COMPARISON OF CHEMICAL AND MAGNETIC STRATIGRAPHY OF BASEMENT ROCKS AT DSDP SITES 332 AND 395

Deep drilling into oceanic basement at Sites 332 (36° 53 'N, 33° 39 "W; 583 m) and 395 (22 ° 45 'N, 46 ° 05 'W; 572 m) provides an opportunity to compare the origin of a major portion of Layer 2 at two places on the Mid-Atlantic Ridge. The sites are similar in that basement at each consists dominantly of relatively few chemically distinct basaltic units (18 in Holes 332A and 332B; 9 in Holes 395 and 395A). These units occur as composites of basaltic pillow lavas, thin flows, and glassy breccia zones as thin as a few meters and as thick as 209 meters. Only two closely spaced dolerite units, totaling 15 meters thick (in Hole 395A), have been confirmed as intrusive at either site. Holes 395 and 395A also contain gabbroic and ultramafic cobbles and boulders, interpreted as talus, both above and below the topmost aphyric basal unit. No plutonic rocks were cored at Site 332. The chemical groups at both sites can be divided into strongly phyric types and moderately, sparsely, or aphyric types (less phyric types). The first group contains 15 to 25 per cent or more of phenocrysts of olivine, Plagioclase, clinopyroxene, or combinations of these minerals. Many of the phenocrysts appear to have accumulated by mechanical processes (gravity settling or floating, and flow differentiation). The proportion of strongly phyric basalts in both Hole 395A and Hole 332B, curiously, is 38 per cent. Among the phyric basalt types in Hole 332B are olivine-rich basalts (oceanites), representing about 5 per cent of the interval cored. All other phyric basalts at both sites have Plagioclase as the predominant phenocryst, with lesser olivine and/or clinopyroxene. For the most part, the less phyric basalt types cannot be related to each other, or to phyric basalts, by crystal fractionation involving phases present in the rocks. Instead, chemical differences appear to have resulted largely from variations in the extent of melting in the mantle. All basalts are low-K and -Ti mid-ocean ridge basalts, but those of Site 332 are typically lower in Ti, Zr, and other low-partitioncoefficient elements than those of Site 395. They thus appear to represent greater partial melts of the mantle than those of Site 395. Basalts of Site 395, however, appear to derive from a distinctly more depleted mantle than those of Site 332, and have La/Sm of about 0.8. At Site 332, less phyric basalts and most strongly plagioclasephyric basalts have La/Sm of about 2.2. The olivine-rich basalts and oceanites have La/Sm of about 1.5. At Site 332, there is evidence for minor crystal fractionation and accumulation between some less phyric types that erupted more or less simultaneously. Less phyric and strongly phyric types are interbedded repeatedly at intervals of tens of centimeters to meters in the deeper parts of Hole 332B. Apparently, either these came from different sources and magma chambers at essentially the same time, or the interval has been repeated numerous times by faulting. A third possibility is that the phyric basalts are closely spaced narrow intrusives into the less phyric basalts. The rock types at both Site 332 and Site 395 suggest that to explain the lava sequences, there must have been at least two magma reservoirs, repeatedly tapped. There is little evidence for magma mixing among less phyric types at either Site 395 or Site 332, and none between strongly phyric and less phyric types. These is considerable mineralogic evidence for mixing within the phyric basalts of Site 395, but mixing has not been evaluated for phyric basalts of Site 332.