Petrogenesis and Isotope Geochemistry of Nepheline Syenites from the Arkansas Alkaline Province, SE USA

Abstract

Silica-undersaturated nepheline syenites are rare, and a great example of outcrops within the Arkansas Alkaline Province (AAP) in central Arkansas, near Little Rock. It has been thought that the AAP formed as part of the Bermuda hotspot. Previous studies have performed geochronological analyses determining an age between 88 – 106 Ma, but the magmatic processes involved and the origin of these rocks remain unclear. This study reports new Nd-Sr-Pb-Hf isotopic compositions plus major and trace element data for nepheline syenites from Arkansas. These nepheline syenites are characterized by depleted Nd and Hf isotopic compositions, radiogenic Pb isotopes (206Pb/204Pb=19.5), and high Na2O (>6%) and K2O (>6%) contents. The high 206Pb/204Pb ratios that were measured for these samples argue against subduction processes and significant upper or lower contributions. Sr and Pb isotope values, as well as depleted Nd-Hf isotopic compositions, indicate that there was a mixing of both enriched mantle (EM2) and high 238U/204Pb (HIMU) mantle components. The trace element data proved to be very useful in determining the tectonic setting of these samples, by first clearly indicating that these samples match the characteristics of a within-plate setting. Positive Nb and Ta anomalies argue against subduction processes or significant involvements of upper crustal materials, while the positive Nb-kick characterize ocean island basalts that form due to mantle plumes. Based on the data, and the ages determined by previous studies this study believes that these nepheline syenites were most likely a part of the Bermuda hot spot in Arkansas.