CONSTRAINTS ON CONDITIONS AND TIMING OF SILURO-DEVONIAN EXTENSION: EVIDENCE FROM THE WESTERN CONNECTICUT VALLEY-GASPÉ TROUGH MARGIN

A swarm of trondhjemitic dikes and associated tonalitic plutons located in the Moretown Formation in central Vermont have been dated by U-Pb methods at 412.7 ± 4 and 409.7 ± 8 Ma, respectively. These intrusions make up the Northfield intrusive complex (NIC), and while generally aligned parallel to the regional fabric, they crosscut Ordovician Taconian fold structures, are weakly metamorphosed at muscovite-chlorite grade, and are variably strained. Ages obtained by U-Pb Laser Ablation ICP-MS isotopic measurements of zircons extracted from these rocks reveal four distinct age populations, three from inherited crystals and one from emplacement crystallization. The oldest population is 1.8-2.0 Ga and the next and largest is 1.0-1.5 Ga, both populations being characteristic of Laurentian basement rocks. The third population centers on 650 Ma, suggesting a Pan-African affinity. The final population in each sample is represented by euhedral rims, with the crystallization ages noted above.

The NIC dike swarm and associated small plutons are only 2.5 km west of the faulted western border of the Siluro-Devonian Connecticut Valley-Gaspé Trough (CVGT). Immediately east of the eastern faulted border of the CVGT is another dike swarm, the Ludlow Comerford Intrusive Suite, consisting of tholeiitic basalts derived by partial melting of asthenosphere and intruded at 419±1 Ma (Rankin et al., 2007). Both dike swarms are thought to represent melt products tapped from depth during early extension of the CVGT. The presence of significant populations of inherited zircons in the NIC intrusives having both Laurentian and Pan-African affinities requires a pre-emplacement tectonic scenario in which some materials beneath the region were derived from the east. A working model of partial melting of a foundered slab following the end of the Taconian orogeny is compatible with the trondhjemitic compositions of the NIC and the presence of their Pan-African zircon population.