16-44 Age of Metamorphism and Deformation in the Cheney Mountain Shear Zone, Eastern Adirondack Highlands, New York

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Abstract:

Recently-recognized shear zones in the eastern Adirondack Highlands have been interpreted as resulting from extensional collapse late in the 1080–1020 Ma Ottawan orogeny. This study focuses on deformed members of the Adirondack anorthosite- mangetite- charnockite- granite (AMCG) suite in the undated extensional Cheney Mountain Shear Zone (CMsz), which is located in the Port Henry 7.5’ quadrangle. The CMsz is a 1-2 km-wide, left-lateral, ductile shear zone that strikes SE that can be traced for at least 14km from near Elizabethtown to Port Henry. Transposed rocks in the hanging wall comprise marble, paragneiss, and Lyon Mountain Granite Gneiss. AMCG lithologies dominate the footwall. In the shear zone slivers of mostly AMCG rocks are separated by anastomosing high-strain zones. 

Seven rocks were selected for in-situ laser ICPMS geochronology of zircon at the Arizona Laserchron lab. Five metagabbros and two leucocratic hornblende granite gneisses represent a variety of metamorphic textures and amounts of strain in the CMsz and footwall.  The metagabbros are Bt + Pl + Grt + Ilm ± Cpx ± Hbl and often preserve coronas. Hbl+Bt coronas around remnant igneous ilmenite contain zircon which is a focus of this study. Two granite samples are coarse, strongly deformed, and contain Hbl + Chl + Grt ± Mag ± Ilm. 

Zircons in coronitic metagabbros have two age populations, either associated with ilmenite breakdown with an age of ca. 1020 Ma, or in matrix with an age of ca. 1145 Ma. Granite zircons do not show a relationship between texture and age. Two generations of zircon growth are seen in these samples: cathodoluminescence (CL)-bright regions are ca. 1055 Ma and CL-dark areas are ca. 1135 Ma. Similar relationships in CL brightness are seen in young versus old ages in mettagabbros. We interpret the ca. 1145-1135 Ma ages in metagabbros and granites to represent (perhaps disturbed) igneous ages, which is similar to the AMCG suite elsewhere in the Adirondacks. The ca. 1060 Ma ages in the hornblende granites represents a metamorphic age and zircon formation during the Ottawan orogeny, and the ca. 1020 Ma age related to ilmenite breakdown in the metagabbros reflect a later hydrothermal event related to mineralization in magnetite deposits in the CMsz footwall during cooling. Deformation in the CMsz post-dates corona formation in mettagabbros, so is constrained to <1020 Ma.

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