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The giant ignimbrites that erupted from the Cerro Galán caldera complex in the southern Puna of the high Andean plateau are considered to be linked to crustal and mantle melting as a consequence of delamination of gravitationally unstable thickened crust and mantle lithosphere over a steepening subduction zone. Major and trace element analyses of Cerro Galán ignimbrites (68-71% SiO) that include 75 new analyses can be interpreted as reflecting evolution at three crustal levels. AFC modeling and new fractionation corrected δO values from quartz (+7.63-8.85‰) are consistent with the ignimbrite magmas being near 50:50 mixtures of enriched mantle (Sr/Sr ~ 0.7055) and crustal melts (Sr/Sr near 0.715-0.735). Processes at lower crustal levels are predicated on steep heavy REE patterns (Sm/Yb = 4-7), high Sr contents (>250 ppm) and very low Nb/Ta (9-5) ratios, which are attributed to amphibolite partial melts mixing with fractionating mantle basalts to produce hybrid melts that rise leaving a gravitationally unstable garnet-bearing residue. Processes at mid crustal levels create large negative Eu anomalies (Eu/Eu* = 0.45-0.70) and variable trace element enrichment in a crystallizing mush zone with a temperature near 800-850°C. The mush zone is repeatedly recharged from depth and partially evacuated into upper crustal magma chambers at times of regional contraction. Crystallinity differences in the ignimbrites are attributed to biotite, zoned plagioclase and other antecrysts entering higher level chambers where variable amounts of near-eutectic crystallization occurs at temperatures as low as 680°C just preceding eruption. Ar/Ar single crystal sanidine weighted mean plateau and isochron ages combined with trace element patterns show that the Galán ignimbrite erupted in more than one batch including a ~ 2.13 Ma intracaldera flow and outflows to the west and north at near 2.09 and 2.06 Ma. Episodic delamination of gravitationally unstable lower crust and mantle lithosphere and injection of basaltic magmas, whose changing chemistry reflects their evolution over a steepening subduction zone, could trigger the eruptions of the Cerro Galán ignimbrites. [ABSTRACT FROM AUTHOR]

Abstract: Voluminous mafic dacitic to rhyodacitic ignimbrite fields linked to giant calderas are distinctive features of the late Miocene to Quaternary magmatic record of the central Andean Altiplano–Puna plateau. The magmas erupted through a compressionally thickened crust over a generally shallow, but variably dipping subducting Nazca plate. New analyses including whole rock major and trace element concentrations (~175), 143Nd/144Nd (22) and 87Sr/86Sr (23) ratios and δ 18O analyses on quartz phenocrysts (18) coupled with data from the literature provide a regional and temporal perspective on the chemical variability of Puna ignimbrites between 22°S and 27°S latitude. Ranges of Pb, Sr, Nd and δ 18O isotopes, variability in Al/(K+Na+Ca), Na/K and trace element ratios, chemical parallels with Paleozoic magmatic rocks and published experimental constraints link crustal contributions in the ignimbrites to a more pelitic crust in the northern Puna, a more igneous gneiss-like crust in the southern Puna and a more amphibolite-like crust near the arc. Melting in the deep crust by injection of mantle-derived melts followed by magma rise, accumulation and evolution at depths near 25–20km is supported by heavy REE evidence for deep crustal garnet-bearing residues, negative Eu anomalies superimposed on steep REE patterns, calculated bulk Sr distribution coefficients and Puna seismic images. Temporal trends towards less evolved isotopic ratios, metaluminous compositions, flatter REE patterns and less HFSE depletion in northern Puna ignimbrites suggest an evolving crustal magma source as mafic melts continued to enter the crust. Assimilation-fractional crystallization models for Sr and modeling of δ 18O data are consistent with the large ignimbrites (>500km3 DRE) forming as near 50–50 hybrids of enriched mantle-wedge derived (87Sr/86Sr~0.7055; 143Nd/144Nd~0.5126) basaltic melts and spatially variable lower to mid-crustal melts with 87Sr/86Sr ratios from 0.715 to 0.745 at 300 to 125ppm Sr and δ 18O from ~+12‰ to ~+15‰. Given a 1:1 mantle to crustal ratio, a 3:1 to 5:1 plutonic/volcanic ratio and an ignimbrite volume near 11,000km3, the mantle magma production rate to produce the ignimbrites is a distinctly non-flare-up-like rate of less than 20km3/km/Ma when averaged across the Puna over 7Ma. [Copyright &y& Elsevier]

Abstract: Late Cenozoic back-arc mafic volcanism in the southern Puna plateau of Argentina offers insights into the state of the mantle under the world’s second largest continental plateau. Previous studies of the mafic magmas in this region proposed a scenario of mantle melting due to lithospheric delamination and/or steepening of the subducting slab. However, few of the centers have been precisely dated, which limits any geodynamic interpretation. We present results of laser incremental-heating 40Ar/39Ar dating of 22 back-arc centers in the southern Puna, with emphasis on the Salar de Antofalla region where volcanic activity was most intense. Three localities yielded ages between 7.3 and 7.0Ma which, along with 2 previous 7Ma ages, firmly establishes that back-arc activity began as early as late Miocene. Volcanism continued through the Pleistocene but the peak was in the early Pliocene. This result has important tectonic implications. If, as previously suggested, magma genesis is related to lithospheric delamination, this process was underway by the latest Miocene in the southern Puna. Furthermore, since the mafic back-arc volcanism is considered to mark a change in fault kinematics from compressional to transtensional, the new age constraints indicate that this change took place in the early Pliocene. The spatial and age distributions of the mafic centers indicate that magmatism began, and remained focussed in, a region between Salar de Antofalla and Cerro Galán. This concentration is probably structurally controlled, as it corresponds to the intersection of the NW–SE striking Archibarca lineament zone and the sets of NNE–SSW faults that run parallel to the Salar de Antofalla basin. [Copyright &y& Elsevier]