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Magmatic Relations in the La Pacana Caldera System, Andean CVZ, North Chile

Magmatic Relations in the La Pacana Caldera System, Andean CVZ, North Chile

Magmatic relations in the System, Andean CVZ, North

Lindsay, J. 11,Trumbull, R. and deSilva, S. 2 1GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany 2 SFB 267 Indiana State University, Terre Haute, IN 47089, U.S.A.

0 ~ 0 0 ~ ~ ~ PURICO ~ ~ ~ ~ ~ ~ 1.4 ~ ~ FILO DELGADO () ~ ~ 10 20 15 2.4 PAMPA CHAMACA TARA FORMATION 20m ? ? -PunaVolcanic Complex ATANA TARA FORMATION 4.0 40 A fractionation model 35 /// \\\ /// TOCONAO 3.8 - 4.1 \\\ /// \\\ 4.5 (indurated ) 50 0 50 /// \\\ /// ~ ~ ~ Zapaliri ATANA 70m ? ? ~ ~ 5.8 PUJSA 75 4.1 The Altiplano-Puna Volcanic Complex (APVC, Fig. 1) is one of the Jarellon The grey, -rich inclusions found in the Atana 50 Purico 100m ? ? Huailitas have compositions similar to the host (e.g. largest ignimbrite provinces on Earth, and represents an Salar TOCONAO de Tara (non-indurated facies) Nevados X de Poquis pumice cores: An ; rims: An , inclusion cores: episode of large-scale crustal melting in the Andean magmatic arc. 0 30-60 30-50 Curutu 100m 9.4 UNNAMED Toconao Paso ? ? IGNIMBRITE An , rims: An ). Bulk compositions plot at the end of The La Pacana Caldera, one of the largest and best-exposed Bola Salar de Jama 35-60 36-40 X Quisquiro ATANA Purifican Morro 0 resurgent in the world, is representative of the large- Talabre Negro ~ ~ ~ ATANA the fractional crystallisation trend defined by the Atana pumice ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ N 20 100 volume, homogeneous systems typical of the APVC. The Aguas Corral de TOCONAO (Fig. 6), but glass compositions are the same (inclusions Calientes Coquena 40 (non-indurated facies) Chamaca ATANA vitrophyre Cordon Puntas Ne 140 and pumice: 76-77 wt% SiO ). We interpret these inclusions as main units associated with this caldera are the Toconao Ignimbrite /// \\\ /// UNNAMED IGNIMBRITE /// \\\ /// 60 2

\\\ /// \\\ \\\ /// \\\ 3 3 0 20 km 70m /// \\\ /// /// \\\ /// gras ?? (500 km ) and the Atana Ignimbrite (1,200 km ), which have both /// \\\ /// /// \\\ /// TOCONAO accumulations of early-formed resulting from \\\ /// \\\ \\\ /// \\\ (indurated facies) Chile Paso /// \\\ /// /// \\\ /// Guaitiquina been dated at 4 Ma. Post-eruption resurgence formed an elongate 4.0 TOCONAO fractionation of now represented by the host pumice, (non-indurated facies) 220 ~ ~ ~ ~ Salar 0 ~ ~ ~ ~ 3.9 ATANA ~ Alluvial deposits/ ~ ~ 10 ~ ~ ~ ~ and note that similar inclusions have been recognised in other block of intracaldera , along the margins of which several ~ deflation surface ~ ~ ~ ~ ~ ~ ~ ~ ~ 5.1 HUAYTIQUINA < 4 Ma 0 15m ? ? 260 ~ ~ ~ PUJSA Dacite and domes were extruded (Fig.2). centers < 4 Ma APVC ignimbrites (deSilva 1989b), where they have been 20 centers < 4Ma = vent ~ ~ ~ ~ ~ Tara Formation Ignimbrites (4 Ma) ~ ~ ~ ~ ATANA ~ ~ 40 interpreted in a similar way.These inclusions provide evidence Atana Ignimbrite (4 Ma) 320 m ?? 0 70 Toconao Ignimbrite (4 Ma) 5.0 HUAYTIQUINA (5.0 Ma) 10 Rhyolitic centres and ignimbrites > 4 Ma for side-wall fractional crystallisation in the Atana magma 65 TOCONAO 20 UNNAMED IGNIMBRITE (10 Ma) P L.Titicaca 0 Andesite and dacite 70 (non-indurated facies) PAMPA CHAMACA centers > 4 Ma 9.3 30m 10 ~ ? ? E Prevolcanic ~ ~ ~ chamber, and can be considered representative of an early ~ UNNAMED 20 ATANA ~ ~ ~ Topographic IGNIMBRITE R BASEMENT Caldera rim 30m ? ~ ~ Welded 100m ? ? ? X Possible vent ~~~ U LaPaz ignimbrite Atana magma. Arequipa BOLIVIA Fig. 2. Geological map and representative stratigraphic sections of The compositional variations within and between the Toconao ignimbrites in the La Pacana region. Where samples from particular and Atana ignimbrites can be explained by fractional sections have been dated, their ages are given in Ma in the section. C crystallisation of the observed mineral assemblages, with accessory playing a greater role above about 70 wt% H Potosi SiO2 . In particular, variations in REE, Zr, TiO225 and P O can be I Pressure, temperature and magmatic 20 Salar de explained by fractionation of allanite, and L Uyuni contents respectively (Figs.6 and 7). APVC E Pastos Grandes - and -plagioclase geothermometers

25°S Atana crystal-rich Guacha (Fig.4) yield temperatures of 730°-810°C for Atana pumices.A greater 16 Panizos 400 Atana crystal-poor pumice Purico Calama 14 Grey inclusions in Atana range was obtained for the grey inclusions (720-870°C). Zircon- Atana glass 300 12 Toconao pumices Salar de saturation thermometry yields minimum temperatures of 750°C and La Pacana 10 Trachyte Atacama 770°C for the grey inclusions and Atana pumices respectively, and 8 Trachyan. 200 50% A Rb (ppm) 22 N 6

La Pacana I 720°C for Toconao pumices. Preliminary results from Al in hornblende Na O + K O (wt.%) T Rhyolite Pl+Hbl+Bt E N +Cpx+Mag A R G 4 Complex barometry yield a range of pressures for the Atana pumices and grey Bas. 25 Andesite Dacite Salar de inclusions of 3-5 kbar, corresponding to 10-17 km depth. SIMS 2 . 0 100 analyses of melt inclusions yield water contents ranging from 3.2 to 40 45 50 55 60 65 70 75 80 40 100 200 300 SiO2 (wt.%) Zr (ppm) 600 Pl+Hbl+Bt+Cpx+Mag 4.6 wt%. 1000 50%

approximate extent 500 Pl+Qtz+Bt Pl+Qtz+Bt +Sa+Zrn 70% of ignimbrite outcrop +Hbl+Sa Pl+Hbl+Bt 100 caldera town +Cpx+Mag 10% 0 100 200 km Ba (ppm) 30% Sr (ppm) comp. by Hendrich, GFZ

zircon saturation (Harrison & Watson 1983) 100

Fig. 1. Map showing the location of the APVC and the extent of Toconao 60 10 Inclusions 40 100 200 300 100 200 300 ignimbrites in the CVZ. Atana Zr (ppm) Rb (ppm) Fig. 6. TAS classification diagram and trace element variation -plagioclase (Holland & Blundy 1994) Project goals diagrams. Arrows represent model fractionation paths of observed mineral assemblages.The gap in Zr can be explained Previous geologic and stratigraphic studies (Gardeweg and by zircon fractionation above 70 wt% SiO . Ramirez, 1987; deSilva, 1989a) of the La Pacana caldera provide a 2 solid basis for our work - the first detailed petrologic and Fe-Ti oxides (Anderson et al. 1993) geochemical study of the ignimbrite-producing magmas. Our error range specific goals are to investigate:

600 700 800 900 1000 500 1. the genetic relationship between eruptive units to determine the o importance of fractionation, assimilation, recharge and zoning in T ( C) Atana pumices large-volume magma chambers, and Toconao pumices Grey inclusions in Atana Fig. 4. Temperatures of the Atana and Toconao ignimbrites and grey 100 2. magmatic pressures, temperatures and volatile contents to inclusions derived by different geothermometers. constrain conditions of melt generation and evolution.

allanite Toconao - Atana relationship fractionation La Pacana caldera-forming ignimbrites Sample/chondrite 10 Age and stratigraphic relationships (Fig. 2) and whole rock ToconaoIgnimbrite compositions suggest that the Toconao Ignimbrite represents the The rhyoliticToconaoIgnimbrite is crystal poor (<10% crystals), and La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu evolved cap of the Atana . Confirming whether these contains abundant glassy, crystal-poor, tubular pumices (Fig. 3) Rare Earth Elements two ignimbrites are cogenetic has implications for size, shape and bearing plagioclase, sanidine, and which range in zonation of the magma chamber, and the nature of the eruption. composition from 76-77 wt% SiO2 . It comprises two main facies: a Fig. 7. REE patterns of the Toconao and Atana ignimbrites.The lower, non-indurated facies, and an upper, vapour-phase altered The isotopic values for Atana and Toconao pumices are lower LREE concentrations in the Toconao relative to Atana indurated sillar facies (Fig. 2). In places, a thin plinian deposit is indistinguishable, consistent with the hypothesis that they are pumices can be explained by allanite fractionation. Normalising preserved at its base. cogenetic. These isotopic values are typical of crustal melts in the values are from Anders and Grevesse (1989). ε 87 86 Atana Ignimbrite CVZ: Nd ranges from -8.14 to -6.56; and Sr/ Sr from 0.7091 to The dacitic Atana Ignimbrite is crystal rich (45% crystals) and 0.7132 (Fig.5). contains variable amounts of crystal-rich pumices bearing plagioclase, quartz, biotite, hornblende, magnetite and accessory Toconao pumices fail to plot on an extrapolation of the Atana pumice ilmenite, zircon and titanite. These pumices range from 67-70 wt% array on most major and trace element variation diagrams (Fig. 6). Synthesis However, the fields for Atana pumice glasses and crystal-poor SiO2 .The Atana Ignimbrite also contains rare crystal-poor pumices (76 wt% SiO ) that lack the tubular texture of the Toconao pumices. pumices overlap the Toconao pumice field (Fig. 6), showing that the We propose that side-wall crystallisation in the Atana magma 2 compositional gap between the Atana and Toconao pumices may be In contrast to the Toconao Ignimbrite, the Atana Ignimbrite is chamber led to the development of crystal accumulations primarily due to differences in crystal content. relatively homogeneous, and lacks a basal plinian unit. In some (represented by the grey inclusions) and released a buoyant localities the Atana Ignimbrite contains significant amounts of grey, residual melt which collected at the top of the magma chamber. crystal-rich (70% crystals), andesitic inclusions in the pumices and This was efficiently separated from the bulk of the convecting as individual clasts (Fig. 3). These have a similar mineral magma chamber prior to its eruption as the rhyolitic Toconao assemblage as the host pumice (but lack quartz), and range from ignimbrite. This eruption was followed closely by that of the 10 homogeneous, crystal-rich Atana ignimbrite, which formed the 59-61 wt% SiO . Oceanic Arc Volcanic chain Basement 2 SVZ, NVZ Andean Basement CVZAnde- gneisses and current caldera. 5 sites (after F. Lucassen, unpub. data) CVZ Ignimbrites

Atana Atana grey inclusions Toconao 0 Post caldera domes

Nd -5 ε -10 References:

-15 Anders E.and Grevesse N.(1989):Geochimica et Cosmochimica Acta 53(1):197-214

0.700 0.710 0.720 0.730 8 7 8 6 Andersen D.J.; Lindsley D.H. and Davidson, P.M. (1993): Computers and Geosciences 19(9): Sr/ Sr 1333-1350

deSilva S.L.(1989a):Journal ofVolcanology and Geothermal Research 37:93-131 400 µm deSilva S.L.(1989b):Geological Magazine 126 (2):159-175

Fig. 3. Photomicrograph of a crystal rich inclusion (right of arrow) in Fig. 5. Nd and Sr isotope data from the Andes. Symbols denote new Gardeweg M.and Ramirez C.(1987):Bulletin ofVolcanology 49:547-566 an Atana pumice (left of arrow). Left = plane-polarized light; right = data. The fields for the CVZ and SVZ+NVZ are based on literature Harrison M andWatson E (1983):Contributions to Mineralogy and 84 (1):66-72 crossed nicols. data, and mainly represent data from stratovolcanoes. HollandT and Blundy J (1994):Contributions to Mineralogy and Petrology 116 (4):433-447

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