AGE AND ORIGIN OF THE KILLINGWORTH DOME, SOUTH-CENTRAL CONNECTICUT: IMPLICATIONS FOR THE LATE PALEOZOIC TECTONIC EVOLUTION OF SOUTHERN NEW ENGLAND

New petrologic and isotopic investigations in the Killingworth dome, southern Conn. comprised of an interior Killingworth complex (KC) and an outer Middletown Complex (MC) reveal a metamorphic and magmatic event of late Paleozoic age. The KC is composed of tonalitic orthogneisses, while associated rocks of the MC are bimodal metavolcanics of calc-alkaline affinity. Rocks of the KC were emplaced at ~460 Ma , at about the same time as eruption and deposition of volcanic-sedimentary rocks of the MC (459 ± 4 and 449 ± 4 Ma). However, the tonalitic core of the dome was intruded by a Mississippian (339 ± 3 Ma) tonalite. Pb and Nd isotopic data suggest that all rocks of the KC, including the younger core resulted from mixing of Neoproterozoic Gander and lower crustal sources, whereas rocks of the MC were derived from mixtures of Gander lower crustal and primitive sources. The less radiogenic component of the KC is similar in isotopic composition to Laurentian (Grenville) crust. However, because Gander terrane is peri-Gondwanan, the isotopic signatures of KC rocks show that they probably were derived from Gander basement that contained detritus from non-Laurentian sources such as Amazonia, Baltica, or Oaxaquia. We suggest that the KC formed above an east-dipping subduction zone on the west margin of the Gander terrane, whereas the MC formed to the east in a back-arc rift environment. The Alleghanian orogeny involved the accretion of the Avalon terrane in the Carboniferous to Permian times. On a regional scale it resulted in the wedging of Avalon crust between Gander cover and basement. On a more local scale, it involved intrusion of the Mississippian core orthogneiss, thrusting of the MC over the KC, and anatexis of the surrounding KC. Despite the similarities of age, structural setting, and geographic continuity of the Killingworth dome with Oliverian domes in central and northern New England, isotopic data suggest that the KC and MC (Gander origin) were not part of the Laurentian-derived Bronson Hill arc. The trace of the Ordovician Iapetan suture (equivalent to the Red Indian line in southwestern New Hampshire) between rocks of Laurentian and Ganderian origin was probably modified during Alleghanian deformation, and may lie west of the Pelham dome of north-central Massachusetts and under the Mesozoic Hartford basin.