General chemical composition of Precambrian crust in the Llano Uplift, central Texas
Johnson, Linda Ann
Rogers, John J. W.
Master of Arts
Many investigations have been made of the late Proterozoic crystalline rocks in the Llano Uplift, central Texas. No comprehensive chemical treatment of this shield area, however, has as yet been reported. Published chemical and modal analyses and planimetric analysis of geologic maps have been applied in this study to generate estimates of major element chemistry for important contributing lithologies and for the Llano Precambrian crust as a whole. Comparison of average compositions derived for metamorphic rock units in the Llano exposure with published compositions for undeformed rock sequences indicates that: (1) the basal pink gneiss (Valley Spring Gneiss) is similar chemically to rhyolite but slightly higher in Fe, Mg, and Ca; and (2) parts of the overlying, more mafic, metasedimentary and metavolcanic unit (Packsaddle Formation) are chemically similar to typical eugeosynclinal lithologies but that it does contain some abnormally high SiO2 and low AI2O3 metasedimentary rocks. Comparison of the average chemical composition of the one-billion-year-old continental crust exposed in the Llano area with average chemical compositions derived for Archean and Proterozoic rocks exposed in the Canadian shield indicates a continuation of trends of crustal chemical evolution proposed by Fahrig and Eade (1971). Decreases in MgO, Na2O, and CaO, and increases in are observed from the Archean to the youngest Precambrian Llano crustal averages. Higher SiO2 and total alkali , and lower Al2O3 and total mafic oxide components are also evident in the Llano area continental crust relative to the older Canadian shield.