GABBRO AND RELATED ROCK EMPLACEMENT BENEATH RIFTING CONTINENTAL-CRUST- U-PB GEOCHRONOLOGICAL AND GEOCHEMICAL CONSTRAINTS FOR THE GALICIA PASSIVE MARGIN (SPAIN)

The thinned continental crust of the west Galicia margin is bound by a belt of serpentinized peridotites ('peridotite ridge') lying about 30 0 km off the coast in the North Atlantic ocean. From this ridge, a gab bro and a chlorite rock were studied in an attempt to substantiate rif t-related subcontinental magmatism, occurring prior to sea-floor sprea ding. U-Pb dating of 13 different zircon fractions yields a precise ag e of 122.1+/-0.3 Ma (2 sigma) for the emplacement of the chlorite rock protolith, from which more than 50% of Si and alkali-calc-alkali elem ents were lost during greenschist facies tectonometamorphism. Sr and N d isotope signatures suggest that the gabbro and chlorite rock protoli ths were derived from mantle sources that were moderately depleted in LILE, relative to a chondritic reservoir. No evidence for the presence of continental material in the magma source regions can be observed. From the new zircon age of 122.1 +/- 0.3 Ma, and the earlier determine d Ar-39-Ar-40 of 122.0+/-0.6 Ma for amphibole from the same locality, it can be documented that magma formation, solidification and unroofin g of the mantle rocks occurred during a short period of time of about 3.4 Ma, which means that the peridotite ridge detached from the contin ent and rose to the surface immediately after, or even coevally with m antle melting.