Effusive Eruption of Viscous Silicic Magma Triggered and Driven by Recharge: a Case Study of the Cerro Chascon-Runtu Jarita Dome Complex in Southwest Bolivia

Home , Complex volcano, Effusive eruption

Bull Volcanol (1999) 60:241–264 Q Springer-Verlag 1999

ORIGINAL PAPER

R. B. Watts 7 S. L. de Silva 7 G. Jimenez de Rios I. Croudace Effusive eruption of viscous silicic magma triggered and driven by recharge: a case study of the Cerro Chascon-Runtu Jarita Dome Complex in Southwest Bolivia

Received: 23 August 1998 / Accepted: 10 March 1999

Abstract The Cerro Chascon-Runtu Jarita Complex is composites of dacitic and rhyolitic compositions. The a group of ten Late Pleistocene (F85 ka) lava domes southernmost dome is dominantly rhyolite with rare located in the Andean Central Volcanic Zone of Boliv- mafic enclaves. The composite domes have lower flanks ia. These domes display considerable macroscopic and of porphyritic dacite with F35 vol.% phenocrysts of microscopic evidence of magma mixing. Two groups of plagioclase, orthopyroxene, and hornblende in a micro- domes are defined chemically and geographically. A lite-rich, rhyodacitic glass. Sieve-textured plagioclase, northern group, the Chascon, consists of four lava bod- mixed populations of disequilibrium plagioclase com- ies of dominantly rhyodacite composition. These bodies positions, xenocrystic quartz, and sanidine with ternary contain 43–48% phenocrysts of plagioclase, quartz, san- composition reaction rims indicate that the dacite is a idine, biotite, and amphibole in a microlite-poor, rhyol- hybrid. The central cores of the composite domes are itic glass. Rare mafic enclaves and selvages are present. rhyolitic and contain up to 48 vol.% phenocrysts of Mineral equilibria yield temperatures from 640 to plagioclase, quartz, sanidine, biotite, and amphibole. 750 7C and log ƒO2 of –16. Geochemical data indicate This is separated from the dacitic flanks by a banded that the pre-eruption magma chamber was zoned from zone of mingled lava. Macroscopic, microscopic, and a dominant volume of 68% to minor amounts of 76% petrologic evidence suggest scavenging of phenocrysts SiO2. This zonation is best explained by fractional crys- from the silicic lava. Mineral equilibria yield tempera- tallization and some mixing between rhyodacite and tures of 625–727 7C and log ƒO2 of –16 for the rhyolite more evolved compositions. The mafic enclaves repre- and 926–1000 7C and log ƒO2 of –9.5 for the dacite. The sent magma that intruded but did not chemically inter- rhyolite is zoned from 73 to 76% SiO2, and fractiona- act much with the evolved magmas. A southern group, tion within the rhyolite composition produced this var- the Runtu Jarita, is a linear chain of six small domes iation. Most of the 63–73% SiO2 compositional range of (~1km3 total volume) that probably is the surface ex- the lava in this group is the result of mixing between pression of a dike. The five most northerly domes are the hybrid dacite and the rhyolite. Eruption of both groups of lavas apparently was triggered by mafic recharge. A paucity of explosive activity suggests that volatile and thermal exchanges between reservoir and re- Editorial responsibility: D. A. Swanson charge magmas were less important than volume increase and the lubricating effects of recharge by mafic Robert B. Watts 7 Shanaka L. de Silva (Y) Department of Geography, Geology, and Anthropology, magmas. For the Runtu Jarita group, the eruption is Indiana State University, Terre Haute, IN 47809, USA best explained by intrusion of a dike of dacite into a e-mail: gesilva6scifac.indstate.edu, chamber of crystal-rich rhyolite close to its solidus. The Fax: c812-2378029 rhyolite was encapsulated and transported to the sur- Guillermina Jimenez de Rios face by the less-viscous dacite magma, which also acted GEOBOL, Casilla 2729, La Paz, Bolivia as a lubricant. Simultaneous effusion of the lavas pro- Ian Croudace duced the composite domes, and their zonation reflects Department of Geology, University of Southampton, the subsurface zonation. The role of recharge by hotter, Southampton, SO9 5NH, England more fluid mafic magma appears to be critical to the Present address: Robert B. Watts, Department of Geology, eruption of some highly viscous silicic magmas. Bristol University, Bristol, BS8 1RJ, UK 7 7 Present address: Guillermina Jimenez de Rios, ISERSIG, Key words Lava domes Recharge Eruption Casilla 3527, La Paz, Bolivia mechanisms 7 Encapsulation 242

Introduction tion and second boiling within the mafic magma. Second boiling may result in an increase in the volume of the recharge magma that may generate vapor Effusive eruptions of silicic lavas are common in the overpressures sufficent to trigger an eruption. Fur- geologic record. Such events, generally accepted to be thermore, crystallization could lead to a density de- extrusions of gas-poor magma, constitute a significant crease in the residual liquid of the mafic magma, proportion of silicic volcanism (e.g., Eichelberger which might promote intimate mixing of the two 1995). Stimulated by such eruptions as Santiaguito, magmas. This process would promote vapor satura- Guatemala (1920s to present), Mount St. Helens, USA tion of the mafic magma, which releases volatiles as (1980 to present), Unzen, Japan (1990–1995), and Sou- a result, particularly water vapor. This may help fur- friere Hills, Montserrat (1995 to present), much atten- ther reduce the viscosity of the reservoir magma. tion has focused on understanding the mechanisms that Both thermal exchange and volatile exsolution may occur during the eruption and emplacement of such continue throughout the eruption. lavas. Considerable progress has been made in devel- 4. Transport of silicic magma. Mafic magma may act as oping concepts concerning the initial or magma cham- a lubricant and as a transporting agent for viscous ber conditions (e.g., Tait et al. 1989; Stasiuk and Jau- silicic magma through the development of core-an- part 1997), the importance of rate of ascent and the nular zonation (Carrigan and Eichelberger 1990; possible role of syneruptive degassing (e.g., Eichelberg- Carrigan 1994). If two magmas of different viscosi- er et al. 1986; Jaupart and Allegre 1991; Fink et al. ties are introduced at the entry end of a conduit sys- 1992; Stasiuk et al. 1993), the eruption and growth of tem, the less viscous magma will migrate to the high- lava bodies (e.g., Huppert et al. 1982; Blake 1990; Fink er stress regions at the margins of the conduit (or and Griffiths 1990, 1992), and the role of mafic magma dyke), producing a zoned system with a silicic core (e.g., Gibson and Walker 1963; Blake and Campbell and mafic annulus. This lubricates the conduit (pipe- 1986; Carrigan et al. 1992; Carrigan 1994; Rutherford et like or dyke) walls and reduces the forces required al. 1998). However, fundamental questions still remain, to move the silicic magma. The silicic magma is particularly about the triggering of an eruption and the therefore transported by the mafic magma. An ex- movement and transport of extremely viscous crystal- tension of this would be a portion of the silicic mag- rich silicic lavas, which form some of the largest individ- ma that was physically separated from the main res- ual lava bodies (e.g., de Silva et al. 1994; Manley 1996) ervoir as it was encapsulated and buoyed upward by and dominate the geologic record for this class of fea- the denser mafic magma. ture. Whereas all of these effects are likely to occur dur- The role of recharge and attendant effects in the ing recharge (mafic into silicic), their relative impor- eruption of crystal-rich silicic lavas may be crucial. Nu- tance would depend heavily on the nature and proper- merous interdependent effects may occur during a re- ties of individual systems. Here we present a case study charge event that juxtaposes dense, hot, low-viscosity of a superbly exposed group of silicic and composite magma into a reservoir of cooler, less dense, viscous lava bodies in the Andes of Bolivia: the Chascon-Runtu (crystal-rich) magma. The most important of these Jarita (CRJ) Complex. Using a combination of field ob- are: servations, petrologic and geochemical data, and volca- 1. Volume increase. A simple increase in the volume of nologic analysis, we show that recharge by mafic mag- magma may induce an increase in magmatic pres- ma probably triggered the eruptions at CRJ and also sure that may exceed the minimum principal stress facilitated the transport and eruption of extremely vis- and tensile strength of the country rock; if so, mag- cous silicic magma. ma may erupt. Bindeman (1993) argues that during recharge a 10% increase in volume might be the maximum expected. This may provide a limiting Regional geologic context condition above which magmatic pressure induces failure and conduit formation. The Chascon-Runtu Jarita Complex is one of a group 2. Superheating. Superheating of magma can have sev- of youthful effusive silicic centers within the Altiplano- eral effects. Firstly, it may result in a reduction of Puna Volcanic Complex (APVC) of the Central Vol- viscosity and may promote convective motion. Fur- canic Zone (CVZ) of the Andes (Fig. 1; de Silva et al. thermore, heating of the silicic magma may lower 1994). The APVC covers approximately 70,000 km2 be- the solubility of the volatiles and promote volatile tween latitudes 217 and 247S. It developed during the exsolution. During convective uprise of the silicic Late Miocene to early Pleistocene ignimbrite flare-up magma, decompression can cause further volatile ex- which produced voluminous silicic ignimbrites during solution. The subsequent increase in vapor pressure massive caldera-forming eruptions (de Silva 1989a, might trigger an eruption. 1989b). This flare-up has been argued to be the re- 3. Volatile exchange during recharge. Thermal ex- sponse to crustal thickening and subsequent thermal re- change at the interface between cooler silicic magma laxation of the Central Andean crust augmented by the and hot, denser mafic magma promotes crystalliza- thermal input from mantle-derived magmas. Produc- 243 68°30’W 67°30’W

Salar de 21°30’S Ascotan Pastos Grandes Caldera 70°

10° 1 V. San Pedro V. Az uf r e NVZ 0° Chascon- Runtu Jarita 10° CVZ Negra Ridge Peru-Chile Trench V. Leon 20° Chao Nazca Plate Rio Loa 2 30° P T

SVZ V. Mi c hi na 40° Chile V. Tat i o S Rise AVZ Calama 3 50° Antarctic 22°30’S V. Putana Plate 70°

Pre-ignimbrite basement Volcano active in Pleistocene

Upper Tertiary ignimbrites Volcano active in Holocene

Quaternary volcanics Active geothermal fields T-EI Tatio, S-Sol de Manana~

Silicic lava flows and domes International frontier Town (Chile/Bolivia)

Fig. 1 Index map of the Chascon-Runtu Jarita region. Other depth (e.g., Lipman 1984) suggesting that an intrusion domes. 1 Cerro Chanka; 2 Cerro Chillahuita; 3 Cerro Tocopuri. P of batholithic proportions may be present below the is Piedras Grandes, a 7.8-Ma dome complex. Small map shows relative position of the Central Volcanic Zone (CVZ) to the APVC (de Silva 1989a, 1989b). Northern Volcanic Zone (NVZ), Southern Volcanic Zone (SVZ), The most recent volcanic activity within the APVC and Austral Volcanic Zone (AVZ) is represented by dominantly andesitic eruptions from stratovolcanoes along the arc front and by scattered extrusions of crystal-rich silicic lavas such as Cerro Chao and the subject of this paper, the Chascon-Runtu Jarita tion of enormous volumes of dacitic magma resulted Complex (CRJ). The silicic lavas share many petro- from a complex combination of melting, assimilation, logic, chemical, and textural characteristics with each and mixing. Diapiric rise into the upper crust subse- other and the voluminous ignimbrites leading de Silva quently led to cataclysmic eruption of the ignimbrites. et al (1994) to suggest that the lavas are the latest prod- Such a scenario implies that the large-scale volcanism ucts from the APVC silicic magmatic system. As such, at the surface reflects large-scale silicic magmatism at they may define the most active portion of the APVC 244 magmatic system and may signal either the waning or a given SiO2 content a low-Y group which consists of rejuvenation of the system. the northern group of domes G through K contrasts The CRJ is a linear group of silicic lavas trending with a high-Y group consisting of the Runtu Jarita from 21753bS and 67754bW at base elevations of alti- domes A though F. We therefore refer to the low-Y tude of 4600 m to 22702bS 67749bW at 4900 m. The CRJ northern group as the Chascon Group, and the high-Y can be divided up into a northern group of four solitary group as the Runtu Jarita Group. There is also a physi- lava bodies, of which the northernmost, Cerro Chascon, cal distinction. Domes K, H, and G are all generally ho- is the largest, and a southern linear chain of composite mogeneous, leucocratic, crystal-rich silicic lava, where- domes known as Runtu Jarita. The CRJ lies within the as the Runtu Jarita Group generally consists of two lav- southern moat and flank of the Pastos Grandes resur- as, partly mixed. Cerro Guichi (Dome J) is somewhat gent caldera (Fig. 1). This caldera defines a crude oval enigmatic in that it is pervasively mixed but relates bet- with a maximum dimension of 60 km and is thought to ter to the northern geographic group. Each group has a be the source for at least two extensive ignimbrites the characteristic shape or form due to the contrasting 8.1 Ma Sifon ignimbrite, and the 5.3 Ma Chuhuhuilla ig- rheologic behaviors of their lavas (Fig. 4). Morphologic nimbrite (de Silva and Francis 1989; Francis and de Sil- parameters of each dome are listed in Table 1. va 1989). A related younger (3.1 Ma) ignimbrite “shield”, Cerro Juvina, lies on the northern flanks of this caldera. Together, these elements indicate a long- The Chascon Group lived silicic magmatic system of which the Chascon- Dome K–Cerro Chascon Runtu Jarita Complex may be the most recent eruption. The CRJ is oriented northwestward and follows Chascon dominates the complex with its classic low the regional structural fabric. However, locally it is dome or “torta” form flanked by 400-m-high blocky ta- probably controlled by the stress field associated with lus slopes. Chascon is a homogeneous, leucocratic and the magmatic system or southern structural elements of porphyritic rock with rare randomly oriented flow- the Pastos Grandes caldera (Fig. 1) and it is most likely banding. We call this the Chascon Silicic Lava (CSL). the surface manifestation of a dyke using faults on the Two key features of CSL are its high crystal content southern margin of the caldera. Six samples of Runtu (up to 48% crystals; Table 2) with dominant pheno- Jarita were used for age estimates. Four 40Ar–39Ar ages crysts of feldspar (plagioclase and sanid- from single crystals of sanidine yield an age range from ine)pquartz1biotite and amphibole. Its poorly, but 88B4 to 89B4 Ma, whereas two from bulk separates of pervasively, vesicular matrix imparts a friable texture to sanidine yield ages of 93B3 and 97B2 (Dunne 1998). the rock. Rare vitrophyric selvages, enclaves, and The relatively young age of the complex and the arid blocks (in scree; CC22, CC23, 96014) are found, some climate combine to result in superb preservation of the with andesitic composition. geology. The top surface of Chascon is irregular and includes many conical mounds ca. 20 m high (Fig. 5B). These mounds are constructed of blocks of the vesicular CSL Geology and separated by sandy trails of loose crystals eroded away from these mounds. The mounds may be similar The ten domes are labeled A–K (south to north) omit- to the ogive ridges seen on other similar domes, such as ting the letter I (Fig. 2). The rocks of the CRJ are calc- the Chao coulée (de Silva et al. 1994), but they do not alkaline and define a high-K suite ranging from ande- form long linear ridges. They may alternatively repre- site to high silica rhyolite (Fig. 3A). Two groups can be sent diapirs of more vesicular lava that rose to the sur- defined on the basis of Y vs SiO2 relations (Fig. 3B). At face (Fink et al. 1992).

Table 1 Morphometric Dome Max. height Average height Area Volume Diameter parameters for the domes of (m) (m) (m2) (m3) (m) the Chascon-Runtu Jarita Complex Chascon Group G 140 50 2.1!105 1.0!107 515 H 85 40 2.1!105 8.3!106 515 Guichi, J 6 6 4.5!102 2.7!103 24 Chascon, K 490 375 12.8!106 4.8!109 4028 Runtu Jarita Group A 110 50 3.2!105 1.6!107 635 B 180 65 6.2!105 4.0!107 886 C 120 60 1.6!106 9.6!107 1427 D 105 50 2.0!106 1.0!108 1596 E 140 70 8.2!105 7.4!107 1020 F 65 40 6.0!105 2.4!107 874 245

Fig. 2 Mosaic of aerial photographs of the Chascon-Runtu Jarita Complex. The lighter shaded material on the domes is the silicic lavas, the darker the dacite. Note the annular (mafic-silicic-mafic) distribu- tion. also note the square plan view of Laguna Khara, the western shore of which is parallel to the trend of Cerro- Runtu Jarita Complex. Aerial photo numbers 2833, 2656, 2634, 2472, and 2633 from the Instituto Geografico Militar, La Paz, Bolivia

Table 2 Representative modal Sample no. Chascon Group Runtu Jarita Group analyses of the Chascon-Run- tu Jarita Group lavas. T trace CC01 CC04 CC30 CC33 CC12 CC10 CC14 CC28 amount present Chascon- Chascon- Dome G Dome H Dome C C-Mafic C-Mafic E-Mafic Dome K Dome K Flowfront Flow Flow

Glass/Matrix 54 55 52 56 53 72 71 72 Phenocrysts Plagioclase 21 25 27 27 25 18 17 16 Quartz 12 7 8 6 10 – 2 3 Biotite 10 7 8 7 7 – 1 2 Sanidine 2 4 2 2 4 – 1 1 Hornblende 1 2 2 2 2 4 3 3 Hypersthene – – 1 – – 4 3 2 Sphene T T T T T – – T Fe–Ti oxides T T T T T 2 2 1 Apatite – – T – T – – – Phenocrysts (%)46 45 48 44 48 28 29 28

All data are volume percent on the basis of 500 points counted 246 6 A juvenile ejecta radiate eastward and westward away Rhyolite from the summit region, further evidence for minor ex- Rhyodacite plosive activity continuing to the final stages of dome 5 formation. Dacite Laguna Khara (Fig. 2), a small square-shaped lake Andesite that probably represents the remnants of a once much larger lake that occupied the moat of the Pastos 2 4 KO Grandes caldera, is located just east of Chascon. Pyro- High-K clastic deposits that form an 8-m-high ridge between Field of late tertiary this lake and Chascon record pre-lava explosive activity 3 volcanics of the CVZ (Fig. 6). Med-K A thin pyroclastic deposit is also found which consists of a 0.2-m-thick matrix-supported layer with sub- 2 rounded leucocratic lithics (up to 5 cm diameter) within 60 62 64 66 68 70 72 74 76 78 SiO a white ash matrix. Above this lies a 2-m-thick clast- 2 supported block and ash flow which consists of large Chascon group Runtu Jarita group Exotic blocks poorly sorted angular blocks (up to 1 m diameter) with Chascon Dome A Granodiorite minor ash (Fig. 5A). Two types of lithics dominate this Pyroclastics Dome B Ig lithics deposit; a predominant leucocratic rock similar to the Guichi Dome C CSL (sample no. CC15) and a minor amount of meso- Domes G & H Dome E cratic, porphyritic rock that is notably vesicular or fro- Dome F 45 thy in texture (CC16). These are variants of the CSL. B At another outcrop is a thin matrix-supported basal 40 layer which consists of 70% pumice (up to 3 cm diameter) and minor andesitic fragments in an ash matrix. 35 Above this, a 1-m-thick matrix-supported heterolithic layer is present with dominant clasts of a red (CC26) 30 and green ignimbrite (CC34), minor andesitic clasts, Y and rare granitic clasts (CC17). These blocks are all of 25 exotic, subsurface origin, and the deposit has all the characteristics of a breccia blasted out during an early 20 phreatomagmatic vent-clearing episode. 15 Scattered remnants of a pumice fall deposit which consists of well-sorted pumice lapilli between 2 and 10 4 cm in diameter are found along the ridge surrounded 60 65 70 75 80 by talus blocks. Similarly, a solitary small outcrop of an SiO2 ash fall (CC32) was noted on the same flank of Chas- con overlying the pumice fall. The variable nature of all Fig. 3A, B Harker diagrams of K2O and Y. A The Chascon-Run- tu Jarita (CRJ) ranges from andesite through to high-Si rhyolite the pyroclastic deposits along this ridge, and their prox- which almost spans the entire compositional range of the Late imity to Laguna Khara coupled with their overall char- Tertiary volcanics of the Central Volcanic Zone (shaded region in acter, suggest that phreatomagmatic activity quickly A). These lavas define a high-K suite. B At a given SiO2 content, followed by effusion of CSL to form Chascon (Fig. 6). Y contents strongly discriminate the two groups of lavas Other pyroclastic deposits are not evident around the rest of the base of Chascon. On the northeastern flank, a fan of talus lies at the base of a large spoon- The highest part of the dome (5190 m) is a roughly shaped scar within the cliffs of the dome. The talus is circular area ca. 200 m in diameter near the geometric composed of two size populations, rounded blocks av- center of Chascon. The sides of this feature are marked eraging 5 cm diameter (CC19) of CSL in a loose ash by steep columnar jointed faces almost 30 m high. This matrix containing small pieces of obsidian. The location plug-like extrusion may have formed over the vent dur- and nature of this deposit at the base of the scar indi- ing the final stages of eruption. Small, rounded, poorly cate that it probably formed from a “cold avalanche” vesicular to non-expanded rhyodacitic blocks and lapilli generated during gravitational dome collapse. are found all around this vent region (CC04, CC21). A blanket of loose small lapilli (CC25) and ash Similar pyroclasts occur around the summit and vent drapes across the base of the northern flanks of Chas- region of many of the silicic lavas and domes in this con. This is believed to be a distal part of the pyroclas- region (cf. Cerro Chao; de Silva et al. 1994). We inter- tic fan that splays westward away from the central vent pret them to be the result of strombolian sensu lato ac- and represents an early explosive “puff.” Another tivity that accompanied the extrusion of the dome. prominent feature seen all around the base of Chascon Analysis of aerial photographs shows that fans of this is a surficial layer of abundant small obsidian fragments 247

Fig. 4A, B Cross-sectional A ~4km profiles across. A Cerro Chas- con; B Dome C. Note the difference in scale in the two dia- Talus apron grams. Vertical scale is exag- gerated for clarity

Monolithologic silicic rock ~400 m

B ~1.6 km

Central rift Near-vertical Magma-mixing magma-mixing

Glassy mafic Silicic core flowfront ~100 m Homogeneous vesicular mafic lava

(CC20) at the foot of the talus slopes. Their abundance long) that are generally aligned parallel to the banding. around Chascon, paucity away from the dome, and ap- The lighter bands are biotite-rich with subordinate feld- parent association with the scattered pumice fall con- spar crystals and look like CSL (Fig. 5C). The flat up- vince us that they are early pyroclastics associated with per surface of Guichi appears to be homogeneous plag- Chason. ioclase and biotite-phyric CSL.

Dome G (Morro Chascoso) and Dome H The Runtu Jarita Group (Pabelloncito Loma) Dome A These solitary domes with steep, blocky talus slopes are far smaller than Chascon but share a similar shape and This small dome, at the southeastern end of the Runtu plan. They have a stubby conical shape truncated at the Jarita Complex, has a circular plan and similar aspect summit with a small central depression. These domes ratio and truncated cone morphology as have domes G are composed of massive and structureless CSL, slight and H. The lava of dome A is a generally massive and differences being the darker groundmass with a slight homogeneous rhyolite with rare flow banding. The gray color and a markedly less friable texture than the rock type is superficially similar to CSL. It contains the typical CSL. Poorly developed banding is seen locally same mineralogy and is equally crystal rich (Table 2), on the surface of these domes. but it is chemically distinct (Fig. 3). We call it the Runtu Jarita Silicic Lava (RJSL). Mafic enclaves are more common than in the CSL. These enclaves (up to 7 cm Dome J (Cerro Guichi) diameter) are of sub-angular and cuspate form, with phenocrysts of plagioclase and pyroxene predominant Although volumetrically insignificant and barely visible in a melanocratic matrix. Other notable features of on aerial photographs, this small feature (only 6 m high dome A are the rare small pumice and rounded lapilli and 25 m long) exhibits most of the features seen at the (CC07) scattered around the base of the talus slopes. other domes. Cerro Guichi lies at the southwestern These are most abundant close to dome A and disap- edge of Laguna Khara, in the shadow of the looming pear rapidly away from the dome. They are among the eastern flank of Chascon, and has a crude tabular most evolved compositions in the entire complex and shape. It is most likely a very shallow-level intrusion indicate that minor pyroclastic activity accompanied that was subsequently exposed by erosion. It has a very dome formation. flat top and there is no evidence that it ever flowed on the surface. The base of the outcrop is composed of near-vertically banded leucocratic and mesocratic rock. Dome B (Cerro Runtu Jara) The mesocratic rock ranges from a fresh to devitrified glassy vitrophyre and obsidian with anomalously large Dome B is broadly akin to dome A, with a circular euhedral plagioclase and sanidine crystals (up to 5 cm plan, truncated conical shape, and blocky talus slopes. 248

Fig. 5 A Block-and-ash flow at the southeastern base of Chascon However, at the western base of this dome it is in con- (in the background). B Upper surface of Cerro Chascon showing tact with a dark brown scoriaceous andesite lava, the the conical mounds that characterize the surface of the dome. C Mingled lava of Cerro Guichi. Note the very large white crystals Runtu Jarita Mafic Lava (RJML), which is melanocrat- of feldspar. D View showing the mafic flanks of dome C in fore- ic with a porphyritic texture and dominant phenocrysts ground and the silicic dome B in the background. The difference of plagioclase1pyroxene. The main dome is the RJSL. in profile (aspect ratio) is evident. E Looking down into the cen- No clear junction between the two rock types was tral rift of dome C with prominent ramp structures present. Vol- can Chico in background. F Near-vertical mingling textures evi- found along the western slopes, where loose talus and dent close to the center of dome C. Note the large feldspar crys- eroded ash and crystals cover the contact. tals in the darker bands

Domes C, D, E, and F

These domes are composite domes with low flanks of RJML surrounding steep craggy central regions of 249

Fig. 6 Composite stratigraphic section for the deposits at the southeastern base of Chascon. Compare with Fig. 5A. Note 350 m Cerro Chascon lava dome the discontinuity of scale in the uppermost part

Sparse layer of pumice lapilli and obsidian ? clasts. Surrounds hase of dome. Rare ash bed Magmatic sequence

Block and ash flow Monomict composed of blocks of CSL Some blocks are vesicular 2 m Crude reverse grading Ash matirx ML~1m Some parts are strongly indurated

2 m Ash-rich layer, minor CSL lithics

Vent clearing episode Matrix-supported, heterolithic breccia of dominantly red and green welded and 1 m indurated ignimbrite blocks

sequence ML = 25 cm Matrix-supported, with fine ash Phreatomagmatic

0.1 m Pumice-rich basal layer

Scree cover- data

RJSL (Fig. 4B, 5D). A traverse across dome C high- ignment of the domes are present, best displayed at lights spectacular textures that provides key insight into Dome C, D, and E, Cerro Runtu Jarita (Figs. 2, 5E). In the formation of the Runtu Jarita Group. this central region the rift flanks are composed of RJSL The outer flanks of RJML form ca. 30-m-high flow- exhibiting exceptionally large plagioclase and sanidine fronts with well-developed ramp structures. A relative- phenocrysts (up to 2 cm long) sitting in a more pumi- ly flat upper surface is covered by a loose blanket of ceous groundmass imparting a friable texture. weathered crystals and small mounds of andesitic Dome F has a flatter, pancake-like morphology with blocks resulting from ramp structures formed during a very small raised center and is predominantly a lava extrusion and flow. flow of RJML with near-horizontal banding textures As the center is approached, the terrain steepens composed of thick bands of RJML and thin bands of and outcrops of banded or streaky lava consisting of RJSL. Only the center forms 20-m-high crags, which the main lithologies on a centimeter to decimeter scale represent ramp structures composed of near-vertically occur (Fig. 5F). Superb examples of magma mingling banded rock (thick RJSL bands and thin RJML bands). are abundant in this zone, where the orientation of the At no place on this dome are outcrops of the silicic banding steepens from sub-horizontal near the contact rock evidenced as a single, homogeneous lithology, and with the pure RJML of the outer flanks to near-vertical no linear rift passes through the center as in the other as contact with the core of rhyolite is approached Runtu Jarita composite domes. (Fig. 4B). At the center, impressive spine-like, ramp The fact that dome A is dominantly RJSL and dome structures and a linear rift trending parallel with the al- F is dominantly RJML strongly suggests lateral zona- 250 tion of the system with the mafic magma more predom- microscope, and their mineral chemistry was analyzed inant toward F and the silicic magma toward A. on a Cameca SX-50 electron microprobe at Indiana University (Bloomington, Indiana). Modal analyses of Petrography and mineral chemistry the phenocryst phases were determined on ten samples by point counting (Table 2). Representative mineral Standard-thickness polished thin-sections for 20 sam- and glass compositions are reported in Table 3. Com- ples were prepared and examined using a petrographic plete data are available in Watts (1995).

Table 3 Representative microprobe analyses of minerals from Chascon-Runtu Jarita. Plag plagioclase; Hbl hornblende; opx orthopyroxene; cpx clinopyroxene; hyp hypersthene

Chascon Group lavas Dome K–Chascon: sample no. CC01

Sani- Sani- Plag Plag Bio- Bio- Hbl Hbl Glass Glass dine dine tite tite

SiO2 64.51 64.69 58.88 59.50 37.05 37.89 44.92 46.40 75.00 75.66 TiO2 4.05 3.98 1.23 0.98 0.05 0.07 Al2O3 19.55 19.04 26.64 26.62 13.81 14.19 8.63 7.82 12.37 12.56 FeO 0.06 0.06 0.17 0.17 17.58 17.80 16.64 1.93 0.64 0.66 MnO 0.26 0.29 0.64 0.57 0.02 MgO 13.20 13.67 11.89 12.34 0.07 0.08 CaO 0.17 0.08 7.51 7.10 0.04 0.07 11.97 12.07 1.03 0.95 Na2O 2.20 2.18 6.66 6.86 0.33 0.34 1.36 1.28 2.87 2.78 K2O 13.15 13.70 0.69 0.65 9.25 9.29 1.10 0.92 5.22 5.04 Total 100.64 100.13 100.59 100.98 99.61 101.65 100.46 99.99 97.29 97.80 Or 77.60 79.65 4.04 3.80 Ab 19.80 19.28 59.10 61.10 An 0.80 0.40 36.80 34.90

Dome H: sample no. CC33

Sani- Sani- Plag Plag Bio- Bio- Hbl Hbl Glass Glass dine dine tite tite

SiO2 64.45 58.59 59.09 58.36 37.49 37.66 43.26 46.02 76.36 76.74 TiO2 3.85 3.99 1.84 1.07 0.10 0.07 Al2O3 19.43 26.44 25.65 25.81 14.10 13.85 9.67 8.00 11.52 12.26 FeO 0.07 0.17 0.16 0.22 17.55 17.36 16.48 15.92 0.50 0.60 MnO 0.34 0.34 0.56 0.52 0.08 0.03 MgO 13.09 13.41 11.02 12.15 0.06 0.05 CaO 0.10 6.79 6.40 6.48 0.06 0.02 11.35 12.04 0.66 0.78 Na2O 2.16 6.72 7.18 6.91 0.30 0.37 1.52 1.31 2.55 2.75 K2O 13.24 0.77 0.75 0.84 8.76 9.03 1.34 0.94 5.28 5.06 Total 100.21 99.52 99.23 98.67 99.62 100.11 99.05 100.01 97.14 98.35 Or 78.60 4.60 4.40 5.00 Ab 19.50 61.20 64.10 62.50 An 0.50 34.10 31.60 32.40

Runtu Jarita mafic lava Dome C: sample no. CC31 mafic flowfront

Plag Plag Plag Plag Plag Hbl Hbl Hyp Hyp Bio- Glass Glass (c) (r) (m) tite

SiO2 59.70 59.06 58.38 48.93 46.29 44.61 45.87 55.22 53.21 36.16 71.62 70.45 TiO2 0.06 1.43 1.58 0.31 0.32 4.08 0.42 0.48 Al2O3 25.82 25.55 24.58 32.79 32.51 8.41 8.73 3.49 2.30 13.46 13.48 13.67 FeO 0.21 0.19 1.28 0.63 0.61 15.50 13.12 14.83 16.72 17.43 1.54 2.39 MnO 0.49 0.39 0.46 0.39 0.41 0.01 0.13 MgO 0.01 12.16 13.51 23.33 24.81 12.85 0.39 0.75 CaO 6.08 5.95 7.94 13.42 16.51 11.57 11.59 1.45 1.44 0.02 1.36 1.67 Na2O 7.00 7.14 3.55 2.57 1.86 1.35 1.48 0.12 0.02 0.48 3.44 3.48 K2O 0.76 0.77 1.74 0.31 0.12 0.82 0.73 8.73 5.01 4.61 Total 99.56 98.70 97.54 98.71 98.19 98.38 99.08 99.21 99.21 97.58 97.39 97.73 Or 4.60 4.60 12.60 2.00 Ab 64.50 65.20 39.00 25.20 An 30.90 30.10 48.20 72.70 251

Table 3 Continued

Dome C: sample no. CC14 mafic flow center

Sani- Sani Sani- Plag Plag Plag Plag Hbl Hbl Opx Opx Bio- Glass Glass dine dine (c) dine (r) (c) (r) (m) tite

SiO2 65.08 64.90 69.53 48.05 58.08 53.33 50.06 42.35 45.59 53.73 54.27 37.54 69.11 69.19 TiO2 0.11 3.16 1.11 0.23 0.25 0.40 0.55 Al2O3 19.39 19.35 17.35 33.71 26.61 29.21 28.94 11.19 7.92 1.73 1.76 14.18 13.51 13.94 FeO 0.03 0.05 0.93 0.64 0.20 0.86 1.03 12.10 15.36 16.58 15.48 17.47 1.82 1.89 MnO 0.03 0.14 0.56 0.41 0.41 0.25 0.03 MgO 0.05 13.64 12.18 25.43 25.92 13.56 0.42 0.58 CaO 0.10 0.09 2.68 14.46 7.39 10.86 13.04 11.61 11.81 1.27 1.38 0.10 1.39 1.62 Na2O 2.93 2.98 4.69 2.32 6.49 3.42 3.45 1.98 1.20 0.03 0.52 3.38 3.47 K2O 12.38 12.36 3.53 0.11 0.81 0.78 0.40 0.85 0.91 8.55 4.70 4.58 Total 99.95 99.72 98.87 99.31 99.58 98.48 97.29 99.07 98.68 99.37 99.51 100.18 94.84 96.00 Or 73.10 72.90 27.20 0.70 4.80 5.10 Ab 26.30 26.70 55.00 22.30 58.40 34.40 An 0.50 0.40 17.40 77.00 36.80 60.40

Runtu Jarita mixed and silicic lava Dome C: sample no. CC13 silicic phase

Sani- Sani- Sani- Plag Plag Plag Plag Bio- Glass dine dine dine tite

SiO2 66.03 66.01 66.60 52.83 59.18 61.48 61.80 36.98 76.02 TiO2 4.22 0.06 Al2O3 19.29 19.15 19.15 30.86 26.28 25.47 24.34 13.08 12.35 FeO 0.03 0.09 0.03 0.18 0.29 0.14 0.11 22.27 0.61 MnO 0.53 0.01 MgO 9.78 0.06 CaO 0.11 0.11 0.12 12.00 7.15 5.87 5.01 0.06 0.98 Na2O 2.94 2.93 2.95 4.19 7.01 7.87 8.07 0.39 2.85 K2O 12.74 12.71 12.50 0.22 0.47 0.76 0.99 8.69 5.12 Total 101.14 101.03 101.37 100.29 100.39 101.63 100.38 100.10 100.10 Or 73.60 73.60 73.10 1.30 2.80 4.30 5.60 Ab 25.80 25.80 26.20 38.20 62.20 67.70 70.20 An 0.50 0.50 0.60 60.40 35.00 27.90 24.10

Dome C: sample no. CC13 mafic phase

Sani- Sani- Plag Plag Plag Plag Opx Cpx Bio- Glass Glass dine (c) dine (r) (c) (r) (c) (r) tite

SiO2 66.34 68.05 46.44 48.05 46.92 47.29 53.95 50.72 37.05 70.95 76.74 TiO2 0.27 0.84 4.12 0.23 0.07 Al2O3 19.32 19.35 33.64 32.12 34.49 34.60 1.30 3.54 13.96 13.43 12.26 FeO 0.13 0.71 0.54 0.59 0.49 0.60 17.68 7.23 17.63 1.02 0.6 Fe2O3 1.60 MnO 0.56 0.17 0.32 0.02 0.03 MgO 24.83 14.02 12.69 0.45 0.05 CaO 0.12 3.79 15.52 13.89 16.71 15.96 1.25 21.43 0.18 2.22 0.78 Na2O 2.94 5.16 2.01 3.06 1.68 1.84 0.01 0.33 0.38 3.83 2.75 K2O 12.63 2.72 0.12 0.24 0.11 0.09 9.01 4.23 5.06 Total 101.49 99.77 98.36 98.03 100.43 100.38 99.85 99.89 99.37 96.38 98.35 Or 73.40 19.80 0.80 1.50 0.70 0.60 Ab 26.00 57.10 18.80 28.00 15.30 17.10 An 0.60 23.20 80.20 70.40 84.00 82.30

Chascon Group ioclase often exhibits undulose extinction along with a rare glomerophyric texture. A feature of the plagio- The CSL is a crystal-rich rock (ca. 45–48 vol.%) with clase in these domes is their homogeneity and small phenocrysts of plagioclase1quartz1biotite1sanidine range in composition. Large crystals of rounded quartz 1hornblende. Seriate textured plagioclase (25 vol.%) is (12 vol.%) reach 5 mm in size (average 2 mm) with generally homogeneous and andesine in composition some examples having corroded rims or an embayed (An33–An44), with sparse normally zoned crystals (Ta- appearance. Phenocrysts of sanidine (4 vol.%) have a ble 3; sample CC01). The euhedral and subhedral plag- compositional range of Or73An01Ab26 to Or80An0.5Ab26 252 and average 2 mm in size, with highly corroded edges difference is the microlite-rich nature of the ground- and a fractured nature. In contrast, ubiquitous laths of mass in these domes. Dome H (50 vol.% microlites) euhedral biotite (average 1 mm) are scattered, some- and G (75 vol.% microlites) have abundant plagioclase times clustering together and generally aligned with the and minor biotite laths randomly oriented in a poorly flow texture of the groundmass. Reaction rims were vesicular glass (76 wt.% SiO2). noted on many, but not all, of the biotite laths. The blocks from the block-and-ash flow are similar Minor proportions of subhedral hornblende to the Chascon Silicic Lava. The abundant leucocratic (2 vol.%) occur, often associated with biotite clusters. blocks (CC15) within the breccia have a moderate crys- Rare examples of completely altered hornblende crystal content (ca. 30 vol.% crystals) and finely porphyritic tals were also observed. Crystals of sphene (~1 vol.%) texture with microphenocrysts of plagio- with typical diamond-shaped habit and rare Fe–Ti ox- clase1quartz1biotite1hornblende. All these crystals ides (commonly Ti-magnetite, rare ilmenite) are also lie within a dominantly glassy, microlite-poor ground- present. All of these crystals lie within fresh rhyolitic mass. The mesocratic blocks (CC16) of the breccia are glass (76 wt.% SiO2) that is overall poorly but perva- the most crystal rich (ca. 60 vol.% crystals) of the sam- sively vesicular and notably microlite free (Fig. 7A). ple set, with phenocrysts (up to 8 mm) of plagio- Domes G and H have similar phenocryst contents clase1quartz1biotite1hornblende. A significant fea- and assemblage to those of Chascon. The only marked ture of this rock is the extremely vesicular nature of the groundmass. All of the vesicles (ca. 75 vol.%) appear stretched along the same direction. The ignimbrite blocks in the breccia have a porphy- Fig. 7 A Photomicrograph (plane-polarized light, PPL) of Chas- ritic texture with predominant seriate plagioclase and con Silicic Lava. Phenocrysts of plagioclase, quartz, and biotite in a fresh vesicular rhyolitic glass. Field of view (FOV) is 4 mm. B quartz, both of which appear fractured. Minor biotite, Photomicrograph (crossed nicols, XPL) of Runtu Jarita Mafic hornblende, opaques, and chert fragments are also lava. Seriate plagioclase, some with sieve textures, and horn- present. This crystal-rich rock (ca. 50 vol.% crystals) blende in a plagioclase-microlite rich groundmass (FOV is 4 mm). has an ashy matrix with a pervasive but indistinct eu- C Photomicrograph (in XPL) of large sanidine xenocryst with ter- taxitic texture. The “granitic” blocks (CC17) have a nary composition rim in the RJM of mixed rock from Runtu Jari- 1 ta (FOV is 4 mm). D Photomicrograph (in XPL) of the boundary phaneritic texture with crystals of plagioclase K-feld- between RJSL and RJM within co-mingled lava (FOV is 4 mm) spar1quartz1biotite1hornblende along with minor 253 chlorite, apatite, and opaques. This rock is a granodio- posed of blocks of a glassy plagioclase-phyric andesite. rite. Seriate plagioclase (18 vol.%) is evident as two discrete Glass in an obsidian sample (CC20) has an average populations, one consisting of fresh euhedral crystals, 73 wt.% SiO2. The obsidian is characteristically aphyr- and contrasting with this, a set of larger subhedral crys- ic, containing only sparse crystals of euhedral biotite tals. The latter population exhibits classic sieve textures and magnetite. Close examination of other obsidian (Fig. 7B). The fresh plagioclase straddles a wide com- clasts highlighted a full spectrum of devitrification tex- positional range from An30–An82 with distinct clusters tures. A pumice sample (CC25) is predominantly com- near each end member (Fig. 8), i.e., andesine posed of vesicular, isotropic glass averaging 76 wt.% (An30–An34; type A) and bytownite (An78–An82; type SiO2. Rare small crystals of angular plagioclase (An28), B). In the latter group, there are common examples of subhedral biotite, and magnetite were also detected. sieved plagioclase exhibiting reverse zoning from Dome J (Guichi) is not only unique for its small size An48–An78 (type C). Microphenocrysts of subhedral but also for the high degree of intimate mixing. The augite and hypersthene (4 vol.%), hornblende two main lithologies are a glassy vitrophyre and the (4 vol.%) commonly associated with clusters of plagio- CSL. The CSL here is characterized by conspicuous, clase, and euhedral biotite (2 vol.%) are also present. large (up to 5 cm) euhedral plagioclase (20 vol.%). These crystals lie in a groundmass predominantly com- These crystals are of andesine composition posed of rhyodacitic glass (70 wt.% SiO2) and plagio- (An34–An42), exhibit undulose extinction, and some- clase microlites. times occur as intergrown clusters. Anhedral quartz The central parts of the mafic flow are similar to the (8 vol.%), sanidine, ubiquitous small laths of biotite flowfront, with seriate-textured plagioclase popula- (3 vol.%), subhedral hornblende (1 vol.%), and rare tions; however, numerous xenocrysts are also evident crystals of sphene and apatite were also noted. All (Table 4). Xenocrysts of rounded quartz (up to 3 mm) these crystals are in a variably vesicular glassy ground- and subhedral sanidine (up to 5 mm) are common; both mass. In the vitrophyre, large xenocrysts of sanidines types have a fractured appearance and display reaction have distinct glassy reaction rims. rims of ternary composition (Figs. 7C, 8). Micropheno- crysts of hornblende (5 vol.%), augite, and hypersthene (4 vol.%), along with minor quantities of biotite (2 vol.%), and ubiquitous opaque minerals (magnetite Runtu Jarita Group and ilmenite) are also present. All these crystals sit in a groundmass with dominantly rhyodacitic glass (70% Sampling across a transect of dome C enabled a de- SiO2; Table 3) containing plagioclase/hornblende mi- tailed analysis of the range in lithologies that character- crolites. Rare rhyolitic glass (76 wt.% SiO2) was also ize the group-II domes. The mafic flowfront is com- found.

Fig. 8 Ternary diagrams of feldspar compositions from rocks of the Chascon-Runtu Jarita Complex. Tie lines con- nect core-rim compositions. Note the presence of ternary composition rims on sanidine in the Runtu Jarita mafic and mixed rocks. Type “A” refers Cores /microphenocrysts to phenocrysts from the silicic Rims magmas; Type “B” is pheno- Or crysts from the mafic magmas; x Intermediate . Type “C” refers to type-A .... . Microlites plagioclase incorporated into the mafic magma and now reversely zoned. See text for further discussion Runtu Jarita - Silicic

Type “A” Mixed Type “B”

Runtu Jarita - Mafic Type “C”

Dome K - Cerro Chascon

An Ab 254

Table 4 Temperature and fO2 estimates based on mineral equilibria for the Chascon-Runtu Jarita Group. Mag magnetite (spinel); Ilm ilmenite (rhombohedral); Usp isulvospinel

Sample no. Runtu Jarita Chascon

Mafic lava Silicic Lava

CC13 CC13 CC06 CC06 CC14 CC14 CC14 CC14 CC04 CC04 CC04 CC04 Mag Ilm Mag Ilm Mag Ilm Mag Ilm Mag Ilm Mag Ilm

SiO2 0.06 0.02 0.06 0.04 0.10 0.03 0.04 0.02 0.34 0.00 0.47 0.00 TiO2 10.80 36.58 10.12 38.01 10.07 38.67 4.30 47.54 4.12 47.36 2.69 47.36 Al2O3 1.30 0.55 3.58 0.55 3.67 0.44 1.07 0.08 1.17 0.02 0.54 0.02 FeOa 38.76 27.86 44.21 32.54 35.95 29.42 33.03 37.95 31.68 37.83 30.91 37.83 a Fe2O3 46.35 30.22 36.66 21.65 45.63 26.32 58.30 8.54 53.82 7.74 56.58 7.74 MnO 0.44 0.34 0.34 0.39 0.42 0.39 1.29 3.41 0.60 3.26 0.40 3.26 MgO 1.20 2.65 2.8 2.42 2.85 2.80 0.27 0.77 0.55 0.81 0.27 0.81 CaO 0.03 0.00 0.01 0.03 0.01 0.02 0.00 0.01 0.12 0.01 0.09 0.01 Total 98.93 98.22 97.61 96.61 98.71 98.08 98.29 98.31 92.39 97.04 91.95 97.04 log(Mg/Mn) 0.69 1.14 0.91 0.82 1.08 1.10 –0.44 –0.40 0.21 –0.36 0.08 –0.36 X(usp)b 0.32 0.31 0.31 0.13 0.13 0.09 X(ilm)b 0.69 0.72 0.73 0.91 0.92 0.92 T(7C)c 956 964 926 683 677 638 log f(O2) –9.77 –9.87 –10.29 –15.79 –16.18 –16.80 2-Feldspar 1 635–727 650–750 2 623–713 620–747 2-Pyroxene 1 1006 961–1225 2 1030 976–1095

Oxides are in weight percent a Calculated assuming charge-balanced stoichiometric formula 2-Feldspar temperatures; 1 after Stormer (1975), 2 after Powell b Calculated following Stormer (1983) and Powell (1977) c Calculated using Andersen and Lindsley (1988) 2-Pyroxene temperatures; 1 after Wood and Banno (1973), 2 after Wells (1977)

Samples of material taken from the banded rock re- Temperature and oxygen fugacity estimates veal abundant evidence of mixing between the two magmas. The boundary between them is sharp and the Estimates for the temperature and oxygen fugacity phases distinct, reflecting the strong contrast in rheolo- (ƒO ) of each group are given in Table 4. gy of the two phases. Complete homogenization has 2 Homogeneity of the Fe–Ti oxides was ascertained by not taken place (Fig. 7D). Close inspection along this multiple analyses of a single grain and investigation of boundary suggests that many larger crystals in the silic- core-to-rim zonation. Equilibrium between coexisting ic phase have been “plucked” away and dragged into magnetite and ilmenite pairs was assessed using the the less-viscous mafic phase. Reaction rims and sieve Mg–Mn partitioning following Bacon and Hirschmann textures are abundant in the large plagioclase xeno- (1988). For the CSL temperatures of equilibration are crysts of the mafic phase, but no such textures are 630–680 7C and log ƒO is –16.17 to –16.8. For the found in the crystals of the silicic phase. The mixing ap- 2 RJML, temperatures of equilibration were between pears to be dominated by the one-way movement of 926–964 7C and log ƒO –9.8 and –10.3, and for the crystals from silicic to mafic lava, as highlighted in a ter- 2 RJSL, 683 7C and log ƒO –15.8. A co-existing pair of nary diagram of feldspar compositions (Fig. 8). 2 Fe–Ti oxide inclusions within a sanidine xenocryst The plagioclase of the mafic phase is more anorthite (found within the mafic lava) yielded a temperature of rich ranging between An and An , dominantly by- 45 83 680 7C and log ƒO of –15.8. These estimates are con- townite (type B; Table 3). Normal and reverse zoning 2 cordant with those from 2-feldspar and 2-pyroxene of the plagioclase in the mafic phase is common, and equilibria from the respective lavas. This lends confi- the large sanidine xenocrysts exhibit strong reverse dence to these estimates. zonation cores of Or73Ab26 to rims of ternary composition Or20Ab57An23 (Figs. 7C, 8; Table 3). Micropheno- crysts of hypersthene, rare Fe–Ti oxides, and biotite with reaction rims are also present. The plagioclase in Summary and interpretation of mineral chemistry the RJSL of the banded rock ranges from An30 to An61 and is dominantly andesine (Type A). Sanidine is ho- The silicic lavas of the two groups are broadly similar, mogeneous at Or73, biotite is relatively Fe rich, and the but there are some subtle differences in mineral chem- glass in the RJSL is typically a high-Si rhyolite (76% istry. Sanidine crystals of the Chascon Group are gen- SiO2). erally more potassic, Or80 vs Or73, whereas biotite is 255 less Fe rich and amphibole less Mg rich than those in tion to unravel the nature of the eruption. Broader as- the RJSL. This confirms the geochemical distinction pects of the geochemistry such as the petrogenesis of (Fig. 3) of the two groups. the magmas in a regional context, have been addressed There is strong evidence for disequilibrium through- elsewhere (de Silva et al. 1994; Watts 1995). out the RJML and mixed rocks of Runtu Jarita Group. Harker diagrams of the major elements illustrate a The mixed rocks and some samples of what appear to general family resemblance among the magmas of the be homogeneous dacite contain xenocrysts of plagio- CRJ. The range of compositions is related by fractional clase with reverse zoning, sieve textures, and sanidine crystallization and/or magma mixing. The trace ele- showing reaction textures and strong reverse zonation. ments, particularly Ba, Rb, Sr, Zr, Nb, Th (Fig. 9), are Some samples also contain two distinct glass composi- more informative and clearly show that the two groups tions, a rhyodacite and the high-Si rhyolite of the RJSL. were very different systems with different histories. Rare pargasitic amphiboles are also present. The presence of evolved plagioclase compositions (type A) and reversely zoned (type C) plagioclase in the RJML of Chascon Group the flow fronts indicates that the RJML is also a hybrid. In order to allow reversely zoned plagioclase and re- The compositions of domes K (Chascon), G, and H are versely zoned and reacted sanidine to develop in the dominantly rhyodacitic, and they define a coherent RJML, there had to have been sufficient time before it group, the CSL. The pyroclastic rocks associated with finally intruded into the rhyolite. We believe that the Chascon, except for the more evolved obsidian and pu- intrusion of the RJML into the RJSL occurred just pri- mice fall, are of this composition as well. The rare an- or to eruption and that the length- and time scale of desitic rocks have relatively low Sr, Zr, and Ba. The this mixing and mingling event is too small to cause the leucocratic (CC15) and mesocratic (CC16) blocks from disequilibrium features seen. This is supported by the the block-and-ash flow are almost identical to each oth- chemical homogeneity of the oxides. If the RJML and er in composition supporting the field and petrographic RJSL mingling event was prolonged or extensive, the evidence that they are the same rock type and that they Fe–Ti oxides could be expected to have recorded the are well within the ranges defined by the CSL. increase in temperature. Generally, Ba increases while Sr, Rb, Th, and less We therefore suggest that the RJML is a hybrid pro- convincingly Nb and Y, all decrease from the rhyoda- duced by mixing a more mafic andesite with the RJSL. cite to the rhyolite. Zr and Sr show divergent behavior This hybrid contained xenocrysts from the RJSL – for the pumice fall and the obsidian; they both decrease type-A plagioclase which develop into the reversely in the latter. Cerro Guichi is unique and does not ap- zoned type-C plagioclase, altered amphiboles, and pear to be directly related to the other domes. reacted and reversely zoned sanidines – as well as its Although the relationships are somewhat complex, own phenocrysts (type-B plagioclase, hypersthene, and domes K, H, and G and the associated pyroclastic rocks augite). Rare paragasite crystals would be artifacts of excluding the obsidian are easily reconciled by fraction- magma mixing. The RJML would then later intrude the ation of observed phenocrysts and inferred minor RJSL to produce the mixed rock and exchange more phases. The lower Zr and Sr of the obsidian are diffi- crystals (type-A plagioclase). Herein we present a mod- cult to fit into a fractionation relationship with the oth- el in which this intrusion event was the trigger and im- er lithologies. petus for the eruption. The andesite of Chascon has lower Sr, Zr, and Ba. The red and green welded ignimbrites from breccia at the base of Chascon are virtually identical geochemical- Whole-rock geochemistry ly. These samples are interpreted as different levels of intracaldera Chuhuilla ignimbrite from the Pastos Thirty samples from the CRJ were analyzed by X-ray Grandes caldera moat exhibiting different degrees of fluorescence for the 10 standard major elements and 17 welding. trace elements. Major element analyses were perform- The composition of the plutonic blocks (CC17) col- ed on fused glass beads, while trace elements were ana- lected from the phreatomagmatic deposits generally lyzed on pressed powder pellets using a fully automat- falls within the ranges of the domes but is notably de- ed Philips PW1400 X-ray fluorescence spectrometer at pleted in Th and Rb. The significance of this remains the University of Southampton, UK. All complete ana- speculative, but an interesting possibility is that this lyses (samples for which major- and trace elements block is part of the Pastos Grandes caldera plutonic were available) are given in Tables 5 and 6; all data are complex. available in Watts (1995) and Dunne (1998). Herein we discuss key aspects of the geochemistry with the aim of illuminating the volcanology of the complex. Our em- Runtu Jarita Group phasis here is to understand the nature of the pre-eruption magma system and to emphasize the geochemical Two end-member magma compositions, dacite (RJML) evidence for recharge and mixing and use this informa- and rhyolite (RJSL), are found in domes A through F. 256

Table 5 Representative major and trace element analyses of Chason-Runtu Jarita lavas

Chascon Group

Chascon (Dome K)

Sample no. CC22 CC01 CCO2 CCO3 CC24 96014 Vitro- Chas. Chas. Chas. Vitro- Enclave phyre Porph Porph Dome phyre

SiO2 69.98 69.39 69.41 72.10 63.19 62.69 TiO2 0.44 0.47 0.47 0.33 0.75 0.76 A12O3 14.90 14.75 15.16 14.02 17.05 17.35 Fe2O3 2.87 2.99 2.96 2.49 5.28 5.33 MnO 0.05 0.48 0.05 0.06 0.07 0.07 MgO 1.06 1.11 1.14 0.87 2.15 2.20 CaO 2.69 2.85 2.85 2.33 4.87 4.99 Na2O 3.42 3.37 3.38 3.46 3.29 3.33 K2O 4.48 4.49 4.48 4.27 3.17 3.12 P2O5 0.10 0.11 0.11 0.07 0.18 0.18 Rb 252 259 266 257 192 193 Sr 258 265 230 271 203 208 Ba 602 601 477 622 592 587 Y 151419151514 Zr 157 148 137 165 144 145 Nb 16 16 17 16 15 15 Th 29 31 30 32 19 20 Ni222125 Cr9236756 V 555849592731 U 121313148 8 Pb 28 31 30 30 25 26 Zn 56 58 50 57 42 44 Ga 21 21 20 22 14 13 Cu353244

Chascon Group

Chascon pyroclastics

Sample no. CCO4 CC21 CC19 CC32 CC15 CC16 CC20 CC25 Chas. Chas. Block Chascon Leuco- Meso- Chas. Chas. Lapil Lapil and ash cratic cratic obsid pumice ash block block

SiO2 70.39 68.39 69.68 69.65 69.18 69.10 75.26 76.15 TiO2 0.42 0.54 0.44 0.48 0.47 0.44 0.17 0.16 A12O3 14.90 15.25 15.10 14.51 14.85 14.81 13.40 12.98 Fe2O3 2.70 3.48 2.94 3.30 2.97 3.08 1.11 1.17 MnO 0.05 0.06 0.05 0.05 0.05 0.05 0.05 0.04 MgO 0.96 1.35 1.11 1.76 1.35 1.37 0.22 0.24 CaO 2.57 2.78 2.81 2.97 3.01 3.17 1.15 1.23 Na2O 3.39 3.31 3.32 3.18 3.36 3.51 3.54 3.19 K2O 4.52 4.74 4.43 3.98 4.65 4.36 5.08 4.81 P2O5 0.09 0.11 0.11 0.11 0.12 0.11 0.02 0.02 Rb 236 283 255 225 259 249 242 204 Sr 287 256 268 302 262 280 154 250 Ba 645 605 609 551 580 579 768 780 Y 1617141314142219 Zr 159 179 151 155 147 150 141 205 Nb 15 17 15 14 15 16 17 15 Th 27 34 30 25 30 28 29 21 Ni33273213 Cr 14 12 12 25 88 14 1 4 V 726659815466822 U 121313131313109 Pb 28 31 31 26 29 28 31 26 Zn 64 66 57 62 57 55 47 60 Ga 21 22 21 20 21 20 17 18 Cu1064533038 257

Table 5 Representative major and trace element analyses of Chason-Runtu Jarita lavas (Continued)

Chascon Group Runtu Jarita Group

Chascon Group domes Mafic flanks

Sample no. CC23 CC30 CC33 Sample no. CC10 CC14 CC31 Guichi Dome G Dome H Dome C Dome C Dome F Dome J

SiO2 72.91 68.55 68.27 SiO2 63.50 63.74 63.66 TiO2 0.28 0.50 0.53 TiO2 0.73 0.71 0.72 AI2O3 14.00 15.41 15.63 A12O3 16.64 16.38 16.66 Fe2O3 1.77 3.29 3.50 Fe2O3 5.18 5.12 5.04 MnO 0.05 0.05 0.06 MnO 0.07 0.07 0.07 MgO 0.80 1.25 1.37 MgO 2.12 2.09 2.01 CaO 1.73 3.07 3.14 CaO 4.71 4.77 4.71 Na2O 3.56 3.46 3.15 Na2O 3.40 3.44 3.48 K2O12 4.84 4.30 4.21 K2O12 3.47 3.50 3.48 P2O5 0.05 0.12 0.14 P2O5 0.17 0.17 0.17 Rb 239 251 244 Rb 179 182 176 Sr 187 279 255 Sr 387 387 397 Ba 684 578 627 Ba 635 608 657 Y 20 16 17 Y 21 23 21 Zr 150 165 156 Zr 185 182 192 Nb 16 16 16 Nb 14 14 14 Th 28 30 29 Th 16 18 16 Ni122Ni231 Cr 5 14 6 Cr 10 10 11 V 18 61 47 V 117 119 125 U 101310U 666 Pb 30 28 30 Pb 22 22 22 Zn 49 59 54 Zn 72 74 74 Ga 18 22 20 Ga 23 23 21 Cu134Cu668

Runtu Jarita Group

Mixed zones Silicic lavas

Sample no. CCO6 CCO8 CC11 CC13 CC28 CC29 CCO7 CCO9 CC12 CC27 Dome A Dome B Dome C Dome C Dome E Dome F Dome A Dome B Dome C Dome E lapilli

SiO2 65.25 69.41 75.46 73.46 67.37 69.30 74.92 73.86 76.27 76.04 TiO2 0.65 0.47 0.19 0.28 0.55 0.48 0.20 0.27 0.13 0.16 A12O3 16.00 15.16 12.98 13.71 15.74 15.47 13.19 13.22 13.00 12.90 Fe2O3 4.62 2.96 1.51 2.06 3.73 3.11 1.67 2.05 1.05 1.20 MnO 0.07 0.05 0.04 0.05 0.06 0.05 0.06 0.06 0.04 0.04 MgO 1.87 1.14 0.35 0.66 1.44 1.20 0.47 0.65 0.18 0.27 CaO 4.42 2.85 1.41 1.96 3.47 2.90 1.43 1.86 1.04 1.28 Na2O 3.40 3.38 3.24 3.21 3.37 3.23 3.42 3.52 3.15 3.19 K2O 3.60 4.48 4.78 4.56 4.15 4.15 4.60 4.47 5.12 4.90 P2O5 0.14 0.11 0.03 0.05 0.12 0.12 0.04 0.05 0.01 0.02 Rb 161 206 328 296 231 249 431 359 352 326 Sr 433 354 86 141 311 272 82 160 55 78 Ba 721 544 161 251 624 518 134 257 118 156 Y 20223026192044382928 Zr 190 173 99 106 161 161 98 116 85 96 Nb 14 16 20 19 16 17 29 25 20 20 Th 15 19 38 32 25 30 30 30 39 40 Ni2222332212 Cr7124412125449 V 116 97 17 32 74 65 21 42 4 7 U 6 72016101325212120 Pb 18 23 37 33 27 29 43 36 39 37 Zn 65 63 33 37 61 60 34 40 28 31 Ga 23 21 18 18 22 22 19 19 16 17 Cu5410331100

Major elements are recalculated on an anhydrous basis 258

Table 6 Representative compositions of juvenile and exotic blocks in deposits associated with Cerro Chascon

Block-and-ash flow Xenolith blocks

Sample no. CC15 CC16 CC17 CC26 CC34 Leucocratic Mesocratic Granitic Ignimbrite Welded ignimbrite

SiO2 69.18 69.11 68.47 68.06 68.20 TiO2 0.48 0.44 0.60 0.55 0.54 Al2O3 14.84 14.81 15.01 15.35 15.52 Fe2O3 2.97 3.08 3.43 3.62 3.30 MnO 0.05 0.05 0.05 0.04 0.04 MgO 1.35 1.37 1.58 1.52 1.12 CaO 3.01 3.16 3.51 3.42 3.39 Na2O 3.36 3.51 3.55 3.44 3.59 K2O 4.66 4.36 3.67 3.90 4.18 P2O5 0.12 0.11 0.12 0.11 0.12 Rb 259 249 169 198 216 Sr 261 280 349 327 326 Ba 580 578 609 577 608 Y1414151614 Zr 147 149 124 137 126 Nb 15 16 14 15 13 Th 30 28 9 24 24 Ni 3 1 3 2 3 Cr 88 13 23 17 16 V5465857791 U 13 12 5 11 13 Pb 28 27 21 25 20 Zn 57 54 50 60 55 Ga 20 20 18 19 20 Cu 2 3 1 5 5

Major element recalculated on an anhydrous basis

The dacite is dominant from C toward F, and the rhyol- cal relations of the Runtu Jarita Group, an incompati- ite grows in dominance from C to A. The field and pe- ble- (Th) vs compatible-element (Sr) diagram can be trologic evidence for mixing is corroborated by the geo- used on which magma mixing gives linear trends while chemical relations. Samples taken from the banded fractional crystallization gives curved trends (Fig. 10). zones exhibit the widest compositional range (67–76% From this it is clear that mixing is the primary mecha- SiO2). Small variations are apparent in some major ele- nism that accounts for the range of compositions within ments including K2O (4.15–4.79%) and Na2O this group. Fractionation within the rhyolitic composi- (3.24–3.37%), whereas larger variations are exhibited tions is evidenced by the most extreme compositions of by CaO (1.41–3.47%) and Fe2O3 (1.51–3.73%). Trace domes A and B, where Rb, Y, and Nb are markedly element variations are most notable for Ba enriched while Th is depleted. This would be consistent (161–623 ppm) and Sr (86–311 ppm), Y (16–29 ppm), with fractionation of accessory phases, such as chevkin- and Th (25–38 ppm). ite or allanite, to remove Th in these minimum melt Close attention to the analyses for dome C, the most compositions. We interpret these evolved compositions rigorously sampled dome in this study, highlights the as the uppermost part of a zoned RJSL magma batch, variations across one dome from mafic flank to silicic which the RJML would have intruded. core. Major elements increase significantly in SiO2 (63.5–76.27%) and K2O (3.48–5.13%), whereas decreases are noted in Al2O3 (16.64–13.00%) and Fe2O3 (5.18–1.04%). Trace elements also display significant Summary and interpretation of geochemistry increases for Rb (179–352 ppm) and Th (16–38 ppm), whereas Sr (386–55 ppm) and Ba (634–118 ppm) exhi- In summary, the geochemical relations of the CRJ show bit considerable decreases. some broad similarities. There is a general family re- The silicic rocks of the core zones are high-K rhyol- semblance of the rocks in this complex to one another ites, the most evolved lava of the entire CRJ complex and to the silicic rocks that have been erupted through- (73–76% SiO2). The coherent linear trend on all these out the history of the APVC. Rocks in both groups of diagrams is in stark contrast to the more complex rela- domes range from andesite to rhyolite, the felsic mag- tions of the Chascon Group. To confirm that mixing is ma was zoned, and there is evidence for recharge; how- the primary mechanism responsible for the geochemi- ever, there are important differences. The two systems 259

450 250

400 4 3 200 350

300 150 Zr Rb 250 Obsidian 100 2 200 1 50 150

100 0 6065 707580 60 65 70 75 80 500 30 450 400 25 350 300

Sr 20 250 Nb 200 150 15 100 50 0 10 60 65 70 75 80 60 65 70 75 80 800 45

700 40

600 35

500 30

400 25 Ba Th

300 20

200 15

100 10

0 5 60 65 70 75 80 60 65 70 75 80 SiO2 SiO2

1. Fractionation within Chascon group magmas. Compositional gap between andesites and rhyodacties is seen as the result of recharge; andesite intrudes into the rhyodacite. 2. Mixing within the Chascon group. Cerro Guichi is best explained by mixing between Chascon rhyodacites and the obsidian. 3. Mixing within Runtu Jarita andesite and rhyolites.

4. Fractionation within the Runtu Jarita rhyolite.

Fig. 9 Harker diagrams of Ba, Rb, Sr, Zr, Nb and Th for the Chascon-Runtu Jarita Complex. Symbols as in Fig. 3. Four (1–4) different evolutionary schemes are required to explain the relationships among the various lithologies. Note the strong similarity of the ignimbrite blocks in the explosion breccia to the Chascon Silicic Lava. The granodiorite is broadly similar but depleted in Rb and Th 260

450 these two magmas produced the observed range of Runtu Jarita mixing model compositions. The strong chemical interaction between 400 0 the mafic RJML and silicic RJSL indicates a strong 350 physical interaction. The Runtu Jarita group threrefore represents a single eruptive event fueled by recharge of 300 50 the RJML into the zoned RJSL magma chamber. 250 Sr 200 150 Eruption mechanism 4 100 Fractional Characteristics of the Chascon-Runtu Jarita Complex crystallization 50 overwhelmingly point to the prominent role of magma 100 mixing in the evolution and eruption of each group of 0 0 10 20 30 40 50 domes. Abundant evidence has been described in the Th form of macroscopic, microscopic, and geochemical data to support this. Fig. 10 Th (incompatible)/Sr (compatible) variation diagram In considering the eruption mechanisms for these highlighting the role of mixing in controlling the compositional range of the Runtu Jarita Group. The trend shown uses end two groups of domes, there are some common factors members from dome C. Contrast this with the trend that would that we need to emphasize in order to better constrain be expected from fractional crystallization. Note that fractiona- the eruptive processes. tion has occurred within the rhyolitic compositions (path 4 from 1) The crystal-rich character of the silicic lava. It is Fig. 9) to deplete Th in the dome A rhyolite. See text for discussion generally accepted that magma with a high degree of crystallinity is difficult to move or erupt and will continue to crystallize and form a pluton (Marsh 1981). For Cerro Chascon, the ca. 48 vol.% crystal content and had very different and separate histories and were high SiO2 content of the interstitial melt suggest that probably separated in time. The Chascon Group is this mass of magma may have been close to its rheo- dominantly rhyodacite with trivial amounts of andesite logic limit. Indeed, estimates for the yield strength and and high-Si rhyolite. The evolved compositions, except apparent viscosity of the RJML and RJSL magmas (Ta- for the obsidian, can be related by simple crystal frac- ble 7) of the CRJ yield extremely high apparent viscosi- tionation to produce a system zoned from the rhyoda- ties for the rhyolitic magma, consistent with those esti- cite of the CSL to more evolved compositions repre- mated for other similar lavas (e.g., Pinkerton and Stev- sented by the pumice fall. The andesite of Chascon can- enson 1992; de Silva et al. 1994; Manley 1996) and high- not be parental to the system because of its lower Sr, light a considerable difference in viscosity spanning six Zr, and Ba; instead, its association can be best under- orders of magnitude that exists between the two mag- stood in terms of a separate late stage recharge into the ma types. CSL magma with little or no chemical interaction. We The advanced state of crystallization of the evolved are conviced that the obsidian clasts were erupted dur- rocks of the CRJ indicates a protracted and advanced ing the short explosive magmatic phase that produced stage of evolution. We believe that the physical proper- the pumice fall. However, the chemistry of one obsid- ties (high crystallinity, high apparent viscosity, relative- ian clast is difficult to reconcile into a simple relation ly low magmatic temperature) of these magmas are with the rest of the evolved rocks. Furthermore, the si- consistent with it being in an early stage of pluton for- multaneous eruption of volatile-rich and volatile-poor mation. We consider the crystal-rich rhyolitic magmas tephra seems paradoxical. We therefore prefer to inter- to have been close to their solidii when recharge oc- pret the obsidian as chilled magma from a previous epi- curred and posit that without recharge these magmas sode. This episode may be represented by Cerro Gui- would not have erupted. chi, whose chemical (and physical) character is best un- 2) The paucity of explosive precursor to the dome derstood in terms of mixing between CSL and the ob- eruptions. Explosive activity driven by magmatic vola- sidian. At the onset of the Chascon eruption, this layer was probably blasted out along with juvenile pumice lapilli and all these clasts settled on the caldera floor be- Table 7 Estimates of magma properties for the Runtu Jarita lav- neath the growing dome. Each of the domes K, H, and as based on method of Pinkerton and Stevenson (1992) G was probably a discrete event, with G and H tapping on the CSL magma. Yield strength Apparent viscosity The chemistry of the Runtu Jarita domes is much ty (Pa) h0 (Pa S) simpler and reveals that a zoned magma chamber con- Silicic lava 473 2.5!1013 taining the RJSL and its evolved derivatives was in- Mafic lava 238 6.0!107 truded by the RJML. Mixing and mingling between 261 tiles is insignificant in the CRJ and is restricted to the 4 explosive release of minor juvenile material in the form of a pumice-and-ash fall. These minor explosive pulses Cerro Chascon erupted the most evolved compositions in their respec- Blocky talus tive groups, so they probably represent small volatile- . Laguna Khara rich caps to the systems. However, the insignificant vol- . . . . ume of these fall deposits and rapid conversion to effusive activity suggests that volatile contents were insuffi- cient to maintain a sustained explosive eruption. Most . of these volatiles may have been transferred from the + + recharge magmas. + + + A significant role for non-explosive degassing of a slowly rising magma column as suggested by Eichel- 3 Endogeneous and exogeneous berger et al. (1986) and Jaupart and Allegre (1991) is dome growth Local strombolian... unclear. While the magnitude of this effect in crystal- activity ... . . Enclaves rich magmas is debatable, it is likely to have played . ... some role in the effusive nature of these eruptions. . . . However, our data indicate that other effects, such as . . the intrinsically volatile-poor nature of the system, . were more important. + + . 3) Influence of the local and regional structural fab- + + + . ric. The CRJ complex is aligned parallel to the regional ...... northwest structural grain. It is located within the Pas- tos Grandes caldera, a complex volcano-tectonic re- 2 . . . . . Phreatomagmatic and minor gion, and it is aligned parallel to the main collapse ...... explosive magmatic activity ...... scarp of the caldera. There is no definitive surface man- ...... ifestation of a fault system controlling the complex, but .. the alignment of the domes and the trace of the apical rifts of the CRJ suggests local structural control. The . + + Ignimbrite bedrock regional structural fabric and pre-existing weaknesses . + + + associated with the Pastos Grandes caldera and its mag- + + + . . . matic system (e.g., Heiken 1976; Bacon 1986) were . . . probably utilized by the intrusion and conduit system . for the CRJ. 1

Eruption of Cerro Chascon and associated domes Remnant Obsidian? The paucity of magmatic explosive activity suggests Granodiorite? that this was not a volatile-driven eruption. We suggest + . + that recharge of mafic magma evidenced by rare inclu- + + + . + + . sions induced a magmatic (Bvapor) overpressure in ...... the magma chamber (Fig. 11). This caused a conduit to ...... Crystal-rich silicic magma . develop along a local structural weakness, up which the Andesite . reservoir silicic magma was forced. At an early stage, the rising magma body encountered and interacted Fig. 11 Representation of the Cerro Chascon eruption. 1 Volume with shallow lake water within the the Pastos Grandes increment due to recharge causes magmatic overpressure that induces a fracture and movement of reservoir magma upward. 2 caldera moat, and explosive phreatic eruptions formed Rising reservoir magma interacts with water table and ancestral the small tuff-ring sequence. This activity opened a Laguna Khara resulting in early phreatic and phreatomagmatic larger pathway to the surface from which minor juve- activity to produce the explosion breccia. Later magmatic explo- nile explosive activity produced the scattered pumice sive activity resulted. 3 Effusion of lava and dome growth. 4 Con- tinued dome growth to produce Cerro Chascon. See text for fur- and obsidian fall. This was most likely fueled by vola- ther discussion tiles from the recharge magma. Extrusion of the viscous silicic lava followed as the magmatic overpressure was sustained by recharge. The lava probably oozed out in of the lava and the persistent local (vent region) strom- a series of sporadic endogenous and exogenous epi- bolian activity throughout the effusion of the lava sug- sodes to eventually form the large low-dome of Cerro gest that volatile exchange with the recharge magma Chascon (Fig. 11). The pervasively vesicular character may have continued throughout. 262

The eruption of domes G and H was also a response Simultaneous emplacement can also result when two to increased magmatic pressure. Their small size, how- magmas of different viscosity are introduced at the en- ever, may reflect either the lack of explosive activity try end of the conduit (dyke) system. In this case the during the initial eruptive stages or smaller recharge lower-viscosity magma always migrates towards the events. margins of the conduit (higher stress) and encapsulates The subvolcanic features, petrologic character, and the core of higher-viscosity silicic magma. A mixed or alignment with the other domes make Cerro Guichi a mingled zone may develop between the outer zone and very intriguing but enigmatic feature. We believe it rep- core. This geometry is transported to the surface and all resents an earlier event that involved magma which the magmas are erupted simultaneously. The surface produced the obsidian and the CSL. The combination zonation reflects the subsurface zonation. of its features allow our suggestion that it affords a view With respect to the CRJ system, choosing between of what part of the subsurface system may look like. the two models is important because they imply very different roles for the mafic magma in the eruption of composite domes. We offer a schematic view of the two transport and eruption scenarios in Fig. 12. Most of the Eruption of the Runtu Jarita Group observations on the Runtu Jarita are equivocal and do not allow definitive choice between the two models. The paucity of explosive activity again suggests that However, two observations favor the idea of simulta- thermal and volatile exchange during recharge were neous emplacement. Firstly, basal contacts of the silicic subordinate to volume-increase effects during the erup- and mixed zones are not concordant, and the silicic tion of the domes. However, the composite nature and does not conform with the upper contact of the mafic the mafic–silicic–mafic zonation (Figs. 2, 4) of most of lava. The contact between the core of RJSL and the the domes suggest that mafic magma played a crucial mixed zone is vertical, whereas the contact between the role in transporting the silicic magma to the surface. mixed and RJML is at a very low angle. We would ex- We interpret a more active role for the mafic magma pect these contacts to be more concordant if they had than for the Chascon Group. been sequentially emplaced on one another. Secondly, The RJML is a hybrid produced by mixing between the mafic–silicic–mafic zonation is the result of the core a more mafic lava with the RJSL. After this mixing of RJSL being surrounded by the mixed zone and the event, a stable zoned system must have existed for a RJML, not of it sitting on top of the mixed zone and time sufficient for the disequilibrium features described RJML. herein to develop in the RJML. Disruption of this sta- We cannot rule out the complicated transport model ble system by a further recharge event, emplacement of of Blake and Campbell (1986), but we feel the evidence a dyke, caused a fissure-like conduit to open due to at CRJ is more consistent with simultaneous emplace- magmatic pressure. The relative dominance of mafic ment and therefore with the conceptually and physical- magma decreases from dome F toward dome A, so we ly simpler model of encapsulation and lubrication put suggest that the final recharge event may have been forth by Carrigan et al. (1992). We might extend this concentrated closer to domes E and F. The dyke may model slightly to suggest that the silicic core of RJSL have utilized a pre-existing structural weakness. was a viscous silicic magma that completely separated Elucidating the subsequent transport and emplace- from the reservoir by buoyancy forces associated with ment dynamics in this dyke is difficult from surface ex- encapsulation. The higher elevation and spinose char- posures alone. Previous workers (Gibson and Walker acter would be consistent with their lower density and 1963; Blake and Ivey 1986; Eichelberger et al. 1986; with buoying up by the transporting and effusing Carrigan et al. 1992; Koyaguchi and Takada 1994) have RJML (Fig. 12). provided insight into some of the processes during eruption and emplacement of similar composite domes and intrusions. Two schools of thought now dominate: Conclusion a sequential emplacement school (e.g., Koyaguchi 1985; Blake and Campbell 1986; Koyaguchi and Takada The lavas of the Chascon-Runtu Jarita Complex afford 1994) and one that advocates simultaneous emplace- insight into the eruption of crystal-rich silicic magmas. ment (Gibson and Walker 1963; Carrigan et al. 1992; Here volatile-poor rhyodacitic and rhyolitic magmas Carrigan 1994). The associated transport mechanisms close to their rheologic limits and solidii apparently are very different. were induced to erupt by recharge of hotter, more fluid For sequential emplacement a reverse zoning ar- mafic magma. The resulting lava domes display a range rangement is required at the entry end of the conduit: of characteristics that are the result of varied interac- mafic magma first with silicic magma trailing behind. tion and process. The central mafic layer breaks through the silicic mag- We interpret the dominant effects of the recharge to ma during the propagation of a dyke to allow eruption be a volume increment and attendant increase in mag- of mafic magma first, followed by mingled magma and matic pressure and the role of mafic magma as a trans- then by silicic magma. porting agent or lubricant. The eruption of the Chascon 263

D D1 Fig. 12 Diagrams of two possible sequences of processes that may have given rise to the Runtu Jarita group lavas. The cross- sectional views are normal to the length of the complex (and the inferred dyke). The sequence on the left depicts events that might have occurred during encapsulation and simultaneous emplacement (c.f. Carrigan 1994). The sequence on the right depicts the penetration and sequential emplacement model of Blake and Campbell (1986) and Koyaguchi (1985). Scales are variable to help highlight key processes. The models start with a recharge event that increases magmatic pressure to initiate fracture and conduit formation. Stage 1. A: Both models start with silicic magma penetrating into the entry end of the conduit first. Stage 2. In C C1 B the less viscous magma (RJML) rapidly migrates to the zones of higher stress at the edges of the dyke. A zonation with silicic magma (RJSL) in the center, and mafic magma on the outside, results. The zones are separated by a zone of mixing and/or mingling. The silicic magma may be completely separated from its reservoir. In B1 the less viscous magma penetrates through and overtakes the silicic magma to produce a zonation with mafic magma in the center and silicic magma on the outside separated by a mixing and mingling zone. Stage 3. In C the zoned structure is maintained to the surface with the silicic magma “raft” being buoyed up, because it has a lower density, by the mafic magma. If the conduit is long enough, the mafic magma may overtake and B B1 completely surround the silicic magma seperating it from the reservoir. In C1 the mafic magma has penetrated through and over- taken the silicic magma to fill the upper conduit and erupt first at the surface. Stage 4. In D effusion of the three zones occurs. In D1 effusion of the mafic magma (RJML) continues and comes to rest. This is then followed by the mixed zone and then the silicic Runtu magma in sequence. Dips of the contacts should be more concor- Jarita dant in this scenario. Note the difference in the conduit zonation mafic at depth in the two models lava

A ly, rather than sequentially, to make up the composite domes. Eventual formation of a pluton appears to have Encapsulation Penetration been interrupted by recharge of hotter more mafic and and simultaneous sequential magma into the base of the chamber. emplacement emplacement Our observations support and extend those at the nearby Cerro Chao (de Silva et al. 1994) that effusive eruption of large volumes of silicic magma is fueled by Runtu Jarita recharge. This may suggest a consistency of process. A silicic lava crucial role for recharge has been proposed for silicic S N domes in other areas (e.g., Bacon and Metz 1984; Carri- gan and Eichelberger 1990; Eichelberger 1975; Murphy Fracture et al. 1998). We posit that viscous (crystal-rich) silicic magma, which is the dominant volume of effusive silicic lava bodies, is unlikely to erupt without the impetus from recharge.

Acknowledgements We gratefully acknowledge the contribu- tions of several people who have helped bring this work to frui- View normal to the dike elongation tion. K. Copenhaver, R. Carrasco, A. Dunne, B. Minsley, E. Ro- que Lobaton, and J. Roque Lobaton for their help and compan- ionship during various field campaigns between 1989 and 1996. The officers of GEOBOL, particularly M. Claure, R. Perez, and Group is thought to be driven by this overpressure. The M. Arduz are thanked for their cooperation, support and logistic support. The authors are deeply indebted to W. Duffield, S. Na- eruption of the Runtu Jarita group was probably trig- kada, and D. Swanson for thorough and constructive reviews that gered and driven similarly, but this eruption is thought have improved this contribution immeasurably. S.L. de Silva is to have been fed by a composite dike with a mafic–silic- also grateful to J. Fink, C. Carrigan, S. Sparks, T. Koyaguchi, and ic–mafic zonation resulting from the encapsulation and S. Blake for discussions about this work. R.B.W. gratefully ac- knowledges an Indiana State University Graduate Studentship lubrication of the cooler, viscous, silicic magma by the and summer funding from NASA grant NAGW-156. R.B.W.’s hotter, less viscous, more mafic lava (c.f. Carrigan et al. 1996 fieldwork was conducted under auspices of NSF- 1992). The different lavas were emplaced simultaneous- EAR9523565 to S.L. de Silva. 264

Fink JH, Anderson SW, Manley CR (1992) Textural constraints References on effusive silicic volcanism: beyond the permeable foam model. J Geophys Res 97:9073–9083 Andersen DJ, Lindsley DH (1988) Internally consistent solution Francis PW, de Silva SL (1989) Application of Landsat Thematic models for Fe–Mg–Mn–Ti oxides; Fe–Ti oxides. Am Mineral Mapper to the identification of potentially active volcanoes in 73:714–726 the Central Andes. Rem Sens Environ 28:245–255 Bacon CR (1986) Implications of silicic vent patterns for the pres- Gibson IL, Walker GPL (1963) Some composite rhyolite-basalt ence of large crustal magma chambers. J Geophys Res lavas and related dikes in eastern Iceland. Proc Geol Assoc 90:11243–11252 74:301–318 Bacon CR, Hirschmann MM (1988) Mg/Mn partitioning as a test Heiken G (1976) Depressions surrounding volcanic fields: a re- for equilibrium between coexisting Fe–Ti oxides. Am Mineral flection of underlying batholiths? Geology 4:568–572 73:57–61 Huppert HE, Shepherd JB, Sigurdsson H, Sparks RSJ (1982) On Bacon CR, Metz J (1984) Magmatic inclusions in rhyolites, con- lava dome growth with application to the 1979 lava extrusion tamination basalts, and compositional zonation beneath the of the Soufriere of St. Vincent. J Volcanol Geotherm Res Coso volcanic field, California. Contrib Mineral Petrol 14:199–222 85:346–365 Jaupart C, Allegre CJ (1991) Gas content, eruption rate and in- Bindeman IN (1993) A practical method for the determination of stabilities of eruption regime in silicic volcanoes. Earth Planet volume proportions of magma chamber refilling. J Volcanol Sci Lett 102:413–429 Geotherm Res 56:133–144 Koyaguchi T (1985) Magma mixing in a conduit. J Geotherm Res Blake S (1990) Viscoplastic models of lava domes. In: Fink JH 25:365–369 (ed) Lava flows and domes: emplacement mechanisms and Koyaguchi T, Takada A (1994) An experimental study on the for- hazard implications. IAVCEI Proc Volcanology, vol 2. Spring- mation of composite intrusions from zoned magma chambers. er, Berlin Heidelberg New York, pp 88–126 J Volcanol Geotherm Res 59:261–267 Blake S, Campbell IH (1986) The dynamics of magma mixing Lipman PW (1984) The roots of ash-flow calderas in western during flow in volcanic conduits. Contrib Mineral Petrol North America: windows into the tops of granitic batholiths. J 94:72–81 Geophys Res 89:8801–8841 Blake S, Ivey GN (1986) Magma-mixing and the dynamics of Manley CR (1996) Physical volcanology of a voluminous rhyolite withdrawal from stratified reservoirs. J Volcanol Geotherm lava flow: the Badlands lava, Owyhee Plateau, southwestern Res 27:153–178 Idaho. J Volcanol Geotherm Res 71:129–153 Carrigan CR (1994) Two-component magma transport and the Marsh BD (1981) On the crystallinity, probability of occurrence, origin of composite intrusions and lava flows. In: Ryan MP and rheology of lava and magma. Contrib Mineral Petrol (ed.) Magmatic systems. Academic Press, London, pp 78:85–98 319–354 Murphy MD, Sparks RSJ, Barclay J, Carroll MR, Lejuene AM, Carrigan CR, Eichelberger JC (1990) Zoning of magmas by vis- Brewer TS, Macdonald R, Black S (1998) The role of magma cosity in volcanic conduits. Nature 343:248–251 mixing in triggering the current eruption at the Soufriere Hills Carrigan CR, Schubert G, Eichelberger JC (1992) Thermal and volcano, Montserrat. Geophys Res Lett 25:3433–3436 dynamical regimes of single- and two-phase magmatic flow in Pinkerton H, Stephenson RJ (1992) Methods of determining the dikes. J Geophys Res 97:377–392 rheological properties of magmas at sub-liquidus tempera- de Silva SL (1989a) Geochronology and stratigraphy of the ignim- tures. J Volcanol Geotherm Res 53:47–66 brites from the 21730bS to 23730bS portion of the Central Powell M, Powell R (1977) Plagioclase-alkali feldspar geother- Andes of northern Chile. J Volcanol Geotherm Res mometry revisited. Mineral Mag 41:253–256 37:93–131 Rutherford MJ, Devine JD, Barclay J (1998) Changing magma de Silva SL (1989b) Altiplano-Puna Volcanic Complex of the conditions and ascent rates during the Soufriere Hills eruption Central Andes. Geology 17:1102–1106 on Montserrat. Geol Soc Am Today 8:3:1–7 de Silva SL, Francis PW (1989) Correlation of large ignimbrites: Stasiuk MV, Jaupart C (1997) Lava flow shapes and dimensions two case studies from the Central Andes of northern Chile. J as reflections of magma system conditions. J Volcanol Geo- Volcanol Geotherm Res 37:133–149 therm Res 78:31–50 de Silva SL, Self S, Francis PW, Drake RE, Ramirez CR (1994) Stasiuk MV, Jaupart C, Sparks RSJ (1993) On the variations of Effusive silicic volcanism in the Central Andes: the Chao da- flow rate in non-explosive lava eruptions. Earth Planet Sci cite and other young lavas of the Altiplano-Puna Volcanic Lett 114:505–516 Complex. J Geophys Res 99:17805–17825 Stormer JC Jr (1975) A practical two-feldspar geothermometer. Dunne JA (1998) Calibration of the method of exposure dating of Am Mineral 60:667–674 geomorphic surfaces on the earth using in-situ produced cos- Stormer JC Jr (1983) The effects of recalculation on estimates of mogenic radionuclides, and applications in the Central Andes. temperature and fugacity from analyses of multicomponent Thesis, Purdue University, Indiana, pp 1–250 iron-titanium oxides. Am Mineral 68:586–595 Eichelberger JC (1975) Origin of andesite and dacite: evidence of Tait SR, Jaupart V, Vergniolle S (1989) Pressure, gas content, mixing at Glass Mountain, California, and at other circum-Pa- and eruption periodicity of a shallow crystallizing magma cific volcanoes. Bull Geol Soc Am 86:1381–1391 chamber. Earth Planet Sci Lett 92:107–123 Eichelberger JC (1995) Silicic volcanism: ascent of viscous mag- Watts RB (1995) Evolution of the Cerro Chascon-Runtu Jarita mas from crustal reservoirs. Ann Rev Earth Planet Sci Complex in southwest Bolivia: implications for silicic dome 23:41–63 formation. Thesis, Indiana State University, Terre Haute, In- Eichelberger JC, Carrigan CR, Westrich HR, Price RH (1986) diana, pp 1–143 Non-explosive silicic volcanism. Nature 323:598–602 Wells PRA (1977) Pyroxene thermometry in simple and complex Fink JH, Griffiths, RW (1990) Radial spreading of viscous-gravity systems. Contrib Mineral Petrol 62:129–139 currents with solidifying crust. J Fluid Mech 221:485–509 Wood BJ, Banno S (1973) Garnet–orthopyroxene and orthopy- Fink JH, Griffiths RW (1992) A laboratory analogue study of the roxene–clinopyroxene relationships in simple and complex surface morphology of lava flows extruded from point and line systems. Contrib Mineral Petrol 42:109–124 sources. J Volcanol Geotherm Res 54:19–32