Beatriz Coira

Young mafic back arc volcanic rocks as indicators of continental lithospheric delamination beneath the Argentine Puna Plateau, central Andes

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 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 0.7055) and Pb and nonradiogenic Nd ( eNd < −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.

UPPER CENOZOIC MAGMATIC EVOLUTION OF THE ARGENTINE PUNA—A MODEL FOR CHANGING SUBDUCTION GEOMETRY

The spatial and temporal distribution and chemistry of Late Oligocene to Recent Central Andean Puna volcanic rocks can be broadly explained by temporal changes in the dip of the subducting Nazca pl...

Magmatic sources and tectonic setting of Gondwana margin Ordovician magmas, northern Puna of Argentina and Chile

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 western Gondwana 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 Cordon 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.

Paleomagnetism of upper Miocene ignimbrites at the Puna: An analysis of vertical‐axis rotations in the Central Andes

The origin of the rotations detected paleomagnetically in the Central Andes is controversial. Tectonic models proposed to explain it involve Late Cenozoic oroclinal bending (the Bolivian Orocline), and/or small-block rotations driven by oblique subduction. In this paper, we report paleomagnetic data from upper Miocene ignimbrites from the Puna, in the presumed southern limb of the orocline. These rocks showed high unblocking temperature and high coercivity magnetization. The resulting pole position (latitude 85.7°S, longitude 80.5°E, A95 = 7.9°, K = 22, N = 17) indicates that the study area has not been significantly affected by vertical-axis rotation since late Miocene onward. This implies that the whole Puna has not undergone significant regional rigid-body rotation during the same time span. A farther analysis based on Neogene paleomagnetic data from 31 Andean localities distributed from 11°S to 31°S shows the presence of rotated and unrotated areas. Systematic counterclockwise rotations in Peru and northern Bolivia are observed, whereas clockwise rotations are present in southern Bolivia, northern Chile, and perhaps northwestern Argentina. No systematic rotations are found farther south. The overall time-spatial distributions of these Neogene paleomagnetic data suggest that if orogenic bending occurred, it must have taken place before the middle Miocene. Thus the oroclinal hypothesis cannot explain the rotations observed in middle Miocene rocks or younger ones, which also show orocline-like declination anomalies. We suggest that small-block rotations driven by distributed shear may be a single process that can account for the whole rotations in the Central Andes. It is hypothesized that horizontal shear in the Andean crust could be controlled by ancient structures.

Implications of Quaternary volcanism at Cerro Tuzgle for crustal and mantle evolution of the Puna Plateau, Central Andes, Argentina

The high-K Tuzgle volcanic center, (24° S, 66.5° W) along with several small shoshonitic centers, developed along extensional Quaternary faults of the El Toro lineament on the east-central Puna plateau, ≈275 km east of the main front of the Andean Central Volcanic Zone (CVZ). These magmas formed by complex mixing processes in the mantle and thickened crust (>50 km) above a ∼200 km deep scismic zone. Tuzgle magmas are differentiated from shoshonitic series magmas by their more intraplate-like Ti group element characteristics, lower incompatible element concentrations, and lower 87Sr/86Sr ratios at a given εNd. Underlying Mio-Pliocene volcanic rocks erupted in a compressional stress regime and have back-arc like calc-alkaline chemical characteristics. The Tuzgle rocks can be divided into two sequences with different mantle precursors: a) an older, more voluminous rhyodacitic (ignimbrite) to mafic andestitic (56% to 71% SiO2) sequence with La/Yb ratios <30, and b) a younger andesitic sequence with La/Yb ratios >35. La/Yb ratios are controlled by the mafic components: low ratios result from larger mantle melt percentages than high ratios. Shoshonitic series lavas (52% to 62% SiO2) contain small percentage melts of more isotopically “enriched” arc-like mantle sources. Some young Tuzgle lavas have a shoshonitic-like component. Variable thermal conditions and complex stress system are required to produce the Tuzgle and shoshonitic series magmas in the same vicinity. These conditions are consistent with the underlying mantle being in transition from the thick mantle lithosphere which produced rare shoshonitic flows in the Altiplano to the thinner mantle lithosphere that produced back-are calc-alkaline and intraplate-type flows in the southern Puna. Substantial upper crustal type contamination in Tuzgle lavas is indicated by decreasing εNd (-2.5 to-6.7) with increasing 87Sr/86Sr (0.7063 to 0.7099) ratios and SiO2 concentrations, and by negative Eu anomalies (Eu/Eu* <0.78) in lavas that lack plagioclase phenocrysts. Trace element arguments indicate that the bulk contaminant was more silicic than the Tuzgle ignimbrite and left a residue with a high pressure mineralogy. Crustal shortening processes transported upper crustal contaminants to depths where melting occurred. These contaminants mixed with mafic magmas that were fractionating mafic phases at high pressure. Silicic melts formed at depth by these processes accumulated at a mid to upper crustal discontinuity (decollement). The Tuzgle ignimbrite erupted from this level when melting rates were highest. Subsequent lavas are mixtures of contaminated mafic magmas and ponded silicic melts. Feldspar and quartz phenocrysts in the lavas are phenocrysts from the ponded silicic magmas.

Central Andean mantle and crustal seismicity beneath the Southern Puna plateau and the northern margin of the Chilean‐Pampean flat slab

Earthquake hypocenters recorded in the Andean Southern Puna seismic array (25–28°S, 70–65°W) provide new constraints on the shape of the subducting Nazca plate beneath the Puna plateau, the transition into the Chilean-Pampean flat slab and the thermal state of the mantle and crust. Some 270 new mantle hypocenters suggest that the subducting slab under the Puna shoals into the flat-slab segment more abruptly and farther to the north than previously indicated. The revised geometry is consistent with the Central Volcanic Zone Incapillo caldera being the southernmost center with Pleistocene activity until reaching the southern side of the flat-slab region. Evidence for the revised slab geometry includes three well-defined hypocenter clusters in the Pipanaco nest (27.5–29°S, 68–66°W), which are interpreted to reflect slab-bending stresses. A few low-magnitude earthquakes with strongly attenuated S waves in the long-recognized Antofalla teleseismic gap (25.5–27.5°S) support a continuous slab under the Southern Puna. The paucity of gap earthquakes and the presence of mafic magmas are consistent with a hot mantle wedge reflecting recent lithospheric delamination. Evidence for a hot overlaying Puna crust comes from new crustal earthquake hypocenters concentrated at depths shallower than 5 km. Two notable short-duration swarms were recorded under the resurgent dome of the ~2 Ma back-arc Cerro Galan caldera and the near-arc Cerro Torta dome. New crustal earthquake focal mechanisms from 17 events in the array along with two existing mechanisms have strike slip, oblique reverse, and oblique normal solutions fitting with regional E-W compression and N-S extension.

Peperitic textures of Ordovician dacitic synsedimentary intrusions in Argentina's Puna Highland: clues to emplacement conditions

Abstract Synsedimentary crystal-rich dacitic sills, laccoliths and cryptodomes emplaced in early Ordovician submarine thin-bedded silt/sandstone sequences show a variety of magma–water interaction features. Megapeperites occurring along the igneous contact contain angular dacitic megaclasts that are dispersed in dominantly structureless host sediment. They result mainly from quench fragmentation of magma and dynamic stressing of cooling magma bodies on contact with wet sediments, combined in some instances with fluidization of the sediments or minor explosive expansion of water incorporated by magma breaching the sediment–water interface or stopping of water-saturated roof sediments by shallowing intrusions. Globular peperites, having bulbous to globular dacitic clasts immersed in the host sediment with evidence of fluidization along the contact zone, are also recognized at the top and base of sills. In this case, a vapor film might have formed at the magma–sediment interface; insulating the magma from direct contact with the sediment and suppressing quench fragmentation and steam explosions. The diversity of peperite textures observed in the same intrusive and sedimentary rocks appears to be mainly the result of different levels of emplacement of the synsedimentary dacitic intrusions. Scarcity of peperites related to mafic sills is attributed to their having been emplaced late during the evolution of the basin.

The Scelidotheriine Proscelidodon (Xenarthra: Mylodontidae) from the Late Miocene of Maimará (Northwestern Argentina, Jujuy Province)

1Dpto. de Paleontología, Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA), CCT–CONICET–Mendoza, Avda.Ruiz Leal s/n, Parque Gral. San Martín, 5500 Mendoza, Argentina. fpujos@mendoza-conicet.gov.ar 2Institut Français d’Etudes Andines, Casilla 18-1217, Av. Arequipa 4500, Lima 18, Peru. fpujos@yahoo.fr 3CONICET, División Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque, B1900FWA La Plata, Argentina. acandela@fcnym.unlp.edu.ar, regui@fcnym.unlp.edu.ar 4Facultad de Ingeniería, Universidad Nacional de Salta, Salta, Argentina. claudiagalli@fibertel.com.ar 5CONICET, Instituto de Geología y Minería, Universidad Nacional de Jujuy, 4600 San Salvador de Jujuy, Argentina. bcoira2004@yahoo.com.ar 6Facultad de Ciencias Naturales y Museo de La Plata, Paseo del Bosque, B1900FWA La Plata, Argentina. delossreyes@yahoo.com.ar 7Laboratorio de Sistemática y Biología Evolutiva (LASBE), Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, La Plata, Argentina. mabello@ fcnym.unlp.edu.ar AMEGHINIANA 2012 Tomo 00 (0): xxx – xxx ISSN 0002-7014

Geochemical and isotopic constraints on Palaeozoic orogenic gold endowment and crustal evolution of the south central Andes, NW Argentina

Major and trace element concentrations, and Nd isotope compositions of Ordovician volcano-intrusive rocks in the Puna of northwestern Argentina point to their formation in a pericratonic setting above thickened and evolved continental crust. Input from juvenile sources was limited and there is no evidence for oceanic crust or the presence of a terrane-bounding suture in the region. In contrast to analogous terrains elsewhere in the Central Andes (e.g., in Peru and Bolivia), relatively minor orogenic gold mineralisation is known to occur in the Palaeozoic succession in northwestern Argentina. Possible explanations for the lack of more substantial orogenic gold endowment include the absence of hydrated oceanic substrate in the Puna, lack of 1st-order conduits and the limited extent of crustal shortening and thickening during Ordovician orogenic events.