Petrology and geochemistry at DSDP Site 89-462

The 16 samples of Deep Sea Drilling Project (DSDP) Leg 89 basalts that we analyzed for whole rock major and trace elements and for mineralogic compositions are identical to some of the basalts recovered during Leg 61. Leg 89 samples are mostly olivine-plagioclase-clinopyroxene sparsely phyric basalts and exhibit a wide variety of textures. These basalts have lower TiO2 at a given Mg/(Mg+Fe2+)*100 than MORB (midocean ridge basalt). We recognize three major chemical types of basalts in the Nauru Basin. We believe that different degrees of partial melting, modified by fractional crystallization and possibly by magma mixing at shallow depths, can explain the chemical differences among the three groups. This petrogenetic model is consistent with the observed downhole chemical-chronostratigraphic relations of the samples. New 87Sr/86Sr and U3Nd/144Nd analyses of basalt samples from DSDP Site 462 indicate that the Nauru Basin igneous complex is within the Sr-Nd isotopic range of ocean island basalt. Thus the Nauru Basin igneous complex resembles MORB in many aspects of its chemistry, morphology, and secondary alteration patterns (Larson, Schlanger, et al., 1981), but not in its isotopic characteristics. If it were not for the unambiguous evidence that the Nauru Basin complex was erupted off-ridge, the complex could easily be interpreted as normal oceanic layer 2. For this reason, we speculate that the Nauru Basin igneous complex was produced in an oceanic riftlike environment when multiple, fast-propagating rifts were formed during the fast seafloor spreading episode in the Cretaceous.

(Table 1) Geochemistry of realatively fresh rocks of ODP Sites 127-794 and 127-797

The flows and sills drilled at Sites 794 and 797 in the Yamato Basin of the Japan Sea are subalkalic, olivine, and/or plagioclase phyric basalts. Compositionally, the rocks can be divided into a depleted, low-K type and an enriched, relatively high-K type. In addition, two contrasting evolution trends are reflected in the rock compositions, which allow four different magmatic suites to be identified. It is suggested that the depleted or enriched nature of these suites represent primary characteristics, while the different evolution trends are related to fractionation processes in crustal magma chambers. A tholeiitic evolution trend, with increasing FeO and TiO2 and decreasing Al2O3, can be modelled by fractional crystallization of 40%-50% plagioclase, olivine, and augite. A mildly calc-alkalic evolution trend, with decreasing FeO, increasing Al2O3, and nearly constant TiO2, can be modelled by 8%-12% olivine fractionation. Mineralogical evidence suggests that these differences may be related to the effect of small amounts of water during crystallization of the calc-alkalic suites. The tholeiitic suites occur in the lower parts of the drill cores, while the calc-alkalic suites occur in the upper parts. This suggests a complex tectonic and magmatic evolution, perhaps reflecting a transition between calc-alkalic magmatism related to subduction zone activity and tholeiitic magmatism related to back-arc spreading. Furthermore, any magmatic model must be able to account for the range in parental magmas from depleted to enriched throughout the tectonic history of the Yamato Basin.