IGNEOUS PETROLOGY

Basaltic rubble was recovered from 10 of 23 holes. The rubble is distinguished by multiple weathered surfaces that were not cut by the drill and semirounded forms. Usually, the rubble can be recognized as pillow basalt based on the criteria outlined above. In many holes rubble was encountered just below the contact between sediment and basalt, providing very poor drilling conditions and requiring that the initial hole be abandoned in 9 out of 13 sites. In those instances a second hole was started within 200 m; in all but one case (Site 1158), this second hole allowed us to satisfactorily complete our drilling objectives. In a few holes rubble was encountered at a deeper level, below intact lava flows, demonstrating that at least some of the rubble deposits formed within the active volcanic zone.

Massive Basalt
Massive basalts, with recovered (and, hence, minimum) thicknesses of as much as 1.5 m were recovered only from Hole 1160B. These can be distinguished by long (up to 50-60 cm), continuous core pieces with uniform texture (Fig. 5). In Hole 1160B each of the three massive basalts is overlain by a pillow flow of similar lithology. In one massive flow, olivine phenocrysts increase in abundance toward the base, suggesting that there has been some crystal settling. No chilled margins were recovered from the massive units. Site 1160 is the easternmost of the Leg 187 drilled sites. It is located at the northern foot of a 1500-m-high seamount in the middle of Zone A. The abundance of massive flows at this site may reflect the generally robust magmatism of the SEIR, or it may reflect increased magmatism related to the seamount itself.

Interpillow Sediments and Basaltic Breccias
Basalts are intimately associated with sediments at 8 of the 13 sites. Sediment may be present as interpillow fill, as fracture fill, and as both small clasts and matrix material in breccias of various types. Interpillow and fracture fills are common in Holes 1155B and 1163A, where micritic limestone is attached to infills fractures in the glassy margins (Fig. 6). The fracture infills extend deep into individual pieces, and they are typically made up of micritic sediment containing sand-sized or smaller glass/palagonite and basalt fragments. Subsequent diagenetic and wall rock reactions, probably related to fluid flow through the fractures, have transformed the simple fracture fill to a composite, partly sedimentary, partly hydrothermal vein fill.

The breccias can be subdivided into (1) volcanic breccias, formed during or soon after eruption, and (2) breccias that formed after extensive basalt alteration. The best examples of volcanic breccia are found in Holes 1159A, 1163A, and 1164A. Hyaloclastite breccia, consisting of angular to subrounded glass/palagonite in a clay matrix, was recovered from Hole 1159A, and hyaloclastite breccia with calcarenitic to calcareous clayey matrix is present in Hole 1163A. Rounded glass fragments included in the carbonate matrix suggest that some of the lava flows sampled in Hole 1159A erupted onto a sedimented surface. Volcanic breccia dominated by aphyric basalt and glass/palagonite clasts was sampled in Hole 1164A. This breccia differs from all other Leg 187 breccias in having only insignificant amounts of nonlithic sedimentary matrix.

Posteruptive, carbonate-cemented breccias are prominent in seven holes (Holes 1155B, 1156A, 1157A, 1161A, 1161B, 1162A, and 1162B). The multistage, posteruptive evolution of these breccias is evident in the common truncation by fracturing of distinct alteration halos around the original clast margins and in the common occurrence of composite sediment-basalt clasts. Breccias of this type range from matrix supported (Fig. 7) to clast supported, with clast sizes varying from several tens of centimeters to a few millimeters. The carbonate-cemented basalt breccia from Hole 1156A has a particularly complex history: much of the clast material has a multistage alteration and fracturing history, and there are two generations of matrix carbonate (Fig. 8) followed by late-stage carbonate veining. The formation of this breccia must have involved deposition and redeposition of talus in a tectonically active setting, accompanied and followed by sediment deposition, lithification, and, ultimately, carbonate veining.

Dolomite-cemented basalt breccias were recovered from Holes 1162A and 1162B. In Hole 1162A, a polymict clast assemblage composed of greenschist facies metagabbro, metadiabase, basalt, and cataclasites is cemented by crystalline dolomite with a subsidiary fine lithic component that imparts variable bright red and green colors to the matrix. All the clasts in Hole 1162B are very highly altered basalts that have been transformed almost entirely to clay under low-temperature conditions.

Petrography of Basalts
Leg 187 basalts range from aphyric (Fig. 9) to moderately phyric (Fig. 10), with small vesicles, up to 1-2 mm in size, in a small proportion of the rocks. Plagioclase and olivine are the typical phenocryst phases, with plagioclase being most abundant. No systematic spatial or temporal variations in the abundances of phenocryst phases were observed. Cr spinel is present in accessory amounts as small, euhedral inclusions in plagioclase and olivine phenocrysts or as discrete subhedral crystals up to 0.5 mm in size. Clinopyroxene phenocrysts and microphenocrysts are present only in Holes 1152B and 1164A. These holes are located in Segments B4 and B5 near the western margin of the depth anomaly. At the spreading axis, these zones are characterized by relatively unevolved lavas with a high degree of compositional variability for a given MgO content. These characteristics suggest an array of primary magma compositions, with minimal mixing and differentiation at crustal levels. The presence of clinopyroxene phenocrysts at these sites, in lavas of relatively high MgO content, may reflect high-pressure fractionation within these more magma-starved segments.

As many as 30% of the phenocrysts are glomerocrysts. These range from centimeter-sized loose clusters of prismatic plagioclase and equant olivine (Fig. 11) to tightly intergrown aggregates. Corroded xenocrysts and xenocrystic aggregates (Fig. 12) are present in several samples from different cores; disequilibrium textures, including discontinuous zoning and sieve-textures (Fig. 13), are common in the larger plagioclase phenocrysts.

Groundmass textures in the pillow basalts are typically microcrystalline, ranging from intersertal to sheaf quench crystal morphologies. Groundmass is usually dominated by acicular to skeletal plagioclase, with equant, often skeletal, olivine forming in subordinate amounts. Clinopyroxene occurs predominantly as plumose quench growths (Fig. 14) or as bundles of bladed crystals, but larger, anhedral to euhedral, granular to prismatic, crystals occur adjacent to and within miarolitic cavities in several holes. The massive flows in Hole 1160B show intergranular to subophitic groundmass textures (Fig. 15).