Basic magmatism in northeastern Puna, Argentina: Chemical composition and

COIRA, B., KIRSCHBAUM, A., HONGN, F., PÉREZ, B. Y MENEGATTI, N.,

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdan; Año: 2009 vol. 28 p. 374 - 382

0895-9811

Two main groups of basic rocks, representative of the bimodal Ordovician magmatism of the northeasternPuna (22–24S, 6630’–6540’W) were revisited on the basis of new geological, petrographic and geochemicaldata. One of them shows geochemical characteristics intermediate between those of MORB andarc-type basalts common in back-arc basins. The other group is represented by low and high Ti basaltsand gabbros with an OIB-type signature. Basalts of the first group (Tremadocian–early Arenigian) alongwith minor dacites are found within a sedimentary–volcanic sequence that hosts the Cobres and the Tanquegranitic plutons (475–480 Ma). This group of basic rocks is also coeval with Cobres Granite, whereasmagmas with OIB-type signature were contemporary with the emplacement of Tanque Granite. Duringthe early to middle Arenigian, dacites and minor amounts of basalts with OIB-type signatures eruptedin 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 wasdominated by extension (Tremadocian–early Arenigian), and the second (until the late Arenigian) duringwhich extensional and contractional conditions occurred diachronously along the basin. The geochemicalcharacteristics of the basic magmas are attributed to a combination of factors such as variably depletedmantle sources, continental crustal contamination and addition of subduction components, as well asvariations in the degree of melting in the sources.These factors probably were controlled by episodical changes both in time and space in the extensionalconditions. These characteristics altogether indicate a back-arc setting for the northeastern Puna duringEarly and Middle Ordovician, in which contractional episodes took place.