MARGIN-PARALLEL TECTONISM AROUND THE MARCY MASSIF, ADIRONDACK HIGHLANDS: IMPLICATIONS FOR THE SUBSOLIDUS EMPLACEMENT OF MASSIF ANORTHOSITE

Pluton and batholith emplacement mechanisms have traditionally been evaluated under two broad categories: diapiric upwelling and piecemeal emplacement of lit-par-lit semi-concordant intrusions. Massif anorthosite and associated mangerite, charnockite, and granite lithologies, collectively referred to as AMCG suites, are a hallmark of the Grenville Province in NY and Canada. Recent mapping along the eastern margin of the Marcy massif, eastern Adirondack Highlands, provides structural and petrologic data that constrain the geometry and conditions of emplacement. This includes the transition from undeformed anorthosite in the core enveloped by a km-thick, highly tectonized zone of gabbroic anorthosite to jotunite. Gneissosity within the gabbroic anorthosite, as well as opx-bearing granitoid gneisses and subordinate paragneisses is parallel with the massif anorthosite margin, with poles to foliation (n = 253) defining a fold-like girdle pattern on stereonets. Quartzofeldspathic gneisses and garnetiferous hbl-bearing migmatitic paragneisses included in the deformational zone display folds that are absent in younger orthogneisses, and therefore predates AMCG plutonism.This indicates complete transposition of older tectonic fabrics into margin parallelism subsequent to folding. Mineral stretching lineations (n=66) display a unimodal distribution (22° to 148°) and are co-linear with the β -pole calculated from the marginal girdle (30° to 154°). Kinematic indicators throughout the anorthosite margin suggest a normal component of relative displacement.The relationship between the stretching lineation and the fold-like geometry of the marginal fabric is interpreted as evidence of syn-emplacement deformation. Preliminary thermobarometry corroborates these field relationships and demonstrates that anorthosite crystallization and early deformation occurred at pressures near or exceeding 1 GPa followed by decompression and recrystallization during emplacement of massif anorthosite to shallower crustal levels, perhaps as a crystal mush. The 3-6 km wide zone of massif-parallel deformation is interpreted as a consequence of anorthosite emplacement during the terminal phase of Shawinigan orogenesis at ca. 1150 Ma.