Position: 32º43.1346'S, 57º16.6518'E
Start hole: 0902 hr, 2 May 1998
End hole: 1837 hr, 10 May 1998
Time on hole: 201.58 hr
Seafloor (drill-pipe measurement from rig floor, mbrf): 714.0
Distance between rig floor and sea level (m): 11.1
Water depth (drill-pipe measurement from sea level, m): 702.9
Total depth (drill-pipe measurement from rig floor, mbrf): 872.00
Penetration (mbsf): 158.00
Coring Totals: 30
Type: RCB
Number: 30
Cored: 143.00 m
Recovered: 118.43 m
Recovery: 82.82%
Formation: Gabbro
Because of delays in the arrival of the supply ship, which inhibited resumption of hammer tests near Hole 735B, Hole 1105A was drilled during Leg 179 for a period of 6 days. The hole was located ~1.2 km east-northeast of Site 735 on the Atlantis platform along the eastern transverse ridge of the Atlantis II Transform. The site is along a near-ridge axial trend with respect to Hole 735B but more distal from the north-south Atlantis II Transform, which lies to the west. The site was chosen to avoid a duplication of Hole 735B efforts that might occur by drilling at proximal Site 1104. At the same time, we wanted to utilize Hole 735B as a reference section to attempt lateral correlation of large-scale igneous units, structural features, and geophysical characteristics over the broader distance represented by the offset in holes in the direction approximately parallel to the former ridge axis. In addition, the site was chosen to help constrain the overall structure of the massif exposed on the platform. If successful, the correlation experiment could yield a minimum measure of the dimensions of subaxial magma chambers and continuity of structure and processes along the strike of the ridge axis at a very slow spreading center. If correlations are unsuccessful, the dimensions of igneous units, former magma chambers, or structures would be limited to a distance smaller than the scale of the offset experiment. Correlation will be attempted on the basis of detailed and integrated data sets including core descriptions and subsequent shore-based laboratory analyses to establish cryptic chemical and mineralogical variations and the alteration and structural framework in the core. A full and highly successful logging program that was completed after the cessation of drilling will aid in the correlation attempts.
The hole penetrated to a depth of 158.00 m, and the cored interval measured 143 m, starting 15.0 m below the seafloor (mbsf, see Table T1). Core recovery included 118.43 m of gabbroic rock for a total recovery of 82.8%. Together with logging results, this recovery provides a rather complete coverage of the rock types and a comprehensive view of pseudostratigraphy in the gabbroic section cored. Shipboard results now indicate a high probability that specific units, structures, and/or geophysical characteristics from Holes 735B and 1105A may be correlated.
The cores recovered record a wide variety of rock types ranging from gabbro, oxide gabbro with as much as 20-25 modal% Fe-Ti oxides and olivine gabbro to scarcer troctolitic gabbro, gabbronorite, and felsic rocks such as trondhjemite. Within the core, 141 rock intervals have been described and defined on the basis of distinct changes in mode, modal proportions, grain size, and/or texture (Fig. F1). Well-defined igneous layer contacts or structural boundaries to these intervals are preserved in many sections of the core. The highly layered nature of the gabbroic rocks documented within the core is supported by high-quality continuous Formation MicroScanner (FMS) logs of the borehole, as well as other logs and whole-core magnetic susceptibility measurements. The scale of the layering in the core varies from a few centimeters to meters. On a broader scale, the intervals define four basic units from top to bottom consisting of (1) a gabbroic unit characterized by more primitive rock types and by a scarcity or lack of oxide gabbro, (2) a gabbroic unit characterized by a high abundance of oxide gabbro and oxide-bearing gabbro, (3) a gabbroic unit characterized by more primitive rock types and a lack of oxide gabbro, and finally (4) another unit rich in oxide gabbro and oxide-bearing gabbro. Rocks are crosscut by millimeter- to decimeter-sized veins of leucocratic gabbro, quartz diorite, trondhjemite, and irregular pegmatitic gabbro intrusions. Irregular veins and bands of oxide minerals have also been observed.
Thin sections indicate typical cumulate textures in the majority of samples that range from adcumulate to orthocumulate and show variable amounts of core-to-rim zoning in plagioclase. Poikilitic textures are also common with pyroxene as the oikocryst phase and plagioclase as the chadocryst phase. Igneous laminations were observed in several samples but were generally scarce or possibly overprinted by crystal-plastic fabrics in some deformed intervals of the core. Preliminary bulk rock geochemical results show a wide range in the chemistry of gabbroic rocks with Mg# varying from ~0.80-0.23, Fe2O3 from ~3.5-24.0 wt%, P2O5 from ~.01-4.1 wt%, Y from 7-192 ppm, Nb from 1-10 ppm, and Cr from 1-1066 ppm.
Alteration of the primary igneous mineralogy in the core is generally low but varies on the scale of a thin section to a meter. Alteration of olivine to chlorite, tremolite-actinolite, and talc is the most common manifestation of alteration, whereas plagioclase and clinopyroxene tend to be less altered. Clinopyroxene, when altered, is commonly partially replaced by patchy brown amphibole, but alteration to brown amphi-bole generally does not exceed 1%-2%. Where alteration is extensive, clinopyroxene is replaced by assemblages of actinolite and chlorite. Plagioclase is generally little altered.
Actinolite and chlorite are also the most common vein assemblages. Scarce high-temperature brown amphibole and low-temperature smectite and carbonate veins have also been observed.
The structure of the core is complex, and structural styles and intensities range from brittle to ductile. Most of the gabbroic samples cored possess igneous textures, but there are several parts of the core that display crystal-plastic fabrics. Mylonitic zones characterized by high oxide-mineral content were observed at ~53 and 71 mbsf. Centimeter- to decimeter-thick zones of ductile shear are restricted to the upper 90 m of core, whereas thicker zones of ductile deformation with weak to strong crystal-plastic fabrics become more prevalent at depths >90 mbsf. Intervals of penetrative ductile deformation in the lower portion of the core exceed 2 m in thickness. Zones of ductile deformation are commonly oxide rich, as are the contact regions between undeformed and ductilely deformed rocks. Oxide-gabbro rich zones tend to be strain localizers as many, but not all, of the crystal-plastic shear zones are rich in oxide minerals. Inclination of the ductile foliations vary from ~18º to 75º in the cored intervals and averages ~30º-35º. Thin sections show a range of textures from strictly igneous to slightly deformed igneous to dynamically recrystallized metamorphic textures with crystal-plastic fabrics. As deformation intensity increases, the effect can be most easily observed in plagioclase, where a progression from strain-free plagioclase to plagioclase with deformation twins, undulose extinction, kink bands, and dynamic recrystallization to neoblasts along grain margins progresses to porphyroclastic textures with small neoblasts of plagioclase and highly strained, kinked plagioclase, pyroxene, or olivine porphyroclasts. Olivine appears to have recrystallized to neoblast grain sizes prior to pyroxene, which tends to be preserved as the dominant porphyroclastic phase unless the intensity of deformation is most severe. Brittle fractures are generally filled with vein material such as actinolite and chlorite, but no large fault zones were recovered in the core. There were several regions of low recovery that could correspond to fault zones based on temperature, sonic, resistivity, and porosity logs. These regions of poor recovery generally sampled little intact core, although recovered gabbroic rocks are highly altered to smectite and contain carbonate veins.
Preliminary analysis of the downhole geophysical measurements from core and logging measurements yields a wide variety of information. Magnetic data indicate that the core possesses a single coherent magnetic direction with an average inclination of ~69º. This is compared with an inclination of -51º expected for the site. As in Hole 735B, these results indicate a consistently reversed polarity for the section and may indicate a significant block rotation of the massif similar in magnitude to rotations interpreted from Hole 735B (15º-20º). The consistency of the magnetic inclination downhole suggests that any relative rotations along ductile shear zones in the section must have occurred before cooling below the blocking temperature and are necessarily high temperature in nature. Magnetic susceptibility measurements clearly define zones of oxide gabbro and oxide-bearing gabbro documented in the core. Likewise, it provides a direct downhole comparison for the FMS logs, which measure resistivity. Oxide-rich zones are conductive, whereas oxide-free zones have high resistivity. Magnetic intensity on split cores ranges from ~0.2-5 A/m.
Lastly, a seismic-while-drilling (SWD) experiment was conducted at Hole 1105A and lasted for the duration of the drilling. The two OBS deployed were recovered and this data, together with accelerometer data from the drill rig, will be employed to test the feasibility of SWD during drilling operations of the JOIDES Resolution.