PARTITIONED STRAIN, STATIC AND DYNAMIC METAMORPHISM, AND FLUID MOBILIZATION WITHIN THE MARCY ANORTHOSITE MASSIF, ADIRONDACK HIGHLANDS: IMPLICATIONS FOR DEEP CONSTRUCTION AND SHALLOW EMPLACEMENT OF MASSIF-TYPE ANORTHOSITE

The emplacement of Proterozoic, or massif-type, anorthosite has been debated for decades with models suggesting a deep or shallow level of emplacement. The Marcy massif (Mm), in the Adirondack Highlands, crystallized at ca. 1.15 Ga near the end of the Shawinigan Orogeny and represents a classic example of massif-type anorthosite. Recent 1:24,000 scale mapping, both along the margin and within the Mm, provides new insight relevant to its origin. A thick (~5 km) high-strain zone envelopes the Mm and is composed of a variety of rock types with varying degrees of mylonitization. Kinematics are consistently dextral/normal, with an average lineation plunging 20° at 145°. Lineations vary around the mean plane suggesting that rocks deformed around the Mm. Tectonites proximal to the Mm consist of gabbro, gabbroic anorthosite, and ferrodioritic gneisses, each containing abundant coarse andesine xenocrysts and a metamorphic assemblage of plag + ilm + cpx + grt +/- opx +/- biot +/- hbl. Thermobarometric analyses paired with forward modelling of several rocks suggests that the rocks were mylonitized during decompression from pressures of  >1 GPa to <0.6 GPa. Within the Mm, strain is localized in conjugate shear zones commonly developed within aplitic gneisses with microporphyroclasts of opx, and associated with metamorphic hbl +/- grt. Shear zones developed within anorthosite contain annealed scapolite (> 15 modal %), plag, and dynamically recrystallized cpx  with little to no grt. The lack of grt and opx in anorthosite mylonite paired with the presence of scapolite indicates high temperatures and high ɑH₂O during deformation. Shear zones containing hydrous phases provide a mechanism to distribute fluids throughout the Mm, facilitating the batholith-wide contamination recorded by O, Hf, and Nd isotopes. Igneous phases within the anorthosite consist of very coarse plagioclase and opx with exsolved plagioclase suggesting Al-rich hypersthene crystallized. Metamorphic cpx is common as a static phase enveloping primary opx. The results suggest that the Mm crystallized at the base of the crust, and was emplaced to higher crustal levels shortly thereafter in a subsolidus state.