Detrital zircons from modern sands in New England and the timing of Neoproterozoic to Mesozoic Magmatism

Abstract

We report detrital zircon ages from modern sands from rivers and beaches of New England. Sand samples were collected from the Connecticut, Thames, Pawcatuck, Blackstone, Merrimack, Saco, Presumpscot, and Androscoggin-Kennebec watersheds, and from Cape Cod. About 1200 detrital zircons were analyzed by LAICPMS; 984 of these passed strict filters for concordance. A composite plot of the 984 detrital zircon ages from sands and a new plot of 230 igneous ages from the literature show similar age distributions. On the basis of both datasets, the times of peak magmatism in the New England Appalachians, in round numbers, are at 580-610, 440-450, 420, 405, 370-380, 180-200, and 125 Ma. Most zircons from individual watersheds can be attributed to dated igneous rocks in the same watershed. A few zircons are probably exotic and can be interpreted as having been transported into the sampled watershed by ice, and a handful of other zircons likely represent undated plutons within the sampled watershed. The correspondence between the detrital and igneous datasets underscores the proxy value of detrital zircons from river sands in regional geologic reconnaissance---even in glaciated regions. Most detrital zircons from the modern passive margin of New England are Phanerozoic in age, reflecting various Appalachian sources; the age distribution of zircons older than 700 Ma closely resembles the age distribution of detrital zircons from the next-older passive margin that flanked this part of Laurentia after the breakup of Rodinia, that is, the Cambrian passive margin of the Appalachians.