The scenic rim of southeastern Queensland, Australia: A history of mid Cenozoic intraplate volcanism

Abstract

Intraplate volcanism was widespread in southeastern Queensland during the mid Cenozoic, leaving a legacy of variably eroded volcanoes and rugged topography known locally as The Scenic Rim. These plume-derived volcanoes provide a detailed record of northward Australian plate velocity, and indicate a major slowdown commencing at 26 Ma and persisting until 23 Ma, correlated with initial collision of the massive Ontong Java plateau with the northern subduction margin of the Australian plate. Despite traversing over 36 km of continental crust, trace element and isotopic signatures indicate little or only minor contamination for most units, with the exception of rhyolites formed during the period of slow plate velocity. Nevertheless, the thick continental crust allowed magmas to stall and fractionate during ascent, often producing highly evolved rocks (e.g., comendites) containing extreme concentrations of incompatible elements, including >2000 ppm Zr. Meanwhile, isotopic and trace element results from mafic units are consistent with melting and mixing of depleted upper mantle and an EM1-like source. Alkaline mafic eruptions also often contain abundant upper mantle and lower crustal xenoliths, providing excellent samples of these otherwise inaccessible regions. Denudation has produced good exposures of the subsurface magmatic architecture, a variety of landscapes, and diverse wildlife habitats; as a result many of the volcanoes are contained in National Parks, including the World Heritage listed Gondwana Rainforests of Australia.