DETACHMENT, ORE MINERALIZATION, AND PROGRESSIVE UPLIFT OF THE MARCY MASSIF DURING THE GRENVILLE OROGENIC CYCLE, ADIRONDACK MOUNTAINS, NY

The Mesoproterozoic Grenville Province of eastern North America is the roots of an orogenic belt that formed during protracted phases of accretion followed by ca. 1.09 –1.04 Ga supercontinent assembly. Differentiating the tectonic significance and timing of structural fabrics associated with the various events within the Adirondack region in the southern Grenville Province persists as a major problem. 1:24,000-scale bedrock mapping paired with detailed structural analysis in landslide chutes, in-situ monazite geochronology, and zircon geochronology within the vicinity of the 3000 km² Marcy massif (Mm) provides new insight into the structural evolution of the Adirondack Mountains.

Localized strain within the Mm was coaxial, whereas (sub)simple shear was accommodated along the margin preserving granulite- to amphibolite-facies metamorphic assemblages. Kinematic indicators on the southeastern margin are consistently oblique normal and plunge to the southeast, whereas kinematic indicators along the northern margin are oblique reverse and plunge to the northwest. These observations suggest that the Mm is overlain by a domed unidirectional detachment with consistent top to the southeast kinematic indicators defining an extensional metaigneous dome. The detachment was folded, particularly to the southeast of the Mm, where late leucogranite was emplaced into dilatant structures during continued extension within discrete shear zones away from the Mm. In-situ monazite geochronology from ca. 1180 Ma augen gneisses within the detachment zone indicates that top to the southeast shear occurred at 1065 ± 4.4 Ma along a retrograde P-T path, followed by emplacement of the ca. 1060 – 1040 Ma Lyon Mountain Granite Gneiss. Monazite also records a phase of extensive fluid-mediated dissolution-reprecipitation from 1050 – 1000 Ma consistent with the influx of fluids during progressive uplift. We interpret that the Ottawan orogeny within the Adirondack region represents collapse of an orogenic plateau that formed after the Shawinigan Orogeny and extensive anorthosite-mangerite-charnockite-granite plutonism (ca. 1190 – 1140 Ma). The Mm detachment zone may have facilitated fluid flow from both magmatic and surficial sources, which set the stage for widespread ore mineralization in the detachment from 1050 – 1000 Ma.