The main rock types are principally distinguished by the presence or absence of the main minerals plagioclase, clinopyroxene, orthopyroxene, olivine, Fe-Ti oxides, and apatite. Sulfide minerals, brown hornblende, and biotite are additionally present in many of the examined samples in total proportions <1%-2%. The classification used in this study contrasts to the usage adopted during Leg 179 (Shipboard Scientific Party, 1999), as well as to IUGS classification (Le Maitre, 1989). The justification is that the relatively coarse grained nature of the gabbros makes it difficult to obtain representative estimates of rock modes. An additional consideration is the need to use a nomenclature that facilitates the petrogenetic discussion.

The relatively coarse grained nature of many of the gabbros and their low modal olivine content question the significance of distinguishing between gabbro and olivine gabbro based on sampling a volume represented by a thin section. Therefore, gabbro and olivine gabbro are grouped together in this work and are described in general terms as olivine gabbro. This contrasts with the nomenclature adopted during Legs 118, 176, and 179.

The Shipboard Scientific Party (1999) defined Fe-Ti oxide gabbros drilled during Leg 179 as containing at least 5% modal Fe-Ti oxide minerals. This definition contrasts with the usage adopted by the Shipboard Scientific Parties of both Legs 118 and 176 (Robinson, Von Herzen, et al., 1989; Dick, Natland, Miller, et al., 1999), which defined petrographic and modal varieties of Fe-Ti oxide gabbros as containing at least 1% Fe-Ti oxide minerals. The present work follows Legs 118 and 176 in the modal definition of Fe-Ti oxide gabbros.

Gabbronorite and apatite gabbro are defined by the presence of orthopyroxene and apatite, respectively, as primary igneous phases irrespective of their absolute modes. The mode of orthopyroxene in many of the rocks, here referred to as gabbronorites, is often too low to classify these as gabbronorites stricto sensu (Le Maitre, 1989). The presence of orthopyroxene is a diagnostic feature for monitoring the petrogenesis of the gabbros, and its inclusion in the nomenclature is thus justified. Similarly, the presence of apatite is used to classify the gabbro (or gabbronorite) as apatite gabbro. Pigeonite, or its low-temperature inversion products, was not detected as a primary igneous phase in this study, despite being described from Hole 735B (Robinson, Von Herzen, et al., 1989; Dick, Natland, Miller, et al., 1999).

The Hole 1105A gabbros are modified to variable degrees by hydrothermal alteration, ductile to brittle deformation, extensive development of secondary and metamorphic minerals, and dynamic recrystallization of primary mineral assemblages. The secondary neocrystallized minerals include talc, serpentine, smectite, magnetite, calcite, epidote, chlorite, green hornblende, and actinolite. Recrystallization of primary igneous minerals (olivine, pyroxene, and plagioclase) is likewise extensive in many gabbros. The secondary and metamorphic mineral assemblages suggest that overall the dominant alteration and metamorphism took place under greenschist facies conditions (Stakes et al., 1991). These modifications of primary igneous textures and mineralogies are not dealt with in this contribution to any significant extent.

Olivine Gabbro

Olivine gabbro is the most abundant rock type in all lithologic units (Fig. F2) (Scientific Shipboard Party, 1999). The average modal composition of olivine gabbros is olivine = 9 vol%, plagioclase = 60 vol%, and clinopyroxene = 31 vol%. All major mineral phases show wide modal variations (Fig. F4), mainly due to variations in amounts of plagioclase and pyroxene, with occasional troctolitic and anorthositic variants. Additional trace amounts of brown hornblende and opaque minerals (Fe-Ti oxide and sulfide minerals) are present in many gabbros in amounts <0.4%. The grain sizes of the olivine gabbros are dominantly medium to mostly coarse grained (~75 vol%). The textures are granular with interlocking plagioclase and clinopyroxene grains (Fig. F5A). Most of the coarse-grained and pegmatitic olivine gabbros, however, have poikilitic or subophitic textures in which large subhedral poikilitic clinopyroxene (Fig. F5B) encloses or partly encloses small euhedral to subhedral plagioclase and, sometimes, olivine. Plagioclase grains are commonly homogeneous and in undeformed gabbros rarely show signs of compositional zoning (Fig. F5C). Because of incipient deformation, plagioclase often contains tapering twin lamellae and has undulatory extinction. Plagioclase of the olivine gabbros shows very wide compositional ranges from An66 to An40 (Fig. F6; Tables T1, T2). Olivine is present as rounded to anhedral homogeneous grains that vary in composition from Fo78 to Fo50 (Fig. F7; Table T1). Olivine is often recrystallized and forms clusters of grains with fine-grained polygonal, mosaic textures (Fig. F5D). Deformation bands and undulatory extinction occur in most olivine grains (Fig. F5E). Clinopyroxene is present as homogeneous grains without obvious compositional zoning. Fine exsolution lamellae of low-Ca pyroxene are ubiquitously present in most clinopyroxene grains. The clinopyroxene is augitic with Mg/(Mg + Fetotal) ratios from 0.84 to nearly 0.60 (Fig. F8; Tables T1, T3). Often, clinopyroxene shows consertal intergrowth and replacement textures with adjacent clinopyroxene (Fig. F5F). Clinopyroxene is commonly clouded by acicular opaque minerals that are aligned and regularly spaced. Brown hornblende is an accessory mineral that is present in most gabbros as rims to clinopyroxene grains as well as patchy blebs in clinopyroxene and as coronas around Fe-Ti oxide and olivine. Biotitic mica is also occasionally present as a late-crystallizing phase. Apatite is characteristically absent from the olivine gabbros.

Fe-Ti Oxide Gabbro

The modal composition of Fe-Ti oxide gabbro averages olivine = 3 vol%, plagioclase = 58 vol%, clinopyroxene = 35 vol%, and Fe-Ti oxides and sulfides = 5 vol%. Additional trace amounts of brown hornblende and biotite are often present. As for the olivine gabbros, the modal variation of the Fe-Ti oxide gabbros varies considerably but is mainly restricted to variations in plagioclase and pyroxene contents (Fig. F4). Olivine appears to be present in the Fe-Ti oxide gabbros in lower modal amounts than in the olivine gabbros. However, the presence or absence and absolute amounts of olivine may have little petrogenetic significance because of the relatively coarse grained nature of the gabbros. The Fe-Ti oxide minerals are mostly ilmenite and are present in amounts that do not exceed a total of 12% (Fig. F4) (Shipboard Scientific Party, 1999).

The undeformed Fe-Ti oxide gabbros are in many respects similar to the olivine gabbros, especially in terms of rock textures, mineral morphologies, and mineral intergrowth. The difference is the presence of Fe-Ti oxide minerals that are generally <0.5 mm in grain size in the olivine gabbro and are present as late interstitial patches often intergrown with brown hornblende and metamorphic amphiboles and other secondary phases. In contrast, the Fe-Ti oxide minerals in the Fe-Ti oxide gabbros may reach grain sizes generally up to 10 mm in an interstitial fabric to silicate minerals and as elongated aggregates forming irregular seams and bands.

The composition of olivine in the Fe-Ti oxide gabbros, when present, is between Fo51 and Fo36 and is significantly more iron rich than that for the olivine gabbros (Fig. F7; Tables T1, T4). Similarly, the coexisting plagioclase (An49-An29) and clinopyroxene (Mg/[Mg + Fetotal] = 0.66-0.58) reveals more evolved compositions compared to the olivine gabbros (Figs. F6, F8; Tables T1, T2, T3).


Orthopyroxene, along with plagioclase and clinopyroxene, is present as a primary igneous phase in a few gabbros without any apparent stratigraphic relations. These gabbronorites contain up to 8% modal orthopyroxene and appear not to contain olivine. Only two gabbronorites were examined in this study; both contain a few percent Fe-Ti oxide minerals. It thus appears that orthopyroxene-bearing gabbros are also Fe-Ti oxide bearing and, further, lack olivine. Orthopyroxene is often slightly prismatic elongated and is present in granular intergrowths with plagioclase and clinopyroxene. Orthopyroxene contains abundant, thin exsolution lamellae and blebs of clinopyroxene (Fig. F9C). Some samples reveal replacement textures of orthopyroxene by secondary clinopyroxenes.

The composition of the primary igneous orthopyroxenes is relatively Fe rich, with Mg/(Mg + Fe) ratios of 0.48-0.52 (Fig. F8; Tables T1, T3) without detectable differences between the apatite-bearing and apatite-free gabbros. The orthopyroxene compositions correspond to the generally more sodium- and iron-rich compositions of coexisting plagioclase (An37-An33) and clinopyroxene (Mg/[Mg+Fetotal] = 0.63-0.51) (Figs. F6, F8; Tables T1, T2, T3). Thus, the gabbronorites appear to be relatively more evolved than the most primitive of the Fe-Ti oxide gabbros.

Low-Ca pyroxenes also appear as secondary minerals in minor amounts in many Fe-Ti oxide gabbros and are relatively more Mg rich than the primary igneous orthopyroxenes. These secondary pyroxenes are not dealt with in this study.

Apatite Gabbro

Apatite was found and analyzed in 11 samples and is present in close association with Fe-Ti oxide seams (Fig. F9D). Three of these samples also contain orthopyroxene. Apatite generally ranges up to 5 vol% and contains saline fluid inclusions. The presence of considerable amounts of apatite is one of the significant features that characterize some of the Fe-Ti oxide gabbros from Hole 1105A. Apatite was not fully recognized in the gabbros of Hole 735B (Robinson, Von Herzen, et al., 1989). Biotitic mica is only found in one sample as prismatic grains enclosed by Fe-Ti oxide minerals that texturally signify a primary igneous origin (Sample 179-1105A-27R-1, 71-75 cm). The apatite gabbros are, in most respects, similar to the Fe-Ti oxide gabbros and the gabbronorites. They are typically medium to coarse grained and have granular textures. The apatite is present as <0.1-mm, large euhedral grains, most often in interstitial Fe-Ti oxide seams and patches, but may also be found included in other primary or secondary minerals.

The compositions of the silicate minerals in apatite gabbro show that these are among the most evolved gabbros. The olivine compositions are relatively restricted around Fo35 (Fig. F7; Tables T1, T4). Similarly, plagioclase compositions are below An31 and clinopyroxene Mg/(Mg + Fetotal) ratios are <0.68 (Figs. F6, F8; Tables T1, T2, T3).


Fine-grained gabbros are occasionally present and are of considerable interest, as they may represent quenched liquid. Three samples were included in this study (Samples 179-1105A-8R-1, 68-70 cm; 13R-1, 24-25 cm; and 15R-2, 71-74 cm). Microgabbros appear to be generally confined to bands or lenses (Fig. F10), but no clear discordant relationships and chill effects were observed against the coarser-grained gabbros (Fig. F10). The microgabbros are mineralogically highly variable but share common fine-grained homogeneous, equigranular textures with polyhedral mutual grain boundaries and are void of phenocrysts (or porphyroblasts) (Fig. F10). The latter distinguish them from mylonitic gabbros. The mineralogies of the microgabbros vary from gabbronorite, gabbro, and olivine gabbro and they appear to represent a range of relatively evolved compositions with olivine compositions of Fo40 (Sample 179-1105A-15R-2, 71-74 cm), plagioclase of An46 to An31, and clinopyroxene of Mg/(Mg + Fetotal) between 73 and 62 (Table T1). The orthopyroxene present in one of the microgabbros has a Mg/(Mg + Fetotal) ratio of 51 (Sample 179-1105A-8R-1, 68-70 cm). The petrographic, textural, and compositional similarities between the microgabbros and the gabbros are striking and suggest that the microgabbros may be recrystallized gabbros and not quenched melts.