ARE THE HARRISON GNEISS AND SISCOWIT GRANITE IN THE STAMFORD AND POUND RIDGE QUADRANGLES, CONNECTICUT, CO-GENETIC, NON-ARC MAGMAS?
Four whole-rock samples of variably deformed Harrison and fourteen whole-rock samples of Siscowit were analyzed for their major, minor, and trace elements. The Harrison, where it preserves an igneous flow fabric and is least deformed, has the primary assemblage PL+OPX>CPX overprinted by HBL+BT+QTZ+EP+SPN. Whole-rock analyses range from subalkaline to slightly alkaline with a composition equivalent to basaltic trachy-andesite. On Sun & MacDonough’s 1989 chondrite-normalized REE diagram, samples of Harrison show enrichment in the LREEs, a small negative Eu anomaly, and negatively slope across the HREEs, with the slope flattening out from Er to Lu. The analyzed granitic rocks are peraluminous, calc-alkaline granite. Chondrite-normalized REE patterns of these granites show two groups: one is enriched in LREEs, has a pronounced negative Eu anomaly, and negatively sloping HREEs; the second is less enriched in LREEs compared to the other group, has flat MREEs without an Eu anomaly, and gently positively sloping HREEs. Compositions intermediate between the Harrison and Siscowit are lacking in southwestern CT, but the chemistry of the Bedford Gneiss in the Pound Ridge quadrangle in NY and the Brookfield and Newtown Gneisses in western CT and adjacent NY may link the mafic Harrison to the felsic Siscowit, and to an age.