New radiometric ages on gneisses of the Oliverian domes in New Hampshire and Massachusetts

摘要: Gneissic plutons of the Oliverian domes, mantled by Ammonoosuc Volcanics, are located along axis Bronson Hill anticlinorium from New Hampshire to Connecticut. The contacts between plutonic and volcanic rocks appear be concordant on a regional scale, but gneiss intrudes volcanics in several domes. Available radiometric fossil evidence suggests that Volcanics have Middle Ordovician age somewhat older than gneisses. U-Pb zircon data gneisses six domes plot concordia diagram as an almost colinear array yields upper intercept about 444 m.y. plotted vary nearly moderately discordant, degree discordance, correlating with /sup 207/Pb//sup 206/Pb ages range 459 415 pattern discordance does not relate uranium contents zircons nor geographic distribution If considered individually without assumed consanguinity units, however, they do find particular support geologic relationships. Thus, prefer +/- 8 for suite best estimate time crystallizationmore » Possibly, Whitefield, Gneiss Jefferson dome represents 10 15 unit, although cautious claiming such resolution present data.« less