Timing of Metamorphism in the Adirondack Lowlands, New York State: Constraints from U-Pb Zircon and Monazite Geochronology

The Adirondack Lowlands terrane (AL) of the Grenville Province is dominated by metaigneous and metasedimentary rocks that were accreted during the Shawinigan (ca. 1210-1160 Ma) phase of the Grenville orogenic cycle. Zircon growth in metapelites of the AL at ca. 1180-1160 Ma (Heumann, et al 2006) overlaps with emplacement of the Hyde School, Rockport and Hermon Granite plutonic suites, and with AMCG magmatism in the Adirondack Highlands. New U-Pb SHRIMP dates on metamorphic zircons from metagabbro near Pierrepont, NY in the southeastern AL are somewhat younger (ca. 1150-1125 Ma). An emplacement age of ca. 1203 Ma for the Pierrepont Gabbro is established by EMP dating of monazite grain cores from a tonalitic phase of the gabbro body. EMP dating of monazite from the metatonalite and associated metapelites reveals growth phases from ca. 1185 Ma to ca. 1110 Ma. These data suggest that metamorphism continued for a protracted interval with heat initially supplied by emplacement of plutons within the AL, and later by heating during thrusting of the AL over the Adirondack Highlands upon the compressional precursor of the Carthage-Colton Shear Zone. Zircons from undeformed pegmatites within the southeastern AL are dominated by xenocrysts from country rock. Titanite and zircon in lower amphibolite facies vein assemblages in the southeastern AL are ca. 1035 Ma, similar in age to the Lyon Mountain Granite suite in the adjacent Highlands, and related to late Ottawan (ca. 1050-1030 Ma) detachment.