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Oxygen isotope trends of granitic magmatism in the Great Basin: Location of the Precambrian craton boundary as reflected in zircons

Elizabeth M. King^,1, John W. Valley^,2, Daniel F. Stockli^,3 and James E. Wright^#,4

¹ Department of Geography-Geology, Illinois State University, Campus Box 4400, Normal, Illinois 61790, USA
² Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA
³ Department of Geology, University of Kansas, 120 Lindley Hall, Lawrence, Kansas 66045, USA
⁴ Department of Geology, University of Georgia, Geography-Geology Building, Athens, Georgia 30602, USA

The ¹⁸O 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 ¹⁸O values in zircon with age reconciles previous differences in interpretation based on ¹⁸O_WR and radiogenic isotope data. The new ¹⁸O_Zrc 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, ¹⁸O_Zrc values also vary systematically with crustal structure as determined by radiogenic isotope systematics. Plutons emplaced east of the ⁸⁷Sr/⁸⁶Sr_i = 0.706 isopleth have higher ¹⁸O_Zrc values than plutons intruded west of the 0.706 line. However, analyses of ¹⁸O values in quartz do not show the bimodal distinction across the 0.706 line owing to subsolidus alteration. On the basis of ¹⁸O_Zrc, 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 ¹⁸O values across the 0.706 line reflects the involvement of high-¹⁸O 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-¹⁸O rocks derived from juvenile volcanic arcs. Crustal boundaries and discontinuities in the Great Basin determined by radiogenic isotope systems agree with discontinuities observed in ¹⁸O_Zrc values; this agreement indicates that the 0.706 line marks a geochronologic and compositional discontinuity in the basement rocks.

Key Words: Great Basin • oxygen isotopes • zircon • terrane boundaries

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