The spatial distribution of some major and trace element and isotopic characteristics of backarc Plio‐Quaternary basaltic to high‐Mg andesitic (51% to 58% SiO2) lavas in the southern Puna (24°S to 27°S) of the Central Andean Volcanic Zone (CVZ) reflect varying continental lithospheric thickness and the thermal state of the underlying mantle wedge and subducting plate. These lavas erupted from small cones and fissures associated with faults related to a change in the regional stress system in the southern Puna at ≈ 2 to 3 Ma. Three geochemical groups are recognized: (1) a relatively high volume intraplate group (high K; La/Ta ratio <25) that occurs over a thin continental lithosphere above a gap in the modern seismic zone and represents the highest percentage of mantle partial melt, (2) an intermediate volume, high‐K calc‐alkaline group ( La/Ta ratio >25) that occurs over intermediate thickness lithosphere on the margins of the seismic gap and behind the main CVZ and represents an intermediate percentage of mantle partial melt, and (3) a small‐volume shoshonitic group (very high K) that occurs over relatively thick continental lithosphere in the northeast Puna and Altiplano and represents a very small percentage of mantle partial melt. Mantle‐generated characteristics of these lavas are partially overprinted by mixing with melts of the overlying thickened crust as shown by the presence of quartz and feldspar xenocrysts, negative Eu anomalies (Eu/Eu < 0.90; most < 0.80), and radiogenic Sr (> 0.7055) and Pb and nonradiogenic Nd ( εNd < −0.4) isotopic ratios. Mixing calculations show that the lavas generally contain more than 20% to 25% crustal melt. The eruption of the intraplate group mafic lavas, the change in regional stress orientation, and the high elevation of the southern Puna are suggested to be the result of the late Pliocene mechanical delamination of a block (or blocks) of continental lithosphere (mantle and possibly lowermost crust). The loss of this lithosphere resulted in an influx of asthenosphere that caused heating of the subducting slab and yielded intraplate basic magmas that produced extensive melting at the base of the thickened crust. Heating of the subducting slab led to formation of the seismic gap and trenchward depletion of the slab component. Backarc calc‐alkaline group lavas erupted on the margins of this delaminated block, whereas shoshonitic group lavas erupted over a zone of relatively thick nondelaminated lithosphere to the north.

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An array of 74 seismological stations was deployed in the Argentine Puna and adjacent regions for a period of two years. The aim is to investigate the seismic structure in the crust and upper mantle in order to address fundamental questions regarding the processes that form, modify and destroy continental lithosphere and control lithospheric dynamics in this part of the Central Andes. This portion of the Central Andes is an ideal locale to address these questions given that there is geologic evidence that there has been recent lower crustal and mantle lithospheric delamination. 2We performed a teleseismic P wave tomography study using seismic events at both teleseismic

Understanding the sources of Ordovician magmatic rocks in the broad western Faja Eruptiva Occidental and eastern Faja Eruptiva Oriental magmatic belts in the northern Puna of Argentina and Chile is important to Ordovician geodynamic models for westernGondwana evolution and Gondwana-Laurentian terrane interactions. A critical evaluation of existing chemical and age data, along with new major trace element data, and field observations leads to a working model in which magmatism in the western belt progressively occurred in an active to waning arc to collisional regime, whereas that in the eastern belt occurred in an oblique fault regime transitional to a subduction zone to the south. The model is hampered by data gaps in all areas, and particularly by lack of ages in western and southern regions. The best understood area is the northern Faja Eruptiva Oriental where dacitic and mafic units with published ages of 476-467 Ma occur in linear fault-controlled trends. Their chemistry is consistent with dacitic magma sources being dominated by sedimentary-type protoliths melted in association with emplacement of mantle-derived alkaline mafic magmas. These dacitic units represent lava-dome complexes emplaced contemporaneously with outer shelf and slope basin sedimentation. Farther south where deeper crustal levels are exposed, volcanic/subvolcanic units grade into poorly studied plutonic facies. Trondhjemitic bodies are reported to the east. Contemporaneous units in the Faja Eruptiva Occidental are plutons and mafic to rhyolitic lavas in sequences of volcaniclastic sediments. Chemical signatures in the poorly dated Cordón de Lila lavas and Choschas dioritic to leucogranodioritic pluton in Chile are consistent with emplacement in a magmatic arc on thinned continental or oceanic crust. Mafic volcanic units in volcanic-sedimentary sequences just to the east could represent arc volcanism behind the front. An Arenig-Llanvirn change to bimodal and predominantly silicic magmatism, the emplacement of shoshonitic and mafic andesitic flows with weak arc signatures followed by alkaline dikes in the eastern Faja Eruptiva Occidental, and the formation of the Faja Eruptiva Oriental dacitic-mafic sequences could signal a change to a very oblique subduction regime. This change would be roughly contemporaneous with the arrival of the Laurentia-derived Precordillera terrane to the south. A post-Arenig-Llanvirn compressional regime seems required to explain Ocloyic deformation in the east and late plutons to the west. The nature of the eastern boundary of a periGondwana Arequipa terrane and the existence of a peri-Gondwana Famatina-Puna Oriental terrane remain unclear.

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