Zoned garnets and bulk rock compositions have been compared in order to examine relationships between the processes that created zoning and the compositional evolution of peridotite xenoliths with high equilibration temperatures and sheared textures. Most xenoliths discussed are from kimberlites in northern Lesotho and from the minette neck at The Thumb on the Colorado Plateau. Zoned garnets in sheared rocks from both suites have characteristic rim enrichment in Ti; rim enrichment in Fe and depletion in Cr are also present in garnets of some samples. Garnets in many peridotite xenoliths, including those typical of coarse-textured rocks, contain too little Ti to be in equilibrium with normal mantle melts. Rim enrichment in Ti is best explained by exchange between garnets and percolating melt.

Single episodes of melt-rock interaction like those responsible for zoned garnets appear insufficient to derive compositions of sheared peridotites from those typical of coarse peridotites. Minimum effects of melt-crystal exchange have been calculated from garnet zonation. Fe and Ti were relatively enriched by at least 10% and over 100%, respectively, in one rock, but the calculated changes are not sufficient to produce the present Fe-Ti values of this rock from those typical of coarse peridotite. In addition, garnet zoning is not correlated with Fe, Ca and Al enrichment in host peridotites. Finally, even core compositions of garnets in most deformed rocks are not as low in Ti as are garnets of most coarse peridotites. Repeated episodes of melt percolation, and consequent exchange with and crystallization of infiltrated melt, are suggested to be responsible for the enrichment of the sheared peridotite xenoliths in Fe and Ti and perhaps in Ca, Al, and other elements. Evidence of multiple events has been recognized in zoning within garnets in a few xenoliths, despite the ephemeral nature of zoning at high temperature, and melt percolation may have affected sheared xenoliths repeatedly. Repeated enrichment processes like those documented by garnet zoning should be most effective in the uppermost asthenosphere and should lead to a compositional divergence of that region from the lithosphere.

The suites from The Thumb and northern Lesotho have distinct differences, as in Mg/Si and Fe/Mg. In each suite, however, similar compositional characteristics, such as increased Ti and Fe/Mg, serve to distinguish most sheared from most coarse-textured peridotites, despite the differences in host magmas, equilibration temperatures, and presumed ages of lithosphere stabilization. These distinctive characteristics of the sheared peridotites in both regions may have been imposed by melt-rock interactions just below the continental lithosphere. ¿ American Geophysical Union 1988