The geochemistry of a dying continental arc: the Incapillo Caldera and Dome Complex of the southernmost Central Andean Volcanic Zone (~28°S)
Abstract
The Pleistocene Incapillo Caldera and Dome Complex (5,570 m) marks the southernmost siliceous center of the Andean Central Volcanic Zone (~28°S), where the steeply dipping (~30°) segment of the subducting Nazca plate transitions into the Chilean "flatslab" to the south. The eruption of the Incapillo Caldera and Dome Complex began with a 3-1 Ma effusive phase characterized by ~40 rhyodacitic dome eruptions. This effusive phase was terminated by an explosive "caldera-forming" event at 0.51 Ma that produced the 14 km3 Incapillo ignimbrite. Distinctive and virtually identical chemical signatures of the domes and ignimbrites (SiO2 = 67-72 wt%; La/Yb = 37-56; Ba/La = 16-28; La/Ta = 30-50; 87Sr/86Sr = 0.70638-0.70669; ɛ Nd = -4.2 to -4.6) indicate that all erupted lavas originated from the same magma chamber and that differentiation effects between units were minor. The strong HREE depletion (Sm/Yb = 6-8) that distinguishes Incapillo magmas from most of the large ignimbrites of the Altiplano-Puna plateau can be explained by the extent and degree of partial melting at lower crustal depths (>40 km) in the presence of garnet. At upper crustal depths, this high-pressure residual geochemical signature, also common to adjacent late Miocene/Pliocene Pircas Negras andesites, was partially overprinted by shallow-level assimilation and fractional crystallization processes. Energy-constrained AFC modeling suggests that incorporation of anatectic upper crustal melts into a fractionated "adakite-like" dacitic host best explains the petrogenesis of Incapillo magmas. The diminution of the sub-arc asthenospheric wedge during Nazca plate shallowing left the Incapillo magma chamber unreplenished by both mafic mantle-derived and lower crustal melts and thus stranded at shallow depths within the Andean crust. Based on its small size and distinctive high-pressure chemical signature, the Incapillo Caldera and Dome Complex provides an endmember model for an Andean caldera erupting within a waning magmatic arc over a shallowing subduction zone.
- Publication:
-
Contributions to Mineralogy and Petrology
- Pub Date:
- January 2011
- DOI:
- 10.1007/s00410-010-0523-1
- Bibcode:
- 2011CoMP..161..101G
- Keywords:
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- Central Andes;
- Dacite;
- Adakite;
- EC-AFC;
- Ignimbrite;
- Radiogenic isotopes;
- Incapillo