Our mineralogical and chemical data provide new information about the nature, metamorphic evolution, and origin of the mafic rocks recovered during Leg 149 in the Iberia Abyssal Plain.
The low-grade metamorphosed or unmetamorphosed lavas and microgabbros present in the cataclastic breccia at Site 899 derived from the crystallization of E-MORB to alkaline-type magmas. These rocks possibly crystallized from magmas that originated from different sources or from a homogeneous melt, later contaminated by an enriched subcontinental mantle. Some of the mafic rocks, and particularly the chlorite-bearing schists, may be considered to be former differentiated Fe-Ti leucogabbros or plagiogranites, which were later hydrothermally modified under low-temperature and -pressure conditions. The associated sheared amphibolites are the hydrated equivalent of the flaser gabbros recovered eastward at Site 900.
The flaser gabbros have no equivalent on shore. Their composition is slightly different from typical mid-ocean ridge gabbros and fits better the compositions of transitional magmas and their differentiated products, which form at the beginning of rifting (Dupuy and Dostal 1984; Sun and McDonough 1989; Arndt et al., 1993). These former gabbros subsequently underwent an intense shearing under relatively high-temperature (high amphibolite to granulite facies) and high-pressure (>0.8 Gpa) conditions, which is not compatible with crystallization under a mature oceanic ridge. This shearing ended under lower-grade metamorphic conditions around 136 Ma (Féraud et al., this volume).
The flaser gabbros would therefore have crystallized from transitional to tholeiitic-type magmas ponded and slowly cooled at the base of continental crust, and subsequently sheared during continental stretching. As thinning proceeded, the decrease of pressure combined with an increase of the permeability and of the amount of pervasive hydrothermal fluids allowed dynamic metamorphism under granulite to amphibolite facies conditions followed subsequently by static green-schist facies conditions. The oldest lavas, presumed to pour out during this stage, at present bear only a low-pressure prehnite-chlorite imprint.