GEOCHRONOLOGY AND CHEMISTRY OF MONAZITE IN A MIGMATITIC PARAGNEISS IN THE EASTERN ADIRONDACK HIGHLANDS ADJACENT TO TICONDEROGA, NEW YORK

The Adirondack Highlands, a terrane within the mid-Proterozoic Grenville Province of eastern North America, was affected by multiple periods of metamorphism, deformation, and plutonism. Major events include the Shawinigan orogeny at c. 1200-1150 Ma, emplacement of anorthosite-mangerite-charnockite-granite (AMCG) magmas at c. 1160-1140 Ma, the Ottawan orogeny at c. 1090-1050 Ma, and late-orogenic collapse and intrusion of Lyon Mt. granite magmas at c. 1060-1030 Ma. Attributing outcrop features to these events is complex but important to understanding regional tectonic patterns and how orogens evolve at all scales. One particularly useful tool in this endeavor is combining monazite electron microprobe U-Th-total Pb ages with monazite chemistry. This technique was applied to a garnet-biotite-quartzofeldspathic migmatitic paragneiss on the eastern edge of the Adirondack Highlands adjacent to Ticonderoga, New York.

Most monazite ages from the outcrop fall into the c. 1060-1030 Ma range, one analysis provides a c. 1140 Ma age, and the remainder fall into the c. 1030-930 Ma range. Yttrium concentration of the 1060-1030 Ma ages is generally below 6000 ppm and is interpreted to indicate garnet growth prior to this time, which is supported by non-systematic, but moderately low U concentrations with respect to age. Whereas the c. 1140 Ma analysis has a distinctly higher Y concentration of ~10,000 ppm indicating garnet growth during or after this time. Ages in the c. 1030-930 Ma range show a progressive increase in Y with time indicating garnet breakdown (retrogression).

Garnet growth from biotite dehydration melting is interpreted to have occurred prior to c. 1060 Ma, likely during the Shawinigan orogeny or the AMCG event as supported by U-Pb zircon data. This was followed by significant monazite recrystallization at c. 1060-1030 Ma associated Lyon Mt. granite emplacement and/or fluid alteration – consistent with microtextures. The youngest monazite ages are associated with the early stages of garnet breakdown and may correspond with fluid infiltration from local pegmatite emplacement.