No abstract

No abstract

The Lípez region of southwest Bolivia is the locus of a major Neogene ignimbrite fl areup, and yet it is the least studied portion of the Altiplano-Puna volcanic complex of the Central Andes. Recent mapping and laserfusion 40 Ar/ 39 Ar dating of sanidine and biotite from 56 locations, coupled with paleomagnetic data, refi ne 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 ignimbriteproducing 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 km 3 ) of magma from sources distributed throughout the volcanic complex. The fi rst 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 km 3 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 km 3 of magma. Pulse 3, the largest, produced at least 3100 km 3 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 vol canic explosivity index (VEI) 8 eruptions, producing the 3.49 Ma Tara (800 km 3 dense rock equivalent [DRE]) and 2.89 Ma Pastos Grandes (1500 km 3 DRE) ignimbrites. In addition to these large calderarelated eruptions, new age determinations refi ne the timing of two little-known ignimbrite shields, the 5.23 Ma Alota and 1.98 Ma Laguna Colorada centers. Moreover, 40 Ar/ 39 Ar 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 fi eld as a whole are consistent with both incremental construction of plutons as well as a composite Cordilleran batholith.

No abstract

No abstract