|Title:||A reconnaissance of U–Pb zircon ages in the Cerro Galan system, NW Argentina: Prolonged magma residence, crystal recycling, and crustal assimilation|
|Authors:||C. B. Folkes, S. L. de Silva, A. K. Schmitt, and R. A.F. Cas|
|Publication:||Journal of Volcanology and Geothermal Research, v. 206, p. 136‐147.|
206Pb/238U ages obtained from Secondary Ionization Mass Spectrometry (SIMS) analyses of zircon crystals in eight ignimbrites from Cerro Galan (Central Andes) reveal insights into magma dynamics and crystallization histories of a sustained (N3.5 Ma), large–volume, crystal–rich magmatic system. This makes the Galan 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 N1 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 Galan magmas. Many ignimbrites also contain zircon antecrysts that crystallized in the preceding cycle in the Galan 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 Galan 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 Galan Ignimbrites are consistent with those detected in other large ’monotonous’ ignimbrites, but Galan 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 Galan and are independent of eruptive volume: pre–eruptive zircon crystallization within b500 ka prior to eruption, with episodic recycling of non–erupted material of older eruptions and assimilation of crustal material local to the region.