A SUBDUCTION SYSTEM MIGRATING ACROSS IAPETUS CAN EXPLAIN TERRANE AFFINITIES ACROSS WESTERN CONNECTICUT

The tectonic model of Waldron et al. (2014, Geology, v. 42) for the evolution of the early Paleozoic Iapetus Ocean and the northern Appalachian orogen provides a valid framework for interpreting the terrane affinities and geochronology of rocks in western CT between Cameron’s Line and the Hartford Basin. Central to the model is that deformation and terrane assembly occurred as a single sinuous subduction system migrated across Iapetus. Subduction initiation along the margins of both Laurentia and Gondwana is explicitly rejected. The model posits that subduction was active by the Middle Cambrian near the Gondwanan margin. Wintsch et al.’s (2015, GSA Abstrs., v. 47, no. 3) report of detrital zircon ages between 515 and 570 Ma on cores of ~610 Ma and 1.0 and 2.0 Ga from Cobble Mtn and Taine Mtn Fm. metasediments is consistent with a peri-Gondwanan arc source, as are U-Pb monazite ages of 535 and 525 Ma from metasediments in the core of the Waterbury dome. After subduction sweeps into the Laurentian margin, delivering deformed forearc-arc-backarc fragments, subduction reversal leads to west-dipping, peri-Laurentian subduction by 450 Ma. There is scant direct evidence in western CT for the existence of a peri-Laurentian arc before this time. Amphibolites of the Collinsville Fm. (better called the Collinsville Intrusive Suite, CIS) are extensive, but their chemistry is dominated by MORB; their age is unknown although the εNd values of 6 samples from the Collinsville and Granville domes (+1.4 to +6.9) are similar to metabasalts along strike as far north as the Mt. Norris Intrusive Suite in VT. Orthogneisses in western CT have arc chemistry but are no older than 450 Ma, and widespread tonalite and trondjemite in the belt of domes have ages of 434-437 Ma, similar to the Newtown Gneiss to the west (Sevigny & Hanson, 1995, GSA Bull., v. 107). In western CT, the Taine Mtn Fm., CIS, and Silurian-Devonian The Straits Schist were thrust over core rocks of the Waterbury dome during Acadian deformation so these rocks only arrived in their present position well after west-dipping subduction was established. Detrital zircon ages in The Straits Schist (Wintsch et al., 2015, ibid), include 600-650 Ma and 1.0-1.6 Ga, consistent with deposition either in a forearc or backarc that received recycled, peri-Gondwanan and peri-Laurentian detritus.