INVESTIGADORES
COIRA beatriz lidia luisa
artículos
Título:
Implications of Quaternary volcanism at Cerro Tuzgle for crustal and mantle evolution of the Puna plateau, Central Andes, Argentina.
Autor/es:
COIRA, B. AND S., MALHBURG KAY
Revista:
CONTRIBUTIONS TO MINERALOGY AND PETROLOGY
Editorial:
SPRINGER
Referencias:
Lugar: Berlin; Año: 1993 vol. 113 p. 40 - 58
ISSN:
0010-7999
Resumen:
Abstract.
The high-K Tuzgle
volcanic center, (24 ~ S, 66.5 ~ W) along with several small shoshonitic
centers, developed along extensional Quaternary faults of the El Toro lineament
on the east-central Puna plateau, 275 km east of the main front of the Andean
Central Volcanic Zone (CVZ). These magmas formed by complex mixing processes in
the mantle and thickened crust (> 50 km) above a ~ 200 km deep seismic zone.
Tuzgle magmas are differentiated from shoshonitic series magmas by their more
intraplate-like Ti group element characteristics, lower incompatible element
concentrations, and lower SVSr/S6Sr ratios at a given eNd. Underlying Mio-Pliocene
volcanic rocks erupted in a compressional stress regime and have back-arc like
calc-alkaline chemical characteristics. The Tuzgle rocks can be divided into two
sequences with different mantle precursors: a) an older, more voluminous
rhyodacitic (ignimbrite) to mafic andesitic (56% to 71% SiOz) sequence with
La/Yb ratios < 30, and b) a younger andesitic sequence with La/Yb ratios
>35. La/Yb ratios are controlled by the mafic components: low ratios result
from larger mantle melt percentages than high ratios. Shoshonitic series lavas (52%
to 62% SiOz) contain small percentage melts of more isotopically
"enriched" arc-like mantle sources. Some young Tuzgle lavas have a
shoshonitic-like component. Variable thermal conditions and complex stress system
are required to produce the Tuzgle and shoshonitic series magmas in the same
vicinity. These conditions are consistent with the underlying mantle being in
transition from the thick mantle lithosphere which produced rare shoshonitic
flows in the Altiplano to the thinner mantle lithosphere that produced back-arc
calc-alkaline and intraplate-type flows in the southern Puna. Substantial upper
crustal type contamination in Tuzgle lavas is indicated by decreasing eNd (-2.5
to -6.7) with increasing STSr/S6Sr (0.7063 to 0.7099) ratios and SiO2 concentrations,
and by negative Eu anomalies (Eu/Eu* < 0.78) in lavas that lack plagioclase
phenocrysts. Trace element arguments indicate that the bulk contaminant was
more silicic than the Tuzgle ignimbrite and left a residue with a high pressure
mineralogy. Crustal shortening processes transported upper crustal contaminants
to depths where melting occurred. These contaminants mixed with mafic magmas
that were fractionating mafic phases at high pressure. Silicic melts formed at
depth by these processes accumulated at a mid to upper crustal discontinuity
(decollement). The Tuzgle ignimbrite erupted from this level when melting rates
were highest. Subsequent lavas are mixtures of contaminated mafic magmas and
ponded silicic melts. Feldspar and quartz phenocrysts in the lavas are
phenocrysts from the ponded silicic magmas.