COMPARATIVE STUDIES OF MAFIC DIKES IN THE NORTHEASTERN ADIRONDACKS, NY

This study compares geochemical and petrographic characteristics of mafic dikes from three localities in the northeastern Adirondacks: Coughlin Brook, Split Rock Falls, and Rand Hill. The Coughlin Brook dike swarm and Split Rock dikes intrude anorthositic country rock and are located in the footwall and hanging wall, respectively, of the Upper Bouquet Valley normal fault, south of Elizabethtown, NY. The Rand Hill dike swarm also intrudes anorthositic country rock and is located northwest of Plattsburg, NY. Published age data estimate Rand Hill dikes as late Proterozoic, which correlates with the time of the opening of the Iapetus Ocean. The Coughlin Brook and Split Rocks Falls dikes are undated.

Comparison of these samples suggests that the tectonic event that caused intrusion of dikes was crustal extension over a rising asthenospheric plume, preceeding the opening of the Iapetus Ocean. Rocks from all three localities lie on “Within Plate Alkali Basalts” or “Ocean Island Alkaline Basalts” fields on paleotectonic discrimination diagrams. Geochemical and petrographic data shows that rocks from Coughlin Brook and Split Rock Falls are transitional basalts, and rocks from Rand Hill are mildly alkaline basalts. Similarities between the Split Rock Falls and Coughlin Brook dikes suggest that they had the same magma source and thus, were intruded at the same time. Rand Hill dikes plot within the same fields as Coughlin Brook dikes on discrimination diagrams, but consistently demonstrate more alkaline nature on classification diagrams. It is possible that the Coughlin Brook magma was derived by partial melting of the same mantle plume as Rand Hill, but at shallower depths, probably at an earlier stage of the plume ascent. A similar pattern of transitional basalts followed by alkaline basalts was described for the early rifting of the East African Rift system. Rand Hill dikes are characterized by a higher degree of alteration and metamorphism compared to Coughlin Brook dikes, which can also explain the slight difference in chemical composition between the two localities.