STRUCTURAL, PETROLOGIC, AND MONAZITE AGE DATA FROM THE EASTERN ADIRONDACKS: CONSTRAINTS ON THE NATURE AND TIMING OF DEFORMATION AND METAMORPHISM

Interpretations of the tectonic evolution of the Adirondack Mountains have been significantly modified during the past 15 years on the basis of new geochronology, field mapping and geochemistry. One major change involves the recognition of a phase of granulite facies metamorphism and deformation associated with the Shawinigan Orogeny (ca. 1200-1180 Ma) that was only cryptically recognized previously. This phase of orogenesis is separate and distinct from the Ottawan event (ca 1050Ma). Both orogenies were accompanied by granulite facies metamorphism and deformation. In addition, Wong and others (2011) and McLelland and others (2011) suggest that at least some of the fabric in lineated, mylonitic rocks in the eastern Adirondacks developed during a period of extensional collapse following the Ottawan orogeny. This may be related to the Tawachiche shear zone recognized to the north. Current work involves detailed analysis rocks in the eastern Adirondacks, including new mapping, petrologic and structural analysis, electron microprobe (EPMA) monazite dating and thermodynamic modeling. Some preliminary results are reported here.

Mylonitic rocks with a well developed lineation are found throughout the eastern Adirondacks. L-tectonite fabrics are found in granitic rocks as well as metamorphosed, ferro-gabbro to anorthositic-gabbroic rocks. Migmatitic, metapelitic rocks commonly contain strongly lineated, coarse grained sillimanite. Kinematic indicators in these various lithologies are mixed but overall suggest normal shearing, i.e. extension. Initial thermobarometric calculations and psuedosection analysis of metapelitic rocks indicate P-T conditions between 6-9 kb and 700-750 C. Partial melting in these rocks occurred via biotite dehydration reactions.

Oriented samples of deformed rocks of variable lithologies were collected for in-situ EPMA monazite age determination. Initial analysis of monazite from samples of deformed mylonitic granitic rocks and metapelitic rocks with a strong sillimanite lineation yields several distinct age populations varying between 1180 and 1030 Ma. Monazite rims (1050-1030 Ma) are synchronous with the normal shearing, and support the post-Ottawan collapse model.