Ordovician K-bentonites in the Argentine Precordillera: relations to Gondwana margin evolution

Abstract

Ordovician K-bcn Ionites have now been recorded from >20 localities in the vicinity of the Argentine Prccordillera. Most occur in I he eastern thrust belts, in the San Juan Limestone and the overlying the Gualcamayo Formation, but a few ash beds are known also from the central thrust belts. The oldest occur in the middle Arenig t. victorias iunatus grnplolite (Oe. evae conodont) Zone, and the youngest in the middle Llanvirn P. etegans (P. .tuecicas) Zone. Mineralogical characteristics, typical of other Ordovician K-bentonites, include a matrix of illite/smectite mixed-layer clay and a typical felsic volcanic phenocryst assemblage: biotite, beta-form quartz, alkali and plagioclase feldspar, apatite, and zircon, with lesser amounts of hornblende, clinopyroxene, tilanilc and Fe-Ti oxides. The proportions of the mineral phases and variations iii their crystal chemistry are commonly unique to individual (or small groups of) K-benlonite beds. Glass melt inclusions preserved in quartz are rhyolitic in composition The sequence is unique in its abundance of K-benlonite beds, but a close association between the Precordillera and other Ordovician sedimentary basins cannot be established.Theash distribution is most consistent with palacogeographical reconstructions in which early Ordovician drifting of the Precordillera occurred in proximity to one or more volcanic arcs, and with eventual collision along the Andean margin of Gondwana during the mid-Ordovician Ocloyic event of the Famalinian orogeny. The Puna-Famatina terrane northeast of the Precordillera might have served as the source of the K-bcnlonite ashes, possibly in concert with active arc magmatism on the Gondwana plate itself.