Ocean Drilling Program, United States

During Ocean Drilling Program (ODP) Leg 149 to the Iberia Abyssal Plain, drilling into the basement recovered mafic rocks near and east of a north-south peridotite ridge that bounds the Iberian ocean/continent transition. Recovered primarily at Sites 899 and 900, 60 km apart, some of these extrusive and intrusive mafic rocks show evidence of low to high metamorphic grade. Those recovered at Site 899 consist of lavas, microgabbros, and metamorphosed rocks that occur with predominantly ultramafic breccia and sedimentary rocks within a mass-flow deposit. The entire basement core from Site 900 consists of brecciated flaser gabbros. The low-grade metamorphosed or unmetamorphosed lavas and microgabbros at Site 899 derive from the crystallization of E-MORB to alkaline magmas. Some of the mafic rocks, and particularly the chlorite-bearing schists, may be considered as formerly differentiated Fe-Ti leucogabbros or plagiogranites, later hydrothermally modified under low-temperature and low-pressure conditions. The flaser gabbros (Site 900) and the sheared amphibolites (Site 899) have tholeiitic to transitional affinities, slightly different from those of typical mid-ocean ridge gabbros. These former gabbros underwent intense shearing under relatively high-temperature (high amphibolite to granulite facies) and high-pressure (> or =0.8 Gpa) conditions that ended around 136 m.y. ago in lower-grade metamorphic conditions (Feraud et al., this volume). The peculiar mineralogy, chemistry, and tectonometamorphic evolution of these mafic rocks combined with the preliminary isotope ages are consistent with an origin during continental rifting. The flaser gabbros would have crystallized from transitional to tholeiitic magmas, which ponded and slowly cooled at the base of continental crust, and then sheared during continental extension. Similar, but weakly alkaline, magmas poured out onto the suboceanic floor as extension proceeded. The older ones were subsequently hydrothermally modified under low pressure.