2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 33
Presentation Time: 9:00 AM-6:00 PM

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


RANGEL, Evelyn1, RICHARDS, Ian2, FERGUSON, Kurt3 and GREGORY, Robert T.2, (1)Huffington Department of Earth Sciences, Southern Methodist University, Dallas, TX 75275, (2)Stable Isotope Laboratory, Southern Methodist University, 3225 Daniel Avenue, Dallas, TX 75275, (3)Stable Isotope Laboratory, Southern Methodist University, Dallas, TX 75275, geodork54@yahoo.com

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 Sri values for two major plutons, Lone Grove and Enchanted Rock (0.7048 and 0.7061 respectively, Garrison et al.,1979).