Origin, sedimentary geochemistry, and correlation of Middle and Late Ordovician K-bentonites: constraints from melt inclusions and zircon morphology

The present study of Paleozoic K-bentonites demonstrates that the geochemistry of melt inclusions and the morphology of zircons can be studied by inexpensive and simple-to-use methods, which rely on phenocrysts. Constraints are obtained that lead to (a) the origin of these altered volcanic ashes, (b) the geochemistry of ash-to-K-bentonite-alteration, and (c) the reliable correlation of extensively altered volcanic ashes (i.e. K-bentonites). Silicic melt inclusions (i.e. non-devitrified) have been found in quartz and zircon phenocrysts contained within Ordovician and Devonian K-bentonites from New York State, the Upper Mississippi Valley, and Pennsylvania. Origin, source, and tectonic setting of the volcanism that produced these Paleozoic volcanic ashes (i.e. K-bentonites) are constrained by the geochemistry of these inclusions. The major element compositions of the inclusions, which are small samples of the non-degassed pre-eruptive melt trapped during growth of the phenocrysts, indicate that the K-bentonites were generated by explosive eruptions of rhyolitic, high-K type magmas in a continental volcanic arc. The geochemistry of melt inclusions may furthermore be used for correlation of these volcanic ashes since stratigraphically distinct K-bentonites contain inclusions with different major element composition. Diagenetic alteration of the rhyolitic ashes to K-bentonites has strongly affected their mineralogy and bulk geochemistry. The major element composition of altered K-bentonites, which apparently depends on the composition of the dominating clay minerals and other authigenic phases (e.g., pyrite, calcite), has been compared with unaltered melt inclusions and shows the direction and magnitude of the geochemical changes that occur during diagenesis. Relative to aluminum, substantial amounts of Si, Na, K and Mn have been lost, whereas Ti, Fe and Mg have been gained in the K-bentonites. The surrounding sediments, which are enriched in SiO2 compared with sediments further away, apparently acted as a sink for the silica released from the volcanic ash. The observed enrichment of TiO2 in the K-bentonites relative to aluminum seems best explained as a result of contamination by pelagic, TiO2-rich clay particles that have settled into the voids within the unaltered volcanic ashes. The morphology of zircon populations from several K-bentonites has been studied using the classification scheme of Pupin and Turco(1972b). Applied as a petrogenetic indicator, the morphologies suggest crystallization of the zircons in I-type magmas at temperatures common for silicic volcanic rocks (i.e. \u3e750ºC). It can be demonstrated that stratigraphically different K-bentonites contain zircon populations that are morphologically distinct and can be used for correlation. At least two different K-bentonites seem to be correlated between New York State and the upper Mississippi Valley based on the morphology of zircons. The trace element abundances of Hf, Ti, P, Y, Yb, Ce, U and Th in individual zircons from several K-bentonites have been analyzed by electron microprobe. Single grains have been selected from layers which can be correlated by stratigraphy and by zircon morphology. It was found, however, that the geochemistry of zircons from stratigraphically different layers is indistinguishable and can not be used for correlation of the Paleozoic K-bentonites