PETROLOGIC AND RHEOLOGIC CONTRASTS IN PROTEROZOIC-AGED LITHOSPHERIC MANTLE: KILBOURNE HOLE, NEW MEXICO

This study reports on temperatures for three different groups of spinel peridotite from Kilbourne Hole, New Mexico. The groups are defined by a combination of texture, modal mineralogy, and chemical composition. The first group is fine-grained lherzolite with tabular, equigranular texture. Olivine and pyroxene have a strong grain shape preferred orientation (GSPO), suggesting large finite strain. The second group is protogranular to porphyroclastic lherzolite. Its chemistry is still fertile, whereas the fabric of the rock is different from the first group. For this group, recovery kept pace with deformation, impeding the development of grain shape preferred orientation. Most previous studies on Kilbourne Hole xenoliths have included only these two groups. A third group is porphyroclastic olivine-rich peridotite. Modal analyses indicate that the group includes lherzolite, dunite, and harzburgite. The xenoliths in this group are characterized by strongly porphyroclastic texture and whole-rock and mineral chemistry consistent with melt depletion. The rocks lack a grain shape preferred orientation and have amoeboid grain boundaries. These textures are consistent with grain boundary diffusion creep in olivine and suggest plastic deformation at high temperature. Temperatures for the three groups are distinct: fine-grained lherzolite has the lowest temperature (approximately 980°, calculated at approximately 2 GPa). Protogranular to porphyroclastic lherzolite has temperatures of 1020° to 1044° C, and porphyroclastic olivine-rich peridotites have the highest temperatures (1052° to 1184° C). The combined textural and thermometric data suggest that the sub continental mantle in the southern Rio Grande rift is both rheologically and chemically layered.