Geochemical and isotopic evolution of the Maggo Gneiss component from the Hopedale Block, Labrador: evidence for Late-Middle Archaean crustal reworking

• Main Author: Finn, Gregory Clement
• Format: Thesis
• Language: English
• Published: Memorial University of Newfoundland 1988
• Subjects: Nain; Hopedale
• Online Access: https://research.library.mun.ca/6809/; https://research.library.mun.ca/6809/1/GregoryClementFinn.pdf; https://research.library.mun.ca/6809/3/GregoryClementFinn.pdf

The Hopedale Block, an informal name for the southern portion of the Nain Structural province, Labrador, preserves evidence of a protracted Archaean to Proterozoic developmental history. This study examines the Archaean development of a segment of the Hopedale block, centered at Hopedale village as recorded in the Maggo gneiss, a typical grey gneiss of tonalitic to trondhjemitic composition. Following field mapping and petrographic examination of all lithologies within the study area, major, trace and rare earth element (REE) lithogeochemical analyses and Rb-Sr isotopic determinations were completed on the Maggo gneiss. -- The geochronological development of the Maggo gneiss, based on the Rb-Sr isotopic studies of twelve suites, identified three distinct Archaean tectonothermal events. Isotopic evidence of the oldest event, the Pre-Hopedalian, dated at 3306 +/- 75 Ma (1 sigma) is preserved at one locality; however structural evidence of this event can be recognized throughout the study area. Inclusions of supracrustal material (Weekes association) and anorthosite within Maggo gneiss, preserve evidence of two periods of deformation prior to the Pre-Hopedalian event. The dominant component of the Maggo gneiss is preserved within the Hopedalian domain. Four sample suites yielding ages of 3,140 +/- 95 Ma to 3,025 +/- 165 Ma date this event. The Hopedalian event produced moderate to steeply dipping NW-SE planar fabric within the gneiss. The Hopedale dykes, which predate the Hopedalian event were rotated into concordance with the Hopedalian fabric at this time. -- The Fiordian, a series of events, which marks the last major Archaean period of tectonism to affect the Hopedale Block lithologies, occurred between 2,927 +/- 50 Ma and 2,632 +/- 285 Ma. The Fiordian is divided into an early and late period characterized by reworking of preexisting crustal material separated by a period during which the Kanairiktok intrusive suite was emplaced. In response to the Fiordian reworking the Maggo gneiss exhibits the following ...