TEXTURAL AND U-PB ZIRCON SHRIMP-RG GEOCHRONOLOGY EVIDENCE IN SUPPORT OF A C. 1060-1040 MA MAGMATIC CRYSTALLIZATION AGE FOR THE SKIFF MOUNTAIN BODY OF THE LYON MOUNTAIN GRANITE, EAST-CENTRAL ADIRONDACK HIGHLANDS, GRENVILLE PROVINCE

The Lyon Mt. Granite (LMG) is found within the granulite facies Adirondack Highlands, the southern-most terrane within the contiguous Mesoproterozoic Grenville Province of eastern North America. The LMG is widely thought of as a late-tectonic granitoid emplaced during the c. 1050-1025 Ma collapse of the c. 1080 Ma Ottawan Orogeny. Most research report LMG magmatic crystallization ages in the c. 1060-1040 Ma range, consistent with this model. However, some recent analysis and reinterpretation of zircon ages and textures suggests a LMG emplacement age of c. 1140 Ma. Our work presents detailed textural analysis and U-Pb zircon sensitive high resolution ion microprobe – reverse geometry (SHRIMP-RG) dating of the Skiff Mt. body of the LMG to determine the emplacement age of this body.

The Skiff Mt. body is located about 15 km west of Ticonderoga, New York, in the east-central Adirondack Highlands. The Skiff Mt. body possesses magmatic microtextures indicating the body did not experience solid-state recrystallization due to deformation or regional metamorphism, though evidence for limited fluid alteration exists. Field relationships indicate the Skiff Mt. body intruded synchronous with F₃ regional folding.

Zircon grains from the Skiff Mt. body are complex with one or more bright cathodoluminescent, oscillatory zoned cores. Zircon rims are relatively dark in cathodoluminescence, but optimized images reveal that rims truncate core zoning patterns and possess oscillatory zoning. The core-rim transition is often associated with a variety of mono- and polymineralic inclusions of quartz, Na-rich plagioclase, K-feldspar, biotite, sphene, and apatite. Based on their character, these inclusions are interpreted as melt inclusions; therefore, zircon rims are interpreted as forming during magmatic crystallization of the Skiff Mt. body.

Uranium-Pb SHRIMP-RG dating of zircon rims produces a weighted mean age of c. 1043 Ma. Whereas a chord fitted through these rim analyses and generally reversely discordant core ages, possible partially reset during LMG emplacement, produce an intercept of c. 1057 Ma. More data will be obtained but the current best estimate of the emplacement age of the Skiff Mt. body is between 1057 Ma and 1043 Ma, consistent with, and thus supporting the 1060-1040 Ma syn-collapse emplacement of the LMG.