NEOGENE AND QUATERNARY IGNIMBRITES IN THE AREA OF AREQUIPA, SOUTHERN PERU: STRATIGRAPHICAL AND PETROLOGICAL CORRELATIONS

Abstract

This study focuses on the correlation of four newly identified rhyolitic ignimbrites located around Arequipa, which were previously described as a single ignimbrite sheet: (1) The Río Chili Ignimbrite (ca. 13.33 Ma) crops out in the Río Chili canyon and consists of a cooling unit of non-welded to partially welded, massive, crystal-rich lapilli tuff. This ignimbrite represents the base of the Neogene ignimbrite succession in the Arequipa area and may be stratigraphically equivalent to the 14-13 Ma Huaylillas Ignimbrites in southern Peru. (2) The second voluminous (~ 20 km3) and widespread La Joya Ignimbrite sheet (ca. 4.87 Ma) filled the Arequipa depression. Its probable source is now buried underneath the Chachani volcanic complex. The La Joya Ignimbrite covers both the Arequipa Batholith toward the Río Vitor piedmont and also part of the Altiplano to the East and North of the Chachani complex. (3) The Arequipa Airport Ignimbrite (AAI) sheet (ca. 1.6 Ma, ~18 km3), the third and latest infill of the Arequipa basin, consists of a lower white, columnar jointed, massive lapilli tuff, indurated by vapor-phase recrystallization, and is overlain by an upper, pink, lithic-rich, non-welded, massive lapilli-tuff. The source of this ignimbrite is also buried underneath the Chachani complex, as indicated by anisotropy of magnetic susceptibility measurements and component lithology. (4) The Yura non-welded pumice-flow deposits (ca. 1.02 Ma, 1.5 km3) are restricted to the northern and western flanks of the Chachani complex and may correlate with tuffs of the Capillune Formation on the Altiplano. All ignimbrites are high-K calc-alkaline to alkaline rhyolites, which contain variable amounts of plagioclase, biotite, quartz, sanidine, and Fe-Ti oxides. The modal composition of mineral phases and chemistry of amphiboles and biotites are useful for distinguishing the ignimbrites. Specific geochemical fingerprints, such as Sr, Y, Rb, Ba, Cs, REE, and Sr isotopes were effective discriminator elements. Magmatic differentiation, crustal residence time and contamination, and source melting processes are discussed in order to explain compositional variations. The older La Joya Ignimbrite and Río Chili Ignimbrite reflect initial wetter crustal melts closer to granite composition, whereas with time the younger Arequipa Airport Ignimbrite and Yura Tuffs initial melts are slightly less evolved and dryer due to more extensive crustal melting. The voluminous (> 40 km3) Neogene ignimbrites of the Arequipa area were produced by three major explosive volcanic pulses in the Central Volcanic Zone of southern Peru, whose genetic relation with tectonic uplift and crustal thickening of the Andean Cordillera are still a matter of debate. © 2006 Elsevier B.V. All rights reserved.