Boletín de la Sociedad Geológica Mexicana

Volumen 70, núm. 3, 2018, p. 731- 760



Geochemistry of the Amazcala Caldera, Querétaro, Mexico: An unusual peralkaline center in the central Mexican Volcanic Belt

Gerardo de Jesús Aguirre-Díaz1, Ofelia Morton-Bermea2

1Centro de Geociencias, Universidad Nacional Autónoma de México, campus UNAM-Juriquilla, Blvd. Juriquilla No. 3001, Querétaro, Querétaro 76230, Mexico.. 
2Instituto de Geofísica, Universidad Nacional Autónoma de México, Circuito de la Investigación Científica s/n, Ciudad Universitaria, Coyoacán 04150, CDMX, Mexico.

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The Amazcala caldera is 30 km northeast of Queretaro City, Mexico, in the central sector of the Mexican Volcanic Belt. It is 14 x 11 km in size and was active from 7.3 to 6.6 Ma. The first caldera event was the Ezequiel Montes Pumice, which was a widespread grey pumice-fall sequence of coarse to fine lapilli, with thicknesses from <1 to ~ 43 m. Colón ignimbrite was next in the evolution of the caldera and consisted mostly of dense pyroclastic density current deposits interbedded with pumice-fall lapilli and minor dilute pyroclastic density current deposits. Several rhyolitic domes form the rim of the caldera, some of which have associated pumice fall and pyroclastic density current deposits. The rim domes still preserve aphyric obsidian, named as Rim Dome Obsidian. A 4 x 2 km intra-caldera lava dome represents the last cal­dera event, but lacks preserved obsidian. All caldera products are peralkaline rhyolites with chemical char­acteristics of comendites. A correction of Na2O and K2O was necessary because of secondary hydration of glasses in pumice deposits. This correction process could also be applied in similar weathered or devitri­fied volcanic deposits at other localities. Al2O3, K2O, Fe2O3, and most REE (except La, Ce) and Y remain practically unchanged from the earliest caldera unit (Ezequiel Montes Pumice) to the last analyzed caldera unit (Rhyolitic Dome Obsidian), and Ba, Sr, La, P2O5 markedly decrease from Ezequiel Montes Pumice to Rhyolitic Dome Obsidia, whereas HFSE Th, Nb, Zr, and TiO2 slightly increase from Ezequiel Montes Pumice to Rhyolitic Dome Obsidian; that is, from pre-caldera collapse to post-caldera collapse. Comparing Amazcala units with calc-alkaline mafic lavas of the area, either pre- or post-caldera, HFSE, REE, and Th are markedly enriched in the Amazcala caldera products. In contrast, Ba, Sr, P2O5, and TiO2 are strongly depleted in the Amazcala units with respect to the mafic rocks. These data suggest a very efficient fractional crystallization process of possible parental mafic magmas. All caldera products are enriched in HFSE, LILE and REE, in particular Nb, Th, Zr and REE, suggesting an input from the fertile mantle during a pulse of continental intra-arc exten­sion in the central sector of the Mexican Volcanic Belt. Amazcala caldera comendites have similar chemical signatures than other caldera comendites at extensional settings within the western sector of the Mexican Volcanic Belt, and with comendites within continental rifts, such as the Kenya rift in Africa.

Keywords: caldera, comendite, peralkaline rhyolite, continental extension, Na-loss and K-enrichment correction.