Early cretaceous tectonic setting of eastern Australia: Evidence from the subduction-related Morton Igneous Association of Southeast Queensland

Abstract

Early Cretaceous magmatic rocks of the central east coast of Australia are recognized as a lithologically, geochemically, and geochronologically distinctive rock package herein named the Morton Igneous Association. The association embraces small plutons and hypabyssal intrusions within the Jurassic and Lower Cretaceous Nambour and Maryborough basins. It also includes volcanics of the Grahams Creek Formation, basal infill of the Maryborough Basin with an indicated volume exceeding 0.25 M km3, representing a much more substantial igneous assemblage. Plutonic rocks vary from hornblende-rich gabbro to granodiorite, and syenite. The Grahams Creek Formation consists of volcanic to volcaniclastic rocks of basaltic to dacitic composition. The association is U–Pb zircon dated at 145–140 Ma, and is characterized by distinctive subduction-related trace element com- positions typical of magmatic rocks from continental arc settings. Oxygen isotope and εHf compositions obtained from zircon are consistent with a largely mantle source for the host magmas. Trace element data for the 130–95 Ma Whitsunday Igneous Province, of more northerly location on the east Australian coast, are consistent with a continental back-arc association. The MIA and Whitsunday Igneous Province represent small parts of an extensive Cretaceous active margin system developed for east Gondwana, mainly located on continental crust comprising northern Zealandia which is now separated from east Australia, largely submerged and little docu- mented. East Australia records both older (145–140 Ma) continental arc magmatism and younger (132–95 Ma) continental back-arc magmatism attributed to progressive Cretaceous roll-back of a consuming plate boundary to east Gondwana.