PEGMATITES OF COBBLE LOOKOUT, NORTHERN MARGIN OF THE MARCY MASSIF, ADIRONDACK MOUNTAINS, NEW YORK, USA

The Marcy Massif of the Adirondack Mountains of New York is a Proterozoic anorthosite batholith of the Grenville Province. The geologic history of the area involves Shawinigan convergent deformation and magmatism, emplacement of the anorthosite as part of the AMCG plutonic suite, and continued orogenesis during the Ottawan phase. The Ottawan ended with late syn-to-post kinematic emplacement of the Lyon Mountain Granite, which included the intrusion of pegmatites.

At the Cobble Lookout site at the northern margin of the Marcy Massif, the meta-anorthosite hosts garnet-rich planar structures. These are similar to shear structures in that they are anastomosing, commonly have a layered appearance, and have an orientation similar to the host rock gneissosity (E strike, N dip). However, field and microscale crosscutting relationships support the interpretation that they are pegmatites.

The pegmatites contain garnet, quartz, and plagioclase with variable amounts of ilmenite, potassium feldspar, titanite, pyroxene, zircon, and iron oxides, which is a more varied mineral assemblage than the host rock (i.e., dominantly pyroxene, plagioclase, and hornblende). They present with the following textures at the microscale: (1) granular and unlayered, (2) granular and compositionally-layered, and (3) layered with corona textures. Garnet typically surrounds pyroxene and/or oxides in the corona texture and is locally symplectic.

Field and microscale observations suggest that the intrusion of the pegmatites was syn-deformational. The compositional layers in the pegmatite are aligned in ways consistent with layer-normal kinematics. This includes an example of layer alignment within what appears to be a boudin neck. Yet, quartz displays only minor undulous extinction and recrystallization textures, and there is no microstructural evidence for significant post-crystallization shear deformation (e.g., disruption of the corona/symplectite texture). The layered vs. unlayered appearance of the microtextures may reflect the variation in strain localization within the pegmatites as they were crystallizing. It is currently unclear whether these pegmatites are related to the Lyon Mountain Granite, but their syn-kinematic nature is consistent with what is known about that stage of the area’s geologic history.