Portal de Periódicos da CAPES

The Oxygen-Isotope Geochemistry of Archean Granitoids

1979; Elsevier BV; Linguagem: Inglês

10.1016/b978-0-444-41765-7.50016-2

2212-4128

Fred J. Longstaffe,

Paleontology and Stratigraphy of Fossils

The δ180 values of Archean granitoids from northwestern Ontario range from 3.2 to 11.7 °/oo, and reflect a myriad of magmatic, metamorphic, sedimentary, and alteration processes. The Pakwash and Footprint gneisses contain some samples with δ180 values of 6 to 7 °/oo, much lower than measured for other portions of these rocks. The low δ180 values probably reflect isotopic exchange with a low 180 external reservoir (basalt?) during partial melting accompanying upper amphibolite facies metamorphism. The metasedimentary Group A Twilight gneiss, which has been metamorphosed to granulite facies, appears to have isotopically exchanged with the surrounding volumetrically more important metaigneous granitoids. Other less metamorphosed parts of these and other gneisses retain oxygen isotope compositions diagnostic of their clastic sedimentary (Pakwash gneiss) or metaigneous (and probably metaplutonic) protoliths (Footprint, Cedar Lake, and Kenora area gneisses). Some Archean granitoids of lower metamorphic grades have also been depleted in 180 but such occurrences are localized. Greenschist facies felsic metavolcanic rocks in the vicinity of the Burditt Lake granodiorite pluton have been chemically altered and depleted in 180, by interaction action with stock-derived magmatic fluids. Portions of the Jackfish Lake Complex granodiorites also have low δ180 values due to exchange with low 18O fluids (probably ground waters). The chemical composition of these rocks, including their LREE abundances also have been affected by this activity. Unaltered portions of the plutonic Jackfish Lake Complex range in δ18O from 6.4 to 9.2 °/oo, higher values accompanying the progressive differentiation of the Complex from dioritic to granodioritic compositons. The unaltered Archean metaplutonic granitoids of tonalitic to granitic composition have δ 18O values of 7.3 to 9.3 °/oo, somewhat lower than Phanerozoic analogues. Such results probably reflect the more juvenile nature of the sialic material added to the continental crust during this period of Archean crustal evolution. Most Archean granitoids while not grossly disturbed, are nevertheless out of oxygen isotopic equilibrium. This disequilibrium is indicated by variable and usually large quartz-microcline fractionations, large quartz-biotite fractionations, and, for many of the gneisses, low δ18O values for biotite (less than 3 °/oo). Quartz-magnetite temperatures calculated for the Kenora area gneisses' provide a reasonably good minimum estimate of the real temperatures achieved near the culmination of metamorphism; the low (400°) quartz-biotite “temperatures” calculated for the gneisses, however, reflect only the preferential isotopic exchange of biotite (relative to quartz and magnetite), most likely with low 18O fluids.