Basic magmatism in northeastern Puna, Argentina: Chemical composition and tectonic setting in the Ordovician back-arc

Abstract

Two main groups of basic rocks, representative of the bimodal Ordovician magmatism of the northeastern Puna (22-24°S, 66°30'-65°40'W) were revisited on the basis of new geological, petrographic and geochemical data. One of them shows geochemical characteristics intermediate between those of MORB and arc-type basalts common in back-arc basins. The other group is represented by low and high Ti basalts and gabbros with an OIB-type signature. Basalts of the first group (Tremadocian-early Arenigian) along with minor dacites are found within a sedimentary-volcanic sequence that hosts the Cobres and the Tanque granitic plutons (475-480 Ma). This group of basic rocks is also coeval with Cobres Granite, whereas magmas with OIB-type signature were contemporary with the emplacement of Tanque Granite. During the early to middle Arenigian, dacites and minor amounts of basalts with OIB-type signatures erupted in the Escaya-Huancar ranges and were intruded as dikes in Cobres and Tanque ranges. Two main stages are recorded in the tectono-magmatic evolution of the region. The first stage was dominated by extension (Tremadocian-early Arenigian), and the second (until the late Arenigian) during which extensional and contractional conditions occurred diachronously along the basin. The geochemical characteristics of the basic magmas are attributed to a combination of factors such as variably depleted mantle sources, continental crustal contamination and addition of subduction components, as well as variations in the degree of melting in the sources. These factors probably were controlled by episodical changes both in time and space in the extensional conditions. These characteristics altogether indicate a back-arc setting for the northeastern Puna during Early and Middle Ordovician, in which contractional episodes took place.