GEOCHRONOLOGICAL CONSTRAINTS ON DUCTILE DEFORMATION, CRUSTAL EXTENSION, AND DOMING ABOUT A BASEMENT-COVER BOUNDARY, NEW-ENGLAND APPALACHIANS

An integrated geochronological/microstructural investigation at the Willimantic dome, Connecticut, has determined the age of gneissic and mylonitic deformation fabrics in Avalonian basement rocks and two generations of metamorphism and deformation in overlying metamorphic cover rocks. Mylonitic amphibolites along the basement-cover contact were totally recrystallized during lower amphibolite-grade, northwest-southeast oriented ductile stretching. Analytically identical Sm-Nd, Rb-Sr, and U-Pb mineral isochrons (272 +/- 7, 274 +/- 15, 258 +/- 6 Ma, respectively) indicate that minerals achieved complete equilibration by ductile deformation processes and underwent only very limited isotopic exchange during cooling. At deep structural levels in the core of the dome, U-Pb analyses of sphene from a granitic gneiss reveal rare sphene porphyroclasts preserving late Precambrian radiogenic components an a dominant population of clear sphene euhedra which grew during penetrative ductile deformation at 304 +/- 2 Ma. Rb-Sr and Sm-Nd mineral compositions form scattered arrays due to relict, pre-300 Ma radiogenic components and to partial exchange during a later non-penetrative deformational overprint related to ductile stretching at higher structural levels at approximately 275 Ma). Within the cover rocks, pelitic gneisses (> sillimanite grade) occur as lozenges surrounded by an anastomosing network of approximately lower- to mid-amphibolite grade extensional shears. Inside one lozenge, concordant U-Pb ages for coarse monazite (403 +/- 1 Ma) and a two-point Sm-Nd isochron for monazite and garnet (405 +/- 13 Ma) demonstrate that the associated metamorphism is Devonian (Acadian) in age. In a lozenge-bounding extensional shear 2 m away, monazites are discordant and have markedly younger U-Pb ages which vary with grain size. All these monazite data form a linear discordia array (intercepts at 392 +/- 7 Ma and 286 +/- 20 Ma) produced by late Paleozoic extensional shearing in the cover. The discordia array records a disturbance of U-Pb radiometric systems in monazite during a discrete deformational/metamorphic event, not mineral cooling ages. These data define a sharp discontinuity in isotopic ages about the basement-cover contact and indicate that basement and cover record different tectonic and thermal histories until they were juxtaposed along extensional shear zones in the late Paleozoic. Combined with other evidence of mid-crustal penetrative strain in Avalonian basement at 290 to 305 Ma throughout south-central New England, it is proposed that Avalonian crust was emplaced beneath the Acadian metamorphic belt in the Carboniferous during tectonic convergence. The subsequent heating and gravitational collapse of the thickened, composite structural section led to crustal melting (270-285 Ma), extensional faulting (265-280 Ma), and possibly the removal of structural section between basement and cover as these rock groups were juxtaposed in the Permian for the first time. Avalonian crystalline rocks were progressively uplifted and cooled during the Permian, leaving a domed basement-cover contact throughout the region.