The metasupracrustal units within the north central Chelan block of the North Cascades Range, Washington, are investigated to determine mechanisms and timescales of supracrustal rock incorporation into the deep crust of continental magmatic arcs. Zircon U‐Pb and Hf‐isotope analyses were used to characterize the protoliths of metasedimentary and metaigneous rocks from the Skagit Gneiss Complex, metasupracrustal rocks from the Cascade River Schist, and metavolcanic rocks from the Napeequa Schist. Skagit Gneiss Complex metasedimentary rocks have (1) a wide range of zircon U‐Pb dates from Proterozoic to latest Cretaceous and (2) a more limited range of dates, from Late Triassic to latest Cretaceous, and a lack of Proterozoic dates. Two samples from the Cascade River Schist are characterized by Late Cretaceous protoliths. Amphibolites from the Napeequa Schist have Late Triassic protoliths. Similarities between the Skagit Gneiss metasediments and accretionary wedge and forearc sediments in northwestern Washington and Southern California indicate that the protolith for these units was likely deposited in a forearc basin and/or accretionary wedge in the Early to Late Cretaceous (circa 134–79 Ma). Sediment was likely underthrust into the active arc by circa 74–65 Ma, as soon as 7 Ma after deposition, and intruded by voluminous magmas. The incorporation of metasupracrustal units aligns with the timing of major arc magmatism in the North Cascades (circa 79–60 Ma) and may indicate a link between the burial of sediments and pluton emplacement.
IntroductionThe development of the SIMS (secondary ion mass spectrometry) technique opened the world of high spatial resolution determination of U-Pb ages from polished zircon surfaces (e.g., Andersen and Hinthorne 1972; Compston et al. 1984; Williams 1998). The SIMS technique is a comparative method and the determination of accurate isotopic ratios in zircon requires calibration of the unknown results against the results of a well-characterized calibrant material measured under identical conditions during the same analytical session. In principle either a natural or synthetic reference material can be used as such a calibrant; unfortunately wellperforming synthetic reference standards for the U-Pb analysis of zircon are not available. This is due to two main reasons. First, no synthesis technique is currently available that allows one to produce homogeneous Pb-bearing zircon crystals, owing to the exceptionally low partition coefficient of Pb in zircon. Second, glass reference materials cannot be used due to their different sputtering behavior under the oxygen-ion beam, compared to that of the zircon crystals to be analysed. Consequently, high-quality natural zircon reference materials must be used, and are therefore increasingly sought.Any potentially suitable reference zircon needs to be characterized thoroughly in order to provide users with the best currently obtainable values, with regards to precision and accuracy, for the isotope ratios and concentrations of the calibrant material. Also, a well-performing reference zircon needs to be exceptionally homogeneous (Pidgeon 1997, Kennedy 2000, because each individual chip, and micro-areas within chips, of the reference material must have the chemical, physical and isotopic properties of the entire, previously characterized, bulk material. Another important characteristic that determines the quality of a reference material is its reliability during analysis. In U-Pb geochronology, higher count rates -in particular for the radiogenic Pb isotope, which is always lower in abundance than the parent U isotope -lead to better Poisson statistics. This may allow the operator to shorten the analysis time and/or to decrease the spot diameter without loss of precision. A suitable SIMS U-Pb reference should therefore contain sufficiently high concentrations of U and radiogenic Pb (whereas the concentration of non-radiogenic Pb should be negligible). However, higher levels of radiogenic Pb correlate with elevated radiation damage and this leads to an analytical difficulty, as the secondary ion sputtering behaviour under the ion beam may be different for the unknowns and reference if the structural states are sufficiently different. In searching for suitable U-Pb zircon references it is therefore advantageous to find gem-quality specimens that have some, but not too much radiation damage. Zircon M127 fulfils all of these important prerequisites. Zircon M127contains notably higher levels of U and radiogenic Pb isotopes than most other SIMS references, resulting in a somewhat ...
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