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Igneous lithostratigraphy, petrology, alteration,
and structural geology

At Site U1369, basaltic fragments were found at the bottom of Holes U1369B and U1369C. The fragments consist of a number of altered cryptocrystalline to glassy pieces in dark pelagic sediment (see “Lithostratigraphy”) in Hole U1369C and one large (3 cm) fragment of altered cryptocrystalline phyric basalt with a large glassy margin in Hole U1369B (Fig. F12). Basaltic fragments recovered from both holes are moderately to highly altered.

Basaltic fragments

Hole U1369B

The basaltic fragments recovered in Section 329-U1369B-3H-CC range from 5 to 35 mm in size and consist of cryptocrystalline to glassy phyric basalt with a large glassy rind. The basalt has a spinifex texture composed of plagioclase with interstitial, partially formed clinopyroxene, Fe-Ti oxide crystals, and glass. Phenocrysts include lath to prismatic plagioclase that range in size from 0.2 to 1.3 mm and minor clinopyroxene (0.2–0.4 mm). Plagioclase phenocrysts make up ~4% of the groundmass, whereas clinopyroxene makes up ~1% of the groundmass. Phenocrysts are partially to nearly completely corroded with a ring of fibrous clay mineral (saponite) propagating perpendicular to the phenocrysts. Vesicles make up 1% of the rock and are filled with saponite and iron oxyhydroxides. The glassy portion of the fragments is 90% fresh with a vitreous, jet-black surface that exhibits conchoidal fracturing.

The basaltic groundmass is moderately altered to highly altered to saponite and iron oxyhydroxides. Alteration is most common around phenocrysts, within the glass, and in interstitial zones between the plagioclase groundmass. It is manifested as several hairline fractures of saponite, iron oxyhydroxide, and quartz that impart curved subhorizontal banding of alteration between the glassy margin and the basalt. The presence of a fresh, curved glassy margin suggests that the fragment at the base of Hole U1369C is the uppermost extrusive lava of the basement.

Holes U1369C and U1369E

The basaltic fragments in interval 329-U1369C-2H-CC, 0–5 cm, and Section 329-U1369E-2H-CC range in size from 10 to 25 mm. They were found intermixed with dark brown pelagic sediment (see “Lithostratigraphy” for detailed description of the sediment) in Section 329-U1369C-2H-CC and with disturbed pebbly sand in Section 329-U1369E-2H-6. Section 329-U1369E-2H-CC contained basaltic fragments within a completely disturbed clay sediment. The basaltic fragments are moderately to highly altered to saponite and iron oxyhydroxides. A number of the fragments in Hole U1369C are flanked by glassy margins that are variably altered to a yellow to pale brown clay mineral, possibly saponite and iron-oxyhydroxides.

The original basaltic groundmass appears to have been cryptocrystalline to glassy and aphyric. Primary mineralogy is spinifex plagioclase with amorphous clinopyroxene and glassy textures. Alteration in the groundmass is patchy but pervasive, with an orange-brown clay mineral (saponite?) partially corroding the plagioclase groundmass and replacing interstitial zones. Detailed petrographic studies will be required to further determine the primary mineralogy at this site. The presence of altered glassy margins and cryptocrystaline to glassy textures within the groundmass suggests that the basaltic chips represent the top of an eruptive basaltic unit (e.g., a pillow lava or sheet flow). However, because they were recovered as flow-in during retrieval of the piston core, their geological relationship to the sediment remains unknown. The surrounding sediment in Hole U1369C does not show any signs of being an alteration product. Sediment in contact with basalt fragments in Hole U1369E is sandy with olive-green patches (clay minerals), which indicates that the olive-green portions may represent completely altered basalt.