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Oxygen and Hydrogen Isotope Studies of Metamorphic Rocks in the Wawa-Kapuskasing Crustal Transect, Ontario, Canada

<p> Oxygen and hydrogen isotopic distributions have been studied for rocks from a 100 km transect in the central Superior Province of Ontario, Canada. The transect represents progressively deeper terrains of the Michipicoten Greenstone Belt (MGB), the Wawa Gneiss Terrane (WGT), and the Kapuskasing Structural Zone (KSZ), which correspond to an increase of metamorphic grade and are interpreted as an oblique section through approximately 20 km of crustal thickness. The rocks in the terrains range in age from ~2.76 to ~2.60 Ga, with fewer later intrusions.</p> <p> Equivalent lithologic types have similar δ18O range at middle and lower crustal levels (WGT and KSZ). Tonalitic to granodioritic rocks range from 6.4%o to 9.5o/oo; Dioritic and anorthositic rocks range from 5.5o/oo to 7.6o/oo; a majority of the mafic gneisses (group 1) range from 5.7o/oo to 7.1o/oo, while group 2 mafic gneisses range from 8. 1o/oo to 9.5o/oo. δ18O values of the rocks exhibit a remarkable correlation with SiO2 values, similar to that observed in unaltered plutonic rocks of equivalent composition. Paragneisses have significantly higher δ18O values, 9.3o/oo to 12.2o/oo. Low-grade metavolcanic and metasedimentary rocks of the MGB are 18O-enriched compared to their high-grade equivalents in the KSZ and WGT, 7.4o/oo to 13.3o/oo for mafic to felsic metavolcanic rocks and 11.4o/oo to 14.7o/oo for clastic metasediments.</p> <p> Coexisting minerals from high-grade rocks exhibit 18O-fractionation closely consistent with isotopic equilibrium, suggesting that the isotopic system has not been grossly disturbed. Isotopic thermometers give uniform apparent temperatures, about 553°C to 584°C, across the entire transect, which are lower than the inferred metamorphic temperatures in the highest-grade (KSZ) terrane.</p> <p> The lack of distinctive isotopic differences between equivalent rock types in the KSZ and WGT suggests that there is no significant gradient in δ18O with depth in the crust or with metamorphic grade. The majority of mafic gneisses (Group 1) have δ18O values similar to fresh basalts and appear to have been emplaced either as subaerial extrusives, intrusive sills, or, less likely, as submarine extrusives that were hydrothermally altered at high temperatures. The less abundant Group 2 mafic gneisses have δ18O values typical of greenstones that were altered at low temperature by sea-water, and isotopically resemble low-grade rocks in the Michipicoten and Abitibi belts. In general, no major changes in whole-rock isotopic composition appear to have occurred during granulite facies metamorphism, implying limited flux of water or CO2.</p> <p> The continuous linear gradient in δ18O vs SiO2 in the high-grade rocks cannot be due to differentiation of a mafic source magma. A model involving an association between mantle-derived mafic magma and 18O-enriched crustal materials is more consistent with the oxygen isotopic data.</p> <p> Hydrogen isotope composition of hornblende and biotite has been analyzed from selected rocks. Mafic and anorthositic rocks from the KSZ have δD values from -58 to -62o/oo, suggesting a possible mantle-derived origin of fluid in the system. Two mafic gneisses, which are 18O-enriched, show lower δD values, -89 and -101o/oo. The depletion of deuterium is consistent with the model of low temperature alteration with seawater in a submarine environment. Mafic and tonalitic gneisses from the WGT are also depleted in deuterium, -87 to -109o/oo. Since these rocks intruded into relatively higher level of the crust and commonly contain secondary alteration minerals, it is possible that hydrothermal alteration took place at late- or post-metamorphism stage, and the source of the fluid is likely meteoritic water, The amount of water involved in the exchange was restricted, and was not enough to disturb the oxygen isotopic system in the rocks.</p> / Thesis / Master of Science (MSc)
Date02 1900
CreatorsLi, Hong
ContributorsShaw, D. M., Schwarcz, H. P., Geology
Source SetsMcMaster University
Detected LanguageEnglish

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