TIMING OF HIGH-GRADE ALLEGHANIAN METAMORPHIC OVERPRINT ON ACADIAN METAMORPHISM

The Bronson Hill terrane of central New England preserves a record of both Acadian metamorphism in New Hampshire and Massachusetts and Alleghanian metamorphism in Connecticut. New U-Pb SHRIMP ages from sphenes, together with 40Ar/39Ar cooling ages from amphiboles allow identification of the time and tectonic style of the Alleghanian overprint in Bronson Hill rocks in southern New England.

In New Hampshire and Massachusetts, both sphene and amphibole ages reported in the literature (Tucker and Robinson, 1990; Spear and Harrison, 1989) are Devonian and Mississippian, and are most easily interpreted as cooling from a Middle Devonian, or older, Acadian metamorphic growth and cooling event. Evidence for Alleghanian metamorphism is conspicuously absent. In contrast, SHRIMP U-Pb ages of metamorphic sphene overgrowths on Ordovician igneous sphene from Connecticut are early Permian; no Devonian metamorphic sphene has been identified. In the same rocks, 40Ar/39Ar amphibole cooling ages drop from 345 and 315 Ma in southern Massachusetts to exclusively latest Pennsylvanian and Permian in Connecticut. We interpret this drop in age to be the result of Alleghanian overprinting. Ages decrease from 294 Ma in northern Connecticut to 248 Ma in southern Connecticut.

Several conclusions can be drawn from these data. The youngest amphibole not reset by Alleghanian metamorphism defines the oldest possible age of the Alleghanian in this region, that is 315 Ma. The oldest reset amphibole marks the minimum age of Alleghanian overprinting, that is 294 Ma. SHRIMP ages of metamorphic sphenes are as old as 300 Ma, and mark the time of prograde Alleghanian metamorphism. Thus the time of prograde metamorphism in the Bronson Hill rocks of northern Connecticut is Middle Pennsylvanian, and the time of peak metamorphism is Late Pennsylvanian. Because of the hinged loading and uplift of Bronson Hill rocks indicated by the amphibole age gradient, the time of peak metamorphism in the south was delayed until Middle Permian. Thus, a combination of U-Pb crystallization ages and 40Ar/39Ar cooling ages allow discrimination of polymetamorphism where mapping and structural analysis have been inconclusive.