Geochemistry and tectonic significance of meta-igneous rocks of the Gneiss Dome Belt, southwestern New England Appalachians

The whole-rock geochemistry of upper epidote-amphibolite/lower amphibolite facies meta-igneous rocks of the Gneiss Dome Belt (GDB) exposed in the southwestern New England Appalachians was analyzed to identify their eruptive setting and test the hypothesis that the GDB represents an island arc that collided with Laurentia during the Taconic orogeny. Ninety-nine metabasites, 4 metamorphosed intermediate, and 18 meta-felsic rocks were analyzed for major oxides and 11 trace elements (Ba, Cr, Cu, Nb, Ni, Rb, Sr, V, Y, Zn, and Zr). A subset of 18 metabasites and 8 meta-felsic rocks was also analyzed for REE, Hf, Ta, Th, and U.; Seventy-six metabasites of the Collinsville Formation (CF) have been subdivided into three major, geochemically distinct groups: boninitic and low-Ti IAT, arc-like, and MORB-like. Sixteen metabasites of the Taine Mountain Formation (TMF) and 7 of the Rowe Schist (RS) resemble the arc-like metabasites of the CF modified by crustal contamination. Four meta-andesites and 12 meta-tonalites with I-type granite signatures accompany CF metabasites and carry boninitinc and arc-like signatures. Six crosscutting granodiorites are S-type granites.; The geochemistry of meta-igneous rocks of the GDB is consistent with the opening and evolution of an ensialic backarc basin formed over a west-dipping subduction zone. Available geochemical and geochronological data suggest the following evolution of the GDB: (1) formation of an arc above a west-dipping subduction zone active beneath an attenuated Laurentian margin by the Late Cambrian/Early Ordovician; (2) rifting of the arc during the Tremadoc, recorded by dykes with arc-like signatures modified by a crustal component (TMF and RS metabasites); (3) initial spreading, opening, and then growth of an ensialic backarc during the Arenig recorded by CF metabasites, with initial spreading marked by a bimodal boninitic suite followed by a major episode of MORB-like mafic volcanism and intrusion of I-type, subduction-related tonalites ca. 470 Ma; (4) closing of the backarc basin and assembly of the GDB terrane during the Middle Ordovician; and (5) intrusion of 453-436 Ma collision-related, S-type granodiorites. This tectonic model suggests that the Taconic orogeny in the southwestern New England Appalachians resulted from the Middle-Late Ordovician accretion of an Early Ordovician arc-backarc complex.