[SCG64-04] The Atlantis Bank Batholith, SW Indian Ridge, the Link Between Lower Crustal Accretion at High and Low Melt Supply
Keywords:Lower Ocean Crust, Gabbro, Melt Transport
The batholith represents dynamic accretion, where extension across the plate boundary is accomplished by a combination of intrusion hypersolidus and sub-solidus crystal-plastic deformation at intermediate depth, and brittle deformation and dike intrusion at higher levels. It has as much in common with crust formed at the East Pacific Rise as it does with typical crust at the Mid-Atlantic Ridge. At the former lower crustal accretion is largely passive by direct intrusion of melt, and likely crystal-mush flow, with extension largely accommodated above the dike-gabbro transition by dike intrusion and brittle deformation. Melt lenses exist at the base of the crust, from where they are intruded to the seafloor with only minor modification occurring in a melt lens overlying a crystal mush zone at the dike-gabbro transition. Due to the melt lens, the dikes are generally cut by the gabbros. Hydrous alteration beneath the axis is very limited. Atlantis Bank is similar to the EPR in that a substantial portion of extension is accommodated by direct intrusion of melt, crystal mush flow, and both the EPR and the Atlantis Bank sections have undergone extensive km-scale late-stage melt transport that modified the overlying gabbros. Like the EPR, magmas are intruded to the seafloor directly from near the base of the crust.
By contrast, MAR lower crust drilled at 23°N and Atlantis Massif experienced little high-temperature crystal-plastic deformation and limited late-stage melt transport. They contain primitive cumulates, and represent episodic intrusion, melt storage, and crystallization of parental MORB in thin crust below the dike-gabbro transition. Primitive gabbros and therefore storage of primitive melt occurred at all levels. Due to only episodic magmatism, individual intrusions crystallized rapidly, so that unlike Atlantis Bank, there were only short periods where extension could be accommodated by high-temperature crystal-plastic deformation, and tectonic extension was by brittle deformation. Consequently, greenschist facies alteration dominates down Hole 1309D at Atlantis Massif, as compared to largely high-temperature high-level amphibolite to granulite facies alteration at Atlantis Bank. There was no km-scale permeable melt transport and systematic enrichment of the upper levels of the massif in Fe and Ti to form massive oxide gabbros, as found at Atlantis Bank. Like the MAR localities, Atlantis Bank had no melt lens, and the dikes cut the gabbros as a result.