SILURIAN STRATIGRAPHIC REVISIONS AND EVIDENCE FOR THE PISCATAQUIS VOLCANIC ARC IN THE GILEAD-NEWRY REGION, WESTERN MAINE

The Gilead-Newry region of western Maine is comprised of migmatized Silurian (?) metasedimentary rocks intruded by Devonian (?) diorite-tonalite suites, Devonian to Permian (?) two mica granite, and widespread pegmatites. The region borders the southern edge of the Piscataquis Volcanic Arc and is NW of the Sebago and Songo Plutons. The bedrock was remapped in 2013, with the goal of reevaluating the region’s stratigraphy and intrusive rocks, and to ultimately gain a more complete understanding of the region’s tectonic history largely influenced by the Salinic and Acadian orogenies. Eight meta-sedimentary units and two igneous units were mapped for this project.

There are a total of 7 diorite-tonalite plutons composed of two large and five smaller sill-like intrusions. A total of 5 two mica granite plutons perpendicular to the stratigraphy made up of two large, two small, and one with alternating layers of grey-schist were mapped. Petrographic analysis will categorize igneous rocks on the QAP ternary diagram. XRF data from these plutons show that the diorite-tonalite fall within the volcanic arc and within plate fields while two mica granites fall within the syn-collisional field.

The metasedimentary rocks from youngest to oldest (?) include the following units: Sgf, a biotite and/or calc-silicate granofels (Madrid Fm?); Ssqr, a rusty schist and quartzite (Smalls Falls?); Sqsc, quartzite and schist with calc-silicate pods (Perry Mountain?); Ssqg, a biotite granofels; Ssq, a greyschist and quartzite with calc-silicate pods; Ssg, a greyschist; Ssr, a rusty weathering schist with calc-silicate pods; Ssqm, grey schist and quartzite with calc-silicate pods. The five oldest units are interpreted to be correlative with the Rangeley Fm. The abundant calc-silicate pods may represent olisotromal features deposited in an active tectonic setting during the Salinic Orogeny. Two generations of folding and abundant migmatization occurred during the Acadian Orogeny.

Preliminary results show that much of what was previously mapped in the region as Littleton Formation now correlates best with the Rangeley Formation. The diorite-tonalite plutons are an extension of the Piscataquis Volcanic Arc. Both findings have significant implications for regional Appalachian tectonics, and validate the importance of creating a new map.