Mafic igneous rocks (basalt, dolerite, gabbro) were recovered from 6 of the 13 holes drilled in the Gulf of California during Leg 64. Sites 474 and 475 are on oceanic and continental crust, respectively, at the southern tip of Baja California; they effectively form secular end-points of a transect from the East Pacific Rise (EPR) completed during Leg 65. Hole 474A penetrated a sequence of olivine-rich dolerite sills before encountering pillow basalts and massive flows typical of Oceanic Layer 2. Although the basement age at this site is estimated to be 3.2 m.y., the sills clearly represent offaxis activity postdating the axis lavas by approximately 1.4 m.y. Detailed X-ray fluorescence (XRF) and instrumental neutron activation analysis (INAA) of samples from Hole 474A demonstrate that the least-altered axis lavas are depleted in the more-hygromagmatophile (HYG) elements (Cs, Rb, K, Ba, Th, Ta, Nb, La, Ce) relative to the HYG elements (middle to heavy rare earth elements [REE], Y, Zr, Hf, Sr, P, and Ti; e.g., Ti/Zr = 85-100, Zr/Nb-30, La N / Yb N-0.9, Th/Hf-0.12, La/Ta-20). These ratios are similar to those found in basalts recovered during Leg 65 (Ti/Zr = 100-130, La N /Yb N-0.5; Th/Hf = O.O3-O.O8) but do not indicate such strong more-HYG element depletion. The off-axis sills, however, have high MgO, Ni, and Cr contents; very low more-HYG/HYG element ratios (e.g., La N /Yb N = 0.2), and high La/Ta ratios (generally in excess of 40). It is difficult to relate the composition of at least one of the sills to that of the basement lavas by fractional crystallization or even by partial melting of chemically similar sources; and it is more likely that the sills were derived from a mantle source that was originally compositionally heterogeneous and that had undergone a previous episode of basalt extraction (possibly during the production of the axis lavas). Hole 475B is in a continental slope basin, 21 kilometers southeast of the tip of Baja California. Drilling resulted, unexpectedly, in recovering fresh, sparsely olivine-phyric basaltic cobbles. The basalts are nepheline normative, with high MgO, Ni, and Cr contents, and probably represent liquid compositions. They exhibit relative depletion of several of the more-HYG elements (particularly Ta, Nb, and Th); La N /Yb N ratios exceed 1, but La N /Sm N ratios are less than 1, and Sr/Zr ratios are high. The petrogenesis of these basalts is obscure, but it is likely that they were derived either from a depleted mantle source that contained residual components and a calc-alkaline chemistry or from a depleted, EPRtype source followed by minor sialic contamination prior to eruption. Sites 477, 478, and 481 are in the Guaymas Basin, a youthful basin containing two small spreading axes beneath a blanket of sediments. Drilling at all three sites failed to recover unequivocal basalt flows; the main igneous bodies were massive basaltic, doleritic, or gabbroic sills. Detailed XRF and INA analysis show that the units from all three sites are chemically similar and characterized by dep...
Major-element, trace-element, and mineralogic data from basalts drilled during Leg 64 indicate that a wide spectrum of N-type MORB have been erupted in the Gulf of California. Leg 64 basalts have chemical and mineralogical characteristics broadly similar to those from the East Pacific Rise and Nazca Plate. They are not nearly so evolved as basaltic lavas recovered from the Galapagos Spreading Center. Trace-element data suggest that several N-type mantle sources or a spatially heterogeneous mantle were partially melted to generate the spectrum of basalts recovered during Leg 64. However, simple, closed-system crystal fractionation does not adequately explain all of the observed chemical and mineralogical features. Mixing of primitive magmas, with observed An-rich Plagioclase xenocrysts and fractionating magmatic liquids, is a plausible mechanism which can help to explain the evolution of basaltic magmas in the Gulf of California.
Fourteen microprobe analyses of basaltic glasses from three drill sites plus two dredged samples from the seafloor at the mouth of the Gulf of California suggest that magmatic liquids that cooled to form the volcanic crust in the Gulf of California have been fractionated, whereas other, more primitive submarine basalts from the Mid-Oceanic Ridge (MOR) have not. The limited chemical diversity among drill sites suggests that magmatic liquids, which erupted along the accretionary boundary in the Gulf of California, are grossly similar. Major-element variations in the glasses from Hole 474A may, in part, however, be attributed to simple crystal-liquid fractionation in a subridge magma chamber. Additionally, the analysis of a glass from Hole 475B, near the eastern continental slope of southernmost Baja California, suggests that magmatic liquids which produced submarine basalts associated with initial opening of the Gulf may have been chemically less evolved than present ones. Trace-element concentrations (Saunders et al., this volume, Pt. 2 [hereafter "this volume"]) in whole rocks from Hole 475B suggest that this magma may have experienced minor sialic contamination on its ascent through lower crustal continental material. Whole-rock and glass major-element concentrations, however, suggest that early Pliocene(?) basalts that erupted in Hole 475B were derived from a magmatic liquid of (deep) mantle origin. The liquid quickly rose to the surface with little shallow-level fractionation.
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