The unmetamorphosed lavas and microgabbros from Site 899 have a wide range of compositions which overlaps the tholeiitic and transitional fields, some lavas having alkaline affinities (Fig. 18). The porphyritic lavas and microgabbros have flat tholeiitic-like patterns that do not exhibit the common LREE depletion of N-MORBs. These flat patterns, with LaN/CeN > 1 and CeN/YbN > 1, fit the characteristics of E-MORB well (Sun and McDonough, 1989). Other lavas are clearly transitional in composition.
Such compositional trends are common in lavas from rifted continental margins. Those recovered on the Galicia Margin display quite similar tholeiitic to alkaline trends (Kornprobst et al., 1988). However, they have higher heavy REE (HREE) content (11 times chondrite, at least) and some of them are LREE depleted, like true MORBs. Lavas and dolerites from Zabargad Island (Petrini et al., 1988) also display the same compositional fan as the Galician lavas, with the same order of HREE contents but with slightly higher LREE ones in the most enriched samples.
Differences between the Leg 149 lavas and those from Galicia Bank and Zabargad Island are slight. Compared to the latter, the Leg 149 lavas do not display any LREE depletion and are HREE poorer (10 times less), even for the more enriched samples. This would indicate that the sources at the origin of the Iberia Abyssal Plain lavas retained HREE better or were more depleted in HREE. Because of the high content of incompatible elements, we suggest that the magmas from which the Leg 149 lavas originated formed at relatively high pressure through low degrees of partial mantle melting that probably involved garnet-bearing zones.
The prehnite-bearing mafic metamorphic clasts recovered at the same site retain textural (porphyritic or intergranular) and mineralogical (pyroxenes) evidence that points to the origins of the lava. Their geochemical signatures are in fact quite similar to those of the unmetamorphosed lavas and microgabbros (Fig. 18). Some have a tholeiitic signature, others have transitional to alkaline affinities. The high Mg# of the pyroxenes in the metamorphic clasts point to a lesser degree of differentiation for these clasts compared to the clasts of unmetamorphosed rocks.
The clasts and boulders dominated by chlorite (ripidolite) are peculiar metamorphic rocks, the origin of which is debatable. Petrological and chemical arguments have shown that such rocks may result from the combined effects of hydrothermal alteration and compressive strain operating on leucogabbros during magma cooling (Essaifi et al., 1995). Those recovered from the Galicia Bank were interpreted as derived from a differentiated and underplated Fe-Ti gabbro, deformed during the late stages of the continental rifting. They provided primitive zircon with a U/Pb age of 122.1 Ma, which is in close agreement with the previous results obtained on this margin (Schärer et al., 1995). Those recovered at Site 899 have flat REE patterns that fit those of leucogabbros or plagiogranite perfectly. Their REE concentrations (20-25 times chondrite), combined with a CeN/YbN of 1.24 and the absence of La and Ce depletion, would be indicative of undepleted tholeiitic or transitional affinities for these differentiated magmas.
The sheared amphibolite sampled at the bottom of Hole 899B have all the mineralogical and geochemical characteristics of the Site 900 flaser gabbros, apart from the occurrence of Cl-rich titanian pargasites. Although the chronology of amphibole crystallization must be determined, it seems that these former gabbros underwent a high-amphibolite-grade metamorphism, close to granulite facies conditions, which allowed crystallization of green spinels (pleonaste) and titanian pargasites. Later, the primary clinopyroxenes and brown amphiboles were replaced by lower-grade amphiboles.
The flaser gabbros from Site 900 and the sheared amphibolites from Site 899 are obviously different from the undeformed microgabbros, whether prehnite-bearing or not, recovered from Site 899. The former indeed contain high-temperature and high-pressure phases such as green spinel, Na- and Al-rich pyroxenes, or titanian pargasite, which are indicative of metamorphism under high-pressure conditions (about 0.8 GPa). Because the syntectonic recrystallization process leading to the transformation of porphyroclasts into neoblasts is not marked by any Al or Na decrease for pyroxene, and Ca for plagioclase, it is inferred that the high-pressure and high-temperature conditions were still operating during the shearing event.
The major- and trace-element compositions of the flaser gabbros correspond to those of tholeiitic magmas. Their relatively high REE concentrations, their LREE content up to 2.7 times chondrite, and their CeN/YbN of 0.81 are unusual in MORB-type gabbros, unless it is assumed that they represent a differentiated fraction of "primitive" MORB-like magma (Allègre et al., 1973). The latter point is consistent with the high Mn contents of their Fe-Ti oxides but not with the high Mg# and Ni content of bulk rocks. Their LaN/CeN of about 1 are also unusual for MORB-like magmas, even if differentiated, and fit better those of transitional magmas that form at the beginning of rifting (Dupuy and Dostal, 1984; Sun and McDonough, 1989; Arndt et al., 1993). The age close to 136 Ma of the last low-grade metamorphic event that imprinted the gabbros at Site 900 (Féraud et al., this volume) is consistent with an origin related to the last magmatic episodes of the continental rifting (Féraud et al., this volume).
At present, no equivalent of these flaser gabbros is known on the Galicia Bank, or on shore from Iberia. The dredges performed immediately northward have produced very different rocks: quartz-, sericite- and chlorite-bearing schists on the Vasco de Gama Seamount, and orthogneiss and coarse-grained garnet-, orthopyroxene-, and biotite-bearing granulites on Vigo Seamount (Capdevila and Mougenot, 1988). These granulites, the origins of which are dubious, have Archean ages (Guerrot et al., 1990; Capdevila, pers. comm., 1994).