Petroi Metabasalt: Alkaline within-plate mafic rocks from the Nambucca Slate Belt, northeastern New South Wales
Title:
Petroi Metabasalt: Alkaline within-plate mafic rocks from the Nambucca Slate Belt, northeastern New South Wales
Author:
Asthana, Deepanker Leitch, Evan C.
Appeared in:
Australian journal of earth sciences
Paging:
Volume 32 (1985) nr. 3 pages 261-277
Year:
1985-09
Contents:
The Petroi Metabasalt comprises approximately 2000 m of massive and pillowed metabasalt flows, breccias, and metadolerite sills that overlie and are intercalated with Early Permian epiclastic rocks of the Nambucca Slate Belt. Both the basaltic rocks and associated sedimentary material were multiply deformed and metamorphosed to pumpellyite-actinolite facies grade at about 255 Ma. Metamorphism and earlier sea-floor alteration of these mafic rocks have led to hydration, carbonation and oxidation and considerable redistribution of the major elements and the more labile traces, notably Rb, Ba and Sr. However, abundances of TiO2, the high field strength trace elements, Ni, Cr and V, the light rare earths and yttrium are interpretable as being the largely unmodified magmatic abundances of mildly alkaline within-plate basalts. This interpretation is supported by the composition of relic Ca-rich pyroxenes in the metadolerites which fall in the fields of mildly alkaline basalts. The field relationships, age and composition of these rocks suggest either eruption on oceanic crust covered by a thick sequence of epiclastic rocks and subsequent incorporation into an accretionary subduction complex, or generation during rifting of the eastern part of the New England Fold Belt and accumulation, together with the associated sedimentary rocks, in a graben. The chemical and mineral characteristics of the igneous rocks indicate that the volcanism was not related to magmatic arc activity, and their presence demonstrates the rocks of the Nambucca Slate Belt are neither fore-arc basin nor slope-basin deposits.
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