LATE SILURIAN DEPOSITION AND LATE DEVONIAN METAMORPHISM OF GANDER COVER AT THE SOUTHERN END OF THE CENTRAL MAINE TERRANE: EVIDENCE FROM SHRIMP ANALYSIS OF DETRITAL ZIRCONS

New SHRIMP U-Pb ages of cores and rims of detrital zircons from samples historically assigned to the Ordovician Brimfield Schist show that the depositional age of the metasedimentary rocks can be no older than late Silurian. Seven analyzed samples of rocks called Brimfield on the Bedrock Geological Map of Connecticut include five samples from between the Killingworth and Lyme domes in southern Connecticut, one sample from the eastern margin of the Hopyard basin, and one sample from the Hamilton Reservoir Formation (HRF) in the Westford quadrangle adjacent to the intrusive dioritic gneiss at Hedgehog Hill (HHG). The age of deposition of these rocks is constrained to be younger than the youngest detrital zircons, which in most samples is about 420 Ma, with a few samples containing grains as young as ~410 Ma. A late Silurian age for the HRF sample is constrained by the 416 ± 3 Ma age of the cross-cutting HHG. All samples contain a large fraction of Ordovician and early Silurian grains, and one sample from the eastern margin of the Killingworth dome is entirely composed of grains younger than ~480 Ma. This age population suggests a provenance area proximal to an emerging Ordovician volcanic arc (presumably to the west in current coordinates). Six of seven samples show moderate to large concentrations of Mesoproterozoic zircons, confirming an eastward (current coordinates) transport direction from Grenvillian provenances. Four of the seven samples show a small component of Ediacaran age (630-542 Ma), consistent with these sediments being deposited upon Gander basement rocks. Rims on zircons from five samples were wide enough to analyze, and show overgrowth events from Middle Devonian through the Carboniferous. Rims as old as ~390 Ma suggest rapid deposition and metamorphism of these sediments.

Based on our new data and previously published results, all metasedimentary rocks west of the Quinebaug Formation and east of the Ordovician intrusive rocks on the west side of the Hartford basin, including the Tatnic Hill, Hebron, Southbridge, and Brimfield formations are probably late Silurian in age. This suite of rocks, now at least 15 km thick, suggests an accretionary wedge of sediments accumulated on the eastern margin of the Gander terrane, and metamorphosed during the arrival of the Avalon terrane in the early Devonian.