MORB-like rocks in a Palaeozoic convergent margin setting, northeast NSW

Blackwell Publishing Asia Pty Ltd
Publication Type:
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 DevonianCarboniferous 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 3.347.98; (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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