Deciphering the Paleoproterozoic cooling history of the northeastern Trans-Hudson Orogen, Baffin Island (Canada), using 40Ar/39Ar step-heating and UV laser thermochronology

Abstract

The previously unstudied cooling and exhumation history of mid-crustal rocks exposed on southeastern Baffin Island (Canada) provides new insights into the post-orogenic evolution of the Paleoproterozoic Trans-Hudson Orogen (THO). New 40Ar/39Ar step-heat analyses of biotite, muscovite and phlogopite and core-to-rim intra-grain 40Ar/39Ar analyses of muscovite have a range of apparent ages compatible with slow regional cooling following peak metamorphism. Twenty-nine amphibolite- to granulite-facies rocks were dated using the 40Ar/39Ar step-heating laser (CO2) method. 40Ar/39Ar spot analyses were performed across muscovite grains from three samples using an ultraviolet (UV) laser to investigate intra-grain 40Ar/39Ar age variations. Step-heating apparent ages range from ca. 1788–1622Ma for biotite, 1720–1630Ma for phlogopite and 1729–1657Ma for muscovite. UV spot 40Ar/39Ar analyses in the three muscovite grains range from ca. 1661–1640Ma, 1675–1645Ma and 1680–1652Ma, with core-to-rim apparent age gradients of 20–30Myr. Previous studies resolved peak metamorphism in this region to between ca. 1860 and 1820Ma and identified late- to post-THO zircon and monazite populations at ca. 1800–1750Ma. Numerical diffusion models for Ar in muscovite were conducted to test different Proterozoic cooling and exhumation scenarios. Comparisons with our 40Ar/39Ar ages attest to cooling rates of ~1–2°C/Myr following peak metamorphism and ~1.5–2.5°C/Myr after ca. 1740Ma. Anomalously old apparent 40Ar/39Ar ages, in cases equivalent to U–Pb zircon rim and monazite ages, likely result from incorporation of excess Ar. The results suggest that mid-crustal rocks on southeastern Baffin Island remained hotter than ~420–450°C for ~150–200Myr after peak metamorphism, with subsequent slow cooling and denudation rates that are typical of Proterozoic orogens. The apparent absence of orogenic collapse implies that, despite high temperatures and estimated maximum crustal thicknesses comparable to those of large, hot orogens, the THO remained gravitationally stable during its terminal phase.