U-PB CHRONOLOGY OF DETRITAL ZIRCONS IN THE IRVING POND QUARTZITE, ADIRONDACK MOUNTAINS, NEW YORK: IMPLICATIONS FOR TECTONIC HISTORY OF THE GRENVILLE OROGENIC CYCLE

The uplift history of orogenic belts can be reflected by the provenance and composition of sedimentary accumulations within them. In the Adirondack Mountains, metapelites that are cross-cut by ca. 1210 Ma granitoids have detrital zircon populations that include only grains with ages in the range 1250 –1340 Ma, constraining deposition to the period 1250 to 1210 Ma. The age range of the zircons suggests deposition in restricted basins that received detritus only from arc-related rocks of the Elzevirian Orogeny. In marked contrast, the detrital zircon population of the Irving Pond Quartzite (IPQ), a thick (~1000m) orthoquartzite with minor pelitic layers, yields an age distribution spectrum with pronounced peaks at 2720, 2650, 1960, 1800, 1670, 1500, 1410, 1360, and 1310 Ma, clearly indicating sources of detritus from the Archean Superior Province as well as Makkovikian, Labradorian, and Pinwarian terranes. The metapelites and the IPQ display ca 1170 - 1200 Ma (Shawinigan Orogeny) as well as ca 1020-1050 Ma (Ottawan Orogeny) metamorphic zircon, both as discrete grains and as distinct overgrowths on older detrital grains. Thus, deposition can be constrained to ca. 1250-1210 Ma for the metapelites and to ca. 1300-1200 for the IPQ ca, i.e. between Elzevirian and Shawinigan arc magmatism. During deposition of the Irving Pond, the orogen must have stabilized in proximity to major river systems that drained regions hundreds, or perhaps thousands, of kilometers to the north. This configuration coexisted with, preceded, or followed the restricted basins in which the metapelites were deposited. Given the composition of the units, it seems most likely that the quartzites represent a late sedimentary blanket deposited on an eroded Elzevirian basement of arc plutons and associated metapelitic flysch.