ISOTOPIC CONSTRAINTS FOR THE AGES OF DEPOSITION AND METAMORPHISM OF METASEDIMENTARY ROCKS, MERRIMACK AND PUTNAM-NASHOBA TERRANES, EASTERN NEW ENGLAND

SHRIMP analyses of detrital, metamorphic and magmatic zircon and metamorphic titanite and monazite provide constraints on the timing of deposition and metamorphism of metasedimentary rocks from the Merrimack terrane, Connecticut and SW Maine, and the Tatnic Hill Formation (THF) of the Putnam-Nashoba terrane, Connecticut. Ages of detrital zircons from three samples of the Hebron Formation (Conn.) and one sample each of the Berwick and Kittery formations (SW Me.) show that all rocks are Silurian (Ludlovian) or younger, and contain Grenvillian and Ordovician components. All units except the Kittery Fm.show bimodal age populations of Ordovician zircons that may correlate with the Shelburne Falls and Bronson Hill arcs. A minimum age of deposition of the Hebron Formation is tightly constrained by the earliest Devonian (Lochkovian, 414±3 Ma) age of the cross-cutting Canterbury gneiss. The Tatnic Hill Formation contains metamorphic zircons and monazites as old as ~402 Ma, showing that high grade metamorphism occurred only about 20 m.y. after deposition. An eastern net transport direction for sediments deposited as the Hebron, Berwick, and Tatnic Hill formations is indicated by the Grenvillian and Ordovician magmatic zircons they contain. Thus, the continental margins of Laurentia and Gander must have merged by late Silurian, closing any oceanic troughs that might have acted as sediment traps. The age distribution of detrital zircons in the Kittery Formation is different from the other older formations, with a near lack of Ordovician, the presence of a small Late Proterozoic (Gondwanan?) and dominant Grenvillian fractions. This age distribution suggests that the western Ordovician arc rocks had been covered by latest Silurian. Westward paleocurrent flow directions suggest an eastern source for these sediments, perhaps from more northerly Ordovician arc rocks or reworking following tilting during the arrival of the Avalon terrane in the early Devonian. Metamorphism of the Tatnic Hill Formation immediately after deposition probably requires deposition and burial in an arc environment rather than the shelf environment of the near time-stratigraphically equivalent Merrimack Group sediments.