In Hole 1162A moderately to highly altered basalt occurs as single fragments in Units 1 and 2 and as basalt clasts within a dolomite-cemented breccia in Unit 2 (see "Igneous Petrology"). Since basalt fragments from Unit 1 have alteration characteristics similar to those of Unit 2, the two are discussed together. Alteration conditions in Hole 1162A range from low-temperature to lower greenschist facies.
Greenschist facies alteration has affected most types of breccia clasts, including both aphyric and sparsely to moderately plagioclase olivine phyric basalt (Sections 187-1162A-3R-1, 5R-1, and 5R-2), microgabbro (Sections 187-1162A-4R-1 and-5R-1), dark green metabasalt (Section 187-1162A-2R-1 [Piece 1]), dark green basaltic cataclasite (Section 187-1162A-4R-1 [Piece 13]), and moderately plagioclase phyric basalt (Section 187-1162A-2R-1 [Piece 2]). Veins are rare, but, where present, they are 0.5-2 mm wide and filled with chlorite, white cryptocrystalline silica (Section 187-1162A-2R-1 [Piece 1]), or colorless crystalline dolomite (Sections 5R-1 and 2), the latter being similar to the matrix of the breccia.
In hand specimen, the aphyric and plagioclase olivine phyric basalts are greenish brown and appear highly altered (40%-60%) throughout (Fig. F9). When viewed with a hand lens, groundmass olivine and clinopyroxene appear to be extensively replaced by Fe oxyhydroxide and clay in most pieces, except for Section 187-1162A-3R-1 (Piece 4), which has a chloritized groundmass. About 40% of the plagioclase phenocrysts in that piece are opaque and cream colored. In some places, plagioclase is discolored (darkened) by Mn oxide linings along crystal faces and microcracks (e.g., Sections 187-1162A-5R-1 and 5R-2); elsewhere it is mostly transparent and relatively fresh. Olivine phenocrysts are totally replaced by Fe oxyhydroxide and/or a pale yellowish mineral, identified in thin section as a mixture of mostly chlorite, talc, clay, and Fe oxyhydroxide (Figs. F10, F11). Plagioclase phenocrysts commonly are clay lined along crosscutting fractures but are otherwise fresh. In thin section, groundmass olivine and clinopyroxene are mostly replaced by chlorite (Fig. F12) and clay. The presence of chlorite indicates that these rocks underwent low-temperature metamorphism at 150°-250°C (Alt et al., 1996), which left the overall textural relationships of the rock intact. Such alteration conditions have not been observed at our earlier sites. The presence of Fe oxyhydroxide in the centers of some olivines that are rimmed by chlorite and talc indicates metamorphic disequilibrium.
Similar observations are made for the dark green to cream, vein-free metadiabase (Fig. F13). In hand specimen the rock appears highly altered, and, except for plagioclase, mineral phases are difficult to identify. In thin section, fibrous actinolite and chlorite mostly replace (20%-38%) the original (45%) clinopyroxene (Fig. F14). Olivine is totally replaced by concentric layers of talc, chlorite, and magnetite with cummingtonite? in the center (Fig. F15). Plagioclase is partially replaced (~20%) by clay + chlorite and recrystallized to albite along crystal margins. Numerous microveins (<25/m) are filled with chlorite and clay. The presence of actinolite + talc + chlorite + albite is consistent with metamorphism in the lower greenschist facies (Alt et al., 1996).
The dark green metabasalt fragment in Section 187-1162A-2R-1 (Piece 1) contains a wide, complex spherulitic quench zone in which the spherulites are partially to completely replaced by actinolite. Actinolite, chlorite, and epidote pervasively replace the groundmass (Fig. F16). Recrystallized quartz veins dissect the groundmass and show strain extinction and preferred orientation of crystallographic axes, indicating temperatures of at least 150°C. Again, the presence of the secondary mineral assemblage actinolite + chlorite + quartz ± epidote indicates lower greenschist facies metamorphism (Alt et al., 1996).
The 10 cm × 5 cm piece of basaltic cataclasite (Fig. F17) is dominated by 1- to 3-mm-wide parallel sets of greenish gray shear zones (see "Structural Geology") oriented 40°-30° from vertical and linked by a network of smaller (<0.5 mm) subvertical shear relay zones. The two sets of shear planes divide the rock into angular domains of undeformed, light, altered aphyric basalt that appear to indicate brittle deformation. These domains are cut by minute veins that are filled with the same material as the larger shear zones. This greenish gray silt-sized matrix probably consists of chlorite and intensely ground basalt clasts.
In Section 187-1162A-5R-1 (Pieces 18 and 20), green chlorite is present as several millimeter-wide layers within the crystalline dolomite matrix. In Piece 18, the chlorite-rich zones appear to reflect small-scale shear zones. The presence of chlorite in these breccia pieces indicates elevated temperatures in the range of 150°-250°C, again suggesting metamorphic disequilibrium.
A brown to orange-brown, very highly altered, moderately plagioclase phyric basalt in Section 187-1162A-2R-1 (Piece 2) is exceptional in alteration mineralization. It is the only basalt fragment in this hole in which the groundmass is mostly replaced by brown clay and Fe oxyhydroxide, indicating low-temperature alteration conditions; this is a distinct contrast to the predominant lower greenschist facies alteration of the other basalt fragments. It may be an intensely weathered greenschist facies basalt.
The poorly sorted, dolomite-cemented breccia has clast:matrix ratios ranging from 80:20 to 90:10. The majority of angular to subangular clasts range from 30 to <1 mm, but occasionally they measure up to 80 mm × 35 mm (Section 187-1162A-4R-1 [Piece 4]). The clasts are predominantly aphyric basalt. Notably, and unlike previous sites, Mn oxide is almost absent from the surfaces of clasts and is rare in the dolomite matrix. Small (<1-4 mm) palagonite clasts are restricted to Section 187-1162A-3R-1. Clasts of epidote are present only in Section 187-1162-4R-1 (Piece 3). Clasts consisting of basaltic cataclasite and plagioclase-olivine phyric basalt, similar to the single pieces described above, are present in Sections 187-1162A-4R-1 (Pieces 4 and 5) and 5R-1 (Piece 17), respectively. A few clasts that appear to be fractured in situ (e.g., Section 187-1162A-3R-1 [Piece 1]) are cut by veins (as wide as 2 mm) that are filled with crystalline dolomite similar to that of the breccia matrix. The aphyric basalt clasts are highly altered, except for a few moderately altered clasts in Section 187-1162A-4R-1 (Pieces 1-6).
No alteration halos are associated with the veins or with the clast margins. This indicates that alteration was pervasive and took place prior to formation of the breccia. Olivine and clinopyroxene in the groundmass of the aphyric basalt clasts are mostly replaced by clay and Fe oxyhydroxide, consistent with low-temperature alteration. The matrix of the basalt breccia consists of crystalline dolomite (Fe stained in places), the mineralogy and chemistry of which are treated in more detail in "Igneous Petrology".
In conclusion, two alteration or metamorphic facies, (1) low temperature and (2) lower greenschist, are recognized in Hole 1162A and do not vary systematically downhole. Because greenschist facies alteration is restricted to individual pieces and a few smaller breccia clasts, whereas low-temperature alteration is pervasive in most breccia clasts, the greenschist facies rocks must have been derived from a source outside the location of Hole 1162A. The secondary mineral assemblage actinolite + chlorite + talc + albite reflects hydrothermal alteration at 250°-350°C (Alt et al., 1996). The setting of Hole 1162A at the margin of a deep (~5500 m), fault-bounded basin next to a comparatively shallow (~4000-3500 m) ridge, together with the presence of cataclasite fragments in the core, suggest that exhumed, lower greenschist facies rocks of the deeper volcanic section were exposed and eroded along the flanks of the ridge. Both greenschist facies and low-temperature altered basaltic clasts formed a breccia that was subsequently cemented by dolomite and then deformed again. The low-temperature rocks would be derived from a shallower part of the volcanic section, either locally or from the upper parts of the ridge.
Unit 1 of Hole 1162B is exclusively pale pinkish dolomite (see "Igneous Petrology"). Unit 2 consists of highly to very highly altered, angular to subangular, basalt clasts in a dolomite-cemented breccia. Because numerous basalt pieces have matrix breccia attached to their margins, basalt and breccia pieces are considered as a single lithologic Unit 2.
Basaltic clasts range from 30 to 70 mm and include both aphyric and slightly to moderately plagioclase-olivine phyric basalts, all of which are highly (40%-80%) to very highly (80%-95%) altered (Fig. F18). Minor fractures and veins are either narrow and Mn oxide lined or as wide as a millimeter and filled with crystalline dolomite, similar to that of the breccia matrix. Alteration halos are rare (e.g., Section 187-1162B-4R-1 [Piece 1]), but the vast majority of basalt fragments show pervasive, low-temperature groundmass alteration of olivine, clinopyroxene, and, in places, plagioclase, to brown clay and Fe oxyhydroxide. Olivine phenocrysts are 100% replaced by Fe oxyhydroxide and clay. In very highly altered sections (such as Section 187-1162B-5R-1), plagioclase phenocrysts are 20%-50% replaced by clay and Fe oxyhydroxide. In less altered sections, plagioclase phenocrysts are relatively fresh. Chilled margins are minor and distributed throughout the hole; within these, fresh glass is rare (e.g., Section 187-1162B-5R-1 [Pieces 17 and 19]). Most of the glass has been replaced by layered palagonite up to a millimeter thick (Fig. F18).
The dolomite-cemented basalt breccia is poorly sorted with a clast:matrix ratio that increases downhole from ~40:60 in Sections 187-1162B-1W-3 through 7R-1 to ~80:20 in Sections 7R-1 through 11R-1, except for Sections 3R-2 and 6R-1 (20:80 and 10:90, respectively). The two principal clast types are <5- to 50-mm aphyric and slightly to moderately plagioclase-olivine phyric basalt and 1- to 15-mm palagonite. In contrast to Hole 1162A, ~30% of the basalt clasts have surface patches of Mn oxide from a few millimeters to a centimeter across. In numerous places, the basalt clasts are dissected by 0.5- to 3-mm-wide veins that are connected with the breccia matrix and filled by crystalline dolomite, similar to that of the breccia matrix (Fig. F19). These relationships suggest in situ fracturing during or after breccia formation. No alteration halos are associated with these veins, suggesting that the pervasive clast alteration predates veining. Throughout the hole, the basalt clasts are highly (40%-80%), often very highly (80%-95%) or even completely, altered. Alteration is manifested by thorough replacement of groundmass olivine, clinopyroxene, mesostasis, and, in places, plagioclase by clay and Fe oxyhydroxide (Fig. F20). Red-brown coloration, softening by clay, and large open fractures that formed after the core was split (as a result of contraction during drying) reflect the high clay abundance in these altered basalt clasts (Fig. F21). Olivine phenocrysts are totally replaced by Fe oxyhydroxide and clay. Plagioclase pheno-crysts are also mostly replaced by clay and Fe oxyhydroxide.
Palagonite clasts are less abundant (10%-20%) and smaller (mostly 1-15 mm) than the basalt clasts. In the 1- to 3-mm range, palagonite dominates over basalt in some sections (e.g., Section 187-1162B-5R-1). Palagonite ranges in color from orange or reddish brown to yellowish green to pale yellow beige. Concentric layering is common; the centers of clasts tend to be reddish and the outer margins yellowish green. The yellowish green palagonite is softer than the reddish brown palagonite, suggesting a higher abundance of clay, which can be interpreted to reflect alteration of palagonite itself (Fig. F8).
The breccia matrix is described in detail in "Igneous Petrology". It consists of two generations of dolomite. The first is clay rich and intensely brecciated and veined by second-generation crystalline dolomite (Fig. F22). Mn oxide spots (0.5-2 mm) appear to be exclusively associated with the first-generation clay-rich dolomite (Fig. F19).
In conclusion, low-temperature alteration is pervasive in Hole 1162B; the most intensely altered basalts recovered during Leg 187 are from this site.