IN SITU THIN SECTION LA-ICP-MS U-PB DATING OF GARNET FROM THE STRAITS SCHIST, WESTERN CONNECTICUT

In situ thin section LA-ICP-MS U-Pb dating of garnet from The Straits Schist, western Connecticut, has produced robust ages that record regional Acadian metamorphism. The Straits underwent amphibolite facies metamorphism at ~600-750 °C and ~0.65-0.9 GPa prior to the late Middle Devonian (>383 Ma). From two samples located south of the Waterbury dome we analysed 13 (sample SS-1) and 18 (sample DSts-00.6.5) garnet grains, and 10 grains from a sample along the western flank of the Bristol dome (sample TSS-89.23) with average diameters of ~1.7, ~1.3 and ~1.5 mm, respectively. Up to 21 analyses per grain were conducted with a total of 199 (SS-1), 105 (DSts-00.6.5) and 97 (TSS-89.23) analyses. Data sets define linear arrays in ²⁰⁷Pb/²⁰⁶Pb vs ²³⁸U/²⁰⁶Pb space, interpreted as mixing lines between initial common-Pb and Pb that formed by in situ U-decay after garnet crystallization. The age of garnet crystallization is defined by the lower intercept with the Concordia. Sample TSS-89.23 did not yield a meaningful isochron age due to little variation in ²³⁸U/²⁰⁶Pb. Inclusions >5-10 µm were detected using optical and scanning electron microscopy and were avoided during ablation. To assess the effects of U-rich inclusions ≤5 µm as well as >5 µm on garnet analysis, we (1) investigated garnet for U-rich inclusions prior to ablation to quantify their abundance and size distribution; (2) determined the trace element concentration of inclusion-free garnet to use its maximum U concentration as a cut-off criteria during U-Pb data evaluation; (3) applied U-Pb dating to zircon, monazite, and xenotime in the matrix and included in major phases to constrain their U-Pb ages and U concentrations to gauge their effect on Pb and U/Pb isotopic ratios; and (4) used ~200 µm laser spots to analyse garnet domains with zircon, monazite, and xenotime inclusions of varying grain sizes to assess their effects on the garnet U-Pb data and the time-resolved U-Pb signal. From two thin sections of sample SS-1, 137 analyses yield an isochron age of 409 ± 10 Ma. From two thin sections of sample DSts-00.6.5, 76 analyses yield an isochron age of 406 ± 15 Ma (uncertainties are 2σ). We interpret our garnet ages to record bulk garnet growth. The fact that in Barrovian-type settings garnet commonly grows over millions of years contributes to the relatively large age uncertainties.