Ocean Drilling Program, United States

Ultramafic rocks were recovered at Sites 897 and 899, drilled during Ocean Drilling Program (ODP) Leg 149 to the ocean/continent transition in the Iberia Abyssal Plain. Such recoveries, in particular that at Site 897, attest to the southward extension of a peridotite ridge previously studied along the Galician Margin. The ultramafic rocks are coarse-grained websterites and depleted peridotites with some minor plagioclase-rich lherzolites. The variations in modal compositions and primary-phase chemistry show that many of the peridotites were subjected to significant partial melting. These rocks also underwent some impregnation by undersaturated alkali magmas that, in many samples, have given rise to local resorption of orthopyroxene, cryptic enrichment in Fe and Ti of most phases, and plagioclase crystallization. Such magmas have left discrete traces composed of clinopyroxene, kaersutite, phlogopite, ilmenite, and rutile. This impregnation occurred during the last stages of the high-temperature deformation event that produced the porphyroclastic foliation in the peridotites. It stopped under static conditions, at a temperature around 965 degrees -880 degrees C and at a pressure below 1 GPa. Subsequently, the websterites underwent a subsolidus reequilibration event, well defined by textural features, which implies that some spinel and plagioclase formed after Al-pyroxenes and Al-spinel, and also defined by some local Ti enrichment of both spinels and pyroxenes. This reequilibration event occurred after the main high-temperature deformation, at a temperature of about 78O degrees C and a pressure below 1 GPa, but before a mylonitic stage well displayed in a few rocks, which ceased at a temperature close to 735 degrees C. The magmatic signature of the Leg 149 ultramafic rocks, their petrological and tectonometamorphic evolution, and their similarities with the peridotites recovered at the western edge of Galicia Bank support the hypothesis that they originated as a piece of asthenospheric mantle that has been accreted to the lithosphere in a rift system during stretching of the continental lithosphere.