CHARACTERIZATION OF MANTLE BENEATH TEXAS: CONSTRAINTS FROM KNIPPA XENOLITHS

Peridotite xenoliths hosted by ~86 Ma nephelinites from the Knippa quarry in Uvalde County, central TX provide snapshots of upper mantle processes and compositions beneath south-central Laurentia. Knippa peridotites consist of three rock types: spinel bearing dunite, harzburgite, and lherzolite (some with very high modal pyroxene). These are all coarse-grained, equigranular, with straight (triple junctions) to smoothly curving mineral boundaries. Most of the olivine grains are cut by serpentine veins and alteration is more pronounced at the boundary with the host rock. Elongate, bleb-shaped, dark brown spinels and CPX grains define weak foliation in some samples. Minor sulphide, apatite, and ilmenite is present in the serpentine veins. Electron microprobe analyses of olivines indicate a limited range of Fo (89-91). OPX compositions are En ₈₉ Fs ₁₀ to En ₉₁Fs ₈ and CPX is En _49-55 Fs _4-5 Wo _44-46 . The Cr# of the spinel varies from 0.14-0.27 although two pyroxene-rich lherzolites have very low Cr# of 0.05 and 0.08. Temperatures of both lherzolites and harzburgites using the Ca in OPX thermometer of Brey and Kohler (1990, Journal of Petrology) range between 900 and 1000°C. Based on phase relations (Takahashi et al., 1993, Phil.Trans.R.Soc.Lond.A) and temperature of equilibration, Knippa xenoliths come from a depth range of 40-70 kms and thus sample mantle lithosphere. Mineral and major element data show that the lherzolites are less depleted than the harzburgites. (Lherzolites were formed by less than 10% melting while the harzburgites were formed by 10-15% melting of Primitive Mantle. Whole rock Primitive Mantle normalized REE and trace element patterns show two distinct trends – LREE enriched and U-shaped. Batch melting as well as fractional melting models, starting from a fertile Primitive Upper Mantle, cannot explain these patterns. The peridotites have undergone cryptic metasomatism or impregnation with fluids as well as very complex enrichment processes that operated on what was originally slightly depleted mantle.