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

Seven intrusive rocks and one volcanic rock from the Bronson Hill arc in west-central New Hampshire yield reliable, magmatic and volcanic SHRIMP U-Pb zircon ages from the Early to Late Ordovician. The Plainfield tonalite (475 ± 5 Ma) intruded mafic rocks of the Ammonoosuc Volcanics (Oa) and contains zircon grains that exhibit oscillatory zoning and lack inherited cores. The Lebanon tonalite (466 ± 8 Ma) also intruded the Oa and zircon contain rare inherited cores dated at ~575 Ma. The Sugar River granodiorite (460 ± 3 Ma) intruded the Oa at the same time as the extrusion of a felsic lapilli tuff (460 Ma ± 2 Ma) in the Oa near Plainfield. The Croydon Dome granodiorite has a U-Pb age of 454 ± 3 Ma. The Mascoma dome granite (450 ± 4 Ma) lacks inherited cores. The Lebanon granite (448 ± 5 Ma) and quartz diorite (445 ± 7 Ma) comprise the Lebanon pluton, with the granite forming the core while the quartz diorite forms a border phase that intruded the Ordovician Partridge Formation.

The new ages indicate that parts of the Bronson Hill arc in western NH are older than reliably dated rocks to the south in MA, although similar in age to rocks in northwest NH, and that the arc was active for at least 35 m.y., from ~475 Ma to 440 Ma. The age of the Plainfield tonalite overlaps with younger ages from the Shelburne Falls arc of VT and MA (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 the Bronson Hill and Shelburne Falls arcs are part of a complex and long-lived arc system. Rare inherited zircon cores in the Lebanon tonalite are similar in age to Peri-Gondwanan zircon and suggest that the Bronson Hill-Shelburne Falls composite arc is built on Ganderian crust. This is consistent with recent geochemical data (Dorias, 2011) and detrital zircon ages from VT (Macdonald et al., 2014) that indicate the Laurentian-Ganderian suture (Red Indian Line) lies to the west of the arc.