The petrogenesis of back-arc magmas, constrained by zircon O and Hf isotopes, in the Frontal Cordillera and Precordillera, Argentina

Abstract

The Andean margin of the South American continent has been magmatically and tectonically active for over 330 million years. It is the type location where “Cordilleran-type” magmatism and orogenesis are manifest. In Argentina and Chile, between the latitudes of 28° and 40° S, magmatism related to the Gondwanan “Cordilleran-type” orogeny is reflected in a series of Carboniferous to Triassic intrusions. A comprehensive model exists for the petrogenesis of such magmas in Chile, however there is relatively little understanding of the nature and timing of Permo-Triassic magmatism in the Frontal Cordillera and Precordillera in Argentina. To address this, we present a new dataset of in situ zircon U–Pb, O and Hf isotopes from 15 felsic intrusions from Argentina. Zircon geochronology shows that magmatism in this region commenced at ca. 285 Ma and continued until ca. 250 Ma. Zircon O and Hf isotopes suggest that the oldest Permian magmas were derived from young supracrustal sources, with elevated δ18O (~ 8.5 to 7.5‰) and negative eHf values (~ − 1 to − 3 eHf). The emplacement of these magmas was coeval with the formation of mantle-derived magmas characterised by mantle-like δ18O (~ 6.0 to 5.5‰) and moderately positive eHf values (~ 4 to 1 eHf). As magmatism continued, transitional isotope signatures became predominant as melts of these disparate sources interacted and hybridised. It is proposed that under a compressional regime, mantle-derived magmas were halted in the lower continental crust, where they exchanged heat and volatiles with an older fertile lithosphere to generate melts from supracrustal sources. A shift in the stress regime at ca. 285 Ma permitted both crustally derived and juvenile mantle-derived magmas to exploit newly formed conduits to rise into the upper crust. A regional compilation of zircon O and Hf isotopes from felsic igneous rocks reveals a coherent secular trend over ~ 100 million years, where the oldest magmatism exhibits a dominant supracrustal component and younger magmas progressively (over 50 Ma) transition towards juvenile mantle-like isotopic compositions. This new dataset from Argentina fills a significant gap in the previous regional models between 285 and 250 Ma and documents the isotopic response of magmas produced in back-arc regions to a transition between compression and extensional/neutral stress regimes. These results give insight into the generation of new, or recycling of, continental crust in a back-arc setting and how the transition from compression to extension is imperative for ore-forming magmas to reach the upper crust.