Geochronological imaging of an episodically constructed subvolcanic batholith; U-Pb in zircon chronochemistry of the Altiplano-Puna volcanic complex of the Central Andes

• Published in: Geosphere (Boulder, Colo.) Colo.), 2016-08, Vol.12 (4), p.1054-1077
• Main Authors: Kern, Jamie M, de Silva, Shanaka L, Schmitt, Axel K, Kaiser, Jason F, Iriarte, A. Rodrigo, Economos, Rita
• Format: Article
• Language: English
• Subjects: absolute age; Altiplano; Andes; Batholiths; Cenozoic; Central Andes; Construction; Crusts; dates; eruptions; geochemistry; Geochronology; igneous and metamorphic rocks; igneous rocks; ignimbrite; intrusions; Magma; magmatism; metals; Miocene; Neogene; nesosilicates; orthosilicates; Petrology; Pleistocene; Pliocene; Puna; pyroclastics; Quaternary; Radioactive age determination; rare earths; Reservoirs; silicates; South America; Tertiary; trace elements; U/Pb; volcanic rocks; volcanism; xenocrysts; Zircon; zircon group; Zirconium
• ISSN: 1553-040X; 1553-040X
• DOI: 10.1130/GES01258.1

Zircons from 15 crystal-rich monotonous intermediate ignimbrites and 1 crystal-poor rhyolite ignimbrite erupted during the 11-1 Ma Altiplano-Puna Volcanic Complex (APVC) ignimbrite flare-up record multiscale episodicity in the magmatic history of the shallowest levels (5-10 km beneath the surface) of the Altiplano-Puna Magma Body (APMB). This record reveals the construction of a subvolcanic batholith and its magmatic and eruptive tempo. More than 750 U-Pb ages of zircon rims and interiors of polished grains determined by secondary ion mass spectrometry define complex age spectra for each ignimbrite with a dominant peak of autocrysts and subsidiary antecryst peaks. Xenocrysts are rare. Weighted averages obtained by pooling the youngest analytically indistinguishable zircon ages mostly correspond to the dominant crystallization ages for zircons in the magma. These magmatic ages are consistent with eruptive stratigraphy, and fall into four groups defining distinct pulses (from older to younger, pulses 1 through 4) of magmatism that correlate with eruptive pulses, but indicate that magmatic construction in each pulse initiated at least 1 m.y. before eruptions began. Magmatism was initially distributed diffusely on the eastern and western flanks of the APVC, but spread out over much of the APVC as activity waxed before focusing in the central part during the peak of the flare-up. Each pulse consists of spatially distinct but temporally sequenced subpulses of magma that represent the construction of pre-eruptive magma reservoirs. Three nested calderas were the main eruptive loci during the peak of the flare-up from ca. 6 to 2.5 Ma. These show broadly synchronous magmatic development but some discordance in their later eruptive histories. These relations are interpreted to indicate that eruptive tempo is controlled locally from the top down, while magmatic tempo is a more systemic, deeper, bottom-up feature. Synchroneity in magmatic history at distinct upper crustal magmatic foci implicates a shared connection deeper within the APMB. Each ignimbrite records the development of a discrete magma. Zircon age distributions of individual ignimbrites become more complex with time, reflecting the carryover of antecrysts in successively younger magmas and attesting to upper crustal assimilation in the APVC. Although present, xenocrysts are rare, suggesting that inheritance is limited. This is attributed to basement assimilation under zircon-undersaturated conditions deepe