MAGMATIC AND SEISMIC EVIDENCE FOR NEOGENE CRUSTAL AND MANTLE EVOLUTION UNDER THE PUNA PLATEAU OF THE CENTRAL ANDES

Crustal and mantle lithospheric delamination along with forearc subduction erosion have become popular mechanisms for explaining processes operating along continental margin orogenic belts and have raised questions of the balance of creation and destruction of continental crust at active subduction margins. Evidence for changes in subduction zone geometry and crustal loss though delamination of thickened dense lower crust beneath the Central Andean Puna plateau comes from petrologic and geochemical studies of <18 Ma magmatic rocks linked with seismic studies. Calculations of crustal loss and new crustal growth rates based on magmatic chemistry and seismic imaging suggest that the Puna is an area of net crustal loss for at least the last 10 Ma, with up to 10% or more of the lower crust removed in that time. A specific example of crustal loss and gain under the southern Puna comes from the chemistry (e.g., Sm/Yb, La/Ta, Ba/La, Nd and Sr isotopes) of Neogene volcanic rocks, which support an 18 to 7.5 Ma period of slab shallowing, followed by slab steepening and forearc subduction erosion linked with backarc lithospheric delamination and large scale ignimbrites. Support for backarc crustal delamination comes from seismic attenuation and Vs tomographic images that reveal an ~100 km wide high velocity anomaly associated with an irregular shear wave splitting pattern, which is interpreted as a delaminated block above a nearly aseismic segment of the subducting Nazca slab at a depth of ~150-200 km. This block underlies the < 7 Ma Cerro Galan ignimbrites and bordering mafic lava flows and glassy andesites and dacites erupted to the west, north and south of the ignimbrites. The chemistry and mineralogy of these surrounding flows support equilibration of the basaltic magmas at > 1350°C at ~2 GPa followed by fractionation and mixing with melts of garnet-pyroxene bearing crust to produce the andesites and dacites. In accord, these lavas along with the Cerro Galan ignimbrites lie above a region where receiver functions indicate a lithosphere-asthenosphere boundary at ~60-80 km that lies below a regionally thin 45-55 km thick crust with a low Vp/Vs (< 1.70) ratio.