A MYLONITIC TERRANE SUTURE AND STITCHING PLUTONS IN THE BRANFORD QUADRANGLE, CONNECTICUT

New mapping in the Branford quadrangle in southcentral Connecticut immediately east of the Mesozoic Hartford Basin reveals a major terrane boundary between the southernmost Bronson Hill arc and peri-Gondwanan terranes, a suture not identified on existing maps. The Bronson Hill lithologies north of the suture are dominated by dioritic, granitic, and tonalitic augen and mylonitic orthogneisses. South of the suture migmatitic peri-Gondwanan (p-G) orthogneisses engulfed in granite are beautifully but rarely exposed in coastal outcrops. Stitching the suture zone is a weakly deformed garnet-bearing granite with two additional granite plutons bordering it to the east and west. The rocks record a complicated sequence of deformations. Mylonitic Bronson Hill gneisses dip gently to the north and contain rare outcrop-scale folds with shallowly plunging ~E-W fold hinges approximately parallel to feldspar augen lineations suggesting early westward under-thrusting of peri-Gondwanan terranes. These gneisses are cut by garnet-bearing granitic dikes that are variably transposed to the south suggesting northward under-thrusting of p-G terranes to the north. The actual contact between Bronson Hill rocks and the garnet-bearing granite consists of steeply dipping lineated biotite-fibrolite schist cut by a complex of garnet-bearing granitic and banded pegmatite-aplite dikes. The undated stitching garnet-bearing granite contains only weakly ductile fabrics indicating that it posts-dates most of the deformation. However, the ages of adjacent rocks suggest suturing was nearly complete by the early Permian. The Lighthouse Point granite to the west gives a U-Pb zircon age of 292 ± 3 Ma, and the eastern Stony Creek granite that intruded p-G terranes at ~360 ± 4 Ma was remelted at ~288 ± 3 Ma. Together these results show that the Honey Hill – Hunts Brook tectonic boundary between p-G terranes and cover rocks east of the Bronson Hill arc does continue to the west, but is cut off by the border fault of the Hartford basin.