Hole 1105A was drilled during Leg 179 in a water depth of 703 m on the block-faulted and wave-cut platform of the Atlantis Bank at approximately longitude 57°E (Fig. F1A), only 18-20 kilometers from the eastern rim of the Atlantis II Fracture Zone, Southwest Indian Ridge (Fig. F1B) (Pettigrew, Casey, Miller, et al., 1999). The position of Site 1105 is offset 1.3 km toward the east-northeast from Hole 735B, drilled during Legs 118 and 176 (Robinson, Von Herzen, et al., 1989; Dick, Natland, Miller, et al., 1999). Both sites are located east of the north-south-trending Atlantis II Transform that left-lateral offsets the Southwest Indian Ridge (Fig. F1C) (Dick et al., 1991b). The age offset is ~20 m.y., and the Atlantis II Fracture Zone grew at an average of 0.4 cm/yr (Dick et al., 1991b). Both ridge-transform intersections display asymmetric rift valleys with inside corner highs, outer corner lows, and nodal basins. The relief of the northern inner rift valley wall is nearly double that of the outer valley wall as a result of low-angle detachment faulting and unroofing of the plutonic footwall block (Fig. F1D).
Both Holes 735B and 1105A sampled gabbroic massifs formed at the inner corner of the northern median rift valley. The crustal section penetrated by Hole 735B represents thin crust relatively unaffected by the adjacent transform and formed beneath the median valley ~15-19 km away from the transform-ridge intersection ~11 m.y. ago (Dick et al., 1991a, 1991b; Muller et al., 1997). The massif was subsequently uplifted 5-6 km as a transverse ridge to above sea level, eroded, and later subsided to its present water depth. Thus, the Atlantis Bank may represent the remains of a fossil magma chamber formed near the midpoint of a volcanic ridge segment and unroofed by transform tectonics (Dick et al., 1991b, p. 396). Site 1105 is located in crust of approximately similar age and origin as that at Site 735 and represents crust formed farther distal from the transform-ridge intersection. Hole 1105A probably penetrated crust that originated from the same volcanic ridge segment as Hole 735B, only slightly farther away from the transform-ridge intersection. The results from Hole 1105A, therefore, can elucidate the lateral variation in magma supply and magma processes along active slow-spreading centers.
Hole 1105A penetrated to a depth of 158 m and recovered a total of 118 m of dominantly medium- to coarse-grained gabbro intervals (Fig. F2). The grain size ranges from coarse to pegmatitic in extreme cases. The lithologic intervals were divided into three major units by the Shipboard Scientific Party (1999). Gabbroic rocks constitute by far the majority of the recovered igneous lithologies with only a very minor component of felsic veins (<0.1%). The gabbros are composed of olivine gabbro (52%), gabbro (16%), Fe-Ti oxide olivine gabbro (14%), Fe-Ti oxide gabbro (16%), and minor amounts of Fe-Ti oxide gabbronorite (2%). The main components can largely be grouped into Fe-Ti oxide-free (78%) and Fe-Ti oxide-bearing gabbros (22%) that are intercalated throughout the recovered gabbro interval on a scale from a few millimeters to meters. Apatite is a characteristic index mineral in some relatively evolved Fe-Ti oxide gabbros and gabbronorites. A minor component of granular textured microgabbros is also occasionally present.
Igneous layering is seen mainly as irregularly developed modal, textural, and grain size variations in all gabbro components. Mostly this layering is defined by grain size variation from medium to coarse grained (Fig. F3A). The gabbros are texturally granular, often with large poikilitic clinopyroxenes, particularly in the pegmatitic components (<60 mm). Rhythmic modal layering and laminations (Fig. F3B) are only occasionally identified. Many of the gabbros show a distinctive penetrative foliation (Fig. F3C) that may have overprinted a primary igneous fabric. The Fe-Ti oxides and sulfides are irregularly distributed as granular to interstitial and disseminated grains that are often highly concentrated in patches and bands. There is a strong tendency for ductile deformation to be channeled along Fe-Ti oxide-rich bands and to be accompanied by grain fining and neoblast and porphyroblast formation (Fig. F3D) and sometimes the development of mylonitic bands (Fig. F3E). In the relatively undeformed gabbros and olivine gabbros, the effects of deformation are seen as kinking of albite twins and the development of tapering twins (Fig. F3F), irregular zoning, and incipient neoblast formation along grain boundaries. The pyroxenes similarly show kinking of exsolution lamellae and twin planes. Most preserved olivine grains show signs of kink banding and recrystallization. The Shipboard Scientific Party (1999) estimated that gabbroic rocks with deformation and recrystallization textures represent >40% of the rock intervals present in Hole 1105A.
The principal divisional units were defined by the Shipboard Scientific Party (1999) by the relative proportion of Fe-Ti oxide gabbro layers and lenses in dominantly massive Fe-Ti oxide-free gabbro and olivine gabbro. The assigned unit divisions are illustrated in Figure F2, together with a schematic breakdown in the two principal gabbroic components.
Lithologic Unit I (15-48.14 meters below seafloor [mbsf]) is composed of medium- to coarse-grained, alternating gabbro, olivine gabbro, and Fe-Ti oxide gabbro. Fe-Ti oxide gabbro (8%) is confined to the lower part of the unit (Fig. F2), whereas the rest of the unit is medium- to coarse-grained massive gabbro and olivine gabbro (92%). The Fe-Ti oxide gabbro is characterized by irregularly distributed, interstitial, and disseminated Fe-Ti oxides and sulfides that may be highly concentrated in localized patches and bands.
Lithologic Unit II (48.14-136.38 mbsf) is composed of alternating gabbro, Fe-Ti oxide gabbro, and olivine gabbro. The upper boundary is defined by the appearance of a higher proportion of Fe-Ti oxide gabbro and is also clearly reflected in the magnetic susceptibility (Fig. F2). Unit II is composed of medium- to coarse-grained Fe-Ti oxide gabbro (29%), gabbro (55%), and olivine gabbro (16%), without systematic distribution of the three main gabbro types. Fe-Ti oxide minerals are a minor component in all gabbros and olivine gabbros. The gabbro and olivine gabbro vary modally widely and are composed of olivine (2%-20%), plagioclase (55%-70%), and clinopyroxene (20%-35%) with anhedral to subhedral granular textures and larger poikilitic clinopyroxene grains. The Fe-Ti oxide gabbros are typically composed of olivine (0%-15%), plagioclase (50%-60%), clinopyroxene (25%-40%), Fe-Ti oxide minerals (5%-15%), minor amounts of sulfides, and, occasionally, orthopyroxene and apatite. The Shipboard Scientific Party (1999) observed that the average grain size in the lower part of the unit appeared to be larger than that in the upper part of the unit (Fig. F2). This allows Unit II to be divided into two subunits. Subunit IIA is defined between 48.14 and 92.79 mbsf and Subunit IIB between 92.79 and 136.38 mbsf. This subdivision corresponds to the magnetic susceptibility measurements, which show a significant drop in average values at this boundary (Shipboard Scientific Party, 1999).
Lithologic Unit III (136.38-150.60 mbsf) is composed of gabbro (9%), olivine gabbro (88%), and minor amounts of Fe-Ti oxide gabbro (3%). The lower boundary is defined by an increase in the modal abundance of Fe-Ti oxide minerals and in the intensity of the magnetic susceptibility (Fig. F2). Unit III is a sequence of dominantly olivine gabbro that is separated from the units above and below, both of which are characterized by the ubiquitous presence of Fe-Ti oxide-rich gabbros.
Lithologic Unit IV (150.60 mbsf to the bottom of the hole at 157.44 mbsf) is composed of Fe-Ti oxide gabbro (37%) and gabbro (63%) in alternating intervals. The contact with Unit III is marked by an abrupt increase in the modal abundance of Fe-Ti oxide and in the intensity of the magnetic susceptibility (Fig. F2). The Fe-Ti oxide content in the Fe-Ti oxide gabbros ranges from 5% to 10%, and the remainder is mainly composed of plagioclase and clinopyroxene, the former volumetrically generally exceeding the latter.
The gabbro units drilled from Hole 1105A are principally defined by the relative proportions of Fe-Ti oxide-bearing and Fe-Ti oxide-free gabbros. Except for a coarsening downward in the average grain size in the core (Fig. F2), there is no additional systematic petrographic and textural variation that can be related to stratigraphic position (Shipboard Scientific Party, 1999).
A total of 99 samples (Table T1) were selected from Hole 1105A to represent the main gabbroic rock types. This number includes 67 shipboard-prepared thin sections for which extensive petrographic and modal data exist (Shipboard Scientific Party, 1999). Primary igneous minerals were analyzed for each thin section with an CAMECA SX-50 electron microprobe operated at accelerating voltage = 15 kV, beam current = 10 nA, and count time = 10 s. Because of extensive exsolution lamellae in pyroxenes and Fe-Ti oxides, these minerals were analyzed with a broad beam (~10 µm). Further, the possibility for loss of sodium from albitic plagioclase also necessitated that this mineral be analyzed with the broad beam. Olivine and apatite were analyzed with a narrow beam (~1-2 µm), the latter because of its relatively small size. Multiple analyses of primary igneous phases were compiled for each individual petrographically distinct part of the thin sections. The analytical results are summarized in Table T1 for plagioclase, clinopyroxene, olivine, and orthopyroxene as averages and standard deviations of characteristic cation ratios. Plagioclase is described by the An content (Ca/[Ca + Na + K]). Olivine is described by the Fo content (Mg/[Mg + Fe]). Clinopyroxene and orthopyroxene are described by either their Mg/(Mg + Fetotal) or Mg/(Mg + Fe2+) ratios, with Fe2+ and Fe3+ estimated based on charge balancing cations (Papike et al., 1974). Fe-Ti oxide minerals (ilmenite and titanomagnetite) are described by the ilmenite-hematite and ulvöspinel-magnetite fractions, respectively, with Fe2+ and Fe3+ estimated by assuming perfect stoichiometries (Andersen and Lindsley, 1988).