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Early Miocene extrusives and shallow intrusives from Small Nggela, Solomon Islands

Published online by Cambridge University Press:  01 May 2009

I. R. Plimer  and
G. Neef
I. R. Plimer
Affiliation:
University of New South Wales, W. S. & L. B. Robinson University College, P.O. Box 334, Broken Hill, N.S.W. 2880, Australia
G. Neef
Affiliation:
University of New South Wales, W. S. & L. B. Robinson University College, P.O. Box 334, Broken Hill, N.S.W. 2880, Australia
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Summary

Chemically similar altered gabbro, tholeiitic basalt, basaltic andesite and calc-alkaline rhyolite of Early Miocene age is present on Small Nggela, Solomon Islands. Large vesicles are invariably lacking from the autochthonous basaltic pillow lava which has narrow chilled margins, but clasts of allochthonous basalt present in coeval Early Miocene sediment are commonly vesicular. This suggests that the autochthonous lava was deposited in deep water (> 2.5 km) whereas the allochthonous lava was formed upslope in shallow water. A progressive increase in the tholeiite in Zr, Ti, K2O/(K2O+Na2O), Rb/Sr and K2O/Rb from N to S confirms that an Early Miocene southward-descending subduction zone, postulated by some authors, was present. Its recognition implies that the subduction zone did not migrate far during the Early Miocene.

Type
Articles
Information
Geological Magazine , Volume 117 , Issue 6 , November 1980 , pp. 565 - 578
Copyright
Copyright © Cambridge University Press 1980

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