TRACE ELEMENT EVIDENCE FOR A TRANSITIONAL ARC TECTONIC SETTING OF THE CANTERBURY GNEISS, EASTERN CONNECTICUT

X-ray fluorescence analysis for trace elements in 20 samples collected from the Canterbury gneiss in eastern Connecticut suggest that the magma evolved from a volcanic-arc to an intra-plate tectonic setting during intrusion. The Canterbury orthogneiss of eastern Connecticut intruded as a sill into the Late Silurian Hebron Formation. The orthogneiss is a medium to coarse-grained, weakly foliated, plagioclase-quartz-orthoclase-biotite granite to granodiorite. The rocks are peraluminous, S-type, and range from alkalic to calcic (Frost et al., 2001). U-Pb ages of concordant zircons show the body to have crystallized in the Early Devonian (~414 Ma). Major and selected trace element concentrations of samples of the Canterbury gneiss have been analyzed. By plotting them on the appropriate discrimination diagrams, the tectonic setting of this sedimentary basin can be constrained during the time of intrusion. Normative compositions of samples from across the 100 km length of the outcrop show that it is transitional between magnesian granite and granodiorite.

The trace element compositions of 20 samples straddle the volcanic arc granite-within plate granite boundary on the Rb-(Y+Nb) diagram (Pearce et al., 1984) suggesting a transitional tectonic setting. However, open system metamorphism might have modified the concentrations of the more soluble trace elements, and their concentrations may not be a reliable indicator of igneous provenance. Application of the diagram using only the refractory elements Y and Nb is a test of such open system behavior, and yields identical results, supporting both this transitional tectonic setting and a nearly isochemical metamorphism. Collectively, the results suggest that the tectonic setting of the Canterbury orthogneiss evolved from a volcanic arc to a within plate environment. This is consistent with a tectonic model calling on magma generation by the high heat flow caused by early Devonian subduction, slab roll-back, and asthenospheric upwelling following the collision of the Gander and Laurentian plates at the early stages of the Acadian orogeny.