ERUPTIVE STYLE AND AGE OF THE SPIDER LAKE FORMATION, NORTHERN MAINE

Previous tectonic models for selected Late Silurian basalts of the Piscataquis volcanic belt have been interpreted as being related to crustal rifting, slab detachment during subduction, or other subduction - related models. The rifting model is possibly related to crustal thinning or a mantle plume. Early Late Silurian (Ludlow) basaltic volcanics are assigned to the Spider Lake Formation which has a northeast trending outcrop belt approximately 65 by 15 km. Additionally rocks to the southwest and northeast of the Spider Lake Formation outcrop belt have correlative units such as the Allagash Lake and Five Mile Brook formations plus a partial correlative to basaltic rocks of the Frontenac Formation. This suggests that the Silurian basaltic volcanic province was widespread and spanned a time interval of approximately 8 Ma from circa 432 to 424 Ma.

Mapping of the Spider Lake Formation at the northern end of the Piscataquis volcanic belt suggests basalt erupted from fissures and/or shield type volcanos together with possible, localized cinder cones. Recent LIDAR imagery clearly shows flows of varying thickness. The basalts exhibit features consistent with eruptions in both marine and subaerial environments. Rocks erupted into the marine environment exhibit pillow structures of varying size and shape. Fossils previously collected from sedimentary rocks locally deposited between flows were assigned to the early Late Silurian (Ludlow). Nonmarine flows exhibit a range of thicknesses from approximately 2 to 20 or more meters. Amygdules or vesicles are common and are locally concentrated near the top of nonmarine flows. Reddish contacts between flows and localized reddish gray slates are interpreted as baking or paleosol horizons are locally present. Non marine flows are commonly porphyritic while pillowed flows lack phenocrysts. Minor rock types include peperite, felsic flow banded tuff and tuff breccias. The felsic tuffs suggest eruption from a fractionated magma chamber. Two felsic tuffs from the Spider Lake Formation yielded U-Pb ages of 424.3 and 427.0 Ma confirming the Ludlow age.