Daniel Pfeiffer

            The ages of monazite crystals from metapelitic rock samples from the southwest Nova Scotia has been examined using in-situ dating methods.  The rocks are from the Government Point and Cunard formations of the Goldenville Group and Halifax Group respectively which are characterized an andalusite- and staurolite-bearng schists.  The protoliths are thought to be turbiditic sediments deposited off the passive margin of Gondwana and subsequently deformed during the assembly of Pangaea.  The peak equilibrium mineral assemblage suggests a lower to medium amphibolite facies under conditions of approximately 550-600^oC and 2.5 kb.  These metasedimentary rocks contain an inclusion-rich staurolite poikiloblastic fabric that has been overprinted by inclusion-poor staurolite porphyroblasts.  These textures are interpreted as evidence of polyphase metamorphism associated with the lower- to mid-Devonian Neoacadian orogeny.

            Monazite, a LREE phosphate, is widely used as a chronometer due to its unique chemical signature and high Th and U contents.  Transmitted light microscopy textural characterization and electron microprobe chemical microanalysis has been performed on samples from the region to constrain the timing of the distinct tectonometamorphic events recorded by these samples.  Compositional mapping of monazite grains in these rocks has identified concentric zonation defined by Y concentration resulting from the breakdown of garnet, staurolite and xenotime under prograde and retrograde metamorphic conditions.  This suggests multiple stages of metamorphism preserved within monazite.

            Chronology of the individual metamorphic monazite grains yielded an age range of 370±6 which represent the post-deformation contact metamorphism associated with the Acadian orogeny that affected the rocks of southwest Nova Scotia.  Analysis has also determined a significant portion of the monazite grains are detrital (ages 1000-450 Ma) with partial resetting corresponding to post-depositional deformation and metamorphism.

Keywords: Monazite, Geochemistry, Geochronology, Devonian, Neoacadian, Orogeny, Metamorphism, Amphibolite.
Pages: 76
Supervisor: Richard Cox