Structural State of K-Feldspar in the Felsic Volcanic Rocks and Ring Pluton Granites of the Butler Hill Caldera, St. Francois Mountains, Southeastern Missouri

Abstract

The K-feldspar in all major rock units exposed in the Middle Proterozoic Butler Hill Caldera, St. Francois Mountains, southeastern Missouri, shows a high, but not maximum, degree of structural order. The structural state parameter Z for phenocryst material from the felsic volcanic rock units averages 0.89, with a standard deviation of 0.06 (Z would be 1 in fully ordered microcline). K-feldspar extracted from the matrix of these same samples yields an average Z value of 0.86, with a standard deviation of 0.07. The Z parameter for the K-feldspar in the ring pluton granites averages 0.87, with a standard deviation of 0.06. The structural state of the K-feldspar in these ring pluton granites is virtually identical to that reported by our group for the Butler Hill -Breadtray Granite, the subvolcanic granite massif exposed in the center of the Butler Hill Caldera. There is no statistically significant variation of K-feldspar structural state with inferred structural elevation, present topographic elevation, or proximity to the Simms Mountain Fault. We interpret the homogeneity of the K-feldspar structural state throughout the Butler Hill Caldera complex, and especially the unexpectedly high degree of structural order in the volcanic K-feldspar, as evidence for a pervasive alteration event, which re-ordered the K-feldspar throughout the entire volcanic-plutonic complex. The most likely cause of the homogenization of the K-feldspar structural state is the late-stage alkaline magmatism inferred to have occurred throughout the St. Francois Mountains at approximately 1.36 Ga.Keywords: K-feldspar, structural state, granite, rhyolite, Butler Hill Caldera, St. Francois Mountains, Missouri, Proterozoic, alteration. SOMMAIRELe feldspath potassique présent dans chaque unité lithologique majeure de la caldeira de Butler Hill, dans les montagnes St. François, dans le sud-est du Missouri, d'âge protérozoïque moyen, fait preuve d'un degré d'ordre élevé, mais non maximum. Le paramètre structural Z des phénocristaux des roches felsiques volcaniques, par exemple, est en moyenne 0.89, avec un écart-type de 0.06 (dans le microcline parfaitement ordonné, Z est égal à 1). Le feldspath potassique dans la matrice de ces mêmes roches possède une valeur de Z de 0.86, avec un écart-type de 0.07. Dans le cas des granites annulaires associés, le paramètre Z du feldspath potassique est en moyenne 0.87, avec un écart-type de 0.06. Le degré d'ordre Al-Si du feldspath potassique dans ces granites annulaires est donc quasiment le même que dans le granite pour le pluton de Butler Hill -Breadtray, le granite subvolcanique affleurant dans le centre de la caldeira de Butler Hill. Du point de vue statistique, il n'y a aucune variation dans le degré d'ordre avec l'altitude structurale dans l'édifice volcanique, l'élévation topographique actuelle, ou la proximité à la faille de Simms Mountain. L'homogénéité dans le degré d'ordre du feldspath potassique de part et d'autre dans le complexe de Butler Hill, et particulièrement le degré d'ordre anormaleme...

A Paleomagnetic and Petrographic Analysis of Unconformity Surfaces in Nevada and Missouri: Possible Implications for Paleoweathering Processes

“…Heavy hematite pigment stain is observed in some thin sections of LMSthis is restricted to LMS samples taken from above the BHG, which suggests that the iron source for hematite precipitation could be the granite itself. Extensive Kmetasomatism has been reported in the BHG by several authors (Braxland, 1974;Sutton and Maynard, 1996;Plymate et al, 2001) and likely required fluid flow to facilitate the upward movement of ions, including iron. That upward movement could have mobilized iron within the granite and transported it to the more permeable, oxygenated Lamotte and triggered precipitation of hematite.…”

“…Alteration of the basement granites has been reported (Braxland, 1974;Sutton and Maynard, 1996;Plymate et al, 2001;Driese et al, 2007) forming (Appold and Garven, 1999), fluids during the Ouachita-Marathon orogeny.…”