Abstract:
The Devonian mafic rocks from the Folly Basalt, northeast New South Wales, were emplaced in the
forearc section of the Devonian–Carboniferous magmatic arc preserved in the western part of
the New England Fold Belt. Trace-element abundances in fractionated metadolerites (maximum
concentration of Ni = 85 ppm) from the Folly Basalt outcropping near Nundle demonstrate that
these rocks have MORB affinity. Chondrite-normalised rare-earth element patterns are smooth and
quasi-horizontal; Ce/Yb ratios are 3.34–7.98; (La/Yb)N ratios range from 0.69 to 2.23; (La/Sm)N ratios
of the rocks range from 0.63 to 1.55. The data are compatible with an origin of the melts from large
degrees (>15%) of partial melting of mantle peridotite. A plausible mechanism for the production
and emplacement of depleted magmas in the forearc zone of the Middle Palaeozoic eastern
Australian magmatic arc involves the subduction of a hot oceanic spreading centre, which could
cause the presence of a region of asthenospheric temperatures below the upper plate. It is also
suggested that sustained high-temperature conditions may have prevailed in the eastern Australian
mantle for at least the last 400 million years.
