Geochronology of detrital zircons from the Elzevir and Frontenac terranes, Central Metasedimentary Belt, Grenville Province, Ontario

Abstract

The Central Metasedimentary Belt (CMB) of the Grenville Province contains metasedimentary sequences belonging to a number of distinct tectono-stratigraphic terranes whose depositional ages are poorly known. This study provides information on not only the provenance, but also the maximum age of clastic rocks in two of these terranes, the Elzevir Terrane on the northwest and the Frontenac Terrane to its southeast, adjacent to the Adirondack Mountains of New York.The Flinton Group, a component of the Elzevir Terrane, is a distinctive, mostly clastic, sedimentary sequence that unconformably overlies igneous and metavolcanic rocks 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 1461 ± 5 and 1877 ± 3 Ma. The quartzite was therefore deposited after ca. 1150 Ma, indicating that the Flinton Group is more than 100 Ma younger than the intrusion of the underlying Elzevir batholith. We speculate that 1150–1180 Ma zircons within the Flinton Group were derived from plutons in the Frontenac Terrane to the southeast, implying that the Elzevir and Frontenac terranes were contiguous during Flinton Group 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. 207Pb/206Pb ages range from 1493 to 2580 Ma, with one analysis (2% discordant) at 1306 ± 16 Ma, another at 3185 ± 3 Ma, and a cluster of ages between 1745 and 1892 Ma. Detrital zircon ages are, for the most part, distinctly older than in the Flinton Group. The age of this quartzite sequence is tentatively regarded as less than ca. 1300 Ma (based on one grain), but is certainly less than 1500 Ma. It could therefore have been deposited during the same time interval as the 1.2–1.3 Ga metasedimentary and metavolcanic rocks of the Elzevir Terrane. Although Frontenac Terrane experienced metamorphism along with Elzevir Terrane around 1.1 Ga, the principle metamorphic culmination in the Frontenac occurred prior to 1170 Ma.