Constraints on deposition in the Trans-Adirondack Basin, Northern New York: Composition and origin of the Popple Hill Gneiss

The Popple Hill Gneiss (PHG) records the transition from back-arc to foreland basin sedimentation in the Trans-Adirondack Back-Arc Basin between ca. 1220–1245 Ma. Composed of upper amphibolite facies gneisses, with the assemblage biotite–plagioclase–potassium feldspar–quartz ± garnet ± sillimanite and occasionally abundant k-spar megacrysts and/or leucosomes, the PHG occurs stratigraphically between the upper and lower marbles, which were deposited in shallow, and sometimes evaporitic, marine waters. Best exposed and most intact in the Adirondack Lowlands, correlative granulite facies rocks, often mylonitic and extensively migmatized, occur in the Highlands. In both areas the PHG is intruded by ca. 1170–1200 Ma calc–alkaline granitic rocks and those of the slightly younger AMCG suite (ca. 1155 Ma). The PHG is geochemically similar to UCC estimates, has a calc–alkaline trend, and incompatible element patterns related to arc magmatism. Variable, but low CIA indices are indicative of a slightly weathered and/or rapidly exhumed source. The PHG contains sand- and silt-sized zircon xenocrysts ranging in age from ca. 1220 to 1400 Ma, lacks an Archean population, has rims related to Shawinigan (1160–1180 Ma) and Ottawan (ca. 1050 Ma) anatexis (eastern Adirondack Highlands only), and yields monazite populations of several ages including some in excess of >1220 Ma. These data are consistent with a rapidly exhumed felsic arc source for much of the unit, likely derived from ca. 1250–1350 Ma rocks of the Southern Adirondacks. The metasedimentary sequence in the Adirondack Lowlands records rifting and carbonate platform deposition associated with the opening of the Trans-Adirondack Back-Arc Basin (lower marble), evolution to, and rapid fill of, a foreland basin flysch sequence (PHG), and cyclic carbonate-evaporite deposition (upper marble) as the basin filled and convergence continued just prior to collisional tectonism associated with the Shawinigan Orogeny (ca. 1160–1200 Ma).

► The upper amphibolite facies Popple Hill Gneiss shows geochemical variability, compositional layering, and contains sand-sized detrital zircon cores.
► An origin as a rapidly deposited greywacke-shale sequence in the Trans-Adirondack Basin, in a flysch-type setting is proposed.
► The source of the PHG was dominated by 1300–1380 Ma arc rocks of the Southern Adirondacks.
► The depositional age of the Popple Hill Gneiss is constrained between 1220 and 1270 Ma; it may be ∼1220 Ma.
► Metasedimentary rocks of the Adirondack Lowlands track the evolution of the Trans-Adirondack Back-Arc Basin through its development and closure.