Oceanic Plateau Basalts Research Articles

Geochemical features of intraplate oceanic plateau basalts

Summary Coeval with the Cretaceous thermal uplift of a large region of the ‘old Pacific’ oceanic lithosphere was the widespread development of two expressions of intraplate volcanism: edifice building (ocean islands, seamounts) and submarine sheet-flows (oceanic plateaux). The geochemical features of the development of oceanic plateaux are reviewed using the Nauru Basin complex as a model. Ocean plateau basalts (OPBs) are mildly altered, poorly phyric, hy - or qz -normative tholeiites, rapidly extruded submarine sheet-flow lavas with a relatively uniform chemistry. Although their eruptive setting is apparently intraplate they have a chondrite-normalized incompatible-element-depleted pattern similar to normal-type mid-ocean ridge basalt (MORB) formed at ocean spreading centres. They are chemically distinguished from normal-type MORB by being enriched in selected incompatible elements and can be matched by various enriched or ‘transitional’ MORB types. Isotopically they have similarities to some types of ocean island basalt (OIB), such as Hawaii. They were probably derived by high degrees of melting from an upper mantle MORB-type source with an enriched OIB-type component; melts being mixtures derived from the two mantle components. In terms of crustal development they are characteristic of anomalously thickened oceanic crust and were formed in an off-axis (or near-axis) eruptive setting shortly after the development of axis-generated oceanic basement over which they flowed. OPB appears to be restricted to the widespread Cretaceous magmatic event.

Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas

The composition of chromian spinel in alpine-type peridotites has a large reciprocal range of Cr and Al, with increasing Cr# (Cr/(Cr+Al)) reflecting increasing degrees of partial melting in the mantle. Using spinel compositions, alpine-type peridotites can be divided into three groups. Type I peridotites and associated volcanic rocks contain spinels with Cr# 0.60, and Type II peridotites and volcanics are a transitional group and contain spinels spanning the full range of spinel compositions in Type I and Type II peridotites. Spinels in abyssal peridotites lie entirely within the Type I spinel field, making ophiolites with Type I alpine-type peridotites the most likely candidates for sections of ocean lithosphere formed at a midocean ridge. The only modern analogs for Type III peridotites and associated volcanic rocks are found in arc-related volcanic and intrusive rocks, continental intrusive assemblages, and oceanic plateau basalts. We infer a sub-volcanic arc petrogenesis for most Type III alpine-type peridotites. Type II alpine-type peridotites apparently reflect composite origins, such as the formation of an island-arc on ocean crust, resulting in large variations in the degree and provenance of melting over relatively short distances. The essential difference between Type I and Type III peridotites appears to be the presence or absence of diopside in the residue at the end of melting.