PRE-OTTAWAN DEFORMATION AND TRANSPRESSIONAL FAULTING IN THE HUDSON HIGHLANDS OF NEW YORK BASED ON SHRIMP ZIRCON AGES OF THE STORM KING GRANITE AND THE SYNTECTONIC CANOPUS PLUTON

New SHRIMP isotopic ages of zircon from the Storm King Granite (of Berkey, 1909) at Dunderberg Mountain indicate an intrusive age of 1174±8 Ma and overgrowth ages of 1114±15 Ma. These ages are considerably older than previous estimates of the Storm King of about 1130 Ma, the Byram Intrusive Suite (~1095 Ma, Drake et al., 1991), and the Vernon Super Suite (1116±41 Ma to 1095±9 Ma, Rb-Sr WR, Volkert et al., 2000) all of which contain the regional YF2 folds and the dominant gneissosity of the Hudson Highlands. The Canopus pluton was intruded into an active right-lateral shear zone following YF2 folding at 1144±13 Ma based on a new SHRIMP zircon age. Overgrowths grew at about 1000 Ma. These data indicate that the intense YF2 folding event and formation of gneissosity occurred between about 1174 and 1144 Ma during the Shawinigan Pulse of the Grenville orogeny rather than in the Ottawan Pulse (as defined by Rivers, 1997). A broad zone of transpressional folding accompanied intrusion of the Canopus pluton both to the west and to the east of the right-lateral Canopus shear zone. Late upright north-plunging YF3 folds and syntectonic intrusion of the Canada Hill granite at 1010±6 Ma (Aleinikoff and Grauch, 1990; Ratcliffe, 1992) may be the only expression of Ottawan Pulse tectonism in the Hudson Highlands. Previous age assignments of Grenville events based on previous U-Pb zircon and Rb-Sr isotopic ages for many of these hard-to-date rocks are reevaluated in the light of these new data and our evidence for pre-Ottawan structures in the Mount Holly Complex of Vermont.