Geochemistry, Thermochronometry, Petrogenesis, and Geometry of Igneous Dikes in Central Arkansas: Implications for Mississippi Embayment Evolution

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The Mississippi Embayment (ME) is a south-plunging sedimentary basin which unconformably overlies the Cambrian Mississippi Valley Graben (MVG) and serves as a natural laboratory for extensional tectonics. The Arkansas Alkaline Province (AAP) lies along the ME’s western border in central Arkansas and was emplaced in the mid-Cretaceous Period as a suite of intrusive bodies. The relationship between the ME and AAP, and a mechanism which generated both, remain unclear. To investigate this relationship and identify a petrogenetic mechanism, a multidisciplinary approach was used that involves geochemistry (major oxide and trace element, Nd isotopes), thermochronometry (U-Th/He), and geometry of 11 igneous dikes from the AAP. These data reveal that dikes are potassic-ultrapotassic and extremely enriched in incompatible elements relevant to OIBs and MORBs. Combined with data from other studies, the Nd isotopic ratio values demonstrate that lamprophyres were derived from asthenospheric magma unlike other intrusions. Despite depleted Nd isotope ratios, lamprophyres and every other sample reveal enrichment in incompatible elements and high Sr and Nb contents (indicative of carbonatite metasomatism). This decoupling between isotope ratios and trace element contents could represent low degrees of partial melting of a depleted asthenospheric source. The remaining samples show similar trace element slopes on normalized diagrams and were likely derived from a similar source with varying degrees of crustal contamination. Despite geochemical similarities between samples and average OIB values, I propose a non-plume evolution for the AAP and ME related to edge-driven convection (EDC) associated with regional tectonics. At the time of the AAP’s emplacement (106-89 Ma), EDC was at its highest velocity (80-100 Ma after the Atlantic Ocean and GOM started rifting). At this time, the proto-ME region was uplifted, and melt was guided along pre-existing weaknesses. This is corroborated by rose diagrams, which indicate strong similarities between dike trends and thrust plane trends. Results and interpretations from this study clarify a potential mechanism for alkaline magmatism during a pulse of volcanism in the Cretaceous. Moreover, this study places the ME and AAP within the framework of global tectonics in the Cretaceous Period and emphasizes the influence of regional processes on nearby lithospheric weaknesses.

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