DID THE SNOWY MOUNTAIN ANORTHOSITE (ADIRONDACKS, NY) BEHAVE AS A MEGA-PORPHYROCLAST DURING REGIONAL DUCTILE DEFORMATION?

Plutons comprise a large volume of the crust and often form dome-shaped structures with the country rock. The Snowy Mt. anorthosite body, and related AMCG rocks, occur within a structural dome (Snowy Mt. dome, SMD) approximately 15 km across in the central Adirondacks, NY. The SMD is defined by a concentric mantle of variably foliated norite-, mangerite- and charnockite-gneisses that are compositionally and texturally gradational with the anorthosite. Foliation that defines the dome consists of aggregates of recrystallized plagioclase that alternate with brittlely deformed pyroxenes. The dome is doubly plunging NW & SE with an amplitude of about 1200 m. Mineral elongation lineations defined by aggregates of plagioclase and pyroxenes occur in the deformed mantle of the dome. Consistenly the lineations trend approximately E-W across the dome. Handsample kinematic analysis was performed on rocks around SMD using plagioclase d-clasts, s-clasts of cpx, S-C fabric, domino structures of broken opx, and shear bands. On the NE & NW flanks of the dome, the kinematic analysis yielded a dominant shear sense of top westward, while on the SE and SW sides of the dome, the kinematic analysis resulted in a top eastward shear sense. The foliation is penetrative in the dome flanks, however, the foliation is pregressively weaker developed toward the interior anorthosite and in the supracrustal rocks way from the dome. Additionally, foliation that wraps around SMD, narrows into the sinistral Moose River Plain shear zone (MRPz) to the west. The map pattern of anorthosite at the dome core shows narrow appendages that extend outward from the main body. These appendages contain the same penetrative foliation as the surrouding AMCG rocks.

A foliation trajectory map shows a complex transition between the MRPz and the SMD. Opposing shear sense on opposite flanks of the SMD suggests that ductile flow occurred around the anorthosite in an overall sinistral shear couple. Considering that SMD foliation merges with the foliation of the MRPz, and appears to have been deflected around the Snowy Mt. igneous suite, we tentatively conclude that the Snowy Mt. anorthosite behaved as a relatively rigid object during deformation. The folded shape of the foliation around the dome looks like that found around sinistral macroscopic porphyroclasts, however, at a much larger scale.