SULFUR ISOTOPE CONSTRAINTS ON PLUTONIC ROCKS OF THE GREAT BASIN: IMPLICATIONS FOR MAGMA-CRUST INTERACTION AND CRUSTAL ARCHITECTURE

Sulfur isotope analyses of Jurassic (J), Cretaceous (K), and Tertiary (T) plutons provide important information on the crustal architecture and development of hydrothermal systems in the Great Basin. Whole-rock sulfur isotope analyses have been conducted for 190 granitoid samples collected in the Great Basin. Rock geochemistry and oxygen isotope data, as well as the range and mean δ³⁴S values reported here, indicate that the samples collected are dominantly S-type granitoids, suggesting a significant sedimentary component for the plutonic melt. The δ³⁴S values obtained range from -3.2 to +20.7‰, with a mean value of +7.0‰. The mean δ³⁴S values for all samples are very similar (J = +6.2‰, K = +7.3‰, and T = +6.8‰). However, the range for Cretaceous samples is much larger (-0.5 to + 20.7‰) than for Jurassic (+1.4 to +11.9‰) and Tertiary (-0.5 to +11.7‰) plutons, possibly indicating variable degrees of magma-crust interaction during the Cretaceous. There is spatial variation in both the Cretaceous and Jurassic samples, but none noted in the Tertiary samples. δ³⁴S values of Cretaceous samples are generally heavier in the more southerly latitudes (~38°N), and lighter in the more northerly latitudes (~42°N); Jurassic samples have lighter δ³⁴S values at the margins of the same zone, and heavier values in the center.

We propose that the variation in δ³⁴S values for these plutons reflects the influence of magma-crust interaction. Geological and geochemical data suggest there is a major crustal boundary that should underlie this region, representing a Precambrian rift zone. The δ³⁴S signatures of plutons that intruded into the rift zone reflect the incorporation of sedimentary sulfate sulfur. Plutons marginal to the rift zone passed through different crust, lacking in seawater sulfate, and thus did not incorporate the same signature. Tertiary plutons may have come through the crust so rapidly that they did not interact significantly with the crustal material, thus their signatures do not appear to reflect the rift zone sedimentary rocks.