Geochemistry and significance of basaltic rocks dredged from the South Tasman Rise and adjacent seamounts

Abstract

Basalts dredged from two seamounts separated by some 200 km and built on ca 50–35 Ma oceanic crust west of Tasmania have slightly LREE‐depleted N‐MORB affinities and were probably generated in a spreading ridge or near‐ridge setting. They are compositionally very similar to 60 Ma basalts drilled at DSDP Site 282, midway between these seamounts. Taken together, these basalts provide a useful dataset for the compositional features of MORB erupted early in the rifting history of Tasmania separating from Antarctica. Well‐preserved intraplate alkaline basalts with HIMU geochemical affinities have been sampled from Cascade Seamount, which forms the peak of the East Tasman Plateau. Far more altered alkaline intraplate basalts dredged at several other locations within the South Tasman Rise south of Tasmania, and on seamounts built on oceanic crust and thinned continental crust south of the East Tasman Plateau, also show typically HIMU affinities for immobile trace‐element ratios (e.g. low Zr/Nb values, 3–5), but have undergone strong rare‐earth element mobility associated with sea‐floor weathering and replacement of glass by chemisorbed biogenic phosphate. Dolerites with some strong affinities to the Jurassic dolerites of Tasmania were dredged in the central section of the South Tasman Rise.