Petrographic and chemical characteristics of abyssal tholeiites from Deep Sea Drilling Project Leg 63 off Baja California.

Tholeiitic basalts were obtained from basaltic basement ranging in age from 6 to 17Ma on IPOD/DSDP Leg 63. The main rock types encountered at all sites but 473 are basaltic pillow lavas. Although many of these pillow basalts are highly or moderately altered, fresh glass is usually present. At Site 473, we recovered coarse- grained, massive basalts; no clearly defined pillowed forms were observed. Phenocrysts or microphenocrysts present in the Leg 63 basalts are plagioclase and clinopyroxene at Site 469; olivine, plagioclase, and spinel at Site 470; and olivine, plagioclase, and clinopyroxene at Sites 472 and 473. Olivines of the basalts from Holes 470A and 472 (Fo_85-88) are generally more magnesian than those of the Hole 473 basalts (Fo_77-81). Also, plagioclases of Holes 470A and 472 basalts (An_70-85) are generally more calcic than those of Holes 469 and 473 basalts (An_66-72). Geochemical study of the Leg 63 basalts indicates that in all cases they are large-ion-lithophile (LIL) element depleted tholeiites like typical abyssal tholeiites. In particular, they are very similar in composition to those described from the E Pacific, although the degree of iron enrichment found in the Leg 63 basalts is not as extensive as in basalts from the Galapagos spreading center. Hence, the geochemical evidence of the Leg 63 basalts is compatible with their formation at a spreading center. -Authors