BH250-95
Title
Subject
Description
Petrography of Rhyolite and Ignimbrite – Ozark Plateau (St. Francois Mountains, Missouri)
The rhyolite and ignimbrite units of the Ozark Plateau, particularly within the St. Francois Mountains of Missouri, record a complex volcanic history tied to Mesoproterozoic felsic magmatism.
Major Minerals: Quartz, Sanidine, Orthoclase, Perthite, Biotite
Minor Minerals:, Devitrified volcanic glass, oxides
Mineralogical Comments: Many quartz phenocrysts exhibit overgrowth textures, indicating secondary crystallization or recrystallization. Feldspars commonly display perthitic intergrowths, while biotite, though minor, is consistently present and readily identifiable.
Texture and Thin Section Characteristics: In thin section, the rock shows a porphyritic texture, characterized by large phenocrysts of quartz and feldspars set within a cryptocrystalline groundmass. Originally composed of volcanic glass, the groundmass has undergone significant devitrification, now appearing as a fine-grained mosaic of recrystallized quartz and feldspar.
Phenocrysts: Quartz phenocrysts are typically rounded, embayed, or resorbed, and often display undulose extinction—indicative of deformation during metamorphism. Sanidine and orthoclase phenocrysts are commonly perthitic and frequently exhibit Carlsbad twinning or signs of alteration.
Groundmass: Initially glassy, the groundmass is now fully recrystallized into quartz and feldspar.
Macroscopic Texture and Rock Appearance: In hand sample, the rock bears a strong resemblance to an intrusive igneous rock due to its coarse phenocrysts, recrystallized mineralogy, and massive structure. However, thin section analysis reveals its true volcanic origin, differentiating between phenocryst phases and the recrystallized groundmass. This contrast is especially evident in lab samples and in a large display specimen in the rock garden, where the deceptive intrusive-like appearance can be appreciated alongside its diagnostic microscopic volcanic features.