Geochemistry of Cambro-Ordovician Sedimentary Rocks of the Northeastern United States: Changes in Sediment Sources at the Onset of Taconian Orogenesis.

Trace element and Nd isotopic compositions of Late Cambrian to Middle Ordovician passive margin metasediments from the northernmost Taconic Allochthon of the northeast United States indicate a provenance consistent with a two-component mixture of an evolved Grenville source and a rift source generated during the Late Proterozoic opening of the Iapetus Ocean. Initial ∊_Nd values for these preorogenic deposits range from -11.3 to -14.6 (average = -13.0). Samples of overlying mid-Ordovician Taconian sediments exhibit initial ∊_Nd values between -6.1 and -9.8 (average = -8.5 ). The trace element and Nd isotopic compositions of these synorogenic samples are consistent with a mixture of a less evolved Grenville Province source and recycled preorogenic passive margin deposits. No mantle-derived (ophiolitic or andesitic) source is unambiguously recognized in the synorogenic samples, consistent with published geochemical studies of other Taconian foreland sequences from this region. Sedimentary rocks from the northern Appalachians record a clear change in provenance at ca. 458 Ma, coincident with the onset of Taconian orogenesis and similar to patterns observed in other sedimentary sequences throughout North America. [ABSTRACT FROM AUTHOR]