EVIDENCE FOR A LATE ACADIAN ARRIVAL OF THE EXTREME WESTERN MARGIN OF AVALON TERRANE

New U-Pb SHRIMP data from zircons from the Stony Creek granite in the extreme SW part of anatectic south-central Connecticut east of New Haven reveal a late Devonian crystallization age prior to an Alleghanian remelting event. Cathodoluminescent imagery shows zircons are universally zoned, with two generations of cores and at least two generations of overgrowths. Oscillatory zoning in inner cores (364 +/- 3 Ma, n=14) are commonly truncated by an outer core (342 +/- 7 Ma, n=11) of similar luminescence. Most core regions contain multiple cross-cutting zircon ‘veins’ that heal fractures in the double cores of grains. These veins are connected to overgrowths wide enough for SHRIMP analysis that yields inner rim and outer rim ages of 289 +/- 2 Ma n=14 and 281 +/- 3 Ma n=6 respectively.

These data reveal two surprises. First the Stony Creek granite has a complicated Late Devonian crystallization history; it is not late Proterozoic as anticipated; even Neoproterozoic inherited cores are absent from the 45 grains analyzed. Second, most zircon cores show multiple fracturing events that are healed by at least generations of Permian zircon. These age relationships are consistent with a protracted Late Acadian event with the late (Devonian) arrival of distal portions of the Avalon terrane to the accretionary margin of proto North America. It also suggests that considerable shortening by tectonic wedging is necessary to bring rocks of this age so far west. Moreover, the intensely fractured mid-Paleozoic zircon grains suggest that rocks in the Stony Creek area were strong enough to break the zircons, probably during a cataclastic event, before the multiple higher temperature resorption and overgrowth events occurred in the early Permian. Given that the Permian event was eventually anatectic, the rocks of the Stony Creek area must have been cold in the Late Carboniferous, such that cataclasis in the silicates could have been propagated into the stronger zircons.