Integrated U-Pb and Hf zircon and whole-rock Nd isotopes studies of Devonian granitic rocks from Sierra de San Luis (Sierras Pampeanas, Argentina): Petrogenetic implications

Previous geochronological data indicate a protracted Devonian magmatic activity developed in the Sierra de San Luis, Sierras Pampeanas of Argentina, with three major crystallization events: 393 & PLUSMN;3, 384 & PLUSMN;2, and 377 & PLUSMN;2 Ma. Previously reported whole-rock Sm-Nd isotopes data define two average distinctive eNdt values:-1.37 and-3.47, and they are consistent with new data presented here. The first signature is assumed for a parental magma with dominant metasomatized subcontinental lithospheric mantle (SCLM) source, whereas the second signature could represent a parental magma derived of a lower continental crust source hybridized with magmas of the first signature. Notably, the new zircon Hf isotopes performed on the same zircon domains that were previously dated, indicate that the contribution of the source was variable over time. In situ Hf in zircon is relevant to evaluate the compositional evolution of the Devonian granitic magmas in the Sierra de San Luis, since the high variability of the eHft values recorded in zircons indicate that the calculated eNdt values for the samples can only be interpreted as a final picture of the petrogenetic process. Zircon Hf isotopes data suggest that the zircon crystallized from a magma with variable composition, recording two major events, yielding two eHft signatures: (1)-3.54 and (2)-6.85. A third composition, yield a less representative eHft value of-5.44, and represent a eHft signature (3).