RYE ON THE SHOALS - NEW AGES, REMAINING QUESTIONS

The early Paleozoic Rye Complex of coastal New Hampshire and SW Maine is correlated with the variably mylonitized quartzo-feldspathic gneisses with a variety of metasedimentary enclaves cut by weakly to unfoliated granite, granitic pegmatite and diorite that comprise the Isles of Shoals, ~10km offshore Portsmouth, NH, and Kittery, ME. Lithic and structural similarity/continuity, shear sense in the more strongly deformed units, and metamorphic character/grade support that correlation. Like the onshore package, all are intruded by northeast-trending Mesozoic diabase dikes.

The initial age determinations of gneisses, granite, and diorite on the Isles of Shoals attempted decades ago were refined in the compilation of the Kittery 1:100,000 quadrangle (Hussey and others, 2016; Dorais and others, 2014). The Appledore diorite yielded a U-Pb_zirconCA-TIMS age of ca. 361 Ma and cuts both gneisses and some granite. In places diorite is intimately intermingled with granite, and both are cut by unfoliated granite dikes. The complexities of zircons extracted from the intermingled granite have so far made it impossible to resolve the question of diorite-granite consanguinity.

We report a new U-Pb_zirconCA-TIMS date of ca. 385Ma that places an upper limit on the age of a medium-grained, weakly foliated granite from an old quarry on Star Island, which adds one more important point in the long historical trajectory of this belt of Rye rocks. Late Devonian magmatism nicely ties these rocks to other dated coastal NH and ME granitic plutons. But geochronology from the gneisses and their metamorphic enclaves remain illusive. Ongoing efforts to isolate high-quality zircons suitable for CA-TIMS analysis and/or LA-ICP-MS analysis will help refine temporal elements of this enigmatic belt in the near future.