The ages and tectonic setting of the Faja Eruptiva de la Puna Oriental, Ordovician, NW Argentina

Abstract

The Ordovician Faja Eruptiva de la Puna Oriental is a magmatic, predominantly intrusive belt in the Puna of northwestern Argentina with a N-S extension of ca. 400km. Scarce isotope geochemical ages and biostratigraphic data on some of the folded Faja Eruptiva country rocks assign the magmatism either to the Lower and lower Middle Ordovician, or to the latest Ordovician. Interpretations of origin and tectonic framework of the Faja Eruptiva are controversial and vary between arc, back-arc and collisional-orogenic settings.We present high-resolution La-ICP-MS U–Pb age and Hf isotope data on zircons from 10 plutonic samples covering the magmatic belt along a length of 200km in the northern Argentinian Puna. The xenocrystic and magmatic zircon age data have a wide spread between 2700Ma and 440Ma. Concordia and weighted mean age data document protracted magmatism in two phases between 480 and 460Ma, and between 453 and 444Ma, and constrain the time of the last intrusions at 444±3Ma and at 445±2Ma thus defining this last and main phase of intrusion at 444Ma.εHf(t) values define a main vertical trend centered at 500Ma with εHf(t) values between +3 and −16 indicating significant mixing of juvenile early Paleozoic melts with Paleoproterozoic crustal components. A second trend is formed by zircons with ages between 1.1Ga and c. 500Ma and predominantly positive εHf values between +8 and −3 and originates in juvenile mantle compositions between 1.6 and 1.1Ga. The spread of the zircon and Hf data document that the Faja Eruptiva intrusives have experienced large-scale contamination by the hosting crustal basement. It follows that the basement of the Puna is formed either by the upper Proterozoic-lower Cambrian Puncoviscana Formation as an erosional product of the Proterozoic orogenic belts of SW Amazonia or that the Puna including its Puncoviscana basement is underlain by a crust shaped by these orogenies.The main intrusive event at 444Ma has been linked to the Oclóyic orogeny in the Late Ordovician. The plutons intruded very likely in a sinistral strike-slip regime after the main folding phase of the Oclóyic orogeny had deformed the Ordovician sedimentary country rocks.