THICKENING, COLLAPSE, AND THE U-SHAPED MONAZITE PROFILE: IMPLICATIONS FOR THE TECTONIC HISTORY OF THE ADIRONDACK AND APPALACHIAN MOUNTAINS (Invited Presentation)

Distinguishing regional compressional and extensional stages in mid-crustal metamorphic terranes can be difficult, especially in multiply tectonized regions. Monazite (and Xenotime) petrochronology can provide critical evidence. Garnet is an almost ubiquitous mineral during prograde metamorphism in mid-and deep-crustal metasediments, and widespread garnet growth commonly reflects periods of regional crustal thickening and heating. Similarly, widespread garnet-consuming reactions can indicate exhumation, cooling and fluid influx. Monazite typically shows a dramatic decrease in Y (and heavy REEs) during garnet growth and a subsequent increase during garnet breakdown because, when xenotime is not present, garnet and monazite are typically the main Y hosts in metasedimentary rocks. Consequently, profiles of Y vs. date have a characteristic U-shape that can represent the period of garnet stability (crustal thickening?) in orogenic rocks. In the eastern Adirondack Highlands, a significant period of garnet growth is marked by decreasing Y in monazite at ca. 1150 Ma. Y in monazite remains low until ca. 1050 Ma and then rebounds significantly. The broad U-shaped profile, combined with inferred garnet growth and breakdown reactions, suggests garnet stability for ~100 m.y. with extensive retrogression (orogenic collapse) after 1050 Ma. Other regions, especially to the southeast, have a much narrower U-shaped profile with the first major period of garnet growth at 1070 Ma and resorption after 1050 Ma. In contrast, monazite from the Popple Hill gneiss in the Adirondack Lowlands show a characteristic V-shaped Y-date profile at ca. 1170 Ma suggesting a rapid cycle of garnet growth and break-down, with no evidence for new garnet growth at 1150 Ma or 1050Ma. Together, the data suggest that the Adirondack Highlands and Lowlands preserve distinct Shawinigan (1190-1140 Ma) histories and even in the Highlands, the exposed Shawinigan grade varies significantly. Further the data lead to questions about the degree of Post Shawinigan collapse/exhumation in the Adirondack Highlands, but support widespread post Ottawan (post-1050 Ma) exhumation. Similar U-shaped profiles document post-Acadian exhumation up to 50 m.y. after thickening in the Appalachian Mountains.