PERIDOTITE XENOLITHS FROM THE KNIPPA QUARRY IN CENTRAL TEXAS

The Knippa quarry in Uvalde County, TX is producing aggregate from Chatfield Hill, a Late Cretaceous (77-86 Ma) melilite olivine nephelinite of the Balcones Igneous province. This quarry exposes mantle xenoliths providing a glimpse of the upper mantle composition beneath south-central Texas. The mineral assemblage for the Knippa peridotite xenoliths consist of olivine, orthopyroxene, clinopyroxene and spinel, indicating they come from <~70 km depth and thus represent the lithosphere. The Knippa peridotites consist of three rock types- dunite, harzburgite and lherzolite, all of them are spinel-bearing. All are coarse grained (> 2mm), 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 and cpx grains are corroded. The spinels are elongate and bleb-shaped and dark brown in plane polarized light. Some spinels show corroded rims. Minor sulphides, either pyrite or pyrrhotite are also present. The modal proportion of cpx in the xenoliths ranges from 2-8% with one exception of 15%.

Electron microprobe analyses of olivines indicate a limited range of Fo 87-91. The Wo content of cpx ranges from 52 -54. The En content of opx ranges from 83- 84 with one exception of En 80. The Cr# of the spinel varies from 0.21-0.27 with one sample having a Cr# of 0.17. These spinel compositions show that the Knippa xenoliths are less depleted than those of Kilbourne Hole, NM and some of the San Carlos, AZ xenoliths. Temperatures determined using the Ca in opx thermometer of Brey and Kohler (1990), are 900-963°C, with two exceptions that show temperatures of 997 and 1007°C. We conclude that the mantle beneath this part of central Texas is relatively undepleted, and we are continuing our studies to better understand the significance of the Knippa xenolith locality.