The basalt and diabase clasts recovered from Hole 899B most closely resemble E-MORB similar to that characteristic of much of the present day Atlantic Ocean floor, or less likely CFB. The lack of CFB on the Iberia and Newfoundland continental margins adjacent to the Atlantic Ocean makes a CFB origin unlikely and would seem to indicate these clasts have a MORB origin. The extensive world wide MORB database has allowed detailed analysis of variations in the major element composition of clean MORB to search for compositional relationships and their causes (Klein and Langmuir, 1987; Brodholt and Batiza, 1989; Klein and Langmuir, 1989; Niu and Batiza, 1993). Even small-scale spatial and temporal variations of a local source are now being defined (Perfit et al., 1994). MORB compositional data for the North Atlantic Ocean have been summarized by Bryan and Moore (1977), Langmuir et al. (1977); White and Bryan (1977); Bryan (1979); Bryan et al. (1981); Schilling et al. (1983); whereas Humphris et al. (1985) have provided compositional data for a portion of the southern Mid-Atlantic Ridge (MAR). The wide range in MORB composition along the length of the Mid-Atlantic Ridge and the enrichment of Azores Plateau basalts has been well illustrated by Schilling et al. (1983).
MAR basalt ranges from E-MORB at and near the plume generated Azores Plateau, including the FAMOUS area, to N-MORB from 49°N to 59°N to the north of the Azores Plateau and south of 33°N to the south of the Azores Plateau (Shibata et al., 1979). The E-MORB tholeiites that form the Azores Plateau are distinct from the incompatible element-rich alkali basalts that make up the Azores Islands themselves (White and Schilling, 1978; White et al., 1979). None of our clasts has either greatly enriched LREE patterns similar to the alkali basalts of the Azores Islands (White et al., 1979) or depleted LREE patterns similar to nonplume-generated N-MORB that characterizes the regions north and south (Bryan et al., 1981) of the Azores Plateau region. All of the Hole 899B basalt and diabase clasts have incompatible element compositions similar to the much younger plume-generated E-MORB basalts from the nearby Azores Plateau (Schilling, 1975), with most basalt clasts resembling basalts from 37°-40°N. The one transitional basalt clast and diabase clasts more closely resemble the average FAMOUS basalt (Davies et al., 1989). Although plume-generated volcanism typically should be expected to result in abundant magmatism and the production of a volcanic rifted margin, the presence of a plume component is the only widely accepted mechanism to produce E-MORB. The presence of a plume component in basaltic rocks from this nonvolcanic rifted margin provides further evidence for multiple sources and magnitudes for plume components.