SHRIMP U-PB AGES OF DETRITAL AND METAMORPHIC ZIRCON FROM HIGH-GRADE METASEDIMENTARY ROCKS: REVISING LITHOSTRATIGRAPHIC CORRELATIONS AND TECTONIC MODELS IN SOUTHERN NEW ENGLAND

Existing tectonic models in SE Connecticut call on complex Acadian fold nappes (the Hunts Brook structure) based solely on lithostratigraphic correlations of upper amphibolite facies metasedimentary rocks in the Merrimack (Hebron Formation, HF) and Putnam-Nashoba (Tatnic Hill Formation, THF) terranes. These two formations, previously mapped as Ordovician rocks, occur both structurally above and within the Avalon terrane. SHRIMP U-Pb ages of detrital zircon cores, metamorphic rims, and magmatic grains show that rocks mapped as HF both above and within the Avalon terrane are characterized by Mesoproterozoic (~1900-900 Ma) and bimodal Ordovician-Silurian (~475-420 Ma) populations. THF samples above the Avalon terrane contain a few Mesoproterozoic zircons, but mostly bimodal Ordovician-Silurian populations (420-480 Ma), suggesting Laurentian sources and Silurian deposition. Very few Neoproterozoic-Cambrian grains were found. A minimum deposition age for HF is constrained by the Canterbury Gneiss that intruded HF at about 414 Ma. A minimum deposition age of ~405 Ma for THF is provided by metamorphic overgrowths on detrital zircons. Thus, HF and THF are Silurian-Devonian, not Ordovician as previously interpreted. U-Pb data from detrital zircons in rocks mapped as Ordovician THF within the Avalon terrane do not support this correlation. A sample from the east side of the Hunts Brook structure contains detrital grains as young as ~425 Ma with overgrowths as old as ~395 Ma, indicating a Late Silurian to Early Devonian depositional age; however, most grains are Neoproterozoic-Cambrian, suggesting Gondwanan rather than Laurentian sources. A sample from the west side contains detrital cores as young as Late Devonian. Our geochronologic results and new mapping suggest that the stratigraphic and structural relationships of rocks from disparate terranes, juxtaposed in narrow fault-bounded belts at high metamorphic grades, are more complicated than previously recognized.