SULFUR ISOTOPE STUDY OF GRANITIC PLUTONS IN THE GREAT BASIN, WESTERN USA: IMPLICATIONS FOR MAGMA-CRUST INTERACTION AND CRUSTAL ARCHITECTURE

Sulfur isotope analyses of Jurassic, Cretaceous, and Tertiary plutons provide important information about the crustal architecture of the Great Basin. Major-element geochemical analyses and whole-rock sulfur isotope analyses have been conducted for 283 granitoid samples collected in the Great Basin. Rock geochemistry data indicate that the samples are dominantly metaluminous, I-type granitoids. The d³⁴S values obtained range from -3.2 to +20.7‰, with a mean value of +6.4‰. There is little difference in the d³⁴S ranges and mean values among each of the three age categories.

Examination of the sulfur isotope data using GIS demonstrates spatial trends in the data not readily apparent from other types of plots. Plots of the data on GIS-constructed maps show that most of the samples with relatively low d³⁴S values (< +4‰) lie to the west of the ⁸⁷Sr/⁸⁶Sr_I = 0.706 line, while the majority of the samples with relatively high d³⁴S values (> +11.2‰) lie to the east of that line. Additional analyses using GIS, especially kriging, show a concentration of relatively high sulfur isotope values close to the center of the Great Basin, specifically in north-central Nevada.

The spatial variation in d³⁴S values for these plutons reflects the influence of magma-crust interaction. Prior isotope and geochemical studies have outlined subsurface crustal boundaries in the Great Basin. Our data support previous findings, in that plutons with relatively low d³⁴S values indicate either limited interaction with surrounding crustal rocks, or interaction with crustal rocks having low d³⁴S signatures; we favor the latter interpretation. Samples with high d³⁴S are inferred to have interacted with rocks of late Proterozoic to Paleozoic age that contain sulfate that is ultimately of seawater origin. Complementary to other isotopic systems (O, Sr, Nd, Pb), sulfur displays potential as a geochemical tracer for subsurface boundaries in the Great Basin, and for other plutonic belts worldwide.