GEOLOGY AND GEOCHEMISTRY OF THE FISH POND VOLCANICS, THE FORKS, MAINE

The Fish Pond Volcanics crop out along the southeastern flank of the Lobster Mountain arch in northwestern Maine. This thin unit is well-exposed along the ridges north and east of Fish Pond, located 4 miles northwest of The Forks. In order to constrain the tectonic setting of these rocks, we are currently documenting field relationships and their major and trace element compositions. The unit includes flows, thin intrusions, and pyroclastic deposits. The flows are dark green and aphanitic. Bimodal intrusions, including mafic and felsic varieties, are interpreted as feeder dikes to the volcanic rocks. The pyroclastic deposits include dark to light green tuffs and lapilli tuffs. The dikes intrude the underlying calcareous strata of The Forks Formation that crops out to the southeast. Crinoids present within this calcareous unit indicate a shelf facies. Along the upper contact, tuffs grade into green slates beneath black slates of the Devonian Carrabassett Formation to the northwest. A Siluro-Devonian age is based on conodonts collected from The Forks Formation. Preliminary XRF data places the unit in the within-plate field on Ti-Zr-Y and Ti-Mn-P tectonic discrimination diagrams. We correlate The Forks Formation and the Fish Pond Volcanics with, respectively, the calcareous Ripogenus Formation and the West Branch Volcanics of the Chesuncook Dome. We are unaware of an obvious equivalent of the red Frost Pond Shale, although minor hematitic slate is present in this stratigraphic position. The tentative within-plate signature and passive margin (lower plate) setting are consistent with a slab breakoff mechanism, as proposed by Schoonmaker et al. (2005) for the West Branch Volcanics. Alternatively, the late Silurian Salinic disturbance may be related to breakoff of the northwest-dipping Ganderian slab.