Abstract:
At ODP site 1070 (Leg 173, Iberia Abyssal Plain), gabbroic pegmatites (124 Ma) intrude upper mantle rocks exposed during Early Cretaceous non-volcanic rifting at the Iberian Margin. The pegmatites record magmatism that occurred during the transition from non-volcanic to volcanic rifting in this part of the North Atlantic.The pegmatites include a large (recovered thickness 4 m) dike or sill (the "main" pegmatite), numerous small (1-5 cm thick) dikelets, and fragments within the overlying, parautochthonous serpentinite breccia. Exclusive of alteration products, the main pegmatite consists of 45% zoned plagioclase (rims An35-47; cores An43-53), 50% red-brown kaersutite (pargasitic amphibole with up to 5.3 wt. % TiO2), and less than 5% each augite (partially replaced by amphibole) and ilmenite. Some amphibole crystals in the main pegmatite are spectacularly zoned from magnesian kaersutite cores to Fe-Na-Al-rich hastingsite rims. This indicates substantial Fe-enrichment during in situ differentiation. The high Ti, Na, K, and H2O content of the main pegmatite, its elevated Mg/Mg+Fe (0.6-0.7 in amphibole, 0.7-0.8 in pyroxene), and the limited ascent capacity of a (presumably volatile-saturated) pegmatitic melt all suggest that it was derived by a modest amount of fractionation from a locally-derived, low-fraction melt. The pegmatoidal dikelets are more magnesian (Mg/Mg+Fe = 0.7-0.8 (amphibole) and 0.8-0.9 (pyroxene)) than the main pegmatite. They may represent the locally derived parent for the main pegmatite, thus supporting the model above. However, subsolidus, metasomatic equilibration with surrounding mantle rocks must be considered a possibility in these thin dikelets. A likely cause of melting was decompression accompanying unroofing during extension.