U-PB LASER ABLATION INDUCTIVELY COUPLED MASS SPECTROMETRY (U-PB LA-ICP-MS) ZIRCON AGE CONSTRAINTS ON THE TECTONIC ORIGIN OF THE AVALON TERRANE IN SOUTHEASTERN NEW ENGLAND

The Avalon terrane of southeastern New England accreted to Laurentia in the middle Paleozoic during the Acadian orogeny. During the late Paleozoic Alleghenian orogeny and assemblage of Pangea, the northwest African part of Gondwana collided with Laurentia, closing the Rheic Ocean along a suture zone. This interpretation suggests that the Avalon terrane lies west of an offshore suture. However, recent U-Pb detrital zircon data from metawacke in drill core from the Georges Bank in offshore Massachusetts showed evidence for a fragment of northwest African crust that remained after the breakup of Pangea. The purpose of this study is to test whether this fragment extends on land, and if so, constrain the location of the suture.

U-Pb LA-ICP-MS analysis and cathodoluminescence imaging was executed on detrital zircon from two metasedimentary samples (100-125 grains each) to investigate their provenance, and on inherited zircon from four igneous samples (50-100 grains each) to test the age(s) of the basement and examine internal structures. All four igneous samples yielded Neoproterozoic crystallization ages. Inherited cores are up to only 30 Ma older than the crystallization ages, which may be consistent with either Avalonian or northwest African basement. Quartzite from the Hoppin Hill Reservoir, Massachusetts, has large Neo- and Mesoproterozoic peaks with some Paleoproterozoic dates, consistent with Avalonia. Phyllite from Newport, Rhode Island yielded large 647 and 745 Ma peaks, one ~1.4 Ga date, and a few ~2.0 Ga dates. Though the 745 Ma peak is atypical for Avalonia, a ~780-630 Ma population was recognized previously in the Hammondvale Metamorphic Suite near the northwestern margin of Avalonian terrane in New Brunswick. Possible source areas for those are suggested to include the ~760 Ma Burin Group in Newfoundland, the ~734 Ma Economy River gneiss in mainland Nova Scotia, and the ~680 Ma Stirling Group on Cape Breton Island. Those or equivalent sources may have contributed to our ~745 Ma zircon population. Non-Avalonian potential sources include Cadomia, a Gondwanan-derived terrane exposed in parts of Europe, or West Africa. However, the 745 Ma peak and Mesoproterozoic ages are atypical for northwest Africa. The location of the Pangean suture zone remains unclear and will be further investigated by ongoing analysis.