MIDDLE ORDOVICIAN K-BENTONITES IN THE ARGENTINE PRECORDILLERA: A REVIEW

Middle Ordovician K-bentonites occur throughout the Northern and Central Argentine Precordillera. These beds are widely distributed units that provide stratigraphic, petrologic, and paleogeographic information about the Argentine Precordillera and surrounding regions during Middle Ordovician time. Mineralogy of these K-bentonite beds is diverse with volcanic phenocryst mineralogy being composed commonly of quartz, sanidine, plagioclase, biotite, pyroxenes, amphiboles, and garnets. Clay mineralogy of these beds are dominated by R1 illite-smectite and kaolinite, indicating a low thermal history for these units.

Immobile element chemistry of these beds show that the original volcanic ash was dominantly highly evolved explosive silicic rocks such as rhyolites, dacites, and trachyites. REE data supports this conclusion and show that most beds are enriched in LREE, depleted in HREE, and have negative Eu anomalies. Tectonomagmatic discrimination diagrams indicate that these K-bentonites arose from a collision related, destructive plate margin setting. Grain size data indicates that the paleogeographic source region for these beds is estimated to be between 200 and 700 km from the north central Precordillera. Stratigraphic patterns of K-bentonite distribution indicate that the source region was north relative to the present configuration of the Precordillera.

K-bentonites found in the Argentine Precordillera are generally older than those found in Laurentia and stratigraphic tie lines between Laurentia and the Precordillera are not yet recognized. However, detailed correlation of K-bentonites within the Precordillera is possible owing to the high preservation of a variety of volcanic phenocrysts in these beds and relatively low thermal maturity. The K-bentonites in the Argentine Precordillera continue to offer many geologic constraints on the paleogeography of the region.