T35 Recent Advances in Studies of Large-Volume Silicic Volcanism: Stratigraphy, Architecture, Evolution (Posters)

Sunday, 9 October 2011
9:00 AM - 6:00 PM
Hall C

GEOLOGY, GEOCHEMISTRY AND GEOCHRONOLOGY OF LARGE VOLUME SILICIC VOLCANISM IN THE TRANS-MEXICAN VOLCANIC BELT

Paper number: T35-4
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Silicic volcanism has been scarcely studied in the Trans-Mexican Volcanic Belt, yet it constitutes a significant part of the volume of magmas emplaced in this arc since the Late Miocene. Silicic volcanism is widespread in the western (West of 103°W) and eastern TMVB (East of 101°W) but it is lacking in the central part of the arc. In the western TMVB silicic domes and minor pyroclastic flows (~370 km3) emplaced north of Guadalajara are the sole volcanic products between 8.5 and 5 Ma. Thereafter volcanism become bimodal, but silicic volcanism constitute a significant part of the Pliocene-Quaternary volcanism with ~500 km3 of rhyolites and ash flow tuffs emplaced between 4.9 to 2.9 Ma in a 30 km wide belt in western Tepic-Zacoalco rift (TZR) (Compostela-Etzatlán area) and ~ 430 km3 of rhyolitic domes emplaced since 1.6 Ma mostly in the eastern TZR (Tequila-Guadalajara area). Rhyolites are mainly metaluminous to peraluminous, have high LILE/HFSE values and Sr-Nd isotopic data similar to those of basalts of the previous Late Miocene episode. In the late Pliocene-Quaternary, small volume peralkaline rhyolitic domes were emplaced along with high-Nb basalts.

In the eastern TMVB silicic volcanism is dominated by major collapse calderas with large volume (>50 km3) ignimbrites, and by rhyolitic dome complexes unrelated to calderas. Early volcanism was exclusively silicic (e.g., 7.3-6.6 Ma Amazcala caldera) but since the end of Miocene (~6 Ma) mafic to intermediate lavas were also emplaced, and evidence of magma mingling is found in early Pliocene calderas. Intermediate compositions also occur in these calderas, in contrast to the bimodal distribution of rocks unrelated to calderas. Peraluminous rhyolite with high LILE/HFSE, and isotopic compositions indicating significant crustal contributions are abundant. Also, small volume early Pleistocene peralkaline rhyolitic domes with low LILE/HFSE are found behind the modern front (ca. 460 km from trench).

Silicic volcanism in both areas started after a late Miocene episode of mafic volcanism related to slab detachment, and afterwards migrated to the south, which suggests a process of slab rollback. Origin of silicic volcanism may be related to partial melting of previously hydrated lower crust, which was progressively exposed to asthenospheric mantle as the slab retreated.