Nine principal lithologies are recognized in Hole 735B: troctolite, troctolitic gabbro, olivine gabbro, olivine microgabbro, gabbro, gabbronorite, Fe-Ti oxide gabbronorite, Fe-Ti oxide gabbro, and Fe-Ti oxide microgabbro (Shipboard Scientific Party, 1999). In addition, small quantities of pyroxenite, anorthositic gabbro, and trondhjemite occur sporadically in the section. Highly deformed mylonites, cataclasites, and amphibole gneisses are also locally present, particularly in the upper part of the hole. By far, the most abundant rock types are olivine gabbro and olivine microgabbro, which together make up ~60% of the core (Fig. F2). Gabbros with less than 5 modal% olivine are the next most voluminous rock type, followed by troctolite and troctolitic gabbro. Although most of the gabbros and olivine gabbros have trace amounts of orthopyroxene, only rarely does this mineral exceed 5 modal%. However, because of its petrologic significance, samples in which orthopyroxene appears as discrete grains have been classified as gabbronorites, even when orthopyroxene is less than 5 modal% (Shipboard Scientific Party, 1999). The gabbronorites tend to be associated with Fe-Ti oxide gabbros and microgabbros, and the three combined make up ~10% of the recovered core.
A total of 952 individual lithologic intervals were recognized in the core on the basis of textural and mineralogical variations (Dick et al., 1991a; Shipboard Scientific Party, 1999). These in turn, are grouped into 12 major lithologic units primarily on the basis of their mineralogy (Fig. F2).
Unit I is a 39.5-m-thick sequence of foliated metagabbro with porphyroclastic to mylonitic textures. The least altered samples consist chiefly of neoblastic plagioclase and clinopyroxene with a few porphyroclasts of feldspar and pyroxene. In most samples the pyroxene has been replaced by greenish brown amphibole, producing banded amphibole gneisses. The protolith of these metamorphic rocks was a two-pyroxene gabbro or gabbronorite based on the common presence of orthopyroxene porphyroclasts. Small amounts of olivine are preserved in some of the freshest intervals.
Although this interval has been intensely deformed and metamorphosed, low-temperature alteration is minor. Some of the olivine is replaced by colorless amphibole and talc, and much of the pyroxene has been replaced by greenish brown amphibole. Amphibole and amphibole + sodic plagioclase veins, up to 1.5 mm wide, commonly cut the foliation at a high angle.
Unit II consists of 140.5 m of olivine gabbro and olivine-bearing gabbro with weak grain size and modal layering. Its upper boundary is marked by a sharp decrease in crystal-plastic deformation and its lower boundary by a relatively rapid increase in the modal proportions of orthopyroxene. A basaltic dike cuts this unit in Core 176-735B-19R, and there are a few thin layers of Fe-Ti oxide gabbro interspersed with the olivine gabbros. Although much less deformed than Unit I, this section contains a number of thin mylonitic and porphyroclastic bands.
Background alteration in Unit II is relatively high, ranging from a low of ~5% to a high of >80% (Stakes et al., 1991). Olivine is partially to completely replaced by Mg-amphibole and talc or by green smectite, and pyroxene is rimmed and partly replaced by greenish brown amphibole. Plagioclase is typically the least altered mineral but is locally replaced by more sodic feldspar or epidote. Alteration is typically most intense in zones of crystal-plastic or brittle deformation.
Unit III is a 44-m-thick sequence of predominantly olivine gabbro with well-preserved igneous textures and a well-developed igneous lamination. A few thin bands of Fe-Ti oxide gabbro are intercalated with the olivine gabbro. The upper boundary is marked by a downward increase in orthopyroxene and Fe-Ti oxides and the lower boundary by a shear zone, across which there is a sharp downward increase in Fe-Ti oxide. The mineralogical composition of Unit III suggests that it is more evolved than the overlying olivine gabbros.
Background alteration in this unit is similar in character to that in the overlying Unit II but is somewhat less intense, largely because the rocks are less deformed. Total alteration rarely exceeds 50%, although it may be higher in narrow intervals.
A unique sequence of Fe-Ti oxide-rich gabbro, ~48 m thick, makes up Unit IV. The rocks are similar in mineralogy to those of Unit III, but the proportion of Fe-Ti oxides is much higher. The gabbro is typically coarse to very coarse grained and commonly has a well-developed subhorizontal magmatic foliation. The abundant Fe-Ti oxides (both magnetite and ilmenite) form intergranular to poikilitic masses enclosing or partly enclosing plagioclase and pyroxene. Overall deformation is weak in this unit, but there are a few thin mylonitic bands in the upper part and the base of the unit is marked by a 2- to 3-m-thick zone of mylonitized oxide gabbro.
Background alteration is also relatively weak in this interval, rarely exceeding 20%, because of both its low olivine content and its weakly deformed nature. Small amounts of secondary sulfide minerals are present in these oxide-rich gabbros, and sodic feldspar and chlorite locally replace plagioclase. Veins are relatively abundant in this interval (locally up to 25% of the core) and are mostly felsic or plagioclase + diopside in composition.
A 131.5-m-thick sequence of relatively homogeneous olivine gabbro makes up Unit V. The upper contact lies at the base of a distinct mylonite zone, and the lower contact is marked by the appearance of pervasive, weak to moderate deformation that characterizes all of Unit VI. Overall, Unit V is marked by an absence of deformation, although some weak grain size layering is visible. Nearly all of the rocks contain 5% or more olivine, and a few are troctolitic in composition. A few bands of Fe-Ti oxide gabbro are present, and one layer of orthopyroxene-bearing microgabbro occurs near the base of the unit.
The olivine in Unit V is partly altered to either colorless amphibole or green smectite, and pyroxene is rimmed and partly replaced by greenish brown amphibole. Most of this unit is relatively fresh with background alteration <20%; however, it may exceed 80% in local shear zones. Likewise, veins are generally sparse, rarely exceeding 5% of the core.
Unit VI is a long sequence of coarse-grained, relatively homogeneous olivine gabbro and troctolite characterized by a weak to moderate deformation. Overall, the rocks of Unit VI are similar to those of Unit V but somewhat more primitive, as indicated by a higher modal abundance of olivine and the common presence of troctolite layers. Thin layers and lenses of olivine microgabbro are locally interleaved with the dominant lithology.
Background alteration in this unit is relatively low except at the base where it can be up to 60%. Again, the most common styles of alteration are colorless amphibole + talc or green smectite after olivine and greenish brown amphibole after clinopyroxene. Sodic feldspar and small amounts of epidote locally replace plagioclase. Veins average ~5% of this interval but can be more abundant locally. Felsic veins are the most common variety, but calcite veins occur below 500 m. Where calcite veins are present, olivine and some pyroxene are typically replaced by red iron oxyhydroxides.
63 m of gabbronorite and oxide gabbronorite, locally crosscut by oxide-bearing rocks, makes up Unit VII. The upper contact is marked by the downward disappearance of the troctolites and olivine-rich gabbro that characterize Unit VI, whereas the base is defined by a marked downward increase in grain size and modal abundance of olivine. Although Unit VII contains relatively evolved rocks like those of Units III and IV, it has only a few oxide layers and bands.
Unit VII is one of the most intensely altered parts of the Hole 735B core, in which the rocks are on average 10% to 40% altered. However, there are many narrow intervals with even higher alteration—up to 90%. Olivine is sparse in this unit and is mostly replaced by Mg-rich amphibole, green smectite, or red iron oxyhydroxides, the latter being particularly common in areas where carbonate veins are abundant. Plagioclase is partly replaced by more sodic feldspar and locally by epidote, whereas pyroxene is typically rimmed with greenish brown amphibole. Some of the orthopyroxene has also been replaced by oxyhydroxides. Local variations in alteration intensity are typically related to vein abundance.
Unit VIII marks a return to relatively coarse grained olivine gabbro. The unit is 71 m thick and has distinct upper and lower contacts, marked by changes in grain size and modal mineralogy. In addition to the dominant olivine gabbro, this unit contains a few intervals of gabbronorite, oxide-rich gabbronorite, and oxide-rich gabbro.
Alteration in Unit VIII is similar to that in Unit VII but generally less intense. Felsic and plagioclase + diopside veins are abundant in a narrow zone at ~640 mbsf, and carbonate veins are also present in small quantities throughout the unit. Olivine is partly replaced by smectite and pyroxene by greenish brown amphibole.
A sharp decrease in grain size and in the modal proportion of olivine and an increase in the amount of gabbronorite distinguish Unit IX from Unit VIII. Except for having somewhat fewer oxide-rich layers, it is similar to Unit VII in lithology and mineralogy. The unit is 44 m thick, and its base is taken as the lowest occurrence of noritic rocks.
In Unit IX, background alteration is generally <20%, although it can be higher in narrow intervals. It chiefly involves replacement of pyroxene by greenish brown amphibole. Plagioclase + amphibole veins are locally abundant, and a few carbonate veins are also present.
The top of Unit X at 714 mbsf is marked by a sharp downward increase in grain size and in the modal abundance of olivine. Unit X consists almost entirely of olivine gabbro and gabbro with very few oxide gabbros. Rhythmic igneous layering is present between 827 and 914 mbsf, and there are both pegmatitic and micrograbbroic intervals in the upper part of the unit. Its lower contact is the top of a 30-m-thick shear zone at 960 mbsf.
A zone with localized bands of intense recrystallization occurs between ~800 and 960 mbsf, where the rocks exhibit extensive high-temperature crystal-plastic deformation. For the most part, alteration involves replacement of olivine by colorless amphibole and pyroxene by greenish brown amphibole. Veins are sparse throughout the unit but a few carbonate veins occur in the upper 100 m.
Like Unit X, this unit consists almost entirely of olivine gabbro but is distinguished from the former by an abundance of thin intervals of oxide gabbro. However, the overall abundance of oxide gabbros is less than in either Unit VII or IX. The sequence is 354 m thick, and its lower boundary at 1314 mbsf is defined by the disappearance of oxide-rich bands. Rhythmic layering occurs in the interval between 1138 and 1220 mbsf, and there are irregular variations in grain size and texture throughout the unit.
Below ~960 mbsf, background alteration decreases markedly, typically to <5% except in rare, discrete intervals. Olivine shows weak alteration, mostly to smectite, and pyroxene is only rimmed with amphibole. Smectite and smectite + carbonate veins are relatively common in the lower 250 m of the unit, but their effects on the adjacent rock are limited.
Unit XII is a monotonous sequence of very uniform, coarse-grained olivine gabbro and troctolitic gabbro with almost no other lithologies—only a few, very thin oxide-rich bands are present. In the upper part of the unit, thin bands of microgabbro intrude and crosscut the coarser-grained host rock. Pegmatitic intervals characterized by very coarse grained olivine are locally present, as are a few patches of leucogabbro.
Most of the rocks in the unit are extremely fresh, commonly with <1% background alteration. Smectite ± carbonate and zeolite prehnite veins occur sparsely throughout the unit.