In Search of the Nature of Mantle beneath South Central Texas

The Knippa quarry in Uvalde County, TX exposes peridotite xenoliths which provide snapshots of upper mantle processes and composition beneath south-central Laurentia. Host rock for these xenoliths are nephelinites of the Late Cretaceous (~85 Ma) Balcones Igneous Province. The mineral assemblage consists of olivine, orthopyroxene, clinopyroxene and spinel, defining three rock types- dunite, harzburgite, and lherzolite (two lherzolites have very high modal pyroxene). The xenoliths are coarse grained, equigranular, with straight (triple junctions) to smoothly curving boundaries. Some olivine grains are cut by small serpentine veins. In some samples, small crystals of OPX and CPX occur inside olivine grains. The spinels are elongate, bleb-shaped and dark brown. Some spinels and CPX show corroded rims. Minor sulphide, apatite, and ilmenite are present in the serpentine veins. Electron microprobe analyses of olivines indicate a limited range of Fo (89-91). OPX compositions are En₈₉Fs₁₀ to En₉₁Fs₈ and CPX has a compositional range of En_49-55 Fs_4-5Wo_44-46. The Cr# of the spinel varies from 0.14-0.27, except the two pyroxene-rich lherzolites that have low Cr# of 0.05 and 0.08. Temperatures determined using the Ca in OPX thermometer of Brey and Kohler (1990), range between 900 and 1000°C. Bulk rock Al₂O₃, CaO and MgO vary from 1.49 to 3.70%, 0.43 to 2.49% and 42.55 to 48.74% respectively. Ternary plots of SiO₂- CaO-Al₂O₃-MgO (SCAM) components for a compilation of mantle samples display boundaries that define the dunite, harzburgite, lherzolite, or wehrlite-pyroxenite fields and provide a graphical basis for classification. Knippa xenoliths plot in the harzburgite and lherzolite field of the SCAM ternary diagram.Primitive mantle normalized REE and trace element patterns show three trends. Lherzolites have concave-downward REE patterns, indicating a lack of metasomatism. Harzburgites are enriched in LREE compared to HREE and the dunites have concave-upward REE patterns, indicating metasomatism.