File: L:/3b2/RIVISTE/GeoActa/9045_Sp2/031-058_DeAstis.3d Ð Pagina: 31 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 (plagpyroxhornblbt)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 Amorosi A., Haq B.U. and Sabato L. (eds.), Advances in Application of Sequence Stratigraphy in Italy, 31 GeoActa, Special Publication 1 (2008), pp. 31-58. File: L:/3b2/RIVISTE/GeoActa/9045_Sp2/031-058_DeAstis.3d Ð Pagina: 32 Gianfilippo De Astis, Federico Lucchi, Claudio Antonio Tranne and Piermaria Luigi Rossi 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 32 File: L:/3b2/RIVISTE/GeoActa/9045_Sp2/031-058_DeAstis.3d Ð Pagina: 33 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