GEOCHEMICAL AND TECTONIC EVOLUTION OF THE ORDOVICIAN BRONSON HILL ARC AND SILURIAN AND DEVONIAN CONNECTICUT VALLEY TROUGH: EASTERN VERMONT AND WESTERN NEW HAMPSHIRE, USA

New major and trace element whole rock geochemistry of 94 samples and synthesis of previously published geochemical data from the Bronson Hill arc (BHa) and Connecticut Valley - Gaspé trough (CVt) provides new information on the evolution of the ancient Laurentian-Ganderian margin in the northern Appalachians of New England during, and following the Taconic orogeny. The whole-rock geochemical data from the Ordovician BHa in western New Hampshire indicate that metamorphosed extrusive and intrusive rocks share the same evolving geochemical fingerprint. Mafic and felsic rocks form a bimodal distribution and consistently plot as island arc magmas on geochemical discrimination diagrams. Greenstone and amphibolite plot as tholeiitic basalt and basaltic andesite. Where ages are known, older felsic rocks (ca. 475-460 Ma) in the BHa tend to be more sodic and less potassic than their younger (ca. 460-445 Ma) counterparts, and the geochemistry trends from volcanic arc granites towards syn-collisional granites through time. The data suggest that continental material was incorporated into BHa magmas as the arc approached Laurentia. There is not a geochemical distinction between the Ammonoosuc Volcanics, Partridge Formation, and Oliverian Plutonic Suite, which suggests they share the same magma source and were part of one evolving island arc system that persisted throughout the Ordovician.

Metamorphosed bimodal volcanic and intrusive rocks are present in the overlying Silurian and Devonian (~430 - 407 Ma) CVt and Silurian cover sequence. Mafic rocks plot mostly as tholeiitic basalts. Tectonic discrimination diagrams show that the mafic rocks in the CVt transition from mid-ocean ridge basalt to within-plate basalts. Felsic rocks from the CVt vary from island arc granite to within-plate granite. The geochemical signature of the CVt is consistent with a post collisional back arc, or intra-arc basin origin, where slab-breakoff or crustal attenuation played a key role before transitioning to a foreland basin at the beginning of the Acadian orogeny.