BEDROCK GEOLOGIC MAP OF THE BRANTINGHAM 7.5 MINUTE QUADRANGLE, WESTERN ADIRONDACK HIGHLANDS, NEW YORK

The Brantingham 7.5-minute quadrangle, in the westernmost part of the Adirondack Highlands, New York State, contains bedrock characterized by northeast-striking gneisses of metagranite and meta-quartz syenite, charnockite, amphibolite, calc-silicate, metapelite, and rare pyroxenite boudins containing REE-bearing chevkinite, allanite and apatite. The gneisses dip both northwest and southeast and in the northern part of the quadrangle comprise the Glenfield Antiform of Buddington (1939, p. 151, GSA Memoir 7), a regional fold that can be traced into the McKeever and Number Four 15-minute quadrangles (Whitney et al., 2002, NYSM Map and Chart 44).

The metagranite and meta-quartz syenite gneisses are dominated by mesoperthite + quartz (10-35 vol.%) with minor plagioclase + oxides and mafics of either clinopyroxene + hornblende, hornblende + biotite, or biotite alone. Charnockite has the same modal mineralogy except orthopyroxene is present and biotite is absent. The amphibolites contain two pyroxenes and are garnet-free as they are located far west of De Waard’s (1967, Jour. of Petrology) garnet-in isograd for mafic rocks. The calc-silicates are characterized by microcline + quartz + diopside + titanite + calcite + scapolite ± wollastonite and are compositionally heterogeneous and strongly isoclinally folded. Metapelite gneisses are characterized by the low variance assemblage of garnet + sillimanite + quartz + hercynite, the latter phase likely stabilized by its gahnite component.

Pyroxenite boudins hosted by metagranite and meta-quartz syenite gneisses in the northwest part of the quadrangle are characterized by Fe-rich clinopyroxene, orthopyroxene, and hornblende, with quartz and REE-bearing chevkinite, allanite, and apatite. The boudins are typically 2 to 10 meters across and occur along a distinct foliation plane on the southeast limb of the Glenfield Antiform. The pyroxenite boudins are interpreted as sheared cumulates originating from the surrounding metagranites and meta-quartz syenites (Goodenough et al., 2021, Jour. of Earth Science). Cumulate processes are thought to have concentrated REE’s in these rocks. The pyroxenite boudins may be similar to pyroxene + biotite rich layers in metagranitic rocks mapped by Buddington (1934, NYSM Bulletin 296) in the Lowville quadrangle to the northwest.