The Campanorco Subvolcanic Complex (25 degrees 38 ' S-65 degrees 33 ' W) is a poorly known unit that comprises phenoandesitic sills and dikes located approximately 600 km east of the Nazca Plate subduction trench, an unusual setting for typical Andean back-arc magmatism. This contribution offers new petrographic, geochemical and geochronological data. U-Pb dating on zircon yielded crystallization ages of 6.8 Ma, confirming its belonging to Upper-Miocene back-arc magmatism. This igneous system consists of amphibole-bearing rocks of the high-K calcalkaline and shoshonitic series, related to an enriched mantle source, likely metasomatized by subductionderived fluids. Magmatic differentiation primarily occurred through fractional crystallization of water-rich parental magmas, with low crustal contamination, typical of steady-state andesites. Magmatic enclaves are widespread and are good physical evidence for the influence of multiple episodes of magma mixing on magma evolution. The formation of the Campanorco Subvolcanic Complex is attributed to slab shallowing during Middle to Late Miocene times, likely driven by the subduction of the Taltal and Copiapo aseismic ridges, which facilitated mantle hydration and magma generation far from the volcanic arc. This unit serves as a key indicator of the maximum extent of slab shallowing during this period in the segment between 24 degrees and 26 degrees S. The study of this system provides critical insights into the petrogenesis, tectonic setting, and mineralization potential of the easternmost Neogene back-arc magmatism in the Central Andes.

The Zapla Range (Jujuy province) represents the southern edge of the central Andean Subandean Ranges in Argentina. In the northernmost area of the Zapla Range, basaltic rocks are interbedded in the Mealla Formation. These volcanic units form part of the post-rift magmatism of the Salta rift basin, which mainly focused on the Lomas de Olmedo sub-basin to the east. In this paper, six of these basaltic occurrences are characterized on the basis of new geological, petrographic and geochemical data, in order to explore their emplacement, petrogenesis and possible tectonic setting during rift evolution. Peperites in the upper contact of sills evidence the interaction between basalts and unconsolidated or poorly consolidated wet sediments. This implies that there was concomitance between sedimentation and subsurface magma emplacement, suggesting a Paleocene age for the magmatic event according to zircon U-Pb data available for Mealla Formation rocks. Other basaltic units show hipocrystalline textures and lack peperites at the top, which points to their extrusive nature. Additionally, some of these lava flows are spatially associated with pyroclastic deposits that reveal the occurrence of explosive hydrovolcanic eruptions. The study rocks are classified as alkaline basalts and basanites with high Mg# and Niand Cr-rich compositions, close to primitive magmas. They show affinity with continental intraplate settings. The analyses of Ba/Nb, Nb/La, (La/Yb)N, N , (La/Sm)N, N , (Gd/Yb)N N and (Dy/Yb)N N ratios, as well as normalized trace and rare earth element patterns, suggests a genesis by low degrees of partial melting of a residual garnet-bearing mantle source. Therefore, basalts from this part of the Subandean Ranges evidence a magmatic episode probably linked to an extensional reactivation stage of the Salta Group rift system in the Lomas de Olmedo depocenter during the Paleocene (similar to 60 Ma).

The Huachichocana Subvolcanic Complex (HSC) is one of the easternmost Miocene magmatic complexes from the Central Andes back-arc region. This complex comprises three major sheet-like igneous bodies and contiguous minor intrusions of andesitic to dacitic rocks distinguished by their adakite-like high Sr and Sr/Y values. Based on new detailed data of this system, we explore the genesis of intermediate magmas in the far back-arc region of the Central Andes and complement work on similar rocks from the Eastern Cordillera. We add petrography and mineral chemistry studies and we report new whole rock geochemical and Sr–Nd isotopic data, and a U–Pb zircon age for the HSC. The results suggest that these intrusive rocks were derived from melting of a hydrated mantle source and that the adakite-like composition was acquired through amphibole-dominated fractionation at deep to moderate crustal pressures and by the suppression of early plagioclase crystallization in precursor magmas. The presence of mafic microgranular enclaves is frequent and, along with variations in the composition of phenocrysts and common disequilibrium textures, suggests a significant role of mixing between mafic and intermediate magmas during their ascent and storage in the crust. U–Pb radiometric dating yielded a concordia age of 8.343 ± 0.042 Ma for the andesites of the HSC, coinciding with the period of maximum shortening of the eastern border of the Puna plateau. Nevertheless, this igneous complex is located at the southern margin of the Lípez fault system, a zone of lithospheric weakness that should favor magma emplacement so far east of the trench.

In typical Andean arc magmas, amphibole appears as a phenocryst phase only after considerable differentiation. However, some near-primitive volcanic rocks (high-Mg andesites and basalts) from monogenetic centers in the Puna plateau of Argentina also contain amphibole phenocrysts, implying special conditions of hydrous magma generation in this back-arc setting. This study documents typical examples from Southern and Northern Puna regions and uses the major and trace-element compositions of amphibole to constrain a petrogenetic model for the hydrous magmas. There are significant differences in the nature of amphiboles and their host lavas depending on location of the volcanic centers in the Southern and the Northern Puna regions. In the Southern Puna, basaltic andesitic lavas have Sr/Y values>40 and amphiboles showskeletal forms and occur in an assemblagewith olivine and pyroxene. The amphibole compositions are relatively Al- and Ti-poor compared to the Northern Puna. Thermobarometry indicates amphibole crystallization temperatures of 960-1000 degrees C at moderate pressure (< 5 kbar). In contrast, the mafic lavas from centers in the Northern Puna show Sr/Y ratios lower than 20 and amphiboles in these rocks coexist with a plagioclase-orthopyroxene assemblage. The Northern Puna amphiboles have higher Ti and Al contents than those in the southern region and the thermobarometry estimates imply generally higher crystallization temperatures (>1000 degrees C) and pressures (6-8 kbar). Furthermore, the chemical composition of amphibole phenocrysts in the Northern Puna Campo Negro center suggests an alkaline affinity of the parentalmagmas which, together with radiogenic isotope data from earlier studies, indicates a significant contribution of the enriched lithosphere in the magma source. The new data collectively suggest high pressure evolution of hydrousmagmas in the Southern Puna, whereas the Northern Puna magmas underwent more differentiation at higher levels in the crust. This contrast in the evolution history ofmagmas belowboth regions can be connectedwith their position relative to partialmelting zones in the mid-upper crust, which are larger and longer-lived in the north than in the south, thus favoring a slower ascent of magmas in that region. (C) 2020 Elsevier B.V. All rights reserved.

Neogene back-arc mafic volcanism in the northern Puna Plateau produced a suite of mainly high-Mg, calc-alkaline, basaltic andesites to andesites that form small scoria cones and lava fields. We present the first comprehensive geochemical study of the mafic suite in the northern Puna that complements work on similar rocks from the southern Puna Plateau. The emphasis is on magma genesis and evolution in both areas, and on a combined interpretation of the two regional datasets in the geodynamic context of back-arc magmatism in the central Andes. The results from the northern Puna suite (bulk-rock and mineral compositions, thermobarometry and radiogenic isotope ratios) are consistent with a predominantly asthenospheric source for the mafic magmas, with variable but locally significant contamination by crustal material. Quantifying the crustal and mantle input fails in most cases because the data display contradictory features, such as high compatible element contents (Mg, Ni, Cr) paired with moderate contents of silica and incompatible lithophile elements that defy classical models of magma mixing, fractionation and assimilation. We suggest that magma evolution involved selective assimilation during turbulent flow, probably at more than one level in the crust. Comparison with the southern Puna mafic suite reveals many features in common (high magma temperatures, textural evidence of rapid magma ascent and cooling, assimilation of crust at different depths). However, the volume of erupted magma is greater in the south than in the north and the volcanism in the south is slightly younger. There is much compositional overlap between the two regions, but the southern Puna suite extends to more primitive compositions. These differences suggest a stronger crustal influence in the northern Puna andesites, which we suggest is due to the presence of an extended upper-crustal melt zone associated with the Altiplano-Puna ignimbrite province. Radiogenic Sr and Nd isotope data from both suites define two diverging trends of variation with MgO that can be explained with a crustal component common to both trends, similar to the silicic ignimbrites, and two contrasting mantle components. The more common and regionally more widespread of the two mantle components (also seen in the frontal arc magmas) has Sr-87/Sr-86 and Nd-143/Nd-144 values of 0 center dot 705 and 0 center dot 5126, respectively, which we attribute to an asthenospheric source enriched by subduction erosion. The less common of the two has Sr and Nd initial ratios (0 center dot 708 and 0 center dot 51235) that we attribute to melting or assimilation of enriched lithosphere. This component has been found only in the northern Puna and it may have an origin in delaminated lithosphere.

The Neogene mafic volcanism of the Northern Puna region in the Central Andes is represented by scoria cones and lava flows dispersed over a wide region (c. 9150 km(2)) as isolated or poorly clustered centres. Although all the products are basaltic andesites to andesites, the behaviour of these magmatic systems resembles that seen in basaltic monogenetic fields. These centres were studied with the aim of defining the main volcanic lithofacies and evaluating the eruptive styles. The results suggest that the eruptions developed under a dry strombolian dynamic, with brief periods of lava fountaining and hydrovolcanism, the latter usually restricted to the early stages of cone construction. Changes in eruptive style are thought to be caused by variations in both the internal (e.g. magma ascent) and external (e.g. surficial water availability) conditions. The transitions do not reflect compositional changes, as evidenced by the small chemical differences observed among the products of the studied eruptive centres. Stratigraphic analysis, in addition to a few pre-existing radiometric dates, suggests that this volcanic activity occurred during the Late Miocene to Early Pliocene. This information supports the inference that these eruptions occurred before the peak of Southern Puna mafic volcanism and that they were coeval with eruptions of some of the most important silicic calderas of the Altiplano-Puna Volcanic Complex. The good preservation of volcanic edifices reveals that erosion rates were extremely low, in agreement with the high aridity conditions that have prevailed in the Puna region since the Mid-to Late Miocene.

The Cerro Bitiche Andesitic Field (CBAF) is one of the two largest mafic volcanic fields in northern Puna (22–24° S) and is spatially and temporally associated with ignimbrites erupted from some central Andean Altiplano-Puna Volcanic Complex calderas. The CBAF comprises seven scoria cones and widespread high-K calcalkaline lava flows that cover an area of 200 km2. Although all erupted rocks have a relatively narrow chemical range (56–62 % SiO2, 3–6 % MgO), there is a broad diversity of mineral compositions and textures. The least evolved lavas (∼58–61 % SiO2) are high-Mg andesites with scarce (<10 %) microphenocrysts of either olivine or orthopyroxene. The small compositional range and low phenocryst content indicate evolution controlled by low percentages (<10 %) of fractional crystallization of olivine and clinopyroxene of magmas similar to the least evolved rocks from the field, accompanied by assimilation during rapid ascent through the crust. Evolved andesites (∼62 wt% SiO2), on the other hand, are porphyritic rocks with plagioclase + orthopyroxene + biotite and ubiquitous phenocryst disequilibrium textures. These magmas were likely stored in crustal reservoirs, where they experienced convection caused by mafic magma underplating, magma mixing, and/or assimilation. Trace element and mineral compositions of CBAF lavas provide evidence for complex evolution of distinct magma batches.

Fil: Maro, Guadalupe. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Salta. Instituto de Ecorregiones Andinas; Argentina

En este trabajo se pretende avanzar en el conocimiento acerca de la manufactura y circulación de vasijas de estilo Inca Pacajes o Saxamar.Para ello se pusieron a prueba dos estrategias analíticas (a) la petrografía de pastas en secciones delgadas (N = 33) y (b) el análisis químico por fluorescencia de rayos X (N = 18).Los resultados obtenidos de las muestras analizadas procedentes de sitios arqueológicos del noroeste argentino (NOA), sur de Bolivia y norte de Chile indicaron que la mayoría de las pastas conforman un mismo grupo composicional caracterizado por la presencia de abundantes inclusiones de pómez y trizas vítreas, aunque en las muestras del NOA se registraron las mayores diferencias composicionales.Se plantean alternativas para explicar dicha variación en un estilo con amplia distribución geográfica pero mínimamente representado en sitios de época incaica en el NOA.Palabras claves: Inca Pacajes, pastas cerámicas, análisis petrográfico, fluorescencia de rayos X.This paper aims to advance knowledge about the manufacture and circulation of Inca Pacajes or Saxamar style vessels.Two analytical strategies were tested: (a) fabric petrography in thin sections (N = 33), and (b) chemical analysis by X-ray Fluorescence (N = 18).The results obtained from the analyzed samples from archaeological sites in northwestern Argentina, southern Bolivia and northern Chile indicated that most of the fabrics are included in the same compositional group characterized by abundant fragments of pumiceous rock and volcanic glass, although more compositional differences were recorded in northwestern Argentine samples.Different alternatives to explain variation in a style with wide geographic distribution but minimally represented in northwestern Argentine Inca sites are provided.

This paper aims to advance knowledge about the manufacture and circulation of Inca Pacajes or Saxamar style vessels. Two analytical strategies were tested: (a) fabric petrography in thin sections (N = 33), and (b) chemical analysis by X-ray Fluorescence (N = 18). The results obtained from the analyzed samples from archaeological sites in northwestern Argentina, southern Bolivia and northern Chile indicated that most of the fabrics are included in the same compositional group characterized by abundant fragments of pumiceous rock and volcanic glass, although more compositional differences were recorded in northwestern Argentine samples. Different alternatives to explain variation in a style with wide geographic distribution but minimally represented in northwestern Argentine Inca sites are provided.

Tertiary mafic volcanism of the Jujuy Puna, the Cerros Negros de Jama. The Cerros Negros de Jama (23°29’ S – 66°56’ W) monogenetic volcanoes belong to a group of eruptive centers that are representative of the most mafic magmatism in the northern Puna during the Cenozoic. They comprise scoria cones and associated lava flows erupted during coeval strombolian and effusive volcanic activity.Aphyric to microporphyritic skeletal textures and microphenocryst assemblages dominated by olivine and/or orthopyroxene suggest inexistent or short residence times in supracrustal magma chambers, as well as magma temperatures higher than 1000º C and near to water saturation conditions. The frequent occurrence of magmatic quartz xenocrysts with different degrees of reaction suggests assimilation of silicic magmas/igneous rocks under variable P-T conditions. A combination of large ascent rates and strong turbulence, together with an overheating of the magmas would have been crucial for this in-route contamination process.The Jama volcanic rocks are basaltic andesites and andesites to trachyandesites which belong to the high-K calcalkaline series. The observed geochemical signature is typical of continental arc magmas, showing negative anomalies of Nb, Ta and P, La/Ta > 30 and Ba/Nb > 25. Two main groups of rocks with different evolution patterns can be distinguished from major and trace element diagrams. Different concentrations of some elements at intermediate degrees of evolution point to variable degrees of incompatibility for them, which in turn may have been caused by magma evolution at different depths from the same or different primary magmas.