NEWLY RECOGNIZED GRANITE MAP UNITS REPLACE MIXED META-SEDIMENTS+GRANITE IN THE NORTHERN STAMFORD AND POUND RIDGE QUADRANGLES, SOUTHWESTERN CONNECTICUT

During recent 1:24000 scale bedrock mapping in the northern Stamford and southern Pound Ridge quadrangles in southwestern Connecticut, we have separated three major granites among the polydeformed gneisses and schists shown on Rodgers (1985) state map as Trap Falls Formation and ?Ordovician granite. Orthogneisses on Frank’s (unpublished 1971) map of the Stamford quadrangle include elongated bodies of grt-bt-ms adamellite gneiss and “reddish colored Siscowit” granite. Also included in our map area are meta-gabbro and meta-diorite, previously correlated with the Harrison Gneiss southwestwards towards New York and with other mafic orthogneiss in the Danbury quadrangle near Bethel. The three major granites we mapped are: (1) fine- to medium-grained, light gray colored, bt-ms granite (well exposed adjacent to Mead Farm in Greenwich at 41°06'20.31"N 73°35'25.48"W); (2) medium- to coarse-grained ms granite with large, typically terra cotta colored K-feldspar phenocrysts (well exposed along the ridge in North Stamford at 41°10'52.78"N 73°35'40.64"W); and (3) medium-grained, leucocratic ms granite characterized by ubiquitous small garnets. On outcrop, the bt-ms granite is intruded by the large K-feldspar granite, and also by the leucocratic grt-bearing granite. The relative ages of the large K-feldspar granite and the leucocratic granite are uncertain. We have also mapped a variety of fine-grained, well-foliated bt-ms-pl-qtz-kfs schists that we interpret as layers of strongly deformed bt-ms granite. Based on our whole-rock geochemical data to date, the bt-ms and leucocratic granites are geochemically similar and have higher Hf, Rb, Sr, Ta, U, and Zr compared to the large K-feldspar granite. Without crystallization ages, correlation of our three main granites with others in the region are not yet possible.