TECTONIC IMPLICATION OF NEW U-PB ZIRCON AGES IN THE BRONSON HILL ARC: WESTERN NEW HAMPSHIRE, USA

U-Pb zircon geochronology by sensitive high resolution ion microprobe–reverse geometry (SHRIMP-RG) on 11 plutonic rocks and two volcanic rocks from the Bronson Hill arc in western New Hampshire yield Early to Late Ordovician ages ranging from 475 to 445 Ma. Ages from older trondhjemitic/tonalitic Oliverian Plutonic Suite rocks that intrude the mostly mafic lower section of the Ammonoosuc Volcanics range from 474.8 ± 5.2 to 460.2 ± 3.4 Ma. Metamorphosed felsic volcanic rocks from within the Ammonoosuc Volcanics are contemporaneous with pluton emplacement and yield ages of 460.1 ± 2.4 and 455.0 ± 11 Ma. Younger granodiorite to granite Oliverian Plutonic Suite rocks that intrude either the upper and lower Ammonoosuc Volcanics or the overlying Partridge Formation range in age from 456.1 ± 6.7 Ma to 445.2 ± 6.7 Ma.

These new data and previously published results document extended magmatism for >30 m.y. The Early to Late Ordovician ages presented here overlap with previously determined igneous U-Pb zircon ages in the Shelburne Falls arc to the west, suggesting that the Bronson Hill arc and the Shelburne Falls arc are part of one, long-lived composite arc system. Magma compositions show a general trend from tonalite-trondhjemite in older plutons to granite and granodiorite in younger plutons. These ages and changes to more evolved magma compositions are consistent with (1) a volcanic arc that formed over a single east-dipping subduction zone and (2) incorporated Laurentian derived material into volcanic arc magmas.