AGE OF THE LYON MOUNTAIN GRANITE REVISITED, AGAIN

The Lyon Mountain Granite (LMG) in the Adirondack Highlands was an important source of iron ore and may have REE potential. Aleinikoff and Walsh (2015; A&W) have presented a revised age for the LMG which is problematic. Because A&W’s revaluation is in direct conflict with decades of field mapping and analytical studies, it requires critical scrutiny. Specifically, A&W claim that the crystallization age of the LMG is 1142 ± 10 Ma as determined from dating zircon cores. Thick rims overgrowing the cores yield overlapping ages of 1055 ± 7 and 1028 ± 28 Ma, and A&W consider them to be successive rims of metamorphic growth. A&W’s interpretation is based on the variation of REEs and U/Th ratios between rims and cores, and reinterpretation of internal features (i.e. oscillatory zoning) using CL images.

In this study, we investigate zircon from the LMG, along its extent across the Highlands, by LA-MC-ICP-MS to document: 1) the age and abundance of cores; 2) variations in U/Th ratios within grains; and, ultimately, 3) the crystallization age of the LMG. Two approaches were followed including analyzing visually identified core-rim pairs and the centers of large numbers of randomly selected grains. The amount of “core” material in the LMG varies substantially from almost none to as much as half the volume of zircon present. While ca. 1150 Ma is the dominant core age in some samples, we find core ages to be variable within, and among, samples. Core ages range from as old as 1848±17 Ma to as young as ca. 1040 Ma. In some samples well defined (in Cl and BSE) rims and cores in individual zircon grains are of the same age. Populations of coreless, zoned, and euhedral zircons yield ages identical to rims and have similar chemical characteristics. Most grains slight show increases in U/Th from center to rim; however, this is also shown in grains whose “core” and “rim” are the same age. Taken together, this information indicates that previous ages ranging from ca. 1060-1040 Ma from multi-grain TIMS and SHRIMP analyses across the region are crystallization ages and that older cores are xenocrystic in origin. This conclusion is in concert with constraints from field relations, observations of fabric development and metamorphism, new field work in the eastern Adirondacks, and the LMG’s intimate association with late cross-cutting pegmatites and iron ores.