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

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• 作者：Chris B.  ; Folkes^a ;   ; ^{chris.folkes@monash.edu} ; Shanaka L.  ; de Silva^b ;   ; ^{desilvas@geo.oregonstate.edu} ; Axel K.  ; Schmitt^c ;   ; ^{axel@oro.ess.ucla.edu} ; Raymond A.F.  ; Cas^a ;   ; ^{ray.cas@monash.edu}
• 关键词：²⁰⁶Pb/²³⁸U ; Antecrysts ; Xenocrysts ; Crystal-rich ignimbrites ; Magma residence times
• 刊名：Journal of Volcanology and Geothermal Research
• 出版年：2011
• 出版时间：15 September 2011
• 年：2011
• 卷：206
• 期：3-4
• 页码：136-147
• 全文大小：664 K

文摘

²⁰⁶Pb/²³⁸U ages obtained from Secondary Ionization Mass Spectrometry (SIMS) analyses of zircon crystals in eight ignimbrites from Cerro Gal¨¢n (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 Gal¨¢n 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 Gal¨¢n magmas. Many ignimbrites also contain zircon antecrysts that crystallized in the preceding cycle in the Gal¨¢n 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 Gal¨¢n 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 Gal¨¢n Ignimbrites are consistent with those detected in other large ¡®monotonous?ignimbrites, but Gal¨¢n 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 Gal¨¢n 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.