DECOUPLING OF ZIRCON-WHOLE ROCK TRACE ELEMENT SYSTEMATICS IN THE IDAHO BATHOLITH AND CHALLIS MAGMATIC SUITE

The Idaho segment of the North American Cordillera is an area of pivotal importance due to its residence along the Laurentian cratonic boundary, denoted by a sharp contrast in ⁸⁷Sr/⁸⁶Sr_(i). The Idaho batholith was emplaced entirely within North American crust, to the east of accreted terranes belonging to the Blue Mountains Province, and has provided significant information about the evolution of continental arc magmatism in the Cordillera during Cretaceous-Paleogene time; specifically, the geochemical manifestation of melting and assimilating different crustal materials. Temporal trends were explored by Gaschnig et al. (2011) through whole-rock major and trace element analysis and Hf, Nd, and Pb isotopic analysis, as well as zircon Hf isotope analysis. The missing piece to this geochemical puzzle is zircon trace element data that can pair with the published whole-rock information and fine tune magmatic processes occurring within this system. Previous conclusions emphasize typical arc magmatism in early Idaho batholith phases, with metaluminous bulk compositions and isotopic tracers that suggest mixing of mantle- and crustal-derived material. In contrast, younger phases show evidence of substantial melting of Archean and Proterozoic crust and are mildly to strongly peraluminous.

In this study, we analyzed 217 zircons for trace element geochemistry via LA-ICP-MS and compared their petrogenetic indicators to similar whole-rock proxies to establish consistency, but decoupling of the two systems was observed. Relationships were evaluated using a correlation matrix of seven whole-rock (La/Yb, Sr/Y, Gd/Yb, Lu/Hf, Eu/Eu* ASI, and ⁸⁷Sr/⁸⁶Sr_(i)) and zircon (ƒO₂, Ti, Th/U, Hf/Y, Nb/Yb, Eu/Eu*, and Gd/Gd*) isotopic and elemental ratios. Whole-rock ratios have strong temporal correlations, while zircon ratios have correlation coefficients near zero. Biplots of zircon trace element ratios vs. age show trends that contrast with similar whole-rock proxies for early Idaho batholith magmatism and the Challis magmatic suite. This observation suggests that Idaho zircons provide transient snapshots of the evolving melt compositions and are privy to more subtle changes in petrologic factors, such as magmatic source material, redox conditions, and crystallization environment relative to the whole-rock record.