DECIPHERING TIMING AND KINEMATICS OF DEFORMATION IN THE EASTERN ADIRONDACKS, NEW YORK STATE: WHEN DID THE ANORTHOSITE-MANGERITE-CHARNOCKITE-GRANITE SUITE EXPERIENCE PENETRATIVE DEFORMATION?

Dating ductile fabrics and their kinematics is critical to understanding orogenesis as fabrics can form, for example, during crustal thickening and/or subsequent crustal thinning. Crustal thickening and thinning cycles have important implications for understanding crustal melt generation through heating or decompression, as well as the growth and demise of orogenic plateaus. Detailed structural and geochronologic analysis of an outcrop in the extreme eastern Adirondacks of northern New York State was undertaken to understand the importance of such fabrics in a polyorogenic terrane.

The Adirondacks are a southern extension of the Mesoproterozoic Grenville Province, and here, two main orogenic events have most strongly influenced the geology. The first is the accretionary c. 1200-1150 Ma Shawinigan orogeny, which at its end included intrusion of anorthosite-mangerite-charnockite-granite magmas (AMCG suite). The second is the collisional c. 1090-1030 Ma Ottawan orogeny, which at its end experienced tectonic collapse and intrusion of the c. 1060-1030 Ma Lyon Mt. Granite. Three deformation phases have been identified in the study area. D₁ is Shawinigan aged and produced a penetrative fabric only seen in pre-AMCG suite paragneisses, D₂ transposes D₁ structures and imprints a penetrative fabric into most AMCG rocks, but is of unknown age. D₃ is associated with tectonic collapse during Lyon Mt. Granite intrusion.

Within the outcrop, newly identified cryptic late-D₂ leucosome contains two generations of oscillatory zoned zircon: c. 1177 Ma Shawinigan aged zircon, and c. 1067 Ma Ottawan aged zircon. The oscillatory zoning of each indicates zircon growth during migmatization of the host rock during both orogenies, the latter constrains timing of D₂. D₂ kinematic indicators are poorly developed but indicate east over west transport, consistent with kinematics of correlated fabrics in AMCG rocks further south. D₂ may be from early Ottawan tectonic collapse, though kinematics is opposite to D₃ kinematics. The favored scenario is that D₂ is from Ottawan crustal thickening immediately prior to tectonic collapse. This scenario is consistent with most models for tectonic collapse as well as plate reconstructions that place a colliding Amazonia to the east of the present day Adirondacks during the Ottawan orogeny.