A Monogenetic Alkali Basalt Field East of the Andean Arc Between 34° and 35° S: Implications for Mantle Composition

Open Access
Author:
Murray, Timothy Talmadge
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 29, 2013
Committee Members:
  • Maureen Feineman, Thesis Advisor
  • Tanya Furman, Thesis Advisor
  • Peter Christopher Lafemina, Thesis Advisor
Keywords:
  • Northern Southern Volcanic Zone
  • NSVZ
  • retro-arc basalts
  • Sr isotopes
  • Nd isotopes
  • Hf isotopes
  • Pb isotopes
  • Payenia
  • Andes
Abstract:
This study analyzes a series of Quaternary basaltic lavas sampled ~50km east of the arc front of the Northern Southern Volcanic Zone (NSVZ) of the Andean arc, as well as two basaltic andesites from Casimiro parasitic cone in the Diamante Caldera, located on the arc front itself, for major element concentrations, trace element concentrations, and radiogenic isotope ratios of Sr, Nd, Pb, and Hf. The basaltic field has previously been classified as a northern portion of the Payenia Volcanic Complex, the more southerly parts of which are interpreted to have resulted from adiabatic melting associated with changes in slab dip during the late Oligocene through Miocene. The retro-arc basalt samples are alkaline basalts, enriched in fluid mobile elements such as Cs, Ba, Pb, Sr, and Li, with moderate relative depletions in Nb and Ta. These characteristics are typical of rear-arc basalts from subduction zones. We propose that these basalts should be classified as retro-arc basalts associated with the Quaternary volcanic arc rather than as an extension of the more extensive Payenia Volcanic field to the south. It has been well documented that arc lavas in the NSVZ have distinctly higher 87Sr/86Sr and lower εNd values than arc magmas found south of 34.5°S. This chemical distinction has been interpreted as indicating a significant crustal contribution to rising magmas within the NSVZ. The mechanism for this crustal contribution, however, has been largely debated. Two models have been proposed to explain the driving force for this increased crustal signature: 1) increased assimilation of crustal material with elevated 87Sr/86Sr isotope ratios and lower εNd values due to thicker, older crust in the NSVZ, or 2) the subducting Nazca plate is eroding crustal material with elevated 87Sr/86Sr isotope ratios and lower εNd values and subducting it beneath the arc, where it mixes with and alters the composition of the mantle wedge, i.e. subduction erosion. If the elevated 87Sr/86Sr isotope ratios and lower εNd values observed in the arc front lavas from the NSVZ were due to crustal contamination of the mantle source from subduction erosion, it can reasonably be expected the isotopic evidence for crustal contamination would be observable in the retro-arc basalts of the NSVZ, located a few tens of kilometers west of the arc front. Conversely, if the isotopic evidence for crustal contamination were due to assimilation of thicker, older crust beneath the arc, then one would not expect to see similar isotopic evidence for crustal contamination in the retro-arc basalts. The retro-arc basalts from this study have 87Sr/86Sr isotope ratios of 0.7037 – 0.7043, which is statistically different than 87Sr/86Sr isotope ratios found in arc lavas of the NSVZ (87Sr/86Sr = 0.7046-0.759). Similarly, εNd values retro-arc basalt samples from this study (εNd = 1.4 to 4.4) is statistically different than εNd values found in arc lavas of the NSVZ (εNd = -1.8 to -0.19). This evidence suggests that the mantle beneath the NSVZ does not contain the elevated 87Sr/86Sr isotope ratios and lower εNd values observed in arc lavas of the NSVZ. Therefore, subduction erosion cannot be the driving mechanism for crustal contribution to ascending magmas in the NSVZ. Instead, assimilation of crustal material within the older, thicker crust beneath the NSVZ must be the driving force for elevated 87Sr/86Sr isotope ratios and lower εNd values observed in NSVZ arc lavas.