OXYGEN ISOTOPIC EVIDENCE FOR THE ORIGIN OF MESOPROTEROZOIC GRANITES OF THE LLANO UPLIFT, CENTRAL TEXAS

Over 60 new δ¹⁸O values have been measured on mineral separates and whole rocks from the granites and some of their country rocks from the Llano uplift, central Texas, an exposed Precambrian terrain associated with the deformation of Laurentian crust during the Grenville orogeny. The results are: 1) quartz: +8.7 < δ¹⁸O < +10.8, mean +9.5 (20 samples); 2) potassium feldspar: +7.2 < δ¹⁸O < +9.7, mean +8.3 (20 samples), and 3) biotite: +2.9 < δ¹⁸O < +4.9, mean + 3.6 (13 samples). The measured fractionations between quartz and feldspar are +0.2 < Δ¹⁸O_qtz-fsp < +1.7, mean Δ¹⁸O_qtz-fsp ≈ +1.2. This suggests limited subsolidus open system exchange for these rocks. Estimated whole rock δ¹⁸O values on the generally coarse-grained granites lie within the range +7.5 to +10.5. A limited number of whole rock values obtained from country rocks are: Packsaddle Schist (+5.9 and +9.1), Valley Spring Gneiss (+8.0 < δ¹⁸O < +10.8, 4 samples) and a single analysis of the Big Branch Gneiss (+7.0). Previous work on the granites indicates that these silica over-saturated K-rich rocks have relatively low values for initial Sr (0.7055-0.7062, Garrison et al. 1979) and positive ε_Nd values (Patchett and Ruiz, 1987). Interpretations of Sr data indicate mantle or lower crustal sources for granitic magmas (Garrison, 1979), and ε_Nd values suggest either mantle-derived components or less-evolved deep-seated equivalents of country rock (Patchett and Ruiz, 1989). The oxygen isotopes seem consistent with melting of ophiolitic or eugeosynclinal source rocks, however the K-rich character of the true granites (K₂O ≈ 5%) runs contrary to the expected trondhjemitic suites for the observed isotope signatures.