JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B9, PAGES 17,805-17,825,SEPTEMBER 10, 1994 Effusive silicic volcanism in the Central Andes: The Chao dacite and other younglavas of the Altiplano-PunaVolcanic Complex S. L. de Silva,1 S. Self,e P. W. Francis,3 R. E. Drake,4 andCarlos Ramirez R. 5 Abstract. The largestknown Quaternarysilicic lava body in theworld is CerroChao in northChile, a 14-km-longcou16e with a volumeof at least26 km3. It is thelargest of a groupof severalclosely similar daciticlavas erupted during a recent(< 100,000year old) magmaticepisode in theAltiplano-Puna VolcanicComplex (APVC; 21-24øS)of theCentral Andean Volcanic Zone. The eruptionof Chao proceededin threephases. Phase 1 wasexplosive and produced -• 1 km3 of coarse,nonwelded dacitic pumicedeposits and later block and ash flows that form an apronin frontof themain lava body. Phase2 wasdominantly effusive and erupted -22.5 km• of magmain theform of a compositecoule6 covering -•53km 2 with a 400-m-highflow frontand a smallcone of poorlyexpanded pumice around the vent. The lavais homogeneouswith rare flow bandingand vesicular tops and selvages. Ogives (flow ridges) reachingheights of 30 rn formprominent features on its surface.Phase 3 produceda 6-km-long,3-km- wideflow thatemanated from a collapseddome. Ogives are subdued,and the lava is glassierthan that producedin previousphases. All theChao products are crystal-rich high-K dacites and rhyodacites with phenocrystsof plagioclase,quartz, hornblende, biotite, sphene, rare sanidine, and oxides. Phenocryst contentsreach 40-60 vol % (vesiclefree) in themain phase 2 lavasbut arelower in thephase 1 (20- 25%)and phase 3 (-40%) lavas.Ovoid andesitic inclusions with vesicular interiors and chilled margins upto 10cm are found in thelater stages of phase2 andcompose up to 5% of thephase 3 lava.There is littleevidence for preeruptivezonation of themagma body in composition,temperature (-•840øC), œ07 (10'•), or watercontent, so we proposethat eruption of theChao complex was driven by intrusionof fresh,hot andesitic magma into a crystallizingand largely homogeneous body of daciticmagma. Morphologicalmeasurments suggest that the Chao lavas had internal plastic viscosities of 10•ø to 10•2 Pa s, apparentviscosities of 109Pa s, surfaceviscosities of 10•5 to 10• Pas, and a yieldstrength of 8 x 105 Pa. Theseestimates indicate that Chao would have exhibited largely similar rheological properties to othersilicic lava extrusions, notwithstanding its highphenocryst content. We suggestthat Chao's anomaloussize is a functionof boththe relatively steep local slope (20' to 3') andthe available volume of magma.The eruptionduration for Chao'semplacement is thoughtto havebeen about 100 to 150 years, with maximumeffusion rates of about25 m• s-• for shortperiods. Four other lavas in thevicinity with volumesof ~ 5 km3 closelyresemble Chao and are probably comagmatic. The suiteas a wholeshares a petrologicand chemical similarity with the voluminous regional Tertiary to Pleistocene ignimbrites of the APVC andmay be derivedfrom a zoneof silicicmagmatism that is thoughtto havebeen active since the lateTertiary. Chao and the other young lavas may represent either the waning of thissystem or a new episodefueled by intrusionsof maficmagma. Introduction youngerthan most of the large late Pleistoceneandesitic volcanoesthat dominatethe region. What makesthese lavas We describehere a group of large solitary, well-preserved exceptionalis their largesize. By far the mostspectacular is silicic lava bodies from the Central Volcanic Zone of the Andes CerroChao, a 14-lorn-longcou16e with a flow front•400 m high (CVZ; Figure1). All are highlyporphyritic, high-K dacites and (Figure3). rhyolites(Figure 2) andmost have a steep-sided,circular plan and Located in northern Chile at 22ø07'S, 68ø09'W, Cerro Chao would be characterizedas "low domes"[cf. Blake, 1990], while (alsoknown as Volean Chao or Chaolava) is a prominentfeature the othersare cou16es[cf. Walker, 1973]. These lava bodies are of the Quaternaryvolcanic front, where its unusualsquat characterand morphologyare in markedcontrast to the older •Departmentof Geographyand Geology,Indiana State andesiticcomposite cones of Paniri and Leon which flank it University, Terre Haute. (Figure3). It wasfirst describedby Guestand Sanchez [1969], 2HawaiiCenter for Voleanology,Department of Geology who estimateda minimumvolume of 24 km3. Suchlarge and Geophysics,SOEST, University of Hawaii, Honolulu. volumesof silicicmagma are usually erupted explosively to form 3Departmentof Earth Sciences,Open University, Milton ignimbrites,but large volume silicic lavas are known from several Keynes, England. regions[e.g., Henry et al., 1988;Bonnischen and Kauffman, 4BerkeleyGeochronology Center, Berkeley, California. 1987]. Unlike Chao,these are sheetlike,poorly exposed bodies, 5ServieioNaeional de Geologiay Mineria,Santiago, Chile. andin manycases their eruptive style is stilldebated [e.g., Henry and Wolff, 1992]. Copyright1994 by theAmerican Geophysical Union. Apart from its sheersize, Chao posesseveral intriguing questionsregarding its eruption,eraplacement, and regional Papernumber 94JB00652. siginificance.Here we addressthese questions and present field 0148-0227/94/94JB-0065•05.00 observations that 1end further insight into the eruption 17,805 17,806 DE SILVA ET AL.: EFFUSIVE SILICIC VOLCANISM 68o30,W 67o30' 66 ø30' t t"' / 10 ø \ NVZ 21 ø ,',• o•/ 30' s // 10 ø 20 ø * / ARGENTINA Nazca Plate 22 ø •0 ø 40 ø / •o Plate •" ,...,)# 22 ø 30' ß i 0 50 100 km • Caldera collapse scarp Individual stratovolcanoes (-10 Ma to present) ...... Major normalfaults Alignmentofstratovolcanoes - - Faults and major structurallineaments Silicicdomes (>1 km 3) ..... Outline of resurgentcenters ':'::•'" Salars _ .- International fronhers Figure1.Altiplano-Puna Volcanic Complex ofthe Central Volcanic Zone (CVZ) of the Andes showing themajor volcano-structuralfeatures. The region of young silicic lavas and domes described here is outlined and shown in Figure2.Inset map shows distribution ofthe Cenozoic toRecent volcanic zones ofthe Andes and their relation to theplate tectonic framework ofthe western margin ofSouth America. NVZ is Northern Volcanic Zone; SVZ is Southern Volcanic Zone. mechanismsof the Chao crystal-rich dacite magmas, petrological volcanicfront built upon the ignimbrite basement. Alignment of data bearingon the origin of the magma,and the first thevolcanic front is approximatelyN-S, but the segment between geochronologicdata. We estimaterheologic properties and about22 ø and23øS trends NW-SE. The locationof thevolcanic effusionrate of thelava, and we presentan integrated model for centersinthis portion appears tobe controlled bya seriesof NW- eruptionand emplacement of the lava. We thencompare Chao SEtrending faults possibly related to the southern margin of the with otherassociated domes and volcanic rocks in the regionand PastosGrandes caldera complex (Figures 1 and2). Chaois discusstheir significance in the evolution of theAltiplano-Puna located on theeasternmost arm of theNW trendingportion of the VolcanicComplex (APVC). volcanicfront on a col betweenthe volcanoes of Paniriand Leon (Figure2). GeologicSetting Geochronology Chao and the associateddomes are locatedwithin the APVC, a largezone of silicicvolcanism occupying the 21 ø to 24øS segment Chaois clearly younger than the flanking volcanos of Paniri of the Central VolcanicZone of the Andes,which is one of three andLeon, on whoseflanks it is constructed.A glacial moraine activevolcanic zones in the Andes [de Silva, 1989a, and b; de fromVolcan Leon which abuts against the eastern flank of Chao Silvaand Francis, 1991] (Figure 1). In theAPVC, eruption of formspart of a complexofmoraines that terminate atabout 4500 regionallyextensive ignimbrites from several caldera complexes m. Thesemoraines are thought to havebeen deposited during dominatedthe volcanicevolution from the late Miocene to the recessionfrom the last glacialmaximum [Hollingworth and Pleistocene.Since then volcanism has been dominantly andesitic, Guest,1967; de Silvaand Francis,1991]. On thesegrounds, forminglarge volcanic cones along a diffuse50-km-wide Chaomust therefore be older than ~11,000 years. DE SILVA ET AL.: EFFUSIVE SILICIC VOLCANISM 17,807 68ø30'W 67o30'W 21 30' S Pastos V. San Pedro Chao V. Michina ;•),oalama V. Putana ,,?Lr•"4JPre-ignimbrite basement Volcanoactive in Pleistocene I I UpperTertiary ignimbrites VolcanoactiveinHolocene ........... •.......... Quatemaryvolcanics T - El Tatio, S - Sol de Mar3ana •• Siliciclavaflows anddomes ,/ Intemationalfrontier r..•' (Chile/Bolivia) B Town Figure2. Mapof thestudy area highlighting the volcanic geology. Locations of the Chao dacite and other young domesin relationto othermajor volcaniccenters are shown.Centers numbered are 1, Cerro Chanka;2, Chascon/RuntuJarita; 3, Chillahuita;and 4, Tocopuri.P is PiedrasGrandes a 7.5 Ma domecomplex associated with the ignimbrites. Threebiotite separates from Chao I lavahave been analyzed Geology byconventional K-Ar andhigh precision 4øAr/39Ar single crystal laserfusion (SCLF) techniques (Table 1). An averageage of PyroclasticDeposits 423,000+ 100,000years is suggestedby theconventional method The most extensive erupted products from Chao are a andmay be a realisticage for Chao. However, individual crystals previouslyunrecognized pyroclastic sequence that forms an apron of biotitefrom each separate yielded anomalously old ageson 24 km wide and34 km in extentbeyond the southernflow front SCLFdating. Thus, the K-Ar