CONSTRAINING THE DEPOSITIONAL HISTORY AND EVOLUTION OF THE TRANS-ADIRONDACK BACK-ARC BASIN USING DETRITAL ZIRCON GEOCHRONOLOGY

Detrital zircon geochronology has been applied to constrain the source(s) and depositional ages of metasedimentary rocks of the Adirondack Lowlands. Thin (1-5 cm) quartzites interlayered with pelitic gneisses at Pyrites, NY occur within a NE-trending 55 km-long belt of pyritic gneisses believed to be of volcanic exhalative origin. The quartzites yield a population of silt-sized, strongly zoned, and euhedral zircons with a single age of 1290+/-10 Ma. They represent unroofing of juvenile volcanic rocks that were reworked and deposited as turbidites on the floor of the developing rift basin. A similarly sized belt of tourmaline-bearing arkoses and quartzites occurs within the Lower Marble. Where sampled at Richville, NY the rock is strongly layered (defined by variation in grain-size and mineralogy) and displays a multi-component Proterozoic source (ca. 1264, 1296, 1456, 1654, 1766, 1848 Ma) with minor Neoarchean components. These rocks are believed to represent an interval of clastic-dominated sedimentation during a period dominated by carbonate sedimentation. Their arkosic nature and prevalence of tourmaline suggests deposition in evaporitic, fault-bounded basins. Quartzite from the base of the Upper Marble from core drilled near Balmat, NY yields a detrital pattern similar to that of the tourmaline-bearing quartzites with peaks at ca. 1300, 1444, 1658, and 1902 Ma, and subordinate Neo- and Mesoarchean (~3.3 Ga) ages. All three of the quartzites examined lack detrital grains younger than ca. 1255-1275 Ma indicating deposition prior to the Elzivirian Orogeny. In contrast to the Popple Hill Gneiss, thought to be derived from unroofing of tonalitic gneisses of the Southern Adirondack Terrane, they contain relatively minor amounts of zircon with ages between ca. 1300-1400 Ma. In addition, they lack discrete metamorphic zircons and volumetrically significant proportions of metamorphic zircon rims. Taken together, the data implies sourcing from the Canadian Shield to the north and a currently unrecognized, or erosionally removed, volcanic terrane (ca. 1290 Ma), likely associated with the initial rifting and opening of the Trans-Adirondack Back-arc Basin.