An estimate of the oxygen isotope composition of a large segment of the Canadian Shield in northwestern Ontario

The O 18 /O 16 ratios of the plutonic and supracrustal rocks from a large segment of the Superior province of the Canadian Precambrian Shield in Ontario (total area 111 000 km 2 ) have been determined, using 8 composite samples prepared from 8076 individual specimens. The δO 18 values (in ‰) relative to SMOW are: granitic rocks, 7.6–8.6; basic intrusive rocks, 7.2; metasedimentary rocks, 9.2; and volcanic rocks, 7.6. The composites from the granitic, basic intrusive, and volcanic rocks display δ values very similar to their contemporary or younger equivalents reported in the literature. The metasedimentary composite shows significantly lower δO 18 than most Phanerozoic and Proterozoic metasediments of comparable metamorphic grades, possibly reflecting the presence of a high percentage of little-altered igneous rock detritus in the Archean sediments. The weighted average δ value (according to percent outcrop of each rock type) of this part of the Canadian Shield is estimated to be 8.1‰.