Migrating ignimbrite flare-ups in the Central Andes - Implications for crustal evolution based on chemical, geochronological and GIS-based volumetric data

Temporal and compositional patterns of Neogene ignimbrite magmatism in the Central Andes were analyzed using GIS and geostatistical modeling. We compiled a comprehensive ignimbrite data base and digitized 203 individual ignimbrite sheets, for which geochemical, isotopic (partly), and geochronological data are available from the literature and own data. Composition, timing, volumes and sources of erupted ignimbrite deposits are constrained and magma volumes through space and time are calculated for five segments of the Central Andes.
The total erupted ignimbrite magma volume of 31,000 km3 for the past 30 Ma is distributed as follows: 2,400 km3 for Southern Peru, 2,700 km3 for Southernmost Peru, 8,400 km3 for the Altiplano, 14,200 km3 for the Northern Puna and 3,100 km3 for the Southern Puna segments. Average magmatic addition per Ma and km arc for ignimbrites range between 20 to 30 km3. This is similar to the basaltic “base”-flux for arc magmatism and suggests that ignimbrite flare-ups do not represent orders of magnitude increased magma production rates but are rather punctuated, short-lived well events separated in space and time. There is a clear N-S “younging” of eruption ages and ignimbrite pulses. Major pulses occurred at 19-24 Ma (e.g. Oxaya, Nazca Group), 13-14 Ma (e.g. Huaylillas ignimbrites), 6-10 Ma (Altiplano and Puna ignimbrites, e.g. Vilama ignimbrite) and 3-6 Ma (e.g. Atana, Los Frailes, Toconao). Small and younger ignimbrites (0-3 Ma, e.g. Lauca-Perez, Purico) do not follow this pattern. We propose that large-volume ignimbrite eruptions occurred in the wake of subduction of the Juan-Fernandez ridge on the Nazca Plate that passed below the Central Andes from N to S during the past 25 Ma. Low angle subduction caused compression and fluid release is followed by massive inflow and melting of asthenospheric mantle when the slab steepened again after the passing of the ridge. This in turn caused massive melting within the crust aided by advective heat transport. Differences in chemcial and isotopic composition of the large-volume ignimbrites are related to changes in crustal thickness, and different “preconditioning” during the Anden orogeny. Isotope data and whole rock compositional data suggest a higher degree of crustal assimilation for the younger Altiplano ignimbrites in the S compared to the older (22-19 Ma) ignimbrites in the North. REE compositions for large-volume ignimbrites reflect changes in crustal thickness with a "transition" at ca. 13-9 Ma that can be related to accelerated crustal shortening at that time (Oncken et al., 2006).
Total volumes for the northern segments and the Northern Puna are similar. However, calderas and intra-caldera ignimbrite volumes in the north are less well constrained due to the lower level of study, higher ages and higher degrees of incision and erosion. In any case, there is no “single” ignimbrite flare up in the Central Andes with one regionally and temporally restricted event of high-magma flux and bath-olith construction (de Silva and Gosnold, 2007). Instead, we suggest a more dynamic scenario, with “flare ups” moving from N to S across the Central Andes during the past 25 Ma. Our database aids to better constrain numerical models of Andean geodynamic processes.
References
deSilva, S., Gosnold, W.D. (2007) Episodic construction of batholiths: Insights from the spatiotemporal development of an ignimbrite flare-up. J. Volcanol. Geotherm. Res. 167: 320-335
Oncken, O. et al. (2006), Deformation of the Central Andean Upper Plate System-Facts, Fiction, and Constraints for Plateau Models, in Oncken, O. et. al., eds., The Andes: Frontiers in Earth Sciences, Springer Berlin Heidelberg: 3-27.