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doi:10.2204/iodp.proc.330.106.2012

Igneous petrology and volcanology

Two short holes were drilled on Achernar Guyot. Hole U1375A was drilled to a total depth of 11.5 mbsf and recovered 1.5 m of sedimentary rocks containing five types of volcanic clasts. The clast types found in Subunits IIA and IIB include aphyric basalt, moderately olivine-augite-phyric basalt, moderately augite-olivine-plagioclase-phyric basalt, and highly olivine-augite-phyric basalt. Hole U1375B was drilled to a total depth of 8.5 mbsf, recovering 57 cm of igneous rock. Unit I, the only unit to be defined for Hole U1375B, is moderately olivine-augite-phyric microgabbro (dolerite) with olivine and augite phenocrysts smaller than 10 mm.

Hole U1375A

Basaltic clasts in sedimentary Subunits IIA and IIB

The larger basaltic clasts within the conglomerate and breccia of stratigraphic Subunits IIA and IIB were divided into five types on the basis of their appearance in hand specimen. These clast types are described below, and the sedimentary subunit(s) in which they were found is noted (see “Sedimentology”):

• Type 1 (Subunits IIA and IIB): aphyric basalt that is mottled gray-brown with no phenocrysts. The groundmass is fine grained and moderately altered, with no vesicles. Brown alteration halos are associated with carbonate-filled veins. Four Type 1 clasts were found in Subunit IIA and one was found in Subunit IIB.

• Type 2 (Subunit IIA): moderately olivine-augite-phyric basalt that is medium gray-orange, with 5% olivine phenocrysts (completely altered to iddingsite; maximum size = 4 mm, modal size = 2 mm) and 3% augite phenocrysts (unaltered; maximum size = 5 mm, modal size = 2.5 mm). The groundmass is fine grained and moderately altered, with 0%–5% vesicles (moderate sphericity, rounded) and carbonate-filled veins. Two Type 2 clasts were found.

• Type 3 (Subunit IIA): aphyric basalt that is mottled brown with no phenocrysts. The groundmass is fine grained and moderately to highly altered, with 10% vesicles (high sphericity, rounded) and veins filled with brown clay and carbonate. One Type 3 clast was found.

• Type 4 (Subunit IIA): moderately augite-olivine-plagioclase-phyric basalt that is mottled gray–cream brown, with 3% olivine phenocrysts (altered to iddingsite and carbonate; maximum size = 3 mm, modal size = 1 mm), 5% augite phenocrysts (unaltered; maximum size = 4 mm, modal size = 2 mm), and 3% plagioclase (altered; maximum size = 2 mm, modal size = 1 mm). The groundmass is fine grained and highly altered, with 1% vesicles (high sphericity, rounded) and veins filled with brown clay. One Type 4 clast was found.

• Type 5 (Subunit IIA): highly olivine-augite-phyric basalt that is medium gray-orange, with 25% olivine phenocrysts (altered to iddingsite and carbonate; maximum size = 5.5 mm, modal size = 2 mm) and 5% augite phenocrysts (unaltered; maximum size = 6 mm, modal size = 3 mm). The groundmass is fine grained and highly altered, with 0%–3% vesicles (moderate sphericity, rounded) and carbonate-filled veins. One Type 5 clast was found.

Hole U1375B

Lithologic and stratigraphic igneous units

Unit I

• Interval: 330-U1375B-1R-1, 0–57 cm

• Depth: 0–0.57 mbsf

• Lithology: moderately olivine-augite-phyric microgabbro (dolerite)

• Lithologic unit: 1

Unit I is 57 cm thick and composed of moderately olivine-augite-phyric microgabbro (dolerite) distinguished from basalt by its groundmass grain size of >1 mm (Samples 330-U1375B-1R-1, 1–3 cm [Thin Section 227], and 1R-1, 56–57 cm [Thin Section 228]) (Fig. F7). This holocrystalline, moderately porphyritic rock was the only medium-grained igneous rock recovered during Expedition 330. It contains 2% altered olivine phenocrysts (up to 10 mm) and 0.8% fresh augite phenocrysts (up to 15 mm). The groundmass is composed of tabular plagioclase crystals, olivine, titanaugite, and accessory apatite (Fig. F7C, F7D).

Interpretation of the igneous succession

The microgabbro in Hole U1375B is the only igneous rock drilled at Site U1375 that might be in situ. However, because neither contact was recovered, it is not possible to determine whether the drilled interval is from a large clast in a sedimentary deposit or part of a basement intrusion exposed at the surface by erosion.

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