INVESTIGATING THE BOUNDARIES OF THE SOUTHEASTERN NEW ENGLAND AVALON TERRANE: NEW LA-ICP-MS U-PB ZIRCON DATES FROM IGNEOUS AND METASEDIMENTARY ROCKS

The SE New England Avalon terrane is part of the microcontinent of Avalonia, which rifted from supercontinent Gondwana in the early Paleozoic and accreted to Laurentia during the middle Paleozoic Acadian orogeny. During the ensuing late Paleozoic Alleghanian orogeny, NW Africa was sutured to Laurentia, forming supercontinent Pangea. This suture zone is currently thought to be the location where Pangea subsequently broke apart. However, the recent discovery of a fragment of NW African crust on the Georges Bank in offshore MA, based on detrital zircon data from metawacke in drill core, suggests that the Pangean suture zone lies to the NW of the Georges Bank, on the continental shelf or on land. This work presents the results of new LA-ICP-MS U-Pb analysis on igneous and inherited zircon in four igneous samples and detrital zircon in seven metasedimentary samples to test whether the SE part of what is currently interpreted as Avalon terrane may instead be part of the NW African crustal fragment.

Igneous zircon yielded crystallization ages of ~613-603 Ma with one older ~686 Ma inherited core, which is consistent with either Avalonian or NW African crust. Two Plainfield quartzite samples from Groton, CT and Hopkinton, RI, and two Blackstone quartzite samples from Cumberland, RI and Warwick, RI yielded large Mesoproterozoic detrital zircon populations with a few Paleoproterozoic and Archean dates, suggesting Avalonian provenance. None of these samples yielded grains younger than ~950 Ma. The Hoppin Hill quartzite from North Attleborough, MA has a large ~600 Ma population, in addition to similar populations as the quartzites described above. Two phyllite samples from the Newport Neck formation in Newport, RI, and the Price Neck formation in Middletown, RI yielded large ~650 and ~745 Ma populations and only a few Meso- and Paleoproterozoic dates. The ~745 population and lack of large Mesoproterozoic populations are atypical for the Avalon terrane, and the most viable source is the Pan-African I orogeny in the Anti-Atlas region of Morocco. Therefore, the two phyllite samples may represent part of the NW African crustal fragment discovered below the Georges Bank. Alternatively, the data may imply that the Avalon terrane was derived from the African part of Gondwana, and not the Amazonian part.