Igneous intrusions in the Bowen Basin

Abstract

Igneous intrusions, in the form of stocks, sills and dykes are abundant in the Bowen Basin. They are predominantly Early Cretaceous in age, exclusively epizonal in origin and range in composition from dolerite to granodioriteldacite. All rock units within the basin, up to and including the Clematis Group, are intruded to some degree. This study assesses the distribution, form, petrology and mode of emplacement of plutons, igneous sills and dykes occurring in the Bowen Basin, and considers their relationship to the prevailing structure. The tectonic implications of the findings are then assessed. Igneous sills occur in two geographically distinct domains, one in the northern Bowen Basin and the other in the central Bowen Basin. The sills emanated from pre-existing, north to north-northwest trending reverse faults, and preferentially intruded coal seams. The boundaries to sill intrusion are marked by major northeast trending basement structures. These basement structures occur at regular intervals throughout the basin, and correspond with the localisation of plutonic and dyke activity, anomalous structural disturbance, and changes in the gross structure of the basin. They are interpreted as transfer faults that were inherited from an Early Permian, basin-forming extensional episode. Petrological evidence indicates that the plutons and sills occurring in the northern Bowen Basin are petrogenetically related, and that a progressive variation in their chemistry occurs across the axis of the basin from east to west. Intrusions in the east belong to the calc-alka1ine rock suite, while those in the west belong to the syenitic suite. This transition is inte1preted in terms of increased crustal contamination as the magma migrated from a source area to the east along a buried, shallow-dipping detachment surface that extends under the basin. This detachment was inherited from the above mentioned extensional phase and is intimately linked to structures that penetrate up-section through the basin succession. Reactivation of the transfer faults during the Early Cretaceous initiated the emplacement of dykes, and the synchronous development of northeast trending normal and wrenchstyle faults. The dykes exhibit characteristics that indicate they were self-propagating, and can be regarded as good palaeostress indicators. This phase corresponded with a major compressional event that involved the reactivation of pre-existing thrust structures, deformation of the Folded Zone and eastern margins of the Nebo Synclinorium and Mimosa Syncline, and the rapid preferential uplift of the central Bowen Basin region