ALBERT

Description: U-Pb ages of detrital zircons in lower Paleozoic sedimentary rocks from New England permit evaluation of the relationships between tectonic activity and provenance of foreland sediments deposited in response to the Taconian and Acadian orogenies. The Lower Cambrian Poughquag Quartzite was deposited on the Laurentian passive margin. Poughquag detrital zircons (n 5 40) are mostly concordant to slightly discordant with 207Pb*/206Pb* ages of 1.67–0.87 Ga and are consistent with a provenance from the nearby Grenville Structural Province. Two Poughquag zircon grains record 206Pb*/238U ages of about 0.64 Ga and 0.55 Ga and could have been derived from riftrelated magmatic rocks. The Middle Ordovician Austin Glen Member of the Normanskill Formation and Lower Silurian Shawangunk Formation represent clastic-wedge sedimentation in the Laurentian foreland deposited in response to the Taconian orogeny (ca. 470–430 Ma). Zircons from these units (n 5 41 for Austin Glen, n 5 29 for Shawangunk) are also predominantly of Grenville age (207Pb*/206Pb* ages of 1.46–0.94 Ga). One zircon from the Austin Glen records an age of 3.29 Ga, consistent with derivation from North American Archean terranes. No zircon from either unit (or reported in the literature) can be interpreted as recording an Ordovician (Taconian) age. Devonian Catskill ‘‘redbeds’’ represent synorogenic to postorogenic foreland sedimentation associated with the Acadian orogeny (ca. 400–360 Ma). Zircons from one sample (n 5 45) are all near concordant and have a bimodal age distribution. Approximately 60% of grains have 207Pb*/ 206Pb* ages of 1.23–1.00 Ga and the other 40% have 206Pb*/238U ages of 0.47–0.42 Ga. Very different proportions of Precambrian and Paleozoic ages are seen in the Catskill sample if zircons are selected on the basis of size or optical clarity, and this observation highlights the possibility of sampling bias if detrital zircons are not randomly selected for U-Pb analysis. The 0.47–0.42 Ga population is consistent with derivation from rocks crystallized during the Taconian orogeny. No analyzed zircon in this sample records an unambiguous Devonian (Acadian) age. Accordingly, during early Paleozoic evolution of the Laurentian foreland basins in New England, detrital zircons do not appear to record contemporaneous orogenic activity but instead are restricted to recycling of the preexisting continental margin. It is possible that substantial topography (e.g., accretionary prism) impeded sediment transport from the active orogen to the foreland basin.