NEW SHRIMP U-PB AGES FOR LATE PROTEROZOIC AND PALEOZOIC IGNEOUS ROCKS IN THE AVALON AND BRONSON HILL TERRANES OF SOUTH-CENTRAL CONNECTICUT: CONFIRMATIONS AND SURPRISES

New SHRIMP U-Pb ages from the Avalon and Bronson Hill terranes of south-central Connecticut support some interpretations of earlier mapping, but reveal a surprisingly more complicated history than previously suspected. In the Avalon terrane, Late Proterozoic ages (617±6 and 618±7 Ma) of tonalitic gneisses in the core of the Lyme dome agree with ages of intrusive (620 Ma) and extrusive rocks (618 Ma) of the Waterford Group, thus confirming the age of this entire block. An age of 604±5 Ma for orthogneiss of the Clinton dome substantiates its proposed Late Proterozoic age, and proves that Avalonian rocks structurally underlie rocks of the Bronson Hill terrane in southern New England, as they do in the Pelham dome of MA. Devonian ages of 387±6 and 351±6 Ma were obtained from the Potter Hill Gneiss and the New London Gneiss, respectively, granitic rocks previously mapped as Late Proterozoic. These ages prove that large bodies of Devonian granitoids exist in this part of Avalon, thus firmly linking the eastern and western domains of the Avalon terrane in southern New England. Surprises emerging from the new data from the Avalon terrane include ages for folded and foliated gneiss previously mapped as Late Proterozoic, including Hope Valley Alaskite Gneiss (292±10 Ma) and Joshua Rock gneiss (280±4 Ma). Weakly deformed, crosscutting Westerly granite is only slightly younger at about 279±3 Ma. These ages very narrowly constrain the time of folding of the Lyme dome to the Permian (~280 Ma), interpreted to be the time of major crustal shortening and terrane accretion. An age of about 263 Ma, recorded in zircon rims and in monazite, is interpreted as the time of tectonic unroofing as the region cooled through about 500 degrees C (based on previous hornblende Ar/Ar data). In the Bronson Hill terrane, ages of 471±7, 467±5, and 457±5 Ma for tonalitic rocks from the northern, eastern, and southern margins of the Killingworth dome, respectively, confirm proposed Middle Ordovician emplacement for some of the rocks of the dome. More surprising is that core rocks of the Killingworth dome, including the central and eastern gneisses of Webster and Wintsch (1986), are 336±3 and 337±3 Ma, respectively. These Mississippian ages are unusual in New England; we speculate that they relate to a volcanic arc associated with the early stages of continental collision.