Internal fabrics of the Idaho batholith, USA

Abstract

The Idaho batholith of the North American Cordillera is a large and long-lived silicic intrusive center. We studied fabrics at the regional scale within the different intrusive suites of the Idaho batholith, using microstructural characterization, anisotropy of magnetic susceptibility measurements, and shape preferred orientation analyses. Each studied outcrop was collocated with existing U-Pb zircon geochronology and ongoing (U-Th)/He zircon thermochronology results. Three new 40 Ar/ 39 Ar biotite ages, collocated with the existing U-Pb zircon ages, constrain the cooling rates within the batholith. The presence of dominantly magmatic microstructures allows us to interpret the results relative to the U-Pb zircon ages and observe spatial and temporal patterns of fabric development. The early (pre-80 Ma) intrusive suites exhibit solid-state microstructures that show a consistent orientation only in the western part of the batholith. Fabrics in the 83-67 Ma Atlanta peraluminous suite are weak and inconsistent in orientation, despite emplacement during regional contraction. We hypothesize that the lack of consistently oriented fabrics in the Atlanta lobe results from either: (1) topographic effects that caused local extensional/neutral strain environments in the upper parts of a crustal plateau; or (2) emplacement in thin, horizontal magmatic sheets. In contrast, fabrics within the 66-53 Ma Bitterroot peraluminous suite are well developed and consistently oriented (NW-striking; NE-dipping), recording localized contraction during magmatism.

Incremental Emplacement of the Late Jurassic Midcrustal, Lopolith‐Like Qitianling Pluton, South China, Revealed by AMS and Bouguer Gravity Data

“…Thus, we suggest that those fabrics developed during cooling and crystallization of the magma without contribution from synor postmagmatic tectonics. This inference is in agreement with studies from other plutons (Byerly et al, 2016;de Saint-Blanquat et al, 2001).…”

Detrital Zircon Geochronology of Upper Cretaceous to Paleocene Sandstones From South‐Central Wyoming: Evidence for Middle Campanian Laramide Deformation

“…The Cordilleran magmatic arc and batholiths along the western margin of Laurentia started as early as the Permian (Dickinson & Lawton, ). Most of the Cordilleran magmatic arc is represented by a discontinuous belt of deeply exhumed granitic batholiths (Byerly et al, ; Ducea & Barton, ; Gaschnig et al, , , ; Paterson et al, ). Even though arc magmatism was widespread during early Late Cretaceous time (Ducea, ; Gaschnig et al, , ), magmatism in the Sierra Nevada in central, and southern California had ceased ~85 Ma (Bateman, ; Coleman & Glazner, ; Ortega‐Rivera, ; Ortega‐Rivera et al, ; Saleeby et al, ), while magmatism in the Idaho and Montana continued (Gaschnig et al, , ).…”

Introduction: EarthScope IDOR project (deformation and magmatic modification of a steep continental margin, western Idaho–eastern Oregon) themed issue

“…95 Ma). Byerly et al (2016) investigate the internal fabrics of the Idaho batholith using a combination of microstructures, shape-preferred orientation of minerals, and the anisotropy of magnetic susceptibility technique. This study utilizes the same sampling locations of the U-Pb zircon age and geochemistry studies of Gaschnig et al (2010Gaschnig et al ( , 2013 and Braudy et al (2016).…”

“…Gaschnig et al (2010) showed that the Atlanta lobe, which constitutes most of the central parts of the Idaho batholith, is distinctly younger than the Bitterroot lobe found only in the northern part of the Idaho batholith. Because the microstructures are mostly magmatic, Byerly et al (2016) utilize the existing geochronologic framework to interpret spatial and temporal patterns of fabric development within the batholith. They conclude that the oldest igneous rocks on the western edge of the batholith contain a consistent north-south-oriented fabric, the Atlanta lobe contains weak and nonconsistent fabric, and the young Bitterroot lobe exhibits a consistent northwest-striking foliation.…”

“…They conclude that the oldest igneous rocks on the western edge of the batholith contain a consistent north-south-oriented fabric, the Atlanta lobe contains weak and nonconsistent fabric, and the young Bitterroot lobe exhibits a consistent northwest-striking foliation. Byerly et al (2016) suggest that the Atlanta lobe may have been intruded during a time of crustal plateau formation, and the lack of consistently oriented fabric resulted from emplacement in a neutral tectonic stress environment (due to topographic effects), in an overall contractional setting, and/or sill-like emplacement. Fayon et al (2017) use (U-Th)/He zircon thermochronometry to constrain the exhumation of the southern part of the Idaho batholith.…”