The nearly intact 500-m section of gabbros recovered at Site 735 during Leg 118 provided a first look at the processes of crustal accretion and simultaneous tectonism, alteration, and ephemeral magmatism at a slow-spreading ocean ridge. Results from the leg showed that the section did not form in a large steady-state magma chamber, but instead by continuous intrusion and reintrusion of numerous small, rapidly crystallized bodies of magma. There is little evidence of the process of magmatic sedimentation important in layered intrusions. Rather, new batches of magma were intruded into a lower ocean crust consisting of crystalline rock and semisolidified crystal mush. This led to undercooling and rapid initial crystallization of new magmas to form a highly viscous or rigid crystal mush, largely preventing the formation of magmatic sediments. Initial crystallization was followed by a longer, and petrologically more important, period of intercumulus melt evolution in a highly viscous crystal mush or rigid melt-crystal aggregate.

Thus, if the 437 m of gabbro recovered from Hole 735B is representative, long-lived magma chambers or melt lenses were virtually absent throughout most of crustal formation beneath the Southwest Indian Ridge (Bloomer et al., 1991; Dick et al., 1991b; Natland et al., 1991; Ozawa et al., 1991). Melts in the highly viscous or rigid intrusions were largely uneruptable throughout most of their crystallization. This could explain the near absence of highly evolved magmas such as ferrobasalts along the Southwest Indian Ridge (Dick, 1989), as opposed to fast-spreading ridges where they are common and a long-lived melt lens is believed to underlie the ridge axis (e.g., Sinton and Detrick, 1992).

Wall-rock assimilation occurring while small batches of melt worked their way up through the partially solidified lower crust played a significant role in the chemical evolution of the section and, therefore, in the chemistry of the erupted basalt (Dick et al., 1992). This process has been largely unevaluated for basalt petrogenesis to date, but raises questions for simple models for the formation of mid-ocean-ridge basalt (MORB).

An unanticipated major feature of Hole 735B was the evidence for deformation and ductile faulting of still partially molten gabbro (Bloomer et al., 1991; Dick et al., 1991a; Dick et al., 1992; Natland et al., 1991). This deformation apparently occurred over a narrow window late in the crystallization sequence (probably at 70% 90% crystallization) when the gabbros became sufficiently rigid to support a shear stress. This produced numerous small and large shear zones creating zones of enhanced permeability into which the late intercumulus melt moved by compaction out of undeformed gabbro intervals. Migration of this late iron-rich intercumulus melt into and along the shear zones hybridized the gabbro there by melt-rock reaction and by precipitation of Fe-Ti oxides and other late magmatic phases. The net effect of these synchronous magmatic and tectonic processes is a complex igneous stratigraphy of undeformed oxide-free olivine gabbros and microgabbros crisscrossed by bands of sheared ferrogabbro. Sheared oxide gabbros, nearly identical to those drilled in Hole 735B, were also recovered at ODP Sites 921, 922, and 923 on the eastern inside-corner high of the Kane Fracture Zone (Cannat, Karson, Miller, et al., 1995). Differentiation by deformation (Bowen, 1920) is probably ubiquitous in the lower ocean crust at slow-spreading ocean ridges, and has recently been postulated for in the Lizard ophiolite (Hopkinson and Roberts, 1995). Evidence of such deformation is absent, however, in high-level East Pacific Rise gabbros drilled at Hole 894G at Hess Deep (Natland and Dick, 1996; MacLeod et al., 1996), suggesting that differentiation by deformation may be a feature that can be used to discriminate between fossil sections of ocean crust formed at different spreading rates.

At Site 735, ductile deformation and shearing continued into the subsolidus regime, causing recrystallization of the primary igneous assemblage under granulite facies conditions, and the formation of amphibole-rich shear zones (Cannat et al., 1991; Cannat, 1991; Dick et al., 1991a, 1992; Mével and Cannat, 1991; Stakes et al., 1991; Vanko and Stakes, 1991). Here again, formation of ductile shear zones localized late fluid flow with the most intense alteration occurring in the ductile faults.

One consequence of simultaneous extension and alteration in the rocks of Hole 735B is that high temperature alteration is far more extensive than is found in layered intrusions, which are typically intruded and cooled in a near static environment (Dick et al., 1991a; Stakes et al., 1991). An abrupt change in alteration conditions of the Hole 735B gabbros, however, occurred in the middle amphibolite facies with the virtual cessation of brittle-ductile deformation (Dick et al., 1991a; Magde et al., 1995; Stakes et al., 1991; Stakes, 1991; Vanko and Stakes, 1991). Mineral vein assemblages change from amphibole-rich to diopside-rich, reflecting different, more-reacted, fluid chemistries. Continued alteration and cooling to low temperature within massive gabbro occurred under near static conditions similar to those found at large layered intrusions. These changes were probably the result of an inward jump of the master faults at the rift valley walls, transferring the section out of the zone of extension and lithospheric necking beneath the rift valley into a zone of simple block uplift at the inside-corner high of the Atlantis Fracture Zone. Hydrothermal circulation, little enhanced by stresses related to extension, was greatly reduced, driven largely by thermal-dilation cracking as the section cooled to ambient temperature.

The results from Hole 735B show the strong influence of deformation and alteration on crustal accretion at ultra-slow-spreading ridges as the critical brittle-ductile transition migrates up and down through the lower crust due to the waxing and waning of magmatism. This is in contrast to the results of the East Pacific Rise crustal section exposed at Hess Deep, where no evidence of crystal-plastic deformation was seen either in the high level gabbros from Hole 894G or in dredged gabbros. This is consistent with the presence of a near steady-state melt lens beneath the East Pacific Rise, which fixes the location of the brittle-ductile transition near the base of the sheeted dikes (e.g., Dick et al., 1991a; Sinton and Detrick, 1992).

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