Description of the Hole 735B section required logging the features of 866 m of rock from a 1-km section of the lower ocean crust, and integrating those observations with those made previously from the overlying 500-m section drilled during Leg 118. This was more than double the highest previous recovery of any hard-rock leg, and some seven times the amount logged during Leg 147 in similar materials at Hess Deep. To accomplish this, the scientific party divided into specialist igneous, metamorphic, and structural teams, rather than into alternating watches. The teams, in turn, divided core description into specific tasks, with one individual responsible for logging a specific observation. This method proved remarkably efficient, allowing a greater number of observations to be made, a greater amount of data to be recorded, and a greater amount of core to be described than previously achieved.

The greatest value of this approach, however, is a dramatic improvement in consistency of observation. As can be seen from the comparisons made using independent observations on thin sections to check the macroscopic observations made on the core, there is a remarkable consistency of observation, both with respect to precision and with respect to accuracy. For example, three individuals estimated the modal abundance of plagioclase (Maeda), olivine (Le Roux) and pyroxene (Holm) in each of the 457 discrete igneous intervals described, whereas modes were independently determined by point counting on 220 representative thin sections (Meyer). Averaged for the hole, the macroscopic modal analysis gave: 59.5% plagioclase, 30.1% augite, 8.8% olivine, 0.33% orthopyroxene, and 0.76% oxide, point counting gave 58.9% plagioclase, 30.6% augite, 8.2% olivine, 0.62% orthopyroxene, and 0.79% oxide. The difference in orthopyroxene abundance is attributable to the fact that half of it occurs as rims around olivine and cannot be distinguished macroscopically. These consistent observations, in turn, allowed direct comparisons between independently observed features of the core. Oxide abundances, for example, logged on a centimeter scale down the entire core (Naslund), proved to have a remarkable positive correlation with the degree of crystal-plastic (Hirth) and magmatic foliation (Yoshinobu). This observation, as will be seen, is important to understanding the evolution of the lower crust at Hole 735B.

The Hole 735B core between 504 and 1508 mbsf was divided by the igneous team into 457 discrete igneous intervals, numbered from 496 to 952 following the succession from the upper 500 m of Hole 735B (Dick et al., 1991a). These were distinguished on the basis of igneous contacts, variations in grain size, and the relative abundances of primary mineral phases (Fig. 8). Individual contacts were then described and logged (Snow). The major lithologies in Hole 735B were gabbro and olivine gabbro, composing 14.9 and 69.9 vol% of the core, respectively. The distinction between the two is arbitrary, set at 5% olivine following the International Union of Geological Sciences (IUGS) classification, and there is complete gradation between them. The separation, however, allowed distinction of areas of lesser olivine content in the core. Generally, this is an equigranular rock (Fig. 9A, 9D), rarely containing a weak magmatic foliation commonly overprinted by crystal-plastic deformation (Fig. 9D). Most commonly, though, it is varitextured, with irregular coarse, medium, fine, and even pegmatitic patches (Fig. 9E, 9F).

The average grain size of samples varies from fine grained (<1 mm) to pegmatitic (>30 mm), with average grain sizes generally in the range of coarse (5 15 mm) to very coarse (15 30 mm). The relative grain sizes for the major minerals are in the order augite > plagioclase > olivine, but are generally similar. Pegmatitic intervals occur sporadically through the core (Fig. 9E). Peaks in average grain size occur at 510, 635, 825, 940, 1100, 1215, 1300, 1425, and 1480 mbsf, and the grain size data for augite, plagioclase, and olivine all follow similar trends.

Weak modal and grain size layering is present in 22 intervals (12 vol% of the core). The types of layering observed include: (1) grain size layering characterized by either sharp breaks in grain size or gradational variations in grain size (Fig. 10A), (2) modal layering marked by distinct changes in the abundance of plagioclase, olivine, clinopyroxene, and Fe-Ti oxide (Fig. 10B, 10C), (3) magmatic foliation (igneous lamination) defined by the preferred orientation of plagioclase and in some cases olivine and clinopyroxene, and (4) layering defined by textural changes such as layers with crescumulate texture. In several intervals, layer types 1 and 2 occur in rhythmic sequences.

The olivine gabbro is locally crosscut by fine- to medium-grained microgabbros whose contacts range from sharp, with a definable slight finer grained margin, to irregular grading and swirling up through and into the adjoining olivine gabbro (Fig. 9B). These range in composition from primitive troctolites in lithologic Unit IV and Unit XII at the top and bottom of the Leg 176 section, to microgabbro and gabbronorite, though the majority are olivine microgabbros, which are nevertheless similar to the olivine gabbros they frequently cut. The origin of these bodies is speculative, as they could represent channels along which relatively hot primitive melt was fed up into the succession of gabbro intrusions, or they could be protodikes through which the typically primitive magmas of the Southwest Indian Ridge erupted to the seafloor.

Oxide-rich gabbros, including oxide olivine gabbro, make up 7 vol% of the recovered rocks, and gabbronorites and oxide gabbronorites make up approximately 8 vol%. These intervals range considerably in size, but decrease noticeably downward in the section and never occur in the abundance they have in the upper 500 m of the hole. There is nothing like the nearly 100-m-thick polygenetic units of sheared disseminated oxide olivine gabbro and oxide olivine gabbro found in Units III and IV between 170.22 and 274.06 mbsf. The latter require a very large flux of melt that has passed through them to account for the massive precipitation of intercumulus Fe-Ti oxides (Natland et al., 1991; Dick et al., 1991a), whereas the former could easily have crystallized from locally derived iron-rich melts sweated out of the crystallizing olivine gabbros. The oxide-rich gabbros in the lower two-thirds of Hole 735B are found as innumerable undeformed and sheared irregular patches and veins in olivine gabbro (Fig. 9C), and as consistently deformed larger intervals several meters or more thick. A consistent and impressive feature of these rocks is their overall positive correlation with areas of magmatic and crystal-plastic foliation as detailed in the structure section, and with the percent oxide found in the core. This is noteworthy, because most of these rocks are cumulates and do not represent a liquid composition. Moreover, as discussed in detail when they were previously described from the Leg 118 section, the liquids with which they are in equilibrium are far more evolved than any pillow basalt that has been dredged along the Southwest Indian Ridge (Dick et al., 1991a).

Dick et al. (1991a) have proposed that this is because the oxide gabbros represent intrusion of late iron-rich melts that have migrated out of the olivine gabbros and into and up shear zones penetrating or originating in partially molten lower crust (synkinematic igneous differentiation). When such melts migrate up section and down temperature, they must decrease in mass as they migrate. In the case of a late Fe-Ti-rich melt near the end of crystallization of the gabbros when they were sufficiently rigid to support a shear stress (80% 90% crystallinity), these liquids would precipitate abundant ilmenite and magnetite as they cooled. Thus, the greater the fluid flux through any area, the greater the enrichment in these oxides, hence a correlation exists between the degree of strain and the amount of precipitated oxide. The correlation between oxides and deformation throughout Hole 735B can be interpreted as evidence that deformation and the formation of shear zones have controlled the flow and transport of late intercumulus melt from depth to the top of the section.

A wide variety of felsic rocks were described (Niu), constituting 0.5% of the core. The majority are leucodiorite, but diorites, trondhjemite, tonalite, and a very little granite occur. They are largely net veins, and rarely were sufficiently massive (5 cm) to be described as an igneous interval. Although many of these veins are clearly of igneous origin, having primary igneous textures and sharp intrusive contacts, many have experienced subsequent high- and low-temperature alteration and developed diffusive or reactive contacts with the host gabbros. Still others may be hydrothermal or metamorphic in origin.

Seven additional major lithologic units (VI to XII) were identified below the Leg 118 section, based on modal mineralogy and the relative abundance of rock types: VI&151compound;olivine gabbro, which continues from 382 mbsf in the Leg 118 core to 536 mbsf; VII&151gabbronorite;and oxide gabbronorite from 536 to 599 mbsf; VIII&151olivine;gabbro from 599 to 670 mbsf; IX&151gabbronorite;and gabbro from 670 to 714 mbsf; X&151olivine;gabbro and gabbro from 714 to 960 mbsf; XI&151olivine;gabbro from 960 to 1314 mbsf; and XII&151olivine;gabbro and troctolitic gabbro from 1314 mbsf to the bottom of the drilled hole at 1508 mbsf. These are shown in Figure 8. The stratigraphy, at first glance, would seem to resemble that of a large layered intrusion, with the proportion of rocks crystallized from differentiated and evolved liquids increasing upward. However, this is entirely misleading. The complex contains little layering, and none that resembles that characteristic of a layered intrusion. Rather it consists of a series of individual olivine gabbro intrusions, best defined geochemically, which are crosscut repeatedly at higher levels in the crust by oxide-rich gabbros. Thus the lower ocean crust here is differentiated kinematically by intrusion of late melts into the top of the section. In fact, as discussed in the geochemistry section, despite irregularly increasing olivine content, the lower olivine gabbros are less primitive, more titanium and soda rich, than those crosscut by the evolved intrusives higher in the section.

The average phase proportions in Hole 735B troctolites and olivine gabbros closely resemble cotectic proportions observed in low pressure experiments on mid-ocean-ridge basalts, suggesting that the main body of olivine gabbro crystallized at relatively shallow depths (<6 km) and solidified after efficient expulsion of residual melts. The more evolved Fe-Ti oxide-bearing gabbros do not have good experimental cotectic analogs. The strong correlation between deformation and oxide-rich gabbros in the Hole 735B section suggests that: (1) lenses of late-stage magma may have acted as zones of weakness along which deformation was concentrated; (2) late-stage magmas may have been concentrated in zones that had been previously sheared, because these zones have greater high temperature permeability; or (3) active shear zones acted as conduits for melt transport through the section. The high concentrations of oxides present in some samples require that large volumes of melt were transported through these zones.

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