OXYGEN ISOTOPE EVIDENCE OF THE PRECAMBRIAN CONTINENTAL MARGIN IN THE NORTHERN GREAT BASIN OF UTAH AND NEVADA

Igneous zircons from Precambrian, Jurassic, Cretaceous, and Tertiary intrusive bodies of the northern Great Basin preserve the best record of magmatic oxygen isotope ratios. Zircons preserve more consistent d¹⁸O values than whole-rock or quartz in samples that may be altered by post magmatic processes. In order to compare the magmatic source of granitic rocks ranging in age from Precambrian to Tertiary, and that experienced variable metamorphism, zircon d¹⁸O values are crucial in order to establish tenable comparisons. The magmatic d¹⁸O of intrusive rocks in the Great Basin is important for determining the magmatic source and potentially identifying the location of the Precambrian continental margin at depth.

Values of d¹⁸O(Zrc) vary systematically in the Great Basin with crustal structure as determined by radiogenic isotopic ratios. Plutons emplaced east of the ⁸⁷Sr/⁸⁶Sr(initial)=0.706 isopleth have higher d¹⁸O(Zrc) than plutons intruded west of the 0.706 line. Average d¹⁸O(Zrc) values east and west of the 0.706 line are bimodal. Cretaceous and Jurassic plutons intruded east of the 0.706 line have, on average, d¹⁸O(Zrc) values that are 2‰ higher than plutons west of the 0.706 line. Analyses of d¹⁸O(Qtz) and d¹⁸O(WR) do not show the bimodal distinction across the 0.706 line.

The difference in d¹⁸O across the 0.706 line reflects the involvement of high d¹⁸O sedimentary rocks derived from the Precambrian craton in magmas intruded east of the 0.706 line while magmas west of the 0.706 line are dominated by lower d¹⁸O rocks derived from volcanic arcs. These data demonstrate the importance of analyzing refractory phases, such as zircon, for regional studies of oxygen isotope ratios investigating the structure of the crust. The correspondence of stable and radiogenic isotopic discontinuities in the Great Basin indicates that the 0.706 line is both a geochronologic and lithologic crustal boundary.