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 mafic 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 mafic 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.

The Steens Formation, or Steens Basalt, is formally recognized as the oldest lithostratigraphic unit of the Columbia River Basalt Group, with an estimated areal extent and volume of 53,000 km 2 and 31,800 km 3 , respectively. We integrate petrochemical, paleomagnetic, and 40 Ar- 39 Ar age data on 13 collected sections to help evaluate stratigraphic and petrogenetic relationships through the Steens succession. We estimate that the overall duration of Steens Basalt volcanism from lingering eruptions could be as much as 300,000 yr, centered at ca. 16.7 Ma, but that the far greater volume erupted in <50,000 yr at an effusion rate ~0.67 km 3 /yr. Lava flows of primitive, homogeneous tholeiite initially erupted over a wide expanse of eastern Oregon during a reversed polarity interval (R 0 ). Later eruptions became more focused at the presumed shield volcano at Steens Mountain, where dikes exploited a NNE-trending zone of crustal weakness related to the northeast extension of the mid-Cretaceous western Nevada shear zone. The Steens Mountain shield volcano generated increasingly more diverse flows of tholeiite, alkali basalt, and basaltic trachyandesite that erupted during a geomagnetic polarity transition culminating in upper flows of normal polarity (N 0 ). The Steens sequence is dominated by compound flows (~10–50 m thick) produced by the rapid eruption of thin (<2 m) pahoehoe flow lobes. Analysis of these stacked sequences in the Catlow Peak section reveals periodic recharge of the magma chamber and ubiquitous fractional crystallization of plagioclase and olivine in each compound flow, accommodated by plagioclase accumulation and selective crustal contamination. The overall flood basalt stratigraphy records a rapid and progressive change in eruption style, from the early, near-continuous eruptions of small-volume Steens Basalt flows to later, more episodic eruptions of large-volume, tabular flows comprising the Imnaha, Grande Ronde, and Picture Gorge Basalts.

Cerro Chorrillos and San Jerónimo Quaternary volcanoes of northwestern Argentina erupted on a metamorphic Precambrian basement mostly covered by Paleozoic sandstones. Thickness of the continental crust is about 50 km and depth to the central part of the subducted slab is about 220 km. The lava series consists of absarokite, shoshonite, and banakite. These lavas do not contain plagioclase phenocrysts. The absarokites and shoshonites contain phenocrysts of olivine (Fo 88 to 82 percent core) and clinopyroxene (with high Al VI and low Ti+Al IV ). Orthopyroxene and pargasitic hornblende phenocrysts appear in the banakites. Ti-rich phlogopite occurs in some shoshonites and banakites. Xenocrysts of quartz (as much as 9 volume percent) and sieve-textured xenocrysts of feldspar (with Ab- or Or-rich core) occur in all the lavas. Lack of iron enrichment with differentiation is noteworthy. Mg numbers (≃70), Sc, Co, Cr, and Ni contents (22, 30, 300, and 200 ppm, respectively) in basic lavas indicate origin of the magma by partial melting of peridotite mantle. Compared to calc-alkaline lava series from the south-central Andes, the shoshonitic lavas have high Ba, Rb, Th, K, Ta, Zr, and Hf contents. They also exhibit enriched light rare earth element patterns. Major and trace element distribution in the series is consistent with fractionation of olivine + clinopyroxene at deep crustal levels followed by orthopyroxene and amphibole fractionation. Sr-isotope ratios are almost constant at 0.7068 to 0.7079. Mineralogy and geochemistry of the shoshonite lavas series from northwestern Argentina indicate a three-stage origin: (1) partial melting of a mica-bearing peridotite; (2) storage at deep-crustal levels, and possible assimilation of lower-crustal materials; and (3) contamination at upper-crustal levels by incorporation of solid felsic materials.