THE GRENVILLIAN TOMIKO QUARTZITES OF ONTARIO: CORRELATIVE WITH THE BARABOO QUARTZITES OF WISCONSIN OR THE MAZATZAL OROGEN OF NEW MEXICO? IMPLICATIONS FOR THE TECTONIC ARCHITECTURE OF LAURENTIA IN THE GREAT LAKES REGION

A thick sequence of paragneiss, including quartzite and iron formation (the Tomiko supracrustal rocks), occurs in Tomiko terrane of the Central Gneiss Belt of the Grenville Province in Ontario. These enigmatic supracrustal rocks have been variously correlated with the Huronian Sgp., the Baraboo quartzites in Wisconsin, and the Hondo Group (Mazatzal orogen) in New Mexico.

In order to resolve these diverse correlations, we report on the results of new mapping, geochemistry and geochronology. Highlights include: 1) discovery of B- and Mn-rich rocks with magnetite and ferrosilite iron formation in northern Tomiko terrane, plus epidote-rich (>20%) rocks & high TiO2 & P2O5 calc-silicate rocks. 2) discovery of dacitic and basaltic metavolcanic rocks or sills with continental tholeiite chemistry interlayered with quartzite. 3) infolding of Archean basement with the supracrustal rocks. At least 3 metamorphic events affect the supracrustal rocks; with intensity increasing to the southeast.

Detrital zircons from quartzite in the lower grade, northern Tomiko terrane indicate a maximum depositional age of 1686 Ma (with ~1875 and 2800 to 2660 Ma grains), confirming a previously obtained age of 1687+/-20 Ma from a more highly metamorphosed quartzite to the south.

Zircon from a grey dacite gneiss interlayered with quartzite in northern Tomiko terrane yields a preliminary U/Pb date of circa 1340 Ma. If indeed volcanic, this suggests sedimentation and volcanism occurred at circa 1340 Ma. Zircon from the Jocko pluton, a deformed monzonite body that cuts the supracrustal rocks yields a preliminary date of circa 1289 Ma, similar in age to some phases of the Powassan batholith south of North Bay, and older than the previously dated, less deformed Mulock monozogranite (1244+4/-3 Ma)

The new U/Pb data do not clearly support the previous far-flung correlations of the Tomiko rocks. The Baraboo quartzites lack the related volcanic rocks and 1350 to 1240 Ma magmatic events. Although there are many lithologic similarities to the Hondo Group, the Tomiko igneous rocks are much younger. The new U/Pb and geochemical data do suggest a possible link with other Central Gneiss Belt rocks near Parry Sound, namely, the Sand Bay and Lighthouse gneiss associations, which have been interpreted to have been deposited in a continental margin setting, likely outboard of an Andean-type margin.