REVIEW OF BROAD-SCALE GEOCHEMICAL CHARACTERISTICS OF THE ORDOVICIAN TACONIAN VOLCANIC ARC OF WESTERN NEW ENGLAND

The Bronson Hill belt (BHB) of gneiss domes and stratified cover rocks in western New England, and its fault-displaced Shelburne Falls belt (SFB) counterpart to the west, hold a complex record of Ordovician development of the Taconian volcanic arc in western New England. This talk will review broad geochemical characteristics of this arc system. The ages of plutonic and volcanic rocks associated with the Taconian arc in one way or another extend from about 504 to 428 Ma, a span of 84 Ma. This included distinct tectonic episodes: arc initiation along the Gondwanan margin, rifting away of the arc with back arc basin opening, arc activity as a volcanic chain in the Iapetus Ocean, collision of the arc with the Laurentian margin, and post-collision igneous activity related to subducted slab detachment and foundering. This resulted in a variety of igneous rock geochemical characteristics, sedimentary sequences, detrital zircons, and complicated structural relationships in part caused by post-Ordovician deformation. Taconian igneous rocks have been interpreted to include back-arc basin volcanics, calc-alkaline gabbroic to granitic plutonic and volcanic rocks, adakitic rocks, mid-ocean ridge basalts (mostly in Taconian thrust slices to the west), and forearc rocks (mostly in the SFB). In terms of geochemistry, the metamorphosed plutonic rocks of the Taconian arc roots now exposed in the cores of structural domes in the BHB, are almost entirely calc-alkaline, but have distinct compositional characteristics along its length. Rocks tend to be more granitic to the north, and are generally tonalites and granodiorites to the south. BHB plutonic rocks have a strong compositional signature indicating contributions from melted subducted sediment, rather than exclusively fluids derived from dehydrating subducted oceanic mafic crust. This suggests that, throughout arc life, sediment was subducted (more in the north, less in the south) and, more to the point, the subducting slab top was at the correct P-T conditions (roughly 760-800° at 40-50 kbars) to melt the sediments and allow melts rise into the mantle wedge arc magma source region. Adakitic rocks, indicative of garnet in the original magma source region, are also found along the BHB length, and in some “Highlandcroft” plutons which lie to the west of the BHB.