GEOCHEMISTRY OF THE TACONIAN VOLCANIC ARC, BRONSON HILL ANTICLINORIUM, WESTERN NEW ENGLAND: TIMING OF MAGMATISM AND COLLISION

The Bronson Hill anticlinorium (BHA) in western New England contains various metamorphosed Ordovician intrusive and volcanic rocks related to east-directed subduction and advance of the Taconian arc complex toward Laurentia. Exposed parts of the batholithic arc roots in the BHA are diverse, but generally calc-alkaline and bimodal with felsic rocks greatly dominant over mafic rocks. Tonalites dominate in the BHA south of the Mascoma dome in west-central New Hampshire, and granites dominate to the north. Geographic variability of the felsic rocks suggests regional differences in deep BHA crustal sources: mafic in the south and intermediate to felsic in the north. Some HREE-depleted felsic rocks in the central and southern BHA imply a deep garnet-rich source. Plutonic rocks exposed in the central and northern BHA have been dated at 456-441 Ma (Tucker and Robinson, 1990; Moench et al., 1995), overlapping the Partridge Formation (449⁺³/_-2 Ma) and Upper Ammonoosuc Volcanics (453±2 Ma; Tucker and Robinson, 1990) in the central BHA. These ages are apparently younger than the Ammonoosuc Volcanics (461±8 Ma) and some intrusions in the northern BHA (e.g., Moench, 1993; Moench et al., 1995) and some dome gneisses in the Shelburne Falls belt west of the BHA (~484-452 Ma; e.g., Karabinos et al., 1998). The BHA rocks are in part older than several plutons within and west of the BHA including the Highlandcroft plutons (452-430 Ma; e.g., Moench et al., 1995), Mt. Hermon gabbro in Massachusetts (Silurian; Elbert, 1984), and some plutons in southwestern Connecticut (453-428 Ma; e.g., Sevigny and Hanson, 1995). These younger rocks post-date the putative time of arc collision and are enigmatic in the Taconian context. Biostratigraphic and structural control in the Taconian foreland and within the orogen suggest that collision continued until at least latest Caradoc (~449 MY). The younger plutons may have been related to detachment and foundering of subducted oceanic crust and mantle lithosphere beneath the arc.