Syn-collisional exhumation of hot middle crust in the Adirondack Mountains (New York, USA): Implications for extensional orogenesis in the southern Grenville province

Extensional deformation in the lower to middle continental crust is increasingly
recognized and shown to have significant impact on crustal architecture, magma
emplacement, fluid flow, and ore deposits. Application of the concept of extensional
strain to ancient orogenic systems, like the Grenville province of eastern North
America, has helped decipher the structural evolution of these regions. The Marcy
massif is a ~3000 km2 Mesoproterozoic anorthosite batholith in the Adirondack
Mountains (New York, USA) of the southern Grenville province. Bedrock geology
mapping at 1:24,000 scale paired with characterization of bedrock exposed by recent
landslides provides a glimpse into the structural architecture of the massif and
its margin. New data demonstrate granulite- to amphibolite-facies deformational
fabrics parallel the margin of the batholith, and that the Marcy massif is draped by
a southeast-directed detachment zone. Within the massif, strain is localized into
mutually offsetting conjugate shear zones with antithetic kinematic indicators.
These relationships indicate that strain was coaxial within the Marcy massif, and
that subsimple shear components of strain were partitioned along its margin. In
situ U–Th–total Pb monazite analysis shows that deformation around and over the
Marcy massif occurred from 1070 to 1060 Ma during granulite-facies metamorphism,
and monazite from all samples record evidence for fluid-mediated dissolution reprecipitation
from 1050 to 980 Ma. We interpret that rocks cooled isobarically after accretionary
orogenesis and emplacement of the anorthosite- mangerite- charnockitegranite
plutonic suite at ca. 1160–1140 Ma. Gravitational collapse during the Ottawan
phase of the Grenville orogeny initiated along a southeast-directed detachment
zone (Marcy massif detachment zone), which accommodated intrusion of the Lyon
Mountain Granite Gneiss, and facilitated substantial fluid flow that catalyzed the
formation of major ore deposits in the Adirondack Highlands.