Lithologic and Chemical Stratigraphy at Deep Sea Drilling Project Sites 417 and 418

Abstract

Chemical compositions of basalts, detailed lithologic data, and stable magnetic inclinations are used to define an eruptive stratigraphy for the basement sections at Holes 417D and 418A. A maximum of 25 eruptive units, comprising three major sequences, are recognized in the 365meter-thick section at Hole 417D, and a maximum of 54 eruptive units grouped into seven eruptive cycles are defined in the 544 meters penetrated at Hole 418A. The rocks in Hole 417A are too altered to permit detailed stratigraphic analysis.The freshness of the rocks at Holes 417D and 418A suggests that the basement was sealed off shortly after formation by sedimentation and growth of secondary phases in the lava pile. Several horizons marked by the presence of breccia, by changes in magnetic inclinations and chemical compositions, and by increased alteration are believed to reflect major quiescent intervals in the eruptive record. These define a pattern of episodic eruption, with each episode possibly derived from a separate magma reservoir system. Variations in downhole magnetic inclinations suggest that crustal deformation occurs during each eruptive cycle, and that oscillatory lateral migration of the active zone may occur before a crustal section is complete.Chemical variation of the basalts is not extensive and is dominated by fractionation of olivine, plagioclase, clinopyroxene, and spinel. Phenocryst movement during eruption and flow is fairly complex, but accumulation of plagioclase at or near the site of initial crystallization is a widespread phenomenon. Most magmas had reached a low-pressure cotectic stage by the time of eruption, but relict clinopyroxenes indicate an earlier stage of fractionation.Dikes were intersected in the lower part of Hole 418A and correspond chemically to flows 130 meters higher in the section. These may represent the top of an underlying dike complex, originally forming nearsurface feeder conduits, but now buried by subsequent eruptions. pret igneous processes at the Cretaceous mid-ocean spreading axis. We proceed by defining eruptive units and major quiescent intervals, and then examine the relation between eruptive chronology and compositional variation of the basalts. From these relationships, the sampled section is interpreted in terms of dynamic crustal construction processes. ANALYTICAL TECHNIQUESEight samples from Hole 417A, 45 from Hole 417D, and 152 from Hole 418A were analyzed for major elements at Bochum, West Germany, while a selection of 71 samples from Hole 418A were analyzed for Sr, Y, Zr, Rb, and Nb at Bedford College, London, England. Samples were carefully selected for analysis from representative portions of all flow units identified onboard ship so as to represent all major lithologic types. A few extremely altered specimens were analyzed, particularly from Hole 417A. These were not representative of original basalt compositions and served only to monitor alteration processes. O w W H > r

An Electron Microprobe Study of Basalt Chemical Variation at Deep Sea Drilling Project Sites 482–485, Gulf of California

“…The glass and whole-rock data usually approximate the overall liquid-type fractionation trend, despite the generally greater scatter of the whole-rock data. This pattern is in contrast to that observed for Atlantic Ocean basalts, where complex phenocryst redistribution patterns involve (notably) the strong accumulation of plagioclase (e.g., Staudigel et al, 1980;Flower et al, 1977Flower et al, , 1980Byerly and Wright, 1978). There are slight but probably significant systematic differences between the Site 482 and Site 483 glass compositional trends.…”

Site Synthesis Report of DSDP Sites 417 and 418

“…Mathez presents sulfur analyses in addition to the more abundant elements. Whole rock analyses were done for Holes 417D and 418A by Staudigel et al (1980a), Flower et al (1980a), and Emmerman and . Flower et al (1980a) also analyzed Hole 417A rocks.…”

“…Whole rock analyses were done for Holes 417D and 418A by Staudigel et al (1980a), Flower et al (1980a), and Emmerman and . Flower et al (1980a) also analyzed Hole 417A rocks. Ui et al (1980) and Rice et al (1980) analyzed only Site 417 rocks.…”

Basement drilling in the western Atlantic Ocean: 1. Magma fractionation and its relation to eruptive chronology

“…Some altered basalts reflect minor leaching of Ca + and Mg + and slight enrichment of SiO2. In accord with previous chemical studies of drilled ocean floor basalts [e.g., Flower et al, 1977Flower et al, , 1980a we have normalized all major element analyses to a H20+-free basis with total iron oxide expressed as FeO, assuming all water to be postmagmatic. A correction was made for secondary carbonate contamination (assumed to be stoichiometric CaCO3) prior to normalization by subtracting CO2-equivalent CaO from each analysis prior to dry weight normalization.…”