EVIDENCE FROM ZIRCONS FOR A COLD AND HARD ALLEGHANIAN COLLISION OF AVALONIA WITH COMPOSITE LAURENTIA IN SOUTHERN NEW ENGLAND

Zircons from four granitic gneisses from southern New England considered to have intruded peri-Gondwanan terranes were analyzed to test for a record of Acadian metamorphism. From east to west the samples are from the Narragansett Pier granite (Rhode Island), Potter Hill gneiss (Stonington, Conn), the Stony Creek granite (Guilford, Conn), and the Light House gneiss (East Haven, Conn.). The cores of some zircon grains in the Light House and Narragansett Pier gneisses are ~ 610 Ma, similar to adjacent orthogneisses, while the majority of cores from the Potter Hill gneiss are ~610 Ma. The Stony Creek gneiss alone has only one core of Mesoproterozoic age, while all others are Late Devonian. These results suggest that the former three were derived from peri-Gondwanan terranes, and the Stony Creek granite is in fact a Devonian intrusive rock into peri-Gondwanan gneisses.

In all four gneisses a fracture event is recorded in the core of some zircons, and these fractures are filled with new zircon that connects to the rims. The rim ages are all early Permian, so that means that the fracturing event predated the overgrowth, and was probably Carboniferous. The big implication of this is that the Avalon rocks were COLD when the metamorphism began, such that the zircons fractured during the early stages of the overgrowth, and that the zircon textures together with ages demonstrate a prograde metamorphism in these rocks.