Ordovician Volcanism in the Puna (Argentina)

Abstract

In the Central Andes is located one of the major Ordovician basins with fully developed stratigraphic records from Lower Tremadoc to Hirmantian (485-442Ma). As part of this main basin, in the NW of Argentina, the high plateau of the Puna (22º-27ªS) records vast and well exposed Ordovician sequences that include representatives of the volcanic activity occurred in submarine to subareal conditions. Those Ordovician volcanic rocks are arranged along two submeridional belts known as Western Magmatic Belt and Eastern Magmatic Belt. The first one extends towards the west in Chile, includes volcano-sedimentary sequences with trilobite faunas of Lower Tremadoc age, as well as others with graptofaunas Arenig s. l. to Early Llanvirn. In the Lower Tremadoc sequences the volcanism is represented by dacitic to rhyolitic lavas, hyaloclastites, tuffs and pyroclastic flows and basaltic lavas associated with sandstones volcaniclastic rocks, siltstones, coquinas and cherts. The Arenigian sequences are characterized by dacitic to rhyolitic pyroclastic flows, lapillites and tuffs intercalated in volcaniclastic turbiditic sequences. They are generally preceded by basaltic to andesitic massive lavas, pillow lavas and pillow fragmented breccias and hyaloclastite breccias. The Eastern Magmatic Belt registers begins at the end of the Cambrian and beginning of the Tremadoc with bimodal character, represented by dacites and basalts to microgabros syn-sedimentary with the clastic host sequences. Dacitic sills present to their roofs levels of peperites and autobreccias. Towards the southern sections outcrop meta-sediments associated with meta-rhyolites, dacites and basalts. Other records of this stage are the basalts and microgabros that integrate the sedimentarymagmatic sequence that hosted the granite plutons of Cobres and Tanque (475-480 Ma.). During the Tremadoc to Early-Middle Arenig the Eastern Magmatic Belt volcanism reached its highest representation with siliceous syn-sedimentary lava-domes and sills associated with autoclastic breccias, and hyaloclastites and minor spilitized basaltic pillow lavas as well as microgabros forming lacoliths and sills. Granitoids with U/Pb 476 Ma-444 Ma ages are also part of the Eastern Magmatic Belt. The geochemical characteristics of the volcanism in both belts indicate during Tremadoc early stages of back-arc basins. During Early-Middle Arenig the volcanism in the Western Magmatic Belt shows geochemical characteristics of arc to back-arc developed on a thin continental crust. In the Eastern Magmatic Belt, basic rocks with intraplate signature and siliceous rocks with cortical filiation, indicate a tectonic environment where the alkaline magmatism is the result of decompression of the mantle along pull-apart regions, while it takes place important cortical fusion due to the thermal anomaly related to thinning.