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Published July 1989 | public
Journal Article

Isotopic evidence for the origin of Mesozoic and Cenozoic granitic plutons in the northern Great Basin


¹⁸O/¹⁶O ratios of granitic rocks in western North America reflect source-rock lithologies, providing insight into the geology of the deep continental crust. In a 400-km-wide, 800-km-long, east-west transect through the northern Great Basin, the Mesozoic and Cenozoic plutons define three zones: western low-¹⁸O zone (WZ), δ¹⁸O = +6 to +8.5; central high-¹⁸O zone (CZ), δ¹⁸O = +9 to +13; eastern zone (EZ) (Utah), δ¹⁸O = +7 to +9. The WZ-CZ boundary is analogous to the δ¹⁸O "step" in the Peninsular Ranges batholith (PRB). A similarly abrupt (⁸⁷Sr/86Sr)i "step" (<0.708 to the west and >0.710 to the east) defined by Farmer and DePaolo (1983) lies 150-200 km east of the WZ-CZ ¹⁸O/¹⁶O step, dividing the CZ into two subzones: V-type in the west, corresponding to the eastern half of the PRB (but much wider due to regional Cenozoic extension); and S-type to the east. A plausible source for the high-⁸⁷Sr, low-ϵ_(Nd), S-type subzone is a late Precambrian (miogeoclinal) metasedimentary section, whereas the V-type source appears to be a Phanerozoic volcanogenic (eugeoclinal) accreted terrane. Both source regions have very high δ¹⁸O (+9 to +16) but radically different ⁸⁷Sr/⁸⁶Sr (and ϵ_(Nd)); thus, we would not place the buried edge of the > 1.5 Ga craton beneath the ⁸⁷Sr/⁸⁶Sr step, but farther east near the EZ boundary. We suggest instead that this step is a sharp lithologic and "age" boundary (a suture-zone?) within the giant prism of sediments and volcanics in Nevada.

Additional Information

Supported by National Science Foundation Grant EAR-83-13106. We thank G. Lang Farmer and Donald J. DePaolo for supplying some of the samples studied in this research, Farmer and Z. E. Peterman for helpful reviews, and Robert E. Criss, Robert I. Hill, Leon T. Silver, and Stephen M. Wickham for their help and collaboration on the problems discussed in this paper, as well as for numerous discussions over the years on the origin of granites. California Institute of Technology, Publications of the Division of Geological and Planetary Sciences Contribution No. 4592.

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October 24, 2023