WHAT IS THE AGE OF THE FRONTENAC FORMATION, NORTHERN NEW HAMPSHIRE AND WESTERN MAINE?

The age of the bimodal Frontenac Formation of northern New England and southern Quebec has long been controversial; New England geologists think the Frontenac is Silurian, lying below the Seboomook Group whereas Canadian geologists reverse the stratigraphy with the Frontenac above the Seboomook. It has been suggested that the meta-igneous rocks are the eruptive equivalent of the 419 + 1 Ma Comerford Intrusive Suite (CIS) of northern Vermont. If this correlation is correct, then the Frontenac Formation is Late Silurian in age. To help resolve the conflict, we analyzed the meta-igneous rocks of the formation for major and trace elements and compared their compositions to the CIS and to the Silurian Chaleur Group and the Early Devonian Dalhousie Group of New Brunswick and the Gaspe Peninsula.

While the major element compositions of the Frontenac, CIS, and Chaleur indicate they are comparable tholeiitic basalts, significant differences in trace element compositions are evident. The CIS differs from the Frontenac in several tectonic discrimination diagrams, suggesting that the two are not equivalent. The Silurian Chaleur Group rocks have steeper chondrite-normalized REE patterns than the Frontenac and is more enriched in incompatible elements on spider diagrams. In contrast, the Frontenac rocks have REE patterns and spider diagram patterns that match the Devonian Dalhousie Group. Ce/Yb versus La/Ta values suggest that the Frontenac and Dalhousie magmas were derived from spinel peridotite sources in the asthenospheric mantle whereas the Silurian Chaleur Group magmas are partial melts of a deeper, garnet peridotite source. The geochemical correlation of the Frontenac with the Dalhousie Group suggests that the Frontenac may be Devonian in age. U-Pb zircon ages are currently being obtained for meta-rhyolite portions of the Frontenac Formation to conclusively resolve the problem.