The age and origin of the Balleny and Scott volcanic provinces, Ross Sea, Antarctica

Abstract

The Balleny and Scott volcanic provinces are two isolated and remote volcanic occurrences located along the rifted margin of Western Antarctica in the Ross Sea. They include oceanic plateaus, seamounts, and volcanic islands. Although volcanic activity has been documented through remote sensing, there is no geochronological data based on radio-isotope methods constraining the duration of the volcanic activity in this region. As a consequence, hypotheses, based on the interpretation of chemical characteristics of the lavas, that suggested the volcanism to be either the result a deep mantle plume or a large-scale shallow melting anomaly have not yet been tested decisively.In order to tackle this issue, we have now dated lavas dredged from seamounts from both Balleny and Scott provinces using the 40Ar/39Ar technique. Our new age dates suggest that the Balleny and Scott volcanic provinces were active since at least 2.8 Ma and 2.4 Ma respectively. This suggests that these volcanic provinces were active simultaneously to the magmatism of the McMurdo volcanic group in the Western Antarctica Rift System that comprises Mount Erebus and Mount Melbourne stratovolcanoes. Our new dates suggest that there is no obvious age trend across the Balleny and Scott provinces. Combined with plate reconstructions since the end of Mesozoic, these data suggest that the Scott province does not coincide with any potential hot-spot track, thus negating a deep mantle plume origin for this volcanism. The case of the Balleny province is more ambiguous. The hypothesis of this province representing the end of the Tasman seamounts chain formed by a hot-spot track with decreasing-age trend is not confirmed by the new geochronological data, since available dates for the region are rather scarce and unreliable for the Tasman seamounts.Therefore, we suggest that the Balleny and Scott provinces originated from shallow mantle melting processes, a hypothesis that seems to be consistent with our re-interpretation of the SrNdPb isotope data of the lavas from the Balleny and Scott provinces. We argue that these provinces were derived from a heterogeneous subcontinental lithospheric mantle metasomatized during ancient subductions and which is now scattered in the shallow mantle of the southwest Pacific realm. This scattered mantle source component has been melting since the beginning of Cenozoic, and was then progressively depleted in the most fusible components. This new working model can explain the presence of several magmatic provinces in the southwest Pacific-Antarctica realm which show similar chemical signatures.