Lyell Collection
Geological Society, London, Special Publications
Lyell Centre  |   Lyell Collection  |   Subscriptions   |   Geological Society  |   Email alerts  |   Online bookshop  |   Help

Keywords:
Author:
Advanced search>>
This Article
Right arrow Full Text (PDF)
Right arrow References
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow Request Permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Chambers, L. M.
Right arrow Articles by Fitton, J. G.
Right arrow Search for Related Content
GeoRef
Right arrow GeoRef Citation
Geological Society, London, Special Publications; 2004; v. 229; p. 325-331;
DOI: 10.1144/GSL.SP.2004.229.01.18
© 2004 Geological Society of London

Volcaniclastic Rocks

Phreatomagmatic eruptions on the Ontong Java Plateau: an Aptian 40Ar/39Ar age for volcaniclastic rocks at ODP Site 1184

Lynne M. Chambers1,2, Malcolm S. Pringle3 & J. Godfrey Fitton1

1 School of GeoSciences, University of Edinburgh Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK
2 NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK
3 SUERC, Rankine Avenue, Scottish Enterprise Technology Park, East Kilbride, Glasgow G75 0QF, UK

The discovery of a thick (337.7 m drilled) succession of volcaniclastic sediments at Ocean Drilling Program (ODP) Site 1184, on the eastern salient of the Ontong Java Plateau (OJP), shows that at least part of the plateau formed at or near sea level. All the other parts of the OJP that have been drilled or are exposed on land are composed of basaltic lava flows erupted in a deep-marine environment. The composition of the volcaniclastic rocks at Site 1184 is essentially indistinguishable from that of basaltic lava flows recovered from the other drill sites on the OJP, suggesting that the eastern salient formed during the same magmatic event (at approximately 120 Ma) that formed the main part of the plateau. A steep (–54°) magnetic inclination preserved in the volcaniclastic succession is consistent with an Early Cretaceous age, but rare nannofossils recovered from the volcaniclastic rocks suggest a much younger Eocene age. In order to resolve this paradox, we attempted to date the succession by the 40Ar/39Ar technique, even though the rocks are highly altered. Two samples of feldspathic material separated from basaltic clasts give minimum age estimates of c. 74 Ma. The basaltic clasts have compositions similar to that of their host rocks and are therefore probably cognate. A weighted average of the results of total fusion experiments on four or five small euhedral plagioclase crystals separated from the matrix of the volcaniclastic rocks at each of four levels in the lower part of the succession gave a value of 123.5 ± 1.8 Ma, and this probably represents a reasonable estimate of the age of eruption. Although not very precise, the 40Ar/39Ar results are the best that presently can be obtained from such altered rocks. They rule out an Eocene age for the volcaniclastic succession at Site 1184, and we suggest that the Eocene nannofossils were introduced through contamination, probably along fractures.