Petrogenesis and Provenance of Epiclastic Volcanic Cobbles From the Cretaceous Woodbine Formation, Southwest Arkansas.

Abstract

Whole rock and mineral compositions of epiclastic volcanic cobbles have been used to reconstruct the evolution of the magmatic system that supplied this volcanic material. Using techniques that have been applied to stratified volcanic deposits, the chemical and mineralogical variation of volcanic material preserved as cobbles in sedimentary rocks has been used to identify a cogenetic suite of igneous rocks and to evaluate possible fractionation mechanisms. Volcanic rocks, preserved as rounded cobbles in the upper Cretaceous Woodbine Formation of SW Arkansas, range from mafic, pyroxene-rich phonolites to felsic, crystal-poor phonolites. Felsic lithologies contain abundant sanidine phenocrysts and lesser amounts of calcic pyroxene and hornblende; sphene is an important minor phase. Intermediate lithologies contain subequal proportions of anorthoclase and pyroxene phenocrysts. Mafic lithologies contain only pyroxene phenocrysts. Discrimination diagrams of whole rock major and trace element concentrations reveal smooth compositional gradients. MgO contents vary from 6.0 wt. % in the most mafic rocks to 0.5 wt. % in felsic cobbles. K$\sb2$O and SiO$\sb2$ are enriched in the felsic rocks: K$\sb2$O 3.1 to 8.0 wt. %, and SiO$\sb2$ ranges from 45.2 to 61.5 wt. % from mafic to felsic rocks, respectively. Compatible trace elements (Ni, Cu) are enriched in the mafic lithologies while incompatible elements (Zr, Rb) are enriched in the felsic samples. Electron microprobe analysis of melt inclusions in sanidine show smooth compositional gradients consistent with whole rock data. Modeling of whole rock and mineral compositions indicate that these volcanic lithologies preserved as cobbles are related to each other by fractional crystallization; felsic lithologies represent the more differentiated level of a magma chamber while mafic samples represent relatively primitive magmas. Melt inclusion compositions have been used with whole rock data to trace a liquid line of descent of the evolving magma. Potential igneous source rocks are present throughout the Gulf Coast. Geochemical and geochronologic data from rocks of these various igneous centers have been compared with similar data of the Woodbine volcanics; rock compositions of the Woodbine volcanics compare favorably to the lamprophyric dike swarm associated with Magnet Cove in central Arkansas.