Regional Chemical diversity, crustal and mantle sources and evolution of Puna Plateau Ignimbrites in the Central Andes.

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 ä18O 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 ä18O 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 25–20 km 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 ä18O data are consistent with the large ignimbrites (N500 km3 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 125 ppm Sr and ä18O 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 11,000 km3, the mantle magma production rate to produce the ignimbrites is a distinctly non-flare-up-like rate of less than 20 km3/km/Ma when averaged across the Puna over 7 Ma.