WHOLE-ROCK GEOCHEMISTRY OF INTERMEDIATE TO FELSIC ORTHOGNEISSES IN WESTERN CONNECTICUT RECORD VOLCANIC ARC MAGMATISM

The ages, sources, tectonic settings, and terrane affinities of high-grade and strongly deformed intermediate to felsic orthogneisses in western Connecticut between Cameron’s Line to the west and the Mesozoic Hartford Basin to the east, including rocks in the cores and covers of the Waterbury (WD), Bristol (BD), and Collinsville (CD) domes, remain largely unknown. Wathen et al. (2015) reported ages of 448±3 and 443±2 Ma for porphyritic Pumpkin Ground meta-granodiorite and Beardsley meta-diorite, respectively, exposed in Dieterich’s (1968) Southeastern Belt south of the WD and Dietsch et al. (2010) reported ages of three, syn-migmatitic orthogneisses from the WD between 434±4 and 437±4 Ma. We have compiled previously unpublished whole-rock geochemical data from orthogneisses (n = 29) of the Collinsville Formation (CF) north of the WD, and the Harrison, Pumpkin Ground, Beardsley (HPGB) gneisses south of the WD. We have also compiled and generated new whole-rock data from orthogneisses (n= 28) from the Waterbury Gneiss (WG) and Hop Brook Gneiss (HBG; Dietsch et al., 2010) which form the core of the WD. Core and cover rocks of the WD are separated by a décollement of Acadian age. CF and HPGB orthogneisses range from 57.6-76.2% and 54.3-67.9% SiO₂, respectively, and on Sun’s (1980) multi-element diagram show depletion in Nb, enrichment in HREE, peaks in K and Zr, and depletions in Ti and Sr. On Pearce discrimination diagrams, CF and HPGB orthogneisses all plot in the field of volcanic arc granite. WG and HBG othogneisses range from 60.4-73.3% and 57.5-73.6% SiO₂, respectively, and cannot be differentiated on the basis of their trace or REEs. Two different compositions are found in both units: one is depleted in Th with low La and Ce, the other is depleted in Nb, Ta, and Sr, and has elevated La and Ce. WG and HBG orthogneisses both plot as island arc granite. These data support a model in which multiple arc rocks were juxtaposed during Acadian, not Taconic, deformation.