Plantilla de artículo 2013
Andean Geology 49 (2): 208-237. May, 2022
Andean Geology
doi: 10.5027/andgeoV49n2-3377
Pliocene-Pleistocene ash-fall tuff deposits in the intermountain Humahuaca and Casa Grande basins, northwestern Argentina: tracers in chronostratigraphic reconstructions and key to identify their volcanic sources
*Beatriz Coira1, Claudia I. Galli1, 2, Suzanne Mahlburg Kay3,
Daniel F. Stockli4, Patrocinio Flores5, Emilio Eveling2

1 Instituto de Ecorregiones Andinas (INECOA), CONICET-Universidad Nacional de Jujuy, Avda. Bolivia 1661, (4600) S.S. de Jujuy, Argentina.

2 Facultad de Ciencias Naturales, Universidad Nacional de Salta, Avda. Bolivia 5150, 4400 Salta, Argentina.;

3 Departments of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY14853, USA.

4 Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, 2275 Speedway Stop C9000 Austin, TX 78712-1722, Texas, USA.

5 Instituto de Geología y Minería, Universidad Nacional de Jujuy, Avda. Bolivia 1661, (4600) S.S. de Jujuy, Argentina.

* Corresponding author:

Ash-fall tuffs of the Pliocene-Pleistocene deposits of Humahuaca and Casa Grande intermountain basins, northwestern Argentina, have been differentiated into two groups based on new geochemical and geochronological data which correspond to the tuffs of the Pliocene-Lower Pleistocene alluvial fan deposits dominated by debris flow, deep sandy gravel braided, and shallow ephemeral lake deposits (Uquia and Mal Paso formations), and those recorded in Pleistocene alluvial fans sheet flood deposits. The two clusters of ages recognized: 4.3 to 2.6 Ma, and 2.2-pre 0.8 Ma, corresponding to these tuff groups, are in agreement with pulses of ignimbrite eruptions in the Altiplano Puna Volcanic Complex (APVC), and Southerm Puna calderas located west of the study region. The ash-fall tuffs of both groups are mainly vitreous to phenocryst-poor of rhyodacite-dacite composition with minor andesites to trachyandesites, characterized by 58-69% SiO2 contents, A/CNK 1-1.4, FeO/MgO (0.8-2.8), which plot in the calc-alkaline range. They can be differentiated based on its immobile trace elements ratios as indicated by a slight enrichment in LREE, higher arc affiliation and somewhat higher Sm/Yb ratios in the case of the younger group, although in both Sm/Yb ratios are indicative of sequestration of HREE in residual hornblende. The new geochemical and geochronological data of those ash fall tuffs point to these as chrono-stratigraphic tracers of the Humahuaca and Casa Grande intermountain basins stratigraphy, during the Pliocene-Pleistocene, also as the key to identify their volcanic sources.

Keywords: Ash-fall tuffs, Pliocene-Pleistocene, Humahuaca-Casa Grande intermountain basins, Northwest-Argentina.



1. Introduction

In the Humahuaca and Casa Grande intermountain basins, Northwestern Argentina (Fig. 1), there are important records of ash-fall tuffs interspersed in their conspicuous Pliocene-Pleistocene sedimentary rock sequences. The characterization of these tuffs offer the possibility to analyze the use of these regional extended and well preserved ash-fall tuffs as chrono-stratigraphic tracers. In this study, we present new petrographic and geochemical (major and trace elements) data and geochronological age constraints from selected ash-fall tuffs from Uquía and Mal Paso formations and from Pleistocene alluvial fans sheetflood deposits outcropping in those basins.




Fig. 1. Geologic Map of the Humahuaca-Casa Grande basins. The red dots show the location of the studied profiles.



The sedimentary rocks of the Uquía Formation exposed in the Quebrada de Humahuaca (Fig. 1), records numerous taxons of fossil mammals belonging to different lineages that have been used to document major events associated with the Great American Biotic Interchange (GABI) in South America during Pliocene and Early Pleistocene (Reguero et al., 2007; Reguero and Candela, 2008), and become a crucial unit in the calibration of the Uquía South America Land Mammal Age (SALMA) (Marshall et al., 1982). This formation (Upper Pliocene-Lower Pleistocene) is also a carrier of well-preserved levels of tuffs widely distributed along the Quebrada de Humahuaca.

The Mal Paso Formation (Fernández et al., 1973), time-equivalent to the Uquía Formation (Fig. 2), also registers frequent tuff levels and has a large areal extent in the Casa Grande basin (Fig. 1). A decrease in the pyroclastic contribution to the Humahuaca basin is recorded in the Pleistocene alluvial fan deposits distributed in it.




Fig. 2. Lithostratigraphic column of the Humahuaca and Casa Grande intermoutain basins area. The sedimentary fill of both basins is shown and the similar stratigraphic position of the Uquia and Mal Paso formations. The formations that constitute the pre-Miocene basement of both basins are also indicated.


Those well-preserved pyroclastic layers, recorded in these Pliocene-Pleistocene sedimentary sequences, provides the opportunity to investigate their possible sources, as well as their applicability in stratigraphic correlations.

In the particular case of the Uquia Formation, considering the central role it has had in the calibration of the Uquia South American Land Mammal Age: SALMA (Reguero et al., 2007), the study and characterisation of its numerous tuff records becomes important for correlation purposes to extend even beyond the Quebrada de Humahuaca and Casa Grande to Bolivia where similar fossil records have been recognised in the Incahuasi Viscachani and Ayo Ayo regions (MacFadden et al., 1993).

The new petrographic, geochemical and geochronological data determined for the selected ash-fall tuff deposits, combined with previously published studies of the Southern Central Andes ignimbrite centers (de Silva, 1989a, b; Wörner et al., 1992, 1994, 2000; Schröder and Wöerner, 1996; Lindsay et al., 2001; Schmitt et al., 2001; Mamani et al., 2010; Folkes et al., 2011; Kay et al., 2010, 2011; Salisbury et al., 2011, and references therein) are used to analyze possible connections between those tuff and the ignimbrite flare-up that took place during Pliocene-Pleistocene to the west-northwest in the Altiplano-Puna Volcanic Complex (APVC, de Silva,1989a), and to the southwest in southern Puna.

2. Geological setting of the Pliocene-Pleistocene sedimentary rock sequences and its ash-fall tuff deposits at Humahuaca and Casa Grande basins

The Uquía Formation crops out along the Quebrada de Humahuaca in the Eastern Cordillera, at an elevation of ~2800 m. The rocks of this Formation, defined by De Carles in Castellanos (1950), are gently folded and faulted and unconformable overlies (Fig. 2) the undifferentiated rocks of Puncoviscana Formation (Neoproterozoic-Lower Cambrian), Meson Group (Lower Cambrian-Upper Cambrian), Santa Victoria Group (Ordovician) and continental deposits of Pirgua Subgroup (Cretaceous) and Maimará Formation (Upper Miocene), (Fig. 2). The Uquía Formation is made up of ~70-300 m of sandstones, siltstones and conglomerates and intercalated whitish, grayish and white-brownish ash-fall tuffs of variable thicknesses (0.10-2 m). These deposits accumulated in a braided river system associated with alluvial fans and in some areas, transitioning into lake accumulations (Galli et al., 2012, 2017; Herrera Oviedo et al., 2017).

Based on paleomagnetic and geochronological studies of the Uquía Formation, ash-fall tuffs in the Esquina Blanca locality were assigned an age of   2.86 Ma (Marshall et al., 1982). Moreover, Walther et al. (1998) determined a zircon fission track age of 2.5 Ma for a dacitic tuff in the laminated mudstone and siltstone deposits, of the upper Uquía Formation in the San Roque profile (Fig. 3), like the 2.55 Ma 40Ar/39Ar age. In addition Pingel et al. (2013) dated tuffs interbedded with sandstones deposits in the Incahuasi sector in 4.3 to 3.52 Ma (U-Pb zircon ages) and those interstratified with conglomerates in the Quitiacara profile (Fig. 3) in 4.38-4.24 Ma (U-Pb zircon ages).




Fig. 3. Stratigraphic correlation of Pliocene-Pleistocene sedimentary deposits of Humahuca and Casa Grande basins, and their pyroclastic levels.



 South of the town of Tilcara (Fig. 1), in the Quebrada de Humahuaca, Pingel et al. (2013) defined the Tilcara Formation which includes conglomerate and fanglomerate deposits interstratified with sandstone and some pyroclastic beds. It reaches a thickness of ~250 m and unconformably overlies the Maimará Formation rocks. The Tilcara Formation, appears to be contemporaneous with the Uquía Formation, based on tuffs that yield U-Pb zircon ages between 4.2 to 2.5 Ma (Pingel et al., 2013).

In the Alonso locality, on the east side of the Quebrada de Humahuaca (Fig. 1), outcrops a thick succession (~100-300 m) of alluvial fan sheet flood deposits containing ash fall tuff levels (Fig. 3) giving U-Pb ages of 2.21 Ma (Pingel et al., 2015) and 2.08 Ma (this paper). It is bounded by clear unconformities at the base with Precambrian or Paleozoic-Mesozoic basement and at the top with upper Pleistocene deposits, dated at 0.87 Ma (U-Pb zircon age, Streit et al., 2015) in the El Molino profile (Fig. 3).

The ash-fall tuffs in the Mal Paso Formation yielded in Casa Grande basin U-Pb zircons ages (Streit et al., 2015) of 3.74±0.4 Ma (northern basin) and 2.95 ±02 Ma and 2.13±0.08 Ma (central basin). In this basin, the Mal Paso Formation unconformable overlies different units, including the Casa Grande, Río Grande and Pisungo Formations as well as Cretaceous deposits of the Salta Group (Fig. 2). Ages for the tuffs from these Pliocene-Pleistocene sequences previously described fall into two distinct clusters corresponding to eruptive pulses, with one ranging from 4.3 to 2.6 Ma in the case of Uquía and Mal Paso Formations tuffs and one giving ages from 2.21 to 2.10 Ma, which corresponds to tuffs in alluvial fans sheet flood deposits.

3. Methodology

In order to evaluate the use of the regional extended and well preserved Pliocene-Pleistocene ash-fall tuffs in the Humahuaca and Gasa Grande basins as chronostratigraphic tracers and to analyze their potential sources, we proceeded to investigate its geochronological, mineralogical, petrographic and geochemical characteristics.

Ash-fall tuffs were sampled from stratigraphic sections studied in detail (1:500 or 1:100 scale) at Esquina Blanca South, Esquina Blanca North, San Roque, Incahuasi, Quitiacara, Molinos and Alonso in the Humahuaca Basin and at Quebrada Salida and Casa Grande river in the Casa Grande Basin (Fig. 3, Table 1).

We carried out careful field observations and stratigraphic section measurements of the volcano-sedimentary sequences in order to collect tuff samples that showed depositional features indicative of either primary air-fall deposition (e.g., homogeneous deposits with few or no internal structures or with normally graded stratification, in some cases reversely graded) or syn-eruptive re-sedimentation (e.g., compositionally uniform layers or with systematically varying structures that indicate rapid deposition, texturally unmodified juvenile clasts, paucity of lithic clasts, and homogeneity in phenocrystals, according to McPhie et al., 1993). Deposits with evidence of post-eruptive sedimentary mixture, reworking and incorporation of previously lithified ash layers were excluded. Detailed microscopic, petrographic and textural analysis of the collected samples was carried out looking for select the ash fall tuffs free of alteration, homogeneous mineral phases and lacking mixed-in vitroclasts.

The selected samples, as whole rock, have been analyzed by X Ray fluorescence (XRF), Neutron activation (INAA), ICP-AES and ICP-MS. The XRF analyses of major and trace elements were performed at the Laboratory of the Institute of Geology and Mining, University Nacional de Jujuy on a Rigaku FX 2000 spectrometer with Rh tube. For major element analysis homogenized samples were pulverized and fused with lithium tetraborate flux. Ba, Sr, Rb, Zr, Nb, Hf, Y, Th and U determinations were performed on rock powder pellets mixed with methacrylate and pressed at 20 t. Operating conditions were 50 kV and 45 mA. Major and trace elements were calibrated against standards from US Geological Survey and Japan Geological Survey. Trace elements compositions were analyzed by INAA in the Ward Laboratory at Cornell University following the methods described by Kay et al. (1987) and by ICP-AES and ICP-MS at ALS Laboratories, employing the ultra-trace method described by ALS Geochemistry (2019) (For all geochemical data see Table 2A and 2B).

40Ar/39Ar analysis on biotite separates from a sample (SR 13b) in the middle section of the stratigraphic profile of San Roque (Fig. 3) was completed at the Geochronology Laboratory of University of Alaska Fairbanks. The samples were irradiated in reactor of McMaster University in Hamilton, Ontario, Canada for 20 megawatt-hours. Taylor Creek Rhyolite standard (TCR-2: 28.619 Ma - Renne et al., 2010) was co-irradiated and used as a neutron flux monitor and to calculate the irradiation parameter (J) for all samples. Upon their return from the reactor, the sample and monitors were loaded in an ultra-high vacuum extraction line. The monitors were fused, and samples were step-heated, following the technique described in Benowitz et al. (2014). The evolved gases were analyzed using a VG-3600 mass spectrometer at the Geophysical Institute of University of Alaska Fairbanks. The argon isotopes measured were corrected for system blank and mass discrimination, as well as calcium, potassium and chlorine interference reactions following procedures outlined in McDougall and Harrison (1999).

Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb dating of zircon from sample MN2 from the Alonso profile (Fig. 3) were carried out the UTChron Laboratory at the University of Texas. We determined the ages of 40 unpolished zircon grains by zircon U-Pb depth profile analysis using a single collector Element2 HR-ICP-MS with attached Analyte2 Photon Machine Excimer laser, employing a laser spot of 30 μm with 10 Hz frequency and a fluence of 6 J/cm2. GJ1 was used as primary zircon standard (206Pb/238U 601.7±1.3 Ma) and interspersed with unknown analyses at a 1:4 ratio for elemental and depth-dependent fractionation. Data reduction was performed using the IgorPro (Paton et al., 2010) based Iolite 3.4 software with Visual Age data reduction scheme (Petrus and Kamber, 2012). A Stacey and Kramer (1975) common Pb correction was applied in Tera-Wasserburg concordia space using IsoPlot R software (Vermeesch, 2018). While showing an inherited component of Cambrian (~15%) and Miocene (<10%) zircons.

4. Results

4.1. Petrography and mineralogy of ash fall tuffs

Tuffs from the Pliocene-Pleistocene Humahuaca and Casa Grande basins, investigated for this study, are characterized for vitro-clastic textures, showing bubble-wall glass shards with cuspate or tubular shapes (0.1 to 1 mm in diameter) and scarce pumice shards (1.4 to 0.7 mm in diameter), immersed into a fine vitreous mesostasis. In the Uquía and Mal Paso Formations, the vitreous tuffs are rich in phenocrysts (5-25% crystals) largely devoid of lithics (<2%). Phenocrysts generally are angular and consist of andesine to oligo-andesine, quartz, brown biotite and green hornblende and as accessories magnetite, zircon and minor titanite.

The young group of tuffs, interspersed in alluvial fans, sheet flood deposits, are vitreous (1-5% crystals) and some of them rich in phenocrysts (20-25% crystals). Phenocrysts are angular to subangular and consist of andesine to calcic andesine, quartz, biotite, and hornblende. A distinct and distinguishing feature is the presence of clinopyroxene-orthopyroxene, mainly as accessory phase, as well as magnetite, zircon and minor titanite.

4.2. Characteristics of the ash fall tuff deposits and associated clastic facies

The studied ash fall deposits display a homogenous internal texture or normally graded stratification, in some cases laminated, exhibit flat to irregular bases, in cases filling alluvial channels and use to show signs of reworking on tops or they are covered by post-eruptive reworked previously lithified ash layers They have variable thickness 0.10 to 1.5 m and use to be tabular or lenticular (Fig. 4 and Table 1).




Fig. 4. A. Basal level of the normal graded stratified tuff in the southern part of the Casa Grande basin (Quebrada De Salida), Gmm facies (matrix-supported, massive conglomerate with a solid sandy and muddy matrix. Plastic debris flow). B. Massive tuff filling channel in Casa Grande river profile (3.74±0.04 Ma), Gmc facies pseudoplastic debris flow. C. Tuff level in the center of Casa Grande basin (2.95±0.02 Ma). CH channel. In the inset, detail showing a massive tuff on top of a laminated reworked tuff. D. Massive level of tuff in Alonso profile, irregular basal contact with fine-grained deposits accumulated in the floodplain (Fm. Facies: massive multi-coloured kaolinic clays with mud-cracks beds) later modified by desiccation. E. Tuff in Incahuasi south profile with flat base on top of Smc facies(medium- to very coarse-grained sandstones; moderately- to well-sorted; massive) F. Normal graded stratified level of tuff, laminated to the roof overlying Smc facies gravity flow deposits of high flatbed regime and single dynamic events such as flash floods. Incahuasi south profile. G. Massive tuff level in the Quitiacara profile with flat base and roof. Ghi facies (fining-upward clast-supported conglomerate with a sandy matrix and imbricated clasts), high energy deposits, longitudinal bars. H. Alternating lapilli and ash levels with irregular contacts. San Roque profile.


Ash-fall tuffs are intercalated into matrix to clast-supported massive conglomerates with sandy and muddy matrix, in cases imbricated associated to medium-to very coarse-grained sandstones which correspond to alluvial fan deposits dominated by debris flow and deep sandy gravel braided, as is observed in Casa Grande, Incahuasi, Quitiacara and Molino profiles. In other cases as in Alonso and San Roque sections they are interspersed between medium to coarse-grained sandstones; moderately to well-sorted and laminated to massive mudstones and siltstones which correspond to floodplain and shallow ephemeral lake deposits (Fig. 5).




Fig. 5. Enlarged partial sections of stratigraphic columns from figure 3 showing detail of ash-fall tuffs and host sediments. T (ash-fall tuff), Fl (clayey siltstone laminated), Gmm (matrix-supported, massive conglomerate with a solid sandy and muddy matrix), Gmi (clast-supported conglomerate with imbrication ), Gmc (clast-supported conglomerate, massive), Ghi (fining-upward clast-supported conglomerate with a sandy matrix and imbricated clasts), Po (mudstones to very fine-grained sandstones; massive). The code to the right of the tuff level indicates the sample number.


4.3. Geochemical characterization

Major and trace element geochemical analyses were performed on whole-rock of dated ash-fall tuffs samples from the studied stratigraphic sections in the Pliocene-Pleistocene deposits (Fig. 3) in order to constrain their sources and evaluate them as potential geochemical tracers to stablish stratigraphic correlations. Results of whole-rock and trace elements analyses are listed in table 2A and 2B. As some samples show high loss of ignition (LOI) values, only immobile elements were chosen for their characterization and interpretation. For the classification of the tuffs we used Zr/TiO2 versus Nb/Y diagram (Fig. 6) (Winchester and Floyd, 1977). The tuff samples mainly plot in the rhyodacite-dacite field with some scattered in the andesite and trachy-andesite fields. Compositionally, they are peraluminous bordering metaluminous (A/CNK=1.05-1.39). The Uquía and Mal Paso Formations tuffs (old group tuffs) show a homogeneous and distinct composition. They are characterized by 58-70% anhydrous SiO2, 2.2-3.5% Na2O and 1.0-4.8% K2O contents. With FeO/MgO (1.0-1.8) they are in the calc-alkaline range (Miyashiro, 1974) and show TiO2 contents of 0.2-0.6%. The young group of tuffs, intercalated in Pleistocene alluvial fans sheet flood deposits, also form a compositional coherent group. They have 61-69% anhydrous SiO2, 2.2-3.1 % Na2O and 1.5-3.0% K2O contents. Their FeO/MgO (1.8-2.8) puts them in the calc-alkaline range Miyashiro (1974) with a TiO2 contents are 0.3-0.7%.




Fig. 6. Plot of Zr/TiO2 versus Nb/Y from Winchester and Floyd (1977) for whole rock analyses (Table 2A, B) of tuff samples from the Uquía and Mal Paso formations (red circles) and from Pleistocene alluvial fans sheet flood deposits (blue squares).


The major difference between the two groups lies in their trace elements contents. The young group of tuffs is distinguished from the old group by its highest ratios (Sm/Yb=2.3-4.2 versus 2.1-3.2; La/Yb=9-19 versus 15-24 and La/Ta ratios=22-44 versus 12-32 (Fig. 7). Moreover the old group shows a major enrichment in LIL, Th, U, and Pb, more pronounced P and Ti negative anomalies and higher Nb and Ta peaks than the young group (Fig. 8).




Fig. 7. Trace element plots showing some general chemical features of tuff samples from the Uquía and Mal Paso formations (red circles) and from Pleistocene alluvial fans sheet flood deposits (blue squares). Data from the table 2A and B.





Fig. 8. Representative trace element patterns of old (red circles) and young group of tuffs. (blue squares). Normalized to primitive mantle concentrations (Sun and McDonough, 1989).


4.4. 40Ar/39Ar and U-Pb zircon ages

Result of 40Ar/39Ar dating of sample SR13 b is summarized in table 3A (Appendix Table S1). The age spectrum of this sample provides a plateau age (Fig. 9A) with three or more consecutive gas fractions that represent at least 50% of the total gas release and are within two standard deviations of each other (Mean Square Weighted Deviation less than 2.5). Based on the isochron regression to initial 40Ar/36Ar there is no evidence this sample had any significant inherited 40Ar (Fig. 9B). The integrated age (2,563±28 ka) is within error of the plateau age (2,556±26 ka) (Fig. 9A) and the isochron age (2,558±52 ka). We prefer the plateau age of 2,556±26 ka because of the higher precision over the isochron age. In table 3A (Appendix Table S1) are listed ages quoted to the ±1 sigma level and calculated using the constants of Renne et al. (2010).

Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb dating of zircon from sample MN2, from the Alonso profile, mainly yielded young concordant zircons with a 206Pb/238U age of 2.08±0.04 Ma (MSWD=2.2) (Fig. 10, Appendix: Table S2).




Fig. 9. A. 40Ar/39Ar age spectrum for SR13b sample. Integrated age (ka)=2,563 ±28. Plateau age(ka)=2,550±26, MSWD=0.6, 99.2% 39Ar released. B. Isochron for SR13b sample. 40Ar/36Ar intercept 295.6 ±2.9 , MSWD=0.68. Isochron age 2,558 ±52 ka.





Fig. 10. Tera-Wasserburg diagram for the U-Pb data from MN2 sample. Best age 2.08 ±0.04 Ma (n=14) Regressed data are in gray.


5. Discussion

5.1. Palaeo-environmental conditions during ash deposition

The geological and paleontological evidences recognized by Reguero et al. (2007), Reguero, and Candela (2008) in their biostratigraphic studies of the Late Neógene deposits of the Quebrada de Humahuaca, suggest that during the Pliocene-Early Pleistocene, the area could have been a wide intermountain valley at 1,400-1,700 m, with a more humid environment than that of the present day and some wet-dry seasonality that permitted the coexistence of forest and open areas, scenario in which the studied ashes would have been deposited.

The sedimentary processes at that time in the Humahuaca and Casa Grande basins were dominated by alluvial fan systems with predominantly debris flow associated with a deep sandy gravel braided fluvial system, floodplains, and shallow ephemeral lacustrine deposition (Galli et al., 2017, 2019). The alluvial fans sheet flood dominated paleoenvironment conditions prevailed throughout the Pleistocene. In these environmental conditions, ash deposition took place covering the relief and being covered by those sedimentary facies, reflection of mass flood activity, which partially limited further erosion (Fig. 5). Although it is frequent to observe evidence of partial erosion and re-deposition by water, especially effective during rainy season. Ash-rich channel fill observe in some cases (Fig. 4b) indicates reworking and deposition during flash floods.

In some sector of the basins, as in San Roque section (Figs. 3, 5), pro-gradation of an alluvial fan into a lacustrine systemprovided better conditions for the preservation of pyroclastic material, which is interbedded with lacustrine/palustrine fine sandstone and mudstone containing gastropods and diatoms (Galli et al., 2017, 2019).

5.2. Eruptive pulses and correlations

Ages of the tuffs from the Pliocene-Pleistocene sequences previously described fall into two distinct clusters, the first includes Uquía and Mal Paso Formations tuffs ranging from 4.3 to 2.6 Ma, and the second one giving ages from 2.21 to 0.8 Ma, which corresponds to tuffs hosted in alluvial fans, sheet flood deposits. These two age clusters show (Fig. 11) correspondence with the eruptive pulses recognized by de Silva and Gosnold (2007) and with the stages of eruptions identify by Kern et al. (2016)within the Altiplano-Puna Volcanic Complex (APVC) flare-up.




Fig. 11. Volume-time relations plot of Altiplano-Puna volcanic complex and southern Puna ignimbrites for the time span of the pulses recognized in Humahuaca and Casa Grande basins.


In the lapse of time of the “old eruptive pulse” recognized in the studied region large ignimbrites were erupted to the west and northwest in the Altiplano-Puna Volcanic Complex (APVC,de Silva, 1989a), and to the southwest in southern Puna (Kay and Coira, 2009; Kay et al., 2011). Such is the case of ignimbrites as Puripicar (4.09 Ma), erupted from Chaxas dome complex, Atana (3.96 Ma) and Toconao: 4.0 Ma, from La Pacana caldera, Laguna Amarga (3.8 Ma,) from homonymous caldera, Cueva Negra ( 3.78 Ma) from Galán caldera, Tara (3.49 Ma) from Guacha caldera, Pastos Grandes (2.89 Ma) from Pastos Grandes caldera (Lindsay et al., 2001; Kay et al., 2006, 2010; Salisbury et al., 2011) (Figs. 11, 12).

During the lapse of time of the recognized “young eruptive pulse” took place the eruption of the ignimbrites Galán (2.6 Ma), Laguna Colorada (1.98 Ma), Puripica Chico (1.70 Ma), Purico (0.98 Ma) (Kay et al., 2010; Salisbury et al., 2011). 

Volume-time relations plot of Altiplano-Puna volcanic complex and southern Puna ignimbrites for the time span of the pulses recognized in Humahuaca and Casa Grande basins (Fig. 11), shows a clear coincidence between those pulses and the most voluminous episodes of the APVC ignimbrite flare-up, (mainly >500 up 2,200 km3, DRE) and the megacalderas of southern Puna located respectively 200-300 km west to northwest and 300 km south-southwest of the study region. So, in terms of ages and proximity the APVC and southern Puna calderas are the most likely sources of the ash fall tuffs recorded in Humahuaca and Casa Grande basins.

The age clusters that correspond to the two eruptive pulses are not only concordant with regionally recognized pulses of ignimbrite eruptions in the APVC and in the southern Puna, but also with the tectonic events that resulted in basin isolation and paleogeographic changes by ca. 4.2- 4.8 Ma, 2.1-1.8 Ma, 0.8 Ma (Pingel et al., 2014; Streit et al., 2015; Pingel et al., 2020).




Fig. 12. Satellite Radar Topography Mission (SRTM) image of the Sothern Central Andean region showing main Cenozoic giant calderas evaluated as possible sources of the studied tuffs.


5.3. Origin and dispersion of the distal ashes

Distal ashes may have been derived from the eruption column of large plinian eruptions (co-plinian ash) or from ash-clouds elutriated from pyroclastic density currents (co-ignimbrite ash) (Walker, 1971; Wilson, 1976; Sparks and Walker, 1977; Chesner et al., 1991). In the case of the distal ashes deposited in Huamhuaca and Casa Grande basins we lean, as Breitkreuz et al. (2014) in the study of Neogene to Quaternary ash deposits of the Coastal Cordillera in northern Chile, towards the co-ignimbrite origin, mainly based on the general lack of basal plinian fall or intra-plinian fall units in the ignimbrites of the APVC. In support of this possible origin, although it has considered that the pyroclastic density currents that deposited the large ignimbrites of the APVC were dense, poorly inflated and sluggish (Ort, 1993; de Silva et al., 2006) and that elutriation of ash to feed large co-ignimbrite ash clouds is not efficient in these flows. The crystal enrichment factor 17-47% (de Silva and Francis, 1989; Folkes et al., 2011) suggests that ash loss through elutriation has likely occurred.

The amount of elutriated material resulted from large eruptions has been estimated by different authors (e.g., Bursik and Woods,1996) in the order of 20 wt%, although Sparks and Walker (1977) and Mason et al. (2004) suggest that co-ignimbrite ash may constitute as much volume as the outflow ignimbrite itself. These estimates agree with large thickness up to 1.5 m of some of the studied distal ash fall tuffs (see Table 1).

The dispersion of these ashes around the volcanic region was tightly controlled by dominant geostrophic winds and topography. Ashes from historic eruptions as Lascar Volcano, northern Chile (Glaze et al., 1989), and from Miocene Ramadas Volcanic Complex, Puna (Viramonte et al., 1994) have been dispersed by northwesterly winds, as other examples of the Southern Central Volcanic Zone (SCVZ). In this sense, important ash-fall records recognized in Upper Cenozoic deposits distributed marginally to the high plateau of the Puna and intermountain basins as Humahuaca and Casa Grande could be the result of eruptions from volcanic centers located northwesterly and westerly in the SCVZ, that belong to the Altiplano Puna Volcanic Complex (APVC).

5.4. Potential volcanic sources-correlations

Considering the contemporaneity and proximity among the APVC and southern Puna conspicuous calderas (Fig. 12) with the Pliocene-Pleistocene ash-fall tuffs of Humahuaca and Casa Grande basins, these centers represent the most likely sources for them. In order to adjust this correlation and to evaluate the potential sources of origin of the tuffs, their mineralogical and chemical composition was compared (Table 4) with those determined in the ignimbrites products of different calderas of the APVC and of the southern Puna, of similar age (Wörner et al.,1992, 1994, 2000; Schröder and Wörner, 1996; de Silva 1989a, b; Lindsay et al., 2001; Schmitt et al., 2001; Mamani et al., 2010; Folkes et al., 2011; Kay et al., 2010, 2011; Salisbury et al., 2011, and references therein). In this comparison, the geochemical data on whole rock were considered, both in the case of tuffs and reference ignimbrites. Although the usual methodology particularly in the study and correlation of proximal and distal ashes contemplates analysis of major element composition of glass shards by EPMA and SEM (Westgate and Gorton, 1981; Froggatt, 1983; Nakagawa and Ohba, 2002; Harangi et al., 2005; Lowe, 2011), it do not always show efficiency in their distinction and correlation, as indicated in cases by the similar composition of tephras produced by different eruptions (Larsen and Eiriksson, 2007; Brendryen et al., 2010; Stokes and Lowe, 1988; Hermanns and Scellenberger, 2008) or by the great compositional variation observed between the glass shards (Breitkreuz et al., 2014).

On the other hand, there are case studies in which correlations of ashes and determination of possible sources have been carried out comparing major and trace-element chemical compositions of bulk-tephras, with whole rock analysis (mayor and trace elements) of samples belonging to possible source volcanoes (e.g., lavas, ignimbrites), obtaining results of manifest concordance (Naranjo and Stern, 2004; Stern, 2008; Watt et al., 2011). Based on the previous considerations, in order to identify the possible sources of the ash-fall tuffs, major and trace-element chemical compositions of them were compared with those of coeval ignimbrites of APVC and southern Puna calderas, adding to it the identification of their mineral associations.

From this comparative analysis it appears that the age range of Uquía and Mal Paso formations tuffs, that belongs to the older eruptive pulse (4.3 to 2.6 Ma), temporally coincides with the eruption age of ignimbrites from La Pacana and Guacha calderas located ~250 km to the WNW, as well as from Pastos Grandes caldera at ~300 km to the NW, from Galán Caldera ~300 km to the SW and the Laguna Amarga caldera at ~400 km SW of the studied ash-fall tuffs outcrops (Fig. 12).

The mineralogical a geochemical characteristics of the Uquía and Mal Paso Formations tuffs (Table 4), correlate well with those of the Atana and Toconao Ignimbrites from La Pacana caldera that yielded zircon U-Pb ages for pumice clasts of 4.12  and 4.66 Ma (Schmitt et al., 2001), also with those of the Upper Tara Ignimbrite from Guacha Caldera, that gave 40Ar/39Ar sanidine and biotite ages of 3.49 and 3.55 Ma, respectively (Salisbury et al., 2011), and with those of the Puripicar Ignimbrite with a 40Ar/39Ar biotite age of 4.09 Ma (Salisbury et al., 2011). In figure 13 is observed a clear coherence in trace elements ratios within the ranges: Sm/Yb=2-3.8; La/Yb=9-20; La/Sm=4.2-8; La/Ta=11-3 of the tuffs and those coeval ignimbrites erupted from APVC calderas.




Fig. 13.Trace element plots showing some general chemical features of Uquia and Mal Paso formations tuffs (red circle points correspond to samples in table 2 (A-B) compare with those of potential emission centers as Pastos Grandes, Galán, Laguna Amarga and Altiplano-Puna Volcanic Complex centers APVC (22º-24º S) represented as fields. The Uquia and Mal Paso formations tuffs show trace elements affinities with ignimbrites from APVC like Atana (vertical diamond lila) and Toconao (open cross) ignimbrites from La Pacana caldera, Upper Tara (light blue circle) from Guacha Caldera and Puripicar.(horizontal diamond lila) all of them included in gray field. Data from (Lindsay et al., 2001; Salisbury et al., 2011; Kay et al., 2006 ).


Other ignimbrites in the region erupted during the same span of time show marked geochemical differences (Fig. 13). The Pastos Grandes Ignimbrite (2.89±0.01 Ma) from the Pastos Grandes caldera exhibits moderately higher HREE (La/Yb=23) and a higher arc signature (La/Ta=44), the Cuevas Ignimbrite (3.78±0.08 Ma) from Galán caldera has increase in Sm/Yb=4.3-4.8 indicative of HREE retention in residual garnet in the source, in contrast to HREE signature of Pastos Grandes Ignimbrite, attributed to residual hornblende (Kay et al., 1987; Kay and Mpodozis, 2001), and La Amarga Ignimbrite, from the homonymous caldera, that is more enriched in LREE elements (La/Sm=7-9).

Radiometric ages of the young group tuffs, (2.21-0.8 Ma), intercalated in Pleistocene alluvial fans sheet flood deposits, suggest that these tuffs coincide with the eruption time of calde ras such as Laguna Colorada (Laguna Colorada Ignimbrite 2.05±0.01 Ma, 40Ar/39Ar biotite) in the Bolivian Altiplano (Salisbury et al., 2011 ), Lauca Pérez (Lauca Pérez Ignimbrite, 2.72±0.01 Ma, 40Ar/39Ar sanidine) of the Western Cordillera-Altiplano of the Chile-Bolivia border region (Wörner et al., 2000), Guacha (Puripica Chico Ignimbrite (1.69±0.06 Ma, 40Ar/39Ar biotite), Purico (Purico Ignimbrite 1.23±0.35 and 1.37±0.6 Ma, biotite K-Ar) in the Atacama region (de Silva, 1989b; Salisbury et al., 2011) and Galán (Galán Ignimbrite 2.09±0.016 Ma, 40Ar/39Ar sanidine) in the southern Puna region (Kay et al., 2011).

On the basis of mineralogical and geochemical data of these mostly Pleistocene ignimbrites (Wörner et al., 1992, 1994, 2000; Schröder and Wörner, 1996; de Silva 1989a, b; Lindsay et al., 2001; Schmitt et al., 2001; Mamani et al., 2010; Folkes et al., 2011; Kay et al., 2010, 2011; Salisbury et al., 2011, and references therein), we compared them with our new data of the young group tuffs to identify potential correlative eruption sources.

Among these ignimbrites, the Purico from the Purico Complex (Schmitt et al., 2001), the Puripica Chico from the Guacha Caldera and the Laguna Colorada from the Laguna Colorada Caldera (Salisbury et al., 2011), located as far as 250-270 km WNW of the studied tephra outcrops (Fig. 12), show strong mineralogical and geochemical similarities (Table 4, Fig. 14), pointing to them as likely potential sources. In contrast, other regional ignimbrites erupted during the same span of time show considerable geochemical differences. The Lauca Pérez Ignimbrite (18.16 S- 69.64 W Western Cordillera-Altiplano Chile-Bolivia) is rhyolitic in composition (72-75% SiO2) compared to dacitic-rhyodacitic young group of tuffs. It also exhibits a higher degree of enrichment in LREE elements, with La/Sm (8-12) and (La/Yb=30-55) and very low La/Ta (3-7), indicating an intraplate signature (Pearce, 1996) (Fig. 14) as well as a steep REE pattern (Sm/Yb=3-4.3) that requires residual garnet in the source (Kay et al., 1987; Kay and Kay 1991). While having similar SiO2 content (66-70%), the Frailes Ignimbrites differs in HREE content with high Sm/Yb (7-8.5) ratios pointing to the presence of garnet in the source (Kay et al.,1987; Kay and Kay, 1991) and higher La/Ta (35-48) suggesting an arc signature (Pearce, 1982; Pearce and Peate, 1995). The dacitic to rhyodacitic Galán Ignimbrite (Fig. 14) shows lower Sm/Yb=4-5.5 ratios than the Frailes Ignimbrite, but is also characterized by a HREE signatures indicative of residual garnet, which contrasts with the lower Sm/Yb=2-3.5 ratios of the young group of tuffs that are suggestive of residual hornblende (Kay and Mpodozis, 2001). The La/Ta (16-25) of Galán Ignimbrite is also lower than those determined for young tuffs (Fig. 14).




Fig 14. Trace element plots showing some general chemical features of tuffs from Pleistocene alluvial fans sheet flood deposits (blue square points correspond those samples in Table 2B*) compare with those of potential emission centers as Frailes, Galán, Lauca Pérez, Altiplano-Puna Volcanic Complex centers APVC (22º-24º S) represented as fields. The Alonso Formation tuffs show trace elements affinities with ignimbrites from APVC like Purico (red circle) from Purico Complex, Puripica Chico (X light blue) from Guacha Caldera and Laguna Colorada (open cruz), from Laguna Colorada Caldera, all of them included in gray field Data from (Salisbury et al., 2011; Schmitt et al., 2001).


The temporal, mineralogical and geochemical correlations observed among the ash fall tuffs deposited in the Humahuaca and Casa Grande basins and APVC ignimbrites highlight “the distinctive signature” of the APVC, which can be recognized in the distal ash fall tuffs. That “distinctive signature” recognized in the APVC is in accordance with the recognition of the “regional geochemical trends” of the Andean ignimbrites according location and time, as have been reported in previous studies (Kay and Coira, 2009; Mamani et al., 2010; Brandmeier and Wörner, 2016 and references therein). These ignimbrites have distinctive compositional characteristics at the regional level linked to basement differences, along with temporal changes in that basement, thermal and structural evolution of the crust, distinct regimes of partial crustal melting and mantle melt compositions, evolved over a changing variable steepening subduction zone (de Silva, 1989a; Coira et al., 1993; Ort et al., 1996; Kay et al., 1999; Lucassen et al., 2000; Lindsay et al., 2001; Zandt et al., 2003; Sobolev and Babeyko, 2005; Trumbull et al., 2006; de Silva and Gosnold, 2007; Schnurr et al., 2007; Kay and Coira, 2008, 2009; Kay et al., 2010, 2011; Folkes et al., 2011; Guzman et al., 2014 and references therein).


Ash-fall tuffs of the Pliocene-Pleistocene deposits of Humahuaca and Casa Grande intermountain basins, northwestern Argentina have been differentiated into two distinctive groups based on new mineralogical, geochemical and geo-chronological data, which correspond to the tuffs recorded in Pliocene alluvial fan deposits dominated by debris flow, deep sandy gravel braided, and shallow ephemeral lake deposits and in those tuffs recorded in Pleistocene alluvial fans sheet flood deposits.

The distinctive characteristics determined for these ash fall tuffs points to these as excellent chrono-stratigraphic tracers of the Pliocene-Pleistocene stratigraphy of the Humahuaca and Casa Grande intermountain basins.

The two clusters of ages identified: 4.3 to 2.6 Ma, and 2.2-pre 0.8 Ma, corresponding to the tuff groups, were recognized as coeval with ignimbrite eruptions pulses of Altiplano Puna Volcanic Complex (APVC), and southern Puna calderas, located to 200-300 km west-northwest and to 350 km south of the study region respectively.

Detailed petrographic, geochemical, and geochronological comparison among those ignimbrites, potential sources according age and proximity with the studied ash fall tuffs, highlighted the “distinctive signature” indicative of the regional compositional trend of the APVC, which can be recognized in the distal ash fall deposits.

The old group of tuffs shows affinity with those ignimbrites erupted from the most voluminous calderas of the APVC, like La Pacana (Atana and Toconao ignimbrites) Guacha (Upper Tara Ignimbrite) and Chaxas dome complex (Puripicar Ignimbrite). The young group of tuffs show greater agreement with the ignimbrites erupted from Purico Complex (Purico Ignimbrite), Guacha Caldera (Puripica Chico Ignimbrite), and the Laguna Colorada Caldera (Laguna Colorada Ignimbrite) during the Pleistocene waning stage of the APVC. The affinities observed were not detected for an exclusive center, but for several centers of the APVC, active during the recognized pulses, indicating the importance of the regional compositional signatures of magmatic complexes of the magnitude of the APVC, in order to identify potential magmatic sources of distal ash fall tuffs.

This research was funded by the AGENCIA (PICT-2017-1010), PUE-INECOA (22920170100027CO), CI-UNSa 2013-2287 and SECTER-UNJu 08/E036-E0035. We thank Dr. J.A. Naranjo and Dr. I. Petrinovic for the positive suggestions that improved this research. We also thank A.J. Pérez for his invaluable collaboration in the field survey and sample processing, as well as for his constructive and positive suggestions, R. Liquin and P. Cachizumba for their extensive support in laboratory work and E.D. González for his efficient assistance with graphics and illustrations.


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Appendix S2