NATURE OF THE OTTAWAN OROGENY AND THE 1050 MA INTRUSIVE AGE OF LYON MT. GRANITE

Aleinikoff and Walsh (A&W) reinterpret Adirondack Highlands (AH) tectonic evolution based on zircon SHRIMP ages for Lyon Mt. Granite (LMG) and propose that ca 1140 Ma zircon cores, rather than mantles, preserve LMG igneous ages. They interpret 1050-1040 Ma Ottawan age zircon mantles as metamorphic despite oscillatory zoning in many. Peak Ottawan P, T are 750-850°C and 7.5-8.7 kbar similar to P, T in the Canadian Grenville where highly deformed rocks south of the crustal scale ca. 1090-1050 Ma Allochthon Boundary Thrust (ABT) reflect double crustal thickness due to collision with Amazonia, and granites identical to LMG are part of ca. 1050 Ma post-orogenic AMCG magmatism. In AH the collision caused E-W recumbent isoclinal nappes that deformed ca. 1155 Ma AMCG and older rocks and produced granulite facies assemblages and anatexis. In AH penetrative ribbon lineation in AMCG cannot be due to Shawinigan deformation which ended by 1160 Ma (e.g., undeformed 1160 Ma Kingston dikes and 1158 Ma Canton, NY veins, and the end of thrusting on the Maberly Thrust) but must be Ottawan. Ultrachron monazite ages show that between ca. 1160-1090 Ma there were no high T, regional tectonic events in AH. Oriented monazites in ductile shear zones within AMCG and axial planar to F2 isoclinal folds yield ca. 1050 ages. Thus ductile penetrative fabrics in ca. 1155 Ma AMCG rocks must be due to Ottawan deformation. Lack of such fabrics in LMG reflects post-contraction intrusion during extensional collapse of the Ottawan Orogen. In contrast to this data-based conclusion, A&W state that absence of penetrative fabrics in LMG indicates that the Ottawan Orogeny was a high P, T regional event devoid of significant deformation, a conclusion in conflict with the Canadian data, as well as evidence presented here, and is incorrect. The A&W model contains other flaws: zircon cores in LMG have many non-overlapping ages, at odds with precipitation from a single magma and are inherited (consistent with zircon saturation temperatures of ca. 850-1000°C). Again, we stress the presence of oscillatory zoning in many ca.1050 Ma zircon mantles, signifying magmatic crystallization. Finally, undeformed LMG crosscuts AMCG rocks containing Ottawan structures and fabrics fixing intrusion of LMG as post-contractional and late Ottawan. Buddington and Postel also cited such examples.