According to Cornen et al. (chapter 26, this volume), the composition of the Site 900 gabbros is close to that of basalts, dolerites, and gabbros from the OCT of the adjacent Galicia Margin, which was probably formed at the end of the Mesozoic continental rifting by interaction between a MORE source and enriched continental mantle (Kornprobst et al., 1988; Schärer et al., 1995). For this reason, and also because no similar rocks of Hercynian affinity have been identified close to Site 900 either on the western Iberia Margin or on land, these gabbros are more likely related to the Mesozoic syn-rift event rather than to the mobilization of Hercynian basement during margin formation, or to true oceanic accretion in the Atlantic domain.
The main petro-structural constraints on the evolution of the Site 900 gabbros, previously discussed in detail, are the following. The oldest recognizable tectonic event is a dynamic recrystallization during a shear deformation which developed the foliation and porphyroclastic or granuloblastic texture of these rocks. This deformation is intense and homogeneously distributed over the whole drilled section. The phase composition (Na- and Al-rich pyroxenes + spinel) suggests high-pressure conditions of dynamic recrystallization (0.4 GPa; Cornen et al., chapter 26, this volume. This ductile deformation was followed by a static retrograde metamorphism under greenschist facies conditions. A late low-temperature deformation event, aided by the circulation of hydrothermal fluids, led to fractures filled with chlorite, fibrous amphibole, epidote, zoisite, and later calcite. This deformation, which is unevenly distributed throughout the cores, locally brecciated the rocks and formed late shear zones in some intervals of high chlorite-vein density. The two investigated samples belong to zones preserved from this late deformation.
This evolution is comparable and compatible with that of the peridotites drilled at Site 897. These mantle rocks underwent a high-temperature shear deformation, a limited partial melting, and a subsolidus reequilibration in the plagioclase-stability field, followed by a mylonitic shear deformation. This high-temperature evolution is overprinted by extensive deformation under sub-surface conditions, aided by hydrothermal fluid circulation, during and after the serpentinization of the rocks. This evolution is compatible with uplift beneath the rift zone (Beslier et al., this volume; Cornen et al., chapter 26, this volume).
On the adjacent northern Galicia Margin (Fig. 1), the mantle rocks drilled at the OCT underwent a similar evolution during their uplift beneath the rift zone (Girardeau et al., 1988; Kornprobst et al., 1988). The partial melting of the rocks is expressed in particular by the occurence of syn-kinematic dioritic dikes, and by a horizon at the top of the mantle of sheared chlorite-bearing schists derived from former gabbros (Galinaute diving sites 9-10; Fig. 1). Geochronological constraints show that emplacement and cooling of these mafic rocks occurred over a short time period (around 3.4 Ma) during the last stages of continental rifting (Féraud et al., 1988; Boillot et al.,1989; Schärer et al., 1995).
Although extra 40Ar/39Ar analyses (in progress) are needed to correctly interpret the data presented here, and for reasons previously given, we may consider the weighted mean age of 136.4 ± 0.3 m.y. as a reasonable estimate of the closure time of the plagioclase (the closure temperature of plagioclase is estimated in the range 200°-250°C; McDougall and Harrison, 1988). This age corresponds to the K/Ar closure of these minerals at the end of the retrograde metamorphic event recorded by the gabbros. Moreover, this age is about 11 m.y. older than the late Barremian age of the oldest sediments recovered in the OCT (Kent and Gradstein, 1986), on the peridotitic basement at Site 897, which clearly postdate the continental breakup of the margin.
Considering the nature and the evolution of the basement in the OCT of the Iberia Abyssal Plain, and by analogy with the adjacent deep Galicia Margin, this age is consistent with emplacement of these rocks in the thinned continental lithosphere during the last stages of continental rifting. The high-pressure conditions of dynamic recrystallization supported by their mineralogical composition suggest that, before the static metamorphism, the gabbros were ductilely sheared at mid-crustal depths. They may represent either (1) gabbros under-plated at the base of the thinned continental crust (according to this hypothesis, the shear may have developed in a normal shear zone having a synthetic relationship to the normal faults delimiting the upper crustal blocks during the formation of the margin); or (2) gabbros trapped as a thick sill in the upper part of the mantle and sheared during the mylonitization of the surrounding peridotite.
The age of 136.4 ± 0.3 m.y. is intermediate between the 122 ± 0.3 Ma 40Ar/39Ar plateau-age measured by Féraud et al. (1988) on a dioritic dike intruding ultramafic rocks of the Galicia Margin (dive sites 9-10, Fig. 1), and 142.1 ± 0.5 Ma and 143.9 ± 0.7 m.y. 40Ar/39Ar plateau-ages measured on the amphiboles from gabbros of Gorringe Bank, to the south (Féraud et al, 1986). These three geochronological data are consistent with a northward propagation of the Atlantic Ocean opening as suggested by Klitgord and Schouten (1986).
Hence, although there is no direct evidence to demonstrate that the Site 900 gabbros formed during the Mesozoic rifting of the margin, petrological characteristics, their tectono-metamorphic evolution, and dating of the latest retrograde metamorphism that they underwent (at the end of the continental rifting) strongly suggest that they formed during the rifting.