IS THE HARRISON GNEISS IN SOUTHWESTERN CONNECTICUT VOLCANIC OR PLUTONIC? A NEW SHRIMP U-PB ZIRCON AGE AND A SUMMARY OF FIELD RELATIONS

The origin of the Harrison Gneiss (HG), exposed in southwestern New England from Harrison, NY northeastwards to Stamford, CT, is uncertain. Hall (1968, 1976) developed a stratigraphy across this area, recognized the HG as a volcanic unit in the Hartland Belt east of Cameron’s Line, and considered it part of an Iapetan sequence accreted during Taconic collision. Recent field studies in the Stamford quad have shown the HG to have some similarities to plutonic rocks in adjacent NY and western CT, particularly rocks of mafic and intermediate composition of the Bedford, Brookfield, and Newtown complexes, which have U-Pb zircon ages of 452±4 (Ratcliffe et al., 2012), and 453±3 and 446±2 Ma (Sevigny and Hanson, 1995), respectively. Where the HG is least deformed, it is a medium-grained quartz diorite with relic ortho- and clinopyroxene overgrown by amphibole and biotite, and small mafic inclusions in it are common; and this rock has a new SHRIMP U-Pb zircon crystallization age of 447.8 ± 3.0 Ma. More widespread throughout rocks mapped as HG in NY and CT is strongly foliated quartz-titanite-biotite-amphibole-plagioclase schistose gneiss; dark colored “inclusions” occur locally in this rock type. In the Stamford quad, sheet-like layers of the schistose gneiss are commonly in contact with, and occur as screens within, various granitic rocks, including biotite granite, muscovite granite, and coarse-grained muscovite pegmatitic granite. Only locally is the granite porphyritic, a common characteristic of intermediate and felsic rocks of the Bedford, Brookfield, and Newtown plutons. Across the Stamford quad, the regional foliation in the schistose gneiss and granite are parallel, and sets of granite boudins are common in the schistose gneiss. Layers of garnet schist and amphibolite locally separate the schistose gneiss and pegmatitic granite. As it has been mapped so far, only small portions of rocks mapped as HG in the southwestern CT have unambiguous plutonic structures and the 447 Ma age of the quartz diorite establishes a common age with definitive plutonic complexes in the region. Much of the belt of rocks mapped by us and others as HG lacks plutonic structures, including cross-cutting contacts, and instead, is characterized by strongly developed tectonic fabrics.