Gabbroic Pegmatite Intrusions, Iberia Abyssal Plain, ODP Leg 173, Site 1070: Magmatism during a Transition from Non-volcanic Rifting to Sea-floor Spreading

On the Iberia Abyssal Plain (Ocean Drilling Program Site 1070),gabbroic pegmatites and related rocks (127 ± 4 Ma, U–Pbzircon) intrude upper mantle that was subsequently exposed andserpentinized during Early Cretaceous non-volcanic rifting.The pegmatites include a 3–4 m dike or sill (the ‘main’pegmatite), numerous dikelets of 1–5 cm thickness, andclasts within the overlying ophicalcite breccia. Exclusive ofrodingitization, the main pegmatite contains 40–70% calcicandesine, 25–35% kaersutitic amphibole (Mg# 60–70),5–25% augite (Mg# 70–80) and 1–2% ilmenite.The dikelets are more magnesian (Mg# up to 82 in kaersutiteand 88 in augite). Most indications are that the high Mg#s inthe dikelets reflect igneous compositions. Isotopic and elementalchemistry indicate that the pegmatite-forming melt was enrichedin incompatible elements relative to normal mid-ocean ridgebasalt, but not as enriched as Azores basalts. The amphibole-bearingplagioclase peridotites of the Iberia Abyssal Plain are an appropriatesource for the pegmatite melts. A combination of decompressionaccompanying unroofing and heating from the upwelling asthenospherebeneath the developing rift caused P–T conditions in theamphibole-bearing lithosphere to exceed the dehydration-meltingsolidus (