ABSTRACT We addressed fundamental questions about the lithology, age, structure, and thermal evolution of the deep crust of the retroarc hinterland of the North American Cordilleran orogen through systematic investigation of zircons from Cretaceous plutons in the Snake Range and Kern Mountains of east-central Nevada. Geochronological (U-Pb) and geochemical (trace element, O and Hf isotopes) characterization of pre- and synmagmatic growth domains of zircons, coupled with traditional petrologic methods (petrography, field relationships, and whole-rock major-element, trace-element, and Sr-Nd and Pb isotope geochemistry), fingerprinted temporal variations in crustal contributions to magmatism. The samples are typical felsic, peraluminous Cordilleran interior granitoids that formed between 102 ± 2 Ma and 71 ± 1 Ma (95% confidence). Over the entire time span of magmatism, 87 Sr/ 86 Sr initial , εNd ( t ) , 208 Pb/ 204 Pb, and εHf ( t ) exhibit incrementally more “crustal” ratios. The oldest and youngest samples, respectively, predate and postdate all published timing constraints of Cretaceous peak metamorphism in the region and exhibit the least and most radiogenic whole-rock isotopic results in the study (87 Sr/ 86 Sr initial = 0.7071 vs. 0.7222; εNd ( t ) = −3.4 vs. −18.8; 208 Pb/ 204 Pb = 38.8 vs. 40.1). Accordingly, the least intrasample variability of εHf ( t ) , δ 18 O Zrc , and trace-element ratios in magmatic zircon domains is also observed in these oldest and youngest samples, whereas greater intrasample variability is observed in intermediate-age samples that intruded during peak metamorphism. The geochemistry of zircon growth in the intermediate-age samples suggests assimilation of partially molten metasedimentary crust led to increased heterogeneity in their magma chemistry. Interaction of magmas with distinctive crust types is indicated by contrasts between four categories of inherited zircon observed in the studied intrusions: (1) detrital zircon with typical magmatic trace-element ratios; (2) zircon derived from high-grade 1.8–1.6 Ga basement; (3) zircon with anomalously low δ 18 O of uncertain origin, derived from 1.7/2.45 Ga basement (or detritus derived thereof); and (4) zircon from variably evolved Jurassic–Early Cretaceous deep-seated intrusions. The progression of zircon inheritance patterns, correlated with evolving geochemical signatures, in Late Cretaceous granitic plutons is best explained by early, relatively primitive intrusions and their penecontemporaneously metamorphosed country rock having been tectonically transported cratonward and superposed on older basement, from which the later, more-evolved Tungstonia pluton was generated. This juxtaposition consequentially implies tectonic transport of synorogenic plutonic rocks occurred in the Cordilleran hinterland during the Sevier orogeny as a result of the interplay of retroarc magmatism and convergent margin tectonism.

Abstract This field guide covers the Precambrian geology of the western portion of the Clearwater complex and surrounding area in north-central Idaho in the vicinity of Marble Creek within the St. Joe National Forest. The regional geology of the Marble Creek area includes Precambrian basement orthogneisses, possible basement metasupracrustal rocks, and overlying metamorphosed Belt Supergroup strata. These rocks are exhumed within the western portion of the Cretaceous-Eocene Clearwater metamorphic core complex. This guide focuses on the western part of the Clearwater complex in the vicinity of Marble Creek. Outcrops of Paleoproterozoic basement and overlying Mesoproterozoic metasedimentary units provide a better understanding of the Precambrian magmatic and metamorphic history of this region. The road log in this guide describes the regional geology in a south to north transect from Clarkia, Idaho, to the confluence of Marble Creek with the St. Joe River.