Timescales of large silicic magma systems : implications from accessory minerals in Pleistocene lavas of the Altiplano-Puna Volcanic Complex, central Andes Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/tx31qp236

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  • Constraining the development, evolution, and timescales of large silicic magma systems is important to understanding the development of granite batholiths, the relationships between volcanoes and their plutonic underpinnings, and the development of the continental crust. The ignimbrite flare up that produced the Altiplano-Puna Volcanic Complex of the Central Andes is characterized by episodic volcanism over a ~11 Ma time-span that climaxed about 4 Ma. Since peak activity, the temporal and spatial record of volcanism suggests a waning of the system with only one other supervolcanic eruption at 2.6 Ma. The most recent phase of volcanism from the APVC comprises a series of late Pleistocene domes that share a general petrochemical resemblance to the ignimbrites. New U-Th/U-Pb data on zircons and high precision 40Ar/39Ar age determinations reveal that these effusive eruptions represent a temporally coherent magmatic episode. The five largest domes (Chao, Chillahuita, Chanka, Chascon-Runtu Jarita, and Tocopuri) have a combined volume >40 km³, and are distributed over a roughly elliptical area of almost 2000km² centered at 22°S 68°W. They are crystal rich (>50%) dacites to rhyolites. ⁴⁰Ar/³⁹Ar ages from biotite reveals eruption ages from 108±6 to 120±5 ka while more accurate sanidine for some of the domes reveal eruption ages from 87±4 to 97±2 ka. SIMS U-series crystallization ages from the rims of 215 zircon crystals from the domes show a similar age spectra from dome to dome, with common peaks in zircon ages at ~100ka and ~220ka. Furthermore, the ages reveal a fairly continuous spread of ages from near eruption to >300ka indicating that the residence time of this magma body was likely over a similar time interval. Ubiquitous andesitic inclusions evidence a vital role for recharge in sustaining and maybe eventual eruption of these magmas. Lastly, the interiors of crystals with rim U-Th secular equilibrium ages were re-analyzed and have yielded U-Pb ages of up to 3.5 Ma. The presence of these older interiors suggests that the source region of these magmas retained a record of an earlier history dating back to the last supervolcanic eruption in the region from the nearby Pastos Grandes caldera. This suggests that the thermal history of the system precluded complete resorption of antecrysts. The volcanological, petrological, temporal and spatial coherence of this series of eruptions combined with the similar ⁴⁰Ar/³⁹Ar and zircon age spectra argue for a long-lived and unitary magma chamber revealing perhaps the waning of this major continental magma system.
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