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