AGE OF THE RYE COMPLEX, SE NEW HAMPSHIRE: A DETRITAL ZIRCON STUDY

The Rye Complex in coastal New Hampshire and adjacent Maine consists of an enigmatic group of variably metamorphosed and highly tectonized metasedimentary, granitic and migmatitic rocks. The Portsmouth fault separates these rocks from lower grade feldspathic metasiltones and metashales of the Kittery Fm. of the Merrimack trough. Previous attempts to date the Rye Complex have been frustrated by its complex history and indicate only that it is Ordovician or older.

Detrital zircons from two samples of the Rye were analyzed by ICPMS. Seventy-four datable zircons from Sample 1, a highly sheared micaceous quartzite, produced a range of ages from 529 Ma to 2175 Ma, with distinct peaks at 627 Ma, 794 Ma, 1224 Ma and 1506 Ma. An additional cluster of grains occurs between 1800 Ma and 2200 Ma. The youngest grain is 529.7 ± 8.8 Ma and the average of the three youngest grains is ca. 539 Ma, indicating the rock is early Cambrian or younger. Sample 2 is a mylonitized and altered felsic rock. Its 78 datable zircons fall in a narrow peak with an age of 482 Ma. Because of its felsic nature and its single zircon distribution peak, we interpret this sample to be a highly tectonized granitic intrusion. The equant and stubby zircon crystal morphologies in this sample (length-to-width ratios in the range of 2 to 3) indicate this rock was not of volcanic origin, thus limiting the age of the Rye metasediments to Cambrian or very earliest Ordovician. The data, however, are not definitive as to whether the Rye is Avalonian or Ganderian. The principal peak at 627 Ma resembles that of many Avalonian sediments in the northern Appalachians, but the secondary peaks at 1221 Ma and 1506 Ma are more characteristic of Ganderian detrital zircon suites.