GEOCHEMICAL AND PETROGRAPHIC COMPARISON OF TWO ADIRONDACK MAFIC DIKES WITH OTHER KNOWN PROTEROZOIC AND MESOZOIC RIFT DIKES AND VOLCANICS FROM THE ADIRONDACKS AND NORTHERN VERMONT

A comparison was conducted between previously published studies on the geochemical and petrographic characteristics of Proterozoic and Mesozoic mafic basalts in the Adirondack region with that of two mafic dike samples collected from two sites in the Adirondack region, basalt from a dike on the peak of Ampersand Mountain in Franklin County, NY, and a diabase from a dike along Golden Staircase Creek at North Lake near Forrestport, NY. The purpose of the analysis was to determine if the geochemical and petrographic characteristics of the samples are more consistent with Proterozoic or Mesozoic origins and under what tectonic conditions they may have formed.

Overall, the geochemistry of both samples is consistent with that of within-plate basalt and their geologic context indicates that they are most likely related to the rift events either during the Proterozoic and/or Mesozoic. Geochemical discrimination between these two events is difficult, but the chemistry of the Ampersand Mountain sample more closely resembles that of other published samples of Proterozoic rift dikes, especially with respect to Th and Sr contents. The chemistry of the Golden Staircase sample is mostly inconclusive, but its higher Th and Sr contents are more consistent with that of Mesozoic basalts.

The Golden Staircase sample displays an ophitic texture with randomly oriented, striated phenocrysts of plagioclase. Petrographic analysis of the Ampersand Mountain sample shows similarities with the Proterozoic suite; original plagioclase, pyroxene, and olivine that shows some metamorphic recrystallization to chlorite, serpentine, actinolite, and epidote.