36 Implications of Lithosphere-Crust interactions within the Northern Cordillera

Friday, 17 May 2024
8:00 AM - 12:00 PM
Cedar Ballroom

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

Paper number: 36-9
Program: Theme Sessions
Time: 10:20 AM – 10:40 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 56, No. 4

doi: 10.1130/abs/2024CD-399875

© Copyright 2024 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.