Geochronology and facies analysis of subaqueous volcanism of lower ordovician, Famatinian arc, Argentina

Accepted Manuscript Geochronology and facies analysis of subaqueous volcanism of lower ordovician, Famatinian arc, Argentina Paula Armas, Eber A. Cristofolini, Juan E. Otamendi, Alina M. Tibaldi, Matías G. Barzola, Giuliano C. Camilletti PII:

S0895-9811(18)30070-1

DOI:

10.1016/j.jsames.2018.04.005

Reference:

SAMES 1904

To appear in:

Journal of South American Earth Sciences

Received Date: 21 June 2017 Revised Date:

4 April 2018

Accepted Date: 4 April 2018

Please cite this article as: Armas, P., Cristofolini, E.A., Otamendi, J.E., Tibaldi, A.M., Barzola, Matí.G., Camilletti, G.C., Geochronology and facies analysis of subaqueous volcanism of lower ordovician, Famatinian arc, Argentina, Journal of South American Earth Sciences (2018), doi: 10.1016/ j.jsames.2018.04.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

GEOCHRONOLOGY AND FACIES ANALYSIS OF

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SUBAQUEOUS VOLCANISM OF LOWER

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ORDOVICIAN, FAMATINIAN ARC, ARGENTINA

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Paula Armas1,2*, Eber A. Cristofolini1,2, Juan E. Otamendi1,2, Alina M. Tibaldi1,2, Matías G. Barzola1,2,

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Giuliano C. Camilletti1,2

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CONICET. Godoy Cruz 2290 – C1425FQB. CABA. Argentina.

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Departamento de Geología, Universidad Nacional de Río Cuarto. X5804BYA Río Cuarto, Córdoba,

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Argentina.

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*Corresponding author:

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Paula Armas

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Email: [email protected].

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Argentina

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Phone: +54 358 4676198

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Western edge of the Famatina. Chuschín Formation.

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ABSTRACT

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The Chuschín formation is a sedimentary-volcanic succession of the Famatina System subduction-related

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arc magmatism. This formation developed along the western margin of Gondwana during the early and

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middle Ordovician. The interpretation of the Chuschín formation is significant for the understanding of

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the Famatinian arc. This study provides a detailed facies analysis, which allows characterizing the

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deposits as well as defining some of the characteristics of the volcanism and sedimentation. Coherent,

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autoclastic, peperitic, pyroclastic and volcaniclastic facies were recognized between the different volcanic

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facies. The characteristics and relationships between these facies indicate events of effusive subaqueous

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volcanism as well as explosive volcanism showing predominantly high temperature features. Both types

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of volcanism are interbedded with volcanogenic deposits and epiclastic sediment that are associated

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mainly with turbidity currents. The magmatic and volcanic rocks are dominantly subalkaline, weakly

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peraluminous and silicic. Two new U-Pb zircon crystallization ages on volcanic rocks from the Chuschín

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Formation show that this section of the Famatinian arc formed between 463 and 469 Ma (Middle

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Ordovician).

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Keywords:

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Geochronology.

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Formation,

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Chuschín

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Famatinian

Arc,

Middle

Ordovician,

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Western edge of the Famatina. Chuschín Formation.

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1. INTRODUCTION Over the past decades, numerous studies were conducted to interpret the tectonic setting and

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evolution of the Ordovician Gondwana margin of southern South America (Ramos, 1988; Pankhurst

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and Rapela, 1998; Astini, 2003; Ramos, 2008; Bahlburg et al., 2009; and references therein). There is

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now a large amount of information on the products of magmatism, the sedimentary processes, the

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biostratigraphy and chronostratigraphy of Ordovician Famatinian rocks (Turner, 1958; Mannheim,

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1988; Clemens and Miller, 1996; Toselli et al., 1996; Mángano and Buatois, 1994; Benedetto, 1998;

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Pankhurst and Rapela, 1998; Astini, 2003; Fanning et al., 2004; Otamendi et al., 2010; 2017; Bahlburg

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et al., 2016; Suzaño et al. 2017). Nevertheless, some Famatinan arc sequences and deposits still miss

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detailed observations with in situ interpretations from a multidisciplinary dataset. For this reason, we

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collected petrological, sedimentary, geochemical and geochronological data in a typical Ordovician

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locality at Chuschín valley on the western part of the Famatina System (Figs. 1a and 1b).

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The Chuschín Formation is a volcano-sedimentary succession of the Famatina System and its

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genesis is linked to the evolution of the western margin of Gondwana during the Lower and Middle

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Ordovician. Mannheim (1988) first defined the Chuschín Formation that provided petrological and

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geochemical data of high quality for the entire volcano-sedimentary succession. Our work builds upon

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the work of Mannheim (1988) but takes a more detailed approach for the facies analyses and also

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provides absolute ages from U-Pb geochronology. Continuous exposures of effusive, pyroclastic,

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volcaniclastic and epiclastic facies are used to reconstruct the volcano-sedimentary succession. The

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acquired information allows us characterizing the nature of volcano-sedimentary deposits and

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understanding the processes of volcanism and sedimentation in this part of the Famatinian magmatic

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arc (Fig. 1c). Facies analysis, internal architecture of erupted and sedimentary deposits, and

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geochronology of volcanic rocks studies were conducted as they potentially give insights on the

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emplacement mechanisms of volcanics under subaqueous environment during the evolution of the

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Famatinian arc. The acquired dataset on the Chuschín Formation contributes to refining geodynamic

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models for the generation and evolution of the western Gondwana margin.

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2. GEOLOGICAL FRAMEWORK

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2.1. Regional geology The Famatina System, located between 27° and 31° S latitude, is part of an Ordovician

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magmatic arc (480-467 Ma; Pankhurst and Rapela, 1998; Ducea et al., 2010) that currently extends for

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more than 2,500 km from Colombia to the center of Argentina (Chew et al., 2007; Chernicoff et al.,

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2010), forming part of the fragmented foreland since the Cenozoic (Dávila and Astini, 2007). The

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Famatina active magmatism along the western margin of Gondwana had a frontal arc belt with a large

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retro-arc basin. Remnants of Ordovician arc magmatism are found in the Sierra de San Luis, Sierras

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Pampeanas of Catamarca and Tucumán, the Cordillera Oriental and in the Puna (Bahlburg, 1990;

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Astini and Dávila, 2004; Lucassen and Franz, 2005; Otamendi, et al., 2010; Larrovere et al., 2011). Along the western slope of the Famatina System, Ordovician rocks crop out in sparse sections

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due to the various tectonic events that affected the western edge of Gondwana (Ramos, 1988; Toselli

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et al., 1996; Pankhurst and Rapela, 1998; Astini, 2003). These outcrops, as well as those in central

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Famatina, have deposits related to an active margin environment of Tremadocian-Llanvirniano age. In

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Sierra de Narvaez, the granodiorite from Las Angosturas (Cisterna, 1994) can be found intruding

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volcanic and sedimentary deposits. The latter displaying fossils of the graptolite faunas, which allows

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dating the system to early Tremadocian age (Cisterna et al., 2006). To the west, in Sierra de Las

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Planchadas lie the Arenigian volcanic sedimentary deposits defined as the Suri Formation (Harrington

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and Leanza, 1957) and Las Planchadas Formation (Turner, 1958; Fanning et al., 2004). Near Valle

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Hermoso are quartzites, slates and chert in a volcanic series that form part of the Suri Formation (de

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Alba, 1980; Clemens and Miller, 1996). Around Cerro Negro, Ordovician rocks comprise La

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Alumbrera Formation, whereas the Portezuelo de las Minitas Formation is composed of

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conglomerates, sandstones and slates interbedded with volcanic rocks. The presence of graptolites in

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the lower midsection indicates Arenigiana age (Esteban, 1999) and makes it possible to correlate it

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partially with the Suri Formation (Astini, 2003). In Cuchilla Negra lie sedimentary successions formed

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by volcanic, volcaniclastic and epiclastic deposits with the occurrence of brachiopods and trilobites

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from the Llanvirn (Lavandaio, 1973; Dahlquist et al., 2008).

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In the southwestern most slope of Famatina, Ordovician rocks comprise the Chuschín

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Formation (Mannheim, 1988). To the east lies the Bordo Atravesado Formation composed of

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siltstones, silty sandstone and calcareous sandstone that display fossils of conodont fauna with low

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species diversity from the upper Tremadocian and an assemblage of trilobites from the upper late

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Tremadocian age (Esteban, 2003). This unit is intruded by rhyolites and dacites from the Middle

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Ordovician period (Toselli et al., 1996).

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2.2 Chuschín Formation

The Chuschín Formation is bordered by tectonic contacts to the east, with Ñuñorco Granite,

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and to the west, with the Paganzo Group. It is typically found in the Quebrada de Chuschín,

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department of General Lavalle, and was characterized by Mannheim (1988) as a unit constituted by

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various ore deposits associated with chert, slate, greywacke, quartzites and rhyolites of more than

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100 m thick. This formation is 2000 m thick and represents a volcano-sedimentary complex intruded

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by porphyry granodiorites, aplites, lamprophyres dykes (Mannheim and Miller, 1996) and the

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Ñuñorco Granite. In the western side of the study area, this formation is sheared by the Chuschín shear

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zone established by Conci et al. (2001). This shear zone is characterized by narrow mylonite belt

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stabilized in the greenschist facies and a mylonitic foliation superimposed on the sedimentary bedding

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with a strikes from N 335° to N 20° and dips between 90° to 59° E. Chuschín shear zone developed

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after the volcano-sedimentary deposition between the Early Ordovician and Early Devonian periods.

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This shear zone shows a reverse kinematics with slight sinistral movement and has been attributed to

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the Ocloyic orogeny (Conci et al., 2001).

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Mannheim (1993) assigns the Chuschín Formation to an Arenigian-Llanvirnian age due to the

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presence of a sequence of intercalated volcanic material. Clemens and Miller (1996) include this unit

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in the Suri Formation of Lower Ordovician, and Astini (1998) proposes a correlation between the

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Chuschín Formation and the Famatina Group given their similarities in facies and thickness.

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Subsequently, this author also argues on the possible link with the Cerro Morado Group, which is

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assigned to the Middle Ordovician (Astini, 2003).

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Western edge of the Famatina. Chuschín Formation.

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In the central sector of the study area, the Chuschín Formation is affected by hydrothermal

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alteration that is characterized by the association of quartz, sericite, kaolinite and pyrophyllite (Schalamuk

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and Curci, 1999).

4 3. PETROLOGICAL CHARACTERIZATION AND FIELD RELATIONS

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The Chuschín Formation presents an alternation of volcano-sedimentary facies with strike north

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south. The mapping and facies analysis as a result of a survey of stratigraphic sections (Fig. 1d and

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Fig. 2) enabled us to identify effusive pyroclastic vulcanite alternating with volcaniclastic and

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epiclastic rocks. Table 1 shows the facies that were defined for the Chuschín Formation, using the

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methodology of Busby-Spera and White (1987), McPhie et al. (1993), and Branney and Kokelaar

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(2003) for volcanic and volcaniclastic rocks. The identified lithotypes and their spatial distribution

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show variations in the east-west direction from the coherent and autoclastic facies to the pyroclastic

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and volcaniclastic facies with epiclastic facies (Fig. 1d).

14 3.1 Coherent facies (A, B, and C)

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The coherent facies extend mostly in the eastern sector of the area next to the Ñuñorco

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Granite. In particular, facies A and B crop out as tabular bodies between 30 and 50 meters of

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thickness, while the facies C crop out in lobular bodies with a thickness greater than 600 m. All the

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coherent facies (A, B, and C – Table 1) have a rhyolitic composition but they display different

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textures. The facies A and C have a microcrystalline texture attesting of high-temperature

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devitrification of silicic glass (McPhie et al., 1993). The facies B is characterized by a spherulitic

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texture (Fig. 3a) that forms in clusters with spherulites reaching up to 0.8 mm in diameter, which

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suggest a rather high temperature (700° C) environment (Lofgren, 1971b). Facies B also displays a

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micro-poikilitic texture that supposedly is the product of early stage glass devitrification with high

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water content or a slowly cooled material (Lofgren, 1971a). These textures and the evidence of high-

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temperature glass devitrification within the coherent facies suggest that the groundmass crystals are

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the result of syn-eruptive crystallization of magma and/or crystallization accompanied by the slow

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cooling during and after the sitting (Lofgren, 1971b; Allen and McPhie, 2003). Facies A also contains

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fragmented phenocrysts of quartz with canalicular filled with recrystallized groundmass (Asvesta and

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Dimitriadis, 2010). The interconnection of these deep channels and embayment of the edges causes the

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fragmentation (Allen and McPhie, 2003). The dimensions, geometries and textures variations suggesting different degrees of

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devitrification (Lofgren, 1971a) allow interpreting a typical model of the edges of the rhyolite domes

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(Armas et al., 2016). Porphyritic massive rhyolite (facies A) and porphyritic flow-banded rhyolite

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(facies C) constitute the internal sector while the microspherulitic massive rhyolite (facies B) is a

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typical texture occurring on the edge of rhyolitic domes (McPhie et al., 1993).

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9 3.2 Autoclastic facies (D)

Facies D is a typical autoclastic facies (Table 1) forming tabular-shaped bodies of up to 15 m

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that show gradational contacts with the porphyritic rhyolite (facies C). The characteristic feature of

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facies D is that the clasts are from facies C and the groundmass preserves the perlitic curved and

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concentric texture (Fig. 3b). Note that clasts can also be elongated in the sample. We suggest that the

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presence of a perlitic texture in the groundmass of this type of breccia is associated with the

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undercooling of glass by hydration generating water diffusion and an increase in the volume within the

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solid glass (McPhie et al., 1993; Asvesta and Dimitriadis, 2010). Through the hydratisation process,

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perlite retains between 2 – 5 % water in the glass structure while maintaining its parental chemical

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composition (Friedman and Long, 1984).

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This type of autoclastic deposit with angular clasts is defined as a hydroclastic breccia that is

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the result of the non-explosive fracturing and disintegration by cooling in the exterior part of the lava

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flows or domes (McPhie et al., 1993). The features associated with the fragments as well as their flat

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and angular shape show in-situ fragmentation (Asvesta and Dimitriadis, 2010), which attest of

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reworking before the deposition.

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3.3 Peperite facies (E and F)

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Facies E and F are the products of mixing between coherent lava (facies A and C) and

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unconsolidated wet sediment (epiclastic facies L, M and N). This attest of subaqueous environment,

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therefore, E and F are interpreted to be a peperite facies. Both facies E and F are arranged in bodies of

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irregular geometry with diffuse borders and thickness from 1 m to 3 m. Facies E (Table 1), found only

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in the area of Mina Delina, extend mainly in the western sector where the largest proportion of

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epiclastic facies remains. Facies F is in the center of the area (Fig. 1d). Clearly the influence of the granulometry of the host sediment can be observed. It allows

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categorizing fluidal and blocky clast types in the peperite textures (Busby-Spera and White, 1987;

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Skilling et al., 2002). Facies E is characterized by the interdigitation of facies C and pelitic beds

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presenting a fluidal aspect as well as angular pelitic clasts and rhyolitic clast less than 4 cm in size.

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These fluidal clasts are irregular, diffuse, forming globules and sometimes connected by thin necks

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(Fig. 3c). Facies F has a blocky appearance with sub-angular and angular clasts with a jigsaw-fit

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texture (Fig. 3d) of rhyolites defined as facies A. The matrix is sandy.

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3.4 Pyroclastic facies (G and H)

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Pyroclastic facies are more present to the western sector of the study area where facies G in

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the Quebrada Chuschín and facies H in Mina Delina dominate. They appear in bodies of tabular shape

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of up to 300 m thickness that show sharp to transitional contact with volcaniclastic facies. Both facies

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display the development of lithophysae between 2 to 4 mm indicating expansion of vesicles by

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exsolved volatiles and plastic deformation of the glass at high temperature (McPhie et al., 1993;

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McArthur et al., 1998; Holzhey, 2001; Breitkreuz, 2013). Moreover, facies G is characterized by a

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eutaxitic texture in the matrix, fiammes with disharmonic folds (Fig. 3e) and clasts rotated by the flow

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action. These features make it possible to interpret the facies G conditions of rheomorphism and/or

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welding (Motelib et al., 2014) called agglutination (Branney and Kokelar, 1992). The characteristics

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of facies G correspond to the high-grade ignimbrites, or reoignimbrite (Walker, 1983), where the

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agglutination and coalescence of the vitroclasts are preserved at temperatures above glass transition

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(White, 2000; Kralj, 2012). Facies H matrix preserves ubiquitous centimetric glass shards (Fig. 3e),

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entirely devitrified pumice fragments and some fiammes. Glass shards of ash size can be produced by

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a number of different processes including abrasion during transport. Both the textural features and the

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absence of eutaxitic texture in facies H suggest low-temperature pyroclastic currents. Besides, the lack

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of internal organization with poor sorting and continuous lateral extent suggest deposition from cold

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water-supported pumice-lapilli/block pyroclastic density currents (Allen and McPhie, 2009;

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Valenzuela et al., 2011).

4 3.5 Volcaniclastic facies (I, J and K)

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The volcaniclastic facies I, J, and K are mainly present in the central and eastern sector of the

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area in the Chuschín Quebrada and Mina Delina. They appear in tabular banks with thicknesses that

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vary between 30 and 100 meters. Monolithologic deposits corresponding to the facies I contain

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rhyolite clasts of facies C with sharp edges or, in some places, diffuse borders in contact with the

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matrix. In the sector of Mina Delina, facies I and C have a concordant contact. The facies is thought to

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be hyaloclastic material deposits. The detritus character of the matrix, the form of the clasts (Table 1)

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and the bedding with facies C suggest that these units were not remobilized. The glass in hyaloclastite

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is unstable and readily hydrates and devitrifies. The monolithologic feature of facies I and the size of

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the clasts suggest that deposition occurred in proximity to the volcanic edifice and the reworking of

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primary deposits occurred in a relatively short time after emplacement. Facies J (Table 1) is polymictic

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with chaotic fabric and sabulite matrix whose characteristics indicate avalanche deposits with quick

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deposition rates (Schmincke et al., 1997) with high slopes due to the blocks of up to 3.5 m in size. It is

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common that facies type J is generated from faults movement on the slopes of volcanic edifices

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(Moore et al., 1989). Facies K is the smallest grain size volcaniclastic facies with the former internal

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primary structure obliterated by mylonitic penetrative anastomosed fabric that demonstrates an intense

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brittle and ductile deformation. However, it preserves the high proportion of completely devitrified

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pumice fragments. In the area of Mina Delina, a transitional passage of facies J to facies K is

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preserved that possibly is the transition from avalanche flows to debris flows turbidity currents

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(Fisher, 1984).

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3.6 Epiclastic facies (L, M, and N)

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The best field relationships and associations of epiclastic facies in the area are mostly

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preserved in the northwestern sector of the area in the vicinity of Mina Delina. Facies L appears in

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lenticular to tabular bodies with an eroded base alternating between beds of facies type N. Facies M

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like in the case of facies N (metagreywacke and metapelite) lie in tabular banks interspersed with the

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sequences of the entire area of study. Both facies are affected by low-grade metamorphism (prehnite-

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pumpellyite facies) and considerable brittle and ductile deformation superimposed attested by marked

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crenulation cleavages.

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Thick banks of metapelites (facies N) are the product of deposition by decantation of

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suspended sediments in low energy marine environment, under level of fair weather wave base. The

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alternation of the facies L and M, in massive, laminated layers with diffuse gradient suggest deposition

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of episodic turbidity currents of low to medium concentration (Pickering et al., 1986) in a deep marine

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environment or below storm wave base in shelf outer environment.

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4. GEOCHEMISTRY, TEMPERATURE AND VISCOSITY OF COHERENT FACIES The coherent facies of the Chuschín Formation are plotted against the alkalis diagram vs SiO2

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of Le Maitre et al. (1989) as the SiO2 content of the rhyolites vary between 75 wt % and 79 wt % (Fig.

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4a). These rocks are sub-alkaline with a felsic peraluminous signature. According to the Shand (1927)

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diagram, rhyolites are weakly peraluminous with ASI values between 1.02 and 1.17 (Fig. 4b). Trace

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elements data, plotted on spidergrams normalized to primitive mantle, point out the enrichment in

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lithophile elements (LILEs) and depletion in high field strength elements (HFSE). The HFSE Nb and

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Ti as well as the LILE Sr show negative anomalies (Fig. 5a). The patterns of rare earth elements

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normalized to chondrite suggest higher content of light REE compare to heavy REE content (Fig. 5b)

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with a negative Eu anomaly (Eu/Eu* between 0.17 to 0.23). The values of the normalized ratio

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(La/Yb)N vary between 8.2 to 11.8 indicating a moderate fractionation between LREE and HREE with

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the only exception of the sample of banded rhyolite that presents an almost flat trend (La/ Yb)N = 3.5.

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The temperature data of coherent facies were estimated from the major elements and Zr

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contents following the method of Watson and Harrison (1983). The zircon saturation data of the

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rhyolites of the Chuschín Formation (Table 2) indicate a temperature interval between 735ºC and

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800ºC. Interestingly, the range of zircon saturation temperatures is similar to the interval of the

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liquidus temperature calculated through the Excel-based MELTS (Gualda and Ghiorso, 2015).

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MELTS liquidus temperature calculated considering a H2O content of 3 % yield values from 769 ºC to

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800 ºC. Supporting estimates from zircon saturation and MELTS temperatures, the low phenocrystals

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content (<6 %) allows us to infer that the composition of major elements in total rock is representative

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of the melt (Hanchar and Watson, 2003). The calculated viscosity for the rhyolites using Giordano et al. (2008) yields values varying

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from 6.05 to 7.08 log h (Pa s) (Table 2). As an initial parameter, we assumed a H2O content of 3%,

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whereas the temperature data are those estimated by the Zr saturation method (Watson and Harrison,

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1983). In addition this model yields glass transition temperature (Tg) values ranging from 714ºK to

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741ºK, and fragility (m) values vary from 17.65 to 18.92. In the case of the anhydrous condition,

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viscosity values vary from 10.7 to 12.43 log h (Pa s), Tg values vary from 1011 ºK to 1056 ºK, and m

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values vary from 22.9 to 24.4.

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5. U-PB ZIRCON GEOCHRONOLOGY

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In order to obtain the ages of the Chuschín Formations, U-Pb geochronologic studies were

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conducted on individual zircon grains from the analyses of two samples from this Formation (Fig. 1d).

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Zircon separation was performed following a conventional routine that uses magnetic separations with

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a Franz magnetic separator apparatus and heavy liquids. Then, zircon crystals were mounted and

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analysed

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(www.chronuscamp.com).

the

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The complete dataset measured in each sample is shown in Pb/U concordia plots that are

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constructed using the routines in Isoplot (Ludwig, 2003). The reported ages are determined from the

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weighted mean of the 206Pb/238U ages of the concordant and overlapping analyses (Ludwig, 2003). The

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reported uncertainty (labeled “mean”) is based on the scatter and precision of the set of

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206

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which includes contributions from the standard calibration, age of the calibration standard,

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composition of common Pb and U decay constants, is generally ~1–2 % (2σ). In both samples of

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Chuschín Formation, the subset of overlapping ages shows a Gaussian curve to the age spectra with a

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maximum peak that coincides with the best age characterizes.

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Pb/238U or

Pb/207Pb ages weighted according to their measurement errors (shown at 1σ). The systematic error,

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The facies A of Chuschín Formation contain homogeneous populations of zircon grains in

2

terms of shape and size. Zircons were studied with SEM in backscatter electron mode and

3

cathodoluminscence at Chronuscamp laboratory. Backscatter electron and CL images show that

4

compositional zoning is similar among all of the specimens. The zircons are euhedral to subhedral

5

with typical prismatic terminations, clear, transparent, a few colourless, and vary in size from 70-200

6

microns. All the zircons show internal oscillatory zoning and simple cathodoluminescence pattern

7

from a bright dominant interior to slightly dark outer zones.

RI PT

1

Specimen QMA24 is a rhyolite taken in the Mina Delina area (Fig. 1d). Twenty-seven spots

9

were analysed, eleven in core and sixteen in rims. The large majority of the zircons has U contents

10

from 200 to 600 ppm but only spot measurements in rims has U contents of 900 ppm and only spot

11

measurements in core has U contents of 800 ppm. All zircons have U/Th ratios > 0.5 with most U/Th

12

values ranging from 0.6 to 0.9. The best age determination for this sample is estimated from a coherent

13

cluster of sixteen points in rims of with a weighted mean

14

with MSWD = 1 and probability = 0.45. There are several clusters of inherited core concordant age

15

around 507.92 + 3 Ma.

M AN U

SC

8

Pb/238U age of 463.3 ± 1.5 Ma (Fig. 6a),

TE D

206

Rhyolite QCH6 was collected in the Quebrada Chuschín (Fig. 1d). Seventeen spot

17

measurements were made in the rhyolite in zircon grains, ten in core and seven in rims. The majority

18

of the zircons have U contents from 800 to 1500 ppm but only spot measurements in rims of one

19

zircon has U contents of 2400 ppm and only spot measurements in core has U contents of 680 ppm.

20

The measured U/Th ratios fall in a narrow range of between 0.55 and 0.8, and one spots in an inherited

21

zircon that have U/Th = 1.17. The Ordovician age is extracted from the seven rims data that give

22

concordance with a weighted mean on 206Pb/238U ages of 469.7 ± 2.6 Ma (Fig. 6b) with MSWD = 0.58

23

and probability = 0.63. Furthermore the rhyolite QCH6 contains zircons with inherited cores that

24

cluster around 515.3 + 6 Ma.

AC C

EP

16

25 26

6. DISCUSSION

27

6.1. Significance of geochronological data from the Chuschín Formation.

13

Western edge of the Famatina. Chuschín Formation.

ACCEPTED MANUSCRIPT

Our new geochronological data show that the volcanic rocks of the Chuschín Formation

2

crystallized between 463 to 469 Ma (Fig 6). The ages for volcanism of the Chuschín Formation are

3

identical to previous geochronological results of a rhyolite dome (468.3 + 3.4 Ma) from the Sierra de

4

Las Planchadas, at Río Chaschuil valley (Fanning et al., 2004). Moreover, other rhyolite from the

5

western flank of the Sierra de Famatina in the area of Cuchilla Negra has a zircon crystallization U-Pb

6

age 477 ± 4 Ma (Dahlquist et al., 2008). In the study area, granitoids from Cerro Toro and Ñuñorco

7

plutons are exposed in tectonic contact with the sedimentary volcanic sequence of Chuschín

8

Formation (Fig 1d). The Cerro Toro plutonic rocks with crystallization age of 481+4 Ma were

9

emplaced at deeper depths than the Ñuñorco Complex with crystallization ages 463+4 Ma (Toselli et

10

al., 1996; Pankhurst et al., 2000; Dahlquist et al., 2008). The Chuschin Formation volcanism was

11

coeval with the emplacement of younger Famatinian granitoids and is consistent to the effusive stage

12

of Las Planchadas Formation in the northern Famatina. At regional scale, the Chuschín Formation

13

corresponds to a time period of relatively high magma production at the central segment of the

14

Famatinian arc (Ducea et al. 2017, Otamendi et al., 2017).

M AN U

SC

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1

One zircon core from specimen QCH6 yields a Brazilian orogenic age of about 616 Ma (100

16

% concordant). Seven inherited ages in specimen QCH6 and four in specimen QMA24 give Middle

17

Cambrian best ages of between 535-505 Ma (Fig. 6) reflecting core inheritance from metasedimentary

18

host successions (e.g. Collo et al., 2009; Cristofolini et al., 2012). The presence of a dominant

19

population of Early Cambrian zircons in the silicic rocks from the western flank of the Famatina

20

System support the inference that they are generated by crustal anatexis (Mannheim, 1993).

22 23

EP

AC C

21

TE D

15

6.2. Volcanic depositional environment of Chuschín Formation. The analysis of facies of the Chuschín Formation reveals the evidence of volcanic events

24

effusive and explosive, which formed the Famatinian arc during the lower Ordovician period. Section

25

A (Fig. 2) reveals the associations between the coherent facies and the epiclastic facies. The well

26

preserved outcrops in the western sector of the field area allow teasing out the environment of

27

deposition of volcano deposit and sediments around the volcanic system of Chuschin (Fig. 1d). The

28

environment of deposition was a relatively deep basin where deposition was dominated by decantation

14

Western edge of the Famatina. Chuschín Formation.

ACCEPTED MANUSCRIPT

(facies N) and episodic flows of turbidity currents (facies L and M). As mentioned before, the

2

characteristics and spatial distribution of the coherent, autoclastic and peperitic facies are interpreted

3

to be different sections of a rhyolitic dome (Cas and Wright, 1997; Mc Phie et al., 1993). From the

4

core to the rim of the dome, massive rhyolites (facies A), flow banded rhyolites (facies C), rhyolites

5

with spherulites (facies B), hydroclastic breccia (facies D) and different peperitic facies (facies E and

6

F) predominate. The obtained temperature and viscosity for the coherent facies are typical of rhyolitic

7

domes (Ewart et al., 1971; 1975; Bacon et al., 1980; Wuitney, 1985; Wright et al., 2015; Loewen et

8

al., 2017). The rhyolite facies presents a very low proportion of phenocryst; therefore, we assume that

9

the bulk rock composition represents the melt composition (Hanchar and Watson, 2003). It is worth

10

highlighting that the estimated temperatures are similar to those established by Mannheim (1993) for

11

the volcanites of the Chuschín Formation based on studies of low-quartz phenocrysts.

M AN U

SC

RI PT

1

The method of Giordano et al. (2008) is used to constrain magma fragility (m) in addition to

13

viscosity, which is used to constrain the sensitivity of the liquid structure and the flow properties to

14

temperature changes. Angell (1985) distinguishes two extreme behaviors: strong and fragile. The data

15

of m of the volcanites of the Chuschín Formation yield values between 17 and 18 (Table 2), which are

16

considered low values and therefore demonstrates a strong behavior. “Strong” liquids show near-

17

Arrhenian temperature dependence and show a firm resistance to structural change, even over large

18

temperature variations.

EP

TE D

12

The different textures that show the coherent facies demonstrate that the dome was affected by

20

at least three stages of devitrification as put forth by Lofgren (1971b): the state of initial hydration

21

with the perlitic texture (facies D), the vitreous state with the felsitic texture (facies A and C) and the

22

spherulitic state with concentric axiolytic spherulitic in fan (facies B and C). The rate of devitrification

23

is a function of glass viscosity, which is mainly controlled by the water content and temperature

24

(Marshall, 1961). Such textures indicate a cooling rate sufficiently slow to allow their development

25

and a setting temperature greater than the glass transition temperature (Tg) according to McArthur et

26

al., (1998). It is worth clarifying that the Tg obtained in the coherent facies of the Chuschín Formation

27

vary from 441 ºC to 468 ºC and the temperatures experienced and measured by Lofgren (1971b) for

28

the development of the textures on site vary between 450 ºC and 700 ºC.

AC C

19

15

Western edge of the Famatina. Chuschín Formation.

ACCEPTED MANUSCRIPT

The formation of the peperitic facies involves fragmentation or disintegration of magma to

2

form juvenile clasts and mingling of these clasts with a sediment host. Lava flowing over the sediment

3

and the interaction process of posterior hydrodynamic mingling of juvenile clasts and sediment

4

explain the formation of a single peperite. An aspect worth highlighting is the reduced thickness of the

5

peperite facies in relation to the coherent facies deposits, which is related to lava flow velocity and the

6

slope dips of ancient surfaces (Waichel et al., 2007). The reduced thickness of these facies can be

7

linked to areas of deposition with gentler slopes in subaqueous environments and low speeds lava flow

8

generating small levels of mix. A direct relation between the small variations in the values of viscosity

9

that present the coherent facies and the different peperitic facies cannot be established in our samples.

10

The fluidal peperites are associated with the more viscous consistent facies (Sample QCH6 - Table 2)

11

and the blocky peperites with lesser viscosity (Sample QMA3 - Table 2). However, as put forth by

12

Busby-Spera and White (1987), Skilling et al. (2002), and Templeton and Hanson (2003) the condition

13

of the type of peperite is closely influenced by the grain size of the sediment with which the rhyolitic

14

flow interacts and, as mentioned in point 3.3, our sample display evidence of such variation. On the

15

other hand, it is expected that facies C generates fluidal peperite because it corresponds to the areas

16

closest to the edge of the dome where the vapor-film layer forms. Caroff et al. (2009) argues that this

17

fluidal-type peperite is favored when a steam-film layer at the magma-sediment interface acts as an

18

insulating barrier. In addition, the latter work suggests that the interface is controlled by the sediment

19

characteristics, the hydrology and the mass interaction ratio of wet sediment and magma. In our case,

20

variations in the H2O content are not a viable explanation as we interpret the system as being a

21

subaquatic environment.

SC

M AN U

TE D

EP

AC C

22

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1

Section B (Fig. 2) represents associations of facies that are preserved in this eastern sector of

23

the area, where volcaniclastic facies mainly in contact with the coherent facies, acquire greater

24

relevance. The characteristics of these volcaniclastic facies suggest deposition from debris flows

25

(facies J) possibly associated with the collapse of the volcanic edifices. This process leads to

26

gravitational instability causing the development of turbidity currents with high content of pumice and

27

rhyolite fragments (facies K). This generates volcaniclastic turbidites in the more distal areas of the

28

submarine volcaniclastic aprons where reworking by bottom currents occurs (Cas, 1992). In these

16

Western edge of the Famatina. Chuschín Formation.

ACCEPTED MANUSCRIPT

1

environments, tuffaceous sandstone closely associated with debris flows, (Lowe, 1982; Kralj, 2012)

2

are found as well as resedimented hydroclastic breccia (facies I). The events of volcanic edifice collapses may or may not be temporarily linked with events of

4

explosive volcanism that gave rise to the pyroclastic facies. In this regard, there are high-temperature

5

ignimbrites (facies G), which, because of their association with the rest of the facies, could support the

6

hypothesis that they are subaquatic ignimbrites, ignimbrites with a subaerial origin or the result of a

7

subsequent entry into a body of water. Given that the welding of this type of hot flow is controlled by

8

the lithostatic load and viscosity of the glass (McPhie et al., 1993), viscosity decreases in subaquatic

9

environments due to the weight of the water column, favoring the reomorphism at great depths (Sparks

10

et al., 1980). The other type of deposit (facies H) shows the pyroclastic density currents enriched in

11

pumice fragments and vitreous shards as well as lithic rhyolites. The low content of fiammes and the

12

absence of welding textures suggest a rapid dissipation of the temperature. The association between

13

the G and H facies could represent the welding zonation formed by a subaquatic pyroclastic deposit

14

considered as a single unit of flow (McPhie et al., 1993).

M AN U

SC

RI PT

3

16

TE D

15

6.3. Geochemical features of volcanic rocks in the Chuschín Formation. Geochemical data for the coherent facies of Chuschín Formation presented here concur with

18

the previous interpretation by Mannheim (1993) in that silicic peraluminous magmas are mainly

19

formed by intracrustal partial melting within the subduction-related setting of the Famatinian arc (Figs.

20

4a and 4b). Moreover, rhyolite rocks in the south-western flank of Famatina System are broadly

21

similar to silicic volcanic rocks in the area of Chaschuil (Cisterna et al. 2010) as well as with the

22

eastern eruptive belt from the Puna plateau (e.g., Coira et al. 1999; Viramonte et al. 2007). This

23

analysis reveals that volcanic rocks of the Chuschín Formation characterized subalkaline-type arc

24

magmatism (calc-alkaline) with a weakly peraluminous imprint suggesting an important sedimentary

25

component in the source.

AC C

EP

17

26

The patterns of trace elements (Fig. 5a) of the coherent facies of Chuschín Formation are

27

largely agree with those presented by Mannheim (1993) for the silicic facies of the Famatina System.

28

The marked negative Nb anomaly proves the affinity with subduction zone magmatism that is also

17

Western edge of the Famatina. Chuschín Formation.

ACCEPTED MANUSCRIPT

consistent with the overall geodynamic setting (Mannheim 1993; Toselli et el. 1996; Astini 1998). The

2

pattern of trace elements is characterized by a negative slope enriched in LILEs and low in HFSE.

3

Negative Ba and Sr anomalies, linked possibly to the fractional crystallization of plagioclase, stand

4

out, as does the negative Ti anomaly associated with the crystallization of ilmenite at the source. In the

5

REE pattern (Fig. 5b), the negative Eu anomaly also suggests early plagioclase fractionation reflecting

6

either crystal fractionation under low pressure or the predominance of an intra-crustal source

7

component.

RI PT

1

8 7. CONCLUSIONS

SC

9

The Chuschín formation is one of the sedimentary-volcanic units that make up the western

11

slope of the Famatina System. Facies analysis indicates that this formation is characterized by

12

coherent, autoclastic, peperitic, pyroclastic and volcaniclastic volcanic facies.

M AN U

10

Coherent facies are rhyolites associated with hydroclastic facies product of non-explosive

14

fracturing and fluid and blocky peperitic facies suggesting lava-sediment interaction. Their spatial

15

distributions make it possible to interpret the different sections of a subaqueous rhyolitic dome. The

16

Chuschín formation also records explosive volcanism with the development of pyroclastic facies such as

17

ignimbrites of high temperature (eutaxitic tuff) as well as lithic and pumice-rich breccia. This suggests

18

that pyroclastic density currents are the main mechanism of sedimentation. These facies lie in sharp to

19

transitional contact with volcaniclastic facies associated with deposits of rapid-deposition such as high-

20

gradient debris avalanches that are linked to the collapse of the volcanic edifice. In regard to epiclastic

21

facies, there is evidence of decantation processes and turbidity currents in low energy sectors within the

22

basin below the level of the fair-weather wave base.

EP

AC C

23

TE D

13

Geochronological information for the coherent volcanic facies of the Chuschín formation

24

indicates an age of crystallization from Ordovician, between 463–469 Ma similar to other plutonic-

25

volcanic sequences of this system. The geochemical data of these lava facies show that they derived from

26

subalkaline weakly peraluminous silicic magma whose major elements and trace elements patterns

27

indicate a strong crustal influence.

28

18

Western edge of the Famatina. Chuschín Formation.

ACCEPTED MANUSCRIPT

1

ACKNOWLEDGMENTS

2

This work was funded by ANPCYT-Argentina through grants PICT 0371/15 and PICT 0958/14.

3

We acknowledge to CONICET and SeCyT – UNRC for funding our research. We are grateful to

4

collaboration of MSc. Julien René Denis Cornet of ETH Zürich. We are also grateful to with the editor of

5

this journal, Dr. Victor Ramos.

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Figures:

Figure 1: a, b) Location maps of Argentina and La Rioja Province. c) Simplified geological

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map of Famatina with study area (after Mannheim and Miller, 1996). d) Detailed geological map of

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Chuschín Formation in the study area.

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Figure 2: Simplified stratigraphic sections of Chuschín Formation.

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Figure 3: a) Spherulitic texture in coherent facies. b) Perlitic texture curved and concentric in

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autoclastic facies. c) Fluidal clasts irregular, diffuse, and globules in peperite facies (ry: rhyolite; s:

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sediment). d) Blocky appearance clasts with a jigsaw-fit texture in peperite facies (ry: rhyolite; s:

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sediment). e) Eutaxitic texture in the groundmass and fiammes in pyroclastic facies. f) Levels with

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glass shards in pyroclastic facies.

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Figure 4: a) Alkalis diagram vs SiO2 of Le Maitre et al. (1989) with samples of Chuschín Formation. b) Shand (1927) diagram with samples of Chuschín Formation.

Figure 5: a) Multi-element diagram normalized to primitive mantle (McDonough and Sun 95)

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with samples of Chuschín Formation of this paper and of Mannheim (1993). b) Patterns of rare earth

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elements normalized to chondrite (McDonough and Sun 95) with samples of Chuschín Formation of

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this paper.

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Figure 6: Cathodoluminescence images of representative zircon crystals, conventional

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concordia diagrams and bars plot displaying the spot data used to calculate the weighted mean age for

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zircons from samples Chuschín Formation. All results are plotted using ISOPLOT (Ludwig, 2003). a)

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QMA24 sample. b) QCH6 sample.

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Tables:

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Table 1: Facies of Chuschín Formation.

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Table 2: Geochemical data of coherent facies of Chuschín Formation.

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Supplementary data table: U-Pb (zircon) geochronologic analyses.

ACCEPTED MANUSCRIPT Table 1: Facies of Chuschín Formation

C Porphyritic, flow banded, rhyolite.

D Non-stratified rhyolitic breccia.

Peperite facies.

E Massive, rhyolitic breccia, rhyolitic groundmass and sediment matrix. F Massive, rhyolitic breccia, sediment´s matrix. G Eutaxitic massive tuff.

Massive tuff. Porphyritic texture, quartz, alkali feldspars, and plagioclase up to 3mm clstas. Undifferentiated lithic clasts (1 to 3 mm) in rotated cases, irregular lithophysae (1 to 3 mm) and lenticular fiammes (2 cm) with spherulitic texture or recrystallized to granular aggregates of quartz and feldspar. Devitrified paste with eutaxitic texture. Massive breccia. Rhyolitic and pumice angular clasts up to 7 cm. Poor sorting. The matrix presents vitreous texture, quartz and undifferentiated lithic clasts (1 to 2 mm), pumice fragments (1 mm), glass shards and lenticular fiammes (2 cm), spherulitic texture in granular aggregates of quartz and feldspars. Clast supported to matrix supported, massive, breccia. Rhyolitic clasts (4 to 45 cm) with angled to subangled forms, with straight and curviplanar edges. Sand size detrital matrix with volcanic lithic and monomineral euhedral of quartz, albite and orthoclase clasts. Angular to subangular, oblate, equants and prolate clasts (5 cm to 35 cm) with chaotic layout. Sabulitic to psamitic matrix. Composition clasts is rhyolitic, andesitic, pyroclastic and mudstone. It has rounded rhyolitic blocks of between 70 and 90 cm, reaching, in some cases, 350 cm. Tabular banks up to 5 m of thickness. No internal primary structure. Pumiceous fragments predominantly devitrified and lithic rhyolitic fragments and crystal-clasts of quartz

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Autoclastic facies.

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H Lithic and pumice-rich, massive breccia.

Volcaniclastic facies.

I Massive Monomictic rhyolitic breccia.

J Massive polymictic rhyolitic breccia. K Tuffaceous sandstone.

Interpretation Facies of inner coherent core rhyolite domes. Facies of edge rhyolite domes.

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B Microspherulitic, massive, rhyolite.

Characteristics Rhyolite massive. Porphyritic texture, euhedral to subeuhedral phenocrystals of quartz and feldspars (albite 4%-7%An and oligoclase 15%-26%An) up to 3mm, fragmented and flow oriented. Microcrystalline to cryptocrystalline groundmass with felsitic texture. Rhyolite massive. Aphanitic texture, euhedral and subeuhedral phenocrystals (<5%) of quartz and feldspars (1.4 mm). Groundmass with micropoikilitic texture in bands that alternates with concentric axiolytic spherulitic textures in open fan. Rhyolite, banded flow structure (15 to 5 mm). Porphyry texture, euhedral and sub-euhedral phenocrystals with engulfed edges of quartz (1.3 mm), and alkaline feldspar (orthoclase) up to 1.5 mm. Groundmass with banding marked by alternation of textures felsitic microcrystalline and spherulitic texture in fan. Massive breccia. Clast: rhyolitic angular clasts (8 cm), with straight to curviplanar edges. Groundmass: microcrystalline with both felsitic and perlite texture of denatured and chloritized glass, and, porphyric texture with euhedral and sub-euhedral phenocrystals (1,7mm) of quartz and feldspar (oligoclase and albite). Massive breccia. Rhyolitic fluidal clasts connected by thin necks and pelitic clast, with straight and curviplanar edges and jigsaw-fit texture. Rhyolitic groundmass and sediment matrix. Present along the upper or lower contacts of coherent facies. Massive breccia. Rhyolitic clast (3 to 8 cm) with straight and curviplanar edges and jigsaw-fit texture. Volcaniclastic felsic sandstone matrix. Gradational contacts whit coherent coherent facies.

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A Porphyritic, massive, rhyolite.

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FACIES Coherent facies.

Facies of inside /edge rhyolite domes.

Hydroclastic breccia.

Fluidal or globular peperite.

Blocky peperite.

Welded ignimbrite.

Lowtemperature pyroclastic flow deposit.

Mass flow resedimented hydroclastic.

Avalanche deposits, or debris flow.

Volcaniclastic turbidity currents.

ACCEPTED MANUSCRIPT Epiclastic facies.

L Subarkose sandstone.

Episodic turbidity currents.

Episodic turbidity currents.

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M Metagreywacke.

and albite. Pelitic matrix. In the clay fraction present 75% of illite / mica; 20% of illite / smectite; and 5% clorite. Tabular banks up to 0.50 m of thickness. Displays alternation of massive sets and diffuse normal gradation sets. Sub rounded clasts, medium grain size, of quartz, feldspars and volcanic lithics. Tangential contacts and complete packing. Pelitic matrix. Tabular banks of 1 m of thickness, integrated by strata of greywacke of between 0,40 and 0,20 m, with net contacts. Internally massive sets or with planar lamination sets. These facies present > 80% of quartz and as for the clay fraction between 75% and 85% of illite/mica and between 25% and 15% of illite/smectite. Tabular banks from 1 to 6 m of thickness. Strata between 15 and 30 cm limited by net contacts, with sets (1 to 0.5 cm) massive or with very thin parallel lamination. Intense silicification. These facies present > 80% of quartz and as for the clay fraction 95% of illite /mica and 5% of illite/smectite.

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N Metapelite.

Low energy environment. Decantation.

ACCEPTED MANUSCRIPT Table 2: Geochemical data of coherent facies of Chuschín Formation.

800.6 830.08 720.13 6.07 18.7

76.3 12.9 1.67 0.031 0.24 1.35 3.38 4.15 0.157 0.03 0.69 100.9

79.2 11.21 0.21 0.004 0.02 0.08 1.57 6.28 0.055 < 0.01 0.38 99.01

89 673 15.8 4 9 1.5 23 36.8 100 175 43.7 88.5 10.2 7.8 1.56 7.3 1.2 7.5 1.5 4.4 0.63 4.2 0.71

149 516 21.7 3.5 7 2 22 15.6 75 83 21.4 44.2 4.7 3.2 0.72 3 0.6 3.3 0.7 2.2 0.37 2.6 0.41

796.7 821.72 714.59 6.05 18.7

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76.36 12.53 1.99 0.09 0.15 0.89 4.57 2.86 0.231 0.02 0.53 100.2

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T °C Sat Zr T °C liquid Tg °K Visc. log η (Pas) Fragility (m)

134 501 18 3.3 11 1.8 9 50.6 71 161 55.8 118 13.7 10.2 1.92 9 1.4 8.1 1.6 4.5 0.68 4.7 0.76

Quebrada Chuschín rhyolite QCH 6

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Trace elements (ppm) Rb Ba Th U Nb Ta Pb Nd Sr Zr La Ce Pr Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

rhyolite QMA 24

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Quebrada Mina Delina rhyolite QMA 10.5

rhyolite QMA 3 Major oxides element (wt%) SiO2 75.41 Al2O3 12.91 Fe2O3(T) 1.77 MnO 0.053 MgO 0.35 CaO 0.68 Na2O 3.78 K2O 3.49 TiO2 0.104 P2O5 < 0.01 LOI 0.81 Total 99.36

734.9 822.08 727.62 6.93 18.9

234 865 16.5 3.6 19 1.9 9 14.5 28 97 11.1 24.1 3.33 4.5 0.86 5.6 1.2 7 1.3 3.5 0.48 3.1 0.5

764.2 761.44 741.77 7.06 17.6

Major and trace elements on whole rock concentrations were determined at ACTLABS, Canada, using the protocols for Lithoresearch code.

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ACCEPTED MANUSCRIPT This study shows facies analysis and types of volcanism for the Chuschín Formation. The distribution of volcanic facies is typical of subaqueous rhyolitic domes. Volcanism involves ignimbrites, debris avalanches and turbidity currents. Silicic volcanic rocks have U-Pb zircon crystallization ages between 463–469 Ma.

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Rhyolites are weakly peraluminous magmas derivate from intracrustal partial melting.