Up to five pulses of alkaline magmatism occurred in Victoria between the Late Triassic and the Middle Jurassic (c. 225–170 Ma) with three of these events recorded by new U-Pb zircon geochronology undertaken as part of this study. The earliest of these events occurred in the Barwite and Mount Leinster Igneous Complexes of east-central and eastern Victoria. From the Barwite Igneous Complex, the Gonzaga Monzonite returned a U-Pb age of 226.5±1.3 Ma, which replicated U-Pb apatite and K-Ar ages reported from these complexes previously. Two samples from the Mount Leinster Igneous Complex returned somewhat younger U-Pb zircon ages of 211.1±0.9 Ma and 212.5±1.6 Ma, while the youngest observed phase of alkaline magmatism was dated from the Coleraine Volcanic Group where a trachyte laccolith from the Den Hills Formation returned a U-Pb zircon age of 170.2±0.7 Ma. K-Ar geochronology and compositional correlations suggest c. 170 Ma trachytic magmatism in the Coleraine Volcanic Group was preceded by mafic alkaline and tholeiitic magmatism at c. 190 Ma and c. 183 Ma, respectively.
When considered in the context of southeastern Australia, the Mesozoic igneous events observed in Victoria represent a sub-set of up to eight pulses of intra-plate magmatism that occurred between the southern margins of the Sydney Basin and southwest Victoria. These events are defined by clusters of age data that yield mean ages of approximately 234 Ma, 226 Ma, 215 Ma, 211 Ma, 205 Ma, 191 Ma, 182 Ma, and 170 Ma. There remains some uncertainty as to whether each of these age clusters represent true magmatic pulses, or if they are better considered part of a long-lived magmatic continuum. This uncertainty reflects the widespread application of K-Ar geochronology to date the Mesozoic alkaline rocks of southeast Australia and the inherent susceptibility of this decay scheme to low temperature radiogenic Ar loss. The events dated at c. 234 Ma, c. 226 Ma, c. 215 Ma, c. 211 Ma, c. 182 Ma and c. 170 Ma are, however, anchored by U-Pb zircon or apatite ages, which provides some confidence as to the episodic nature of magmatism.
When the timing and distribution of Mesozoic alkaline magmatism is considered, the southern margin of the Sydney Basin appears to have remained the long-term locus for magmatism, with igneous centres forming to the south and west of this area sporadically through time. The distribution of magmatism broadly parallels the coeval Panthalassan active margin of Gondwana. We interpret the timing of igneous activity to reflect changes in the intra-plate stress field associated with the evolution of this margin, with periods of upper plate extension facilitating an increase in magmatic activity. More speculatively, we interpret the termination of Mesozoic magmatism in southeast Australia to also reflect the Middle Jurassic reconfiguration of plate motions that marked the onset of the long-lived process of Gondwana fragmentation.
Understanding the timing and spatial occurrence of alkaline rocks in southeast Australia has important mineral systems considerations. The c. 220–210 Ma trachyte-hosted Toongi HFSE-REE deposit (hosted by the Benolong Volcanic Suite) near Dubbo in New South Wales overlaps with two new ages presented here for the Mount Leinster Igneous Complex, with the Gonzaga Monzonite (Barwite Igneous Complex) only slightly older. The occurrence of alkaline igneous complexes with similar timing to the Benolong Volcanic Suite indicate the source rocks to such systems also exist in Victoria.
