Description: The Lipez region of southwest Bolivia is the locus of a major Neogene ignimbrite flare-up, and yet it is the least studied portion of the Altiplano-Puna volcanic complex of the Central Andes. Recent mapping and laser-fusion 40Ar/39Ar dating of sanidine and biotite from 56 locations, coupled with paleomagnetic data, refine the timing and volumes of ignimbrite emplacement in Bolivia and northern Chile to reveal that monotonous intermediate volcanism was prodigious and episodic throughout the complex. The new results unravel the eruptive history of the Pastos Grandes and Guacha calderas, two large multicyclic caldera complexes located in Bolivia. These two calderas, together with the Vilama and La Pacana caldera complexes and smaller ignimbrite shields, were the dominant sources of the ignimbrite-producing eruptions during the [~]10 m.y. history of the Altiplano-Puna volcanic complex. The oldest ignimbrites erupted between 11 and 10 Ma represent relatively small volumes (approximately hundreds of km3) of magma from sources distributed throughout the volcanic complex. The first major pulse was manifest at 8.41 Ma and 8.33 Ma as the Vilama and Sifon ignimbrites, respectively. During pulse 1, at least 2400 km3 of dacitic magma was erupted over 0.08 m.y. Pulse 2 involved near-coincident eruptions from three of the major calderas resulting in the 5.60 Ma Pujsa, 5.65 Ma Guacha, and 5.45 Ma Chuhuilla ignimbrites, for a total minimum volume of 3000 km3 of magma. Pulse 3, the largest, produced at least 3100 km3 of magma during a 0.1 m.y. period centered at 4 Ma, with the eruption of the 4.09 Ma Puripicar, 4.00 Ma Chaxas, and 3.96 Ma Atana ignimbrites. This third pulse was followed by two more volcanic explosivity index (VEI) 8 eruptions, producing the 3.49 Ma Tara (800 km3 dense rock equivalent [DRE]) and 2.89 Ma Pastos Grandes (1500 km3 DRE) ignimbrites. In addition to these large caldera-related eruptions, new age determinations refine the timing of two little-known ignimbrite shields, the 5.23 Ma Alota and 1.98 Ma Laguna Colorada centers. Moreover, 40Ar/39Ar age determinations of 13 ignimbrites from northern Chile previously dated by the K-Ar method improve the overall temporal resolution of Altiplano-Puna volcanic complex development. Together with the updated volume estimates, the new age determinations demonstrate a distinct onset of Altiplano-Puna volcanic complex ignimbrite volcanism with modest output rates, an episodic middle phase with the highest eruption rates, followed by a decline in volcanic output. The cyclic nature of individual caldera complexes and the spatiotemporal pattern of the volcanic field as a whole are consistent with both incremental construction of plutons as well as a composite Cordilleran batholith.

Description: Contamination of ascending mantle-derived magmas by the continental crust was investigated and modeled for a suite of volcanic rocks and entrained crustal xenoliths from the Central Andes using bulk geochemical compositions for mantle-derived and crustal end-members as dictated by traditional approaches. The assumption that the crustal contaminant in these open magmatic systems is a single composition was assessed through in situ analysis of quenched anatectic melt trapped within its crustal xenolith. Our results show for the first time significant chemical and Sr-isotopic disequilibrium between melt and source over submillimeter-length scales in a natural system. Sampled glass is rhyolitic in nature, enriched in large ion lithophile elements (LILE) and depleted in heavy rare earth elements (HREE). Analysis of the melt for its 87Sr/86Sr composition revealed isotopic heterogeneity ranging from 0.7164 to 0.7276. The isotopic disequilibrium between melt and source is understood to reflect the melting of minerals with different Rb/Sr (and therefore 87Sr/86Sr) more quickly than the isotopic composition can diffusively equilibrate between melt and minerals. Our results suggest that the mechanism of crustal anatexis produces contaminating melts which are geochemically heterogeneous both spatially and temporally. Furthermore, time scales of Sr diffusion and anatectic melt segregation promote the preservation of isotopic disequilibrium at the micro (submillimeter) and macro (crustal) scale. This highlights the need for detailed microscopic investigations coupled with petrogenetic modeling in order to develop more robust characterization and quantification of contamination in open magmatic systems.