TIMING OF PARTIAL MELTING USING COMBINED IN-SITU MONAZITE AND ZIROCN GEOCHRONOLOGY, EASTERN ADIRONDACK HIGHLANDS, NY

The eastern Adirondack Highlands contain abundant monazite- and zircon-bearing migmatites. This study focuses on six migmatite domains within the eastern Adirondack Highlands, utilizing both in-situ monazite geochronology and zircon geochronology to constrain the character and timing of anatexis in these domains. The currently interpreted thermo-tectonic events in the eastern Adirondack Highlands include the ca. 1170 Ma Shawinigan orogeny, emplacement of the ca. 1150 Ma AMCG igneous suite, the ca. 1080-1050 Ma Ottawan orogeny, and a ca. 1050-1020 Ma extensional collapse phase. Previous studies utilize ages of igneous and metamorphic zircon, commonly as out-of-context separates, to interpret these events. Zircon geochronology from these domains indicate multiple generations of zircon growth, but it is difficult to interpret if these generations reflect melting, metamorphism, or hydrothermal activity. To gain more insight into these processes, we combine monazite geochemistry and geochronology to link monazite to specific reactions, especially those related to garnet growth and melt production. While we analyze many elements in monazite, here we focus on Y, Dy and Gd, as these elements partition preferentially into garnet over monazite, as well as U. Decreases in concentrations of Y, Dy and Gd in monazite indicate periods of garnet growth, either from prograde reactions or biotite dehydration melting. Decreasing U is interpreted to reflect melting, as U is strongly partitioned into the melt phase. Increases in Y, Gd and Dy in monazite indicate periods of garnet break-down. In some of the domains, monazite from migmatites show a distinct decrease in Y, Dy, Gd and U around 1170-1150 Ma, indicating anatexis associated with the Shawinigan to AMCG event. In other domains, a distinct decrease occurs around 1050 Ma, indicating anatexis associated with either the Ottawan orogeny or extensional collapse. While zircon geochronology can provide the number of and age of tectonic (igneous and metamorphic) events, in-situ monazite petrochronology enables us to integrate monazite textural setting, geochemistry and age data to date metamorphic reactions and to constrain the nature and significance of tectonic events.