NEOPROTEROZOIC DIKES OF THE NORTHEASTERN ADIRONDACK MOUNTAINS

Mesoproterozoic basement rocks of the Adirondack Mountains are cut by numerous Neoproterozoic dikes whose composition is basaltic or trachytic. The relative age of the dikes is known because they pre-date the Great Unconformity, but determination of their absolute age has been problematic. Similar dikes are common along the Proterozoic Laurentian margin from North Carolina to Norway and mark the onset of rifting of Rodinia. In the southern Appalachians, dike ages range from >700 Ma to 550 Ma. Most dated dikes in the northeastern U.S. and eastern Canada are in the range of 600 to 550 Ma.

One basaltic and three trachytic dikes from Rand Hill (RH) and Dannemora (DM) in the northeastern Adirondacks were sampled for U-Pb zircon and ⁴⁰Ar/³⁹Ar biotite geochronology. High bulk rock Zr contents (600 – 1500ppm) suggested that the trachytes should have zircon, but conventional mineral separation methods were unsuccessful. However, full thin section mapping demonstrated that in fact zircon was abundant, but typically small, inclusion-rich, and irregular in shape, likely contributing to the poor yields by standard methods. The basaltic sample contained very few zircon grains, and these were too small to date. Trachyte sample RH-1 yielded an age of ~670 Ma, while RH-2 and DM-1 both dated to ~640 Ma. The data, especially from samples DM-1 and RH-2, exhibited moderate scatter as expected from their petrographic characteristics. The apparent older age from sample RH-1 may be due to high-U concentrations in the zircon (~2000 ppm).

Biotite was also separated from trachyte samples RH-1 and RH-2. Thin section observations support a magmatic origin for biotite and showed variable alteration of biotite to chlorite. Grains hand-picked for ⁴⁰Ar/³⁹Ar analysis exhibited no visible alteration. Step-heating experiments yielded slightly disturbed age spectra in early degassing due to chlorite, but most of spectra are flat. Weighted average ages from each sample overlap with the 640 Ma U/Pb age. Taken together, the U/Pb and ⁴⁰Ar/³⁹Ar data indicate dike crystallization at approximately 640 Ma. Our results confirm the initial age reported by Sinton and others (2018). The Cryogenian age of these dikes is > 50 m.y. older than rift-related dike ages reported in central Vermont. This suggests that the Adirondack dikes represent an earlier failed rift.