Normal Faulting and Magmatism In the Northeastern Adirondack Highlands, NY: Implication of Geochemical and Structural Data of Upper Bouquet Valley Mafic Dike Swarm

A swarm of mafic dikes cut Grenville metamorphic rocks on both the footwall and hanging wall of the ~N45E striking Upper Bouquet Valley normal fault, south of Elizabethtown, NY. More than a dozen dikes ranging in thickness from 30 cm to 4 m have been mapped in the footwall along lower Coughlin, Slide, and Stevens Brooks. They are characterized by sharp intrusive contacts, apophyses, and narrow screens of host meta-anorthosites. These sub-vertical dikes strike ~N45E, parallel to the fault, and have been traced along the strike for at least 3.5 km. Dikes range in composition from alkaline, olivine and nepheline normative to transitional, quartz normative basalts. They plot in the Within-Plate and Ocean Island Alkali basalts fields on paleo-plate tectonic discrimination diagrams, which together with the trace element spider diagrams indicate enriched asthenospheric mantle as a magma source. Dikes cutting the garnet-bearing anorthositic gneiss of the hanging wall are exposed in a roadcut at Split Rock Falls. In contrast to footwall dikes, they exhibit wavy contacts with an anastomosing internal shear planes, but are geochemically identical to the dikes from the footwall. Occurrence of dikes on both sides of the fault and their orientation parallel to the fault suggests that intrusion was associated with normal faulting. Chemical similarity of the Upper Bouquet Valley dikes to the late Proterozoic Rand Hill mafic dikes, approximately 75 km to the north, further suggests that normal faulting and magmatism resulted from late Proterozoic extension of Grenville crust prior to rifting of Rodinia.