40AR/39AR GEOCHRONOLOGY OF BIOTITE FROM DUCTILE SHEAR ZONES OF THE ELLESMERE-DEVON CRYSTALLINE TERRANE, NUNAVUT, CANADIAN ARCTIC

We present a ⁴⁰Ar/³⁹Ar geochronological analysis of biotite from thin ductile shear zones in Paleoproterozoic granulite-facies gneisses. Quartzofeldspathic basement rocks in southeast Ellesmere Island display centimeter-scale E- to NE-striking sinistral and dextral mylonite zones offsetting pegmatitic dikes. Samples were taken from nearshore outcrops at Hayes Fiord, Pim Island, NE of the Leffert Glacier and NW of Cape Isabella. Biotite clusters pseudomorph orthopyroxene as the result of post-granulite facies metamorphism in the gneisses. Biotite in mylonitic and ultramylonitic fabrics is found as flattened clusters and also as individual crystals in shear bands. This fabric-oriented biotite is synkinematic green biotite related to ductile shearing. Eight samples were dated, including biotite from five mylonites, one deformed pegmatite, one tonalite and muscovite from a pegmatite. Major element x-ray maps produced by wavelength dispersive spectroscopy via electron microprobe analysis demonstrate that all biotite samples are chemically homogenous. Backscattered electron images and electron dispersive spectroscopy via scanning electron microscopy confirm biotite grains lack intercrystalline layering with other K phases. A recent U-Pb date shows that the Hayes Fiord gneisses are 1941 ± 4 Ma. Step heating analyses of mica at the University of Vermont yielded Paleoproterozoic dates. Biotite from the deformed pegmatite was 2051 ± 26 Ma. The pegmatitic muscovite was 1977 ± 35 Ma. Biotite dates ranged from 1874 ± 12 Ma to 1838 ± 14 Ma for the six mylonites. The apparent age spectra formed plateau ages in all but one mylonite sample. The intrusion date of the deformed pegmatitic biotite date implies protolith emplacement before 2077-2025 Ma. This date is untenable because it predates the oldest zircon ages and implies excess Ar. Protolith emplacement before 2012-1942 Ma as implied by the pegmatitic muscovite corroborates the aforementioned U-Pb date. Biotite dated from ductile shear zones signals the latest deformation in the basement, which was active by as early as 1886 Ma.