SILURIAN STRATIGRAPHY AND GEOCHRONOLOGY IN THE MIGMATITE TERRAIN OF WESTERN MAINE

New detailed bedrock mapping and detrital zircon (DZ) and crystallization zircon (CZ) geochronology in the southern Gilead 7.5' Quadrangle in Western Maine supports a Silurian designation for the depositional age of metasedimentary units and their inclusion in the Late Devonian-Carboniferous Migmatite-Granite Complex (MGC) (Solar and Tomascak, 2016). Located within the Silurian to Devonian Central Maine Belt and intruded by the Devonian Songo diorite, numerous small, not dated, two-mica granites and quartz diorites, and ubiquitous pegmatite, the study area preserves a diverse geological setting instrumental in understanding the paleotectonics of the Northern Appalachians.

The revised stratigraphy (oldest to youngest) consists of: Unit 1, gray migmatized schist with calc-silicate pods; Unit 2, moderately rusty migmatized schist with calc-silicate pods; Unit 3, gray migmatized schist, quartzite, and discontinuous layers of gray granofels; and Unit 4, deeply rusty weathered, migmatized schist and rusty granofels. DZ dating of 2 samples from the gray granofels (Unit 3) at Bog Brook and Wheeler Mine yielded youngest zircon ages of 433 to 435 Ma. suggesting these are part of the Silurian Rangeley Formation. Age population density histograms for both granofels samples show similarities to other Silurian, Ganderian-derived, units and correlate well to the first Acadian DZ cycle, inboard-derived, Rangeley, Perry Mtn., Smalls Falls composite of Bradley and O'Sullivan (2016). The deeply rusty weathering schists of Unit 4 are correlated to the Smalls Falls Formation. Between the granofels of Unit 3 and rusty schists of Unit 4 is a narrow section of well-bedded gray schist and quartzite that may be correlative to the Perry Mtn. Formation. Units 1 and 2 are correlative to the Rangeley Formation.

Crystallization ages for two-mica granite sills from Wheeler Mine yielded ages of 352 to 359 Ma. supporting their inclusion as small mappable granites within the Late Devonian-Carboniferous MGC of Solar and Tomascak (2016). These granites cut stromatic layering in the migmatized metasedimentary rocks as well as the macroscale Late Acadian or Neoacadian D₃ open, reclined folds defined by the map pattern. The DZ and CZ ages bracket the deposition, deformation, and metamorphism of this area to within an 80 Ma period.