Potential field modelling and U–Pb geochronology reveal the pluton emplacement dynamics of the Lower Devonian Tarnagulla Granodiorite, southeast Australia

Abstract

The Tarnagulla Granodiorite in the western Lachlan Fold Belt is poorly exposed; however, magnetic data reveal its distinct U shape (in plan view) and patterns characterised by multiple nested cusp-shaped anomalies of contrasting magnetic intensity. We interpret this magnetic response to reflect incremental assembly of the pluton above a persistent magma point source that migrated laterally. Here, we use potential field data, geophysical modelling and high-precision zircon U–Pb dating to constrain the shape of the intrusion at depth and deduce the migration path of the magmatism epicentre. Potential field profile modelling across three traverses, constrained by petrophysics, resolves the subsurface geometry of the Tarnagulla Granodiorite. In cross-section, the intrusion is funnel-shaped, with a lower boundary that varies from ∼2 km depth in the north to ∼6 km depth in the southwest, reaching a maximum depth of ∼8 km, which may coincide with a magma feeder zone. New precise U–Pb geochronology provides ages that confirm the interpreted magnetic phases were emplaced sequentially along a clockwise intrusion path that progressed stepwise from the north to the southwest between 406.62 ± 0.27 Ma and 399.3 ± 1.1 Ma. We attribute migration of the epicentre of magmatism within the Tarnagulla Granodiorite to syn-intrusion (Early Devonian) reactivations of the underlying Tarnagulla Fault, with relative displacements between the fault hanging wall (which hosts the pluton) and footwall (which we interpret to have hosted a persistent point-source magma feeder zone during pluton emplacement). Our model explains the intrusion’s U-shaped magnetic response and reveals critical information about the paleo-stress and paleo-strain conditions during emplacement, characterised by three main displacement vectors and fault slip magnitudes. The geophysical characteristics, path of emplacement and mineralisation distribution all suggest that progressive fractional crystallisation contributed to the formation of an enriched final pulse of magma that resulted in Mo, Cu and W mineralisation hosted in the Moliagul Granodiorite. KEY POINTS High-precision U–Pb zircon geochronology reveal that the Tarnagulla Granodiorite was emplaced along a clockwise pathway from the northeast to the southwest between 406.62 ± 0.27 Ma and 399.3 ± 1.1 Ma. Active faulting during pluton emplacement is the suggested explanation for the Tarnagulla Granodiorites’ U-shaped magnetic response. Fractional crystallisation throughout pluton emplacement produced the varying internal geophysical response of the pluton as well as the enrichment of Mo and W at Mt Moliagul.