DETRITAL ZIRCON GEOCHRONOLOGY OF THE BRONSON HILL ANTICLINORIUM AND WESTERN FACIES OF THE CENTRAL MAINE TERRANE, SOUTHERN NH–VT

Detrital zircon U-Pb geochronology (SHRIMP-RG and LA-ICP-MS) of Ordovician to Devonian metasedimentary rocks of the Bronson Hill anticlinorium (BHA) and central Maine terrane in southern VT–NH provides new insight into the provenance, age, and crustal affinity of these units. Samples include: (1) Ordovician Partridge Fm. (n=2), Silurian Clough Quartzite (n=1) and Silurian Fitch Fm. (n=1) from Skitchewaug Mtn., VT, (2) Partridge Fm. black, sulfidic quartzite and a felsic granofels from the core of the Skitchewaug nappe 4–6 km west of Acworth, NH (n=2), (3) Devonian Littleton Fm. in the Monroe nappe near Bellow Falls, VT, at Saxtons River, VT, and the Skitchewaug nappe near Claremont, NH (n=3), and (4) Silurian Rangeley Fm., central Maine terrane, at Fall Mountain and ~1 km to the east of the Brennan Hill thrust on Grout Hill (n=2).

Detrital zircon age spectra are similar for all samples: abundant 1.3–1.0 Ga zircon with varying proportions of mid-Paleozoic (0.47–0.43 and 0.43–0.41 Ga), Neoproterozoic (0.65–0.56 Ga), Paleoproterozoic (1.8–1.6 Ga), and Archean zircon (2.8–2.5 Ga; oldest 2.9 Ga). The maximum depositional age of the Partridge Fm., defined by the youngest detrital zircon, is 452 Ma. Mesoproterozoic zircon are typical of the Grenville orogen and common to many cratons; however, 1.8–1.6 Ga and 2.8–2.5 Ga grains suggest Laurentian mid-continent sources. Neoproterozoic ages in the Partridge Fm. suggest peri-Gondwanan sources such as Gander or Avalon terranes, and/or recycled zircon from the Rowe-Hawley zone. Age populations in the Clough and Fitch Fms. suggest recycling from the Partridge and other parts of the BHA. In the Rangeley and Littleton samples, mid-Paleozoic zircons are common and peri–Gondwanan ages occur in all samples; Mesoproterozoic ages are less abundant. Silurian zircon (~422 Ma) in the Rangeley Fm. and a large population of ~408 Ma zircon from the Littleton Fm. constrain the maximum depositional ages of these units, respectively. The decrease in abundance of 1.3–1.0 Ga zircon and input of Silurian and Devonian zircon suggest fundamental changes in provenance of the BHA, including derivation of sediment from magmatic arcs formed during collision of Avalon with Laurentia; however, a peri-Gondwanan source(s) is required for the Partridge, Rangeley and Littleton Fms.