NEW U-PB ZIRCON AGES FROM THE BRONSON HILL ANTICLINORIUM, WEST-CENTRAL NEW HAMPSHIRE

Four intrusive rocks from the Bronson Hill anticlinorium in west-central New Hampshire yield reliable SHRIMP U-Pb zircon ages from the Early Ordovician to Early Silurian and Middle Devonian. The Plainfield tonalite (473 ± 5 Ma) intrudes the Ammonoosuc Volcanics and contains zircon grains that are brown, euhedral, have healed fractures, and exhibit oscillatory zoning and a lack of inherited cores.. The Sugar River granodiorite (460 ± 3 Ma) also intrudes the Ammonoosuc Volcanics and contains zircon grains that are clear to pink, euhedral, and also exhibit oscillatory zoning and a lack of inherited cores. The Lebanon granite and quartz diorite are the two rock types that make up the Lebanon pluton. The granite cores the pluton while quartz diorite forms a border phase. The quartz diorite intrudes the Ordovician Partridge Formation. Preliminary zircon analysis from the quartz diorite suggests the rock is ~447 Ma, but the data are not statistically robust. Zircon grains from the Lebanon granite (441 ± 5 Ma) are clear, euhedral, and lack inherited cores. The Bald Mountain granite (395 ± 4 Ma) intrudes the Devonian Littleton Formation and marks the onset of magmatism related to the Acadian orogeny in this area. Zircon grains are small (< 100 µm), poorly formed, and exhibit oscillatory zoning without inheritance. The Bald Mountain granite is truncated by a lower greenschist facies shear zone within the Littleton Formation that places staurolite-grade rocks over garnet-grade rocks.

These new ages indicate that the Bronson Hill arc in western New Hampshire is older than previously thought and was active for at least 30 m.y., from 473 Ma to 441 Ma. Acadian orogenic magmatism occurred in the Middle Devonian and about 11 m.y. after documented volcanic layers in the Littleton Formation. The age of the Plainfield tonalite overlaps with younger ages from the Shelburne Falls arc of Vermont and Massachusetts (502 to 470 Ma). Previous workers suggested that the Shelburne Falls arc was an older and separate arc from the younger Bronson Hill arc based largely on the lack of dated magmatism between about 470 and 450 Ma. The new data suggest that there may have been one long-lived composite arc along the Laurentian-Ganderian margin.