Geochemistry and geochronology of the c. 1585 Ma Benagerie Volcanic Suite, southern Australia: Relationship to the Gawler Range Volcanics and implications for the petrogenesis of a Mesoproterozoic silicic large igneous province

Abstract

New ion microprobe (SHRIMP) U–Pb zircon data indicate that voluminous A-type volcanic rocks were extruded c. 1585Ma in the Benagerie Ridge region of the north-central Curnamona Province in southern Australia. Incompatible trace element ratios and whole-rock Sm–Nd isotope compositions (ɛNd (1585)=−4.3 to −2.2) suggest a dominant crustal source region for the felsic rocks. Incompatible trace element ratios in the basalts, enriched light REE/heavy REE ratios (La/Yb=6–7.5), and ɛNd (1585) values between −1.5 and 0.2 suggest that the basalts were derived from a HFSE-enriched magma from within the mantle. These new data show that the Benagerie Volcanic Suite share geochronological, geochemical, and isotopic affinities with the upper Gawler Range Volcanics of the Gawler Craton indicating these two volcanics suites can be considered part of a formerly contiguous Mesoproterozoic silicic large igneous province. We invoke aspects of the two-phase model for the generation of this A-type silicic large igneous province presented by previous workers and attempt to place the initial ‘developmental’ and subsequent ‘mature’ phases of this event into a tectonic framework for the early Mesoproterozoic of the Curnamona Province and the adjacent Gawler Craton. Initiation of the developmental phase likely resulted from lithospheric extension and was accompanied by localised basaltic magmatism. The mature phase, which involved eruption of voluminous felsic volcanic rocks, was a result of widespread crustal melting, potentially induced by the elevated geotherm caused by extension, by ponding of mafic melt in the lower crust, or a combination of these factors. The high geothermal gradient that resulted from extension and magmatism likely primed the crust for the early Mesoproterozoic regional deformation and metamorphism that occurred across the Curnamona Province and Gawler Craton. The far-field tectonic drivers for this silicic large igneous province remain uncertain, although the association between extensive bimodal A-type magmatism, high-temperature metamorphism, and localised compressional deformation, is suggestive of an intracontinental setting, possibly mechanically connected to a far-field subduction zone.