The fundamental descriptive component of the rocks recovered during both Legs 118 and 176 is the lithologic interval. This formally is a facies or lithology in the core of consistent mineralogy and bounded by other rock with different mineralogy, different texture, or both. Time restraints on board ship forbade breakdown of the core into lithologic intervals of gabbro smaller than 5 cm thick, although felsic veins smaller than this were readily noticed and separately annotated. The boundaries between intervals are variously igneous or tectonic and gradational or sharp, although some were not recovered. A total of 953 lithologic intervals were identified and described over the entire hole (Dick et al., 1991a; Shipboard Scientific Party, 1999c). The average thickness of an interval is thus ~1.7 m, but because few are very thick, most are much smaller than the average.
Rock names were given to each lithologic interval based on the International Union of Geological Sciences (IUGS) classification (Streckeisen, 1974) with some modifications (Shipboard Scientific Party, 1999a). More than a dozen different rock names were applied to the majority of rocks in Hole 735B. Only two broad groupings of primitive gabbro and differentiated gabbro need concern us, however. The first group consists of rock that contains essential olivine but <0.5% oxide minerals (ilmenite and magnetite). It includes troctolite, troctolitic gabbro, and olivine gabbro. We term this the olivine gabbro suite. These rocks comprise nearly 80% of the recovery, and of these, olivine gabbro is by far the most abundant. There are 504 intervals of this material containing 175 intact interval boundaries of these rocks against themselves; an additional 12 were not recovered but inferred from contrasting lithologies. Assembling these end to end and leaving out all of the more differentiated gabbros, the accumulated thickness of the olivine gabbro suite is 1153 m; thus, the average thickness of rock between contacts of primitive gabbro is ~6.5 m. Technically, this is the average expanded thickness, which apportions total curated thickness by core to the core length, even if core recovery was less (or sometimes more) than 9.5 m apiece (Dick et al., 1991a, 2000). About 94% of the intact contacts are igneous, in the sense that they are irregularly sutured, gradational, graded, or obviously intrusive in character. Only 6% are tectonic, variously being sheared, mylonitic, or sharply planar with deformed rock on one side.
Most of the remaining 766 interval boundaries represent places where the olivine gabbro suite is crosscut by differentiated rocks of the second group, termed the oxide gabbro suite. Many of these rocks lack olivine, and they usually contain fairly large crystals of low-Ca pyroxene. Most of them also contain readily visible amounts of the magmatic oxides, ilmenite and magnetite. They include simple gabbro, gabbronorite, oxide-bearing gabbro, and oxide-rich gabbro. The oxide-bearing gabbros contain 0.5%-1% of the oxide minerals and are termed disseminated-oxide gabbros in the site report. They are usually physically associated with the oxide-rich gabbros, but the latter stand out visually because they abruptly have much more abundant ilmenite and magnetite, in many cases comprising 5%-10% and, at the extreme, >30% of the rock. Most seams of oxide gabbro have sharp, usually planar contacts with adjacent rock, whether it is olivine gabbro or another oxide gabbro. They thus appear to be generally intrusive in character, although some sharp contacts represent narrow deformation surfaces. Very few have diffuse boundaries; thus, the melts from which the oxide minerals were not emplaced in a preexisting, spongy, porous matrix of more primitive minerals. Each lithologic interval was encountered sequentially and numbered downward beginning with "one" at the top of the hole. Over the course of the two legs, coherent groupings of these intervals, each having some predominant set of characteristics, allowed the combining of them into lithologic units, numbered again downward from I to XII (Fig. F3). The lithologic units include some dominated by intervals of olivine gabbro and troctolite and others by oxide-bearing + oxide-rich gabbro. Lithologic Units III and VI, made up of alternations of both, were termed "compound olivine gabbro." Lithologic Unit I at the top of the hole was based not on lithology but on an average high extent of crystal-plastic deformation.
There is therefore little genetic connotation to either the intervals or the lithologic units, largely because they were intended to be simply descriptive but also in part because they were identified one at a time, from the top of the hole downward as coring proceeded, with no knowledge of what would lie underneath. This circumstance of the logistics of receiving core on deck extended to every observation made on the core. Therefore, however thorough the descriptions might be, attempts to assemble a unified picture were consistently colored by the piece-wise way in which the cores were described. Furthermore, outside of contacts that clearly demonstrate intrusive relationships on the scale of the diameter of the core, there was no way to establish sequence. Thus, although the cores have a stratigraphy, they have almost no definable succession.
The term interval is not one usually encountered in stratigraphic descriptions of plutonic rocks, especially those of layered intrusions, and the term unit is generally used in a different way (Irvine, 1982). An interval here has no more meaning than the rock name given to it and the fact that it occupies a precise portion of the core. The use of the word unit as described above is partly an accident of history, the borrowing of a term normally applied to sediments in time-stratigraphic sequence and of a particular lithology. In sediments, this is partly to avoid the use of the word formation when considering a core only 6 cm in diameter. However, use of the word for lithologic units means that another term must be found if and when we need something comparable to its meaning in layered intrusions.
Intervals of oxide-bearing and oxide-rich gabbro are usually quite narrow, and many are less than the minimum 5-cm thickness used to define an interval. Based on routine shipboard measurement of magnetic susceptibility, which is strongly sensitive to the presence of magmatic magnetite (and with it magmatic ilmenite) in the rocks during Leg 176, there are 476 seams of oxide gabbro in the lower 1004 m of the core (Natland, Chap. 11, this volume). Only 192 of these were thick enough to be defined as intervals. This is responsible for one of the principal problems of interpreting the chemical data, namely that some samples chosen for analysis inadvertently combine two or more very different gabbro facies. Hart et al. (1999) attempted to get around this by deliberately obtaining narrow strip samples from 1.13 to 4.15 m long in an effort to secure representative analyses combining multiple lithologies along putatively representative portions of the core obtained during Leg 118. Some problems with this approach are considered in "Appendix A."
Many seams detected by magnetic susceptibility also are thinner than the 4-cm spacing of the measurements, but assuming that this is about the resolution of the technique, then the average thickness of oxide seams measured during Leg 176 is 13.4 cm with a large standard deviation. The thickest oxide-rich seam revealed in this way is only 1.7 m thick. The seams tend to be clustered, and from 700 to 900 mbsf, they make up the majority of the rock. Even so, each is still clearly discrete in this part of the core and is bounded by primitive gabbro above and below.
In the upper 504 m of the core described during Leg 176, only the lithologic observations of Dick et al. (1991a) record the presence of oxide-rich seams at this scale. Most are isolated, but a large concentration of oxide-rich seams is present between 223 and 274 mbsf. The sequence is sufficiently thick and distinctive enough to have been accorded the status of a lithologic unit (Unit IV) during Leg 118 (Fig. F3). It contains 32 intervals, mostly disseminated-oxide, oxide-olivine gabbro, and oxide gabbro, within which there are 4 intervals of olivine gabbro totaling 1.35 m, the largest of which is 0.72 m thick (Dick et al., 1991a). These are evidently screens in the larger body of oxide gabbro. The remaining 28 intervals of oxide gabbro average 1.76 m thick—far thicker than the average thickness of seams measured by magnetic susceptibility during Leg 176—and most are in contact with each other. Most of the contacts are igneous in character but a few are undeformed, and there are three cycles, each ~10 m thick, in which the rocks change downward from undeformed through gneissic then porphyroclastic and finally to mylonitic degrees of deformation. In the lower 20 m of Unit IV, 6 of 10 contacts are foliated and are judged to be tectonic in origin; the rest are foliated igneous contacts (Dick et al., 1991a). The basal contact is with a nearly homogeneous body of olivine gabbro >100 m thick. The contact is an intrusion breccia of the olivine gabbro riddled with anastomosing felsic veins that lend the bulk rock an average composition of diorite (Hertogen et al., Chap. 6, this volume).
This one large mass represents the acme in the core of aggregation of seams of oxide gabbro. There are dozens of narrow seams of similar material above it and fairly strong, albeit fluctuating, concentrations of oxide gabbro in the core beneath. In general, however, the proportion of oxide-bearing and oxide-rich seams diminishes with depth away from the massive oxide gabbro; below 1000 mbsf they are only 3.6% of the core (Dick et al., 2000). In any case, there is no immediate relationship between the seams or the massive lithologic Unit IV and the compositions of host olivine gabbros. They were not generated by differentiation in situ, rather they came in from somewhere else (i.e., deeper in the cored section, below even that, or from the side) and are thus, at least in the broad sense, intrusive rocks.
In the core described during Leg 176, many of the contacts between narrow seams of oxide gabbro and their more primitive hosts and with each other are tectonic, in the sense that they are sheared, mylonitic, or sharply planar with more deformed rock on one side. Orientations of shear fabric and planar surfaces are usually conformal to the larger-scale deformation fabrics in adjacent cored rock. Shipboard scientists noted a strong association of the oxide-bearing and oxide-rich gabbros with zones of strong crystal-plastic deformation in the core, although over an entire interval or series of adjacent intervals the most oxide-rich gabbros are not always the most strongly deformed.
This tendency of the oxide gabbros to be rather strongly deformed is not easy to quantify, especially since many olivine gabbros are as strongly deformed. However, olivine gabbros are much more abundant than oxide gabbros through the entire core, and most of them are either undeformed or only very weakly deformed. The proportion of strongly deformed primitive gabbro is thus considerably less than in differentiated gabbro. Over the lower 1000 m of the core described during Leg 176, the smaller proportion of oxide gabbro is about 15 times as likely as olivine gabbro to exhibit gneissic, porphyroclastic, or mylonitic textures (Natland, Chap. 11, this volume). Deformation is often strongest at interval boundaries, whether or not the oxide minerals are most concentrated at those boundaries.
A general conclusion of the scientists of both Legs 118 and 176 is that crystal-plastic deformation occurred at very high temperatures, at the minimum under granulite-facies metamorphic conditions, but very likely also at magmatic temperatures, as the resulting fabrics themselves are crosscut by granitic veins. In this light and given that the oxide gabbros are all in the broad sense intrusive rocks, were the deformed but oftimes very sharp "tectonic" contacts between oxide gabbros and more primitive hosts originally igneous in origin? The question is partly semantic. The modifier "igneous" should mean that at least some melt was present on one side or the other of a contact, however the present gabbro cumulates on either side may have formed. The only thing it precludes is the case where solids formally are juxtaposed across sharp boundaries, presumably in the course of subsolidus crystal-plastic deformation and plastic flow. Thus, among the rocks of Hole 735B, crystal-plastic deformation continued to act while melt was present, modifying many contacts between primitive and differentiated gabbros after and however they formed.
One rock type rarely included in the sequence of lithologic intervals is granitic veinlets. During Leg 176, some 203 of these were sufficiently prominent in the lower 1000 m of the core to be tabulated as "felsic veins" in the site report. Because they consist mainly of quartz and sodic plagioclase, they stand out starkly against their gray or dark gray host gabbros. Few of these are >1 cm thick, and many are sharply bounded, with straight edges standing at highly oblique angles to the core. From estimates of their volumes, they comprise about 0.5% of the material recovered during Leg 176.
Based on modes and chemical analyses (Shipboard Scientific Party, 1999c; Hertogen et al., Chap. 6, this volume; Niu et al., Chap. 8, this volume), the purest of these veins are trondhjemite and tonalite; one is granodiorite. Usually, however, some amount of matrix gabbro is either caught up in the veins themselves or was not completely removed from the samples selected for chemical analysis. The compositions of the resulting hybrids thus are dioritic. Also, there are many places in the core where such material is rather finely dispersed, either in sworls or vein breccias (Natland, Chap. 11, this volume). These instances were not included in estimates of vein volume, and not all are noted on the core barrel sheets in the site report.
Felsic veins crosscut all lithologies, but they are more decidedly associated with oxide gabbros than olivine gabbros. Even felsic veins in many olivine gabbros are immediately associated with narrow seams of oxide gabbro, sometimes splitting them down the middle (Natland, Chap. 11, this volume). Along the core, felsic veins are present at a rate of approximately one per meter in oxide gabbro, as opposed to one per 5 m in olivine gabbro. Almost certainly this means that rather than being anatectectites, most felsic veins represent an end product of magmatic differentiation following crystallization of oxide minerals, as demonstrated experimentally for abyssal-tholeiite liquids (Dixon and Rutherford, 1979; Juster et al., 1989); thus, they are preferentially associated with oxide gabbros, which are nearest to them in stage of differentiation. From these locations, some of them branch into olivine gabbros.
The likely sequential cumulates of such extended magmatic differentiation, namely troctolite, olivine gabbro, gabbro, gabbronorite, disseminated-oxide gabbro, and oxide-rich gabbro, are all present in the core, and most, indeed, are represented in any given 50-m portion of the core. However, the complexly crosscutting relationships prevent tracing of the process of differentiation through any simple sequence or stratigraphic succession of these rocks in any part of the core, in the manner, for example, of the stratigraphy of upwardly more differentiated layered intrusions. Among the rocks of Hole 735B, no simple pattern of cryptic mineralogical variation exists (e.g., Ozawa et al., 1991; Dick et al., Chap. 10, this volume). The most striking and perhaps most fundamental feature of the core is that all of the later differentiates are simply not in place in the following sense: they do not encase a "sandwich horizon" including granophyres as they do at the Skaergaard Intrusion (Wager and Deer, 1939; Wager and Brown, 1967); indeed, they are not concentrated anywhere but are present at all levels in the core, invariably in small, sharply bounded bodies. Consequently, they attained their stages of differentiation elsewhere and were interleaved by some combination of intrusion and deformation in a matrix of primitive gabbros (Natland and Dick, 2001). They are examples of what Bowen (1920) termed "discontinuous differentiation" in a paper where he first outlined the general consequences of differentiation in masses of deforming, partially molten, igneous rock. Discontinuous differentiation is but one manifestation of what Bowen termed "differentiation by deformation." Much later, Dick et al. (1991a) used the term "synkinematic differentiation" to describe the same thing. Although differentiation by deformation has not been considered important in the crystallization of layered igneous intrusions, in the gabbros of Hole 735B it was a major process (Natland and Dick, 2001).
Another contrast with layered intrusions is that <2% of the total section of the core from Hole 735B has modal or graded layering of the type that is produced either by mechanical sorting of minerals separating from moving magmas or from compositional changes in magma during deposition of the minerals. The longest sequence of such material is only 9 m, within which there are 34 normally graded layers of olivine gabbro, each with plagioclase proportions and grain sizes increasing downward and ranging from 6 to 22 cm in thickness. The cause of this layering has not been determined. To some extent, this goes along with the small size of intervals of primitive gabbro, many of which are only a few tens of centimeters to a few meters thick. If these represent the typical dimensions of bodies of magma individually added to the lower crust, then they were never large enough to generate or sustain convective currents of magma within which minerals of different density might have separated.