A reconnaissance of U-Pb zircon ages in the Cerro Galán system, NW Argentina: Prolonged magma residence, crystal recycling, and crustal assimilation

Abstract

206Pb/ 238U ages obtained from Secondary Ionization Mass Spectrometry (SIMS) analyses of zircon crystals in eight ignimbrites from Cerro Galán (Central Andes) reveal insights into magma dynamics and crystallization histories of a sustained (> 3.5 Ma), large-volume, crystal-rich magmatic system. This makes the Galán volcanic system an excellent example of repeated large-volume ignimbrite eruptions from a single magmatic system with eruptive recurrence intervals between ~ 50 ka and possibly > 1 Ma. 40Ar/ 39Ar sanidine and biotite ages indicate eruption ages between ~ 2.0 and 5.6 Ma, although biotite results are likely affected by extraneous 40Ar. In each ignimbrite, interiors of zircon autocrysts frequently crystallized up to several 100's ka prior to eruption, consistent with long upper-crustal residence times of the pre-eruptive Galán magmas. Many ignimbrites also contain zircon antecrysts that crystallized in the preceding cycle in the Galán eruptive sequence. Finally, rare (~ 540 to 500 Ma) zircon xenocrysts are also present, derived from basement rocks that comprise the regional upper crust into which the Galán magmas intruded. In some cases, near eruption-age zircon has overgrown xenocrysts and antecryst cores. We infer that silicic magmas were repeatedly intruded into the shallow crust, whereby antecrysts became recycled from non-erupted progenitors to the earlier ignimbrite eruptions. Complex crystal populations observed in the Galán Ignimbrites are consistent with those detected in other large ‘monotonous’ ignimbrites, but Galán differs from many other silicic systems in that it represents a well-preserved system that frequently tapped silicic magma from the same long-lived spatially-focused system. The magma dynamics revealed by our reconnaissance zircon geochronology are consistent throughout the magmatic cycles at Galán and are independent of eruptive volume: pre-eruptive zircon crystallization within < 500 ka prior to eruption, with episodic recycling of non-erupted material of older eruptions and assimilation of crustal material local to the region.