Site 900 is located 75 km east of Site 897 where the basement of the OCT is made of mantle rocks, and 45 km west of Site 901 located on a large tilted block of the margin presumably made of continental crust (Shipboard Scientific Party, 1993, fig. 1). At Site 900, a total of 56.1 m of basement was drilled and 27.7 m of material was recovered. The basement consists mainly of fine-grained gabbros which are locally highly fractured and brecciated. A detailed description of the cores is given by Sawyer, Whitmarsh, Klaus, et al. (1994) and Cornen et al. (chapter 26, this volume). Only the main characteristics of the series are given here.
The gabbros appear to be mainly homogeneous fine-grained rocks, grading downcore from greenish-white to a grayish-green and brownish color. They locally display a clear foliation marked by alternating mafic and felsic bands (e.g., Section 83R-2). In such intervals, the rocks show a typical structure of flaser gabbros with porphy-roclasts rotated and aligned along the foliation plane. The development of C-S features attests to shear deformation of the rocks. Although less obvious, the same foliation characterizes the finegrained dark intervals, in bands less than 1 mm thick. The coarse-banded intervals are distributed over the whole thickness of the drilled section. The transition between the fine-grained dark facies and the coarser banded facies is often overprinted by the late fracturing. In some sections however, this transition is clearly primary, as the coarser banded intervals grade rapidly over a few millimeters into finer-banded zones (e.g., Sample 149-900A-82R-1, 70-85 cm).
The two samples selected for preliminary dating are representative of these two main facies. Sample 149-900A-83R-2, 77-82 cm (labeled here 83R77-82) is located in the thickest coarse-banded interval recovered in the cores (interval 149-900A-83R-2, 45-110 cm). Sample 149-900A-85R-5, 18-23 cm (labeled here 85R18-23), recovered 16m below, is representative of the fine-grained facies.
Throughout the whole drilled series, the primary mineralogy is composed of clinopyroxene, plagioclase, and a few oxides as accessories. A secondary paragenesis developed as amphibole, chlorite, epidote, and locally, sericite.
In thin section, the texture of the gabbros is granuloblastic to porphyroclastic. In sample 83R77-82, 5% to 10% of porphyroclasts of both pyroxene and plagioclase are embedded in a groundmass of small-sized equant neoblasts (0.2 mm) of the same minerals (Fig. 2). The foliation is well-marked by alternating beds (a few mm thick) of pyroxene and grain boundary plagioclase neoblasts, which exhibit frequent triple junctions. In the plagioclase bands, this mosaic texture is locally underlined by thin rims of chlorite at grain boundaries, and sericite patches are observed in or at the periphery of plagioclase crystals. The proportion of sericite is low (estimated at less than 1% in volume) and plagioclase appears dominantly fresh. Both plagioclase porphyroclasts (which are elongated in the foliation), and neo-blasts display mechanical twins and undulating extinctions. Green spinel, approximately 100 to 150 µm in size, occurs in places in the plagioclase neoblasts. Pyroxene porphyroclasts are also preserved in pyroxene neoblast lenses and bands. They are up to 6 mm in size and display thin exsolution lamellae which are locally bent. Pyroxene is partially retrometamorphosed to fibrous amphibole, which locally forms continuous bands in the foliation.
The texture of sample 85R18-23 is granuloblastic. Except for the absence of porphyroclasts, the texture and primary mineralogy of this sample is comparable to that of sample 83R77-82. The same alternation of plagioclase and pyroxene neoblast bands defines the foliation. Although the bands are thinner than in the previous sample (1-2 mm), the size of the neoblasts is comparable (0.2 mm). Apparently, no spinel is preserved. Secondary mineralogy includes chlorite-rimming plagioclase neoblasts, amphibole as a replacement for clinopyroxene (although to a lesser extent than in sample 83R77-82), and actinolite and epidote as fracture fillings. Secondary mineralogy differs from the previous sample by the absence of sericite and by the noticeable occurrence of albitic framework inside plagioclase neoblasts.
Such textures clearly result from dynamic recrystallization of the gabbros during intense shear deformation. Experimental data on silicates (olivine, quartz and pyroxene) show that during plastic deformation under steady state conditions, the grain size produced by dynamic recrystallization depends on the applied deviatoric stress (e.g., Mercier et al., 1977). No experimental data are available for plagioclase. However, the similar size of the neoblasts in the two samples strongly suggests that the variations in the thickness of bands do not result from varying degrees of deformation, but rather represent an initial heterogeneity of the gabbros. Textural relationships show also that the retrometamorphism to greenschist facies conditions is static and clearly occurs after the dynamic recrystallization of the rock.
Phase compositions have been obtained through a CAMEBAX (SX50) microprobe (Microprobe Ouest, Brest, analytical details are given by Cornen et al., chapter 26, this volume). Selected data are listed in Table 1.
Pyroxene porphyroclasts and neoblasts are all diopside that displays a slight evolution toward augite (Wo48.8En42.9Fs8.3 to Wo35.1En48.6Fs16.3). Their crystallization under higher pressures than pyroxenes from gabbros recovered in presumably comparable settings, that is, slow-spreading ridges (Helmstaedt and Allen, 1977; Honnorez et al. 1984; Bonatti and Seyler, 1987; Bloomer et al., 1989), has been proposed by Cornen et al. (chapter 26, this volume) on the basis of their noticeably higher content of CaO, A12O3 and Na2O. The discrete presence of aluminous spinel with low Cr content (pleonaste) inside plagioclase neoblasts would have the same significance.
In sample 83R77-82, feldspars range from Or0.4Ab35.6An64 in the porphyroclast core to Or0.9Ab42.5An56.5 in the neoblasts without a compositional gap. The average Or content of 0.8% (values range from 0.3% to 1.2%) corresponds to a K2O content between 0.058 wt% and 0.219 wt%. A slight difference appears between the neoblasts and the porphyroclast core which is more anorthitic and less potassic. In sample 85R18-23, which is devoid of porphyroclasts, neoblasts have a composition close to that of the previous sample: Or0.5 Ab29.4An70-Or0.5 Ab47.4An53 with an Or content centered on 0.8% (0.4% to 1.2%).
The major difference between the two samples is the existence in the fine-grained sample (85R18-23) of neoblasts with an albitic framework with a composition centered on Or0.4Ab88.2An11.4. In these albitic zones the K2O content is lower, with a range of 0.02% to 0.06% K2O by weight. In the coarse-banded sample (83R77-82), patches of sericite flakes (muscovite) occur, and albite is not apparent. Aside from the high K2O content of sericite (between 8% and 10% by weight; Table 1), microprobe analyses show the existence of Ca and Mg, which is most likely due to contamination by the surrounding plagioclase and chlorite, probably because of the small size of the sericite flakes.
This secondary mineralogy (which includes actinolite to actinolitic hornblende replacing pyroxenes, chlorite, epidote, albite and sericite) is typically that of greenschist grade metamorphism which overprints and postdates the main shearing event.