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

Finn, Gregory Clement (1988) Geochemical and isotopic evolution of the Maggo Gneiss component from the Hopedale Block, Labrador: evidence for Late-Middle Archaean crustal reworking. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

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 characteristics: -- 1) reorientation of earlier fabrics into a NE-SW direction, -- 2) low to middle amphibolite facies metamorphism which retrogresses Hopedalian middle to upper amphibolite facies assemblages, -- 3) large ion lithophile element (LILE; Rb, Ba, K) depletion, high field strength (HFS) element (TI, Zr, P, Y and V) enrichment and no change in either Sr or REE contents, -- 4) resetting of the Rb-Sr isotopic systematics of the gneiss in response to Rb depletion. -- Geochemical reworking associated with the Fiordian event occurs independently of structural reworking. -- The application of the concept of areals and areal isochrones as applied to the Hopedalian and Fiordian suites yields ages of 3,366 +/- 212 Ma and 2,930 +/- 82 Ma, respectively. These ages date the timing for a common period of Sr homogenization for each group of suites at a specific point in their past. -- Petrogenetic modelling, using LILE, REE and HFS element and a variety of possible lithologic compositions has been carried out to determine the source to the Maggo gneiss precursor. An intermediate composition parent, metamorphosed to granulite and/or amphibolite facies, on 25% melting will yield REE and HFS element abundances in agreement with those observed for the Maggo gneiss. The source compositions modelled all require trace amounts (<0.5%) of minor, residual, phases, e.g. apatite, titanite and zircon, which control single elements, e.g. P, Ti and Zr respectively, and REE abundances in the resulting melts. The major and trace element composition of the parent used is comparable to published results for Early Archaean crust as represented by the Amitsoq gneiss of southern West Greenland. -- These results extend the history of the Hopedale Block beyond that presently recognized. A proposed correlation between the recorded development for other areas of the North Atlantic Craton (NAC) and the Hopedale Block is presented.

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