Abstract

The U-Th-Pb radiometric age of zircons demonstrates that much of Rhode Island consists of a late Proterozoic plutonic complex that subsequently was intruded by a large Devonian alkalic to subalkalic igneous complex.

Two groups of late Proterozoic rocks can be recognized: (1) the Esmond Granite and related plutonic rocks in northern Rhode Island, ranging in composition from gabbro to granite and yielding an upper concordia intercept of 621 ± 8 m.y., and (2) quartz-rich, recrystallized gneissic rocks (Ten Rod Granite Gneiss and Hope Valley Alaskite Gneiss) in southern Rhode Island, giving an upper concordia intercept age of 601 ± 5 m.y. The lower intercept of the zircon discordia for the gneissic rocks indicates the isotopic systems were disturbed during the late Paleozoic Alleghanian Orogeny, whereas zircons from the Esmond Granite and related plutonic rocks have been subjected mainly to loss of lead attributable to recent dilatancy.

Rocks determined to be of Devonian age include the Scituate Granite and plutonic rocks of the East Greenwich Plutonic Suite. The Quincy Granite of Quinn (1971) in northeastern Rhode Island may be of a similar Devonian age, or it may be slightly younger. Zircon data from these rocks fall upon a chord that gives an upper concordia intercept of 370 ± 7 m.y. and a lower intercept close to the origin. Collectively, these rocks form a composite pluton with an area >700 km2. The Devonian rocks exhibit petrologic characteristics distinct from slightly older Acadian orogenic rocks that occur in lithostratigraphic zones to the west and are more comparable to anorogenic Ordovician to Devonian alkalic granitoids that trend north-northeast across Massachusetts and the Gulf of Maine.

The new age determinations confirm that much of Rhode Island contains plutonic rocks intruded during the Avalonian Orogeny but that a hertofore unrecognized major Devonian plutonic episode also occurred. The distinctly different ages probably had gone unrecognized because of generally similar lithologic and textural features exhibited by the late Proterozoic and Devonian rocks. We present new petrologic and geochemical criteria that should assist in distinguishing the rocks of different ages as additional mapping and geologic study proceeds.

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