The Signature of Metasomatized Subcontinental Lithospheric Mantle in the Basaltic Magmatism of the Payenia Volcanic Province, Argentina

Abstract

The Payenia region of Argentina (34.5–38°S) is a large Pliocene-Quaternary volcanic provinceof basaltic compositions in the Andean Cordillera foothills representing the northernmost extent of back-arcvolcanism in the Andean Southern Volcanic Zone (SVZ). Although the chemical diversity of the Payeniabasalts has been characterized previously, the processes and sources responsible for such variation remaincontroversial. Here, we report new whole-rock major and trace element concentrations, Sr-, Nd-, Hf-, and Pb-isotope ratios and high-precision olivine oxygen-isotope ratios in a suite of 35 alkaline basalts from Payenia.These lavas have major and trace elements that define a compositional range from arc-influenced to intraplatesignature. Variable crustal contamination and/or recent slab-derived inputs inadequately account for elementaland isotopic systematics and spatial compositional variations of Payenia lavas. We present a simple forwardmodel indicating that early metasomatism and subsequent melting of the metasomatized subcontinentallithospheric mantle (SCLM) has significantly contributed to the Payenia lava compositional range. Isotopicingrowth calculations of radiogenic Sr, Nd, Hf, and Pb suggest that the SCLM metasomatism occurred at50–150 Ma, consistent with the timing of the breakup of Gondwana and the development of the proto-PacificAndean arc. Variations in δ 18Oolivine values from modeled melts indicate that the metasomatism and meltingwithin the SCLM can fractionate oxygen isotopes even when the metasomatizing melt has MORB-like δ 18 Ovalues, providing a different explanation for the low-δ 18O signatures observed in continental arc settings.