MORB-like rocks in a Palaeozoic convergent margin setting, northeast New South Wales

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Journal Article
Australian Journal of Earth Sciences, 2002, 49 (2), pp. 367 - 374
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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.
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