RECENT MAPPING IN THE EAST ANDOVER, ME QUADRANGLE AND DETRITAL ZIRCON U-PB GEOCHRONOLOGY REFUTE EXISTENCE OF THE PRE-METAMORPHIC PLUMBAGO MOUNTAIN FAULT, NEWRY, ME

The Central Maine Basin is composed of late Ordovician to early Devonian metasedimentary rocks, deposited in a deep marine basin prior to the Acadian Orogeny in the Devonian. As shown by detrital zircon ages, these metasedimentary rocks are derived from two sources: the Silurian Rangeley, Perry Mountain, and Smalls Falls Formations from the inboard Laurentian margin, and the younger Silurian and Devonian Madrid, Carrabasset, and Hildreths Formations from the outboard terrane. In the East Andover, ME 7.5’ quadrangle, the Rangeley, Perry Mountain, Smalls Falls, Madrid, and Hildreths Formations have all been mapped (Moench and Hildreth, 1976). These rocks show early Devonian deformation and amphibolite facies metamorphism, episodes of Middle Devonian contact metamorphism and late folding, and have been intruded by plutons including the Plumbago Mountain gabbro, the Mooselookmeguntic Igneous Complex, a trondhjemite, and Carboniferous Li-pegmatites.

When Moench and Hildreth (1976) mapped the Rumford 15’ quadrangle, they delineated the Plumbago Mountain Fault, which they interpreted as a pre-metamorphic, listric, normal fault based on stratigraphic truncations. Moench and Hildreth (1976) suggested that the fault separated the Devonian Hildreths Formation from the Silurian Rangeley and/or Perry Mountain Formations. We collected samples for detrital zircon geochronology from both sides of their fault to determine its existence. The sampled units were both interbedded schists and quartzites, ± calc-silicate pods. We found maximum depositional ages of 440 ± 4 and 443 ± 4 Ma, reflecting the erosion of the Bronson Hill/Boundary Mountain arc(s). We now correlate the units to the Perry Mountain and Rangeley Formations in a stratigraphically conformable sequence. The similarity of these ages and our mapping do not support the existence of the Plumbago Mountain Fault. In addition, the detrital zircon bar codes are similar and reflect the 1^st Acadian detrital zircon cycle of Bradley and O'Sullivan (2017) consistent with their designation as Silurian. Our interpretation in the East Andover ME 7.5’ quadrangle is consistent with mapping in the Berlin, NH 7.5’ quadrangle where Eusden et al. (2021) refuted the existence of the Plumbago Mountain Fault using detrital zircon geochronology coupled with detailed mapping.