Petrology of submarine basalts from the central Caribbean: DSDP Leg 15

Abstract

Basalts recovered during leg 15 of the Deep Sea Drilling Project in the central Caribbean are finegrained plagioclase tholeiites which were erupted in the Late Cretaceous, at a time when North America was shifting from South America and Africa, and emplaced as thin flows and sills near the water-sediment interface. The topmost of these units is responsible for seismic horizon B″, and gradually increasing P wave velocities with depth are due to increasing proportions of basaltic rocks relative to interlayered sediments. Basalts from these sites have similar subalkaline characteristics and contain normative hypersthene and 5–15% normative olivine. The major element character of the basalts indicates that they have evolved from more primitive compositions, probably through olivine fractionation. Intersite variations can be explained largely through both olivine and plagioclase fractionation from similar parental liquids. However, olivine is probably not on the liquidus at any pressure for anhydrous melts of these compositions. A small amount of water in the peridotitic source regions may result in olivine being a near-liquidus phase at pressures of 5–8 kbar. Minor element (e.g., TiO2) variations which cannot be accounted for through crystal fractionation reflect variations in the primary liquid compositions which are probably a consequence of degree of partial melting. Plagioclase was the low-pressure liquidus phase in all the samples examined but was quickly followed in the paragenetic sequence by calcic clinopyroxene and finally titanomagnetite. Olivine or pseudomorphs after olivine are not observed. Intratelluric crystals, predominantly calcic plagioclase but more rarely clinopyroxene, can be distinguished by their minor element characteristics. Metastable crystallization trends, the lack of pyroxene exsolution, and the presence of interstitial glass (now altered) attest to the rapid cooling of these liquids. Two distinct REE patterns (LREE (light REE) enriched and LREE depleted, the latter being typical of the majority of the samples) require chemically contrasting upper mantle source regions during the Late Cretaceous in the Caribbean Basin. Restrictions on the permissible degrees of partial melting of a garnet-bearing source preclude derivation of the LREE-enriched basalts in a one-stage process, and it is suggested that the source region acquired a LREE-enriched character prior to the generation of the liquids through 20–30% partial melting. The LREE-depleted liquids were derived from slightly deeper source regions (70–100 km) previously depleted in the LREE. Contrasting tectonic regimes required to produce these REE characteristics conflict with present thinking on the style of Caribbean submarine volcanism.