THERMOCHRONOLOGY OF ANATEXIS IN METAPELITES, ADIRONDACK HIGHLANDS AND LOWLANDS, NORTHERN NEW YORK

Extensively migmatized metapelites are abundant in both the Adirondack Lowlands and Highlands. They are intruded by the ca. 1200 Ma Antwerp granitoids in the Lowlands, but the upper age limit, source, and tectonic setting of their accumulation is not yet known. Further, the timing of anatexis that produced quartz-plagioclase, K-spar, garnet leucosomes, and the age of deformation of the migmatites, has been controversial but commonly thought to be associated with ca. 1050 Ma Ottawan high-grade metamorphism. SHRIMP U-Pb analysis of zircons from metapelite leucosomes indicates that euhedral zircons are ca. 1170 Ma, but many have cores that are ca. 1300-1350 Ma. Melanosomes contain similar ca. 1170 Ma euhedral zircons, but also discreet, rounded, ca. 1300-1350 Ma grains, suggesting that these, as well as cores in younger zircons, are detrital grains derived from the source of the primary sediment. Interestingly, no Archean zircons were found in metapelites, although Archean grains are abundant in quartzites in the Adirondacks. Ca.1050 Ma Ottawan overgrowths about ca. 1170 Ma cores were found in only one locality in the eastern Adirondacks. These data indicate: (1) metapelites were accumulated in relatively restricted basins and probably derived from Elzevirian arc terranes whereas quartzites received detritus from the more distant Superior craton as well; (2) the ages of euhedral zircons in metapelite leucosomes and melanosomes indicate that anatexis occurred ca. 1170 +/- 10 Ma and was associated with the ca. 1190-1170 Ma Shawinigan arc-continent orogeny and associated upper amphibolite facies metamorphism, rather than later, ca. 1060-1020 Ma, Ottawan continent-continent orogeny and associated granulite facies metamorphism. Localized Ottawan anatexis was apparently restricted to the eastern Adirondacks. These interpretations are supported by electron microprobe Th-U-Pb elemental ages of monazites from metapelites in the southeastern Adirondacks which record, commonly within single grains, growth events at ca. 1180 Ma, 1155 Ma, 1140 Ma, 1110 Ma, and 1020 Ma.