ABYSSAL THOLEIITES FROM DSDP LEG 34 - NAZCA PLATE

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. With these and other minor flow units at this site, plagioclase 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. Whole rock and mineral chemical data from this study support the idea of a chemically similar worldwide source region of abyssal tholeiites through time.