Evidence for Different Processes of Magma Evolution in El Tatio Volcanic Region (22Ë16' to 22Ë30' S, Central Volcanic Zone, Andes)

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Evidence for different processes of magma evolution in El Tatio volcanic region (22ë16' to 22ë30' S, Central Volcanic Zone, Andes)

GIANFILIPPO DE ASTIS1, FEDERICO LUCCHI2, CLAUDIO ANTONIO TRANNE2 and PIERMARIA LUIGI ROSSI2 1 INGV-Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italy. E-Mail: [email protected] 2 Dipartimento di Scienze della Terra e Geologico-Ambientali, Piazza di Porta S.Donato 1, 40126 Bologna, Italy. E-Mail: [email protected]; [email protected]; piermaria- [email protected]

Abstract We report new petrographic and geochemical data on volcanic rocks erupted over the last ~9 Ma in El Tatio volcanic region (Western Cordillera ± Central Volcanic Zone). They originated from compound volcanism al- ternating composite volcano activities, lava domes formation and minor low-mild explosive eruptions, whereas ignimbrite-like deposits outcropping in the region originated from external caldera systems (Altiplano Puna Volcanic Complex). Volcanic rocks ± mostly erupted in the last 1 Ma - have composition ranging between calcalkaline (CA)and high-K calcalkaline (HKCA)basaltic andesite to rhyolite, but most of them are andesites and dacites. Petrographic features of studied rocks can be frequently related to strong disequilibrium conditions in the crystallizing system: deeply resorbed and rounded mineral phases, reaction rims, skeletal habits, large ranges of mineral compositions with direct and reverse zoning, oxidations and uralitizations phenomena are observed. Most of these crystal disequilibrium features may be explained by convective self-mixing processes in magma reservoirs cooling from above and characterized by a mafic magma batch at the base as a probable heat source. A minor role for magma mixing between compositionally different magmas is suggested. On the other hand, the high crystallization degree of the rocks together with evidence provided by geochemical data suggest that fractional crystallization (FC)of recurrent mineral assemblages (plag‹pyrox‹hornbl‹bt)in closed magma chambers must be considered the most significant differentiation process for several volcanic edifices in the El Tatio area. Although FC holds a leading role, the correlation of geochemical and few available isotopic data also point to AFC processes as a further important evolutionary mechanism. Isotopic trends and patterns of in- compatible elements (i.e., LILE enrichment, LILE/HFSE ratios)mirror the interaction of magmas with crustal contaminants occurring both in the mantle wedge (material transported via subduction)and during the ascent/ ponding of parental magmas along the thick crust beneath central Andes. In other words, starting from already modified parental magmas, further modification of variable type affected magmas at higher level. Finally, we suggest that the plumbing systems (high level magma reservoirs)hosting magmas of El Tatio region were characterized by multistage evolutionary processes consistent both with open- and closed-system regimes.

Keywords: Andes, Tatio, Disequilibrium textures, Magma evolution

Introduction stricted latitude/longitude span. Magmatism of the CVZ (28ë-16ëS)- where our study area is lo- The South American Andes are one of the best cated - is the result of a complex interplay of examples of active continental margins. Here, magma genesis plus fractional crystallization, subduction-related magmatism and volcanism within the mantle, and various evolutionary pro- have been extensively studied by several Authors cesses derived by the interaction of magma with (see Harmon and Rapela, 1991; Reutter et al., wall-rocks in intra-crustal reservoirs. This Andean 1993; for a review)and four different Volcanic segment is characterised by large andesite to da- Zones (VZ)are currently recognised: northern, cite stratovolcanoes, interbedded with voluminous central, southern and austral (NVZ, CVZ, SVZ, dacitic to rhyolitic ignimbrite deposits. These AVZ, respectively). Besides a general agreement stratovolcanoes are formed upon the continental of the scientific community with this four-fold keel of central Andes, where seismicity is active up division, a quite large variability exists in the to ~140 km and the continental crust is much choice of the limits and criteria able to char- older (Precambrian ± Lower Paleozoic)and acterize the different segments. Moreover, each thicker (50770 km - Zandt et al., 1994; Schmitz et Andean segment demonstrates different tectonic al., 1999)if compared with the other Andean features, volcanic systems and processes, in a re- zones. The eruption of mantle-derived mafic

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Fig. 1 - Sketch map showing the localisation of El Tatio volcanic area in the Central Andes (South America). Main morphotectonic units are inferred from Munoz and Charrier (1996)and modified on the basis of lineaments evidenced in the DEM shaded relief image obtained by processing GTOPO30 data (http://edc.usgs.gov/products/ elevation/gtopo30/README.html). Shortcuts: TA=El Tatio volcanic area; SA=- Salar de Atacama; PG=Pastos Grande caldera; LC=Laguna Colorada caldera; CG=Cerro Guacha caldera.

magmas in the CVZ is rare and limited to some volcanic products. The present study is part of a monogenetic centres of unknown age (de Silva broader investigation aimed at mapping the El and Francis, 1991)or some Quaternary strato- Tatio volcanic area, at studying major tectonic volcanoes (e.g., OllaguÈe volcano; Feeley et al., structures and regimes acting during time (~9 1993; Feeley and Davidson, 1994; Mattioli et al., Ma)and at characterizing volcanic products in 2006). The volcanic arc of CVZ developed above a terms of both petrographic features and chemical 30ëE-dipping Wadati-Benioff Zone (Barazangi composition. The processes involved in the gen- and Isacks, 1976)and has migrated eastward from esis of various rock compositions are then briefly the modern Pacific coast of northern Chile to the discussed in the context of magmas genesis and current Andean chain since the Jurassic (de Silva, differentiation processes occurred in the CVZ. 1989; Pichowiak, 1994). Therefore, it is the result Finally, a short summary of the magmatological of a very long and complex geologic history that, in setting associated to the different stages of vol- general, shows systematic variations of magma canism described in the study area is presented composition from W to E (i.e., along strike - Hil- (conforming to Lucchi et al., 2009). dreth and Moorbath, 1988, and ref. therein), si- milar to those reported for the other Andean volcanic zones (NVZ, SVZ, AVZ). The present Magmatism in the Central Volcanic Zone arc region of CVZ corresponds to the Western Cordillera (Fig. 1). In a geological setting broadly driven by plate- Despite the numerous studies carried out in boundary forces, most of the central±southern central Andes, there are still large areas poorly portion of the CVZ (i.e., the arc segment between investigated both for geological mapping and latitudes 17.5ë-22ëS; Fig.1a)displays some con- compositional characterization of outcropping stant features: i)crustal thickness, ii)distance

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Evidence for different processes of magma evolution in El Tatio volcanic region (22ë16' to 22ë30' S, Central Volcanic Zone, Andes)

from the trench, iii)height of volcanic edifices Most of the stratovolcanoes located along the above the seismically active subduction zone and Chile±Bolivia band overlie an ignimbrite substrate iv)sediment supply to the trench (WoÈrner et al., cropping out between latitudes 21ë-24ëS and ex- 1993). By contrast, the age of crust in this latitude tending from the APVC to the western Cordillera interval changes from Proterozoic to the north to (de Silva, 1989). These widespread ignimbrite Paleozoic toward the south. Between latitudes sheets were mostly originated from calderas lo- 21ë-26ëS, a late Precambrian-Paleozoic crystalline cated in the APVC and are mainly early Miocene basement and Mesozoic sedimentary and plutonic in age in the northernmost part of the CVZ rocks underlie volcanoes of SW Bolivia, N Chile (<21ëS), and late Miocene to Pliocene between and NW Argentina, together with ignimbrite latitudes 21ë-24ëS (Worner et al., 2000). Small sheets related to the Altiplano-Puna Volcanic volcanic centres characterized by mafic magmas Complex (APVC). In this framework, geological and usually formed during a single eruption are and chronological data indicate that different relatively rare and occur in small clusters (e.g., stages of volcanism (magmatism)occurred in the Andagua/Humbo fields in southern Peru; Dela- CVZ since Middle-Upper Jurassic, giving rise to cour et al., 2007 - Negrillar field in northern Chile; different volcanic fronts (arcs)subparallel to the Deruelle 1982)or in the back arc region (Da- Pacific coast and progressively migrating toward vidson and de Silva, 1992). They usually generate east, up to late Pleistocene. porphyritic basalts and basaltic andesites. Magmatism corresponding to the present arc Late Cenozoic to Recent magmatism in the region of CVZ is localized in the Andean segment CVZ has been the subject of numerous studies called Western Cordillera (Fig. 1), a volcanic chain (e.g., Harmon et al., 1984, Francis et al., 1989; Kay composed of about five hundreds major volcanoes et al., 1994; Trumbull et al., 1999; Lindsay et al., and several minor volcanic centres (Thorpe, 1984). 2001; Delacour et al., 2007). Overall, magmas The chain is delimited to the west by the Preandean erupted in the CVZ show a very large composi- Depression, a morpho-tectonic unit characterized tional range (SiO2=51-72 wt.%)with largely by transpressional and/or compressional activity dominant andesite and dacite terms, and a med- (Hartley et al., 2000 and ref. therein)producing ium-K to high-K calcalkaline character (Deruelle, several roughly N-S-oriented tectonic basins (i.e., 1982; Harmon et al., 1984). Trace element pat- Salar de Atacama), infilled by thick sedimentary terns are typical of subduction-related settings, sequences (Mpodozis et al., 2005). The APVC with depletion in Nb and Ta and enrichment in borders the Western Cordillera to east (Fig. 1b). large ion lithophile elements relative to patterns As a whole, the Western Cordillera consists of an observed in MORB compositions (Davidson et articulated geological architecture derived from al., 1991). If compared with magmas erupted in the interbedding between large andesitic to dacitic the SVZ or NVZ, magmas from the CVZ have stratovolcanoes, dacitic to rhyolitic lava domes and significantly higher 87Sr/86Sr ratios (ranging from widespread dacitic ignimbrite sheets (Davidson et 0.7056 to 0.7149)and d18O(»8,12 %, in the 21ë- al., 1991). Its geological and tectonic setting is still a 26ë S range), and lower 143Nd/144Nd, which in- resuming subject, continuously under discussion. dicate variable degrees of crustal contribution Several authors (e.g., Davidson and de Silva, 1992; (Harmon et al., 1984). Pb-isotope data of volcanic de Silva et al., 1994)indicate a series of NW- rocks of the Western Cordillera show very good striking faults as dominant tectonic lineaments correlation with those of the underlying basement controlling the area. However, recent structural (Worner et al., 1993). Overall, chemical and iso- studies carried in the region between ~19ë and 23ëS topic data (see Thorpe et al., 1984; Hildreth and (Salar de Uyuni±Atacama area; Tibaldi et al., Moorbath, 1988; Stern, 1991; Davidson et al., 2009)depict a more complex setting according to 1991)reflect the occurrence of subduction-mod- which magma upwelling in the majority of Late ified mantle-derived magmas and their modifica- Miocene and Plio±Quaternary volcanoes have tion through the interaction with a thick crust been controlled by NW-SE trending faults and (Ramos and Aleman, 2000). Temporal variations fractures (especially in the northern and central of CVZ magma compositions have suggested an part of the studied area), whereas latest Pliocene± increasing influence of crustal contamination over Quaternary volcanoes (concentrated in the time, from Miocene to Recent (Lucassen et al., southern part of the region)are aligned in a N±S 2001). direction. Beyond this general and generally accepted

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petrological evidence, such a complex geological formal Paleo-Tatio unit. These UBUs set up a region requires more accurate geochemical data general stratigraphic framework into which the for restricted areas and a deeper knowledge of volcanic bodies are included and described in substrate rocks to discriminate between compo- terms of lithostratigraphic units (i.e., rock types) sitional features inherited from the source and and lithosomes (i.e., eruptive vents). The overall different processes responsible for magma mod- unconformity-bounded architecture allows de- ification along the path from source to surface. scription of El Tatio's eruptive history in terms of The intracrustal differentiation processes are the successive vertical time-stratigraphic units (sensu crucial topics of most recent studies (e.g., Mamani Fisher and Schmincke, 1984)reported in Fig. 2B. et al., 2008). Moreover, a large isotopic data set The Rio Salado and Tatio Sur synthems include (Pb and Nd), recently investigated for the in- volcanic bodies deposited during two distinct and trusive, volcanic and metamorphic rocks of the successive eruptive (or constructional)periods, Central Andes (13ë-28 S), shows a correlation separated by periods of dormancy lasting million- with the crustal structure derived from a 3-D years, whereas the Aguada Chica Synthem com- density model, indicating that changes of these prises ignimbrite deposits of external provenance isotopic compositions are due to variations in the from the APVC. The 2nd eruptive period is fur- proportions of light-felsic to dense- mafic material therly subdivided into four eruptive epochs (cor- of the crust in the central Andes (Mamani et al., responding to the subsynthems), separated by a 2008). major episode of deposition of external ignimbrite products and distinct stages of volcanic inactivity developing over tens to thousands years. El Tatio volcanic area: volcanology and strati- The oldest volcanic rocks outcropping in the graphy (resume) study area are those included within the Rio Sal- ado Synthem (1st eruptive period). They are re- The complex stratigraphic succession char- presented by widely eroded lava flows (Tukle acterizing the El Tatio volcanic area derives from formation), which are the remnants of the NE- the interbedding between local stratovolcanoes, SW-elongated Tukle stratocone. These products lava cones and lava domes emplaced in the last ~9 lie in unconformity above a substratum re- Ma and flattish ignimbrite sheets originated from presented by a strongly folded sedimentary suc- the APVC (Fig. 2A-B). According to available age cession mostly consisting of continental and mar- measurements, volcanic products in the study area ine products deposited from late Cretaceous to range in age from Upper Miocene to Late Pleis- Oligocene (Paleo-El Tatio informal unit and Rio tocene. Based on new field work and remote Hojalar synthem). Tukle lavas are largely sealed sensing analysis (Lucchi et al., 2009), the strati- by the Rio Salado ignimbrite deposits, dated at graphic succession has been subdivided into a 9.3‹0.6 Ma and related to an undefined external series of unconformity-bounded units (UBU)se- eruptive source located westwards the study area. parated by unconformities with variable duration These deposits are covered by the widespread and areal distribution (Fig. 2B). These un- Sifon ignimbrite sheet originated from the Pastos conformities formed during major periods of Grande caldera complex (APVC, SE Bolivia) volcanic inactivity, when erosional processes and around ~9-7.5 Ma. The intrusion of the 7.35-ka- regional tectonic events were dominant. Four old Co. Copacoya laccolith-type body closes the st major unconformities (T1,T2,T3,T4), with re- 1 eruptive period. gional stratigraphic outcome and million-years After a prolonged period of non-deposition, the duration, and four minor unconformities (ta,tb,tc, the large-scale ignimbrite-type Puripicar products td), associated to tens-thousands-years hiatuses, (Aguada Chica Synthem)of Pliocene age (ca. 4 have been identified (Fig. 2B). Together with the Ma)are emplaced. They are originated from the topographic surface (which is by definition a ma- Cerro Guacha caldera, in the APVC. jor unconformity; Chang, 1975), they allow in- The 2nd eruptive period (Tatio Sur Synthem is troduction of four synthems (Rio Hojalar, Rio responsible for the emplacement of most of the Salado, Aguada Chica, Tatio Sur), five sub- volcanic products in the study area, which are synthems (Paso Las Vizcachas, Portezuelo del dated to Middle-Late Pleistocene. We recognized Cordon de Panizo, Quebrada Agua Brava, four distinct eruptive epochs separated by periods Quebrada La Torta and Rio Blanco)and the in- of dormancy and regional tectonic events. The 1st

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Evidence for different processes of magma evolution in El Tatio volcanic region (22ë16' to 22ë30' S, Central Volcanic Zone, Andes)

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Fig. 2 - Stratigraphy and eruptive history of the El Tatio volcanic area from a simplified reduction of Lucchi et al. (this volume). A) Geological map and geologic sections showing the distribution of main stratigraphic units. B)Stratigraphic succession of the El Tatio volcanic area (described by 9 UBUs, 12 lithosomes and several lithostratigraphic units)and corresponding time-stratigraphic vertical units (eruptive periods and epochs). Shortcuts: *= toponyms from outside the study area; PDC=pyroclastic density currents; HK=high-K; CA=calcalkaline. References cited: (1)Baker, 1977; (2)De Silva, 1989a; (3)De Silva and Francis, 1989; (4)De Silva and Francis, 1991; (5)Francis, 1993; (6)Guest, 1969; (7)Lahsen, 1982; (8)Lahsen and Munizaga, 1979; (9)May et al., 1999; (10)Marinovic and Lahsen, 1984; (11)Renzulli et al., 2006; (12)Rutland et al., 1965; (13)Tomlinson et al., 2004.

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Evidence for different processes of magma evolution in El Tatio volcanic region (22ë16' to 22ë30' S, Central Volcanic Zone, Andes)

Table 1 - List and localisation of analysed samples relevant to volcanic rocks of the study area.

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eruptive epoch (Paso Las Vizcachas Subsynthem) the list of selected samples representative of the determines the effusion of the oldest Pleistocene designated stratigraphic units. They have been lava domes and flows (El Negral de Jauna and Co. studied in thin section and analysed. Major and Negro formations), dated at 0.8‹0.1 Ma, and the trace elements, not including Rare Earth Ele- construction of the lower portion of the Cordillera ments (REE), were performed at the Earth Sci- del Tatio stratovolcanoes (Quebrada Chucula ences Department laboratories of Bologna Uni- formation). These volcanoes are mostly aligned versity by X-ray fluorescence, using a full matrix along NNE-SSW trends. The 2nd eruptive epoch correction method and traditional wet chemical (Portezuelo del Cordon de Panizo Subsynthem)is techniques (LOI, MgO, Na2O). Major and trace characterized by the growth of Vn. Tocorpuri elements determination in samples including stratocone (Vn. Tocorpuri 1 formation), which is REE, Th, U, Cs, Ta, Hf and Sc were obtained by the inner portion of Cerros de Tocorpuri com- ICP-MS at the CRPG-SARM laboratories posite volcano and marks an important shifting of (CNRS, Nancy, France). Selected data are re- eruptive vents toward SE. These volcanic products ported in Table 2. In order to avoid inter-labora- are unconformably sealed by widespread ignim- tories discrepancies, some samples were analysed brite sheets corresponding to the Tatio formation by both laboratories, showing that analytical dif- (Quebrada Agua Brava Subsynthem), which is ferences are within precision limits better than 5% dated as younger than 1 Ma and originated from for major elements, and better than 10% for all the Laguna Colorado caldera complex in the other trace elements. Mineral chemistry analyses APVC. During the 4th eruptive epoch (Quebrada both on glass and/or mineral phases have been la Torta Subsynthem)the effusion of several performed on 10 thin sections from selected massive and blocky lava flows and minor lava samples by electron microprobe JEOL JXA-8200 domes determines the construction of the upper at INGV - Roma 1 and by electron microprobe at portion of Cordillera del Tatio stratovolcanoes Actlabs, Ontario (Canada). In particular, these (Vn. Tatio and Cerros del Tatio formations)and analyses were obtained for samples relevant to the Cerros de Tocorpuri composite volcano (Paso de Tukle, Co. Negro, Tatio, Vn. Tatio, Cerros del Tocorpuri formation). These volcanic edifices Tatio and Co. La Torta formations, both on mi- mostly develop along NE-SW to N-S alignments neral phases and glass (just for Co. La Torta and represent a branch of the modern volcanic arc rhyolite - see Tabs. 3 and Figs. 4-5). of central Andes. The following period of volcanic Petrographic and geochemical features of col- dormancy is marked by an intense contractional lected rocks are hereafter described according to tectonic phase outlined by the development of the stratigraphic subdivision above summarized in four major NNE-SSW-oriented, east-dipping and terms of synthems and formations. Many of these west-verging thrust faults. The 5th eruptive epoch petrographic and geochemical features are based (Rio Blanco Subsynthem)is the youngest phase of on previous studies reported in D'Ugo (2003). volcanic activity in El Tatio region and is char- Main petrographic features are schematically re- acterized by the effusion of the 34-ka-old Co. La ported in Table 4. As a whole, the analysed sam- Torta low-profile lava dome. Most of volcanic ples consist of lavic bodies and subordinately products in the study area are affected by traces of pyroclastic products (obsidian fragments, pumices major glaciation periods from Middle-Late Pleis- and scoriae)ranging in composition from cal- tocene to the Last Glacial Maximum. calkaline (CA)to high-K CA (HKCA)andesite to rhyolite (Table 2; Fig. 3). Two microvesicular dark enclaves (magmatic inclusions), sampled in the Petrography, mineral chemistry and classifica- Co. Copacoya formation (Rio Salado Synthem), tion of El Tatio volcanics plot in the basaltic trachy-andesite and trachyan- desite field, respectively (Fig. 3).

Sampling and analytical techniques Rio Salado Synthem During some different geological surveys car- Volcanics of the Rio Salado Synthem corre- ried out from 2003 to 2007 in the El Tatio volcanic spond to the Tukle, Rio Salado and Co. Copacoya area, we collected more than 100 rock-samples for formations, whereas we report data from litera- petrographic and compositional study. Table 1 is ture for the Sifon formation (Guest, 1969). Our

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Gianfilippo De Astis, Federico Lucchi, Claudio Antonio Tranne and Piermaria Luigi Rossi Sr isotopic data for Tocorpuri lava and La Torta dome are from Harmon et 86 Sr/ 87 Table 2 - Representative chemical analyses of the El Tatio volcanic rocks: major (wt%)and trace (ppm)elements. al. (1984).

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Evidence for different processes of magma evolution in El Tatio volcanic region (22ë16' to 22ë30' S, Central Volcanic Zone, Andes)

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Table 3 - Representative microprobe chemical analyses of amphibole and biotite from selected samples of El Tatio volcanic rocks.

Table 4 - Main minero-petrographic features of selected samples from El Tatio rocks.

data range in composition from basaltic trachy- some of them are largely replaced by dusty opa- andesite to dacite (Fig. 3). que grains; groundmass is microcrystalline and The CA andesite lavas of Tukle formation show shows scattered opaque minerals. Fresh portions porphyritic seriate texture (P.I.= 10-20%), with of amphibole phenocrysts exhibit a compositional phenocrysts of plagioclase (~An63-79 ± Fig.4), variability ranging from Fe-pargasitic hornblende clino- and ortho-pyroxene (En62-77 ± Fig.5), and to Mg-hornblende (Table 3e). several glomeroporphyritic aggregates formed by The pyroclastic rocks of Rio Salado formation the same minerals; subordinate amphibole phe- are basically represented by highly welded and nocrysts are characterized by opacitic rims and crystal-rich ignimbrite-type deposits, made up of

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Evidence for different processes of magma evolution in El Tatio volcanic region (22ë16' to 22ë30' S, Central Volcanic Zone, Andes)

Fig. 3 - Classification of El Tatio volcanic rocks according to the TAS classification diagram (Le Bas et al., 1986)(a) and the K2O vs. SiO2 diagram of Peccerillo and Taylor (1976). Volcanic rocks are classified in terms of main UBUs (synthems and subsynthems) from Fig. 2B. Full triangles represent ignimbrite rock-samples of the Rio Salado and Sifon formations (from Guest, 1969; De Silva and Francis, 1989). Full circles represent ignimbrite rock-samples of the Tatio formation (present work).

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HKCA dacitic pumices, phenocrysts or frag- These magmatic inclusions show intersertal tex- mented crystals in a glassy groundmass with eu- ture formed by a network of mostly prismatic/ taxitic texture. Crystals are generally enclosed in a acicular plagioclase, strongly opacitized biotites vitrophyric matrix with fluidal texture and, in de- and subordinate amphiboles. creasing order of abundance, are twinned and zoned plagioclase, euhedral biotite, quartz, pleo- Aguada Chica Synthem chroic amphibole, pyroxene and Fe-Ti oxides. We note that: a)plagioclase crystals are mainly an- This unit only includes the pyroclastic products desinic in composition (Marinovic and Lahsen, of Puripicar formation, which corresponds to the 1984)and frequently broken and/or irregularly widespread ``Puripicar Ignimbrite member'' of shaped due to the transportation, with common Guest (1969)and De Silva (1989).They consist of concentric zonatures and sieved textures; b) very thick and massive beds of pumiceous lapilli quartz phenocrysts are anhedral with rounded tuffs, generally welded and crystal-rich. The ju- shape and/or embayed through resorption; c)both venile fragments are mostly represented by dense ortho- or clino-pyroxene phenocrysts are present, and whitish pumiceous clasts with ~50-55% with the former showing a light pleochroism that crystal content. Pumice composition is HKCA is a common feature in volcanic products of the dacitic (Fig. 3). In most distal outcrops (Northern continental margins. Pumice analyses from these Chile), ``glassy'' pumices (P.I.=25-30%) are re- rocks plot in the field of dacites (Fig. 3). ported by De Silva (1991), locally reaching ~20% The Sifon formation (corresponding to the of the juvenile material. De Silva and Francis ``Sifon Ignimbrite member'' of Guest, 1969)in- (1989)reported the occurrence of crystal-poor, cludes variably-welded ignimbrite-type deposits dark pumices nearby the top of the unit, in dis- made up of HKCA dacitic pumices (Guest, 1969; tinctive coarse concentration zones: they have De Silva and Francis 1989)and a very high major-element dacite composition similar to that amount of crystals (up to 70% vol.). Crystals are of whitish pumices but yield different trace-ele- mostly represented by andesinic plagioclase, an- ment whole-rock and glass compositions and mi- hedral quartz, and frequently resorbed horn- neral chemistry. Rock texture is mostly eutaxitic, blende; biotite, altered along rims, hyperstene and with plagioclase and biotite micro-phenocrysts oxides are also present in limited amount. Rock and diffused ``fiamme''-type structures often af- texture is mainly fluidal and subordinate lithics fected by re-crystallization processes. As a whole, are present. the mineral assemblage consists of plagioclase, The subvolcanic laccolith-type lava body of Co. quartz and minor biotite, hornblende and Fe-Ti Copacoya formation displays compositions ran- Oxides; allanite, pyroxenes and opaques are rare. ging from dacites to trachytes (Fig. 3)and por- Crystals are often fragmented and dispersed in a phyritic seriate texture, with very high crystal glassy groundmass that locally shows flow texture content (P.I.60%). Mm- to cm-sized phenocrysts marked by microlites orientation. Plagioclase of plagioclase, rounded quartz, amphibole and phenocrysts are both euhedral and subhedral, and biotite are set in a microcrystalline to phaneritic to lesser extent rounded, and often show poly- groundmass composed of plagioclase, quartz and synthetic twinning or opaque inclusions; con- minor opaques microlites. It should be noted that: centric zonations are less common. Quartz is a)plagioclase phenocrysts have variable shapes present as subhedral crystals, with rounded rims from euhedral to subhedral and even resorbed; and resorptions. Biotite phenocrysts frequently they show polysynthetic twinning or concentric show break-down and frayed rims or are highly zonation, with common enclosure of melt inclu- resorbed. Amphibole crystals show green to sions as well as biotite, opaques and apatite mi- brown pleochroism and are mainly anhedral, lo- crolites; b)mm-sized biotites are both euhedral cally subhedral. and subhedral, locally with opacitic rims, or ske- letal habit or completely altered crystals; c)most Tatio Sur Synthem of the amphibole phenocrysts are partially or to- tally replaced by opaques. Dark, spherical to The Tatio Sur Synthem is subdivided into the elongated, holocrystalline and micro-vesicular Paso Las Vizcachas, Portezuelo del Cordon de enclaves (with variable shape and size, up to 50 cm Panizo, Quebrada Agua Brava, Quebrada La in diameter), are embedded in the lava body. Torta and Rio Blanco Subsynthems.

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Evidence for different processes of magma evolution in El Tatio volcanic region (22ë16' to 22ë30' S, Central Volcanic Zone, Andes)

Fig. 4 - Compositional diagrams for feldspar crystals analysed in volcanic rocks from the reported formations (from Fig. 2B). a) Volcanic rocks belonging to the Rio Salado and Tatio Sur synthems. b)Rocks of the Rio Blanco subsynthem (Tatio Sur synthem),i.e. Co. La Torta formation. Shaded circles are sanidine crystals.

Fig. 5 - Classification diagram fro pyroxene crystals: En=enstatite; Wo=wollastonite; Fs=ferrosilite ternary plot (Rock, 1990). a) Crystals in- cluded within volcanic rocks of the Tukle and Co. Negro formations. b)crystals from other formations (belonging to the Tatio Sur synthem).

The Paso Las Vizcachas Subsynthem includes polysynthetic twinning. Pyroxene phenocrysts and volcanic products of the El Negral de Jauna, Co. micro-phenocrysts are both ortho- (usually pleo- Negro and Quebrada Chucula formations. chroic)and clino-pyroxenes and mostly have eu- The andesitic lava dome of El Negral de Jauna hedral habit. Glomeroporphyritic aggregates formation displays a porphyritic to glomero-por- consist of plagioclase and pyroxene crystals. phyritic texture (P.I.~30%)consisting of plagio- The Co. Negro formation is formed by lava do- clase and pyroxene crystals, set in cryptocrystal- mes and lava flows yielding HKCA andesitic to line to vitrophyric groundmass consisting of dacitic compositions (Table 2; Fig. 3). These rocks abundant plagioclase microlites. Plagioclase phe- show porphyritic to glomero-porphyritic texture nocrysts are both euhedral and subhedral, with (P.I.~20-40%)and a mineral assemblage of pla- frequent sieve textures, concentric zonation and gioclase, pyroxene, amphibole, biotite and quartz

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phenocrysts or micro-phenocrysts, in decreasing texture varies from microcrystalline to vitrophyric order of abundance. The groundmass is crypto- and shows microlites of the same mineral phases crystalline, contains the same mineral phases listed listed among the phenocrysts. At places, iso-or- above and shows a fluidal texture due to the large ientation of feldspar microlites provides fluidal presence of isoriented plagioclase microlites. Fresh texture and groundmass shows both re-crystal- and euhedral plagioclase crystals (~An48-78 ± Fig. lization and spherulitic micro-structures. 4)can frequently be close to 1 cm in size, but sub- The Portezuelo del Cordon de Panizo Sub- hedral crystals with rounded rims are also present. synthem only includes the volcanic rocks (lava Several crystals display strong concentric zonation flows and minor pyroclastic products)of Vn. To- and/or zoning marked by pyroxenes microlites and corpuri 1 formation, representing the older por- melt inclusions (coarse to fine sieved micro-struc- tion of the Cerros de Tocorpuri composite volcano. ture); analyses reveal oscillatory zoning (e.g. An74- These products crop out on the summit portion of 51-62). Pyroxene crystals (Fig. 5) are present both as the 1000-m-high Vn. Tocorpuri stratocone and clino- (Wo40-47)and ortho-pyroxene (light pleo- have not been sampled due to logistic problems. chroism, higher resorption; En58-70), often forming We infer their main petrographic and chemical glomero-porphyritic clusters; at places, thin clin- features by assuming that they correspond to the opyroxene overgrowth along ortho-pyroxenes so-called ``Tocorpuri E'' morphostratigraphic unit crystal rims can be present. Amphiboles included of Marinovic and Lahsen (1984). This unit is re- within lavas of the Co. Negro formation are in part ported to be andesitic in composition (57-59 wt.% typical brown basaltic hornblende (oxy-horn- SiO2)and characterized by a mineralogical as- blende), in part common green hornblende. In semblage similar to that of Paso de Tocorpuri 1 terms of compositions, both edenitic hornblende formation (see later in the text), except for a and Mg-hornblende have been recorded (Table slightly lower olivine content (Deruelle, 1977). 3e). Crystals mostly show opacitic edges and break- The Quebrada Agua Brava Subsynthem in- downreaction rims; some ofthem arerecrystallized cludes the ignimbrite-type deposits of Tatio for- at the nucleous or show plagioclase and pyroxene mation, corresponding to the ``Ignimbrita Tatio'' inclusions. Reddish-brown, pleochroic biotites unit of Lahsen (1982). These deposits consist of show a prismatic habit and frequent evidence of thickly to very thickly bedded, massive, crystal- reaction to opaques and glass. Quartz crystals are rich, generally indurated and welded, pumiceous anhedral, usually fresh and often fractured, deeply lapilli tuffs and tuffs. The juvenile component is resorbed at places. Glomeroporphyritic aggregates that typical of ignimbrite deposits, represented by consist of ortho-pyroxene and minor clino-pyr- pumices and highly fragmented crystals. These onene, plagioclase and opaque crystals. fragments are set in a fluidal, glassy, eutaxitic The Quebrada Chucula formation is made of groundmass (Table 4), together with lava xeno- several massive and blocky lava flows with mostly liths of dacitic composition. In decreasing order of HKCA dacite bulk composition (a couple of abundance, the observed phenocrysts are plagio- samples are close to the andesite field; Fig. 3). clase, euhedral biotite, clino- and orthopyroxene, Lava generally shows porphyritic texture (P.I. and anhedral to resorbed quartz. Remarkably, ~30-40%)with plagioclase, amphibole and biotite plagioclase crystals exhibit a rather homogeneous phenocrysts; frequent glomeroporphyritic ag- An content (An50-42 - Table 3b; Fig. 4), and some gregates and minor amounts of ortho- and clin- of them have normal zonation (An48-42 - Table opyroxenes are present, together with rounded 3b), whereas a single analysis indicates a higher and/or resorbed quartz and opaques microlites. An content (An62). Clinopyroxene phenocrysts Plagioclase is the most abundant phase, displays (Wo38-44 ± Fig. 5)indicate a certain compositional andesinic composition (Marinovic and Lahsen, variability ranging from Mg#0.79 to Mg#0.65 1984), variable size (up to 7-8 mm), sieve textures, (Table 3d), whereas orthopyroxene crystals show reaction rims, concentric zonation and poly- the presence of two populations (Table 3d): one synthetic twinning. Amphibole phenocrysts are group formed by large crystals with variable and euhedral to subhedral with a rhombic habit, lo- decreasing Mg/Fe ratios from core to rim cally twinned and showing pleochroism. Euhedral (Mg#0.80-0.60)and another group formed by to subhedral biotite phenocrysts display variable pheno-, microphenocryst and groundmass micro- sizes and some break-down rims, with plagioclase lites compositionally more homogeneous, with and clino-pyroxene formation. The groundmass lower MgO content (Mg#0.69-0.63). According

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Evidence for different processes of magma evolution in El Tatio volcanic region (22ë16' to 22ë30' S, Central Volcanic Zone, Andes)

to De Silva and Francis (1989), pumices from the Lahsen (1984), displays andesite composition and widespread Tatio formation range in composition mineralogical assemblage similar to that of the from HKCA dacite to rhyolite, although samples older Vn. Tocorpuri 1 formation, with slight pet- from the study area reveal an homogeneous dacite ro-chemical differences (see above in the text). bulk composition (Fig. 3). The Vn.Tatio formation includes massive and The Quebrada la Torta Subsynthem includes blocky lava flows ranging in composition from volcanic products forming the Paso de Tocorpuri HKCA andesites to dacites (Table 2; Fig. 3). They 1, Vn. Tatio and Cerros del Tatio formations. are characterized by a porphyritic texture (P.I.= The Paso de Tocorpuri 1 formation corre- 10-40%)where the clinopyroxene is almost com- sponds to the upper portion of the Cerros de pletely absent and phenocrysts of plagioclase, Tocorpuri composite volcano and includes mas- amphibole, orthopyroxene (both as euhedral and sive and blocky lava flows with homogeneous skeletal crystals)and biotite are dispersed in a HKCA andesite composition, except for a lava groundmass formed by microlites of plagioclase, xenolith (sample TA08/07; Tabs. 1-2)with dacite orthopyroxene, opaque minerals and interstitial composition. Lavas show porphyritic seriate tex- glass. Plagioclase phenocrysts of different com- ture with phenocrysts of largely dominant plagi- position (Table 3a; Fig.4)are present: there are oclase with minor amounts of biotite, pyroxene, euhedral crystals with rather homogeneous com- amphibole and quartz set in a microcrystalline to position (An52-48), whereas other crystals exhibit cryptocrystalline groundmass; glomeroporphyritic both fresh and sieve-textured sectors with differ- aggregates of various crystals (dominated by pla- ent An contents (about An50 and An70 respec- gioclase)are present. Euhedral to rounded pla- tively). Plagioclase microlites (both in groundmass gioclase phenocrysts, up to 6 mm in size, show or enclosed in amphibole phenocrysts)have variable features: concentric zonation and poly- homogeneous compositions (An ). Phenocrysts 52-45 synthetic twinning, sieved-textures, resorbed and of orthopyroxene (Table 3c; Fig.5), both euhedral reaction rims. Microphenocrysts of biotite are (Mg#0.70)and strongly embayed (Mg# 0.80- both euhedral and subhedral, often with opacitic 0.70), from core to rim respectively), are asso- rims and/or skeletal habit; in several crystals, ciated to very fresh microlites (Mg#0.60)in the opaques replace biotite almost completely. A few groundmass. Fresh phenocrysts of highly pleo- pyroxene phenocrysts and micro-phenocrysts chroic (from yellow to red-brown)amphibole mostly show euhedral habit and are both ortho- (Table 3e), frequently with opacitic rims, exhibit and clino-pyroxenes. The few recognizable am- homogeneous compositions corresponding to phibole phenocrysts are almost completely opa- edenitic terms, but largely transformed pheno- citized. Darker lava xenoliths are occasionally crysts with plagioclase‹oxide‹pyroxene inter- present in the Paso de Tocorpuri 1 lavas, and growths are also present. display an intersertal texture made up of domi- The coulee-type lava flows of Cerros del Tatio nant plagioclase microphenocrysts and few phe- formation range in composition from HKCA an- nocrysts, opaque minerals, anhedral or skeletal desite to dacite (Fig. 3). They are characterized by biotite, almost completely replaced by opaque a porphyritic texture (P.I.= 10-30%)with plagio- minerals; opacitic edges and patches also affect clase and pyroxene phenocrysts, associated to the few amphibole crystals. The Paso de To- lower amount of biotite and amphibole, set in a corpuri 1 formation also contains a few pyroclastic cryptocrystalline groundmass with fluidal texture; rocks composed of dark or banded scoriae and glomeroporphyritic clots are present, occasionally bread-crust bombs. Although characterized by made of a single phase (plagioclase or pyroxene). vesiculation and more glassy texture, these pyr- Plagioclase crystals vary from euhedral to roun- oclastic rocks show a mineralogical assemblage ded and display variable features: strong con- similar to that observed in lavas. In these pyr- centric zonation and polysynthetic twinning, fine oclastic products, we remark the presence of mm- and coarse sieved-textures. Both ortho- and clino- sized and euhedral amphibole crystals that imply pyroxene phenocrysts are present with euhedral high water partial pressures able to provide the to subhedral habit. Biotite and amphibole are stable crystallization conditions. Overall, the Paso found as prismatic microphenocrysts, often with de Tocorpuri 1 formation, which is assumed to altered or opacitic rims. Amphiboles frequently basically correspond to the so-called ``Tocorpuri show skeletal habit and/or coarse pyrox- W'' morphostratigraphic unit of Marinovic and ene‹plagioclase‹oxide intergrowths.

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The Rio Blanco Subsynthem includes the existent phases can suggest low-pressure condi- youngest volcanic rocks in the study area re- tions and/or the polybaric nature of fractionation presented by the rhyolitic low lava dome of Co. La processes. In some cases (i.e., Paso de Tocorpuri 1 Torta formation (known as ``torta'' due to its ty- formation), the presence of euhedral amphibole pical shape). These rocks have generally a por- suggests volatiles-rich magma and/or a closed phyritic texture (P.I. = 30%)with phenocrysts of magma system, being this phase stable only at high plagioclase, quartz, sanidine, amphibole and bio- water partial pressures. The abundance of deeply tite in a glassy, fluidal groundmass (Table 4); ac- resorbed and rounded mineral phases is indicative cessory phases such as titanite and apatite are of strong disequilibrium between magma and the present. On the field, variable lithologies (facies) crystallizing system (i.e., minerals are not in were recognized on the basis of different pro- equilibrium with the surrounding rock matrix). portions between glass and crystals: i)a mainly Similar indications come from the occurrence of glassy, light facies; ii)an intermediate facies other crystals features such as reaction rims, ske- showing an higher P.I. and darker colour if com- letal habits, wide ranges of mineral compositions, pared with the previous one; iii)a dark-grey facies oxidations, and occurence of hydration processes with holocrystalline texture and the highest P.I. acting on mafic phases as pyroxenes. Among these Very fresh plagioclase represents the main mi- attributes, we recall: a)plagioclases having com- neral phase and exhibits both euhedral and an- plex and variable zoning patterns, which is typical hedral crystals; phenocrysts are twinned and of calc-alkaline andesites (e.g., Kawamoto, 1992, zoned with a wide compositional range (An64-37), and ref. therein), and widely resorbed sieved-tex- whereas subhedral sanidine (Or79)is much less tured zones; b)plagioclases with fine sieve-tex- abundant than plagioclase (Table 3b; Fig. 4). tures, which should testify for dissolution episodes Rounded, and locally strongly embayed, crystals due to interference between melts of different of quartz are also abundant. Subhedral to skeletal composition (Kawamoto, 1992; Nelson and phenocrysts of yellow-red pleochroic amphibole Montana, 1992; Tepley et al., 1999; Castro, 2001); (edenite)and euhedral but partially oxidised c)hornblendes ranging from euhedral, to slightly phenocrysts of biotite (Table 3e)represent the resorbed, to those completely replaced by coarse mafic mineral components. Our whole-rock and pyroxene‹plagioclase‹oxide intergrowths; d) microprobe analyses confirm the bulk rhyolitic some orthopyroxenes showing thin overgrowth composition already reported in literature by De rims with high-Ca content, a feature consistent Silva et al. (1994), and reveal that all the reported with crystallization from higher-temperature lithofacies have a similar glass composition ran- magma than the core; e)frequent rounded and/or ging between SiO2 = 74.9778.4 % (Fig. 3b). resorbed quartz crystals within andesitic-dacitic rocks, which suggests the early interaction of this phase with an higher-T, less evolved magma. In Petrographic features and their significance general, mineral zonings and other disequilibrium features may reflect large scale disturbances of the Hereafter we summarize some recurrent pet- crystallizing system such as: i)mixing and/or rographic features of analysed products with the mingling between magmas of contrasting compo- aim of better understanding the conditions of sition and temperature (e.g. Heiken and Eichel- corresponding magmatic systems acting on mag- berger, 1980; Clynne, 1999), ii) gas-driven magma ma evolution. overturn (e.g. Stevenson and Blake, 1998; Ru- Most of studied rocks range in composition precht et al., 2008), or iii) significant changes in from andesites to dacites. Rhyolitic composition pressure and volatile content accompanying characterizes only the Co. La Torta lava dome, magma ascent and eruption. Other Authors (e.g., whereas mafic samples indicative of primitive Couch et al., 2001)explain the features of many magma involved in the eruptions are absent. This porphyritic andesite and dacite lavas having high may suggest that advanced evolutionary processes P.I. (35-55%)and the wide spectrum of dis- have largely modified the original magma com- equilibrium textures listed above as the result of positions leading to generation of intermediate- convection within a magma body with a single acid melts. For several rocks, especially those composition, that is heated from below and cooled forming the Tatio Sur Synthem, the mineralogical from above (self-mixing or convective mixing). assemblage and textural relationships between co- Actually, in absence of further studies and mod-

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elling on glass and mineral compositions, the en- cases, the eruption from upper crustal magma countered features can be ascribed either to in- chambers implicates a mingling/mixing process teraction of magmas with different composition or between the differentiated residing magma and a to variation of physical-chemical conditions hotter mafic melt coming from depth (Takeuchi (depth/pressure, volatile contents, late action of and Nakamura 2001). hydrothermal fluids)in the reservoir(s).At the current state of knowledge, both these processes could have been active during the ponding and Chemical composition of erupted magmas ascent of magma upraising from crustal reservoirs. Literature data for the volcanic products of El Tatio area and adjacent stratovolcanoes provide Major elements clear evidence of mingling/mixing just for the close Lascar volcano (Soncor Ignimbrite - Mat- In the classification diagrams reported in Fig. thews et al., 1999). In our studies, field evidence of 3b, the HKCA character of most samples is clearly mingling (and possibly mixing)processes emerges highlighted, the only exceptions being represented for the 7.35-ka-old Co. Copacoya subvolcanic by few samples of Tukle and Co. Negro forma- laccolith-type body. In this case, the presence of tions that fall near the boundary with the CA variously-shaped and -sized holocrystalline en- series and two mafic microgranular enclaves em- claves in variable state of quenching could docu- bedded within the Co. Copacoya formation that ment a disaggregation process acting on a mafic plot along the limit with the shoshonitic (SHO) magma, which entrains a rhyodacitic magma series. Few other samples belonging to the Co. chamber, so that it gives rise to (hybrid)basaltic Copacoya formation (both lava samples and em- andesite enclaves. A comparison of the petro- bedded enclaves), which is one of the older mag- graphic features observed in our enclaves and matic rocks in the study area, are dacitic in com- those reported for the magmatic inclusions found position with higher potassium contents and plot in rhyodacitic lavas of the Lassen volcanic center out the main trend. This could be related to the (Cascade Range, California - Tepley et al., 1999) presence of higher amounts of biotite crystals due reveals many similarities both in the mineralogical to its early crystallization in high H2O pressure assemblage of the host lava (pl, bt, anf, qz, Fe-Ti condition, but also to a specific line of evolution oxides)and the textures or micro-structures re- related to older phases of activity (~9-7 Ma), corded by the rocks: partially resorbed plagio- which we can only partially appreciate. clase, biotite and amphiboles with opacitized rims, The variations of rock-samples chemical com- enclaves characterised by intersertal texture made position have been classically studied by plotting of acicular plagioclase and biotite network (in the geochemical data on Harker's diagrams according Lassen lava the horneblende dominates the tex- to main UBUs (Fig. 6). These diagrams also re- ture of enclaves). As a conclusive remark, the port literature data for i)the least evolved rocks laccolith presence testifies for magma ponding, erupted in the closer volcanic systems around the crystallization and differentiation in crustal re- El Tatio area and ii)the two volcanic centres servoirs beneath El Tatio area, whereas the en- forming lava domes (Chao and Chillauita domes/ claves testify for different phases of closed and tortas ± de Silva et al., 1994)lithologically and open systems during time. physiographically similar to the Co. La Torta lava Another possible inference from the petro- dome. Variation diagrams of most major element graphy concerns the usually very high crystal oxides with increasing SiO2 show rather typical content observed in lavas and even in pyroclastic positive and negative correlations (Fig. 6), which rocks of dacitic composition (although the latter are in agreement with fractional crystallization are originated from caldera systems several-tens- (FC)processes (see Discussion).Al 2O3,P2O5 and of-kms far from the El Tatio area). This indicates Na2O (the latter not shown)are the diagrams with that the erupted magmas were already largely the highest data scattering. Together with rock- crystallized before the eruption. The ascent and samples from literature, we may recognize two outpuring of such crystal-rich and highly viscous roughly distinct trends for Al2O3, TiO2, MgO, residing magma is easily explained by assuming K2O oxides. Femic microgranular enclaves play a the occurrence of volcano-tectonic collapses or role in this hypothesis, forming a linear array to- magma injections as triggering events. In such gether with their host-lava in some diagrams

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Fig. 6 - Selected major elements vs. SiO2 (wt.%)for volcanic rocks of the El Tatio volcanic area. Symbols as in Fig. 3, full and open St.Andrea's crosses are samples from literature (Volcanoes: Ollague, Pircus Negras, La Porunita, Chela, Carcote, SanPedro-SanPablo, Nevados de Payachata, Lascar - Data from: Deruelle, 1982; Harmon et al., 1984; Davidson et al., 1990; Kay et al., 1991; de Silva, 1991 for Puripicar Ignimbrite; Feeley and Davidson, 1994; de Silva et al., 1994; Worner et al. 1994; Matthews et al., 1999; Mattioli et al., 2006).

(MgO, CaO, K2O)and curvilinear in others ( as differentiation index being the most in- Al2O3, TiO2,P2O5). compatible element in Andean rock (Deruelle, 1982), show both rough correlation trends and scattering. For elements as Rb, Ba, Sr, Pb, Zr, Cr Trace elements and radiogenic isotope and - not shown - K, Y and Ni the data distribu- Selected trace elements vs. Th (Fig. 7), chosen tion is rather scattered; notably, both Ba and Nb

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Evidence for different processes of magma evolution in El Tatio volcanic region (22ë16' to 22ë30' S, Central Volcanic Zone, Andes)

Fig. 7 - Selected trace elements vs. Th (ppm)for volcanic rocks of the El Tatio volcanic area. Symbols as in Fig. 3. Literature data like those reported in Fig. 6.

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(the latter not shown)show flattened trends going considerably thick (= 70 km)continental crust towards higher enrichment in Th. Nevertheless, beneath this Andean sector (e.g. Thorpe et al., considering the plots which include rock-samples 1984). Otherwise, it could be considered expres- from literature, both some LIL Elements (Rb, K, sion of different petrogenetic processes (Hildreth Sr, Pb)and HFS Elements (Zr, Ta, Hf)show fairly and Moorbath, 1988). good positive correlations with increasing differ- entiation degree as well as the compatible ele- ments as Ni, Cr, V, Sc, and Co show negative Discussion correlations with increasing Th contents. Slightly scattered but basically flat-linear trends as those Geochemical data indicate that the composi- shown by Ba, Nb, Y could suggest mixing pro- tional range of El Tatio volcanics is within the cesses between (basaltic-)andesite and dacite HKCA series, ranging from andesite and dacite magmas in crustal magma chambers. fields, with only a few rhyolitic terms (Fig. 3). The Incompatible trace element signatures of some evolutionary degree and K-enrichment closely studied andesitic rocks (SiO2= 59-62.3 wt.%)are match the character of typical Andean margin summarized in the spiderdiagrams normalised to magmatism (Thorpe et al., 1984)and in particular primordial mantle and chondrites reported in Fig. that related to stratovolcanoes of CVZ (de Silva et 8, where samples with different ages (eruptive al., 1994; Feeley et al., 1993; Matthews et al., 1994; periods)have been reported. Overall, the in- Davidson and de Silva, 1995; Ort et al., 1996). compatibile trace elements patterns of these an- Basically, major and trace element trends are desites show a typical subduction-related sig- consistent with differentiation of andesitic mag- nature, with Ta-Nb trough and P, Ti negative mas by FC of main mineral phases (plag‹pyr- spikes and are similar (sub-parallel)to those of ox‹hornblende‹bt), dominantly represented by the andesitic rocks from surrounding areas (Ol- plagioclase phenocrysts in most of studied pro- laguÈe and/or Tocorpuri lavas), with high LILE/ ducts. Going more in detail, we have seen that HFSE ratios (e.g. Ba/Nb ratio ranging between 35 major and trace elements variations in El Tatio and 97)or Ba/La > 15 (typical of island arc or volcanics - as observed in other stratovolcanoes of continental margin volcanic suites). HFSE ratios the same segment of CVZ - are affected by: a) are less scattered (Zr/Nb ratio is comprised be- scattered distribution (Fig. 7), especially for vari- tween 10-18)as expected for immobile in- able LIL Elements (Rb, Th, U, Pb)enrichment; b) compatible elements. some correlated trends with positive or negative REE patterns (Fig. 8b)are usually fractionated slopes, if we consider a magma series related to a (with a LREE/HREE, e.g., La/Yb, comprised single area (volcano); c) weak indications of mix- between 10-30)and their slightly lower or higher ing (e.g., Co. Copacoya products). The enrichment fractionation is related to their SiO2 content (in in LILE may reflect either a larger contribution andesites La/Yb< 20; in dacites is comprised be- from fluids released by the subducted slab to tween 18-30; in rhyolites is about 30). Overall, magmas generated in the mantle wedge or an andesitic samples display a small Eu anomaly. additional contribution from the crust (intra- The 87Sr/86Sr isotope data reported in Figure 9 crustal contribution). Since we basically have in- are from literature and available just for a limited termediate-evolved rocks, it's difficult to dis- number of the studied rocks: Tocorpuri lavas have criminate mantle source contribution inherited by 87Sr/86Sr ratios ranging from 0.70812 to 0.70894 the parental mafic magmas. Previous studies on (Harmon et al., 1984), whereas the 87Sr/86Sr ratios CVZ (e.g., Worner et al., 1988: Davidson et al., measured on the Co. La Torta rhyolites range 1991)have pointed out that contamination during from 0.70801 to 0.70857 (Harmon et al., 1984; de differentiation from basalt to rhyolite may have Silva et al., 1994). Overall, 87Sr/86Sr data from occurred, but most of the CA-HKCA mafic rocks neighbouring volcanoes and ignimbrites (e.g., Ol- erupted in the region are already enriched in in- lague, San Pedro±San Pablo, Lascarand other compatible trace elements and reveal a ``baseline'' volcanoes located between 21ë16 S and 23ë22 S) isotopic composition around 87Sr/86Sr=0.706- vary from 0.70560 to 0.70990 (Deruelle, 1982; 0.707. Moreover, we deal with rocks having largely Harmon et al., 1984; Feeley and Davidson 1994; variable 87Sr/86Sr ratios, but showing a rather re- Mattioli et al., 2006). Such a feature could be in- stricted range if we consider a single stratovolcano herited by magmas through the passage along the (e.g., Tocorpuri - 87Sr/86Sr=0.70812-0.70894).

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Evidence for different processes of magma evolution in El Tatio volcanic region (22ë16' to 22ë30' S, Central Volcanic Zone, Andes)

Fig. 8 - Spiderdiagrams for incompatible trace elements (Wood et al., 1979)and REE (according to Nakamura, 1974)measured in andesitic rocks from El Tatio. Andesite from Ollague volcano (Mattioli et al., 2006)is reported for comparison.

Therefore, on the basis of our data, differentia- magma differentiation. Various volcanoes within tion trends observed in studied rocks can be the CVZ provide the first kind of trend (e.g., San broadly subdivided into two types: a)closed-sys- Pedro-San Pablo volcanoes; Hawkesworth et al., tem trends in which radiogenic isotopic trends do 1982), which can be broadly attributed to closed- not change or slightly change with differentiation system FC magma recharge/mixing processes and b)open-system trends in which isotopic com- within magma reservoirs located in the upper crust. positions vary together with geochemical indices of The second type of differentiation trend has been

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recorded for other volcanoes of the CVZ (e.g., plumbing system of the El Tatio area. Our data Sierra de Lipez; Harmon et al., 1984). We sum- from Puripicar rocks just provide indication for marize these trends in Fig. 9, where incompatible magma differentiation in a large and quite shallow elements and 87Sr/86Sr ratios are reported to depict magma chamber (caldera collapse)characterised magma differentiation processes. Miocene to by slight zonation (SiO2=65.72-67.14 wt.% - Ta- Quaternary volcanic rocks of the southern CVZ ble 2; cfr. de Silva, 1991). (~21-28ëS)from our data set and literature are The 2nd eruptive period, dated at Middle-Late plotted in these diagrams and illustrate the co- Pleistocene, is articulated into four successive variation of the chosen parameters, as a con- eruptive epochs and is the more complex in terms sequence of combined FC, AFC and bulk mixing of of volcanology and magma composition. It shows magmas with crustal rocks (i.e., xenoliths found in the most large magma range, spanning from CA the sampled lavas or felsic rocks constituting the N andesites to HKCA rhyolites, which form lava Chile and NW Argentina basement). These lines of flows and domes associated to minor pyroclastic evidence concerning the occurrence of crustal episodes. On average, the K2O content is lower contamination processes at large scale are basically than that recorded by the 1st eruptive period. Our qualitative. In particular, the limited amount of data well display that magma evolution during this available isotope analyses poses severe limitation period was basically controlled by FC and AFC on the modelling. However, our petro-chemical processes (Fig. 9), playing a variable role through data compared with the huge geochemical and time. Most of the volcanic products exhibit pl, cpx isotopic data set available from literature support and opx crystals with a wide compositional varia- the general interpretation we provide. bility (Table 3 and Figs. 4-5), suggesting the re- In order to shed light on the magmatic plumb- current co-existence of chemically different gen- ing systems over time, we have grouped samples erations of crystals in the magmas. Mixing pro- according to their chemical features and volca- cesses (trend 4 in Fig.9)are also geochemically nological history. In particular, the subdivision compatible for part of the considered rocks, al- into two major eruptive periods is taken as basis though more samples and more data should be for magmatological considerations. As such, we considered to go deeper in this topic. Interest- can draw the following interesting inferences. ingly, the mixing processes of andesitic magmas During the 1st eruptive period (~9-7 Ma; Upper with felsic rocks can be considered to generate Miocene to Pliocene), magma compositions range intermediate and rhyolitic compositions (i.e., Co. from basaltic andesites to dacites (SiO2 = 58-66 La Torta dome). Noteworthy, the Chao and wt.%). Among the studied rocks with similar SiO2 Chillauita lava domes (similar to Co. La Torta, content, these products show the highest K2O, Rb although less acid in composition)lie along the and Pb contents. Unfortunately, the limited same possible mixing trend. number of samples representing this eruptive Beyond the further geochemical evidence, the period prevents any detailed indication on the large diffusion of direct and/or reverse zoning in corresponding magma plumbing system. Based on phenocrysts and the presence of resorption phe- petrographic (high P.I. values, disequilibrium nomena indicate the existence of disequilibrium textures in mineral assemblages)and chemical conditions in these volcanic systems. Moreover, data of the Co. Copacoya laccolith body, we sug- the presence of quartz in some andesites might gest that early FC and following mixing processes indicate that these melts have interacted by mixing have occurred. The lacking of 87Sr/86Sr data with others, more sialic. In the same region of the doesn't allow further considerations about mag- CVZ, for the products of the neighbouring Lascar ma-crust interaction to be developed. volcano, Matthews et al. (1994)have proposed a Successive Puripicar ignimbrite products (4.5-4 model of FC plus mixing with anatectic melts. In Ma; Pliocene)yield HKCA dacite composition this fan of possibilities, the importance of FC, with lower K-enrichment if compared with volca- mixing/recharge and overturn processes within nic rocks previously discussed. It is worthy to note ``shallow'' magma chambers is supported by both that these pyroclastic products are originated petrographic (high P.I. values, disequilibrium from the Cerro Guacha caldera system that is si- textures in mineral assemblages)and chemical tuated in southern Bolivia, outside our study area. data (large compositional variability of the same Therefore, we cannot take into consideration mineral species in the same unit). Among those these samples in the reconstruction of the magma studied, few volcanic bodies are characterized by

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Evidence for different processes of magma evolution in El Tatio volcanic region (22ë16' to 22ë30' S, Central Volcanic Zone, Andes)

Fig. 9 - A)Illustrated changes in K/Rb ratio vs. Rb (ppm)produced by AFC (trend 1: D Rb=0.1, DK= 0.8, r=0.3; trend 2: DRb=0.1, DK= 0.8, r=0.5), plagioclase- and biotite-dominated FC (trend 3; DRb=0.1, DK= 0.8), mixing with felsic rocks (trend 4, Eocambrian sediments average from Lucassen et al., 2001). Parent composi- tion is provided either by T03/7 sample (hi- gher K/Rb ratio)or T12/17 sample, both from 3rd eruptive period; assimilated material is re- presented by xenolith TA08 (grey latin cross); B) 87Sr/86Sr vs. Rb/Sr diagram, depicting qualitative AFC trends that suggest interaction between mafic magma and crustal material able to further modify the ``baseline'' isotopic 87 86 composition. C) Sr/ Sr vs. SiO2 diagram: as shown in the diagram B, the comparison bet- ween the fields representative of the Nevado de Payachata volcanics (field with dashed line, Davidson et al., 1990)and Cerros de Tocorpuri composite volcano (including Co. La Torta dome)indicates that individual volcanic cen- ters related to closed plumbing system cha- racterise El Tatio region (see text for further explanation). Literature data like those re- ported in Fig. 6; black latin crosses are litera- ture data for Cerros de Tocorpuri composite volcano.

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low P.I. values and assemblages of homogeneous during the last 1 Ma, have a tipycal subduction- minerals, which could testify for a low grade of related signature and are representative of the fractionation and solid/liquid equilibrium condi- modern volcanic arc in Central Andes (Western tions, respectively. By contrast, most of the vol- Cordillera). Their complex petrographic attri- canic products are characterized by unusually high butes (e.g. disequilibrium features and multiple crystals contents and sizes (P.I. up to 70, and textures array)can be explained as the result of frequently higher than 35%; Table 3), emphasiz- convective self-mixing processes in magma re- ing that FC within the magma chamber(s)played servoirs, with a minor role for mixing of magmas a main role in magma evolution. Isotopes avail- with different compositions. Magma mingling able for the Cerros de Tocorpuri composite vol- (and probably mixing)can be suggested for the cano confirm this FC leading role, without or with Co. Copacoya rocks that, however, are related to low assimilation, since they are poorly variable the first synthem (7-9 Ma). Magma differentia- (Fig. 9). A comparison between the evolution of tion during the last eruptive period (3rd synthem) the Nevado de Payachata volcanics (Davidson et results from dominant fractional crystallization al., 1990)and the Cerros de Tocorpuri composite (FC), as shown by the high crystallization degree volcano (including the Co. La Torta rhyolitic of rocks and evidence provided by geochemical dome)indicates that El Tatio region includes in- data. Notably, such high crystallization degree dividual volcanic centers related to closed system and consequent high density could require hotter where parental melts have a relatively homo- magma injection and interaction in the magma geneous isotopic composition acquired before chamber to trigger ascent and eruption. In other their arrival in the magma chamber and differ- words, after FC in the magma chamber, mingling entiation. On the other hand, if we consider the of magmas can also occur. Moreover, geochem- whole group of volcanic rocks related to this 2nd ical data together with available isotopic trends eruptive period (including literature data from and patterns of incompatible elements (i.e LILE surrounding areas)we observe that, starting from enrichment, LILE/HFSE ratios), suggest that the ``baseline'' composition, typical AFC trends AFC processes (assimilation plus FC)played an are present (Fig. 9). Indeed, they indicate that important role in magma evolution. The inter- magma interaction and assimilation of crustal action of magmas with crustal contaminants (se- material occurred in deeper reservoirs or during diments, felsic rocks of the crust, acid melts) their ascent to shallower levels. Lack of significant could have occurred both in the mantle wedge isotopic data for our rocks doesn't allow this in- (material transported via subduction)and/or terpretation to be adequately supported, even if during the ascent/ponding of parental magmas in the use of both literature and original data suggest the higher level storage chambers across the thick that AFC occurred due to the interaction of crust beneath Central Andes. Therefore, we magma with the exceptionally thick crust of CVZ. suggest that multiple reservoirs located in the upper crust possibly at variable depths, fed El Tatio volcanism and are the site of multistage Conclusions evolutionary processes occurring both through open and closed regimes. Petrographic and geochemical data relevant to volcanic rocks of El Tatio region provide both their whole compositional characterization and a Acknowledgements contribution to the understanding of magmatic The project was supported by grants from the University of Bologna (University Long-term Project 2/23/07)and the Carisbo systems active beneath this area, especially dur- Foundation (FS 2/27/03 Project). ing Pleistocene. These HKCA intermediate and silicic volcanic rocks, which are mostly emplaced

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Gianfilippo De Astis, Federico Lucchi, Claudio Antonio Tranne and Piermaria Luigi Rossi

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