CONTINENTAL BASEMENT CORES

Direct evidence of continental basement rocks comes from three sites (Sites 900, 1067, and 1068) drilled within 1400 m of each other along the east-west Lusigal-12 seismic profile (Fig. F4) and from scattered clasts in the breccias and mass-flow deposit at Site 899. The first three sites lie on a north-south trending, elongate basement high (called Hobby High by Leg 173 scientists) now known to form part of a ridge that forms the western half of Vasco da Gama Seamount (Fig. F3). The two Leg 173 sites were drilled using an offset strategy designed to sample above and below a 15° east-dipping strong intrabasement reflector (M) that is offset downward by a steep east-dipping normal fault (Whitmarsh et al., 2000) to join a deeper reflector (H_L, H_H, and H in Fig. F5) identified as a major synrift detachment (Beslier et al., 1995; Krawczyk et al., 1996) or décollement (Brun and Beslier, 1996) at the base of tilted continental blocks. Reflectors M and H_L (immediately adjacent to Hobby High) probably represent a tectonic crust/mantle boundary (Whitmarsh, Beslier, Wallace, et al., 1998; Whitmarsh et al., 2000).

At Site 1067, amphibolite with minor tonalite gneiss and meta-anorthosite was cored. Relict textures indicate that the amphibolite protolith may have been either hornblende gabbro or retrometamorphosed pyroxene gabbro. A heterogeneous ductile shear deformation developed a clear locally folded foliation at the top and bottom of the cored section. The foliation is overprinted by mainly static retrometamorphism and heterogeneous fracturing, which locally grades into brecciation in the middle part of the cored section. R. Rubenach, N. Froitzheim, P. Wallace, M. Fanning, and R. Wyzsoczanski (unpubl. data) dated magmatic zircons, which were separated from a metagabbro, at 270 ± 3 Ma; this most likely represents the gabbro intrusion age and is coeval with many other late Hercynian rocks from Western Europe. Further support for this conclusion was obtained from hornblende geobarometry on igneous amphiboles included in plagioclase porphyroclasts in a metatonalite clast from Site 1068 and a tonalite vein from Site 1067; this technique yielded intrusion pressures of 0.73 and 0.60 GPa, respectively. Therefore, they concluded that the metagabbros and metatonalites encountered at Sites 1067 and 1068 intruded and were metamorphosed at lower to mid-crustal depths in the late Hercynian and do not represent igneous or metamorphic rocks contemporaneous with continental breakup. It is interesting to note that the Iberian Basin in Spain was undergoing extension at this time (Arche and Lopez, 1996).

Clasts of amphibolite, metagabbro, and meta-anorthosite were encountered at Site 1068 within a sequence of poorly sorted sedimentary and tectonic breccias. The lower breccias show evidence of cataclasis and hydrothermal metasomatism. The breccias are separated by a tectonized zone from an underlying commonly weakly foliated peridotite, which, although up to 99% serpentinized, contains relict primary textures suggesting the protolith was foliated spinel- and plagioclase-bearing peridotite. Clasts of basalt, microgabbro, diabase, and chlorite-bearing mylonite were also found in the breccias and mass-flow deposits of Site 899.

Metagabbros cored at Site 900 during Leg 149 were metamorphosed under amphibolite to granulite grade conditions (Cornen et al., 1996b) and have trace element and rare-earth element (REE) concentrations that are similar to transitional mid-ocean ridge basalt (MORB) (Seifert et al., 1996, 1997). Based on these geochemical characteristics and MORB-like _Nd values (+6 to +11), the metagabbros have been interpreted as cumulates that formed at a spreading center active during the opening of the Iberia Abyssal Plain OCT (Seifert et al., 1997). However, the late Hercynian ages of zircons from metagabbros at Site 1067, on the same basement high as Site 900, are not consistent with the spreading center interpretation. Alternatively, the metagabbros may be cumulates that were emplaced and sheared at the base of slightly thinned continental crust at pressures of <0.8 GPa (Cornen et al., 1999). ⁴⁰Ar/³⁹Ar dating of plagioclase from Site 900 metagabbros yielded a date of 136.4 ± 0.3 Ma (Féraud et al., 1996), but given that the closure temperature of plagioclase is 200°-250°C (McDougall and Harrison, 1988), this date may simply indicate the time when these rocks were tectonically uplifted and cooled through the 200°-250°C isotherm during the last phase of rifting.

Despite their proximity to Site 900, amphibolites and metagabbros from Sites 1067 and 1068 have higher trace element and REE abundances (Smith Nagihara and Casey, Chap. 10, this volume). The Site 1067 and 1068 rocks also differ from Site 900 metagabbros in lacking certain geochemical traits that are diagnostic of gabbroic cumulates, such as strongly positive Eu/Eu* and Sr/Sr* anomalies that result from plagioclase accumulation. On this basis, Smith Nagihara and Casey (Chap. 10, this volume) interpreted the protoliths for Site 1067 and 1068 amphibolites and metagabbros to have originally formed by direct crystallization of basaltic magmas without significant crystal accumulation. In support of this interpretation, these authors note that the Site 1067 and 1068 rocks have similar trace element compositions to metabasalt and diabase clasts, respectively, recovered at Site 899, located ~60 km to the west. The evidence from Site 1067 and 1068 rocks for the existence of incompatible trace element-enriched melts suggests a simple genetic relationship with the more incompatible element-depleted metagabbros at Site 900, because crystal accumulation generally results in cumulate gabbros that are more depleted than the melt from which the crystals precipitate. The large range in trace element and REE abundances in Site 1067 and 1068 amphibolites and metagabbros, as well as the variations in _Nd values of Site 900 cumulate gabbros, indicate that mantle source regions for the original melts were heterogeneous in both trace element and isotopic compositions.

In conclusion, therefore, current evidence points to the gabbros, amphibolites, and tonalite gneisses encountered at Hobby High (Sites 900 and 1067 and the equivalent clasts at Site 1068) having undergone a long history. A late Hercynian (270 Ma) extensional phase was accompanied by the generation of melts from heterogeneous mantle sources, emplacement and crystallization of the melts to form cumulate and noncumulate gabbros in the lower continental crust or at the base of slightly thinned continental crust, followed by ductile shearing at middle to lower continental crustal depths. There is no direct evidence at the Hobby High sites of magmatism that accompanied the last phase of rifting and eventual continental breakup (i.e., 130-140 Ma).