NEW U-PB ZIRCON AGES AND FIELD STUDIES SUPPORT SHAWINIGAN DEFORMATION IN THE EASTERN ADIRONDACKS

Regional E-W trending penetrative gneissosity and D2 structures in the eastern Adirondacks are conventionally attributed to the Ottawan event of the Grenville orogeny at ca. 1090-1050 Ma. New U-Pb zircon ages for rocks interpreted as post-, syn-, or pre-tectonic relative to D2 include weakly deformed pink garnet-microperthite granite previously interpreted as post-D2 Lyon Mountain granite gneiss (1163 ± 6 Ma, Putnam Station), moderately deformed Hawkeye granite gneiss (1160 ± 10 Ma, type locality), and strongly deformed ferrodiorite gneiss (1153 ± 8 Ma, Dresden Station). New ages for largely late- to post-tectonic, weakly foliated, or flow-foliated rocks include Lyon Mountain granite gneiss (LMG) at the type locality (1142 ± 10 Ma), LMG fayalite granite at Au Sable Forks (1140 ± 5 Ma), metagabbro dike at Dresden Station (1134 ± 3 Ma), and light-gray magnetite-quartz-albite granitoids at Hammondville (1117 ± 18 Ma) and Skiff Mountain (1116 ± 9 Ma). Many, but not all, rocks contain zircon with ca. 1050 and 1000 Ma metamorphic or hydrothermal rims that correspond to the well-dated high P-T Ottawan and terminal Rigolet events, respectively. Although the Ottawan event probably was responsible for formation of ubiquitous zircon rims, it likely was only a localized penetrative fabric-forming event. Most of the widespread strong deformation likely occurred prior to about 1140 Ma. The new ages suggest that some granitoids previously assigned to the Ottawan LMG are older (ca. 1160-1110 Ma), and span the ca. 1160-1110 Ma time period between the Shawingan and Ottawan events. The late or post-Shawinigan granitoids typically show flow-foliation and only limited, localized, solid-state deformation, instead of widespread penetrative gneissosity common in the older rocks. The process of penetrative migration and emplacement of granitoid sheets parallel to older foliations at high grade accompanied by volatile-driven intrusion and subsequent fluid alteration can explain the largely layer-parallel post-tectonic nature of the younger granitoids.