Holes 1162A and 1162B were cored into igneous basement at this site. Hole 1162A penetrated 31.4 m into basement, recovering 2.58 m of rock in four cores (Cores 187-1162A-2R to 5R), a net recovery of 8.22%. Hole 1162B penetrated 58.9 m into basement, recovering 9.94 m of rock in 11 cores (Cores 187-1162B-1W to 11R), a net recovery of 16.88%. The dominant lithology in these two holes is a dolomite-cemented basalt breccia that contains a variety of highly altered basaltic clasts, some of which have undergone greenschist facies metamorphism.
The recovered material from this hole has been assigned to two lithologic units: mixed igneous clasts (Unit 1) and a dolomite-cemented basalt breccia (Unit 2).
This unit consists of the two pieces in Section 187-1162A-2R-1. Piece 1 is a greenschist facies metabasalt. It is part of a complex glassy chilled margin which has red-purple and brown-red subvertical planar bands of coalesced spherulites (Fig. F1) in a dark olive-green matrix. The spherulites increase in size (from <0.4 to ~2 mm) with distance from the outer edge of the piece.
A single, ~4-mm-long plagioclase xenocryst can be seen in thin section (Fig. F2). It has an oval outline, suggesting that it is out of equilibrium with its host, and it contains two large (~0.1-0.6 mm) irregularly shaped melt inclusions. Rare altered olivine microlites (up to 0.2 mm) are also present in areas of coalesced spherulites. The spherulites have been partially to totally replaced by a mixture of chlorite and actinolite, as has much of the groundmass. Epidote (~2%) is present as discrete, irregularly shaped, equant grains (<0.2 mm) in cloudy regions of the groundmass. Minor amounts of chlorite occur in association with quartz replacing mesostasis between spherulites in part of the sample. Based on the presence of crystal pseudomorphs (as long as 0.4 mm) having swallowtail prism shapes, we conclude plagioclase was originally present in the groundmass (~10%), where it usually occurs in the centers of spherulites. Whisker-like quench growth crystals nucleated around the perimeter of the xenocryst discussed above (Fig. F3), testifying to a rapid cooling rate for the protolith. Quartz with undulose extinction is present in veins.
Piece 2 is a brown to orange-brown, very highly altered, moderately plagioclase phyric basalt. Rounded to prismatic plagioclase is the sole phenocryst phase, constituting ~2% of the rock. The groundmass is fine grained and is replaced by brown clay and locally by Fe oxyhydroxide.
Twenty-eight of the 48 pieces in Cores 187-1162A-3R to 5R are dolomite-cemented basalt breccia. The remaining 20 pieces are single clasts, some with adhering matrix. Consequently, all three of the remaining cores from Hole 1162A (Cores 187-1162A-3R to 5R) are included in one lithologic unit: a dolomite-cemented basalt breccia. The three types of clast are basalt (14 pieces), metagabbro and metadiabase (five pieces), and cataclasite (one piece). The polymict breccia is generally red or green and poorly sorted. The clast:matrix ratio is ~80:20 throughout this hole. Unlike breccias from earlier sites, there is little or no Mn oxide coating on the clasts in this breccia, and Mn oxide is rare in the matrix.
Subangular to subrounded clasts of aphyric to highly phyric basalt, cataclasite, metagabbro, metadiabase, and palagonite range in size from <0.5 mm to >3 cm. The relative proportions of the clast types vary throughout the core. For example, Section 187-1162A-4R-1 is dominated by basaltic clasts, whereas Section 187-1162A-5R-2 is dominated by basalt clasts with minor metagabbro and metadiabase clasts. Palagonite is rare, usually present as highly altered (see "Alteration") small (<1 mm) grains. Crystalline epidote clasts (see "Alteration") are small (<1.5 mm) and rare (<3%) (e.g., Section 187-1162A-4R-1 [Piece 3]). Clast size also varies within the recovered section. For example, in Section 187-1162A-3R-1, clasts are bimodal, ranging from <0.5 to 10 mm and 20 to >30 mm, whereas clasts in Section 187-1162A-4R-1 range from <0.5 to 80 mm. All clasts are moderately to very highly altered and are commonly fractured in situ into three or more pieces; the fractures are infilled with matrix, separating the subpieces by ~2 mm (e.g., Section 187-1162A-3R-1 [Piece 1]). There are no alteration halos around clasts, and clasts with different degrees of alteration from different metamorphic facies are intermixed.
The color of these clasts ranges from brownish gray in the fresher pieces (moderately altered) to brown in the very highly altered pieces. Pieces 5-8 in Section 187-1162A-5R-2 are highly plagioclase-olivine phyric basalt, Pieces 8-14 and 16 from Section 187-1162A-5R-1 are aphyric to moderately plagioclase-olivine phyric basalt, and Pieces 1-5 in Section 187-1162A-3R-1 are sparsely plagioclase phyric basalt.
Olivine phenocrysts vary in abundance from 0% to 4% and are totally altered (e.g., Section 187-1162A-5R-1 [Piece 15]; see "Alteration") but appear to have been equant, ranging from 0.5 to 1.5 mm in size.
Plagioclase has a larger range in both abundance and size (0%-20% and 0.5-8 mm). Plagioclase phenocrysts commonly display disequilibrium textures (such as concentric oscillatory zoning and corroded cores) that are partially replaced by clay mixtures (e.g., Section 187-1162A-5R-1 [Piece 15]). In Section 187-1162A-5R-1 (Piece 16), clusters of large (~2 mm) plagioclase phenocrysts have undulose (strain) extinction and have formed new subgrain boundaries across which the original oscillatory zoning continues (Fig. F4).
The groundmass is fine grained (generally <1 mm) and originally contained plagioclase and clinopyroxene in nearly equal proportions along with ~3% opaque minerals; the texture is subophitic. Approximately 25% of the clinopyroxene is variably replaced by clay, prismatic actinolite as long as 1 mm, and some chlorite.
Piece 14 from Section 187-1162A-4R-1 and Pieces 1, 2, 5, 6, and 7 from Section 5R-1 are metagabbro and metadiabase, distinguished from the basalts by their larger grain size and more granular textures (~4 mm in Section 187-1162A-4R-1 [Piece 14] vs. ~1.5 mm in Section 5R-1 [Piece 6]). The color is speckled dark green and cream, reflecting the greenschist facies assemblage of amphibole and plagioclase. The original igneous mineralogy of this clast type was ~49% plagioclase, ~45% clinopyroxene, 5% opaque minerals (mainly ilmenite), and 1% olivine.
As much as 3% of the plagioclase crystals in some pieces of the metadiabase (e.g., Section 187-1162A-5R-1 [Piece 7]) are coarse grained (~3 mm), relative to the medium-grained matrix plagioclase (~1 mm). The large crystals are subhedral laths that have disequilibrium textures, such as disrupted twinning, oscillatory zoning, and corroded cores. There is some replacement by clay along fractures. The remaining plagioclase (~46%) in the matrix is subhedral and tabular to prismatic; some crystals show zoning, radial growth, or undulose extinction. In one metagabbro piece (Section 187-1162A-4R-1 [Piece 14]), plagioclase varies from ~1 to 8 mm in size.
Olivine was present only as relatively large crystals (~3 mm) and has been totally replaced by complex concentric halos of talc and opaque minerals around prismatic cummingtonite? (Fig. F5).
In the metadiabase, clinopyroxene has an average size of ~1.2 mm and partially encloses matrix plagioclase (~1 mm). However, ~85% of the clinopyroxene has been replaced by actinolite and some chlorite (Fig. F6A, F6B).
The cataclasite clast (Section 187-1162A-4R-1 [Piece 13]) is dark green and consists of parallel sets of 1-3 mm wide gray-green shear bands that are oriented at ~30° to the core axis and connected by subvertical ~0.5-mm-wide shear bands to form a conjugate set (see "Structural Geology"). The clasts in the cataclasite are highly altered aphyric basalt ranging from 1 mm to 4 cm in size. The matrix is green-gray, silt-sized mylonite that is possibly altered to actinolite/chlorite.
The matrix consists of ~75% granular to crystalline rhombohedral dolomite (does not react with dilute HCl [~5%-10%] until crushed and heated) and clay- to silt-sized lithic particles of igneous material. Two main matrix colors (red and green) are observed, and these occur together in individual pieces as bands (e.g., throughout Pieces 1A, 1B, and 1C in Section 187-1162A-5R-2). We assume the green color is due to the presence of chlorite and the red color to the presence of Fe-stained clays. In Pieces 1A, 1B, and 1C in Section 187-1162A-5R-2, the largest clasts in the red matrix (1.5-2.0 cm) are smaller than those in the green matrix (2.5-3.5 cm). The clasts in the red matrix are mainly highly altered, brown, fine-grained, aphyric to plagioclase phyric basalt, whereas clasts in the green matrix are metagabbro and metadiabase. In Section 187-1162A-5R-1 (Piece 18), the chloritized green matrix has a linear fabric that is aligned in oblique shear bands ~5 mm wide which appear to partially wrap around clasts.
Irregular cavities (as wide as 10 mm) are lined by distorted, colorless rhombohedra (~0.8 mm) of dolomite, commonly forming saddle-shaped crystals (e.g., Section 187-1162A-5R-2 [Piece 4]).
The recovered material from this hole has been assigned to two lithologic units: dolomite and dolomite-cemented basalt breccia.
This unit consists of Pieces 1-3 from Section 187-1162B-1W-3, which are pink-brown to mottled pink and cream, fine-grained dolomite. The three pieces are increasingly lithified downhole. Piece 1 consists of a gray-pink mottled soft sediment mostly composed of crystals of dolomite <0.2 mm across. The irregular gray blotches are scattered Mn oxide grains associated with Mn oxide-stained halos up to 1.5 cm wide. Piece 3 is coarser grained (~0.2 mm) than Piece 1 and has clasts of highly altered, concentrically banded, yellow palagonite (<1 mm) in areas of coarser grained (~0.3 mm) dolomitic sediment. These coarser grained areas have borders of Mn oxide dendrites that project into the finer-grained sediment.
This unit is a poorly sorted dolomite-cemented basalt breccia, similar to Unit 2 in Hole 1162A. However, there are some differences between the two holes. In Hole 1162B, the clast:matrix ratio varies from 20:80 (e.g., Section 187-1162B-3R-2) to 80:20 (e.g., Section 11R-1) but averages ~50:50. The matrix is lighter in color, being white to buff rather than red and green.
In contrast to Unit 2 of Hole 1162A, all of the breccia clasts in Hole 1162B are basaltic. They are subangular to subrounded and aphyric to highly phyric, and in this sense are similar to the basalt clasts in Hole 1162A. They are, however, more highly altered, the groundmass having been ~95% replaced by clays throughout this hole. Consequently they are brown. Up to 50% of the phenocrysts occur in glomerocrysts (as long as 6 mm) of olivine and plagioclase that consist of numerous (>10) small (<0.5 mm) euhedral crystals (e.g., Section 187-1162B-1W-3 [Pieces 4-7]). Some basalt clasts have chilled margins (e.g., Section 187-1162B-3R-1 [Piece 22]; Fig. F7). Palagonite is also present as a clast and ranges from orange brown near the top of the unit to yellowish green and green toward the bottom, the latter color being an indication that the palagonite has been replaced by clay (see "Alteration"). The palagonite tends to be concentrically zoned (Fig. F8).
In general, basalt clasts are larger (3-70 mm) than the palagonite clasts (<1-17 mm). They also commonly show signs of in situ disaggregation similar to that seen in Hole 1162A (e.g., Section 187-1162B-3R-1 [Piece 22]; Fig. F8). Possible slickensides were identified by the presence of cryptocrystalline silica on the side of a basalt clast in Section 187-1162B-3R-1 (Piece 21). Approximately 30% of all clasts have Mn oxide coatings, which contrast with the breccia in Hole 1162A.
The matrix is white to buff in color and is made up of granular dolomite + clay to silt-sized particles of dolomite and detrital igneous material. Thin (~1 mm) halos of intergrown rhombohedral dolomite outline many of the clasts in the breccia (e.g., Section 187-1162B-3R-1 [Piece 22]; Fig. F7). In addition, the matrix surrounding some basalt clasts below Section 187-1162B-7R-1 (e.g., Piece 1) is stained red, particularly where the red-brown highly altered basalt clasts have complex reaction rims. These zones in the matrix are 1-2 mm wide, with colorless crystalline dolomite next to the clast, followed by a red-stained halo in the matrix. This is observed even for small clasts (~1-2 mm). This process of Fe staining also affects the green palagonite clasts, partially altering them to an orange color. Mn oxide nodules (~0.5-2 mm in diameter) are common. The matrix is dissected by a meshwork of numerous colorless crystalline dolomite veins 0.2-2 mm wide. In Section 187-1162B-3R-1, intense net-veining by clear crystalline dolomite has isolated angular remnants of pink dolomite matrix. In places the matrix reacts more readily with dilute HCl than does the Hole 1162A matrix. This may indicate the matrix contains a significant amount of calcite, although chemical analysis of two samples (Section 187-1162B-5R-1 [Pieces 3 and 24]) by ICP-AES indicates a Ca:Mg ratio appropriate for dolomite; an X-ray diffraction scan of the same samples also displayed dominant dolomite peaks (see "Geochemistry").