Geochemical and isotopic constraints on the petrogenesis of the Puesto La Peña undersaturated potassic complex, Mendoza province, Argentina: Geodynamic implications

Abstract

Major and trace element and radiogenic and stable isotope data are reported for the Miocene Puesto La Peña undersaturated potassic complex, located in the Argentine Precordillera. The complex is composed of a pyroxenite core surrounded by a malignite–borolanite association, followed by radial and ring dikes composed of alkaline lamprophyres, tephrite to phonolite and intermediate varieties, porphyritic microledmorite, benmoreite, alkaline trachyte and a swarm of ultrabasic ouachitite dikes. Late trachytic to phonolitic volcanic necks and associated dikes crop out surrounding the malignitic–borolanitic massif.Major and trace element data are consistent with their derivation by partial melting from an enriched, probably lithospheric, metasomatized heterogeneous mantle involving spinel- and garnet-facies mantle sources. The trace element and isotope data indicate that all rock types are derived from a common parental magma and are thus cogenetic.Sr, Nd and Pb variations in the different studied lithologies are restricted, except for the evidence of Sr decoupling in the ouachitite samples. Their Sr–Nd isotope compositions follow the “mantle array” defined by oceanic basalts, within the OIB field, consistent with an intraplate depleted source. The genesis of the ouachitite is interpreted to be related to mixing between asthenospheric magma and melts from the lower lithosphere consisting of K-rich metasomatic layers. The Dupal like Pb isotopes signature suggests a mantle modification by introduction of continental crust material in the upper mantle; this is consistent with the Sr–Nd isotope data from the late silica-undersaturated felsic dikes and volcanic necks.Multistage mantle extraction would have occurred by the end of Neoproterozoic to lower Paleozoic times as indicated by the isotopic data. Partial melting was initiated by mantle upwelling decompression during lithosphere extension. Back-arc extensional conditions during the latest Early Miocene (19Ma), related to the geometry of the Pacific subducted plate beneath the South American plate, favored the tectonic conditions for magma ascent from a magmatic reservoir located at around 30km depth in the crust. A fractional crystallization process, primarily of clinopynoxene, plagioclase, alkali feldspar, nepheline and lesser biotite, magnetite, and apatite, first yielded the cumulate clinopyroxenite, followed by the malignite–borolanite association and the late dikes and volcanic necks. The morphology of the complex, as well as the relationship between their different facies, points to a cauldron subsidence model of emplacement.