CONSTRAINING THE TIMING OF MELTING IN THE EASTERN ADIRONDACK MOUNTAINS, NY USING A COMBINATION OF IN-SITU MONAZITE AND ZIRCON U/PB DATING TECHNIQUES

Migmatitic gneisses are common in the Adirondack Mountains, particularly in the eastern Adirondack Highlands, but the timing and tectonic setting of melting are not clear. Previous research suggests that significant melting occurred during the Shawinigan orogeny (1190-1160 Ma), the anorthosite-mangerite-charnockite-granite (AMCG) plutonic event (1160-1130 Ma), as well as the Ottawan orogeny (1080-1050 Ma). We analyzed an outcrop of biotite-garnet-sillimanite paragneiss with foliation parallel leucosome along Route 22 south of Whitehall, NY to determine the timing of melting using both monazite and zircon U/Pb geochronology. In-situ U-Th-total Pb monazite analyses from the leucosome yielded monazite data with a unimodal population at approximately 1050 Ma, suggesting partial melting during the Ottawan orogeny. In contrast, monazite data from the restite yielded six compositionally distinct populations near 1170, 1150, 1065, 1050, 1030, and 980 Ma. The 1170 and 1150 Ma populations occur as inclusions in garnet and in cores of matrix monazite. The 980 Ma population is only present as outer rims on matrix grains. Yttrium and heavy REEs decrease in monazite grains in two steps, one at 1150 and another at ca. 1050 Ma, which is interpreted to reflect two periods of garnet growth and probably melting. The 1030 and 980 Ma monazite has higher yttrium concentrations suggesting garnet breakdown and monazite growth during post orogenic collapse. Zircon from both leucosome and restite were analyzed via LA-ICP-MS (in-situ and mounted mineral separates) from the same rock. The restive zircon mineral separate yielded ages biased towards larger metamorphic zircon grains with a significant single peak at 1050 Ma. The in-situ data from the restite and leucosome, in addition to the leucosome mineral separate data records zircon growth during the Shawinigan orogeny, intrusion of the AMCG suite, as well as the Ottawan orogeny. We suggest that these rocks underwent melting at two times (ca. 1150 Ma and ca. 1050 Ma), but monazite crystallized in the leucosome only in the second event. Restite monazite preserves the most complete record of the tectono-metamorphic history, but a combination of monazite and zircon dating techniques are necessary to constrain leucosome-restite relationships and the full petrotectonic history