# Two million years of evolution of the southern central andes retroarc, Payenia Volcanic Province, Argentina, from the study of the Cerro Nevado Volcanic Complex

Carlos Pallares, Philippe Sarda, Xavier Quidelleur, Clémentine Antoine, Pablo Tchilinguirian, Diego Winocur, Andrés Folguera, René Maury

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## To cite this version:

Carlos Pallares, Philippe Sarda, Xavier Quidelleur, Clémentine Antoine, Pablo Tchilinguirian, et al. Two million years of evolution of the southern central andes retroarc, Payenia Volcanic Province, Argentina, from the study of the Cerro Nevado Volcanic Complex. Journal of South American Earth Sciences, 2023, 123, pp.104229. 10.1016/j.jsames.2023.104229. hal-04352362

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1 **Two million years of evolution of the southern Central Andes retroarc,**

2 **Payenia Volcanic Province, Argentina, from the study of the Cerro**

3 **Nevado Volcanic Complex**

4

5 Carlos Pallares<sup>a*</sup>, Philippe Sarda<sup>a</sup>, Xavier Quidelleur<sup>a</sup>, Clémentine Antoine<sup>a</sup>, Pablo

6 Tchilinguirian<sup>b, c</sup>, Diego Winocur<sup>b, c</sup>, Andrés Folguera<sup>b, c</sup>, René C. Maury<sup>d</sup>

7

8 <sup>a</sup> Université Paris-Saclay, CNRS, GEOPS, 91405, Orsay, France

9 <sup>b</sup> CONICET, Argentina

10 <sup>c</sup> FCEyN, Universidad de Buenos Aires, Argentina

11 <sup>d</sup> Université de Bretagne Occidentale, CNRS, UMR 6538 Géosciences Océan, IUEM, 29280

12 Plouzané, France

13

14 * corresponding author: Tel.: (33) 169156771

15 E-mail address: carlos.pallares@universite-paris-saclay.fr

16

17

18 Word count:

19 abstract: 178

20 text: 10129

21 references: 88

22 Figures: 12

23 Tables: 3

24

25

26 Submitted to SAMES: November, 2022

27 Revised and resubmitted: February, 2023

28
29 **Abstract**

30 New K-Ar ages, major and trace element analyses, and Sr and Nd isotopic data for the Cerro
31 Nevado Volcanic Complex (CNVC) coupled with those previously published allow us to
32 better document the hot thermal anomaly which affected the upper mantle beneath the
33 Andean back-arc Payenia Volcanic Province and generated alkaline magmatism. These data,
34 along with previously published ones from this and neighbor volcanic centers, suggest that
35 this anomaly stagnated there, during at least the last 2 million years. Geochemical features
36 from the CNVC basaltic magmas indicate that they were sourced within an OIB-type mantle
37 with an EMI signature, similar to that underlying the neighboring Auca Mahuida Shield
38 Volcano to the south, and attributed to the rise of a mantle thermal anomaly. We identify at
39 least five events of partial melting and seven eruptive phases from 1.88 to 0.75 Ma.
40 Continental crust contamination in the San Rafael Block crustal basement occurred coupled
41 with fractional crystallization processes, probably in magma chambers and across several
42 cycles, producing the trachybasaltic, trachyandesitic and trachytic magmas of the Cerro
43 Nevado volcanic cone (1.34-1.22 Ma).

44
45 **Keywords :**

46 Alkaline lavas, crustal contribution, Quaternary, K-Ar geochronology, Andean back-arc,
47 Cerro Nevado, Argentina.

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## 1. Introduction

The magmatic evolution recorded in the Cerro Nevado Volcanic Complex (CNVC), as part of the Southern Central Andes retroarc, during the last two million years, is closely associated with the tectonic evolution of the San Rafael Block (SRB) within the Andean back-arc Payenia Volcanic Province (PVP; Figs. 1 and 2; annex 1) in Argentina. The PVP has been subdivided into northern, central, and southern segments, evidencing the close links between regional tectonics and magmatic evolution (Ramos and Folguera, 2011; Fig. 1). Located in the central segment at 35°34'47.17"S and 68°29'23.56"W, the large polygenetic CNVC center reaches 3810 m.a.s.l. (Figs. 1 and 2; annex 1). Overall, the basaltic and trachyandesitic lava flows of the CNVC erupted between 1.878 ± 0.028 and 0.944 ± 0.016 Ma (Quidelleur et al., 2009), although other undated potassic calc-alkaline andesitic products such as lava flows, lapilli beds and lapillitic breccias exist (Bermúdez, 1991). The Cerro Nevado Volcanic Complex was emplaced over the pre-Carboniferous San Rafael Block successions, which are part of the Cuyania composite terrane (Cingolani, 2017a). Some geologic units of the whole block include the magmatic protolith of Cerro La Ventana Formation, the Ordovician silico-carbonate sequences, the Devonian Rodeo de la Bordalesa tonalites, the Agua Escondida Mining District granitoids, La Horqueta, Rio de los Castaños, and El Imperial Formations, and the Lower Paleozoic MORB-type tholeiitic El Nihuil dolerites (Fig. 2; Cingolani and Ramos, 2017; Cingolani et al., 2017b).

The CNVC lavas have been interpreted either as subduction-related arc rocks (Bermúdez, 1991; Kay et al., 2006a) or as MORB-type tholeiites belonging to the northern and central segments of the PVP (Søager et al., 2013; 2015a). However, two other large volcanic fields, i.e. the Payún Matru Volcanic Complex (1.72 ± 0.025 – 0.007 ± 0.001; Quidelleur et al. 2009; Germa et al., 2010) and the Auca Mahuida shield volcano (2.03 ± 0.03 – 0.88 ± 0.03; Rossello et al., 2002; Kay et al., 2006a; Pallares et al., 2016), occur ~100 and ~200 km to the SW and S from the CNVC, respectively (Fig. 1). They are located within the central and southern segment of the PVP (Fig.1), which is mainly dominated by intraplate magmatism with an OIB affinity (Kay et al., 2004, 2006a, 2013; Bertotto et al., 2009; Germa et al., 2010; Søager et al., 2013; Søager and Holm, 2013; Hernando et al., 2014a; Søager et al., 2015b; Pallares et al., 2016).
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In this paper, we present new K-Ar ages, major and trace element analyses, and Sr and Nd isotopic data for the Cerro Nevado Volcanic Complex. We use these data combined with previously published, with the aim to better understand its temporal and magmatic evolution and the associated petrogenetic processes, as well as to clarify the magma source question, i.e. arc, MORB-type, or OIB-type. We also investigate the relationships between the CNVC, its basement, and the adjacent back-arc intraplate OIB-type volcanoes, Auca Mahuida and Payún area volcanoes.

## 2. Geological and tectonic setting

### 2.1. *The Cerro Nevado Volcanic Complex basement*

The Pleisto-Holocene magmas of the Cerro Nevado Volcanic Complex ascended through the thick pre-Carboniferous basement of the San Rafael Block (SRB) and were finally emplaced over its central part. The SRB developed south of the Andean flat-slab subduction zone and represents the southern part of the Cuyania composite terrain (Ramos, 2004; Cingolani and Ramos, 2017; Cingolani, 2017a; and references therein). Overall, a stratigraphic sequence from the Mesoproterozoic crustal basement to Silurian-Devonian magmatic and sedimentary rocks are the main components of the San Rafael Block, an isolated range emerging in the Andean foreland zone as part of a broken foreland.

The oldest exposed unit of the Mesoproterozoic basement, the Cerro La Ventana Formation (Cingolani and Ramos, 2017), outcrops in the southeastern part of the SRB. This formation is mainly characterized by metamorphic rocks derived from plutonic, volcanic, and sedimentary protoliths (Cingolani et al., 2017a, b; Cingolani and Ramos, 2017). Tonalites, gabbros, quartz diorites, and amphibolites are the most important lithologies, but granodioritic-dioritic orthogneisses also outcrop (Cingolani et al., 2017a). The southern part of the Cuyania terrane, coincident with the San Rafael Block, has been interpreted as a sequence of island arc terrains juxtaposed during the Grenvillian orogeny (Ramos, 2004; and references therein). Later, the Famatinian orogeny occurred with subduction and collision between the Cuyania microcontinent and Gondwana, stopping magmatic arc activity at about 460 Ma (Ramos, 2004; and references therein). From the Late Ordovician to Early Silurian, an extensional event likely occurred, which built the El Nihuil Mafic Unit, including the tholeiitic MORB-type El Nihuil dolerites (Cingolani et al., 2017b). During the Silurian-
Devonian (~440-390 Ma), the convergence between Cuyania-Gondwana and Chilenia terranes generated new subduction with a magmatic arc: the Devonian Rodeo de la Bordalesa tonalites and the granitoids of the Agua Escondida Mining District (Cingolani and Ramos, 2017; Cingolani et al., 2017c) were produced then.

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## 2.2. Late Miocene to Quaternary back-arc magmatism to the north of the Cortaderas Lineament

Among the main volcanic complexes emplaced in the Late Triassic to Early Cretaceous Neuquén extensional basin, between the Foreland Border and the Fold and Thrust Belt of Malargüe, to the north of the Cortaderas Lineament, only the Mio-Pliocene Chachahuén Volcano and the easternmost volcanic centers of San Rafael Block display typical subduction-related geochemical signatures (Kay et al., 2006b; Ramos et al., 2014; Litvak et al., 2015; Figs. 1 and 2). These arc magmatism characteristics are considered as generated by hydrous fluxing and melting of the mantle wedge during the subduction of the Nazca plate under the South America Plate (Kay et al., 2006b and references therein). However, the PVP includes several Plio-Pleistocene composite volcanoes which missing subduction-related geochemical signatures, not only in the Neuquén extensional basin/Central Depression (Payún area volcanoes and Auca Mahuida shield volcano), but also to the southwest of the Fold and Thrust Belt of Malargüe (El Tromen Volcanic System), and on the San Rafael Block (the Cerro Nevado Volcanic Complex; Figs.1 and 2; annex 1; Kay et al., 2006a, 2013; Søager et al., 2013; Søager and Holm, 2013; Hernando et al., 2014a; Pallares et al., 2016; Pallares et al., 2019; and references therein).

The subduction-related geochemical signature of the Pre-Quaternary volcanoes in the PVP resulted from a previous flat slab episode, which affected the foreland region about 13.5-3.5 Ma (Kay et al., 2006b; Litvak et al., 2015). During the Late Miocene, the compressional tectonic setting triggered by this flat subduction built the Malargüe fold and thrust belt and warped the Andean foothills next to PVP (Ramos and Folguera, 2005; Ramos and Kay, 2006; Ramos et al., 2014; Folguera et al., 2015a; Fennell et al., 2015; Fig.1). The Plio-Quaternary was marked by a steepening of the Nazca subducting plate, which reached ~30° dip angle since about 4 Ma, ending the shallow subduction system and producing a complex interaction between asthenosphere and lithosphere within the mantle wedge beneath the Payenia
152 Volcanic Province (Kay et al., 2004, 2006a, 2006b; Ramos and Kay, 2006; Ramos et al., 2014). These processes triggered an extensional tectonic regime generating the Las Loicas 153 trough in the western rear arc zone, thinning the crust to less than 42 km and increasing the 154 magmatic activity in the PVP since the Pleistocene (Kay et al., 2006a; Ramos and Kay, 2006; 155 Llambías et al., 2010; Germa et al., 2010; Ramos and Folguera, 2011; Gudnason et al., 2012; 156 Ramos et al., 2014; Pallares et al., 2016, 2019).

158 The petrological and geochemical characteristics displayed by the Pleisto-Holocene basaltic 159 rocks from the Auca Mahuida shield volcano, the Payún area volcanoes, and El Tromen 160 Volcanic System indicate a similar magmatic source involved in their genesis, with some 161 different degrees of interaction between the overlying crust and subjacent mantle (Pallares et 162 al., 2016, 2019). These volcanic rocks do not display subduction-related affinity, and their 163 origin has been related to the ascent, during the slab steepening process, of a hot thermal 164 anomaly, which is still present beneath the zone and produces OIB-type intraplate volcanic 165 rocks (Kay et al., 2004, 2006a, 2013; Germa et al., 2010; Bertotto et al., 2009; Søager et al., 166 2013, 2015a, 2015b; Søager and Holm, 2013; Hernando et al., 2014a; Burd et al., 2014; 167 Pallares et al., 2016, 2019). Overall, most authors agree with the hypothesis of magmas 168 generated in the asthenospheric mantle that interact with the overlying crust, and a northward 169 migration of the thermal anomaly during the last 2 Ma, based on the comparison of the 170 geochemical, geochronological, and geophysical characteristics of lavas from the southern 171 and northern Payenia Volcanic Province (Gudnason et al., 2012; Søager et al., 2013; Søager 172 and Holm, 2013; Burd et al., 2014; Folguera et al., 2015a; Pallares et al., 2016, 2019).

173 The magmatic sources of southern PVP basaltic rocks are thought as being either 1) an EMI- 174 like mantle where eclogite and peridotite-derived melts interacted at distinct pressures, with 175 a small or null contribution of subduction-related components, or 2) a mixture of an EMI 176 OIB-type pyroxene-rich source and a peridotitic source with more radiogenic Pb, which was 177 metasomatized by subduction-zone fluids and/or melts (Søager et al., 2013; Søager and 178 Holm, 2013; Søager et al., 2015b), or 3) an EMI-like garnet-bearing mantle or two garnet- 179 spinel OIB-type mantle sources experiencing 4-7% of melting above steepening subduction, 180 followed by mixing of extracted magmas with the continental lithosphere, and finally storage 181 near the base of a 65-70 km-thick lithosphere (Kay et al., 2006a, 2013; Pallares et al., 2016, 182 2019). On the contrary, the magmatic sources of northern PVP basaltic rocks are thought of
183 as sourced from a normal MORB-like mantle, which was modified by added fluids and melts
184 from the subducting plate (Søager et al., 2013, 2015a).

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186 2.3. *The Cerro Nevado Volcanic Complex (CNVC)*

187 The Cerro Nevado Volcanic Complex is located to the east of the Llancanelo Basin/Los
188 Huarpes Depression on the pre-Carboniferous basement of the San Rafael Block (Figs. 1 and
189 2; annex 1). This volcanic complex is mainly composed of lava and pyroclastic flows,
190 volcanic breccias, subvolcanic rocks, and dykes (mafic and intermediate lavas) that erupted
191 in two distinct volcanic episodes (Holmberg, 1973; Bermúdez, 1988, 1991; Ramos and Kay,
192 2006; Sepúlveda et al., 2007; Quidelleur, 2009; and references therein; Fig. 2; annex 2). One
193 comprises a Late Pliocene to Quaternary volcanic unit, named the Nevado Formation
194 (Sepúlveda et al., 2013; Fig 2 and annex 1) while the other is named Chapúa group (in an
195 older classification, Chapúa formation: Basalt III, Maipo formation: Basalt IV, El Puente
196 formation: Basalt V; Holmberg, 1973; Sepúlveda et al., 2007 and references therein). The
197 magmatic series from the Nevado Formation display potassic calc-alkaline signatures, while
198 those from the Chapúa group are alkaline (e.g. Basalt V; Bermúdez, 1991; Bermúdez et al.,
199 1993). Overall, the geochemical variations in the Cerro Nevado Volcanic Complex were
200 associated by some authors with temporal changes of mantle sources over a steepening
201 subduction zone, with subducting slab imprints strongest in the Mio-Pliocene rocks and
202 attenuated in the Quaternary ones (Bermúdez, 1991; Bermúdez et al., 1993; Kay et al., 2006a;
203 and references therein). Søager et al. (2013, 2015a) consider that the steepening of the
204 subduction slab may have generated an upwelling of both OIB-intraplate and MORB-like
205 mantle sources. They propose that the youngest CNVC magmas were from a MORB-like
206 source, previously modified by subduction-related fluids and melts, and including a
207 contribution of lower crustal components.

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209 3. Material and methods

210

211 3.1 *Sampling*

212 We report new data for 14 samples from the western, southeastern, and northeastern
213 surroundings of the Cerro Nevado volcano, and from the southern part of the volcano itself
214 (Fig. 2). These 14 samples were analyzed for major and trace elements (Table 1), Sr and Nd
215 isotopes (Table 2) and 5 of them were dated by the unspiked K-Ar method (94BH, 94H, 94K,
216 94J, 94I2; Table 3; annex 2), while the other ages showing symbol "t" are from Quidelleur
217 et al. (2009): 94BB, 94BD, 94M, 94BJ, 94D2, 94I1, 94L (Table 1; Fig. 2; annex 2). Our set
218 complements the previously available data from Kay et al. (2006a), Folguera et al. (2009),
219 and Søager et al. (2013, 2015a). Following the geologic mapping made by Sepúlveda et al.
220 (2007; Fig. 2), our samples are mainly located in the Cerro Nevado Formation (94H, 94I1,
221 94I2, 94J, 94K, 94L, 94G, 94G), and in the Chapúa Group (94BJ, 94BH, 94BD, 94D2, 94M,
222 94BB).
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224 3.2. *Major and trace elements*

225 Major and trace elements were obtained at Service d'Analyse des Roches et Minéraux
226 (SARM) of the Centre de Recherches Pétrographiques et Géochimiques de Nancy (CRPG),
227 France. The Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES:
228 Thermo® Electron IRIS Advantage) has been used to measure major elements, while trace
229 elements have been obtained by Inductively Coupled Plasma-Mass Spectrometry (Thermo®
230 Elemental X7). First, we broke and crushed the rock to reach 5mm fragments size and
231 secondary minerals such as carbonate and zeolite that sometimes fill the vesicles of analyzed
232 volcanic rocks have been removed during sample preparation, which means that
233 recrystallized materials are absent. Afterward, we used an agate grinder to powder samples
234 before geochemical analysis. At SARM, 200 mg of powder was melted with LiBO₂ in a Pt-
235 Au crucible, then dissolved with HNO₃. The measurements then followed the analytical
236 techniques described by Carignan et al. (2001), Govindaraju and Mevelle (1987) and
237 Govindaraju and Potts (1994). The international standards (AN-G, BR, UB-N, DR-N, GH)
238 were used for calibration, and they underwent the same process as the analyzed samples.
239 Some more detailed descriptions and precisions about analytical uncertainties for each
240 element can be found on the SARM-CRPG website, at
241 http://helium.crpg.cnrsnancy.fr/SARM/index.html.

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243 3.3. *K-Ar technique*
We obtained 5 new ages from the CNVC using the unspiked K-Ar Cassignol dating technique (Cassignol and Gillot, 1982; Gillot and Cornette, 1986; Table 3), which we have previously used to date volcanic rocks in the Neuquén and Mendoza provinces (Argentina): El Tromen Volcanic System (1.922 ± 0.029 - 0.007 ± 0.002 Ma; Pallares et al., 2019), the Auca Mahuida shield volcano (1.61 ± 0.02 - 1.17 ± 0.02 Ma; Pallares et al., 2016), the Payún area volcanoes (285 ± 5 - 7 ± 1 ka; Germa et al., 2010) and the Cerro Nevado (1.88 ± 0.03 - 0.94 ± 0.01 Ma; Quidelleur et al., 2009). This technique allows precise dating of old and young volcanic rocks, including those with low radiogenic Ar content.

The rocks were analyzed on groundmass within a narrow density range to avoid K loss resulting from weathering, as well as excess argon carried by mafic minerals and low K content plagioclases (Table 3). The specimens were crushed and sieved using a typical 125-250 µm or 100-200 µm sieve. Afterward, the sieved fragments of rock were washed in an ultrasonic bath of deionized water and a 10% nitric acid solution. The groundmass was kept in a narrow density range using heavy liquids to remove the phenocrysts and any slightly weathered fraction (if present) using heavy liquids. Magnetic separation was performed to further improve the purity of the groundmass fraction. Flame absorption spectroscopy was used to measure K content, compared to MDO-G, ISH-G (Gillot et al., 1992) and BCR-2 USGS standards. An analytical uncertainty of about 1% is obtained for K contents, within the range 0.1 - 15%.

The ⁴⁰Ar and ³⁶Ar have been measured using a multi-collector mass spectrometer, which determines with precision the radiogenic Ar content (%⁴⁰Ar*), comparing the sample with an air aliquot measured in the same analytical conditions (Gillot et al., 2006). The detection limit of the used mass spectrometer is 0.1% for ⁴⁰Ar* (Quidelleur et al., 2001), which allows to date rocks as young as 2 ka with only a few centuries of uncertainty (Gillot et al., 2006). The frequent measurements of the HD-B1 standard dated at 24.21 ± 0.32 Ma (Hess and Lippolt, 1994) and refined at 24.18 ± 0.09 Ma (Schwarz and Trieloff, 2007), allow calibration of the ⁴⁰Ar signal, combined with frequent measurements of an air pipette compared with the standard. K and Ar measurements have been duplicated for all samples (Table.3). Finally, mean age and uncertainty (reported here at the 1σ level) have been calculated by weighting each analysis with the amount of ⁴⁰Ar*. The decay constants and K isotopic ratio used here are those from Steiger and Jäger (1977).
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## 3.4. *Sr and Nd isotopes*

14 Fourteen samples were analyzed for Sr and Nd isotopes. About 5 ml of the mother solution prepared for trace element analyses was evaporated to dryness and dissolved in HNO₃ 1N.

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Sr, Nd and Pb fractions were chemically separated using the Triskem specific resins TRU-spec, Sr-spec and Ln-spec, following a conventional column chemistry procedure modified from Pin and Santos Zalduegui (1997) and Gale (1996).

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Sr and Nd isotope compositions were measured in static mode on a Thermo TRITON at the Pôle Spectrométrie Océan (PSO) in the Institut Universitaire Européen de la Mer (IUEM), Plouzané, France. All measured Sr and Nd ratios were normalized to ⁸⁶Sr/⁸⁸Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 respectively. Values measured for reference solutions NBS987 and JNdi are reported in table 2.

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# 4. Results

## 4.1. *Major and trace element compositions*

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The new geochemical data obtained for 14 new samples from the Cerro Nevado Volcanic Complex are listed in Table 1. They include major elements (oxide, wt%), CIPW normative mineral contents, Mg# values, compatible and incompatible trace elements (ppm), and trace elements ratios. Overall, the freshness of the studied rocks is evidenced by their loss on ignition (LOI) values that are lower than 1 wt%, except for the pumice sample 94G, which displays a LOI of 4.74 wt%. In this work, our new data from the Cerro Nevado Volcanic Complex (CNVC) were combined with those previously published by Folguera et al. (2009) and Søager et al. (2013, 2015a). We also included the MORB average (Gale et al., 2013), the data from the OIB-type back-arc lavas from the area (Payún, Payún Matrú, Los Volcanes and Auca Mahuida volcanoes; see caption to Fig. 3 for references), as well as those from the Lower Paleozoic tholeiitic MORB-type El Nihuil dolerites (Cingolani et al., 2017b), which are part of the San Rafael block basement crosscut and overlain by the studied lavas. All these data are shown in Fig. 3 plots.

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Fig. 3a (Na₂O + K₂O vs. SiO₂ from Le Bas et al., 1986) indicates that the CNVC lavas belong to the alkaline series; among our new samples, there is one trachytic pumice (94G) and thirteen lavas: three trachyandesites (94I2, 94F, 94L), four basaltic trachyandesites (94J,
306 94K, 94H, 94I1), one trachybasalt (94D2) and five alkali basalts (94BJ, 94M, 94BD, 94BB,
307 94BH). All the mafic lavas are nepheline-normative (Table 1). The previously published data
308 are mainly trachybasalts and alkali basalts, but some rocks are basanitic. Considering the
309 geologic map from Sepúlveda et al. (2007), our new samples show the Cerro Nevado
310 formation as essentially built by basaltic trachyandesites, trachyandesites and trachytes,
311 while the Chapúa group is mainly made of alkali basalts. Although the CNVC lavas are more
312 similar to the OIB-type rocks from shield volcanoes than to MORB-type lavas (Fig. 3a), the
313 plots of TiO₂ vs. SiO₂ (Fig. 3b) and TiO₂ vs. alkalis (Fig. 3c) show that the studied rocks are
314 less Ti-rich than the neighboring OIB from Payún and Auca Mahuida. The Harker diagrams
315 (major elements vs. SiO₂; not shown) suggest an evolution trend mainly controlled by
316 differentiation, with a progressive decrease of TiO₂, Fe₂O₃, FeO, MgO, MnO and CaO, and
317 an increase of Na₂O and K₂O together with increasing SiO₂ contents.

318 Compatible trace element contents (Table 1) of mafic lavas are moderate or low (Ni < 200
319 ppm, Co < 50 ppm, Cr < 400 ppm). Incompatible trace element patterns (Fig. 4) were
320 normalized to the primitive mantle (PM), to the normal mid-Atlantic ridge basalts (N-
321 MORB), and to the ocean island basalts (OIB) all from Sun and McDonough (1989). We also
322 included the all-MORB average values of Gale et al. (2013).

323 The multielemental patterns of CNVC magmas normalized to the PM show, for the most
324 incompatible elements, moderately to highly fractionated patterns, with strong enrichments
325 (except Ba) and clear positive Pb (and to a lesser extent Zr) anomalies. Middle and heavy
326 rare earth elements (MREE and HREE) and Ti and Y are about 6 times the PM. The trachytic
327 pumice 94G shows a trace element pattern with exceptional depletions in Ba, Sr, P, Eu, and
328 Ti (Fig. 4), which might result from particularly advanced crystal fractionation processes.

329 Curiously, this sample plots in the field for samples of the Payún area volcanoes, and this is
330 true for all the geochemical diagrams used in this study. One possibility would be that pumice
331 94G comes in fact from one of the Payún volcanoes, although this was not observed in the
332 field. This hypothesis has to be investigated with new field observations. In the following,
333 we thus do not use pumice 94G anymore for our thought process.

334 Globally, the Cerro Nevado patterns normalized to N-MORB are more enriched than N-
335 MORB, either the all-MORB average or the Lower Paleozoic tholeiitic MORB-type El
336 Nihuil dolerites (Fig. 4). They are strongly enriched, i.e. from 20 to about 2500 times N-
337 MORB for the most incompatible elements, and close to those of Auca Mahuida and Payún
338 areas. The latter similarities also appear clearly in OIB-normalized patterns, where the Cerro
339 Nevado patterns are rather flat (Fig. 4).

340

341 **4.2. Sr and Nd isotopic data**

342 Isotopic data for the 14 samples of the Cerro Nevado Volcanic Complex are given in Table
343 2 and shown in Fig. 5, which also includes those previously published by Søager et al. (2013,
344 2015a). ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr ratios vary from 0.51269 to 0.51284, and from 0.7038 and
345 0.7042, respectively. Pumice 94G has a ⁸⁷Sr/⁸⁶Sr as high as 0.70455 and plots close to some
346 datapoints from Payún volcanoes. Overall, the data show that the lavas from the CNVC (as
347 well as most of the Payún ones) plot in the mantle array, showing a negative correlation from
348 MORB-like signatures towards EMI and/or continental crust poles (Fig. 5). Only pumice
349 sample 94G lies outside the correlation. This correlation points to mixing between a MORB-
350 like source and an isotopically “crustal” end-member, which could be the EMI OIB type
351 (rather than EMII) as in Søager et al. (2013) or the crustal rocks from the local San Rafael
352 Block basement such as the Cerro La Ventana Formation or the Rodeo de la Bordalesa
353 Tonalites.

354 It can be seen in Fig. 5 that the samples from the Cerro Nevado Formation plot to the lower
355 right side of the correlation, that is, closer to the “crustal” end-member, which is certainly
356 related to the more evolved character of these rocks, where melts must have suffered both
357 crystallization and assimilation in a magma chamber. On the other hand, the Chapúa group
358 samples display quite variable Sr and Nd isotopic ratios along the mantle array.

359

360 **4.3. K-Ar dating**

361 The new ages from the Cerro Nevado Volcanic Province are displayed in Table 3, Figs. 2, 5,
362 and 6 and annex 2. The freshness of the samples and the homogeneity of the mineralogical
363 preparation allowed us to obtain highly reproducible duplicates. K contents range from 0.987
364 to 3.386%, while ⁴⁰Ar* varies between 12.3 and 43.2%. The new ages obtained here are
365 between 1.336 ± 0.019 and 0.925 ± 0.015 Ma.

366 Our new ages combined with those previously published by Quidelleur et al. (2009) and
367 Folguera et al. (2009) allow us to establish that the two main rock formations, which built
the CNVC, erupted during an activity period between 1.878 ± 0.028 and 0.750 ± 0.051 Ma.
We also determine that the Chapúa group and Cerro Nevado Formation experienced
simultaneous volcanic activity (Figs. 2 and 6), which started in the southern part of the
complex (Cerro de los Menucos; Fig.2) and in the northern zone (Puntano volcano: 1.780 ±
0.110 Ma; Folguera et al., 2009), and was followed by that of other peripheral zones close to
Cerro Nevado Formation (Fig. 2). However, the Chapúa group was emplaced in two periods:
from 1.878 ± 0.028 to 1.352 ± 0.020 Ma, and between 1.044 ± 0.015 and 0.750 ± 0.051 Ma.
The Cerro Nevado itself was built during an intermediate stage (1.336 ± 0.019 - 1.224 ±
0.018 Ma). Finally, the youngest volcanic activity is documented in the northern part of the
complex (Cerro Ponon Trehue: 94BJ = 0.944 ± 0.016 Ma; Fig. 2).

378

379 4.4. Geochemical features

380 The CNVC rocks display typical features of the alkaline series, evolving from alkali basalt
to trachyte, similarly to those from Auca Mahuida and the Payún area volcanoes. They are
381 different from the all-MORB average (Gale et al., 2013) and the Lower Paleozoic tholeiitic
382 MORB-type El Nihuil dolerites (Fig.3a).
383

384 Using MgO contents, we have grouped the mafic lavas from the CNVC into two main groups:  
385 high-MgO > 8 wt.% and low-MgO < 8wt.% (Fig. 6a and Table 1). Those with MgO contents  
386 between 12 and 8 wt.% will be referred to as primitive basalts (dark green area), those with  
387 MgO contents between 8 and 5% as evolved basalts (light green area), those with MgO  
388 contents from 5 to 3 wt.% as basaltic trachyandesitic rocks (yellow area), and those rocks  
389 displaying MgO values lower than 3 wt.% as trachyandesitic and trachytic rocks (Fig. 6).

390 The CIPW normative mineral contents from studied basaltic rocks show silica-
391 undersaturated characteristics (Fig. 7; Table 1) typical of alkali basalts and basanites
392 (Thompson, 1984; Yoder and Tilley, 1962) and different from the silica-saturated MORB and
393 the Lower Paleozoic El Nihuil olivine tholeiites. This normative classification is very similar
394 to that suggested by Pallares et al. (2016) for the Auca Mahuida magmas and for other alkali
395 magmas from the Payenia Volcanic Province displayed in Fig. 7.
396

396

397 **5. Discussion**

398 5.1. Calendar of CNVC eruptions and synchronous magmatic activity of other volcanoes
Combining our new data (ages and geochemistry) with datings by Quidelleur et al. (2009) and Folguera et al. (2009), petrologic data from Kay et al. (2006a) and Søager et al. (2013, 2015a), and petrographic and stratigraphic data from Sepúlveda et al. (2007) and Folguera et al. (2009) allow us to better document the spatial magmatic activity within the CNVC during the last 2 Ma (Figs. 2 and 6). We identify seven eruptive phases using temporal and rock type (primitive and evolved basalts, basaltic-andesitic and trachytic rocks) data (Fig. 6), several of which started with the emplacement of high-MgO basalts belonging to the Chapúa group. More evolved rocks, i.e. low-MgO basalts, trachybasalts, trachyandesites and trachytes, which match the definition of the Cerro Nevado Formation (Sepúlveda et al., 2007), were emplaced during phases 3 and 4, but without associated high-MgO basalts (Fig. 6). We remark that the Chapúa group, as defined in the geological map (Sepulveda et al., 2007), should probably be separated into two parts (we can call them lower and upper Chapúa group) according to our geochronological data (Fig. 6b).

In the southern part of the complex, the oldest alkali basalt lava from the Chapúa group erupted at 1.878 ± 0.028 Ma (94BB; Figs. 2 and 6; annex 2) and it marked the beginning of phase 1 and the Pleistocene activity of the CNVC. However, in the northern part, a few kilometers away from the studied zone (see Fig. 2 and Fig. 3 from Folguera et al., 2009), the alkali lavas from the Puntano volcano were emplaced almost simultaneously at 1.780 ± 0.110 Ma. These northern and southern eruptions of the Cerro Nevado Volcanic Complex coincide with the emplacement of trachybasalts and alkali basalts from the Pampa de Las Yeguas Formation at about 1.78 ± 0.10 Ma in the Auca Mahuida shield volcano (Kay et al., 2006a; Pallares et al., 2016). However, the CNVC lavas are high-MgO basalts, while those from Auca Mahuida shield volcano are low-MgO evolved basalts, but TiO₂ contents are higher in the latter (Fig. 6a).

About 0.3 Ma later, the magmatic activity of phase 2 started in the northern part with the alkaline eruption of the Cerro Nihuil at 1.474 ± 0.089 Ma (Figs. 2 and 6), followed by the alkali basaltic lavas of Cerro Solo, which were emplaced at 1.352 ± 0.020 Ma (94BD; Fig. 2; annex 2). These alkaline eruptions of high-MgO basalts have similar geochemical compositions and are synchronous with the Rincon del Infiernillo and Pampa de Leon basalts (Auca Mahuida), which were emplaced between 1.44 ± 0.02 and 1.35 ± 0.02 Ma (Pallares et al., 2016).
430 Phase 3 occurred between 1.336 ± 0.019 and 1.316 ± 0.019 Ma, and it marked the beginning
431 of the Cerro Nevado Formation magmatic activity, in the central part of the complex (Fig. 2;
432 annexes 1 and 2). This central activity resumed during phase 4 at 1.244 ± 0.018 Ma (Fig. 6).
433 Concomitantly, basaltic trachyandesitic and trachyandesitic activities occurred in Rincon del
434 Infiernillo, Auca Mahuida and Puesto Retamal formations (Auca Mahuida volcano) after
435 1.35 ± 0.02 Ma and before 1.18 ± 0.02 Ma (Pallares et al., 2016).

436 The trachybasalts (94D2) and low-MgO alkali basalts (94M) of phase 5 erupted at 1.044 ±
437 0.015 and 1.022 ± 0.015 Ma, respectively, marking the resumption of the Chapúa group
438 activity, especially in the western part of CNVC (Fig. 2). The Aguada Lastra Volcanics (Auca
439 Mahuida) show similar compositions, but are slightly older (1.17 ± 0.02 Ma) than phase 5
440 lavas of the Cerro Nevado Volcanic Complex. High-MgO basalts dominate phase 6 between
441 0.944 ± 0.016 and 0.801 ± 0.049 Ma, especially in the northern part of the CNVC, including
442 the low-MgO Cerro Ponon Trehue sample 94BJ (Fig. 2 and 6b), as well as the low-MgO El
443 Puntudo and Guadalito volcanoes (Fig. 3 in Folguera et al., 2009). The emplacement of low-
444 MgO basalts from El Negro volcano at 0.801 ± 0.049 Ma in the northern part of the Cerro
445 Nevado Volcanic Complex (Folguera et al., 2009) marked the end of this phase.

446 Finally, phase 7, the northernmost and youngest eruption in the studied area, was marked by
447 the eruption of high-MgO primitive basalts from the Solo volcano at 0.750 ± 0.051 Ma (Fig.
448 6). Temporally or geochemically equivalent activities are not recorded from Auca Mahuida,
449 but somewhat younger volcanoes were reported by Germa et al. (2010) in the Payún area
450 (Fig. 1).

451 Overall, our data suggest that the Pleistocene magmatic activity (about 1.9-1.8 Ma) occurred
452 throughout the PVP (Auca Mahuida/El Tromen/Payún area volcanoes; Fig. 1) including the
453 CNVC. The hypothesis involving its progressive migration towards the north (Fig. 2) does
454 not explain the existence of middle Pleistocene alkali basalts younger than 0.5 Ma in El
455 Tromen Volcanic System (Pallares et al., 2019) and those from the Payún volcanoes younger
456 than 0.3 Ma (Germa et al., 2010).

457

## 5.2. *Magmatic and geodynamic processes controlling the evolution of the CNVC*

458 The genesis of magmatic processes which resulted in the emplacement of the Pleistocene
459 lavas at the Cerro Nevado Volcanic Complex is intricated because the evolution of this area,
460
located between the San Rafael Block and Payenia Volcanic Province (Fig. 1), has been controlled by important geodynamic events, at least during the last 4 Ma, as shown by Kay et al. (2004, 2006a, 2006b), Ramos and Kay (2006), Ramos et al. (2014). According to these authors, the end of the shallow subduction that characterized this zone, followed by the steepening of the Nazca subducting plate and a consequent thinning of the continental crust, increased the magmatic activity, and complex interaction between asthenospheric and lithospheric mantles occurred, especially in the PVP south of the CNVC. In this area, OIB-type intraplate magmatism is clearly identified in the Payún area volcanoes, the Auca Mahuida shield volcano and El Tromen Volcanic System (Kay et al., 2004, 2006a, 2013; Germa et al., 2010; Bertotto et al., 2009; Søager et al., 2013, 2015a, 2015b; Søager and Holm, 2013; Hernando et al., 2014a; Burd et al., 2014; Pallares et al., 2016, 2019). The case of the Cerro Nevado Volcanic Complex is more debated. Some authors identified a subduction-related signature evidencing the farthest off-trench influence of fluids from the Nazca subducting plate but suggested that the later magmatic features changed and magmas became progressively alkaline in the most recent eruptions (Bermúdez, 1991; Bermúdez et al., 1993; Kay et al., 2006a; and references therein). Others considered that the magmas of the CNVC were sourced from both intraplate OIB-type and MORB-like mantles, which were transformed by fluids and/or melts derived from the subduction zone and the lower crust (Søager et al., 2013, 2015a).

Our results, developed in the following sections, allow us to consider that the genesis and evolution of magmas from the Cerro Nevado Volcanic Complex is intimately linked to an EMI-like garnet-bearing (Kay et al., 2013) or perhaps a garnet-spinel bearing OIB-type mantle, as proposed for Auca Mahuida (Pallares et al., 2016). The melts generated were modified by crustal components from the San Rafael Block basement during their upwelling and residence in the crust. In addition, considering the activity and steepening of the Nazca subducting plate, subduction-related processes could explain the MORB-like signature (Søager et al., 2013, 2015a; Pallares et al., 2019).

### 5.2.1. *Timing of partial melting and fractional crystallization from measured ages and spatial distribution of eruptions*
Overall, Fig. 3 and Fig. 6a suggest that the studied rocks experienced fractional crystallization *sensu lato*, marked by an increase of Na₂O and K₂O together with SiO₂ contents while other major elements decrease. Also, given their strong enrichment in incompatible elements shown in Figs. 3 and 4, compared with the all-MORB average and the Lower Paleozoic tholeiitic MORB-type El Nihuil dolerites, the CNVC alkali basaltic and basanitic lavas likely result from lower partial melting rates.

The partial melting and fractional crystallization patterns of the Cerro Nevado Volcanic Complex lavas are shown in Figs. 6a and 8 and compared with those from the Auca Mahuida shield volcano and the Payún area volcanoes. To better characterize them, we use the compatible MRFE (mantle rock-forming elements) Mg, the HFSE (high field strength elements) Ti, Th and Hf, the LREE (light rare earth element) La and the HREE (heavy rare earth element) Yb.

Even if the three volcanic complexes might seem to have similar behaviors, Fig. 8a displays a different pattern for the CNVC magmas. The abrupt drop of MgO coupled with quasi-constant Th for CNVC rocks (Fig. 8a) can be related to fractional crystallization, as for Auca Mahuida and Payún. However, the linear MgO-Th correlation (Fig. 8a) shown by the low-MgO magmas from the CNVC suggests mixing, either magma mixing or crustal contamination, known to generate such linear trends. This low-slope correlation is clearly different from the pattern displayed by the most silicic samples of Auca Mahuida and Payún even if some magma mixing has also been suggested for them (see e.g. Kay et al., 2013). In the Th/Hf versus Th plot of Fig. 8b, because Hf is only slightly less incompatible than Th in basaltic magmas, the steep linear trend shown by the CNVC basalts indicates partial melting (Pallares et al., 2016), while the linear trend with lower slope at high Th contents again points to fractional crystallization or mixing or both (Fig. 8b). The latter trend is formed by intermediate and evolved CNVC lavas, and, if ascribed to fractional crystallization, indicates involvement of Hf-bearing mineral phases such as zircon or apatite. Indeed, thin sections of basaltic lavas show olivine and pyroxenes with apatite inclusions but no zircon, and this is confirmed by geochemical diagrams (not shown) where Zr clearly behaves as an incompatible element while P₂O₅ behaves much less incompatibly. At the same time, the trend possibly involves some crustal assimilation, as confirmed by data points for the basement (Fig. 9 and 10, not shown for clarity in Fig. 8).
Based on the data generated in this study, we can get information on partial melting by considering the high-MgO primitive and low-MgO evolved basalts (as defined in section 4.4), assuming that such lavas with relatively elevated MgO remained in a magma chamber during a negligible time. The timing of partial melting is thus recorded by the high-MgO and evolved basaltic rocks 94D2, 94BB, 94BH, 94M, 94BD and 94BJ and its rate increases from 94D2 to 94BJ as shown by the arrow in Fig. 8b. The oldest partial melting event is dated at 1.878 ± 0.015 Ma (94BB) and produced high-MgO basaltic magmas in the southern part of the complex (Figs. 2, 6 and 8). At 1.352 ± 0.020 Ma (94BD), a second melting event also produced high-MgO basaltic magmas, which erupted in the northern part (Figs. 2, 6 and 8).

Therefore, we suggest that either the thermal anomaly responsible for melting would have migrated from the south to the northern part of the CNVC, probably from Payún area volcanoes (Figs. 1 and 2), or alternatively, that it was present beneath the whole PVP during the last 2 My, generating eruptions randomly located in the complex.

The third partial melting episode generated the first low-MgO basaltic magmas (indicating shallower melting, see e.g. Lee and Chin, 2014) at 1.044 ± 0.015 Ma (94D2), followed by increasing partial melting rates (Fig. 8b) leading to the fourth melting event and eruption of basaltic rocks at 1.022 ± 0.015 Ma (94M), both erupted in the western area. Considering the uncertainties of these two ages, one might consider them as related to a single extended melting event. Later, primitive and evolved basaltic magmas erupted mainly in the northern part of the complex (Cerro Ponon Trehue; Fig. 2) suggesting either a local migration of the thermal anomaly or the rise of a new one: the most important partial melting rate identified in this work (Fig. 8b) characterizes the fifth event of partial melting, which produced low-MgO basaltic magmas again at 0.944 ± 0.016 Ma (94BJ), followed by the sixth melting episode and last high-MgO basaltic eruption also recorded in the Cerro Ponon Trehue at 0.925 ± 0.015 Ma (94BH).

In the table of Fig. 6b, we also report ages from the literature for 6 more samples. Overall, we observe an important volcanic activity, with a series of partial melting events and quiescent periods of 50-300 ka. This succession of partial melting events, with higher and lower melting rates, produced lavas between 1.88 Ma to 0.75 Ma (Fig. 6b). Considering the whole PVP, volcanism continued until much younger times, as shown by the very recent activity of Los Volcanes in the Payún area, measured at 7±1 ka (Germa et al., 2010). We also
remark that the MgO values measured here are at a maximum of 9.8% (Table 1) and reach 11% in the literature (Fig. 6a). According to Lee and Chin (2014; see their Fig. 1 and 2), such values correspond to a temperature of ~1300°C and pressure of ~1 GPa in cratonic peridotite melting. Such values roughly correspond to depths of ~30-35 km, in perfect agreement with the depths of the asthenospheric plume head seen below Payún and Auca Mahuida from magnetotelluric data by Burd et al. (2014), and shown in Richarte et al. (2018; Fig. 1). Therefore, the PVP volcanism, active between 2.03 ± 0.03 Ma (Rossello et al., 2002) and ~7000 years ago, suggests that a thermal system is still there beneath the northern Payenia Volcanic Province and especially the Cerro Nevado Volcanic Complex (Fig. 1; Richarte et al., 2018).

The timing of fractional crystallization is difficult to determine with precision due to the relatively small number of dated intermediate and evolved lavas. However, Figs. 6a and 8b suggest an overall temporal evolution from low-MgO basaltic and trachybasaltic/trachyandesitic to trachytic magmas (Fig. 6a) which emplaced the Cerro Nevado Formation (Fig. 2 and 6b) during eruptive phases 3 and 4 (1.336 ± 0.019 to 1.244 ± 0.018 Ma Fig. 6b, samples 94H, 94I1, 94J, 94L, 94I2, 94F, and 94K).

Finally, even if the ages shown in Fig. 6b could suggest that the Chapúa group is built at two different periods, the old and young members being separated by the Cerro Nevado Formation, only eruption times are distinct, while geochemical characteristics are very similar.

573

574 5.2.2. *Partial melting rates*

In order to evaluate the partial melting rates, we adapted the petrologic model from Luhr et al. (1995) using their La/Yb vs. Yb plot (Fig. 8c), where relative garnet and spinel percentages of a hypothetical enriched (lherzolitic) mantle source are shown. Yb is less incompatible than La and, like other HREE and Y, it is significantly incorporated into garnet. According to Fig. 8c, basaltic lavas from the CNVC would have been generated by partial melting of a lherzolitic mantle with melting rates of F= 4-12% (the evolved lavas fall between 4 and 3% but were perturbated by assimilation, see below) where the smallest F values (literature data) correspond to melting of a deeper mantle.

575

576

577

578

579

580

581

582
The samples 94BJ and 94BD of this study (green triangles close to the dashed line at F=0.1) were generated with the same highest 10% melting rate, but at different periods of time (Fig. 8b-c and Fig. 6), 94BD, 1.352 Ma ago and 94BJ, 0.944 Ma ago. This model thus shows that melting probably started in a deeper mantle source, which contained more garnet, between 50 and 30%, and where the melting rate was 4%-8% (this produced mainly literature samples) and continued in a shallower mantle source, with less than 30% of garnet, with melting rates up to between 8 and 12% (Fig. 8c). We know that values above 8-10% are less consistent with alkali magmas genesis, however, these characteristics could strongly suggest a rising zone in the mantle, probably a thermal plume (see below and section 5.2.4.)

592

593 **5.2.3. Trace elements, HFSE, isotopes and nature of the magma sources**

594 **5.2.3.1. Trace element patterns**

Fig. 4 shows the trace element patterns normalized to Primitive Mantle, N-MORB and OIB, all from Sun and McDonough (1989). These kinds of diagram are widely used to study sources of volcanic rocks and series. These normalizations strikingly show that the present patterns are much closer to OIB than the two other source types, as the shape of the OIB-normalized pattern is globally horizontal (Fig. 4). In addition, the patterns normalized to OIB all have the same shape, but more evolved rocks have indeed higher normalized concentrations compared to less evolved ones, as expected. It is also seen that the oldest and youngest Chapúa group samples have the same pattern and the same normalized concentrations. Trace elements thus clearly point to an OIB source.

604

605 **5.2.3.2. HFSE Ti, Nb and Ta**

The slightly lower Ti, Nb and Ta contents of the CNVC *basaltic* magmas (not the evolved compositions) compared to basaltic magmas from Auca Mahuida and the Payún area volcanoes (Figs. 3b and c, 4, 6a and 9) might be influenced by the low K<sub>D</sub> of these elements in spinel (K<sub>D</sub>(Ti) = 0.048; McKenzie and O'Nions, 1991) relative to garnet (K<sub>D</sub>(Ti) = between 0.1 and 0.688; McKenzie and O'Nions, 1991; Hauri et al., 1994). The source for the Payún area volcanoes is seen as slightly more spinel rich in Fig. 8c, thus having generated melts with more Ti, while it appears identical to the source for Auca Mahuida in this figure.
This effect is also seen inside the present database in Figs. 8b and c, where samples 94BJ and 94BD result from higher partial melting rates of a more spinel-rich shallower mantle than the others, and also display the highest TiO₂ contents (1.66 and 1.53 respectively; Table 1, Fig. 6a).

Alternatively, the Ti differences between the basalts of the Cerro Nevado Volcanic Complex and those of the Auca Mahuida and Payún volcanoes could result from a source effect (Figs. 3b and c, 6a, 9a). The average concentration of Ti (Fig. 9a) in OIB magmas is about 17,200 ppm (Sun and McDonough, 1989). In *basaltic* CNVC magmas, the average Ti is about 9,000 ppm, close to but slightly lower than the average Ti for Auca Mahuida and Payún area basaltic magmas (averages not shown in Fig. 9), all being close to the ALL MORB Ti of Gale et al. (2013), although the Auca Mahuida and Payún basalts are considered to be of OIB-type (Kay et al., 2004, 2006a, 2013; Bertotto et al., 2009; Germa et al., 2010; Søager et al., 2013; Søager and Holm, 2013; Hernando et al., 2014a; Søager et al., 2015b; Pallares et al., 2016). Such slight relative depletions in HFSE such as Ti, Nb and Ta (Figs. 4 and 9) in the CNVC alkali basalts include Nb/La ratios lower than 1 (Fig. 9a), which is very different from Auca Mahuida and Payún basalts. This slightly depleted character of the CNVC basalts could be due either to metasomatism of typical OIB mantle sources (e.g. Auca Mahuida and/or Payún type) by processes related to subduction of the Nazca plate (low Nb and Ta), or to contamination by continental crust (Bermúdez, 1991; Bermúdez et al., 1993; Kay et al., 2006a; Søager et al., 2013, 2015a), or both.

Altogether, trace elements, especially HFSE, point to an OIB-type mantle source probably modified by subduction and mixing with continental material.

### 5.2.3.3. *Sr and Nd isotope, major elements and mantle sources*

The ¹⁴³Nd/¹⁴⁴Nd (between 0.51270 and 0.51285) and the ⁸⁷Sr/⁸⁶Sr (from 0.7037 to 0.7047) isotope ratios of CNVC samples (Table 2) are shown in Fig. 5 (including Auca Mahuida and Payún area data). The studied rocks form an inverse trend in a limited region of the MORB array, but plot relatively far from the depleted MORB mantle (DMM) or from the ALL MORB point in Fig. 5, in a region of the diagram hosting data for Indian Ocean MORB and Crozet Island (Hofmann, 1997; Meyzen et al. 2005). This inverse trend can be considered to result from mixing between at least two end-members being DMM (or perhaps HIMU, but
this is less likely) and either the Bulk Silicate Earth (BSE), or more likely, the EMI mantle component (see Fig. 5) or the continental crust basement (see Cerro La Ventana formation in Fig. 5). Most likely, we see a mixture of DMM mantle and another end member being either an OIB mantle source with an EMI character (Soager et al., 2013) or the local San Rafael Block basement, or both.

Combining LILE (K), HFSE (Ti) and MRFE elements (Ca and Al), we plot in Fig. 10a the basaltic samples in the K₂O/TiO₂ vs. CaO/Al₂O₃ diagram from Jackson and Dasgupta (2008).

Overall, the CNVC samples, together with those from Auca Mahuida and the Payún area volcanoes, display low CaO/Al₂O₃ and high K₂O/TiO₂, and plot close to the EMI point. This plot confirms that the Payenia Volcanic Province back-arc alkaline magmas bear an EMI signature, as noticed from the Sr and Nd isotopic ratios shown in Fig. 5. In addition, the MgO average value of the EMI component (10.73; Jackson and Dasgupta, 2008) and the Th average for EMI from Tristan da Cunha (6.8; Hofmann, 1997) or from Pitcairn Seamounts (2.55), where sedimentary recycling was evidenced (Eisele et al., 2002), are consistent with an EMI signature for the CNVC basaltic magmas (Figs. 8a and 10b), especially when compared with the OIB-type Auca Mahuida magmas, which are considered deriving from an EMI-type garnet-bearing or garnet-spinel bearing OIB-type mantle source (Kay et al., 2006a, 2013; Pallares et al., 2016). The MORB-like imprint of the CNVC basaltic magmas suggested by Søager et al. (2013, 2015a) is barely perceptible in some diagrams (e.g. Figs. 4, 7), while in other diagrams as in Fig. 5 or 9c, the data plot within the tholeiitic and MORB fields or close to average continental crust, including the local San Rafael Block (La Horqueta and La Ventana formations in Fig. 9c from Fitton et al., 2007).

Altogether, an OIB mantle source is confirmed by Sr and Nd isotopes, which further indicate an OIB source with EMI flavor, mixed with ambient depleted MORB mantle, together with an influence of continental crust assimilation.

#### 5.2.3.4. *Sr and Nd isotopes and assimilation of continental material in magma chambers*

We remark that the Sr and Nd isotopic ratios of the Cerro Nevado Formation samples point to a more crustal signature, at the lower right-hand side of the Sr-Nd isotopic trend (see dark brown labels in Fig. 5 and 6). These samples also are the most evolved since they include basaltic thrachyandesite and thrachyandesite compositions. This strongly suggests that
isotopic ratios do correlate with differentiation for the Cerro Nevado *Formation*, where magma stays in a magma chamber suffering both crystallization and wall digestion. Further, this can also be observed for the Chapúa group: in the Lower Chapúa group, our oldest sample 94BB (dated at 1.878 ± 0.028 Ma) lies at the upper left edge of the Sr-Nd isotopic correlation (Fig. 5) while sample 94BD (1.352 ± 0.020 Ma) lies in the middle of the correlation. Although both are alkali basalts, the isotopes record the influence of assimilation. Then, in the upper Chapúa group, sample 94D2 has again almost the same Sr and Nd isotopic ratios as 94BB: this suggests that the magma chamber was replenished with the same magma composition (or very close). Then, samples 94M and 94BJ again exhibit mixing with more crustal signatures. In addition to this, 94BH (0.925 ± 0.015 Ma) is again more OIB-type pointing to another replenishment. This is shown in Fig. 5 with the same color code as in Fig. 6, and the ages indicated.

### 5.2.3.5. Additional trace element constraints on mantle sources and magmatic processes

Up to now, the geochemical characteristics such as normalized trace elements (Fig. 4), depletions in Ti, Nb and Ta (Figs. 4 and 9), the Nb/La ratios lower than 1 (Fig. 9a), the geochemical signature shown by major element ratios (Fig. 10a), the Sr and Nd isotopic ratios (Fig. 5), among others, show that the Cerro Nevado Volcanic Complex alkali basalts should be associated either with the metasomatism of mainly an OIB-type mantle source by aqueous fluids or magmas during subduction of the Nazca plate or to its contamination by the continental crust (Bermúdez, 1991; Bermúdez et al., 1993; Kay et al., 2006a; Søager et al., 2013, 2015a), especially the local basement San Rafael Block (Figs 5, 9c, 10b).

In the Ba/La vs. Ta/Hf plot shown in Fig. 11, the CNVC basaltic magmas plot between those from Auca Mahuida and the arc magmas of the Southern Volcanic Zone, which this diagram indicates to have respectively OIB-type ‘intraplate or enriched’ and ‘MORB or Arc’ mantle sources, again pointing to a mantle source being a mixture of OIB-type and MORB-like. The use of Ce/Pb and Nb/U ratios (Fig. 12a) does not help to better distinguish between OIB-type and MORB-like (Hofmann et al., 1986; Hofmann, 2014).

When compared with the Auca Mahuida shield volcano, which has La/Ta ratios <18, the La/Ta ratios of ~30 (not shown) measured in the CNVC basaltic rocks rather support that subduction-related processes affected these magmas.
To summarize, based on available data, the genesis and evolution of the basaltic magmas of the Cerro Nevado Volcanic Complex appear to involve an EMI-like garnet-bearing or garnet-spinel bearing OIB-type mantle source with a small contribution of the MORB-like mantle, modified by subduction-related processes, and having produced melts ultimately affected by crustal assimilation.

711

### 5.2.3.6. Trace elements constraints on the crustal contribution from the San Rafael Block

Both OIB-type and MORB-like mantle are relatively depleted in incompatible elements such as K, Rb, Th, U, Pb and of course Si, when compared with continental crust (Hofmann et al., 1986; Hofmann, 2014). Hence, these elements are sensitive to any admixture of continental crust material to melts derived from OIB or MORB sources. Conversely, HFSE like Ti, Nb and Ta display negative anomalies with respect to elements of similar incompatibility in the continental crust and arc-derived rocks, while they are relatively abundant in basaltic magmas extracted from N-MORB (Nb: 2.33 ppm; Ta: 0.132 ppm), E-MORB (Nb: 8.3 ppm; Ta: 0.47 ppm) and even more in OIB (Nb: 48 ppm; Ta: 2.7 ppm) (Sun and McDonough, 1989). These elements are also sensitive to any mixing of OIB or MORB with continental material.

Fig. 8a shows that the high-MgO basalts from the Cerro Nevado Volcanic Complex have MgO contents similar to those from the Auca Mahuida and Payún area volcanoes, and their Th contents are just slightly more important than for the latter two. These basalts spread over an area that can be ascribed to crystallization, as for Payún and Auca Mahuida samples shown in Fig. 8a (see above, section 5.2.1). However, in Fig. 8a, the low-MgO and trachytic samples do not follow a typical fractional crystallization trend as may be expected from Fig. 3a, but rather display a linear trend, which suggests that the evolved magmas from the CNVC incorporated a continental crust component during their magmatic evolution (Figs. 8a and 10b). This crustal involvement is also seen in Fig. 5 with Sr and Nd isotopes and shown to be concomitant to crystallization, as described in sections 5.2.3.3. and 5.2.3.4. This continental crust component should be searched in the local basement, being here the so-called San Rafael Block, which includes (Fig. 10b) the Rodeo de la Bordalesa Tonalite (Rio de los Castaños Formation), the siliciclastic La Horqueta Formation, and the metagranitic Cerro La Ventana Formation (see also Fig. 2 and 5). The same logic can be seen in the Nb/Y
vs. Zr/Y diagram (Fig. 9c), where these continental geological formations plot rather close to some of the studied lavas.

Hofmann et al. (1986) established that very similar Ce/Pb (25 ± 5) and Nb/U (47 ± 10) ratios characterize both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB), as well as values of Ce/Pb=4 and Nb/U=10 for the average continental crust (Fig. 12; see also Hofmann, 1997, 2014). The continental crust is also characterized by high ⁸⁷Sr/⁸⁶Sr ratios of about 0.706 and higher (Fig. 12). The Ce/Pb (< 10) and Nb/U (< 20) ratios of the CNVC magmas displayed in Fig. 12 are thus much lower than those of MORB and OIB. These low Ce/Pb and Nb/U ratios are correlated (Fig. 12a), again consistent with a contribution of the San Rafael Block continental crust to the evolved CNVC magmas during their ascent through this block and stay in magma chambers (from Fig. 12a, this is also true for the Auca Mahuida and Payún alkaline lavas). The ⁸⁷Sr/⁸⁶Sr ratios of the CNVC lavas (from 0.7037 to 0.7042, excluding pumice 94G) follow the same logic, as shown by the correlation diagrams of Figs. 12b and c. In the three diagrams of Fig. 12a,b and c, especially the Cerro la Ventana Formation and La Horqueta Formation appear to be possible crustal end-members. The most important magma – crust interactions were mainly possible during at least 112,000 years, between 1.336 ± 0.019 and 1.244 ± 0.018 Ma (Fig. 6), when the basaltic magmas hosted within the San Rafael Block underwent fractional crystallization, either in magma chambers or upon magma ascent, before the eruptions which built the Cerro Nevado Formation (Fig. 2).

Although this does not rule out the involvement of subduction-related processes, the trace elements, therefore, confirm that assimilation of continental crustal rocks occurred and allow identifying which local continental rocks are likely to have been the end-members for this.

### 5.2.4. Relationships with the neighboring intraplate back-arc volcanoes and thermal plume

#### 5.2.4.1. Crustal density below the several volcanoes

Although shallow asthenospheric plumes like those identified at depths of 33 or 52 km beneath El Tromen, Auca Mahuida and the Payún area volcanoes (where underlying continental crust is thinned) have not been evidenced beneath the San Rafael Block, it is known that deeper hot thermal anomalies exist below the Payenia Volcanic Province at 129 and 200 km depth (Fig. 1; Burd et al., 2014; Richarte et al., 2018; Astort et al., 2019).
767 Gravimetric inversion by Richarte et al. (2018) indicates that the crust of the San Rafael
768 Block has densities between 2.67 and 2.89 g/cm³ while, in the rest of the Payenia Volcanic
769 Province, crustal densities strongly decrease (down to 2.47 at ~10-15 km depth). This is
770 attributed to the rise of upper mantle hotter material, i.e. a thermal plume, below the thinned
771 lithosphere in the southern Payenia region by Richarte et al. (2018), a plume that would have
772 brought heat from below, together with upper mantle materials, certainly magmas, thus
773 decreasing crustal density. Such low crustal densities beneath the Payenia Volcanic Province
774 (and Payún area volcanoes) are similar to those measured under the CNVC (Fig. 5 in Richarte
775 et al., 2018). Similarly, Geoid undulation anomalies in the southern part of the Payenia
776 Volcanic Province show that uprising material from at least 600 km deep is feeding the Auca
777 Mahuida shield volcano (Astort et al., 2019). The latter arguments and the geochemical
778 characteristics shown by the CNVC alkaline magmas (Figs. 3a, 4, 7, 9 and 12a) support the
779 hypothesis that a thermal mantle plume rose below the thinned Payenia lithosphere, providing
780 the OIB-type magmatic source similar to that of Auca Mahuida and Payún lavas (see also
781 section 5.2.2).

782

### 5.2.4.2. Assimilation, lithospheric thickness and geographical position

783 As shown above, the hypothesis of continental crust contamination of the CVNC magmas
784 during their ascent through the San Rafael Block, and occasional storage within it, is
785 consistent with their enrichments in K, Rb, Th, U, Pb and Zr, as well as low Nb (Fig. 4),
786 important variations of Zr/Y ratios at relatively constant Nb/Y ratios (Fig. 9c), low Ce/Pb
787 and Nb/U ratios and relatively high ⁸⁷Sr/⁸⁶Sr (from 0.7037 to 0.7042) isotopic ratios (Fig.
788 12). The fact that such contamination effects are not documented for Auca Mahuida and
789 Payún volcanoes may be related to the geographical position of the CNVC. Indeed, compared
790 to these two volcanic centers, the CNVC is the easternmost one, thus farthest from the
791 Andean arc. This could be related to differential steepenings of the Nazca plate after the
792 Payenia flat slab event: While the subducted oceanic plate barely bent under the CNVC, it
793 got steeper beneath the Payenia region and the Auca Mahuida and Payún Volcanoes further
794 South. Therefore, the CNVC magmas were not emplaced in a thinned crust and lithosphere,
795 like for the PVP region, but rather, over a still thicker San Rafael foreland region. Therefore,
796 magmas erupted at the Cerro Nevado Volcanic Complex had more chance to be affected by
797
798 crustal contamination than those erupted at the Auca Mahuida and Payún volcanoes, where
799 extension produced rapid ways to transit through.

800

## 6. Conclusions

802

803 Geochemical data show that the CNVC magmas are alkaline and OIB-type. Major elements
804 and CIPW compositions indicate that the CNVC series derive from silica-undersaturated
805 magmas (basanites, alkali basalts, trachybasalts). The geochemical behavior of the CNVC
806 lavas is similar to those of Auca Mahuida and especially of the Payún area volcanoes, except
807 for their slightly lower TiO₂, Nb, and Ta contents. The CNVC lavas would be derived from
808 the melting of an OIB-type mantle source metasomatized by fluids during subduction of the
809 Nazca plate, possibly mixed with melts from the MORB-like asthenosphere, and finally
810 contaminated by continental crust.

811

812 1. Our new unspiked K-Ar ages range from 1.316 ± 0.019 to 0.925 ± 0.015 Ma. Combined
813 with those previously published, they indicate that the Chapúa group and Cerro Nevado
814 Formation of the CNVC have emplaced between 1.878 ± 0.028 and 0.750 ± 0.051 Ma.

815

816 2. We identify seven eruptive phases: 1) alkali basaltic lavas from the Chapúa group
817 emplaced at 1.878 ± 0.028 Ma, 2) alkaline eruptions of the Cerro Nihuil at 1.474 ± 0.089 Ma
818 and the Cerro Solo at 1.352 ± 0.020 Ma, 3) beginning of the Cerro Nevado Formation
819 emplacement from 1.336 ± 0.019 to 1.316 ± 0.019 Ma, followed by phase 4) at 1.224 ± 0.018
820 Ma, 5) resumption of the Chapúa group activity at 1.044 ± 0.015 and 1.022 ± 0.015 Ma, 6)
821 eruption of high-MgO basalts between 0.944 ± 0.016 and 0.805 ± 0.051 Ma, and 7) final
822 stage, being the emplacement of high-MgO basalts from the Solo volcano at 0.750 ± 0.051
823 Ma.

823

824 The whole eruptive sequence is ascribed to five partial melting events at 1.878 ± 0.015 Ma,
825 at 1.352 ± 0.020 Ma, at 1.044 ± 0.015 Ma-1.022 ± 0.015 Ma, at 0.944 ± 0.016 and at 0.925
826 ± 0.015 Ma.

826

827
3. Based on major and trace elements, as well as Sr and Nd isotopes, we suggest that the CNVC magmas derive from continuous partial melting rates of an EMI-like OIB-type garnet-spinel source rising from mantle depths, and are similar to those of the neighboring Auca Mahuida shield volcano (Kay et al., 2006a; 2013; Pallares et al., 2016).

4. The magmas of the CNVC were affected simultaneously by differentiation processes, including fractional crystallization, and crustal contamination, in magma chambers of the San Rafael Block basement, with several replenishments of fresh magma (from the systematics of the low Ce/Pb and Nb/U ratios and ⁸⁷Sr/⁸⁶Sr ratios between 0.7037 and 0.7042).

5. The temporal and spatial characteristics of the CNVC suggest that the Pleistocene magmatic activity started 1.9-1.8 Ma ago within the whole back-arc PVP area, including Auca Mahuida/El Tromen/Payún area volcanoes. Their origin is attributed to a hot thermal mantle anomaly that remained stagnant beneath the PVP at least during the last 2 My, as suggested by trace element ratios such as Ba/La (5-35) and Ta/Hf (0.17-0.52) and densities about 2.67 g/cm³ displayed by the lithosphere beneath the Payún area volcanoes and the CNVC.

## Acknowledgments

This work was supported by the INSU-CNRS DyETI program, and RADAKIFA (n°239869), a research project funded by French INSU-CNRS in 2020. We thank anonymous Reviewer 1 for his/her comments, which helped us to improve this manuscript.

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1048 Lopez-Escobar, L., Vergara, M.M., Frey F.A., 1981. Petrology and Geochemistry of Lavas from Antuco Volcano, a Basaltic Volcano of the Southern Andes (37°25′). Journal of Volcanology and Geothermal Research 11, 329-352, doi: 10.1016/0377-0273(81)90030-5.

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1055 Macdonald, G.A., Katsura, T., 1964. Chemical composition of Hawaiian lavas. Journal of Petrology 5, 82-133.

1057 McKenzie, D., O'Nions, R.K., 1991. Partial melt distributions from inversion of rare Earth element concentrations. Journal of Petrology 32: 1,021-1,091.

1059 Meyzen, C.M., Ludden, J.M., Humler, E., Luais, B., Toplis, M., Mével, C. and Storey, M., 2005. New insights into the origin and distribution of the DUPAL isotope anomaly in the Indian Ocean mantle from MORB of the Southwest Indian Ridge. Geochem. Geophys. Geosyst., 6, Q11K11, doi:10.1029/2005GC000979.

1063 Muñoz, J.B., Stern, C.R., 1988. The Quaternary Volcanic Belt of the Southern Continental Margin of South America: Traverse Structural and Petrochemical Variations Across the Segment Between 38°S and 39°S. Journal of South American Earth Sciences 1, 147-161, doi: 10.1016/0895-9811(88)90032-6.

1067 Muñoz, J.B., Stern, C.R., Bermúdez, A., Delpino, D., Dobbs, M.F., Frey, F.A., 1989. Plio-Quaternary Volcanism between 34-39°S of the Andes. Revista de la Asociación Geológica Argentina 44, 270-286

1070 Pallares, C., Quidelleur, X., Gillot, P.Y., Kluska, J.M., Tchilinguirian, P., Sarda, P., 2016. The temporal evolution of back-arc magmas from the Auca Mahuida shield volcano
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1074 Pallares, C., Quidelleur, X., Debreil, J.A., Antoine, C., Sarda, P., Tchilinguirian, P., Delpech, G., Gillot, P.-Y., 2019. Quaternary evolution of the El Tromen volcanic system, Argentina, based on new K-Ar and geochemical data: Insights for temporal evolution of magmatic processes between arc and back-arc settings. Journal of South American Earth Sciences 90, 338-354. https://doi.org/10.1016/j.jsames.2018.12.022

1079 Pin, C., Santos Zalduegui, J.F., 1997. Sequential separation of light-rare-earth elements, thorium and uranium by miniaturized extraction chromatography: Application to isotopic analyses of silicate rocks. Anal. Chim. Acta 339, 79-89.

1082 Quidelleur, X., Gillot, P.-Y., Soler, V., Lefèvre, J.C., 2001. K/Ar dating extended into the last millennium: application to the youngest effusive episode of the Teide Volcano (Spain). Geophysical Research Letters, 28(16), 3067-3070.

1085 Quidelleur, X., Carlut, J., Tchilinguirian, P., Germa, A., Gillot, P.-Y., 2009. Paleomagnetic directions from mid-latitude sites in the southern hemisphere (Argentina): contribution to time averaged field models. Physics of the Earth and Planetary Interiors 172, 199-209.

1089 Ramos, V.A., 2004. Cuyania, an exotic block to Gondwana: review of a historical success and the present problems. Gondwana Research 7, 1009-1026.

1091 Ramos, V.A., Folguera, A., 2005. Tectonic evolution of the Andes of Neuquén: constraints derived from the magmatic arc and foreland deformation. In: Veiga, G. (Ed.), The Neuquén Basin: a Case Study in Sequence Stratigraphy and Basin Dynamics, vol. 252. The Geological Society, Special Publication, 15-35.

1095 Ramos, V.A., Kay, S., 2006. Overview of the tectonic evolution of the southern central andes of mendoza and neuquen (35°-39°S). In: Kay, S.M., Ramos, V.A. (Eds.), Evolution of an Andean Margin: a Tectonic and Magmatic View from the Andes to the Neuquén Basin (35°-39°S Lat), vol. 407. Geological Society of America, Special Paper, 1-17.

1099 Ramos, V.A., Folguera, A., 2011. Payenia volcanic province in the Southern Andes: An appraisal of an exceptional Quaternary tectonic setting. Journal of Volcanology and Geothermal Research, 201, 53-64. Ramos, V.A., Folguera, A., 2011. Payenia volcanic
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1104 Ramos, V.A., Litvak, V.D., Folguera, A., Spagnuolo, M., 2014. An Andean tectonic cycle:
1105 from crustal thickening to extension in a thin crust (34°-37°SL). Geosciences Frontiers
1106 5, 351-367.

1107 Richarte, D., Lupari, M., Pesce, A., Nacif, S., Gimenez, M., 2018. 3-D crustal-scale gravity
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1109 Geoscience Frontiers 9, 239-248. http://dx.doi.org/10.1016/j.gsf.2017.03.004

1110 Rodríguez, C., Sellés, D., Dungan, M., Langmuir, C., Leeman, W., 2007. Adakitic Dacites
1111 Formed by Intracrystal Crystal Fractionation of Water-rich Parent Magmas at Nevado
1112 de Longaví Volcano (3628S; Andean Southern Volcanic Zone, Central Chile). Journal
1113 of Petrology 48, 11, 2033-2061, doi:10.1093/petrology/egm049.

1114 Rossello, E.A., Cobbold, P.R., Diraison, M., Arnaud, N., 2002. Auca Mahuida (Neuquén
1115 Basin, Argentina): a Quaternary shield volcano on a hydrocarbon-producing substrate.
1116 5th International Symposium on Andean Geodynamics (Toulouse), Extended
1117 Abstracts, pp. 549-552.

1118 Schwarz, W.H., Trieloff, M., 2007. Intercalibration of Ar-40-Ar-39 age standards NL-25,
1119 HB3gr hornblende, GA1550, SB-3, HD-B1 biotite and BMus/2 muscovite. Chemical
1120 Geology, 242(1-2): 218-231.

1121 Sepúlveda, E.G., Bermúdez, A., Bordonaro, O., Delpino, D., 2007. Hoja Geológica 3569-IV,
1122 Embalse El Nihuil, provincia de Mendoza. Instituto de Geología y Recursos Minerales,
1123 Servicio Geológico Minero Argentino. Boletín 268, segunda edición revisada, 52p.
1124 Buenos Aires.

1125 Søager, N., Holm, P.M., Llambías, E.J., 2013. Payenia volcanic province, southern Mendoza,
1126 Argentina: OIB mantle upwelling in a backarc environment. Chemical Geology, 349-
1127 350, 36-53. https://doi.org/10.1016/j.chemgeo.2013.04.007

1128 Søager, N., Holm, P.M., 2013. Melt-peridotite reactions in upwelling eclogite bodies:
1129 constraints from EMI-type alkaline basalts in Payenia, Argentina. Chemical Geology
1130 360-361, 204-219. https://doi.org/10.1016/j.chemgeo.2013.10.024.
1131 Søager, N., Holm, P.M., Thirlwall, M.F., 2015a. Sr, Nd, Pb and Hf isotopic constraints on
1132 mantle sources and crustal contaminants in the Payenia volcanic province, Argentina.
1133 Lithos 212–215, 368–378.

1134 Søager, N., Portnyagin, M., Hoernle, K., Holm, P.M., Hauff, F., Garbe-Schönberg, D.,
1135 2015b. Olivine major and trace element compositions in southern Payenia basalts,
1136 Argentina: evidence for pyroxenite-peridotite melt mixing in a back-arc setting.
1137 Journal of Petrology 1–24. https://doi.org/10.1093/petrology/egv043.

1138 Steiger, R.H., Jäger, E., 1977. Subcommission on Geochronology: convention on the use of
decay constants in Geo and Cosmochronology. Earth and Planetary Science Letters,
1139 36(3), 359-362.
1140

1141 Stern, C.R., 2004. Active Andean volcanism: its geologic and tectonic setting. Revista
Geológica de Chile, 31-2, 161-206.
1142

1143 Sun, S.S., McDonough, W.F., 1989. Chemical and Isotopic Systematics of Oceanic Basalts: Implications for Mantle Composition and Processes. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism in the Ocean Basin, vol. 42. Geological Society of London Special Publication, pp. 313-345.
1144
1145
1146

1147 Thomas, W.A., Astini, R.A., 1999. Simple-shear conjugate rift margins of the Argentine
1148 Precordillera and the Ouachita embayment of Laurentia. Geological Society of
1149 America Bulletin 111, 1069-1079.

1150 Thompson, R.N., 1984. Dispatches from the basalt front; l, experiments. Proc. Geol. Assoc.
1151 95 (3), 249-262.

1152 Varekamp, J.C., Maarten deMoor, J., Merrill, M.D., Colvin, A.S., Goss, A.R.,, Vroon, P.Z.,
1153 Hilton, D.R., 2006. Geochemistry and isotopic characteristics of the Caviahue-
1154 Copahue volcanic complex, Province of Neuquén, Argentina. In: Kay, S.M., Ramos,
1155 V.A. (Eds.), Evolution of an Andean Margin: A tectonic and magmatic view from the
1156 Andes to the Neuquén Basin (35-39°S): Geological Society of America, Special Paper,
1157 407, 317-342. doi: 10.1130/2006.2407(15).

1158 Yoder, H.S., Tilley, C.E., 1962. Origin of basalt magmas: an experimental study of natural
1159 and synthetic rock systems. Journal of Petrology 3, 342-532.

1160

1161 **Figures captions**
1162 Fig. 1. Location of the Cerro Nevado Volcanic Complex and surrounding volcanoes with respect to Main Cordillera and the Payenia Volcanic Province. Modified from Richarte
1163 et al. (2018). Magnetotelluric depths of asthenospheric plume are from Burd et al.
1164 (2014). Tectonic features are from Ramos and Kay (2006), Llambías et al. (2010) and
1165 Ramos and Folguera (2011). Purple and light pink: the San Rafael Block, brown light
1166 color: Pleisto-Holocene volcanism, brown: Pleistocene Payún Matru, Payún
1167 volcanism, dark brown color: Pleisto-Holocene Los Volcanes volcanism.
1168

1169 Fig. 2. Shown are the main geologic formations within the studied area according to the
1170 Embalse el Nihuil geological map (Hoja Geológica 3965-IV from Sepúlveda et al.,
1171 2007), Province of Mendoza, Argentina. Geological formations are listed in
1172 stratigraphic order. Green triangles: location of studied samples; text and labels in dark
1173 green: high-MgO primitive basalts; text and labels in light green : low-MgO evolved
1174 basalts; yellow labels: basaltic-andesitic rocks; white labels: trachytic rocks. The ages
1175 for volcanoes or cerros when displaying an asterisk were measured by Folguera et al.
1176 (2009). The ages showing a cross are from Quidelleur et al. (2009).

1177 Fig. 3. (a) Total-Alkali-Silica (TAS) plot for the CNVC studied lavas, according to Le Bas
1178 et al. (1986). The dotted line separating the fields for tholeiitic (Thol.) and alkali (Alk.)
1179 basalts is from Macdonald and Katsura (1964). (b) TiO₂ vs. SiO₂, (c) TiO₂
1180 vs. Na₂O+K₂O. Green triangles : this work; empty triangles : other data; light-blue field
1181 with blue border: Auca Mahuida volcano; light red field: Payún area volcanoes; empty
1182 field with green border: El Nihuil MORB-type dolerites (San Rafael Block); black star:
1183 ALL MORB average (Gale et alL, 2013). Geochemical data are from: Pallares et al.
1184 (2016), Kay et al. (2006a), Kay et al. (2006b), Kay et al. (2004), Kay et al. (2013),
1185 Gudnason et al. (2012), Germa et al. (2010), Søager et al. (2013), Hernando et al.
1186 (2014a,b), Frey et al. (1984), Ferguson et al. (1992), Feeley et. (1998), Costa et al.
1187 (2002), Dungan et al. (2001), Stern (2004), Jacques et al. (2013), Rodríguez et al.
1188 (2007), Dixon et al. (1999), Deruelle (1982), Deruelle et al. (1999), Lopez-Escobar et
1189 al. (1981), Muñoz and Stern (1988), Muñoz et al. (1989), Varekamp et al. (2006), Abre
1190 et al. (2017), Cingolani et al. (2017abc).
1191 Fig. 4. Multi-element patterns for Plio-Quaternary CNVC lavas normalized to Primitive Mantle, NMORB and OIB, all from Sun and McDonough (1989). Symbols as in Fig. 3.

1192
1193

Fig. 5. ¹⁴³Nd/¹⁴⁴Nd vs. ⁸⁷Sr/⁸⁶Sr isotopic ratios for 14 volcanic rocks from the CNVC (see Table 2). The HIMU, BSE, EM1, EM2 and continental crust localization are from Hofmann et al. (1986), Sun and McDonough (1989) and Hofmann (1997, 2014). Symbols as in Fig. 3, label colors as in Fig. 2; light-green field with green border: Rodeo de la Bordalesa Tonalite (San Rafael Block); empty field with blue border: Cerro La Ventana Formation (San Rafael Block).

1200 Fig. 6. (a) MgO (wt.%) vs. TiO₂ (wt.%) diagram showing the CNVC back-arc alkaline magmas, the Lower Paleozoic tholeiitic MORB-type El Nihuil Dolerites (San Rafael Block), and the back-arc alkaline magmas from Auca Mahuida and Payún area volcanoes. This figure allows distinguishing the fields of high-MgO magmas (primitive basalts), low-MgO magmas (evolved basalts and basaltic–andesitic rocks) and trachytic rocks. (b) Chronologic and stratigraphic features of the seven volcanic eruption phases identified in the CNVC and nearby volcanoes (see map of Fig. 2). Dark green field: primitive basalts, light green field: evolved basalts, yellow field: basaltic–andesitic rocks, white field: trachytic rocks. Colors for rock formations as in Fig. 2. Ages from: this study, Folguera et al. (2009) and Quidelleur et al. (2009). Modified from Pallares et al. (2016). Symbols as in Fig. 3. Note that the Chapúa group appears heterogeneous.

1212 Fig. 7. CIPW normative mineral classification diagram of mafic lavas for back-arc alkaline magmas of the CNVC, the Lower Paleozoic tholeiitic MORB-type El Nihuil Dolerites (San Rafael Block) and ALL MORB average (Gale et al., 2013). After Thompson (1984) and Yoder and Tilley (1962). CIPW norm is from Irvine and Baragar (1971). Symbols and fields as in Fig. 3.

1217 Fig. 8. Shown are datapoints and domains for the back-arc alkaline magmas of the CNVC, Auca Mahuida and Payún volcanoes. (a) MgO (wt.%) vs. Th (ppm) diagram, where the main curved trends are due to the fractional crystallization, and the linear correlation indicates mixing with a crustal component which only affected the CNVC (the straight line is only a visual guide). Modified from Pallares et al. (2016). (b) Th/Hf ratio vs. Th
1222 (ppm) diagram allowing identification of partial melting and fractional crystallization (from Luhr et al., 1995). The arrow indicates increasing melting rate. (c) La/Yb ratio vs. Yb (ppm) diagram with dashed lines showing the partial melting trends for likely mantle sources with variable garnet – spinel mineralogy (from Luhr et al., 1995). F = partial melting rates, boxed S: La-Yb composition of the source. Colors for rock formations as in Fig. 2 and symbols as in Fig. 3.

1228 Fig. 9. (a) Ti (ppm) vs. Nb/La diagram showing the low contents in Ti and Nb of the CNVC back-arc alkaline magmas compared to those of Auca Mahuida and Payún volcanoes, as well as MORB and OIB averages. (b) La/Nb vs. Th/Ta diagram showing the low contents in Ta and high contents in Th and La of the CNVC back-arc alkaline magmas compared to Auca Mahuida and Payún volcanoes, as well as MORB and OIB averages. (c) Nb/Y vs. Zr/Y diagram for the back-arc alkaline magmas of the Payenia Volcanic Province, also displaying the fields of Icelandic picrites, tholeiites and alkali basalts as well as N-MORB and continental crust average (Fitton, 2007). ALL MORB and OIB average are also plotted for comparison and some San Rafael Block formations. Symbols as in Fig. 3.

1238 Fig. 10. (a) K₂O/TiO₂ vs. CaO/Al₂O₃ diagram for the CNVC basaltic lavas together with the fields of the other back-arc alkaline magmas of the PVP. Also shown are the EMI, EMII, HIMU, MORB mantle signatures using these major elements (modified from Jackson and Dasgupta, 2008). (b) MgO (wt.%) vs. Th (ppm) diagram for the CNVC lavas only. Also shown are the fields for several formations of the San Rafael Block. The variability is due to both differentiation processes and mixing with the continental rocks of the San Rafael Block, the latter being indicated by the linear correlation (only a visual guide). Modified from Pallares et al. (2016). Symbols as in Fig. 3 and fields as in Fig. 6.

1247 Fig. 11. Ba/La vs. Ta/Hf diagram displaying the CNVC data and fields for the Auca Mahuida and Payún volcanoes. Also shown are the main mantle sources: MORB, Arc, Southern Volcanic Zone and intraplate-OIB. Modified from Kay et al. (2006a) and Pallares et al. (2019). The Payenia Volcanic Province volcanism has a source closer to intraplate and OIB than to MORB or Arc. Geochemical data are from: Pallares et al. (2016), Kay et al. (2006a), Kay et al. (2006b), Kay et al. (2004), Kay et al. (2013), Gudnason et al.
1253 (2012), Germa et al. (2010), Søager et al. (2013), Hernando et al. (2014a,b), Frey et al.
1254 (1984), Ferguson et al. (1992), Feeley et. (1998), Costa et al. (2002), Dungan et al.
1255 (2001), Stern (2004), Jacques et al. (2013), Rodríguez et al. (2007), Dixon et al. (1999),
1256 Deruelle (1982), Deruelle et al. (1999), Lopez-Escobar et al. (1981), Muñoz and Stern
1257 (1988), Muñoz et al. (1989), Varekamp et al. (2006), Abre et al. (2017), Cingolani et
1258 al. (2017abc).

1259 Fig. 12. (a) Ce/Pb vs. Nb/U ratios, (b) Ce/Pb vs. ⁸⁷Sr/⁸⁶Sr ratios and (c) Nb/U vs. ⁸⁷Sr/⁸⁶Sr ratios diagrams showing the crustal contribution of the San Rafael Block to the CNVC
1260 and to other Payenia Volcanic Province magmas. Also shown are fields for several
1261 formations of the San Rafael Block. Modified from Hofmann et al. (1986) and
1262 Hofmann, (1997; 2014).
1263
1264

1265

Table captions

1266 Table 1. Chemical analyses of Pleisto-Holocene lavas from the Cerro Nevado Volcanic Complex. ICP-AES: Thermo® Electron IRIS Advantage; ICP-MS: Thermo® Elemental X7; analyses according to the procedures used at CRPG. Fe₂O₃*: total iron
1267 as Fe₂O₃; LOI: Loss on Ignition; Q: normative quartz; an: normative anorthite; di: normative diopside; ne: normative nepheline; ol: normative olivine; hy: normative hypersthene; Mg#=Mg/(Mg+Fe²⁺). CIPW norm and Mg# calculated with 15 at. % of
1268 total iron as Fe³⁺ and 85% as Fe²⁺. 1 Ages from Quidelleur et al. (2009).
1269
1270
1271
1272
1273 Table 2. Sr and Nd isotopic ratios for the Cerro Nevado Volcanic Complex rocks. Data obtained at PSO (IUEM, Plouzané, France).
1274
1275 Table 3. K-Ar ages. Column headings indicate sample names, potassium (K) concentration
1276 in percent, proportion of radiogenic ⁴⁰Ar (⁴⁰Ar*) in percent, concentration of radiogenic
1277 ⁴⁰Ar (⁴⁰Ar*) × 10¹² in number of atoms per gram, age ± 1-sigma uncertainty (in Ma),
1278 weighted mean ages ± 1-sigma weighted uncertainty (in Ma).
1279
1280

1281

Annexes

1281 Annex 1. Pictures of the Cerro Nevado Volcanic Complex.

1282 Annex 2. Picture of the outcrops for ten dated samples with ages displayed.
<figure><img src="image_3.png" /><figcaption>Figure 1 (Pallares et al.)</figcaption></figure>
LEGEND

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

Quaternary Deposits

Pleistocene

Chapua Group

Cajon de Mayo Formation

Cerro Nevado Formation

Mio - Pliocene

Cortaderas Formation (Plateado)

Middle Miocene

Aisol Formation

Upper Cretaceous

Punta del Agua Formation

Upper Permian - Lower Triassic

Choque Mahuida Formation

Quebrada del Pimiento Formation

Lower Permian

Cochicó Group

Upper Carboniferous

El Imperial Formation

lower Carboniferous

Piedras de Afilar Formation

Devonian

Rio Seco de los Castaños Formation

Ordovician-Silurian

Lindero Formation

Loma Alta Gabbro (El Nihuil Dolerites)

La Horqueta Formation

Ponon Trehue Formation

Precambrian

Cerro La Ventana Formation

Fracture

Covered

fault

• Ore deposits

Permanent River

Intermittent River

Lake, lacuna

Road

Track
<figure><img src="image_6.png" /><figcaption>Figure 3 (Pallares et al.)</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 4 (Pallares et al.)</figcaption></figure>
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>Figure 5 (Pallares et al.)</figcaption></figure>
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>Figure 6 (Pallares et al.)</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Figure 7 (Pallares et al.)</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Figure 8 (Pallares et al.)</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Figure 9 (Pallares et al.)</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Figure 10 (Pallares et al.)</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Figure 11 (Pallares et al.)</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Figure 12 (Pallares et al.)</figcaption></figure>
<table><caption>Table 1.</caption><thead><tr><th>Sample</th><th>94G</th><th>94I2</th><th>94F</th><th>94L</th><th>94J</th><th>94K</th><th>94H</th></tr><tr><th>Coordinates</th><td>35°37'53.30"S<br>68°31'56.70"W</td><td>35°35'21.10"S<br>68°30'0.40"W</td><td>35°37'50.50"S<br>68°31'58.00"W</td><td>35°37'45.80"S<br>68°31'49.30"W</td><td>35°35'35.10"S<br>68°30'13.00"W</td><td>35°37'45.80"S<br>68°31'49.30"W</td><td>35°35'17.10"S<br>68°29'59.20"O</td></tr><tr><th>Rock type</th><td>Trachytic<br>Pumice</td><td>Trachyandesite</td><td>Trachyandesite</td><td>Trachyandesite</td><td>Basaltic<br>Trachyandesite</td><td>Basaltic<br>Trachyandesite</td><td>Basaltic<br>Trachyandesite</td></tr><tr><th>Age (Ma)</th><td></td><td>1.316 ± 0.019</td><td></td><td>1.320 ± 0.019<sup>1</sup></td><td>1.325 ± 0.019</td><td>1.244 ± 0.018</td><td>1.336 ± 0.019</td></tr></thead><tbody><tr><td colspan="8"><strong>Major elements (wt%)</strong></td></tr><tr><td>SiO<sub>2</sub></td><td>65.17</td><td>58.96</td><td>57.87</td><td>57.87</td><td>55.20</td><td>52.80</td><td>52.05</td></tr><tr><td>TiO<sub>2</sub></td><td>0.39</td><td>0.81</td><td>0.89</td><td>0.91</td><td>1.14</td><td>1.33</td><td>1.26</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>14.71</td><td>18.20</td><td>18.08</td><td>18.04</td><td>17.91</td><td>18.03</td><td>17.2</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>3.08</td><td>5.89</td><td>6.49</td><td>6.34</td><td>7.76</td><td>9.24</td><td>8.70</td></tr><tr><td>FeO*</td><td>2.77</td><td>5.30</td><td>5.84</td><td>5.70</td><td>6.98</td><td>8.31</td><td>7.82</td></tr><tr><td>MnO</td><td>0.12</td><td>0.13</td><td>0.16</td><td>0.16</td><td>0.15</td><td>0.20</td><td>0.16</td></tr><tr><td>MgO</td><td>0.38</td><td>1.78</td><td>2.17</td><td>2.38</td><td>3.60</td><td>3.21</td><td>4.30</td></tr><tr><td>CaO</td><td>0.81</td><td>4.72</td><td>5.00</td><td>5.09</td><td>6.72</td><td>6.79</td><td>8.41</td></tr><tr><td>N<sub>2</sub>O</td><td>5.03</td><td>4.96</td><td>4.95</td><td>4.65</td><td>4.63</td><td>4.80</td><td>4.08</td></tr><tr><td>K<sub>2</sub>O</td><td>5.15</td><td>3.69</td><td>3.76</td><td>3.60</td><td>3.05</td><td>3.03</td><td>2.24</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>0.09</td><td>0.55</td><td>0.53</td><td>0.48</td><td>0.58</td><td>0.92</td><td>0.52</td></tr><tr><td>LOI</td><td>4.74</td><td>0.14</td><td>0.22</td><td>-0.04</td><td>-0.07</td><td>0.00</td><td>0.65</td></tr><tr><td>Total</td><td>99.68</td><td>99.85</td><td>100.14</td><td>99.48</td><td>100.67</td><td>100.35</td><td>99.57</td></tr><tr><td colspan="8"><strong>CIPW minerals</strong></td></tr><tr><td>Q</td><td>12.38</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>an</td><td>2.40</td><td>15.74</td><td>15.22</td><td>16.90</td><td>17.79</td><td>17.33</td><td>20.72</td></tr><tr><td>di</td><td>0.86</td><td>2.73</td><td>4.34</td><td>3.57</td><td>8.26</td><td>7.07</td><td>12.99</td></tr><tr><td>ne</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>1.88</td><td>5.50</td><td>3.15</td></tr><tr><td>ol</td><td>0.00</td><td>0.96</td><td>9.18</td><td>3.94</td><td>15.35</td><td>17.93</td><td>16.80</td></tr><tr><td>hy</td><td>5.66</td><td>13.65</td><td>4.34</td><td>11.71</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>Mg#</td><td>19.64</td><td>37.46</td><td>39.96</td><td>42.66</td><td>47.87</td><td>40.77</td><td>49.48</td></tr><tr><td colspan="8"><strong>Trace elements (ppm)</strong></td></tr><tr><td>Cs</td><td>14.95</td><td>3.77</td><td>2.62</td><td>3.31</td><td>7.67</td><td>4.41</td><td>1.62</td></tr><tr><td>Rb</td><td>259.9</td><td>149.9</td><td>149.7</td><td>120.0</td><td>108.8</td><td>99.7</td><td>69.7</td></tr><tr><td>Ba</td><td>33</td><td>1241</td><td>1302</td><td>1233</td><td>1081</td><td>1039</td><td>1075</td></tr><tr><td>W</td><td>5.49</td><td>2.51</td><td>2.21</td><td>1.98</td><td>1.68</td><td>1.62</td><td>1.30</td></tr><tr><td>Th</td><td>45.13</td><td>26.50</td><td>25.69</td><td>26.12</td><td>20.30</td><td>20.76</td><td>14.03</td></tr><tr><td>U</td><td>12.43</td><td>7.47</td><td>7.00</td><td>7.86</td><td>5.12</td><td>4.65</td><td>4.33</td></tr><tr><td>Nb</td><td>108.3</td><td>25.0</td><td>25.9</td><td>31.3</td><td>21.4</td><td>25.0</td><td>18.56</td></tr><tr><td>Ta</td><td>8.87</td><td>2.39</td><td>2.35</td><td>2.34</td><td>1.99</td><td>2.16</td><td>1.41</td></tr><tr><td>La</td><td>65.1</td><td>45.1</td><td>46.5</td><td>53.3</td><td>39.3</td><td>48.5</td><td>39.9</td></tr><tr><td>Ce</td><td>129.0</td><td>92.9</td><td>99.3</td><td>94.3</td><td>83.3</td><td>108.0</td><td>74.9</td></tr><tr><td>Pb</td><td>24.34</td><td>22.38</td><td>26.71</td><td>19.07</td><td>20.82</td><td>21.14</td><td>17.37</td></tr><tr><td>Pr</td><td>13.34</td><td>10.20</td><td>11.06</td><td>10.60</td><td>9.44</td><td>12.72</td><td>8.94</td></tr><tr><td>Mo</td><td>17.13</td><td>3.51</td><td>3.03</td><td>3.39</td><td>2.91</td><td>3.59</td><td>2.86</td></tr><tr><td>Sr</td><td>13</td><td>902</td><td>772</td><td>771</td><td>898</td><td>948</td><td>998</td></tr><tr><td>Nd</td><td>43.9</td><td>36.6</td><td>40.2</td><td>37.8</td><td>35.4</td><td>48.5</td><td>34.5</td></tr><tr><td>Sm</td><td>8.23</td><td>6.53</td><td>7.50</td><td>6.85</td><td>6.70</td><td>9.35</td><td>6.91</td></tr><tr><td>Zr</td><td>757</td><td>334</td><td>379</td><td>400</td><td>279</td><td>325</td><td>220</td></tr><tr><td>Hf</td><td>19.71</td><td>7.60</td><td>8.33</td><td>8.25</td><td>6.34</td><td>7.02</td><td>4.98</td></tr><tr><td>Eu</td><td>0.29</td><td>1.78</td><td>1.92</td><td>1.84</td><td>1.90</td><td>2.50</td><td>2.05</td></tr><tr><td>Sn</td><td>8.30</td><td>1.36</td><td>1.91</td><td>0.77</td><td>1.80</td><td>2.23</td><td>1.30</td></tr><tr><td>Gd</td><td>6.46</td><td>5.18</td><td>5.89</td><td>5.48</td><td>5.40</td><td>7.38</td><td>5.76</td></tr><tr><td>Tb</td><td>1.15</td><td>0.74</td><td>0.87</td><td>0.80</td><td>0.78</td><td>1.07</td><td>0.82</td></tr><tr><td>Dy</td><td>7.27</td><td>4.26</td><td>5.00</td><td>4.59</td><td>4.50</td><td>6.10</td><td>4.62</td></tr><tr><td>Y</td><td>44.8</td><td>23.3</td><td>27.2</td><td>25.6</td><td>23.6</td><td>32.2</td><td>24.6</td></tr><tr><td>Ho</td><td>1.47</td><td>0.85</td><td>0.99</td><td>0.84</td><td>0.88</td><td>1.21</td><td>0.83</td></tr><tr><td>Er</td><td>4.65</td><td>2.27</td><td>2.66</td><td>2.41</td><td>2.30</td><td>3.12</td><td>2.28</td></tr><tr><td>Tm</td><td>0.80</td><td>0.33</td><td>0.39</td><td>0.37</td><td>0.32</td><td>0.43</td><td>0.33</td></tr><tr><td>Yb</td><td>5.88</td><td>2.37</td><td>2.80</td><td>2.56</td><td>2.24</td><td>3.08</td><td>2.24</td></tr><tr><td>Lu</td><td>0.96</td><td>0.37</td><td>0.43</td><td>0.40</td><td>0.34</td><td>0.47</td><td>0.34</td></tr><tr><td>Sc</td><td>0.0</td><td>5.7</td><td>9.3</td><td>0.0</td><td>13.5</td><td>12.3</td><td>0.0</td></tr><tr><td>V</td><td>8</td><td>72</td><td>92</td><td>97</td><td>145</td><td>135</td><td>204</td></tr><tr><td>Cr</td><td>6</td><td>0</td><td>17</td><td>20</td><td>45</td><td>0</td><td>59</td></tr><tr><td>Co</td><td>1.6</td><td>9.94</td><td>13.40</td><td>12.20</td><td>19.97</td><td>18.75</td><td>26.00</td></tr><tr><td>Ni</td><td>3</td><td>0</td><td>13</td><td>13</td><td>26</td><td>8</td><td>32</td></tr><tr><td>Cu</td><td>7.2</td><td>15.5</td><td>26.8</td><td>6.1</td><td>32.3</td><td>37.3</td><td>39.9</td></tr><tr><td>Zn</td><td>93</td><td>84</td><td>93</td><td>94</td><td>90</td><td>112</td><td>94</td></tr><tr><td>Be</td><td>12.28</td><td>3.22</td><td>3.56</td><td>3.46</td><td>2.61</td><td>2.99</td><td>2.08</td></tr><tr><td>Ga</td><td>25.73</td><td>22.52</td><td>23.72</td><td>23.92</td><td>23.15</td><td>22.78</td><td>23.05</td></tr><tr><td>Ge</td><td>1.92</td><td>1.39</td><td>1.46</td><td>1.46</td><td>1.47</td><td>1.46</td><td>1.49</td></tr><tr><td>La/Nb</td><td>0.60</td><td>0.60</td><td>1.79</td><td>1.70</td><td>1.84</td><td>1.94</td><td>2.14</td></tr><tr><td>Ba/La</td><td>0.51</td><td>27.51</td><td>28.00</td><td>23.15</td><td>27.49</td><td>21.42</td><td>26.89</td></tr><tr><td>La/Yb</td><td>11.06</td><td>19.02</td><td>16.62</td><td>20.78</td><td>17.55</td><td>15.76</td><td>17.84</td></tr><tr><td>Ba/Th</td><td>0.74</td><td>46.83</td><td>50.68</td><td>47.20</td><td>53.25</td><td>50.05</td><td>76.62</td></tr><tr><td>Zr/Hf</td><td>38.42</td><td>43.94</td><td>45.57</td><td>48.47</td><td>44.07</td><td>46.28</td><td>44.29</td></tr><tr><td>Zr/Nb</td><td>6.99</td><td>13.36</td><td>14.65</td><td>12.77</td><td>13.06</td><td>12.97</td><td>11.88</td></tr></tbody></table>
<table><caption>Table 1.</caption><thead><tr><th>Sample</th><th>94I1</th><th>94D2</th><th>94BJ</th><th>94M</th><th>94BD</th><th>94BB</th><th>94BH</th></tr><tr><th>Coordinates</th><td>35°35'21.10"S<br>68°30'0.40"W</td><td>35°28'24.60"S<br>68°37'54.70"W</td><td>35°12'56.20"S<br>68°15'21.60"W</td><td>35°35'57.80"S<br>68°38'23.70"W</td><td>35°19'32.60"S<br>68°23'18.90"W</td><td>35°46'26.60"S<br>68°17'55.20"W</td><td>35°15'0.50"S<br>68°15'12.50"W</td></tr><tr><th>Rock type</th><td>Basaltic<br>Trachyandesite</td><td>Trachybasalt</td><td>Alkali basalt</td><td>Alkali basalt</td><td>Alkali basalt</td><td>Alkali basalt</td><td>Alkali basalt</td></tr><tr><th>Age (Ma)</th><td>1.324 ± 0.019<sup>†</sup></td><td>1.044 ± 0.015<sup>†</sup></td><td>0.944 ± 0.016<sup>†</sup></td><td>1.022 ± 0.015<sup>†</sup></td><td>1.352 ± 0.020<sup>†</sup></td><td>1.878 ± 0.028<sup>†</sup></td><td>0.925 ± 0.015</td></tr></thead><tbody><tr><td colspan="8"><strong>Major elements (wt%)</strong></td></tr><tr><td>SiO<sub>2</sub></td><td>52.03</td><td>49.45</td><td>48.96</td><td>47.86</td><td>47.73</td><td>46.6</td><td>46.38</td></tr><tr><td>TiO<sub>2</sub></td><td>1.33</td><td>1.41</td><td>1.66</td><td>1.42</td><td>1.53</td><td>1.49</td><td>1.43</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>17.40</td><td>17.03</td><td>17.30</td><td>16.46</td><td>15.46</td><td>14.38</td><td>15.72</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>9.05</td><td>9.56</td><td>11.13</td><td>11.23</td><td>11.7</td><td>11.56</td><td>11.31</td></tr><tr><td>FeO*</td><td>8.14</td><td>8.60</td><td>10.01</td><td>10.10</td><td>10.52</td><td>10.40</td><td>10.17</td></tr><tr><td>MnO</td><td>0.16</td><td>0.16</td><td>0.17</td><td>0.17</td><td>0.17</td><td>0.17</td><td>0.17</td></tr><tr><td>MgO</td><td>4.73</td><td>6.80</td><td>6.02</td><td>7.74</td><td>8.95</td><td>9.76</td><td>9.52</td></tr><tr><td>CaO</td><td>8.55</td><td>9.66</td><td>10.70</td><td>9.96</td><td>9.59</td><td>9.75</td><td>9.79</td></tr><tr><td>N<sub>2</sub>O</td><td>4.16</td><td>3.72</td><td>3.71</td><td>3.34</td><td>3.27</td><td>3.18</td><td>3.3</td></tr><tr><td>K<sub>2</sub>O</td><td>2.15</td><td>1.63</td><td>1.03</td><td>0.96</td><td>0.89</td><td>0.99</td><td>0.96</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>0.56</td><td>0.61</td><td>0.38</td><td>0.37</td><td>0.36</td><td>0.44</td><td>0.39</td></tr><tr><td>LOI</td><td>-0.22</td><td>-0.22</td><td>-0.19</td><td>0.11</td><td>-0.07</td><td>1.21</td><td>-0.12</td></tr><tr><td>Total</td><td>99.89</td><td>99.82</td><td>100.86</td><td>99.62</td><td>99.57</td><td>99.52</td><td>98.84</td></tr><tr><td colspan="8"><strong>CIPW minerals</strong></td></tr><tr><td>Q</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>an</td><td>20.86</td><td>23.11</td><td>24.94</td><td>24.90</td><td>22.76</td><td>20.43</td><td>23.30</td></tr><tr><td>di</td><td>12.78</td><td>14.68</td><td>17.66</td><td>15.33</td><td>15.54</td><td>18.09</td><td>15.96</td></tr><tr><td>ne</td><td>4.04</td><td>6.03</td><td>6.91</td><td>5.97</td><td>5.72</td><td>7.23</td><td>8.30</td></tr><tr><td>ol</td><td>17.91</td><td>21.53</td><td>21.44</td><td>26.46</td><td>28.98</td><td>29.63</td><td>29.47</td></tr><tr><td>hy</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>Mg#</td><td>50.88</td><td>58.51</td><td>51.74</td><td>57.74</td><td>60.26</td><td>62.60</td><td>62.53</td></tr><tr><td colspan="8"><strong>Trace elements (ppm)</strong></td></tr><tr><td>Cs</td><td>1.62</td><td>2.33</td><td>0.57</td><td>0.69</td><td>0.33</td><td>01.26</td><td>0.81</td></tr><tr><td>Rb</td><td>65.1</td><td>47.2</td><td>20.5</td><td>18.3</td><td>16.44</td><td>24.21</td><td>21.8</td></tr><tr><td>Ba</td><td>973</td><td>718</td><td>393</td><td>397</td><td>421</td><td>423</td><td>388</td></tr><tr><td>W</td><td>1.16</td><td>0.65</td><td>0.00</td><td>0.27</td><td>0.00</td><td>0.34</td><td>0.30</td></tr><tr><td>Th</td><td>12.51</td><td>6.93</td><td>2.95</td><td>3.83</td><td>3.32</td><td>4.88</td><td>4.13</td></tr><tr><td>U</td><td>3.36</td><td>2.24</td><td>0.67</td><td>0.86</td><td>0.80</td><td>1.22</td><td>0.78</td></tr><tr><td>Nb</td><td>15.33</td><td>17.14</td><td>10.65</td><td>8.33</td><td>8.00</td><td>9.29</td><td>9.57</td></tr><tr><td>Ta</td><td>1.34</td><td>1.21</td><td>0.73</td><td>0.58</td><td>0.55</td><td>0.66</td><td>0.67</td></tr><tr><td>La</td><td>32.3</td><td>32.4</td><td>17.37</td><td>18.81</td><td>16.75</td><td>20.39</td><td>19.97</td></tr><tr><td>Ce</td><td>70.5</td><td>63.8</td><td>36.2</td><td>39.2</td><td>34.9</td><td>41.5</td><td>40.1</td></tr><tr><td>Pb</td><td>15.93</td><td>10.88</td><td>4.25</td><td>5.43</td><td>5.29</td><td>6.44</td><td>5.85</td></tr><tr><td>Pr</td><td>8.30</td><td>7.82</td><td>4.67</td><td>5.04</td><td>4.56</td><td>5.24</td><td>5.12</td></tr><tr><td>Mo</td><td>2.03</td><td>2.34</td><td>1.05</td><td>0.95</td><td>0.84</td><td>1.12</td><td>1.01</td></tr><tr><td>Sr</td><td>986</td><td>1016</td><td>575</td><td>627</td><td>642</td><td>648</td><td>613</td></tr><tr><td>Nd</td><td>32.4</td><td>31.64</td><td>19.96</td><td>21.53</td><td>19.83</td><td>22.32</td><td>21.32</td></tr><tr><td>Sm</td><td>6.64</td><td>6.68</td><td>4.70</td><td>4.95</td><td>4.78</td><td>5.21</td><td>4.76</td></tr><tr><td>Zr</td><td>199</td><td>179</td><td>111</td><td>116</td><td>109</td><td>115</td><td>105</td></tr><tr><td>Hf</td><td>4.70</td><td>4.10</td><td>2.76</td><td>2.91</td><td>2.77</td><td>2.92</td><td>2.76</td></tr><tr><td>Eu</td><td>1.99</td><td>2.15</td><td>1.65</td><td>1.63</td><td>1.65</td><td>1.71</td><td>1.61</td></tr><tr><td>Sn</td><td>1.66</td><td>1.30</td><td>1.17</td><td>1.07</td><td>1.07</td><td>1.21</td><td>1.06</td></tr><tr><td>Gd</td><td>5.61</td><td>5.80</td><td>4.56</td><td>4.76</td><td>4.65</td><td>4.95</td><td>4.50</td></tr><tr><td>Tb</td><td>0.80</td><td>0.84</td><td>0.68</td><td>0.70</td><td>0.68</td><td>0.72</td><td>0.66</td></tr><tr><td>Dy</td><td>4.63</td><td>4.63</td><td>3.97</td><td>4.09</td><td>3.98</td><td>4.07</td><td>3.84</td></tr><tr><td>Y</td><td>23.7</td><td>24.3</td><td>20.9</td><td>21.6</td><td>20.86</td><td>21.26</td><td>19.18</td></tr><tr><td>Ho</td><td>0.92</td><td>0.83</td><td>0.75</td><td>0.77</td><td>0.73</td><td>0.74</td><td>0.71</td></tr><tr><td>Er</td><td>2.32</td><td>2.22</td><td>2.07</td><td>2.08</td><td>1.96</td><td>1.99</td><td>1.89</td></tr><tr><td>Tm</td><td>0.32</td><td>0.31</td><td>0.29</td><td>0.29</td><td>0.27</td><td>0.27</td><td>0.27</td></tr><tr><td>Yb</td><td>2.20</td><td>2.04</td><td>1.88</td><td>1.90</td><td>1.79</td><td>1.77</td><td>1.71</td></tr><tr><td>Lu</td><td>0.34</td><td>0.30</td><td>0.28</td><td>0.29</td><td>0.26</td><td>0.27</td><td>0.25</td></tr><tr><td>Sc</td><td>21.4</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td></tr><tr><td>V</td><td>205</td><td>231</td><td>236</td><td>225</td><td>229</td><td>208</td><td>223</td></tr><tr><td>Cr</td><td>63</td><td>176</td><td>108</td><td>194</td><td>368</td><td>337</td><td>326</td></tr><tr><td>Co</td><td>26.22</td><td>34.70</td><td>34.31</td><td>41.49</td><td>47.53</td><td>48.48</td><td>46.61</td></tr><tr><td>Ni</td><td>32</td><td>97</td><td>36</td><td>92</td><td>131</td><td>219</td><td>169</td></tr><tr><td>Cu</td><td>42.8</td><td>42.4</td><td>25.9</td><td>39.1</td><td>34.5</td><td>50.98</td><td>46.72</td></tr><tr><td>Zn</td><td>93</td><td>96</td><td>96</td><td>100</td><td>106</td><td>109</td><td>97</td></tr><tr><td>Be</td><td>1.92</td><td>1.63</td><td>1.05</td><td>1.03</td><td>0.97</td><td>1.02</td><td>0.97</td></tr><tr><td>Ga</td><td>21.99</td><td>22.89</td><td>21.59</td><td>20.94</td><td>21.60</td><td>20.11</td><td>19.93</td></tr><tr><td>Ge</td><td>1.47</td><td>1.61</td><td>1.47</td><td>1.51</td><td>1.50</td><td>1.49</td><td>1.34</td></tr><tr><td>La/Nb</td><td>2.11</td><td>1.89</td><td>1.63</td><td>2.25</td><td>2.09</td><td>2.19</td><td>2.08</td></tr><tr><td>Ba/La</td><td>30.07</td><td>22.14</td><td>22.65</td><td>21.12</td><td>25.11</td><td>20.75</td><td>19.44</td></tr><tr><td>La/Yb</td><td>14.73</td><td>15.86</td><td>9.25</td><td>9.89</td><td>9.36</td><td>11.49</td><td>11.64</td></tr><tr><td>Ba/Th</td><td>77.76</td><td>103.55</td><td>133.39</td><td>103.57</td><td>126.87</td><td>86.70</td><td>93.95</td></tr><tr><td>Zr/Hf</td><td>42.31</td><td>43.55</td><td>40.35</td><td>39.75</td><td>39.29</td><td>39.28</td><td>38.05</td></tr><tr><td>Zr/Nb</td><td>12.96</td><td>10.42</td><td>10.44</td><td>13.91</td><td>13.62</td><td>12.34</td><td>10.99</td></tr></tbody></table>
<table><thead><tr><th>Sample</th><th><sup>87</sup>Sr/<sup>86</sup>Sr</th><th>Standard Error 1σ</th><th>Standard Error 2σ</th><th><sup>143</sup>Nd/<sup>144</sup>Nd</th><th>Standard Error 1σ</th><th>Standard Error 2σ</th></tr></thead><tbody><tr><td>94BB</td><td>0.703928</td><td>0.000003</td><td>0.000006</td><td>0.512827</td><td>0.000004</td><td>0.000008</td></tr><tr><td>94BH</td><td>0.703975</td><td>0.000003</td><td>0.000006</td><td>0.512813</td><td>0.000003</td><td>0.000006</td></tr><tr><td>94F</td><td>0.704185</td><td>0.000003</td><td>0.000006</td><td>0.512779</td><td>0.000002</td><td>0.000004</td></tr><tr><td>94G</td><td>0.704523</td><td>0.000003</td><td>0.000006</td><td>0.512769</td><td>0.000004</td><td>0.000008</td></tr><tr><td>94G Bis</td><td>0.704542</td><td>0.000003</td><td>0.000006</td><td></td><td></td><td></td></tr><tr><td>94H</td><td>0.704142</td><td>0.000003</td><td>0.000006</td><td>0.512783</td><td>0.000004</td><td>0.000008</td></tr><tr><td>94I1</td><td>0.704114</td><td>0.000003</td><td>0.000006</td><td>0.512792</td><td>0.000003</td><td>0.000006</td></tr><tr><td>94I2</td><td>0.704022</td><td>0.000004</td><td>0.000008</td><td>0.512803</td><td>0.000005</td><td>0.00001</td></tr><tr><td>94J</td><td>0.703997</td><td>0.000003</td><td>0.000006</td><td>0.512806</td><td>0.000003</td><td>0.000006</td></tr><tr><td>94J Bis</td><td>0.703996</td><td>0.000004</td><td>0.000008</td><td></td><td></td><td></td></tr><tr><td>94K</td><td>0.704055</td><td>0.000003</td><td>0.000006</td><td>0.512808</td><td>0.000003</td><td>0.000006</td></tr><tr><td>94L</td><td>0.704169</td><td>0.000003</td><td>0.000006</td><td>0.512772</td><td>0.000004</td><td>0.000008</td></tr><tr><td>94M</td><td>0.704068</td><td>0.000004</td><td>0.000008</td><td>0.512786</td><td>0.000004</td><td>0.000008</td></tr><tr><td>94BD</td><td>0.704077</td><td>0.000004</td><td>0.000008</td><td>0.512772</td><td>0.000003</td><td>0.000006</td></tr><tr><td>94BD Dup</td><td>0.704085</td><td>0.000003</td><td>0.000006</td><td>0.512803</td><td>0.000004</td><td>0.000008</td></tr><tr><td>94BJ</td><td>0.704023</td><td>0.000004</td><td>0.000008</td><td>0.512788</td><td>0.000004</td><td>0.000008</td></tr><tr><td>94BJ Bis</td><td>0.704025</td><td>0.000003</td><td>0.000006</td><td></td><td></td><td></td></tr><tr><td>94D2</td><td>0.703922</td><td>0.000003</td><td>0.000006</td><td>0.512835</td><td>0.000004</td><td>0.000008</td></tr><tr><td>BCR-2 Std</td><td>0.705029</td><td>0.000004</td><td>0.000008</td><td>0.512639</td><td>0.000003</td><td>0.000006</td></tr><tr><td></td><td>0.705</td><td>GEOREM recommended values</td><td></td><td>0.512636</td><td>GEOREM recommended values</td><td></td></tr></tbody></table>

<table><thead><tr><th>Standard Sr Triton</th><th>87Sr/86Sr</th><th>Standard Error (1 sigma)</th><th>Standard Error (2 sigma)</th></tr></thead><tbody><tr><td>NBS987_1</td><td>0.710277</td><td>0.000003</td><td>0.000006</td></tr><tr><td>NBS987_2</td><td>0.710272</td><td>0.000003</td><td>0.000006</td></tr><tr><td>NBS987_3</td><td>0.710273</td><td>0.000003</td><td>0.000006</td></tr><tr><td>NBS987_1 Bis</td><td>0.710279</td><td>0.000003</td><td>0.000006</td></tr><tr><td>NBS987_2 Bis</td><td>0.710271</td><td>0.000003</td><td>0.000006</td></tr><tr><td>NBS987_3 Bis</td><td>0.710273</td><td>0.000003</td><td>0.000006</td></tr><tr><td>Mean</td><td>0.7102741666666</td><td>0.000003</td><td>0.000006</td></tr><tr><td>Std dev. (1 sigma)</td><td colspan="3">3.12516666222351E-06</td></tr><tr><td>Std dev. (2 sigma)</td><td colspan="3">6.25033332444702E-06</td></tr></tbody></table>

<table><thead><tr><th>Standard Nd Triton</th><th>143Nd/144Nd</th><th>Standard Error (1 sigma)</th><th>Standard Error (2 sigma)</th></tr></thead><tbody><tr><td>JNdi</td><td>0.512116</td><td>0.000003</td><td>0.000006</td></tr><tr><td>JNdi</td><td>0.512116</td><td>0.000002</td><td>0.000004</td></tr><tr><td>JNdi</td><td>0.512114</td><td>0.000003</td><td>0.000006</td></tr><tr><td>JNdi</td><td>0.512106</td><td>0.000003</td><td>0.000006</td></tr><tr><td>Mean</td><td>0.512113</td><td>0.00000275</td><td>0.0000055</td></tr><tr><td>Std dev. (1 sigma)</td><td colspan="3">4.76095228572331E-06</td></tr><tr><td>Std dev. (2 sigma)</td><td colspan="3">9.52190457144663E-06</td></tr></tbody></table>
<table><thead><tr><th>Sample</th><th>K%</th><th>Taux 40Ar*</th><th>40Ar* (at/g)</th><th>Age (Ma)</th><th>± 1σ (Ma)</th></tr></thead><tbody><tr><td rowspan="3">94H</td><td rowspan="3"><strong>2.368</strong></td><td>24.7%</td><td>3.3184E+12</td><td>1.341</td><td>0.020</td></tr><tr><td>31.9%</td><td>3.2952E+12</td><td>1.332</td><td>0.019</td></tr><tr><td></td><td></td><td><strong>1.336</strong></td><td><strong>0.019</strong></td></tr><tr><td rowspan="3">94J</td><td rowspan="3"><strong>3.376</strong></td><td>29.3%</td><td>4.6804E+12</td><td>1.327</td><td>0.019</td></tr><tr><td>28.8%</td><td>4.6681E+12</td><td>1.323</td><td>0.019</td></tr><tr><td></td><td></td><td><strong>1.325</strong></td><td><strong>0.019</strong></td></tr><tr><td rowspan="3">94I2</td><td rowspan="3"><strong>3.386</strong></td><td>31.9%</td><td>4.6707E+12</td><td>1.320</td><td>0.019</td></tr><tr><td>30.5%</td><td>4.6406E+12</td><td>1.312</td><td>0.019</td></tr><tr><td></td><td></td><td><strong>1.316</strong></td><td><strong>0.019</strong></td></tr><tr><td rowspan="3">94K</td><td rowspan="3"><strong>2.624</strong></td><td>43.2%</td><td>3.4543E+12</td><td>1.260</td><td>0.018</td></tr><tr><td>43.2%</td><td>3.3653E+12</td><td>1.227</td><td>0.018</td></tr><tr><td></td><td></td><td><strong>1.244</strong></td><td><strong>0.018</strong></td></tr><tr><td rowspan="3">94BH</td><td rowspan="3"><strong>0.987</strong></td><td>12.6%</td><td>9.5201E+11</td><td>0.923</td><td>0.015</td></tr><tr><td>12.3%</td><td>9.5662E+11</td><td>0.928</td><td>0.015</td></tr><tr><td></td><td></td><td><strong>0.925</strong></td><td><strong>0.015</strong></td></tr></tbody></table>