LINKING IN-SITU MONAZITE GEOCHRONOLOGY, METAMORPHISM, AND DEFORMATION OF MIGMATITIC PARAGNEISS, EASTERN ADIRONDACK MOUNTAINS, NY

Garnet-bearing paragneiss from an outcrop in the eastern Adirondack Mountains show evidence for anatexis followed by ductile shearing. The outcrop has a well developed foliation oriented approximately 011,14 and a strong, southeastwardly trending, gently plunging lineation. The leucosomes are stromatic and contain garnet + K-feldspar + plagioclase + quartz, while the host rocks contain the same minerals plus biotite +/- sillimanite. Garnet in the leucosomes is texturally distinct from garnet in the host rocks and is also generally larger. K-feldspar megacrysts are common in leucosomes and in the host gneiss. These observations are consistent with melting via a biotite dehydration reaction such as Bt + Pl + Sil +/- Qtz = K-feldspar + Grt + melt. Calculated P-T conditions for this reaction suggest metamorphic temperatures of 720-760 °C and pressure of 0.6-0.8 GPa.

These rocks also show evidence of ductile shearing following anatexis. K-feldspar meagacrysts, some of which may have formed during anatexis, and others presumably represent crosscutting pegmatites, now form elongate trains in the foliation with fine-grained dynamically recrystalized margins.

Preliminary in-situ electron microprobe monazite ages range from approximately 1.08 Ga to 1.02 Ga, suggesting that the anatexis and subsequent ductile deformation recorded by these rocks took place during the Ottawan Orogeny and perhaps post-Ottawan extension, consistent with the data of Bickford et al (2008). However, in-situ microprobe monazite ages from similar rocks less than five kilometers to the north, have a strong Shawinigan signature in addition to a strong Ottawan signature. The timing of melting and deformation are critical to interpreting the tectonic history of the Adirondacks. P-T-t-D modeling and monazite geochronological studies are currently underway to constrain the nature and timing of these events.