hodgkinson Province basalts are interpreted to reflect deposition in an evolving back-arc basin setting. The onset of basin extension was initiated in the Silurian. Accelerated basin subsidence occurred throughout the Devonian and was halted by basin inversion in the Late Devonian. Basin evolution was controlled by an eastward stepping subduction zone outboard of the Australian Craton.