INVESTIGATING THE ORIGIN OF LAMPROPHYRES IN THE CORTLANDT-BEEMERVILLE COMPLEX

The igneous formations of the Ordovician Cortlandt-Beemerville Trend encompass roughly 100 km from the Cortlandt complex of southern New York to the New Jersey Beemerville complex. Throughout the igneous trend are a multitude of dikes that have been suggested to be directly associated with either/both of the Beemerville, Rosetown (small intrusion in-between) or the Cortlandt complexes. The geological maps of the area have identified some dikes as specifically lamprophyres, some as mafic dikes, and others as general igneous dikes. One of the main goals of this project is to more properly identify the character and types of these dikes. Another goal of this project also is to examine some dikes that are labeled as older diabase dikes that have orientations similar to those of the lamprophyre dikes that are more properly mapped within the Beemerville complex. Importantly, the origin and association of these lamprophyres and igneous dikes is poorly understood as only those in the immediate vicinity of the Beemerville complex have been examined in detail petrographically and geochemically (Eby, 2004). Lamprophyres are porphyritic textured igneous rocks with predominantly mafic compositions that have an excess amount of alkali elements. 32 dikes were sampled and examined in 2019-2021 throughout New York and New Jersey. These dikes were sampled and examined in the field for texture and mineralogical similarities to those of lamprophyres. These samples were also examined for major bulk rock geochemistry and petrographic analysis. Field observations indicated that some dikes have been improperly mapped as they either could not be found in the field or were misidentified in the field as other rock types. More critically, the results of bulk rock geochemistry and petrographic analysis clearly indicate that many of these dikes are not lamprophyres but a variety of other igneous rock types including diabase, andesite, and rhyodacite. The geochemistry also suggests that many of these rocks have no association with either the Beemerville, Rosetown or Cortlandt complexes.