COMPARATIVE STRUCTURAL, METAMORPHIC, AND GEOCHEMICAL TRANSECTS ACROSS THE DUNNAGE AND INTERNAL HUMBER ZONES IN THE NORTHERN VERMONT AND SOUTHERN QUEBEC APPALACHIANS

We will review key similarities and differences in structural, metamorphic, and geochemical data and interpretations across the Internal Humber and Dunnage zones in northern Vermont and Southern Quebec. We will focus on the ophiolitic complexes, metasedimentary rocks exposed within Acadian antiformal structures, and correlative faults and formations.

Relatively intact Ordovician boninitic ophiolites such as the Thetford Mines ophiolite (TMO) of southern Quebec were emplaced upon dynamothermally metamorphosed amphibolites and Caldwell Group metasediments during the Arenigian. The ophiolitic Belvidere Mt. Complex (BMC) of northern Vermont lies in tectonic contact with the Hazens Notch Fm of the internal Humber Zone indicating the BMC likely lies at a deeper structural level than the TMO and may be correlative with the older Pennington Sheet ultramafics. An 40Ar/ 39Ar age from the BMC amphibolite (505 Ma; Laird et al., 1993) suggests it is considerably older than the TMO. Boninitic mafic rocks in the Warner Hill Complex of northern Vermont may be correlative with TMO boninites.

Acadian antiformal structures such as the Carineault and Bécancour antiforms (Qc) and Worcester Mountains (Vt) are windows exposing continental metasedimentary and metaigneous rocks from underlying thrust sheets in the Dunnage Zone. Coarse-grained schists from the cores of these structures (Caldwell Group/Stowe Fm) are very similar as are 40Ar/ 39Ar ages on muscovites and amphiboles (~460-470 Ma). The trace and REE geochemistry of the Worcester Mt mafic rocks is nearly identical to that of many Caldwell Group greenstones in the Quebec Humber Zone.

Recent work in Quebec suggests that down-to-the-east, composite normal faults, locally parallel to the Humber-Dunnage contact (St. Joseph Fault and Baie Verte-Brompton Line), postdate a major episode of west-over-east backthrusting. The Burgess Branch Fault in Vt is thought to be correlative with the St. Joseph Fault. A preliminary analysis of structures in Vermont and Quebec suggests potential similarity in relative fabric chronology and fold generations. Cooperative 40Ar/ 39Ar work between the Vermont Geological Survey and Geological Survey of Canada is currently underway in order to address compelling structural and metamorphic questions.