MAGMATIC AND SEISMIC EVIDENCE FOR DELAMINATION OF CONTINENTAL LITHOSPHERIC UNDER THE CERRO GALAN IGNIMBRITE CENTER IN THE SOUTHERN PUNA OF THE CENTRAL ANDES

The Andean southern Puna plateau has been proposed as a site of delamination (foundering) of thick continental crust and lithosphere over a steepening subduction zone leading to the generation of the large volume 6 to 2 Ma backarc Cerro Galán (68–71% SiO₂) ignimbrites and nearby < 7 Ma mafic lavas. The proposal is consistent with the chemical and mineralogical characteristics of the ignimbrites, which can be interpreted as reflecting a combination of lower, middle and upper crustal processes, and can be explained as ~50:50 mixtures of enriched mantle (⁸⁷Sr/⁸⁶Sr~ 0.7055) and crustal (⁸⁷Sr/⁸⁶Sr ~ 0.715–0.735) melts. Evidence for a thick crust in the region at 7-6 Ma before delamination began comes from ~ 9-8 Ma andesitic lavas with Sm/Yb ratios > 5 that must have equilibrated with residual garnet in a thickened crust. Other evidence for delamination comes from new tomographic images generated with data from the 2007-2009 southern PUNA passive seismic experiment, which show an ~ 100 km long high Vs, Vp and Qp anomaly above the subducting Nazca plate, east of Cerro Galán that can be interpreted as a delaminated block. Evidence for a modern crust as thin as ~45 km under the Puna near Cerro Galán comes from P to S and S to P receiver function images, which also indicate a continental lithospheric base at ~70-80 km, a depth that is consistent with equilibration depths for young mafic lavas based on their chemistry and mineralogy. The seismic images also show that the youngest southern Puna magmatic rocks are concentrated above gaps in well-located earthquakes in the subducting slab and in zones of low Vs, Vp and Qp crustal and upper mantle anomalies, interpreted as regions of partial melt. A one day seismic swarm at < 15 km under the Cerro Galán resurgent dome could suggest a potential for continued activity at Cerro Galán.