The Oxygen Isotope and Elemental Geochemistry of Archean Rocks From Northern Ontario

Abstract

<p>The oxygen isotope and chemical compositions (including rare earth elements) of Archean rocks from northern Ontario have been interpreted in terms of various magmatic, sedimentary, metamorphic and/or alteration processes that have occurred during the history of these samples.</p> <p>The greenschist facies clastic metasedimentary rocks from the Wabigoon, Abitibi, and Uchi granite-greenstone belts have chemical compositions that indicate derivation from dominantly felsic source terrains. Their δ¹⁸O values range from 8.0 to 13.3 °/.., about 2 to 3 °/.. lower than Phanerozoic clastics, a result of the immature nature of the Archean metasediments. The most immature samples analyzed show no recognizable chemical or isotopic effects of residence in a sedimentary environment. More mature samples, however, have δ¹⁸O values (>9 °/..), and chemical discriminant function values that indicate chemical and isotopic modification during sedimentary processes. These characteristics are useful in the recognition of clastic sedimentary protoliths in Archean gneisses.</p> <p>The felsic metavolcanic rocks, also from the granite-greenstone belts, have δ¹⁸O values of 7.5 to 13.0 °/... The higher values are probably due to low temperature alteration. Because of the overlap in δ¹⁸O values between the clastic metasedimentary rocks and the felsic metavolcanic rocks, oxygen isotope compositions cannot be used to distinguish between the two rock types.</p> <p>The δ¹⁸O values of metabasalts (4.4 to 9.2 °/..) suggest that isotopic alteration has accompanied their metamorphism, but no consistent relationships between δ¹⁸O and usually mobile elements such as K, Rb, Sr and Ba were observed. The chemical composition of least-altered mafic metavolcanic rocks from the Burditt Lake - Lake Despair area is compatible with the formation of these rocks in an island-arc type environment.</p> <p>Least-altered felsic metavolcanic rocks interlayered with the Burditt Lake - Lake Despair metabasalts have rare earth element compositions consistent with formation by moderate degrees of melting of quartz eclogite. Other samples of felsic metavolcanic rocks from the area have been chemically and isotopically altered by magmatic fluids accompanying the intrusion of the Burditt Lake granodiorite.</p> <p>The Jackfish Lake Complex (diorite, quartz monzodiorite and monzodiorite, leucodiorite and leuco quartz diorite, granodiorite, and Na-syenite), which was intruded along the contact between the Burditt Lake - Lake Despair metavolcanics and the Footprint gneiss, differentiated along a 'gabbro-trondhjemite' trend (Na-enrichment). The most abundant rock type, diorite, has a chemical composition compatible with its formation by partial melting of eclogite. The other rock types were probably differentiated from the dioritic magma along a path first controlled by the crystallization of hornblende, and then by the precipitation of feldspar, especially microcline. The δ¹⁸O rock (6.4-9.2 °/..) and mineral values of the Complex result from this differentiation, except for a few samples, which have been depleted in ¹⁸O by low temperature interaction with meteoric water.</p> <p>The Footprint gneiss also has a chemical composition that suggests that its protolith formed by partial melting of LREE-enriched quartz eclogite. Thus, most of the tonalitic to granodioritic rocks of the Burditt Lake - Lake Despair area are best interpreted as primitive additions to the Archean sialic crust derived from mantle depths.</p> <p>The somewhat low average δ¹⁸O value (7.5 °/..) of many of the isotopically unaltered Archean meta-igneous gneisses and plutonic rocks also could be the result of formation from less differentiated, more mafic sources than Phanerozoic batholiths, which have δ¹⁸O values of 8 to 10 °/...</p> <p>The δ¹⁸O values of ortho- and paragneisses (5.9-11.7 °/..) from the English River gneiss belt, and the Wabigoon granite-greenstone belt, span the range observed for least-metamorphosed meta-igneous and metasedimentary rocks. Up to middle amphibolite facies, the oxygen isotope compositions of the rock usually reflect the protolith of the gneiss. During the metamorphism, quartz-magnetite oxygen isotope temperatures are quenched at 500-600°C; quartz-biotite temperatures are lower (400 °C) and reflect continued isotopic exchange, mostly of biotite. This process results in the formation of low ¹⁸O biotites (<3 °/..).</p> <p>Above middle amphibolite facies, the oxygen isotope composition of Archean gneisses cannot be relied upon to reflect the isotopic nature of protolith rock types. Partial melting seems to encourage isotopic exchange of the gneisses with a low ¹⁸O reservoir, probably basalt, and results in δ¹⁸O values for the gneisses of 6 to 7 °/.., regardless of their previous isotopic composition.</p>