Zircon grains from metasiltstone beds in the Precambrian Ashburn Formation were dated to constrain the age of the stromatolite Archaeozoon acadiense. The largest group of analyzed grains consists of mostly well-rounded detrital zircon no younger than 1060 Ma with significant peaks at about 1200–1100, 1500, 1800, 2050, and 2700 Ma, as well as a few Mesoarchean ages. A second population of apparently euhedral grains gives ages in the range 680–400 Ma. The distribution of detrital zircon ages along with field relationships suggest that the Ashburn and Martinon formations are proximal and distal parts of the Green Head Group, respectively, and hence a previously reported maximum depositional age of ca. 602 ± 8 Ma for the Martinon Formation also applies to stromatolite in the Ashburn Formation. Paleozoic zircon recovered from the Ashburn Formation cannot be detrital because the rocks were intruded and contact metamorphosed by the Golden Grove Plutonic Suite at ca. 550–525 Ma. The Paleozoic zircon grains were found in four independently processed samples and have also been seen in other studies in the region, showing that they are not due to laboratory contamination. Zircon formed during younger magmatic activity may have been transported in hydrothermal fluids and emplaced into these metasedimentary rocks in veins. A single euhedral zircon recovered from an atypically thick vein gave an age of 659 ± 33 Ma. Hydrothermally transported zircon might provide a window into younger igneous processes for which evidence is not otherwise preserved in this and other areas.

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