Petrology and origin of basalts at DSDP Hole 66-487

The volcanism of Central America, according to current theory (Pichler and Weyl, 1973; Stoiber and Carr, 1974; Hey, 1977), is related to the subduction of the Cocos Plate under the North American lithospheric plate and the melting of ocean crust material in the subduction zone (Green and Ringwood, 1968; Dickinson, 1970, Fitton, 1971). Since Cocos Plate subduction occurs at the rate of more than 7 cm/y. (Hey et al., 1977), basalts underlying upper Miocene sediments of the Middle America Trench outer slope, penetrated in Hole 487 (Fig. 1) during Leg 66 (Moore et al., 1979), should have formed far from their present position if current theory is accurate. Present manifestations of basaltic magmatism in adjacent areas of the Pacific derive from the axial part of the East Pacific Rise, the Galapagos spreading center, and transform fracture zones. The question arises: Are there analogs of the Middle America Trench basalts among magmatic cock associated with these modern features, or do the trench basalts have some other origin?

Major and trace elements and interelement ratios at DSDP Leg 81 Holes

Fifty-two samples of basalt from the four holes drilled on the Leg 81 transect across the Rockall margin were analyzed by X-ray fluorescence for Rb, Sr, Y, Zr, and Nb. On the basis of these results 13 samples were chosen for major and supplementary trace-element analysis. The results show no progressive change in the character of the volcanism, from Hole 555 in the continental domain through Holes 552 and 553A in the dipping reflector sequence to Hole 554A on the outer high. Two distinct magma types are present, apparently reflecting heterogeneity of the underlying mantle, but both types are present in both Holes 553A and 555, while Hole 552 and Hole 554 are each composed of a single type. Both magma types have a clear ocean-floor basalt signature when examined by discrimination diagrams, as does the basalt from Deep Sea Drilling Project Site 112, which formed at the same time as the Leg 81 basalts slightly farther south along the spreading center. In contrast, the basalts of East Greenland, formed at the same time, are more enriched in incompatible elements and have a within-plate geochemical signature, as is found in some basalts of Iceland today. Clearly the present distinction in geochemistry between the basalts of Iceland and those erupting well south on the Reykjanes Ridge was already established when continental splitting took place.

Rare-earth elements and isotope ratios at DSDP Leg 82 Holes

We report 48 analyses of rare-earth elements (REE) and 15 143Nd/144Nd and 87Sr/86Sr analyses for basalts from the eight holes drilled during Leg 82. Discrete and distinct REE patterns and 143Nd/144Nd ratios characterize the eight holes, with little variation observed downhole except in Holes 561 and 558, thus suggesting dominantly long-term temporal and large-scale spatial variations in the mantle source of these basalts beneath the Mid-Atlantic Ridge over the last 35 Ma of its spreading activity. There is a good inverse correlation between 143Nd/144Nd and (La/Sm)EF with one exception in Hole 558 (approximately 35 Ma), the latter suggesting a recent (35 Ma) light REE depletion event, perhaps caused by dynamic or fractional melting. Short-term temporal and small-scale spatial mantle source variability is also evident in Hole 561 (approximately 18 Ma), which has rapid fluctuations in REE patterns and 143Nd/144Nd ratios (suggesting rapid transfer of magma from the time of melting) and is evidence contrary to the presence of a well-mixed magma chamber at this particular site and time. The mantle source variations noted can be interpreted within two extreme models. The first model invokes a convecting mantle depleted in large ion lithophile elements (LILE) and containing lumps (or veins) of LILE-enriched material of various shapes and sizes, passively and randomly distributed throughout. A second more restrictive model considers the interaction of fixed mantle plumes and the LILE-depleted asthenosphere flowing towards a migrating Mid- Atlantic Ridge (MAR) axis. With the exception of Hole 558 and the uncertainties of reconstructions of absolute plate movements in the region, the observed variations can be explained by two hot spots; the nearly ridge-centered Azores hot spot (plume) and another hot spot located beneath the African plate that may be affecting the source of basalts currently erupting at the MAR axis at 35°N and which, in the past, would have produced the New England chain of seamounts on the North American plate and (later) the Atlantis-Great Meteor chain on the African plate. Basalts erupted south of the Hayes Fracture Zone have not been affected by either of these two hot spots over the last 35 Ma and appear to have been continuously derived from the LILE-depleted source. Subaxial flow downridge from the Azores plume appears to have started 9 Ma, on the basis of the southward converging V-shaped time-transgressive ridges branching from the Pico and Corves Island, or not earlier than 16 Ma, on the basis of the geochemical results. Variations within Hole 558 remains unexplained by the latter model, unless we hypothesize a third hot spot.

Nature, chemistry, and origin of late Cenozoic megascopic tephras in Legs 89 and 90 Cores from the Southwest Pacific

Several thin (1-10 cm) megascopic vitric tephras occur in the late Cenozoic calcareous oozes on Lord Howe Rise in the Tasman Sea and off eastern South Island, New Zealand. Of the 18 tephras analyzed 15 are silicic (75-78% SiO2) with abundant clear glass shards and a biotite ± hypersthene ± green hornblende ferromagnesian mineralogy. The Neogene silicic tephras were derived from the now-extinct Coromandel volcanic area in New Zealand, and the Quaternary ones from the presently active Central Volcanic Region of New Zealand. On the basis of glass chemistry and age, several of the Quaternary tephras are probably correlatives, and at least two can be matched to the major on-land Mt. Curl tephra (-0.25 m.y.). The occurrence of correlative silicic tephras both northwest and southeast of New Zealand may result from particularly violent eruptions, the ash below and above an altitude of -20 km being dispersed in opposite directions toward the Pacific Ocean and Tasman Sea, respectively. Ash drifting eastward into the southeasterly trade wind belt off northeastern New Zealand could also be carried into the central and northern Tasman Sea. Three megascopic tephras consist of altered basic shards and common labradorite crystals. They record Neogene explosive basaltic to andesitic activity from nearby ocean island or ridge sources in the Ontong-Java Plateau and Vanuatu regions. The megascopic tephras are a very incomplete and biased record of late Cenozoic explosive volcanism in the southwest Pacific because the innumerable, thin, green argillaceous layers in the cores (Gardner et al., this volume) probably represent devitrified intermediate to basic tephras derived mainly from oceanic arc volcanism along the Pacific/Australia plate boundary. In contrast to the New Zealand-derived silicic glass shards, the preservation potential of these more basic shards in Leg 90 calcareous sediments was low.

Geochemistry, mineralogy, and 40K-40Ar radiometric dating of DSDP Leg 84 basalts - Guatemala Trench

Geochemical (atomic absorption, neutron activation analyses), mineralogical (microprobe), and radiometric (40K - 40Ar) data are presented for five basalts from the Guatemala Trench area (Deep Sea Drilling Project, Leg 84). Strong geochemical and mineralogical differences distinguish two types among these basalts: (1) One basalt (Sample 567A-19,CC), recovered below Upper Cretaceous limestone has the following characteristics: it is quartz normative and has low TiO2, content, as well as low amounts of Cr, Ni and other transition metals, an LREE depleted pattern, and affinities of clinopyroxene phenocryst plotted into the field of tholeiitic and calc-alkalic pyroxenes. (2) Four alkaline basalts, recovered from the mafic and ultramafic acoustic basement, are nepheline normative and show high TiO2 content, high amounts of Cr, Ni and so on, an LREE enriched pattern and compositions of clinopyroxene phenocryst plotted close to or within the field of alkali basalt pyroxenes. These basalts are comparable to those recognized in the lower part of the Santa Elena complex and are clearly different from the oceanic basalts of the Cocos Plate. The radiometric age of the orogenic basalt seems to be close to 80 Ma. The alkaline basalts are clearly older, even if a discrepancy appears between the results of different analyses because of the secondary effects of alteration.