MID-PROTEROZOIC GRANITES OF THE LLANO UPLIFT: VARIED SOURCES AS REFLECTED BY STABLE ISOTOPES

Mid-Proterozoic A-type granites of the Llano uplift, central Texas were emplaced along the southern margin of Laurentia during the final stages of the ~800 Ma assembly of the Laurentian supercontinent. An extensive oxygen and hydrogen isotope study has been carried out for all of the major syn- and post-kinematic plutons of the Llano uplift in order to determine these sources. According to Taylor (1968), mantle-derived granites or those containing both mantle and crustal-derived melts commonly display values between +6.0‰ and +7.8‰, whereas those derived predominantly from metasediments display values >+7.8‰. δ¹⁸O and δD values of these types of "true" granites form the basis of the original normal igneous rock range (e.g. Taylor, 1968). δ¹⁸O values for thirty-seven whole rock, quartz, potassium feldspar and biotite samples have been determined as well as a number of δD values of biotite. Values of quartz separates range from +8.4‰ to +11.6‰ (mean = 9.4‰); potassium feldspar from +7.0‰ to +10.2‰ (mean = +8.1‰); biotite from +2.0‰ to +5.1‰ (mean = +3.6‰); δD from -77.1‰ to -104.8‰ (mean = -93‰). Measured fractionations between quartz and potassium feldspar range from +1.0‰ to +2.6‰ (mean = 1.4‰), suggesting equilibrium isotope fractionation during crystallization of the granitic magmas and limited subsolidus open system exchange. Measured whole rock values for all plutons of the uplift (both coarse- and fine-grained) range from +7.0‰ to +10.1‰. The δ¹⁸O and δD data suggest that magmas intruded into the eastern uplift contain a significant mantle component, whereas those intruded into the western uplift contain a more substantial crustal component. Therefore, the magmas intruded into the Llano region during the Grenville orogeny reflect heterogenous sources. This conclusion is supported by εNd data from previous work: +3.0 - +4.3 (Patchett and Ruiz, 1987; Smith et al., 1997; Roller, 2004) and Sr_i values for two major plutons, Lone Grove and Enchanted Rock (0.7048 and 0.7061 respectively, Garrison et al.,1979).