PRELIMINARY BEDROCK GEOLOGIC MAP AND RELATED STUDIES OF THE ROXBURY AND WOODBURY 7.5-MINUTE QUADRANGLES, WESTERN CONNECTICUT UPLANDS

New bedrock mapping of the Roxbury and Woodbury 7.5-minute quadrangles, combined with igneous and detrital zircon U-Pb geochronology, updates 1:24,000-scale mapping and provides new insights into the Paleozoic tectonic evolution of western CT. Rocks in Roxbury consist of a thick succession of interlayered pelitic schists and foliated quartzose to feldspathic meta-sandstones. The schists contain syn- to post-tectonic euhedral garnets, staurolite, and kyanite signifying peak amphibolite-facies metamorphism roughly concurrent with the dominant regional S2 tectonic foliation. The Mine Hill Granite in Roxbury quad is strongly foliated by S2 and recent U-Pb sampling will provide a maximum age for the tectonic fabric. Detrital zircons (dZ) indicate these rocks likely comprise a single, Laurentian-derived litho-tectonic package that was deposited prior to early Paleozoic convergence with terranes to the east. The adjacent western Woodbury quadrangle contains the same Laurentian-derived metasedimentary package, but a tectono-metamorphic boundary separates these rocks from higher-grade migmatitic, sillimanite-bearing rusty schist and black and white banded paragneiss to the east. A foliated metadiorite that intruded the higher-grade rocks yielded a U-Pb zircon age of 442+4 Ma. Between the Laurentian rocks and the migmatitic rocks lies a distinctive belt of interlayered coarse-grained biotite-muscovite schist, well-layered metasandstones, and amphibolites. Distinctive lithologies and dZ spectra indicate these rocks are part of the Silurian and Devonian (S-D) Straits Schist belt. The mostly unfoliated Nonewaug Granite intruded the S-D belt and recent U-Pb zircon sampling should provide a minimum age for the belt. A foliated granite, separate from the main pluton, yielded a U-Pb zircon age of 433+5 Ma.

The early Mesozoic Pomperaug rift basin extends north into the Woodbury quad with an eastern border fault that is nearly coincident with the older tectonic boundary described above. The border fault terminates to the north within the Nonewaug Granite, suggesting that the original throw of the fault was not great.