Abstract

The Trap Falls Formation is a sequence of interlayered quartzites and schists that crops out in the Appalachian belt in southern Connecticut, and was deformed and metamorphosed to middle amphibolite grade during Acadian orogenesis. Schists have high Al2O3 and low CaO, Na2O, and K2O (chemical index of alteration CIA = 68–70), consistent with a significant weathering history in the sediment source. Rare earth element (REE) patterns for both schists and quartzites parallel post-Archean average Australian Shale, with light REE enrichment and well-developed Eu anomalies, suggesting an average upper crustal source. Whole-rock Nd and Pb isotopic analyses indicate old sources, with depleted mantle model ages (TDM) from 1880 to 1660 Ma, 207Pb/204Pb from 15.62 to 15.87, and 206Pb/204Pb from 19.11 to 22.08. U–Pb ages for single-grain and multigrain populations of detrital zircons range between 1113 and 992 Ma, the youngest of which defines a maximum depositional age for the Trap Falls Formation. U–Pb zircon ages indicate a late Grenvillian source for the zircons. Nd and Pb isotopic compositions are consistent with a source that is dominated by Grenville-age rocks with some component of older crust. Combining all of the data, we interpret that the protolith of the Trap Falls Formation was comprised of aluminous muds interbedded with clean quartz arenites, and suggest that they were deposited on the stable, trailing-edge margin of North America sometime during the Late Proterozoic to the Early Cambrian. The sediments were derived from a weathered source with an upper continental crust composition. Isotopic data and zircon ages indicate that this source was dominated by Grenville-age rocks.

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