THE EVENT STRATIGRAPHIC AND TECTONOMAGMATIC APPLICATION OF LOWER PALEOZOIC K-BENTONITES

Explosively erupted volcanic ash preserved as bentonite and K-bentonite can serve as ideal regional stratigraphic markers as well as recorders of tectonomagmatic events. Paleozoic K-bentonites contain unaltered primary phenocrystic mineral grains whose morphology, composition, and mineral character retain information generated by the magmatic and tectonic origins of the source volcanoes. This information can also serve as a chemical fingerprint for stratigraphic applications. During the past two decades these features have been successfully used for solving regional event-stratigraphic, tectonic, and magmatic problems. Examples from the North American Midcontinent, Baltoscandia, the East European Platform, and the Argentine Precordillera illustrate the utility of chemostratigraphic studies. Beta-form quartz with glass melt inclusions, biotite, feldspar, amphibole, pyroxene, spinel, garnet, zircon, and apatite frequently occur in K-bentonites, maintaining constant proportions within beds but varying between beds. In Arenigian K-bentonites from the Argentine Precordillera, for example, melt inclusions represent glasses of rhyolitic composition. Clinopyroxenes are high Ca and Fe-poor augites. Si content is usually between 40 and 48 % and average Al content is 1-5 weight %, all of which reflects some range of differentiation in the source magmas from which these clinopyroxenes crystallized. Microprobe data on biotites from the Caradocian Deicke and Millbrig beds in North America suggest that the Deicke represents a compositionally homogeneous magma. However, the Millbrig shows considerable within-bed variability and has biotites which range from only slightly different from Deicke to those characteristic of a more highly evolved parental magma. The magnesium number (Mg2/(Mg2+Fe2)) of the biotites, which serves as a differentiation index, suggests that the Millbrig variability is the product of a continuously evolving magmatic source. Finally, the Al content of amphiboles in Caradocian K-bentonites in Sweden records magma pressure at the time of crystallization, and thus permits the estimation of depths to the magma chamber.