AV俱乐部

The Canada-Nunavut Geoscience office completed the first of two field seasons of a regional bedrock and surficial mapping project on Hall Peninsula, eastern Baffin Island, during the summer of 2012. Hall Peninsula is situated within the core of the Himalayan-scale Paleoproterozoic Trans-Hudson Orogen and consists of an eastern Archean orthogneiss domain and a western Paleoproterozoic metasedimentary and plutonic domain. The Newton Fiord study area is on the southwestern coast of Hall Peninsula and comprises two regions of contrasting styles of deformation within close proximity (ca. 4 km²). The eastern study area lies on the limb of a regional F₂ fold and has experienced near-complete transposition of pre-existing fabric elements. The western study area lies in the hinge zone of a regional F₂ fold and provides a window into early deformation structures. Both areas consist generally of metasedimentary rocks, primarily pelite, intruded by orthopyroxene-monzogranite and late garnet-leucogranite.
Linked field observations and microtectonic analysis have revealed two main tectonometamorphic events each of which can be further characterized based on relative chronology of mineral growth and fabric formation. The first event, D₁/M₁, is characterized by aligned fibrolitic sillimanite inclusions (M_1A) in garnet porphyroblasts, followed by voluminous garnet-cordierite-bearing leucosome formation (M_1B). Field evidence for D₁ fabric elements is subtle but convincing where S₁ is recorded in the orthopyroxene-monzogranite but is cut by the garnet-leucogranite. Within the pelitic rocks, the M_1A assemblage is only preserved in garnet porphyrblasts and consists of garnet + sillimanite + K-feldspar + ilmenite + cordierite + quartz +/- rutile, whereas the M_1B melt assemblage is plagioclase + cordierite + K-feldspar + quartz +/- sillimanite +/- garnet. Both assemblages suggest equilibration above the biotite dehydration melting reaction. The second event, D₂/M₂, consists of moderately to shallowly west-dipping gneissosity (S_2A) defined by interlayered leucosome and sillimanite-biotite mesosome in the pelites. Progressive D₂ deformation resulted in moderately west-inclined F_2B folds associated with a strong hinge-parallel lineation defined by biotite and coarse matrix sillimanite (L_2B). The M₂ mineral assemblage associated with both phases of D₂ deformation consists of garnet + sillimanite + biotite + ilmenite + K-feldspar + plagioclase + quartz +/- melt, indicating equilibration below the biotite dehydration melting reaction and the addition of H₂O most likely from the breakdown of cordierite and crystallization of melt. These data suggest a relative chronology of events beginning with a weak S₁ foliation at granulite facies, which has been attributed to contact metamorphism as a result of orthopyroxene monzogranite emplacement. This led to a voluminous garnet-bearing partial melting phase. Progressive D₂ deformation evolved from the development of a strong flattening fabric into regional-scale, east-vergent folding and eventual shearing of F₂ fold limbs. D₂ deformation is likely a result of early crustal stacking transitioning into more complex basement-involved folding.

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Pages: 92
Supervisor: Michael Young