EARLY SILURIAN IGNEOUS U-PB ZIRCON AGES IN COVER ROCKS CONSTRAIN TIMING OF TECTONIC EVENTS WEST OF THE WATERBURY DOME, CT

New SHRIMP U-Pb zircon ages from a meta-igneous rock in the basal member of the Taine Mountain Formation and the late-tectonic Nonewaug Granite provide constraints on pre-Acadian deformation within cover rocks west of the Waterbury dome. Zircon grains separated from a medium-gray, fine-grained, weakly foliated but unlayered, titanite-hornblende-biotite-quartz-plagioclase gneiss, mapped as a metaigneous rock of intermediate composition, yielded an age of 442 ± 3 Ma. The zircon grains are euhedral, have aspect ratios of ~8:1, exhibit oscillatory zoning, and lack metamorphic rims. Whole rock geochemistry indicates the rock is andesitic/dioritic in composition. The unit as mapped has a limited extent but occurs adjacent to amphibolite in a NW-terminating belt of meta-igneous rocks and paragneiss west of the Waterbury dome that is surrounded to the NE, N, and W by rusty pelitic schist (basal Taine Mountain Fm. of Rodgers, 1985). The zircon morphology suggests rapid crystallization, and the 442 Ma age is interpreted as dating intrusion of the protolith during a late-Taconic (D2) phase of regional metamorphism and tectonism.

The Nonewaug Granite is a medium- to coarse-grained, weakly-foliated to massive, muscovite-biotite-microcline-quartz-plagioclase granite that intruded well-foliated staurolite-kyanite-grade schist (Ratlum Mtn Fm. of Rodgers, 1985) and complexly-deformed, migmatitic, kyanite- and Kspar-bearing (and staurolite-absent) schist of above-mentioned basal Taine Mtn. affinity. Zircons separated from the Nonewaug have oscillatory-zoned dark CL cores with Th/U ratios of > 0.1 typical of igneous zircon, and yield an age of 431± 5 Ma, interpreted to be the time of crystallization.

The Nonewaug Granite crosscuts a contact between the staurolite-kyanite and kyanite-Kspar-bearing rocks that is interpreted as a low-angle thrust (decollement) of pre- or early-D2 age. The granite forms foliation-parallel, weakly-foliated sills in S2/L2-deformed Ratlum Mtn. Fm. schist, suggesting late-D2 intrusion of the Nonewaug at 431 Ma. Together the field relationships and geochronology indicate that D2 deformation, including tectonic juxtaposition and high-grade metamorphism, occurred between 442 and 431 Ma in the rocks west of the Waterbury dome.