RARE EARTH ELEMENT ABUNDANCES OF CHEVKINITE, ALLANITE, AND FLUORO-APATITE IN ULTRAMAFIC BOUDINS FROM THE WESTERN ADIRONDACKS, NEW YORK

Ultramafic boudins hosted by mesoperthite + quartz gneisses in the Brantingham 7.5 minute quadrangle contain rare earth element (REE)-bearing chevkinite, allanite, and fluoro-apatite. The boudins are up to 10 meters across and are dominated (~70%) by Fe-rich clinopyroxene (Hd = ~75%), with smaller quantities (<20%) of quartz + oxides + Fe-rich orthopyroxene (Fs = ~80%) ± pargasitic amphibole. Chevkinite and fluoro-apatite together comprise about 10 vol. % of the rock; allanite = 1%. Fluoro-apatite occurs as both a rock-forming phase as well as inclusions in chevkinite. The surrounding mesoperthite + quartz gneisses are interpreted to have originated as hypersolvus granites, quartz syenites and rare syenites.

X-ray diffraction and Raman analysis demonstrate chevkinite metamictization. Electron microprobe analyses of chevkinite (n = 64) yield an average composition of 44.1 wt.% total REE oxides. FeO and CaO abundances fall within both the perrierite and chevkinite subgroup fields but the majority are perrierite (MacDonald et al., 2009, Mineralogical Magazine). Allanite has an average REE oxide composition of 26.5 wt.% (n = 105 analyses). LA-ICP-MS analyses of fluoro-apatite (n = 10) yield an average composition of 9.6 wt.% total REE oxides. All three phases show negative Eu anomalies, and high LREE abundances. Moderate abundances for all HREE’s are documented in fluoro-apatite, but only for Gd, Dy, and Ho in chevkinite and allanite.

The clinopyroxene-rich boudins are interpreted as sheared cumulates from the surrounding hypersolvus granites, quartz syenites and rare syenites (see Goodenough et al., 2021, Journal of Earth Science). These document a previously unknown type of REE occurrence in the Adirondacks, distinct from that associated with iron oxide-apatite (IOA) deposits in the eastern Adirondacks (Lupulescu et al., 2017, Canadian Mineralogist; Taylor et al., 2019, Economic Geology).