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|A Geochemical and Isotopic Study of Plutonic and High Grade Metamorphic Rocks from the Chandos Township Area, Grenville Province
|McNutt, Robert H.
Shaw, Denie M.
Heaman, Michael Larry
|<p>The geological history of the Chandos township region in the Grenville Province of Ontario and the behaviour of the Rb-Sr system in response to an amphibolite grade regional metamorphism have been investigated by comparing Rb-Sr whole rock and U-Pb zircon and sphene ages determined, in most cases, from the same sampling site. A variety of sampling techniques have been employed in order to evaluate the geological significance of Rb-Sr whole rock ages in complex, high grade terrains.</p> <p>In the case of the layered sodic Apsley gneiss, samples collected from widely spaced sampling sites result in a more precise but anomalously old (1402 Ma) and geologically meaningless age; a consequence of combining genetically unrelated samples. Although Rb-Sr w~ole rock ages determined using samples collected from homogeneous portions of individual sampling sites are in better agreement with the inferred age of the Grenville Supergroup (ca. 1250 Ma), the best estimate for the age of the sodic Apsley gneiss is 1333±24 Ma, obtained by combining all the samples that have an igneous origin. A minimum estimate for the deposition and extrusion of these supracrustal rocks is the precise U-Pb zircon age of 1254 +11/-4 Ma (the Rb-Sr whole rock age for this site is 1244±32 Ma) obtained for the Tallan Lake sill.</p> <p>Precise U-Pb zircon ages for two other intrusive bodies in the study area, the Methuen quartz mozonite and Loon Lake monzonite, indicate igneous activity at 1242 +22/-6 and 1090 +7/-4 Ma, respectively. Most of the granitoid rocks in the Central Metasedimentary Belt have low initial strontium ratios (0.703 to 0.704) and anomalously high oxygen isotopic compositions (> 8%) indicating that these units were derived from a source with a low average Rb/Sr and/or a source with a relatively short crustal residence history, and subsequently interacted with high-¹⁰O rocks (eg. marble) at depth, but did not interact with "old", high Rb/Sr crustal material).</p> <p>The timing of amphibolite grade metamorphism is unclear, however, a minimum estimate is provided by the 1090 Ma U-Pb zircon age obtained for the post-tectonic Loon Lake monzonite. At present, there is no reliable maximum estimate but the 1150 Ma Rb-Sr study determined using thin slabs from the sodic Apsley gneiss is interpreted to be close to the time of metamorphism. If the occurrence of 1085 Ma U-Pb sphene ages in this region reflects new sphene growth or resetting during metamorphism then these ages, combined with the lack of evidence for new zircon growth, indicate that the region cooled slowly from approximately 600 to 650°C (ie. the blocking temperature for zircon) at 1150 Ma to approximately 500°C (ie. the blocking temperature for sphene) at 1085 Ma.</p> <p>The final geological activity in this region was late-stage faulting, injection of pegmatite and associated hydrothermal activity. The effects of this event are recorded in the Rb-Sr whole rock system of the SiIent Lake gneiss, the potassic Apsley gneiss and fracture-bearing samples of the Tallan Lake granodiorite. The best estimate for the timing of this activity is the Rb-Sr whole rock age of 1023±17 Ma determined using fracture-bearing samples from the Tallan Lake granodiorite.</p> <p>The different sampling techniques employed in this study indicate that the scale of Rb and/or Sr migration in the sodic Apsley gneiss, during metamorphism, must be less than one meter (the minimum distance between samples collected from one sampling site) and greater than 1 centimeter (the average width of the slabs). A more profound effect on the mobility of these elements was found in samples that have been affected by the late-stage hydrothermal activity. In certain rock units of the Rb-Sr system has responded to at least two distinct events, a regional amphibolite grade metamorphism at about 1150 Ma and hydrothermal activity at about 1025 Ma; so some caution should be execized in interpreting the geological significance of Rb-Sr whole rocks ages determined in this region.</p>
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