Petrology and Geochemistry of Basalts from DSDP Leg 34, Nazca Plate

Geochemical studies of basalts from Sites 319, 320, and 321 of Leg 34 indicate that in all cases they are large-ion-lithophile (LIL) depleted tholeiites typical of ocean floors; that is, they are depleted in light rare earth elements (LREE) relative to chondrites and have low abundances of K, Sr, Ba, and Zr relative to island and continental basalts. Samples from Hole 319 and the upper cooling units of Hole 319A have K2O (0.4%) and TiCh (1.17%) concentrations similar to those of tholeiitic basalts from the East Pacific Rise. Compared to these upper flow units, the lower units of Hole 319A have higher concentrations of elements such as V, Y, Zr, Sr, and REE and lower concentrations of Ni, Co, and Cr. In Hole 32OB the upper units are similar to the upper units of Hole 319. One lower unit in Core 5 of Hole 320B is texturally and petrographically different and shows higher concentrations of elements such as K, Ti, Fe, V, Y, Zr and lower concentrations of Mg, Cr, and Ni. Site 321 is similar in composition throughout. Compared to the other units, these basalts have higher concentrations of Fe, Ti, Ba, V, Y, Zr, and heavy REE (30× chondrite) and lower concentrations of Mg, Ni, and Cr. The geochemical and petrographic evidence is compatible with an origin of these flows at a spreading center. Major element compositional differences between flow units can be explained by shallow level fractional crystallization of Plagioclase, clinopyroxene, and minor olivine. Although the compositional trends of most trace elements are consistent with the direction of fractionation indicated by the major elements, models based on major elements are not able to explain the large range in K2O, TiCh, and LIL trace element abundances. Similar genetic problems are encountered at other ocean ridge spreading centers. Apparently, the parental magmas for Leg 34 basalts differed significantly in LIL element abundances; such differences probably reflect different degrees of melting of the source material and/or source heterogeneity. INTRODUCTION Petrographic and geochemical data are presented for 20 basalt samples obtained from cores at DSDP Leg 34 Sites 319, 320, and 321 on the Nazca plate (Figure 1). Details of drilling sites and hole summaries are given elsewhere in this volume (Chapters 3-5). Sample locations and the numbering system used in this report are shown in Figure 2 with reference to lithologic units defined by the shipboard scientists; complete DSDP sample numbers, which are not used in this report, are given in Table 1. Laboratory procedures described by Frey et al. (1974) were used. Analytical precision and data for reference standards are given in Table 2. PETROGRAPHY The samples can be divided into five petrographic and geochemical groups (Figure 2). Samples not petrographically studied were placed in specific groups in Figure 2 on compositional characteristics. Compared to most recent ocean ridge basalts, the Leg 34 basalts are remarkable for the paucity or lack of olivine and the abundance of clinopyroxene microphenocrysts and magnetite. Modal compositions of some of the holocrystalline samples are given in Table 3. Group I (Represented by Samples 1, 2 ,3, From the Upper Parts of Holes 319 and 319A) Plagioclase microphenocrysts, in "bow-tie" intergrowths with clinopyroxene (Figure 3a), lie in a variolitic, pyroxene-rich groundmass with interstitial magnetite (Figure 3b). Traces of quench olivine appear only in Sample 1. Apparently these samples lie on or close to a plagioclase-pyroxene cotectic, but do not intersect the magnetite field until a significant amount of crystallization has taken place. Group II (Represented by Samples 5, 6, 8, 9, 10, 11, 12 From the Lower Part of Hole 319A) These are microdolerites of variable texture and grain size, presumably due to different positions in the cooling units (Figure 2). Plagioclase and clinopyroxene pre-