HIGH-TEMPERATURE SHEARING AND PEGMATITE FORMATION DURING OTTAWAN EXTENSIONAL COLLAPSE, NORTHWESTERN ADIRONDACK MOUNTAINS, NEW YORK

A set of high temperature, NE-oriented shear zones and associated NW-oriented pegmatites occurs near the boundary zone between the Adirondack Highlands and Lowlands. The shear zones and pegmatites sharply cross-cut primary foliation in the host ca. 1155 Ma Diana syenite, and dominantly consist of K-spar and clinopyroxene plus or minus quartz and calcite. A notable feature of these mineralized zones is lenticular masses of titanite that are up to 10 cm. in length and occur mostly in pegmatitic material that has been modified in the shear zones. Because initial observations suggested that the titanite masses were large single crystals, an initial attempt to date the titanite involved casting one of the masses in epoxy, sectioning it, and analyzing portions by standard TIMS methods. These analyses yielded ages ranging from 1089 to 1022 Ma. Subsequently, SEM imaging revealed that the masses are in fact granular aggregates of millimeter-scale euhedral titanite crystals that are intergrown with quartz, calcite, and zircon. SHRIMP-RG analyses of individual titanite crystals within one of these aggregates yielded two 207Pb/206Pb age populations, one at ca. 1050 Ma and another at ca. 950 Ma, with two outliers at ca. 1150 Ma. SHRIMP-RG analyses of included zircon yielded 207Pb/206Pb ages ranging from 1165 Ma to 970 Ma, but regression of the data yields an upper intercept age of ca. 1050 Ma, consistent with the titanite data. Application of the zirconium-in-titanite thermometer yields values consistent with formation in granulite facies. These data are consistent with the 1044 Ma age of deformation on the Carthage-Colton shear zone, and indicate that active, high-temperature shearing and concomitant pegmatite formation occurred during extensional collapse of the Ottawan orogen.