HR: 0800h
AN: V31C-0629    [Abstracts]
TI: Extreme High Field Strength Element Depletion and Chondritic Nb/Ta in Central Andean Adakite-like Lavas (~27° S, ~68° W)
AU: * Goss, A R
EM: arg32@cornell.edu
AF: Department of Earth and Atmospheric Sciences, Snee Hall Cornell University, Ithaca, NY 14853 United States
AU: Kay, S M
EM: smk16@cornell.edu
AF: Department of Earth and Atmospheric Sciences, Snee Hall Cornell University, Ithaca, NY 14853 United States
AB: We present new high precision ICP-MS HFSE data on ~30 samples from the adakite-like Pircas Negras (SiO2 =57-62 %; La/Yb= 20-60; Sr = 600-900 ppm, Cr to 200 ppm) and Dos Hermanos (SiO2 =56-59 %; La/Yb= 57-61; Sr = 1200-1500 ppm, Cr to 60 ppm) andesites from the southernmost Andean Central Volcanic Zone (CVZ). These andesites erupted in a broadened arc as the volcanic front was displaced about 50 km to the east between 8 and 3 Ma and likely reflect melting of a garnet bearing crustal source. Our data reveal a discrete change from chondritic Nb/Ta ratios (19-21) and low Zr/Sm (25-30) in older 8-7 Ma Dos Hermanos lavas to dominantly subchondritic Nb/Ta ratios (11-18) and higher Zr/Sm (30-45) in <6 Ma Pircas Negras lavas. These ratios are uncommon since frontal arc lavas worldwide typically have subchondritic Nb/Ta ratios (<19.9 ± 2) consistent with flux melting of uniformly subchondritic sources (i.e. depleted mantle, subducted sediments, crustal assimilation). The highest Nb/Ta ratios begin to approach those of kimberlitic eclogites (mean Nb/Ta = 24) from Siberia and western Africa where residual rutile controls the budget of eclogitic Nb and Ta (Rudnick et al., 2003). Extreme depletions in HFSE in both Pircas Negras (La/Ta = 40-80) and Dos Hermanos (La/Ta= 90-100) lavas are best explained as hydrous and oxidizing conditions within the mantle wedge initially stabilized Ti-oxides (i.e. rutile) as the arc front migrated. These steeper REE patterns and high La/Ta are a transient feature measured in lavas erupting during the peak of arc migration, as mafic <2 Ma CVZ lavas to the north (~26° S) have lower La/Yb (< 20) and La/Ta (< 40). The observed temporal shift in Nb/Ta coupled with a general increase in Zr/Sm suggests a change from residual rutile to low-Mg amphibole within an eclogitic/granulitic crustal residue during the course of arc migration. A potential factor in explaining these data is that a transient period of increased subduction erosion associated with the migration of the frontal arc transported forearc crustal material into the progressively dehydrating mantle wedge. Partial melting of this eclogitized forearc material contributed to the transient adakite-like nature of the magmas erupted along the margin.
DE: 1065 Major and trace element geochemistry
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3619 Magma genesis and partial melting (1037)
DE: 9360 South America
DE: 9605 Neogene
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005