U–Pb geochronology of Late Cretaceous and early Tertiary plutons in the northern Coast Mountains batholith

Abstract

U–Pb geochronologic studies demonstrate that steeply dipping, sheetlike tonalitic plutons along the western margin of the northern Coast Mountains batholith were emplaced between ~83 and ~57 (perhaps ~55) Ma. Less elongate tonalitic–granodioritic bodies in central portions of the batholith yield ages of 59–58 Ma, coeval with younger phases of the tonalitic sheets. Large granite–granodiorite bodies in central and eastern portions of the batholith were emplaced at 51–48 Ma. Trends in ages suggest that the tonalitic bodies generally become younger southeastward and that, at the latitude of Juneau, plutonism migrated northeastward across the batholith at ~0.9 km/Ma. Variations in the age, shape, location, and degree of fabric development among the various plutons indicate that Late Cretaceous – Paleocene tonalitic bodies were emplaced into a steeply dipping, dip-slip shear zone that was active along the western margin of the batholith. Postkinematic Eocene plutons were emplaced at shallow crustal levels. Inherited zircon components in these plutons range in age from mid-Paleozoic to Early Proterozoic and are coeval with detrital zircons in adjacent metasedimentary rocks. These old zircons, combined with evolved Nd isotopic signatures for most plutons, record assimilation of continental crustal or supracrustal rocks during the generation and (or) ascent of the plutons.

Zircon (U-Th)/He thermochronometry: He diffusion and comparisons with 40Ar/39Ar dating

“…This sample was collected near sea level, in Tracy Arm of Holkham Bay, approximately 200 km south of Juneau. Gehrels et al (1991) obtained a zircon U/Pb age of 60.4 ϩ1.3…”

U-Th-Pb geochronology of the Coast Mountains batholith in north-coastal British Columbia: Constraints on age and tectonic evolution

“…More recent geologic and/or geochronologic studies in the region were conducted by Douglas (1986), Heah (1990Heah ( , 1991, Ingram and Hutton (1994), Klepeis et al (1998), Crawford et al (1987Crawford et al ( , 1999Crawford et al ( , 2000, Saleeby (2000), and Gehrels (2001) in southernmost southeast Alaska and northernmost coastal British Columbia, by Armstrong and Runkle (1979), Harrison et al (1979), Parrish (1983), van der Heyden (1989van der Heyden ( , 1992, Gareau (1989Gareau ( , 1991aGareau ( , 1991bGareau ( , 1991c, Gehrels et al (1991), Gareau et al (1997), Chardon et al (1999), Andronicos et al (1999Andronicos et al ( , 2003, Klepeis and Crawford (1999), Gareau and Woodsworth (2000), Hollister and Andronicos (2000, and references therein), Rusmore et al (2000Rusmore et al ( , 2001Rusmore et al ( , 2005, Butler et al (2001aButler et al ( , 2002Butler et al ( , 2006, Friedman et al (2001), Chardon (2003), Davidson et al (2003), and Hollister et al (2004) in central portions of the study area, and by Rusmor...…”

“…The western margin of the axial portion of the Coast Mountains is characterized by distinctive steeply dipping sills of foliated tonalite that can be traced for tens of kilometers along their northwest-southeast strike, but that are only a few kilometers (locally as much as 10 km) in width (Brew and Ford, 1978;Gehrels et al, 1991;Wheeler and McFeely, 1991). These bodies continue northwestward, apparently continuously, from south of Bella Coola to north of Skagway, a distance of at least 1000 km.…”

“…A regional analysis of U-Pb ages (Gehrels et al, 1991) indicates that the tonalitic sills become younger southeastward, from mostly 70 Ma ages in the northwest to mostly 60 and 50 Ma ages in the southeast. There is also a consistent eastward younging of several million years within individual bodies and in successive parallel bodies.…”

Logan Medallist 1. Seeking the Suture: The Coast-Cascade Conundrum

“…1, 2; e.g. Gehrels et al 1991;Saleeby 2000;Gehrels 2001). The Late Cretaceous-early Cenozoic plutons of the northern Coast Mountains intrude metamorphic rocks whose protoliths are late Neoproterozoic-early Paleozoic, quartzrich sedimentary rocks correlated with rocks of the Yukon-Tanana terrane and volcanogenic rocks correlated with those of Stikinia (Currie and Parrish 1997;Gehrels and Kapp 1998;Gehrels 2001;Gehrels et al 2009).…”