Oxygen isotope studies of Grenville metamorphism and magmatism (Ontario, Quebec).

摘要

Oxygen isotope ratios of metamorphic rocks can provide important information about premetamorphic history. Cordierite and gedrite bearing lithologies from the Central Metasedimentary Belt boundary thrust zone (Grenville Province, Ontario) have δ18O(whole-rock) of 6–8‰, consistent with seawater-altered volcanic rock protoliths. These rocks mark the site of Mid-Proterozoic rifting at the margin of Laurentia. The Morin Complex (Grenville Province, Quebec) consists of two anorthosite plutons with distinct δ18O values (9.6 ± 0.7‰ and 8.7 ± 0.6‰), reflecting magmatic compositions from assimilation of crustal materials by the anorthosite's parent magma.; Empirical studies of δ18O values of zircon from high-grade metamorphic rocks shows that oxygen diffusion in zircon is slow, and that zircons preserve pre-metamorphic δ18O values. Granitic rocks related to anorthosite-suite plutonism intrude three terranes in the Southern Grenville Province, and oxygen isotope ratio of dated zircons from these rocks allow magmatic δ18O values to be calculated. Variability in δ18O(Zrc) of this suite corresponds to geographic location, and isotopically distinct regions in the base of the crust. Archean Superior Province zircons have relatively low δ 18O values (5.7‰ ± 0.6‰), while Proterozoic Grenville Province zircons have elevated δ18O values (8.2‰ ± 1.7‰). These data reflect fundamental differences in crustal evolution between these Provinces. In the Grenville, supracrustal material was subducted or buried to the deep crust, causing high δ 18O values.; Ion microprobe analysis of δ18O in detrital Early Archean zircons reveals high δ18O values relative to the mantle. Oxygen isotope ratios of 3.3 to 3.6 Ga, 4.0 Ga, and 4.15 Ga zircon age populations from a Jack Hills (Western Australia) metaconglomerate average 6.28 ± 0.15‰, 6.97 ± 0.25‰, and 5.74 ± 0.23‰, respectively. A 4.40 Ga grain is zoned with respect to δ 18O (7.4 and 5.0‰) and trace element composition. This is the first evidence of continental-type crust as early as 4.40 Ga. These studies show that zircon is a robust reference mineral to ascertain igneous processes in rocks which have experienced complex, high-temperature histories.