Serpentinized peridotite breccias recovered at Site 899 correspond to a cemented, high-angular, monomictic dark green-to-brown breccia, composed of ultramafic material and subordinate metabasites, with clasts of different sizes. A major breccia body was recovered through an interval of 95 m (Subunit IVA, Upper Breccia, Shipboard Scientific Party, 1994b) from Cores 149-899B-16R to 25R. Smaller breccia elements are intercalated with sediments, mafic rocks (basaltic lavas, fine-grained, undeformed microgabbros, sheared gabbros and fine-grained chlorite schists), and large blocks (maximum length in core of around 1.5 m) of unbrecciated and fresh peridotite downsection to total depth (Subunit IVB) (Fig. 2).
The ultramafic rocks inside the breccias (boulders, clast and fragments) vary in both composition and degree of alteration, and range from serpentinized peridotites to relatively fresh harzburgite and lherzolite (Cornen et al., this volume). Serpentine clasts are mainly composed of lizardite and magnetite, and display mesh, hourglass and bastite textures (Wicks and Whittaker, 1977). Some clasts containing metamorphic minerals such as tremolite and chlorite (clinochlore) occur locally. Within the fresh peridotite elements, minerals are finely hatched by microfractures (Sample 149-899B-19R-3, 9–14 cm), and serpentine minerals develop mainly in funnels that are zoned and composed of several generations of serpentine formed at different temperatures (Agrinier et al., this volume). None of these peridotites appears to have undergone more than greenschist grade metamorphism.
Fragments and clasts of undeformed microgabbros and basalts associated with sediments were recovered in the lower section of Hole 899B (Subunit IVB, from Core 149-899B-26R to 37R at total depth, Fig. 2). Aside from rare microgabbro clast, these rocks were not found inside the ultramafic breccia of Unit IVA. The basaltic rocks display porphyritic, intergranular or variolitic textures, the latter implying an emplacement onto the seafloor. The microgabbros mostly display equant textures. Though not metamorphosed, most of these rocks are deeply altered (clays and adularia).
According to their mineralogy (Ca, Ti-rich clinopyroxene, kaersutite or Ti-richterite, and biotite in the groundmass) and geochemistry, the variolitic, intergranular basalts and microgabbros have alkaline affinities. The porphyritic basalts recovered (Sample 149-899B-27R-1, 27–30 cm), which exhibit former olivine phenocrysts, Ca-rich plagioclase and titanomagnetite, have tholeiitic composition. Their origin is discussed elsewhere (Cornen et al., this volume). Such close association of different magmas has already been identified in the Galicia Margin (Kornprobst et al., 1988).
Clasts of metabasite up to 10 cm in diameter are included within the serpentinite breccias at Site 899 from Core 149-899B-16R to 25R, and they form a minor component of the breccia (<5% of the clasts). Two main types of clast were found:
Type 1: metabasite clasts with a formerly porphyritic microgranular texture. They are mainly composed of prehnite and chlorite and, locally, pyroxenes, amphiboles and oxide remnants. In these clasts, feldspar has been entirely replaced by prehnite (Samples 149-899B-20R-1, 131–134 cm, 23R-3, 17–20 cm, and 27R-2, 7–11 cm). The compositions of the pyroxenes and the oxide remnants have similarities with those of the equant alkaline gabbros found in Hole 899B (from Core 149-899B-26R downsection). The unusual presence inside poikilitic prehnite of a tight microfracturation is probably related to rapid decompression. No pumpellyite was found in these prehnite-bearing rocks and the P-T conditions of the metamorphism, inferred from the mineralogical assemblages, are in the range of 200° to 400°C with a maximum pressure of 0.22 GPa (Liou et al., 1985).
Type 2: metabasite clasts with an equant association of chlorite and titanite. Boulders of similar but schistose rocks were recovered at the base of Hole 899B. They also locally show the presence of rutile, apatite, zircon, and hydrogarnet. The widespread chlorite with a bright blue interference color is a ripidolite. These rocks are exactly similar to those sampled on the Galicia Margin at the top of the peridotite section. They are interpreted as a Fe-Ti gabbro that was later sheared and metamorphosed under greenschist facies conditions (Schärer et al., 1995).
The fragments (up to 20 cm maximum size) of sheared mafic rocks found at the base of Hole 899B (lower part of Subunit IVB) are strongly foliated amphibolites. They are mainly composed of plagioclases and brown and green amphiboles, replacing clinopyroxene, with accessory green spinels (pleonaste). The assemblage of magnesio-hornblende and chlorite after pargasite (brown amphibole) and spinel is indicative of a retrogressive metamorphism from high amphibolite down to a low amphibolite to greenschist grade. The sheared texture and the mineralogy evoke former flaser-gabbros. Trace-element chemistry shows that these amphibolites are also similar to the flaser gabbros recovered to the east in Hole 900A. The composition of the Site 900 gabbros (slightly differentiated and enriched, in comparison to gabbros with MORB affinities), their degree of metamorphism, and the 136 Ma age of the last deformation event, agree well with a synrift underplated origin (Cornen et al., this volume; Féraud et al., this volume).