The nature of the basement of nonvolcanic passive continental margins is poorly documented. Their structure and lithology have, however, a critical importance in constraining the beginning of the rifting process. Recent studies of ocean/continent transitions have shown that serpentinized peridotites occur frequently on these nonvolcanic margins. The presence of such mantle fragments at ocean/ continent transitions proves that continental break-up does not necessary imply subsequent and immediate oceanic accretion, (i.e., extrusion of a large volume of lavas producing new crust). This was well illustrated on the west Galicia Margin where successive dredges (Boillot et al., 1980), drill holes (ODP Leg 103; Boillot, Winterer, Meyer, et al., 1988; Boillot et al., 1988a, b), and dives using the French submersible Nautile (Galinaute cruise; Boillot et al., 1988a) have emphasized that the ocean/continent transition was bordered by a peridotite ridge.
Geophysical survey data (Beslier et al., 1993; Whitmarsh et al., 1990, 1993) and drilling data from Sites 897 and 899, acquired during Leg 149 in the Iberia Abyssal Plain, suggest that this ultramafic ridge extends more than 300 km south of the Galicia Margin (Fig. 1). West of this ridge, an abnormally thin crust with, in places, a seafloor-spreading magnetic signature, remains unsampled. Mafic rocks were recovered in minor amounts near the peridotite ridge itself (Sites 897 and 899), but mainly on a basement high at Site 900, farther east toward the continent. These rocks are irregularly distributed and varied; they are mostly gabbros with minor basalts and dolerites.
On the Galicia Margin, very few mafic rocks were sampled together with the peridotites. They were recovered from five dive sites on the northwestern edge of the bank and consist of a few dioritic veinlets that crosscut the peridotites, 10-m-thick chlorite-bearing schists, supposedly derived from a Fe-Ti-rich gabbro (Beslier et al., 1990; Schärer et al., 1995), a single pod of gabbro, and eight occurrences of basalts and dolerites (Boillot et al., 1988a; Beslier et al., 1990). The chemical signatures and ages of the mafic rocks support a formation during the continental rifting phase (Féraud et al., 1988; Kornprobst et al., 1988; Schärer et al., 1995).
In the Iberia Abyssal Plain, the close association of mafic and ultramafic rocks strongly suggests that magmas were also generated during the rifting stage. To test this hypothesis, petrological data, including phase analyses along with bulk-rock, trace-element, and rare-earth-element chemistry, are discussed and compared to similar occurrences.