DETRITAL ZIRCONS FROM MODERN SANDS IN NEW ENGLAND AND THE TIMING OF PROTEROZOIC TO MESOZOIC MAGMATISM

We report detrital zircon ages from sands collected from rivers and beaches of New England. The primary objectives were to date Paleozoic igneous pulses and to assess the correspondence between detrital ages and conventional igneous ages. About 1200 detrital zircons from 10 samples were analyzed by LAICPMS; 984 ages met strict concordance filters. Sample locations and maxima in zircon age distributions (in Ma) are as follows, with numbers in boldface corresponding to the largest populations: Kennebec and Androscoggin watersheds combined—1104, 996, 436, 371 and 189 Ma; Presumpscot watershed—373 and 299; Saco watershed—1350, 1112, 441, 388, 291, 185, and 124; Merrimack watershed—601, 404, 371, and 206; upper Connecticut watershed —1385, 1209, 1041, 432, 388, 185, and 132; Blackstone watershed —604, 439, and 365; Pawcatuck watershed —595, 377, and 278; Thames watershed —931, 586, 375, and 279; lower Connecticut watershed —1206, 1059, 953, 588, 447, 388, 359, and 271; and Cape Cod—1331, 1009, 413, 367, and 199. A composite plot of 984 detrital zircon ages from modern sands and a new plot of 200 igneous ages from the literature show similar age distributions. The correspondence between the two shows that in this glaciated region, detrital zircons from rivers and beaches faithfully reflect known bedrock zircon sources. This is encouraging because it is far easier to scoop up a sample of sand from the mouth of a river and date 100 or so zircons from it than it is to map, sample, and date each of the zircon-bearing bedrock units in the river's watershed.

Based on both datasets, the times of peak magmatism in the New England Appalachians are at 580-610, 440-450, 420, 405, 370-380, 291, 180-200 and 125 Ma. Most detrital zircons from the modern passive margin 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, i.e., the Cambrian passive margin of the Appalachians. The Cambrian and modern detrital zircon samples both show what might be expected for peri-cratonic passive margins: most zircons were derived from what at the time was the youngest nearby orogenic belt; there is only a muted zircon signal from older sources.