GEOCHRONOLOGY OF DETRITAL ZIRCONS FROM THE ELZEVIR AND FRONTENAC TERRANES, CENTRAL METASEDIMENTARY BELT, GRENVILLE PROVINCE, ONTARIO

The Central Metasedimentary Belt (CMB) of the Grenville Province conta ins metasedimentary sequences belonging to a number of distinct tecton o-stratigraphic terranes whose depositional ages are poorly known. Thi s study provides information on not only the provenance, but also the maximum age of clastic rocks in two of these terranes, the Elzevir Ter rane on the northwest and the Frontenac Terrane to its southeast, adja cent to the Adirondack Mountains of New York. The Flinton Group, a com ponent of the Elzevir Terrane, is a distinctive, mostly clastic, sedim entary sequence that unconformably overlies igneous and metavolcanic r ocks of the main part of Elzevir Terrane of the CMB. Analyzed zircons from quartzose metasediments of the Flinton Group are 0-2% discordant and range in age from 1150 to 1335 Ma, with older rounded grains at 14 61 +/- 5 and 1877 +/- 3 Ma. The quartzite was therefore deposited afte r ca. 1150 Ma, indicating that the Flinton Group is more than 100 Ma y ounger than the intrusion of the underlying Elzevir batholith. We spec ulate that 1 150 - 1180 Ma zircons within the Flinton Group were deriv ed from plutons in the Frontenac Terrane to the southeast, implying th at the Elzevir and Frontenac terranes were contiguous during Flinton G roup deposition. Subsequent metamorphism of the Flinton Group occured between 1150 and 1080 Ma. The high-grade Frontenac Terrane of the CMB lies southeast of Elzevir Terrane, and contains marble associated with pelitic gneiss and quartzite, as well as granitic intrusive rocks; it resembles a metamorphosed continental margin sedimentary sequence. U - Pb analyses of zircons from quartzites from two different localities are generally less than 5% discordant, but show stronger evidence for Grenvillian Pb loss than zircons from the Flinton Group. Pb-207/Pb-20 6 ages range from 1493 to 2580 Ma, with one analysis (2% discordant) a t 1306 +/- 16 Ma, another at 3185 +/- 3 Ma, and a cluster of ages betw een 1745 and 1892 Ma. Detrital zircon ages are, for the most part, dis tinctly older than in the Flinton Group. The age of this quartzite seq uence is tentatively regarded as less than ca. 1300 Ma (based on one g rain), but is certainly less than 1500 Ma. It could therefore have bee n deposited during the same time interval as the 1.2 - 1.3 Ga metasedi mentary and metavolcanic rocks of the Elzevir Terrane. Although Fronte nac Terrane experienced metamorphism along with Elzevir Terrane around 1.1 Ga, the principle metamorphic culmination in the Frontenac occurr ed prior to 1170 Ma.