The nature of the crust along the west Iberia margin (Fig. F1) has been investigated during Legs 103, 149, and 173 and shown to consist generally of sedimentary sequences overlying tilted fault blocks of sedimentary rock, metaplutonic rock, and/or serpentinized peridotite (Boillot, Winterer, Meyer, et al., 1987; Sawyer, Whitmarsh, Klaus, et al., 1994; Whitmarsh, Beslier, Wallace, et al., 1998). The objectives of both Legs 149 and 173 were to establish the nature and boundaries of the ocean-continent transition (OCT) crust between the Iberian continental crust and the Atlantic oceanic crust in order to understand the development of a nonvolcanic or magma-starved passive margin. The OCT zone and thinned continental crust, which consists of a number of basement fault blocks, are overlain by rift-to-drift related sedimentary rocks (Fig. F2). Rifting in this region proceeded from the Gorringe Bank area in the south (143 Ma) (Feráud et al., 1986) to the Galicia Bank area in the north (122 Ma) (Schärer et al., 1995). Continental rifting was determined to have ended by 136 Ma, based on 40Ar/39Ar analyses of plagioclase from the Leg 149 metagabbros (Feráud et al., 1996). Initiation of seafloor spreading for this region has also been estimated at 126 Ma based on anomaly M3 (Whitmarsh and Miles, 1995). Prior to Leg 173, three hypotheses for the origin of the OCT zone were proposed. The first model, presented by Sawyer (1994), suggests that the OCT zone is composed largely of oceanic crust formed by ultra-slow spreading. The second model, proposed by Whitmarsh and Miles (1994, 1995), suggests that the OCT zone is formed by magmatic and tectonic disruption of the continental crust. The third model, proposed by Krawczyk et al. (1996), suggests that the OCT zone is underlain from east to west by a progressive sequence of upper continental crust, lower continental crust, and then subcontinental upper mantle material unroofed by low-angle detachment faulting.
The main results from Leg 173 are (1) amphibolites and metagabbros were recovered on the same continental structural high so that metamorphism probably relates to rifting and detachment faulting, (2) the origin of these metamorphic rocks has been shown to be unrelated to rifting during the opening of the Atlantic ocean basin (Manatschal et al., in press), (3) basaltic rocks were not recovered and have only been encountered as clasts from one site on Leg 149, and (4) rift-related magmatism or early seafloor spreading appears to be represented by gabbroic dikes within serpentinized peridotite at the western edge of the OCT zone. All of these findings reinforce the view that this margin was largely nonvolcanic during the rifting stage and that low-angle detachment faulting probably emplaced the sequences of rocks recovered during Legs 149 and 173.
This report presents whole-rock geochemistry for Leg 173 amphibolites and metagabbros from Sites 1067 and 1068. These compositions are used to show how the Leg 173 samples are distinct from the Leg 149 metagabbros and basalts. Possible protoliths for the Leg 173 amphibolites are also proposed in order to better understand the origin and subsequent history of these metamorphic rocks from the west Iberia margin.