LAUSR

Within-plate basalts are present across the Connecticut Valley - Gaspé trough in New England and Canada. Proposed mechanisms for the extensional magmatism include back-arc basin extension, slab breakoff, and extensional collapse. The age and composition of the bimodal Frontenac Formation of northern New England and southern Quebec has bearing on the topic. We analyzed zircons from the meta-felsic rocks of the formation and determined a Silurian age of 432 ± 8 Ma. We also analyzed the Frontenac rocks for major and trace elements and compared their compositions to the other mafic rocks along strike in the Connecticut Valley - Gaspé trough. These include the Comerford Intrusive Suite and the Standing Pond Volcanic member of the Waits River Formation of Vermont, the Silurian Chaleur Group and the Early Devonian Dalhousie Group of New Brunswick and the Gaspé Peninsula, and to the Piscataquis belt rocks of northern Maine. All these rocks are post-Taconic, within-plate, tholeiites that span an age range from 432 Ma (Frontenac Formation) to 419 Ma (Comerford Intrusive Suite). In spite of their similar tectonic setting, subtle differences in trace element compositions are evident, indicating a multiplicity of sources in the production of the Siluro-Devonian magmas. Ce/Yb versus La/Ta values suggest that the Frontenac magmas were derived from spinel peridotite sources in the asthenospheric mantle. We suggest that slab rollback and back-arc basin extension permitted asthenospheric mantle upwelling to the spinel peridotite stability field, causing the Frontenac partial melting event at depths as low as 35 km. Sediments of the Frontenac Formation were deposited as turbidite successions into a developing trough resulting from extension. The coeval influx of sediment and intrusion of basalt produced a sediment-sill complex because the upward migration of the basaltic magmas was hindered by the rapidly thickening sediment overburden. Subsequently, slab breakoff may have opened an asthenospheric window, generating the younger within-plate magmas that occurred from 425 to 418 Ma across the Connecticut Valley – Gaspé trough.