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Volcaniclastic Rocks |
1 Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK rvw1{at}le.ac.uk
2 Geosciences Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0212, USA
3 Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556, USA
4 School of GeoSciences, University of Edinburgh, Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK
5 Laboratoire de Tectonophysique — CNRS UMR 5568, Case 49, Institut des Sciences de la Terre, de l’Eau et de l’Espace de Montpellier, Université de Montpellier II, Place Eugéne Bataillon, F-34095 Montpellier Cedex 5, France
The compositions of glass clasts in volcaniclastic rocks recovered from drilling at Site 1184 on the eastern salient of the Ontong Java Plateau (OJP) are investigated using microbeam analytical methods for major, minor and trace elements. These data are compared with whole-rock elemental and isotopic data for bulk tuff samples, and with data from basalts on the high plateau of the OJP. Three subunits of Hole 1184A contain blocky glass clasts, thought to represent the juvenile magmatic component of the phreatomagmatic eruptions that generated the volcaniclastic rocks. The glass clasts have unaltered centres, and are all basaltic low-K tholeiites, with flat chondrite-normalized rare earth element (REE) patterns. Their elemental compositions are very similar to the Kwaimbaita-type and Kroenke-type basalts sampled on the high plateau. Each subunit has a distinct glass composition and there is no intermixing of glass compositions between subunits, indicating that each subunit is the result of one eruptive phase, and that the volcaniclastic sequence has not experienced reworking. The relative heterogeneity preserved at Site 1184 contrasts with the uniformity of compositions recovered from individual sites on the high plateau, and suggests that the eastern salient of the OJP had a different type of magma plumbing system. Our data support the hypothesis that the voluminous subaerially erupted volcaniclastic rocks at Site 1184 belong to the same magmatic event as the construction of the main Ontong Java Plateau. Thus, the OJP would have been responsible for volatile fluxes into the atmosphere in addition to chemical fluxes into the oceans, and these factors may have influenced the contemporaneous oceanic anoxic event.
