Foundering-driven lithospheric melting: The source of central Andean mafic lavas on the Puna Plateau (22°S–27°S)

Abstract

Investigations of lithospheric foundering and related magmatism have long focused on the central Andes, where there are postulated links between the eruption of mantle-derived lavas and periodic loss of the lower lithosphere. Whole-rock elemental and Nd-Sr-Pb isotopic results from a suite of late Miocene-Quaternary mafi c lavas erupted onto the Puna Plateau clarify the relationship between this hypothesized process and lava composition. Zinc and Fe provide a critical perspective because they are partitioned differently during the melting of asthenospheric and lithospheric mantle. All Puna lavas have Zn/Fe T (×10 4) values >13, which requires clinopyroxene and perhaps garnet to be the dominant phase(s) in the melt source; this precludes a melt source of typical mantle asthenosphere. This result is contrary to classic models of delamination magmatism that suggest asthenospheric peridotite melts to generate these lavas. Pyroxenite (±garnet)-bearing lithospheric materials in the central Andes are likely common and heterogeneous in age, volatile content, and mineralogical composition, and if they are the melt source, this can explain the diversity in the elemental (La/Yb = 11-45; La/Ta = 22-40) and isotopic (87 Sr/ 86 Sr = 0.7055-0.7080; ε Nd =-1 to −7) compositions of these mafi c magmas (MgO > 8%, Mg number > 60). We propose that compositionally diverse, gravitationally unstable pyroxenites both drive "dripping" of the lower lithosphere and are the source of the resulting melt. We also postulate that mantle-derived lavas erupted on the Puna Plateau were generated during localized foundering and melting of these materials. The cumulative effect of these drip events is a modern Puna Plateau with geodynamic anomalies including thin lithosphere and anomalously high surface elevation.

Magmatic expressions of continental lithosphere removal

“…[] and Murray et al . [] use the Zn/Fe ratio to understand the origin of the Neogene magmas in this region. They find that the earliest magmas (1.5–1 Ma) are mainly sourced from lithospheric pyroxenites, and later magmas show a progressive increase in the asthenospheric peridotite component.…”

“…A xenolith study shows that a garnet pyroxenite root, which was ~40 km thick and 200 kg/m 3 denser than asthenospheric peridotite, existed beneath the central Sierra Nevada before the Pliocene removal event [ Ducea and Saleeby , ]. A link between pyroxenite and lithosphere removal is further supported by magmatic evidence, which shows a pyroxenite magma source in several regions with inferred removal events, including the Puna plateau [ Ducea et al ., ; Murray et al ., ] and the North China Craton [ Gao et al ., ]. The solidus of pyroxenite is ~150°C lower than that of peridotite [ Hirschmann and Stolper , ; Pertermann and Hirschmann , ; Lambart et al ., ].…”

Shallow melting of MORB‐like mantle under hot continental lithosphere, Central Anatolia

“…Such convection could permit intermittent mantle upwelling to shallow depths and draining of decompression melts, and episodic volcanism, potentially reconciling differences between seismic and geochemical estimates of shallow melt fractions. The >1 Ma interval sampled by the Hasan Monogenetic Cluster basalts is sufficient for heating and dehydration melting of hydrated blocks of foundered lithosphere [ Elkins‐Tanton , ], as has been shown in the Altiplano Puna [ Ducea et al ., ; Murray et al ., ]. Consequently, melts could hybridize components from foundering lithosphere—modified during late Cretaceous to Eocene subduction—and shallowly convecting asthenosphere, a scenario that is permitted by the largely two‐component trace element and isotope characteristics of the Hasan Monogenetic Cluster basalts.…”

Lithospheric Architecture and Mantle Metasomatism Linked to Iron Oxide Cu‐Au Ore Formation: Multidisciplinary Evidence from the Olympic Dam Region, South Australia

“…An additional possible source of mantle melts are the descending drips of foundered lithospheric mantle, which theoretically could result in hydrous alkali-rich melts from the volatile-rich drip itself or from wet melting of adjacent asthenosphere (Ducea et al, 2013;Elkins-Tanton & Foulger, 2005;Murray et al, 2015). The extent to which such processes may have contributed to magmatism in the Gawler Craton and Curnamona Province is unknown but could be tested with further petrogenetic modeling and radiogenic (Nd, Sr, and Os) isotope analysis of mafic igneous rocks.…”