INVESTIGADORES
CAFFE Pablo Jorge
congresos y reuniones científicas
Título:
The Neogene Los Frailes Ignimbrites and Evolution of the Andean Crust and Mantle Beneath the Altiplano
Autor/es:
KATO, J. ; KAY, S. ; COIRA, B., HARRIS, C.; CAFFE, P.J., JIMENEZ, N.
Lugar:
Sacramento
Reunión:
Congreso; Godschmidt Conference; 2014
Institución organizadora:
Geochemical Society
Resumen:
The chemistry of the andestic-rhyodacitic Los Frailes
Ignimbrite Complex records the last 25 million years of crustal
thickening and delamination under the Andean Altiplano. The
strongly peraluminous character of these ignimbrites indicates
a metapelitic crust, while seismic velocities reveal a
predominantly silicic lower crust at present. Geochemical data
and modeling suggest the ignimbrites are ~50:50 mixtures of
enriched mantle and crustal-derived melts based on δ18O
ratios of quartz phenocrysts (+9.73-11.09?) and Sr-Nd AFC
calculations that assume an enriched mantle melt with 500-950
ppm Sr; 25-30 ppm Nd; δ18O=+5.8?, 87/86Sr=0.7055 and
143/144Nd =0.51260. Sr and Nd isotopic ratios in the
ignimbrites are 0.710-0.713 and 0.5121-0.5123. The AFC
calculated crustal end-members have δ18O=+13-16?,
87/86Sr=0.730-0.750 at 95-160 ppm and 143/144Nd =0.51190
at 25-40 ppm. Evidence for melting and mixing near the Moho
comes from steep HREEs (Sm/Yb=4-12) and high Sr ppm
(400-650). Negative Eu/Eu* (0.6-0.9) anomalies require
subsequent mid to upper crustal plagioclase fractionation, and
cordierite in some reflect pre-eruption crystallization in the
uppermost crust. Temporal trends in trace element ratios track
a pattern of general and episodic crustal thickening related to
crustal shortening since 25 Ma that is inferred from steepening
HREE ratios. Two temporally distinct decreases in Sm/Yb
ratios could correlate with discrete delamination events at 12-
10 Ma and 4-2 Ma just prior to the major eruptions of the 8-7
Ma Livicucho/Condor Nasa and 2-1 Ma Los Frailes
ignimbrite. A shift to more enriched isotopic compostions at
12-10 Ma could be related to delamination related loss of the
mantle lithosphere and basal crust that both contaminates the
mantle wedge and exposes mantle melts to more isotopically
enriched crust. The voluminous ~1.5 Ma Los Frailes
ignimbrite is linked with seismic evidence for missing
lithosphere related to the latest delamination event.