Geology, geochronology and emplacement structures associated with the Jimbu Microgranite, McArthur Basin, Northern Territory

The Jimbu Microgranite, a series of high-level microgranite plutons exposed in central Arnhem Land in northern Australia, was emplaced into flat-lying sandstones and lesser mudstones, carbonates and basalts of the Katherine River Group during the latter stages of deposition. New SHRIMP U-Pb zircon results indicate a crystallization and emplacement age for the Jimbu Microgranite of 1720 ± 7 Ma. Intrusion took place in an essentially static tectonic setting, and accommodation space was not created by regional extension or by consumption of the host package by the intrusion. Rather, pluton emplacement was facilitated by three distinct mechanisms: the thick basal sandstone package was updomed; overlying lutite units were laterally shortened by outward gravity slide; the uppermost sandstone unit was raised vertically above the pluton by way of a monoclinal margin and steep reverse detachment faults. Decollement at ductility transitions in the rock package is required for the coexistence of these three structural styles. The three resulting structural elements include: relatively steeply and radially outward dipping domes of lower Katherine River Group sandstones with granite cores; irregular, short-wavelength fold patterns in the overlying incompetent lutites; discrete flat-lying blocks of the uppermost sandstone unit with steeply upturned or downturned margins. Plutonism is also suspected of being the cause of similar domes elsewhere in the McArthur Basin and in the adjacent Mount Isa Inlier. The model presented here may also be partly applicable to other basins with similar geometries and volcano-sedimentary histories, where tectonism is currently invoked to explain deformation.