Abstract:
The ca 280 Ma Alum Mountain Volcanics and Werrie Basalt were erupted in northeast New South
Wales, Australia, during Early Permian lithospheric extension that separated discrete episodes of
subduction of Carboniferous and Late Permian - Triassic age. The Alum Mountain rocks, which are
preserved in two major synclines in the southeast Tamworth Belt, are mostly basalt, but andesite and
rhyolite are also present. The Werrle Basalt found further north in the Tamworth Belt and in the floor
of the Gunnedah Basin iscomposed mainly of basalt, but includes more evolved rocks in the vicinity
of several eruptive centres. The Alum Mountain rocks have REE abundances similar to N-MORB. with
flat REE patterns,(La/Sm)N ratios ranging from 0.54 to 1.07, and (La/Yb)N ratios from 0.94 to 2.78.
suggesting an origin by large degrees of partial melting of asthenosphere at a depth < 75 km. The
ENd values range from +5.61 to +8.73. The latter value corresponds to that of the depleted mantle
at 0.2 Ga. Werrie Basalt samples have positive ENdvalues, ranging from +2.05 to +6.00, suggesting
an asthenospheric origin for these rocks. Spider diagrams show a clear negative Nb anomaly, typical
of subduction zones, but LREE/HREE((La/Sm)N = 1.61 to 2.20; (La/Yb)N = 5.07 to 8.81). Ti/Zr, and Tl/P
ratios are close to OIB values. The enriched character of the Werrie Basalt has resulted from either
asthenospheric melts being progressively modified during ascent of fractionating magmas through
the lithosphere, or by partial melting of a mantle metasomatised by subduction. The presence of a
significant depleted-mantle component in the signature of the Lower Permian volcanic rocks indicates
rise of the local mantle geotherm to allow extensive melting. We therefore propose a model
of asthenospheric upwelling and lateral flow following breakoff of the Carboniferous subducting slab.
Our model of asthenospheric convection as derived from eastern Australia suggests a major role for
the asthenosphere in subduction zones: not only is the asthenosphere the reservoir from which
magmatic arc melts originate, but we surmise that the behaviour of asthenospheric mantle at
subduction zones may have far-reaching implications for the overall thermal state of the planet.
