PETROLOGY AND GEOCHEMISTRY OF BASALTS FROM THE SOUTHERN JUAN-DE-FUCA RIDGE - CONTROLS ON THE SPATIAL AND TEMPORAL EVOLUTION OF MIDOCEAN RIDGE BASALT

Three morphologically distinct regions within the neovolcanic zone of the Cleft segment of the southern Juan de Fuca Ridge were investigated and sampled in detail using the DSRV Alvin. Additional along-strike and off-axis samples were recovered by dredge. The southernmost region, the Southern Cleft site, is characterized by a 3-km-wide axial valley floored by ponded sheet flows and bisected by a 30- to 50-m-wide cleft. Farther north at the ''Young Sheet Flow'' site, the ridge axis is characterized by a distinct 500-m-wide inner graben that is largely covered by distinctly younger looking sheet flows. The northernmost of the three regions is defined by a linear series of discontinuous constructional pillow mounds that extend the trend of the Cleft segment well into the zone of overlap with the neighboring Vance segment. The pillowed lavas at the ''Young Pillow Mound'' site represent the most recent episode of volcanism along the Cleft segment. Strong correlations exist between degrees of fractionation, relative ages of lavas, and latitude; lavas are progressively younger looking and more mafic to the north. The compositional range of mid-ocean ridge basalts from the neovolcanic zone can generally be accounted for by 35-40% low-pressure fractional crystallization of relatively primitive, but not primary, depleted (N-type) melts. Scatter of the geochemical data about calculated liquid lines of descent is probably the result of mixing of magmas with slightly different parental compositions, generated from small-scale mantle heterogeneities. Furthermore, the chemical variability may be the result of mixing of very depleted and more enriched sources or melts that are present beneath the southern Juan de Fuca Ridge. The more primitive nature of the young pillow mound basalts and their slightly different chemical characteristics indicates they cannot be simply related to the older southernmost lavas by along-axis flow in a continuous axial magma chamber or conduit. Rather, the data suggest lavas were derived from discrete magma chambers or lenses, each in a different stage of evolution. The youngest events may be associated with a new influx of magma into the northern part of the segment and subsequent northward diking and propagation to form the new pillow mounds. The oldest stage (approximately 40% additional crystallization of the most mafic composition) is associated with focused hydrothermal activity and tectonic extension, whereas the youngest events are characterized by ridge inflation, diking, and dispersed hydrothermal activity. Geochemical and tectonomagmatic features observed along the Cleft segment are similar to those recently documented along the East Pacific Rise from 9-degrees-30'N-10-degrees-N suggesting the scales, processes, and stages of magmagenesis are similar along medium to fast spreading ridge segments.