MONAZITE-ALLANITE-FLUORAPATITE RELATIONSHIPS FROM THE CHEEVER FE-OXIDE APATITE (IOA) DEPOSIT, EASTERN ADIRONDACK MOUNTAINS, NY

Monazite is a useful petrochronometer for linking metamorphic reactions to absolute time. Despite its routine use in relatively high grade rocks, monazite is also known to participate in lower grade, fluid-mediated, reactions. Understanding the composition, textures, and mineral associations of monazite produced during fluid-mediated dissolution-reprecipitation will provide further context for interpreting U-Th-Pb monazite analyses, and help distinguish fluid-related reactions in complex petrologic settings. Monazite within the >1.0 Ga Cheever Fe-oxide apatite deposit in the eastern Adirondack Mountains is a natural example of monazite growth and recrystallization during metasomatism. It occurs in reaction textures with allanite and fluorapatite; both primary and fluid-altered monazite are interpreted to coexist.

Two textural varieties of monazite are recognized. Type I monazite is associated with coarse fluorapatite crystals, and commonly rims or is immediately adjacent to the fluorapatite. This subset of monazite commonly hosts local symplectitic intergrowths of monazite, allanite, and fluorapatite. Type II monazite occurs as intricate monazite-allanite symplectites, which define mm-scale pseudomorphs of an earlier phase. The rim of these pseudomorphs consist of fluorapatite and allanite with no monazite. Type I monazite contains 3.5 – 6.5 % ThO₂ and >1.2 wt % CaO + SiO₂. The monazite is compositionally zoned, with decreasing Ca, Si, HREE, and actinides toward symplectite domains. In-situ U-Th-total Pb geochronology of Type I monazite yielded an age of 980 +/- 5.8 Ma (MSWD: 3.3). Type II monazite is compositionally distinct. It contains far less solid solution components (Ca, Si, Th, U, HREE) than Type I monazite. Type II monazite contains < 1.4 % ThO₂ and an average of 0.64 % CaO + SiO₂. Due to a low actinide content, only two analyses had enough Pb to produce robust results and yielded ages within error of Type I monazite. These results indicate that monazite formation and symplectite development occurred significantly later than (>20m.y.) ore formation during a multi-step reaction history, which favored pure-end member compositions. The textural and compositional evolution of these accessory phases tracks the evolving P-T-fluid conditions during exhumation of the Adirondack Mountains.