Ignimbrite magmatism related to the Central Andean subuction zone through space and time: compositions, magma production rates and net crustal growth

Abstract:

A compilation of geochemical and geochronological data for 194 ignimbrites from the Central Andes forms the basis to quantify eruption volumes and compositions through time and space. Sr- and O-isotope data constrain the mass balance between juvenile addition to the Andean crust vs. crustal melting and recycling, these volumes can be converted into net crustal growth rates. So-called “ignimbrite flare-ups” represent pulses of additional magma flux from the mantle to the crust of 70 km3/km*Ma, about twice as large as the typical background arc magma flux. Ignimbrite trace element compositions change through time during the thickening Andean crust. Characteristic differences in trace element signatures are documented for the Eu anomaly, depletion of MREEs and variable enrichment in LREE.

Older (>14 Ma) plateau-forming ignimbrites in northernmost Chile and southern Peru are less depleted in HREEs and less radiogenic in Sr isotopes, indicating crustal contributions of up to only 25%. Younger (<10 Ma) Altiplano-Puna-Volcanic-Complex ignimbrites in the S have larger crustal contributions (up to 70%). These compositional variations indicate a relation to crustal thickening with a “transition” from plagioclase to amphibole and garnet residual mineralogy between 13 to 9 Ma. The spatial and temporal distribution of ignimbrites follows the N-S passage of the Juan-Fernandéz ridge and a flat-slab subduction regime. Slab steepening thus is the principal trigger of increased input of mantle magmas into the crust. However, ignimbrite “flares” tend to follow (by a few Ma) pulses of crustal shortening. Different crustal contributions reflect changing crustal thermal states as a consequence of crustal thickening with time. Older ignimbrites in the N of the Central Andes formed in relatively thinner and colder crust. By contrast, younger ignimbrites in the south formed at < 10 Ma after the crust had thickened and was considerably hotter and “primed” for more intense crustal melting. Volume and composition of ignimbrites are controlled by the interplay of both, deep processes in the mantle wedge and processes of melting (and recycling) of the continental crust at shallower levels. The link may be found in crustal diapirs where mantle and crustal melts mix and form uniform ignimbrite magmas that erupt from shallow reservoirs.