THE ROWE-HAWLEY ZONE IN WESTERN CONNECTICUT: CORRELATIONS AND CONUNDRUMS

The along-strike correlation of the Rowe-Hawley Zone (RHZ) from northern New England into the high-grade metasediments of western Connecticut (wC) has historical roots and is supported by regional geological relations. The RHZ in wC is a belt of mainly Lower Paleozoic suprasubduction volcanics and subduction complex rocks, and various quartzo-feldspathic gneisses and pelitic schists, all recording Acadian amphibolite facies metamorphism. Cameron’s Line in wC is the southern extension of the major thrust that marks the western boundary of the RHZ along which Iapetan rocks were emplaced on peri-Laurentian slope and rise sediments. The RHZ in wC is truncated on its eastern side by the Mesozoic Hartford Basin: the Triassic New Haven arkose overlies pre-Silurian mafic volcanics and schist for 10 km from Long Island Sound northward; from there, the unconformity is cut out by the Mixville normal fault. The Bronson Hill arc is exposed on the eastern side of the Hartford Basin. The RHZ in wC was emplaced along an Acadian décollement above thrust sheets of migmatitic gneisses exposed in the core of the Waterbury dome (WD); both structural levels share a common post-Acadian history beginning at 378 Ma.

There are geochemical and geochronological data showing the map units of the RHZ in wC do not constitute a single terrane. Amphibolites of the Collinsville Fm., correlated with the Hawley Fm., are boninitic and arc-like, but dominantly MORB, a combination just like modern back-arcs in the western Pacific. εNd_{(475 Ma)} values for Collinsville amphibolites range between 1.4 and 6.9 and form part of an array between depleted mantle and Avalonian crustal values of various suprasubduction zone and arc rocks located along and across strike. Quartzite of the Traps Falls Fm., in the heart of the RHZ south of the WD, contains zircon with ²⁰⁷Pb/²⁰⁶Pb ages between 1113 and 992 Ma, and Trap Falls quartzite and pelitic schist have εNd values ranging between -9.9 and -13.6, and Pb isotopes consistent with derivation from Grenville crust (McDaniel et al., 1997, Can. J. Sci. 34).

A valid pre-Acadian tectonic model of the RHZ in wC might include rifting of Grenville crust, (?-facing) subduction and arc spreading leading to a backarc basin, crustal imbrication of Taconic age, and Early Silurian magmatism and migmatization produced by further crustal thickening.