PRELIMINARY RESULTS FROM BEDROCK MAPPING IN THE SPECKLED MOUNTAIN QUADRANGLE, MAINE

The Speckled Mountain quadrangle is in the easternmost part of the White Mountain National Forest (WMNF) along the ME-NH border. Until recently, there were few modern 1:24,000-scale bedrock geologic maps in the region. Map units and contacts on the state geologic maps of Maine and New Hampshire do not match across the ME-NH border. As a result of national critical mineral resource interest and the lack of detailed investigations, the WMNF is a priority mapping zone for the USGS, National Cooperative Geologic Mapping Program.

The bedrock geology of the Speckled Mountain quadrangle is dominated by Silurian to Carboniferous rusty and non-rusty migmatite schist, 2-mica granite, granodiorite (Songo pluton), and minor granofels and quartzite. Due to pervasive migmatization, recognition of formal stratigraphic units is difficult. Instead, lithodemic units are mapped based on weathering style (rusty or non-rusty), the presence of calc-silicate pods, granofels blocks or layers, and the presence of quartzite. All Silurian to Devonian metasedimentary units were intruded by Devonian to Carboniferous muscovite-biotite granite. The amount of exposed 2-mica granite increases southward, and this spatial relationship suggests that much of the 2-mica granite is part of the northern end of the Carboniferous Sebago pluton.

Associated with the 2-mica granite are numerous pegmatite bodies, one of which was mapped continuously for over a kilometer and is at least 300 m wide. Most of the pegmatites are barren of critical minerals. However, historical data show that there are a number of beryllium-bearing pegmatites and prospects in the SE corner of the map. It is uncertain if these pegmatites are related to the 2-mica granite, anatexis of the country rock, or intrusion of the Devonian Songo pluton; the area is a target of ongoing mapping.

The map pattern is dominated by E-W to NE-SW trending, second generation antiform-synform pairs. These F2 structures post-date recumbent F1 folds, and D1 ductile shear zones are observed at the outcrop scale. The 2-mica granite postdates F2 folding.

Mapping in the WMNF will help resolve outstanding scientific questions on the stratigraphy, protolith age, and timing of magmatism in the region, and will also provide greatly needed detailed mapping to support critical mineral resource assessments.