The δ18O values of magmas in the Great Basin help to decipher the tectonic assembly of North America and to determine the magmatic source and potentially the composition of the crust at depth. Igneous zircons from Precambrian, Jurassic, Cretaceous, and Tertiary intrusive bodies of the northern Great Basin best preserve the rec ord of magmatic oxygen isotope ratios. The variation of δ18O values in zircon with age reconciles previous differences in interpretation based on δ18OWR and radiogenic isotope data. The new δ18OZrc data support the results of previous radiogenic isotope studies documenting the increased availability of crustal and sedimentary components to magmas in the Cretaceous during the Sevier orogeny and a return to a larger proportion of mantle-derived components in the Tertiary during Basin and Range extension.

In the Great Basin, δ18OZrc values also vary systematically with crustal structure as determined by radiogenic isotope systematics. Plutons emplaced east of the 87Sr/86Sri = 0.706 isopleth have higher δ18OZrc values than plutons intruded west of the 0.706 line. However, analyses of δ18O values in quartz do not show the bimodal distinction across the 0.706 line owing to subsolidus alteration. On the basis of δ18OZrc, plutons intruding the Walker Lane belt are indistinguishable from other plutons emplaced west of the 0.706 line despite significant tectonic displacement and rotation of the belt.

The difference in δ18O values across the 0.706 line reflects the involvement of high-δ18O Precambrian and Paleozoic sedimentary rocks of the continental margin and Precambrian craton in magmas intruded east of the 0.706 line, whereas magmas west of the 0.706 line are dominated by lower-δ18O rocks derived from juvenile volcanic arcs. Crustal boundaries and discontinuities in the Great Basin determined by radiogenic isotope systems agree with discontinuities observed in δ18OZrc values; this agreement indicates that the 0.706 line marks a geochronologic and compositional discontinuity in the basement rocks.

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