Document Type


Date of Degree

Spring 2018

Degree Name

MS (Master of Science)

Degree In


First Advisor

Gilotti, Jane A

First Committee Member

McClelland, William C

Second Committee Member

Webb, Laura E


This thesis presents a 40Ar/39Ar geochronological analyses of biotite from thin ductile shear zones in Paleoproterozoic granulite-facies gneisses from the Ellesmere-Devon crystalline terrane, Nunavut, Canada. The gneisses are part of the Paleoproterozoic Thelon tectonic zone. U-Pb dates of zircon show that the gneisses have magmatic protolith ages ranging from 2007–1958 Ma. The quartzofeldspathic gneisses in southeast Ellesmere Island display centimeter-scale E- to NE-striking sinistral and dextral mylonite zones offsetting pegmatitic dikes that are the last stage of ductile deformation of the basement rocks. Samples were taken from nearshore outcrops at Hayes Fiord, Pim Island, NE of the Leffert Glacier and NW of Cape Isabella. Biotite clusters replace 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 defining shear bands related to mylonitization. Eight samples were dated, including biotite from five mylonites, one deformed pegmatite, one tonalite and muscovite from a pegmatite. Major element X-ray maps demonstrate that the biotite is chemically homogenous. Backscattered electron images and electron dispersive spectroscopy via scanning electron microscopy confirm that biotite lacks intercrystalline layering with other K phases. Step-heating analysis of mica at the University of Vermont yielded Paleoproterozoic 40Ar/39Ar ages. The apparent age spectra form plateau ages in all but one mylonite sample. Biotite from a protomylonite was 2051 ± 26 Ma, older than the protolith ages obtained from U-Pb zircon geochronology, and most likely indicates excess Ar. Pegmatitic muscovite was 1977 ± 35 Ma. Biotite dates range from 1874 ± 13 Ma to 1838 ± 14 Ma for the five mylonites without excess Ar. Biotite dated from ductile shear zones signals the latest deformation in the basement, which was active as early as 1887 Ma.


40Ar/39Ar, biotite, ductile shear zones, Ellesmere-Devon crystalline terrane, Taltson-Thelon orogeny, Thelon magmatic zone


ix, 40 pages, 34 unnumbered pages


Includes bibliographical references (pages 24-28).


Copyright © 2018 Brandon Christopher Caswell