10 or alkali syenites or granites if the plagioclase is albitic (anorthite < 10). Since the proportion of quartz to feldspar is difficult to determine in hand sample and the feldspar composition is unknown, we use the more general term granophyre.
Four holes were drilled at Site 1275, separated by a total lateral distance of 91 m. Holes 1275A and 1275C provided very little recovered material. Hole 1275A was drilled to a depth of 5 m, and a curated length of 0.13 m was recovered consisting of fine-grained mafic rock (diabase and limestone-cemented diabasic breccia). Hole 1275C was drilled to a depth of 20.8 m, and a curated length of 0.22 m was recovered consisting of diabase and troctolite. Holes 1275B and 1275D yielded substantial amounts of gabbroic rock and lesser amounts of other lithologies (Hole 1275B: troctolite [5.5%], gabbro [85.1%], diabase [8.6%], and granophyre [0.8%] and Hole 1275D: troctolite [18.3%], olivine gabbro [4.7%], gabbro [64.4%], diabase [10.5%], and granophyre [2.1%]). Therefore, in the following description of Site 1275 we focus on Holes 1275B and 1275D. We first define the stratigraphic variations and hand-sample characteristics of the lithologies in each hole. We then describe the rocks in detail, coupling hand-sample and thin section observations. Because many of the rocks from Holes 1275B and 1275D are similar, they are described together.
Unit I includes the first five cores of the hole and a small part of the sixth, extending to 18 mbsf (Section 209-1275B-3R-2). It contains diabase (48.3%), gabbro and oxide gabbro (51.3%), and granophyre (0.4%) (Fig. F4). We use the term granophyre to refer to rocks that are composed mainly of plagioclase feldspar and quartz. According to formal igneous nomenclature (Streckeisen, 1974), these would either be trondjhemites if the plagioclase has anorthite 10 or alkali syenites or granites if the plagioclase is albitic (anorthite < 10). Since the proportion of quartz to feldspar is difficult to determine in hand sample and the feldspar composition is unknown, we use the more general term granophyre.
The remainder of Unit I is a mixture of variably altered microgabbro to fine-grained gabbro. The grain size of the gabbroic rocks varies on the millimeter scale. The different grain sizes define bands, lenses, and irregular blobs with sharp boundaries (Fig. F5). These rocks have a variable but, in many places, well-developed igneous foliation. Where sharp boundaries in grain size cut the core it can be seen that the igneous foliation parallels these boundaries. No reaction rims or chilling are evident at any of these boundaries. The granophyre in Unit I occurs exclusively as small irregular bodies that crosscut the gabbroic rocks of Section 209-1275B-5R-2.
Unit II is a ~4.5-m-thick horizon composed of "troctolite" (96.4%) cut by a small number of diabase dikes (3.6%). The troctolite is medium grained with equant, rounded olivine surrounded by a gabbroic matrix (Fig. F6). The gabbroic material is heterogeneous and is composed of poikilitic pyroxenes as large as 1.5 cm (both clinopyroxene and orthopyroxene) and interstitial plagioclase, so rock types in this unit vary from plagioclase dunite and troctolite to plagioclase-harzburgite, plagioclase-wehrlite, and plagioclase-lherzolite at the thin section scale. For simplicity, we refer to all these olivine-rich, texturally similar rocks as "troctolites" throughout this report. Olivine makes up >75% of the troctolite and typically accounts for >85%. The troctolites are cut by gabbroic dikes and in many places these dikes appear to be continuous with gabbroic material interstitial to the olivine (Fig. F6).
Unit III is the last unit in Hole 1275B and is composed primarily of oxide gabbro with some oxide gabbronorite in the lower portions. Small intervals of gabbro and gabbronorite with <5% oxides can also be found. Gabbro is the dominant lithology (96.4%), cut by diabase (2.7%) and granophyre (0.9%).
Unit III makes up the bulk of Hole 1275B (76.1 m) and is extremely heterogeneous—the greatest extent of homogeneous material is <50 cm. Heterogeneity is defined by variations in mineralogy and modal proportions. Thus, this unit contains gabbro, gabbronorite, oxide gabbro, and oxide gabbronorite. Heterogeneity is also defined by variations in grain size (Figs. F7, F8) that may or may not be accompanied by changes in mineralogy and/or modal proportions. The style of the grain-size variation can be grouped into three loosely defined categories:
The diabase in Unit III has sharp boundaries with the gabbroic rocks and is distributed throughout the unit with no particularly large concentrations. Grain sizes in the gabbros of Unit III increase downhole (Fig. F10).
Granophyre also occurs throughout Unit III, mostly as thin (2–3 mm) dikelets that compose <4% of the volume of any given section (Fig. F11A). About half of the granophyres and all the larger masses of granophyre are oriented subparallel to the drilling direction (Fig. F11B); more quantitatively, 49% of measured dips of granophyre/gabbro contacts have dips steeper than 60°. Because a vertical drill hole preferentially intersects subhorizontal features rather than near-vertical features, we infer that the near-vertical granophyres are, in fact, much more common than those with dips <60°. For more discussion, see "Structural Geology." Section 209-1275B-18R-1 contains 18% granophyre, and in this section the granophyre includes pieces of the gabbro. The granophyre has reacted with the inclusions, generating large crystals of amphibole in the former gabbro (Fig. F12).
Unit I includes the first ten cores of the hole and most of the eleventh (Fig. F13). Unit I is composed almost entirely of troctolite (85.4% of Unit I) with the exception of the very top of the hole in Section 209-1275D-1R-1, which contains diabase (5.5% of Unit I), and Core 7R, which contains a significant amount of gabbro (9.1% of Unit I). The troctolite is similar to that described in Hole 1275B, with rounded olivine crystals in a gabbroic matrix. As in Hole 1275B, the troctolite is cut by numerous gabbroic dikes and veins that vary in geometry and volume. In interval 209-1275D-9R-2, 27–40 cm, there is a poikilitic pyroxene crystal that is partially enclosed in the troctolite and partially enclosed in a crosscutting gabbro, suggesting co-crystallization of the gabbroic matrix in the troctolite and the crosscutting gabbroic dikes.
Unit II is gabbroic (86.6%) with some diabase (11.4%) and minor granophyre (2%). Unit II contains ~30 m of oxide-rich gabbro and gabbronorite sandwiched between troctolite-rich Units I and II. Unit II gabbros and gabbronorites are mineralogically and texturally similar to oxide-rich gabbroic rocks found in Unit III of Hole 1275B. Texturally they are extremely heterogeneous, with abrupt grain-size changes that define bands or parts of crenulate blobs. The attitude of the igneous foliation in Unit II is variable. Most foliations cross the core at small angles, but in certain places (e.g., Section 209-1275D-13R-1) the foliation is parallel to the drilling direction. The granophyre in Unit II occurs both as thin stringers and in larger, more irregular masses that are generally elongate parallel to the length of the core. In Section 209-1275D-13R-2 there is a granophyre that is ~8 mm wide where it first cuts the core but increases in width upward. As the granophyre widens, its color changes from white to gray and it ultimately becomes diffuse within the gabbro (Fig. F14).
Unit III contains a mixture of the rock types found in Units I and II. Unlike Unit I, which contains 20-m sections of troctolite uninterrupted by oxide gabbro, the troctolite in Unit III is interspersed with oxide gabbro at the meter scale. The contacts between the two lithologies show clearly that the oxide gabbro postdates the troctolite, but no fragments of the troctolite are found as inclusions in the oxide gabbro. The oxide gabbro does not appear to be chilled against the troctolite, but alteration invariably obscures these relations. In addition to troctolite (40.7%) and gabbro (51.7%), Unit III contains diabase dikes or sills (6.3%), which cut both troctolite and gabbro, and granophyre (1.3%), which is exclusively associated with the gabbro.
Unit IV is similar to Unit II in most respects. It contains slightly less gabbro (76.8%) but more diabase (18.6%) and granophyre (4.6%). The base of Unit IV corresponds to the first occurrence of olivine-bearing gabbroic rocks. The two main distinctions between Unit II and Unit IV are the larger proportion of granophyre and the larger average grain size in Unit IV compared to structurally higher gabbros. The granophyre in Unit IV is more massive than elsewhere in the core, and in some intervals (e.g., Section 209-1275D-29R-2) it constitutes a significant portion of the core (i.e., >75%). The grain size of the gabbros in Unit IV varies at small scales as in the upper part of the core but there is an overall increase downhole (Fig. F15). The maximum fraction of coarse-grained gabbro in Hole 1275D is found at the lower boundary in Unit IV, comprising >50% of Cores 209-1275D-31R and 32R. However, it should be noted that microgabbro is present throughout the core and most sections contain fine-, medium-, and coarse-grained gabbros.
Unit V consists of oxide gabbro and oxide gabbronorite (83.8%), olivine gabbro (7.3%), diabase (7.4%), and granophyre (1.5%). The olivine gabbros are more primitive than gabbroic rocks encountered elsewhere in Units I and II in Hole 1275B or Units II and IV in Hole 1275D. The olivine gabbros contain as much as 20% olivine, which is obvious in hand sample where altered to iddingsite (Fig. F16). In Section 209-1275D-35R-4, the olivine gabbro contains bands of coarse-grained euhedral orthopyroxene as wide as 2.5 cm (Fig. F17A). In Sections 209-1275D-36R-1, and 36R-2, the olivine gabbro is associated with an olivine and plagioclase phyric diabase that is mixed with granophyric material (Fig. F17B). The rest of Unit V is similar to the oxide gabbros elsewhere at Site 1275. There is a general decrease in maximum grain size in Unit V oxide gabbros downhole (Fig. F15).
Significant proportions of troctolitic rocks were recovered in Holes 1275B and 1275D, and they are essentially identical in their characteristics in both hand sample and thin section. The troctolite-rich portions of the cores (Unit II in Hole 1275B and Units I and III in Hole 1275D) are cut by numerous gabbros. These gabbros tend to be rich in pyroxene relative to plagioclase and typically lack oxides. In hand sample, the troctolites can be seen to contain large (up to 4 cm) poikilitic pyroxenes that enclose olivine crystals. Plagioclase grains fill the interstitial space between the pyroxene-rich domains. Several of the thin sections studied sampled only interstitial clinopyroxene or orthopyroxene. Both pyroxenes were found together in a thin section, where clinopyroxene encloses olivine and rims orthopyroxene. It is clear from the hand samples that these thin sections do not provide an adequate sampling of the modal proportions in the rock. Some thin sections are classified as plagioclase-harzburgite (interstitial orthopyroxene) or plagioclase-wehrlites (interstitial clinopyroxene), but at the scale of the core the rocks are clearly troctolitic with plagioclase as the most abundant interstitial mineral.
In thin section, most of the olivine grains in these samples are isolated so that olivine/olivine contacts are scarce. Where olivine grains remain in contact it can be seen that the gabbroic material extends as thinning tendrils between the grains (Fig. F18). In these samples the olivine grains are rounded and embayed. Optically continuous, spatially separate grains of olivine are present within interstitial pyroxene or plagioclase (Fig. F18). Plagioclase, clinopyroxene, and orthopyroxene are all found in contact with olivine in various thin sections. No igneous reaction rims or corona textures are present between these phases. Subhedral to euhedral spinel is also found in these samples, and reflected-light microscopy reveals some of these spinels have rims of magnetite.
Only one olivine-bearing gabbro was recovered in Hole 1275B in Section 209-1275B-19R-1. This rock is very fine grained (<0.1 mm) with ~15% fine-grained plagioclase (as large as 3 mm). The larger plagioclase grains in this sample have strong optical zonation and some are dramatically bent (Fig. F19A). A later generation of smaller plagioclase crystals have skeletal morphologies and include olivine and clinopyroxene crystals in their core (Fig. F19B). We infer that this sample contained a small fraction of plagioclase crystals that were variably deformed during emplacement, after which the remainder of the melt crystallized rapidly.
Some of the olivine-bearing rocks from Hole 1275D are also inferred to have crystallized rapidly and might be described as olivine-plagioclase phyric diabase (Fig. F20A, F20B). This is intimately associated with granophyric material with boundaries that are suggestive of liquid-liquid interaction. However, other olivine-bearing rocks probably crystallized more slowly, have uniform granular textures (Fig. F20C, F20D), and are appropriately described as olivine gabbro and olivine gabbronorite. The olivine gabbro is present at the top of Unit V (Section 209-1275D-33R-1). It contains ~20% olivine, 15% clinopyroxene, and 65% plagioclase. Contacts between olivine and clinopyroxene and clinopyroxene and plagioclase are preserved, but no olivine/plagioclase contacts are present. Instead, olivine and plagioclase are always separated by radiating amphibole crystals that may be interpreted as evidence of an earlier igneous reaction (Fig. F20C, F20D). Small subhedral oxides and brown amphibole that may be magmatic are also present in this sample, but not between olivine and plagioclase. The olivine-bearing rocks deeper in the hole (Section 209-1275D-35R-4) contain subophitic orthopyroxene in addition to clinopyroxene, making them olivine gabbronorites, but are otherwise similar to those in Section 209-1275D-33R-1.
The gabbroic rocks that make up the bulk of Holes 1275B and 1275D share a variety of similar features. In both holes, these rocks are mixtures of gabbros and gabbronorites and oxide gabbros and oxide gabbronorites that are mixed at scales ranging from a few grain diameters to tens of centimeters. The mineralogical variation in these samples is primarily in the proportions of orthopyroxene and oxides. Whereas the proportion of orthopyroxene is difficult to estimate in hand sample, the proportion of oxides can be readily estimated visually or by using the bulk magnetic susceptibility.
The oxide distribution is especially heterogeneous in these gabbros and provides a good proxy for other types of variation. This heterogeneity can be observed in thin section, where the oxide is often segregated into bands and associated with fine-grained pyroxenes (Fig. F21). The proportions of the oxides were estimated visually and compared to bulk magnetic susceptibility on selected large core pieces. In general the magnetic susceptibility correlates with the visual estimated modal proportion of oxides, although with significant scatter (Fig. F22). The correlation is not strongly dependent on the grain size of the rocks (Fig. F23), and this, coupled with thin section observations of significant amounts of ilmenite, indicates that the bulk magnetic susceptibility provides only a crude proxy for the total oxide content. Comparing the oxide proportions and mineralogy in large thin sections would ameliorate some of the ambiguity of the bulk susceptibility interpretations. In any case, the susceptibility provides a good measure of the amount of magnetite present and so provides a measure of heterogeneity when plotted against depth (Figs. F24, F25). More detailed examination of the estimates of magnetite contents reveals that the scale of variation is <6 cm in the oxide gabbros (Fig. F26A, F26B, F26C, F26D). The variation documented by changes in the estimated magnetite content is smoothed compared to that seen in grain-size variations because of the inherent averaging done by the susceptibility loop (effectively over at least 3 cm) (see "Igneous and Mantle Petrology" in the "Site 1270" chapter).
Although extremely variable in hand sample, the gabbros share many features in thin section. Almost all of the gabbros have an igneous foliation defined by tabular plagioclase and in places by elongate clinopyroxene as well. These fabrics generally parallel grain-size contacts but locally can be nearly orthogonal to such contacts. The foliations also typically define a uniform direction in a single sample, but in several thin sections the foliations can be seen to change orientation over the length of the section. Deformation of individual crystals is common. Examples include kinked and broken crystals (Fig. F27A) and development of neoblasts at the ends of crystals oriented orthogonal to the main foliation (Fig. F27B). This deformation is interpreted to have occurred in the presence of interstitial liquid, perhaps during igneous compaction, because it is localized, the neoblasts are completely recovered, and nearby grains are completely strain free.
Another common feature of these gabbros is the presence of a few conspicuously large plagioclase crystals in many sections. These plagioclase crystals have strong optical zonation and irregular grain boundaries, may be optically discontinuous, and lack polysynthetic twinning (Fig. F28). These characteristics suggest that these plagioclase grains reacted with the crystallizing liquid and were partially resorbed. Other evidence for reaction is found in the relation of the oxide minerals to the silicates. In the majority of the thin sections, the oxides have textures that suggest that they are replacing pyroxenes and plagioclase (Fig. F29). Chemically this does not seem plausible, but the textural features are quite compelling.
Coarsely exsolved orthopyroxene is present in some clinopyroxene (Fig. F30). In detail it can be seen that many of the exsolved orthopyroxenes also contain exsolved clinopyroxenes. This texture may result from breakdown of original igneous pigeonite. Another relatively common but curious feature is the distribution of accessory apatite in these gabbros. In most, apatite is not conspicuous, but when present it is found as relatively large grains that occur in clusters (Fig. F31).
Diabase is found throughout Holes 1275B and 1275D. It is invariably aphanitic, and phenocrysts are mostly absent. Chemical analyses suggest that the diabases rapidly crystallized and preserve liquid compositions (see "Geochemistry"). Diabase crosscuts all other lithologies and is interpreted to reflect a late magmatic event.
Granophyre is common in Site 1275 gabbros. When present as small irregular veins, the granophyre appears to be replacive rather than crosscutting. Where the granophyre traverses plagioclase, the plagioclase is modified to a more albitic composition and clinopyroxene is pseudomorphed by amphibole that is in optical continuity with the original clinopyroxene (Fig. F32). The granophyre illustrated in Figure F32 also contains zircon and apatite, and the vein formation process appears to have completely dissolved an oxide grain. Wider granophyres have sharper boundaries, are composed predominantly of plagioclase and quartz, and contain accessory titanite and apatite. In Hole 1275D there are examples of granophyre that is mixed with olivine-plagioclase phyric diabase. The boundaries between the diabase and granophyre are crenulate and suggestive of liquid-liquid interaction. This granophyre is distinct in that it contains more abundant amphibole, apatite, and titanite than the granophyre associated with the gabbros (Fig. F33).
Drilling at Site 1275 sampled a complex assemblage of gabbroic rocks. We discuss their genesis by first considering the crystallization history of the troctolite, the oxide gabbros, and the granophyre. The relationship of the diabase to these lithologies is discussed where appropriate. We then consider the larger question of the assembly of these rocks as constrained within Holes 1275B and 1275D and by comparisons between them.
The crosscutting relations indicate that the troctolite and its associated gabbroic material formed the host rock into which the liquid parental to the oxide gabbros was intruded. Based on analogy with other olivine-rich rocks associated with abundant gabbro, we infer that these troctolites were generated by the addition of interstitial plagioclase and pyroxenes to a peridotite via crystallization from a liquid migrating along olivine grain boundaries. This process resulted in isolated olivine grains surrounded by a gabbroic matrix. Where olivine grains are in contact it can be seen that the gabbroic material extends as thinning tendrils between the grains (Fig. F18). In these samples, the olivine grains are rounded and embayed. Optically continuous patches of olivine are separated by interstitial pyroxene or plagioclase. The contemporaneous crystallization of the gabbroic material between the olivine crystals and in the crosscutting gabbros is suggested by the growth of poikilitic pyroxenes across the gabbro/troctolite contact.
The oxide gabbros have complex internal relations and clearly did not crystallize from a single pulse of magma. The presence of regions with discrete grain sizes and irregular crenulate boundaries suggests that multiple injections, magma mingling, and, perhaps, magma mixing were important processes. The variably deformed plagioclase crystals and changes in the orientation of the magmatic foliations suggest that partially solidified gabbro was disrupted on numerous occasions. The combination of periodic intrusion, magma mixing, and disruption of partially solidified gabbros produced the complex textural relations observed. The apparent reaction relationship between the oxides and silicate grains indicates a complex crystallization history, whereas the exsolution features in the pyroxenes imply protracted and possibly nonmonotonic cooling.
The granophyre is closely associated with the oxide gabbros and may represent the product of extreme fractional crystallization. That the granophyre replaces and reacts with the gabbro clearly indicates that the two were not in equilibrium at some times and places. However, this is an expected consequence of fractional crystallization and should not be taken as evidence that the granophyres are exotic to the system.
Simple correlations between Holes 1275B and 1275D are not possible. It is clear that the troctolite in both holes is similar and probably represents the "country rock" into which the oxide gabbros were intruded. Grain size in Hole 1275B consistently increases downhole (Fig. F10), whereas in Hole 1275D the grain size increases to the base of Unit IV and then decreases below (Fig. F15). If the intervening troctolite was removed, the sequence of gabbroic material in Hole 1275B might correspond to that seen in Units II, III, and the upper part of IV in Hole 1275D. The correspondence is somewhat crude and the gabbros in Hole 1275D are on average coarser grained than those in Hole 1275B. If the gabbros in these two holes represent a single pluton, then it may be that Hole 1275B sampled closer to the margin of the intrusion and cooled more rapidly.
