Crustal reworking during a long-lived magma pulse: 11 m.y. isotopic record from the Aucanquilcha Volcanic Cluster, central Andes
Abstract
Since ~11 Ma, successive eruptions from the Aucanquilcha Volcanic Cluster (AVC) in northern Chile document the magmatic evolution of a long-lived subduction system. Situated within the central volcanic zone of the Andes, the AVC is constructed upon remarkably thick (~70 km) crust—a heterogeneous filter through which all central Andean lavas are extensively processed and modified. The 11 m.y. history of the AVC is characterized by sluggish eruption rates from ~11-5 Ma, with an increase in eruptive output between ~5-2.5 Ma, and a return to modest eruption rates from ~2.5 Ma to present. This pattern is attributed to the waxing, climactic, and waning stages of a magmatic ‘pulse’. Eruptive pulsing in the form of long-lived magmatic systems appears to be not uncommon (cf. APVC, Tuolumne, SRMVF), and we exploit the AVC lavas to explore the geochemical signal accompanying the evolution of such a system. More specifically, isotopes (whole rock Sr, Nd, Pb; O from plagioclase) and trace elements of the AVC lavas are employed to investigate the compositional influence of the crustal filter on the production of arc lavas. 87Sr/86Sr of AVC andesite to dacite lavas ranges from 0.70509 to 0.70680, with a broad increase through time. Three analyses from nearby, recently erupted basaltic andesite scoria cones yield relatively high ratios of 0.706347 - 0.706826. 143Nd/144Nd ranges from 0.512262 - 0.512590 (scoria cones: 0.512300 - 0.512323), and decrease through time, consistent with the Sr data. δ18O ranges from 6.47 to 7.47, with the lowest values associated with the onset of AVC volcanism. 206Pb/204Pb ranges from 18.4679 to 18.7039, with a small, but distinguishable, increase through time. Dy/Yb ranges from 1.79 - 3.45 and Sm/Yb ranges from 2.18 - 6.66, with a marked increase from 11 Ma to present. The AVC is situated on the boundary between two distinct Pb domains (Arequipa and Antofalla) of the central Andean crust. The minor fluctuation seen in Pb isotopes through time likely results from the tapping of these two different (and probably stacked) crustal Pb reservoirs. Trends in the Sr and Nd (and to a lesser extent, O) isotopic ratios suggest an increase in the crustal influence (or changing crustal source) with progressive magmatism at the AVC. Increasing Dy/Yb and Sm/Yb (MREE/HREE) ratios through time indicate an increasing role of garnet in the residuum of crustal melting, suggesting an increasing crustal thickness with time. Despite the overall trend toward more crustally contaminated compositions through time, there is a focusing of the isotopic values (at ~low Sr and ~high Nd) during the eruptive flare-up from ~5-2.5 Ma. During this time of high mantle flux, the crustal isotopic signature is likely being diluted in the lavas by mantle components. As the influx from the mantle wanes, the crustal signature becomes more pronounced with successive eruptions. In this sense, the balance between mantle power and thermal maturity of the crust, in addition to crustal character, bears heavily on the compositions of the lavas and the reworked arc crust.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V14A..03W
- Keywords:
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- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 8178 TECTONOPHYSICS / Tectonics and magmatism;
- 8413 VOLCANOLOGY / Subduction zone processes;
- 8439 VOLCANOLOGY / Physics and chemistry of magma bodies