SHRIMP GEOCHRONOLOGY OF DETRITAL ZIRCONS FROM METASEDIMENTARY ROCKS IN SOUTH-CENTRAL CONNECTICUT: PRELIMINARY INTERPRETATIONS OF THE TECTONIC EVOLUTION OF THE AVALON TERRANE OF SOUTHERN NEW ENGLAND

New SHRIMP U-Pb ages of detrital zircons from several lithostratigraphic units in south-central Connecticut provide age constraints for their times of deposition and metamorphism, and show that traditional correlations of these high grade rocks with lower grade equivalents to the north require major revision. Previous geologic mapping, reinterpreted in the modern context of assembly of Bronson Hill, Central Maine, Merrimack, Putnam, and Avalon terranes, leads to unacceptable correlations of units in these different terranes.

Age distributions of detrital zircons from four samples of Hebron Fm. are similar to those from the Berwick Fm. of southern Maine, supporting the correlation of these rocks in the Merrimack terrane. These data indicate a Silurian deposition age, and are consistent with earlier mapping of rocks assigned to the Merrimack terrane south and east into the Avalon terrane. Detrital zircons from the Putnam terrane, near the type locality of the Tatnic Hill Fm., probably were derived from Laurentian sources (0.95-1.8 Ga), deposited after 490 Ma, and were severely overprinted by Acadian metamorphism. Two samples previously mapped as correlatives of the Tatnic Hill Fm. on the limbs of tight folds extending into Avalon are not correlative. Zircons from a biotite schist near Old Saybrook are surprisingly young (<350 Ma, i.e. Mississippian), and were derived from both Laurentian (0.9-1.7 Ga) and Devonian sources. Zircons from a biotite schist from the east side of the Lyme dome are <440 Ma and derived primarily from Avalonian sources (0.50-0.65 Ga). Quartzo-feldspathic gneiss previously mapped as Ordovician Brimfield Fm. is younger than 400 Ma, and was derived entirely from sources in the Bronson Hill terrane (0.40-0.49 Ga).

On the basis of these new data, previous structural interpretations of antiforms and synforms must be abandoned. We suggest that parts of the Merrimack terrane and Mississippian metasedimentary rocks occur as fault slivers around the Lyme dome, separating two domains of Avalon terrane that differ in age by ~20-30 m.y. (590-600 Ma vs. 620 Ma). Other faults isolate Devonian to Mississippian rocks between Avalon and Bronson Hill terranes. The identification of these late Paleozoic metasedimentary rocks constrains the final assembly of Avalon terrane with Laurentia to the Alleghanian.