Northeastern Section - 37th Annual Meeting (March 25-27, 2002)

Synthesis of geochronologic and geochemical data from the tectono-stratigraphic rock units and plutons that compose the Hartland and Gneiss Dome Belts of western Connecticut show that this deeply eroded part of southwestern New England is a collage of fragments, including rocks formed in continental margin, backarc, and arc settings. At least some of these fragments were structurally juxtaposed prior to 470 Ma, based on electron probe U-Pb ages of monazite from migmatite in the Waterbury Dome (WD). Their final assembly and accretion to Laurentia was early Silurian based on concordant TIMS U-Pb ages of xenotime from blastomylonitic granite (Sevigny & Hanson, 1995) that forms part of the ductile shear zone of Cameron's Line. Whole-rock geochemistry, detrital zircon ages, and Nd isotopic data of pelitic schist and quartzite of the Trap Falls Formation are consistent with a Grenville provenance and deposition on a passive continental margin (McDaniel et al., 1997). The geochemistry of boninite and low-Ti IAT, arc-like, and predominant MORB-like metabasites of the Collinsville Formation (CF) is consistent with generation in a backarc basin. The chemistry of arc-like metabasites and the association of the CF with evolved meta-tonalites suggest this backarc had an ensialic character and involved attenuated continental crust. Emplacement of the CF above rocks of the WD indicates its age is pre-Llanvirn. Plutonic rocks that intrude the Hartland Belt represent a Caradoc to Llandovery arc (Sevigny & Hanson, 1993, 1995). Discordant zircons and Pb isotopic data from some of the pre-Silurian arc plutons show them to have Grenvillian inheritance. A fragment of crust -- here named the Hartland Block -- analogous to the Dashwoods Block of western Newfoundland (Waldron and van Staal, 2001) was rifted from the Laurentian margin. On the outboard side of the Hartland Block, a backarc basin developed in a SSZ setting. Based on analogy with modern marginal basins in the western Pacific, this backarc developed over a W-dipping subduction zone with the oceanic plate dipping beneath the Hartland Block. Arrival at this subduction zone of the more distant Bronson Hill arc initiated E-dipping subduction west of the Hartland Block, which allowed a composite Taconic arc to advance upon the Laurentian margin.