Abyssal tholeiites from DSDP leg 34: The Nazca Plate

Abstract

Basalts recovered from the Nazca Plate at sites 319A and 321 of DSDP (Deep-Sea Drilling Project) leg 34 are hypersthene normative abyssal tholeiites with chemical compositions similar to abyssal tholeiites of worldwide occurrence. These basalts, among the freshest cored by the DSDP, may be the products of ocean ridge volcanism along the extinct Galapagos rise at about 14 and 40 m.y. B.P. The basalt section at site 319A includes two major flow units of porphyritic intersertal to ophitic plagioclase tholeiites. Within these and other minor flow units at this site, plagioclase (An88–24 and Or ≤ 1) and clinopyroxene (augite to pigeonite) exhibit complex optical and chemical zoning, the result of metastable crystallization. Basalts from site 321 are porphyritic to aphyric and vesicular and intersertal; and they exhibit complex zoning in plagioclase and clinopyroxene similar to those at site 319A. Chemical compositions of the leg 34 basalts indicate evolution from a more primitive magma generated in a peridotitic upper mantle. The evolved character of the leg 34 basalts is a consequence of varying degrees of fractionation of olivine and plagioclase from the ascending primary magmas. The site 321 magmas are more fractionated than site 319A magmas. This is indicated by their higher TiO2, lower average Sr, higher REE (rare-earth element) concentrations, lower Mg/(Mg + Fe2+) ratios, and more sodic plagioclase core compositions (An71) and the presence of clinopyroxene microphenocrysts in the site 321 basalts. Whole rock and mineral chemical data from this study support the idea of a chemically similar worldwide source region of abyssal tholeiites through time.