Zircon U-Pb ages in the Sey Plutonic Volcanic Complex: Implications for the magmatic activity in Eastern Magmatic Belt, Puna, Argentina.

ORTIZ, AGUSTÍN; HAUSER, NATALIA; BECCHIO, RAÚL

Cochabamba

Simposio; 11th South American Symposium on Isotope Geology. Cochabamba, Bolivia.; 2018

The Sey Plutonic Volcanic Complex (CPVS by its Spanish name) is located in the Northeastern Magmatic Belt, Puna Argentina. The plutonic, subvolcanic and volcanic rocks of this complex vary from felsic to mafic composition, with magma mixing evidence in the plutonic rocks. We present new U-Pb zircon ages of the CPVS. We have selected two plutonic rocks (monzogranite and granodiorite) and a volcanic rock (rhyodacite). The objective was to obtain the crystallization ages for the different levels of emplacement in the complex in order to understand the evolution of the Eastern Magmatic Belt in the Lower Paleozoic Arc. Zircon U-Pb analyses were carried out by LA-MC-ICP-MS at the Geochronology Laboratory, University of Brasilia. Zircon crystals are similar in all samples; these are brown-pinkish, between 100-300 µm, with various crystal shapes, such as elongate euhedral and subhedral bipyramids, while others are subrounded. Th/U values are typical from a magmatic origin, varying from 0.04-0.30. The analyses gave for the monzogranite a 206Pb/238U weighted average age of 475±4 Ma, the granodiorite yield a 206Pb/238U weighted average age of 482±7 Ma and the rhyodacite gave a 206Pb/238U weighted average age of 437±7 Ma. The analyses for the zircons age populations of all samples reveals various age groups. A young group of ~445 Ma, a most frequent group of ~470-480 Ma, a significant group of ~490-500 Ma, an older population of ~515-520 Ma. Finally, older populations and single crystals of ~560 Ma, ~600 Ma, ~850 Ma, ~930 Ma, and ~1500 Ma. The obtained ages of the monzogranite and granodiorite overlap within error and are considered the crystallization age of the plutonic rocks of the CPVS. The age of the rhyodacite is considered as the crystallization age for the volcanic rocks of the CPVS. The age population of ~445 Ma -observed only in the rhyodacite- is interpreted as the volcanic activity of the complex, whereas the most frequent age populations of ~470-480 Ma and ~490-500 Ma can be interpreted as the apex of the magmatic activity in the complex. The smaller age group of ~515-520 Ma probably signify zircon antecryst, representing the early start of the magmatic activity in the CPVS. Finally, the older zircon populations could represent inherited zircon crystals. This interpretation allows us to describe that a long-lived magmatic event took place from ~520 Ma through ~445 Ma in the CPVS. Moreover, this corresponds with the long-lived magmatic activity in the Diablillos Intrusive Complex and Pachamama Igneous Metamorphic Complex (~540-490 Ma), Southeastern Magmatic Belt, Puna (Ortiz et al., 2017a; 2017b). Emphasizing the premises proposed by Ortiz et al. (2017a), that the Eastern Magmatic Belt in Puna was formed by a long-lived magmatic activity from ~540 until ~440 Ma.