A Stable Isotopic Study of Mid-Proterozoic Granites of the Llano Uplift, Central Texas

Oxygen and hydrogen isotope analyses have proven to be effective indicators of the sources of granitic magmas, particularly in establishing relative contributions of mantle versus crustal sources. In this study, oxygen isotope ratios of whole rock, quartz, potassium feldspar, and both oxygen and hydrogen isotope ratios of biotite have been measured for the major plutons of the Mid-Proterozoic Llano uplift, central Texas in order to determine the sources of the A-type granites. These syn- and post-kinematic plutons were emplaced 1.19 – 1.07 Ga ago along the southern margin of the Laurentian continent during the Grenvillian orogeny.

Values for quartz separates are +8.4‰< d¹⁸O>+11.6‰, mean +9.4‰ (n=37); potassium feldspar: +7.0‰< d¹⁸O>+10.2‰, mean +8.1‰ (n=37); biotite: +2.0‰< d¹⁸O>+5.1‰, mean +3.6‰ (n=26) and -77.1‰<dD<-104.8‰, mean -93.0‰. Measured fractionations between quartz and potassium feldspar separates are +0.1< d¹⁸O_qtz-fsp<+2.6, mean +1.4, suggesting that subsolidus open system exchange was limited for these rocks.

Mantle-derived granites or those containing a combination of both mantle and crustal derived melts commonly display d¹⁸O values between +6‰ and +7.8‰ (Taylor, 1968), whereas those containing d¹⁸O-rich metasediments display values >7.8‰. Measured whole rock d¹⁸O values for the major plutons of the uplift (both coarse- and fine-grained) range from +7.0< d¹⁸O>+10.1‰. The data suggest that the magmas intruded into the eastern uplift contain a significant mantle component whereas those intruded into the western uplift contain a higher amount of crustal component, suggesting heterogenous sources. This conclusion is supported by ε_Nd values from previous work (+3.0 – +4.3; Patchett and Ruiz, 1987; Smith et al., 1997; Roller, 2004) and Sr_i values for two plutons (0.7048 and 0.7061; Garrison et al., 1979). However, these silica-oversaturated rocks are extremely potassium-rich, which runs contrary to their mantle-like ε_Nd and Sr_i values.