Date of Award
Doctor of Philosophy (PhD)
Gary Philip Beakhouse
An integrated geological, geochemical and geochronological study of granitoid rocks from the Winnipeg River belt has revealed a complex geologic history spanning at least 500 Ma. Detailed Rb-Sr and U-Pb zircon isotopic investigations have revealed complexities that caution against simplistic application of these geochronological techniques in similar terrains.
Rb-Sr whole rock isochron ages are sufficiently imprecise and/or inaccurate that they can not be used to establish relative and absolute-time scales. Late to post-tectonic plutons, classically considered perfect candidates for Rb-Sr geochronology commonly have non-ideal Rb-Sr isotope systematics and suggested ages are slightly to significantly younger than U-Pb zircon ages for the same units. There is no independent evidence for a geologic event that could reset the Rb-Sr system and these younger ages should not be interpreted as representing discrete geologic events. The U-Pb zircon system is inherently a more precise geochronometer and these ages are interpreted to represent the age of crystalization.
Detailed studies of a biotite tonalite gneiss layer from the Kenora area reveals a complex redistribution of mineralogical, chemical and isotopic species during metamorphism. For certain elements, including Rb and Sr, parts of the layer have acted as an open system suggesting that whole rock Rb-Sr isochron ages for such rocks may not be reliable. Model initial ratios offer a more reliable, though more qualitative, approach to the problem of identifying ancient crust. Several different ages of zircon growth occurred in this tonalite but it is feasible to select specific morphological types of zircon and to determine more than one age from the same rock.
The earliest rocks in the Winnipeg River belt were a "greenstone-type" supracrustal sequence that are now preserved as highly metamorphosed and deformed inclusions in younger plutonic rocks. These supracrustal rocks are not abundant, but are widespread, and their occurrence as inclusions in tonalite gneiss interpreted to be as old as 3.0 Ga, indicates that a greenstone belt significantly older than those in the Wabigoon subprovince was once approximately coextensive with the Winnipeg River belt.
Massive sedic plutons and complex, banded, predominantly tonalitic gneisses are slightly to considerably older (2.76 to >3.0 Ga) than volcanism in the Wabigoon subprovince (2.70-2.75 Ga). The tonalite gneiss and sodic plutons are geochemically similar to other early Precambrian tonalites and are interpreted to be a product of the partial melting of metamorphosed tholeiitic basalt.
During the interval 2.7-2.75 Ga, most terrains in the western Superior province received voluminous juvenile additions from the mantle, either directly (basalt) or indirectly (felsic plutonic and felsic volcanic and volcaniclastic rocks derived from the melting of the basalts). The consequences of this thermotectonic event (the Kenoran orogeny) in the Winnipeg River belt was to partially melt and upwardly mobilize the earlier basalt-tonalite association with little addition of new material from the mantle.
Late to post-tectonic mafic plutons and voluminous potassic plutons were emplaced between 2.65 and 2.7 Ga. The potassic plutons have geochemical signatures and elevated Sr isotope initial ratios that indicate they originated from the partial melting of the early tonalites. The volume of these plutons requires that the early tonalites were extensive and it is probable that much of the present volume of the Winnipeg River belt represents relatively old (3.0 Ga) crust reworked during the Kenoran orogeny. This implies that the Winnipeg River belt could have acted as a sialic microcontinent during the deposition of younger (2.7-2.75 Ga) supracrustal sequences.
The pattern of pre-existing sialic nuclei and younger, coeval, interdigitating volcanic arcs and sedimentary basins is geometrically, though not necessarily genetically, analogous to modern plate tectonic regimes.
Beakhouse, Gary Philip, "Geological, Geochemical and Rb-Sr and U-Pb Zircon Geochronological Investigations of Granitoid Rocks from the Winnipeg River Belt, Northwestern Ontario and Southeastern Manitoba" (1983). Open Access Dissertations and Theses. Paper 1528.