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doi:10.2204/iodp.pr.305.2005
A total of 1043 m of mafic and ultramafic rock was recovered from Hole U1309D during Expeditions 304 and 305, ranging in composition from basalts and diabase to dunite. The following major types of rocks were recognized (Figs. F7, F8):
- dunitic troctolite
- troctolite
- troctolitic gabbro and olivine gabbro
- gabbro group (olivine-bearing gabbro, gabbro, microgabbro, gabbronorite, and orthopyroxene-bearing gabbro)
- oxide gabbro
- diabase/basalt
These were distinguished on the basis of primary mineral mode, igneous contacts, and variations in grain size. A total of 770 igneous units were defined in Hole U1309D, including 221 from 20.5 to 401.3 mbsf (Expedition 304) and 549 from 401.3 to 1415.5 mbsf (Expedition 305).
The most abundant rock type recovered is from the gabbro group, composing 60.8% of the core recovered during Expedition 305. This group has a wide range of modal compositions, including minor volumes (rarely exceeding 10%) of olivine, Fe-Ti oxides, and/or orthopyroxene. This group exhibits significant variations in grain size from microgabbro (<1 mm crystals) to seriate medium-grained to pegmatitic (grain size exceeding 10 cm), in places within a single section of core. Gabbronorite and orthopyroxene-bearing gabbro are included in this gabbro group. Since unambiguous identification of orthopyroxene requires careful thin section observation, the amount of orthopyroxene-bearing gabbro is a minimum estimate. Gabbronorites show the same textural relationships as the associated gabbros. In the lower part of Hole U1309D (from Cores 305-U1309D-243R to 272R), low-Ca pyroxene appears, as either orthopyroxene and/or inverted pigeonite.
Olivine gabbro is a major rock type in Hole U1309D (25.5%) (Fig. F7), with modal olivine 20%. However, the modal composition of this rock type is highly variable on a submeter scale as well as, in places, commonly grading into troctolitic gabbro. Together, olivine and troctolitic gabbro form the secondmost abundant lithology, 23.1% of the recovered rock types during Expedition 305.
Troctolite is generally spatially associated with olivine and troctolitic gabbros (Figs. F8, F9). The texture of troctolite is irregularly seriate, locally with poikilitic clinopyroxene. Troctolite is less common and constitutes only 2.2% of the rocks recovered during Expedition 305. Troctolite units are commonly intruded by late-stage dikes of both coarse-grained gabbro and microgabbro.
Olivine-rich rocks with relatively low modal plagioclase and clinopyroxene are grouped as "dunitic troctolite." The recovered dunitic troctolites are very olivine rich, containing >70% olivine by mode, and are commonly intercalated with olivine and troctolitic gabbro (Fig. F10). The downhole abundance of the dunitic troctolites is >5% of the rocks recovered during Expedition 305, with the thickest interval between 1092 and 1236 mbsf (Fig. F8). In contrast to troctolite, dunitic troctolite displays subhedral to rounded medium-grained olivine and interstitial to poikilitic plagioclase and clinopyroxene in variable proportions (Fig. F11). This texture is commonly interpreted to suggest a cumulate origin.
Oxide gabbro, defined by the presence of >2% by mode Fe-Ti oxide minerals, is also a ubiquitous lithology, making up 7.7% of the rocks recovered during Expedition 305. The most common occurrence of oxide minerals (~80% of the oxide gabbros) is as randomly dispersed patches in undeformed, generally coarse-grained gabbros. Concentrations of oxide minerals are also present as discrete dikelets/layers cutting other rock types with either sharp or diffuse boundaries. Oxide mineral concentrations are commonly associated with intervals of ductile deformation (Fig. F12). Many oxide gabbros contain apatite and zircon as accessory minerals. Ubiquitous occurrences of oxide gabbros were also noted in ODP Hole 735B, Atlantis Bank (Dick, Natland, Miller, et al., 1999); in the MARK area (ODP Leg 153; Cannat, Karson, Miller, et al., 1995); and at 15°N at the MAR (ODP Leg 209; Kelemen, Kikawa, Miller, et al., 2004).
Diabase intrudes other rock types in several places throughout Hole U1309D. The deepest recovered diabase was from 1377.6 mbsf (interval 305-U1309D-287R-1, 0–48 cm).
Almost all gabbroic rock types are cut by veins/dikes of variable thickness and composition. These veins/dikes include gabbro dikelets, oxide-bearing dikelets, and trondhjemite dikelets. These dikelets may be partly magmatic in origin, partly metamorphic, partly deformation-related, or the result of a combination of all three processes.