the volcanic and magmatic evolution of volc n ollag e a high k late quaternary stratovolcano in the...

Journal oI lbk'anologv and Geothermal Research. 54 ( 1993 ) 221-245 22 l Elscx ier Science Publishers B.V., A m s t e r d a m

The volcanic and magmatic evolution of Volcfin Ollagiie, a high-K, late Quaternary stratovolcano in the Andean Central

Volcanic Zone

T o d d C. Fee ley a, Jon P. D a v i d s o n a and Adol fo A r m e n d i a b 1)eparlment q/Earth and Space Science.s. University of( "a/~li)rnia, Los lngek'~. ( . 1 90024. ~ .SI

b ,5"o'vicio Geologieo de Bolivia. Callc k)ederico Zua:~ 1673, ('asi//a 5'729. La Par. B~divm

( Received March 23, 1992: revised version accepted June 23, 1992

ABSTRACT

Fecley, T.C., Davidson, J.P. and Armendia, A., 1993. ]-he volcanic and magmatic evolution of Volc~in Ollagfie, a high-K iale Quaternary' stratovolcano in the Andean Central Volcanic Zone. J. ! blcanol. (i'eoH~erm. Res.. 54:221 245.

Volcfin Ollagfie is a high-K, calc-alkaline composite volcano constructed upon extremely thick crust in the :\ndean Cen- tral Volcanic Zone. Volcanic activity commenced with the construction of an andesilic to dacitic composite cone composed of numerous lava flows and pyroclastic deposits of the Vinta Loma series and an ox erlying coalescing dome and coulee sequence of the Chasca Orkho series. Following cone construction, the upper western flank of Ollagiie collapsed toward the west leaving a collapse-amphitheater about 3.5 km in diameter and a debris avalanche deposit on the lowe~ western flank of the volcano. The deposit is similar to the debris avalanche deposit produced during the Ma? 18. 1980 eruption of Mount St. Helens, U.S.A., and was probably formed in a similar manner. It presently covers an area of 100 km 2 and extends 16 km from the summit. Subsequent to the collapse event, the upper western flank was reIbrmed via eruption of several small andesitic lava flows from vents located near the western summit and growth of an andesitic dome within the collapse-amphitheater. Additional post-collapse activity included construction of a dacitic dome and coulde ol the La Celosa series on the northwest flank. Eield relatinns indicate that vents for the Vinta Loma and post-collapse series were located at or near the summit of the cone. The Vinta Loma series is characterized by an anhydrous, two-pyroxenc assemblage. Vents for the La Celosa and Chasca Orkho series are located on the flanks and strike N55 W, radial to the volcano. The pattern of flank eruptions coincides with the distribution in the abundance of amphibole and biotite as the main marie phenocryst phases in the rocks. A possible explanation for this coincidence is that an unexposed fracture or fault beneath the volcano served as a conduit for both magma ascent and groundwater circulation. In addition to the lava flows at Ollagfie, magmas are also present as blobs of vesiculated basaltic andesite and marie andesite that occur as inclusions in nearly all of the lavas. All eruptive activity at Ollagiie predates the last glacial episode ( ~ 11.000 a B.P. ), because post-collapse lava flows are overlain by moraine and are incised by glacial valleys. Present activit~ is restricted to emission of a persistent, 100-m-high fumarolic steam plume from a vent located within the summit andesite dome.

Sr and Nd isotope ratios for the basaltic andesite and marie andesite inclusions and lavas suggest that they haxe assimilated large amounts of crust during cDstal fractionation. In contrast, narrow ranges in 143Nd/144Nd and sTSr/~%r in the andesitic and dacitic lavas are enigmatic with respect to crustal contamination.

Introduction

Previous discussions of magmatism in the

('orrcspondence to. T.C. Feeley, Depar tmen t of Earth artd Space Sciences. Universi t? of California, Los Angeles. CA 90024, USA,

Andean Central Volcanic Zone (CVZ) have focused on regional-scale isotopic and whole- rock geochemical studies of the volcanic rocks (e.g., Siegers et al., 1969; James et al., 1976; Thorpe et al., 1976, 1982; Francis et al., 1977; Klerkx et al., 1977; James 1981, 1982; Har- mon et al., 1984; de Silva, 1989; Rogers and

0377-0273 /93 /$06 .00 ~ 1993 Elsevier Science Publishers B.V. All rights reserved.

222 "T'.(. FEELf'~ ET At..

Hawkesworth, 1989; W6rner et al., 1991 ). These discussions center on the relative contri- butions and compositions of mantle wedge, continental crust, and slab-derived sources to the andesitic volcanic rocks in light of high 87Sr/S6Sr ratios, ~180, and incompatible element concentrations. To date, there is no con- sensus regarding the nature of the mantle source (s) of these magmas or the location and mechanism of crustal contamination of primary mantle-derived melts (c.f., Davidson, 1988; Rogers and Hawkesworth, 1989; Dav- idson et al., 1990b; Davidson et al., 1991b; Stern, 1991 ). In addition, on the basis of the isotopic compositions of young volcanic rocks collected during a north-south traverse of the 17.5-22 S segment of the CVZ, W6rner et al. ( 1991 ) suggested a major crustal lithologic and age boundary at about 20S, which may cor- relate with the southern limit of Proterozoic basement beneath the CVZ.

In contrast to these predominantly regional- scale studies, comprehensive studies of individual CVZ volcanoes combining detailed field, geochemical, and mineralogic data on the same set of samples are few. North of the 20 S crustal boundary only Volcfin Parinacota ( 18 S) has received detailed examination (Fig. 1; W/Srner et al., 1988; Davidson et al., 1990b). Volcanoes studied in detail south of the 20 S boundary include Purico-Chascon at 23 S (Hawkesworth et al., 1982; Francis et al., 1984), Cerro Galan at 26S (Francis et al., 1980, 1983) and San Pedro at 22S (Fig. 1; Francis et al., 1974; O'Callaghan and Francis, 1986). Purico-Chascon and Cerro Galan are atypical of late Cenozoic CVZ volcanic centers, however, because both are large ignimbrite shield volcanoes surmounted by later intermediate and silicic composition domes and flows. They, therefore, may not record the same volcanic and magmatic processes operative at the more abundant andesitic stratovolcanoes during the late Cenozoic in the CVZ.

Because of the lack of comprehensive studies at individual stratovolcanoes in the CVZ,

74" i I I

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Fig. 1. Central Volcanic Zone of the Andes. Stars show the locations of Volc~ins Parinacota, Ollagtie, and San Pedro. After de Silva and Francis ( 1991 ).

we embarked on a detailed field, petrologic, and geochemical study of Volc~in Ollagtie (5863 m) located at 2137'S (Fig. 1). The purpose of this study is to better understand the volcanic evolution of an individual stratovolcano, and to quantify petrogenetic processes that affect magma compositions in the CVZ. Our choice of Ollagtie for detailed study is a result of reconnaissance investigations that showed the lava suite to have a large compositional range and isotopic ratios that appear to be correlated with indices of differentiation, unlike San Pedro and Parinacota (Francis et al., 1977; Davidson et al., 1990b; W6rner et al., 1991 ). The apparent correlation of isotopic ratios may be linked with the geographic position of Ollagiie, which is located slightly to the east of the main axis of Quaternary volcanoes in this region of the CVZ. In this report we dis- cuss the volcanic history, field relations, and petrography of volcanic rocks at Ollagtie, and present a simple geochemical model to explain

V()lX 'ANIC A N D MAGMATIC E V O L U T I O N OF VOL( ~N ( ~L[ AGl ;E 223

their compositional diversity. Detailed discussions of the geochemistry, petrology including mineral chemistry, and petrogenesis of the rocks are left to a forthcoming paper.

Regional tectonic and geologic setting

]-he Central Andes at 21 S are divided into three NW-SE-trending geological provinces (Fig. 1 ). From west to east they are: ( 1 ) the Cordillera Occidental composed of the active volcanic arc bounded on the west by a west- ward-dipping monocline and on the east by (12) the Altiplano, a broad plateau where unde- formed late Miocene and younger ignimbrites overlie variably folded and faulted mid-Mio- cene and older sedimentary and volcanic rocks; and (3) the Cordillera Oriental, a major east- verging thrust complex involving Paleozoic to Mesozoic sedimentary and metamorphic rocks. The belt of active composite volcanoes at 21 S lies approximately 130 km above the Wadati- Benioff zone, which dips about 30E (Bara- zangi and Isacks, 1976). The crust here is extremely thick, averaging about 70 km (James, 1971 ). Uplift of the modern Central Andes and development of the present-day anomalou,;ly thick crust likely resulted from Miocene and younger tectonic episodes (Isacks, 1988). De- formation occurred along the western margin of South America during the Paleozoic to Eocene (Coira et al., 1'982; Jordan and Gar- deweg, 1989), although tectonic quiescence and erosion during the Oligocene beveled the Central Andes to a low-lying, subdued landscape (Noble et al., 1979; Tosdal et al., 1984).

Volcfin Ollagfie is part of a broad NW-trending belt of late Cenozoic calc-alkalic and ai- kalic volcanic centers that extends from southern Peru to northern Chile, southwest Bolivia, and northwest Argentina (Fig. 1 ). Late Ceno- zoic volcanic activity appears to have initiated during the Miocene (Baker and Francis, 1978 ). Older igneous rocks (Jurassic through Eocene) are exposed at lower elevations to the west of the currently active volcanic front, although

they are usually considered separate in most models of magmatism and tectonism for the central Andes because of a period of Oligocene magmatic quiescence followed by eastward migration of the arc (e.g., Coira et al., 1982).

The late Cenozoic volcanic rocks in the CVZ can be divided into three broad groups on the basis of composition and eruptive style (Thorpe et al., 1982). First, large-volume ( >/10,000 km ~' in total ), regionally extensive ignimbrite volcanism has persisted almost continuously since 23 Ma, although units older then 15 Ma are only present north of2 l S (de Silva, 1989). The tufts are calc-alkaline dac- ires to rhyolites that form ignimbrite shield volcanoes sometimes with well-defined central collapse structures (e.g., Cerro Galan). Fhe second group consists of basaltic andesitc to dacitic lavas ranging in age from 23 Ma to the present, although the largest volumes were erupted during the past 6 million years. Units are not-regionally extensive, usually form large stratovolcanoes, and are generally confined to the Cordillera Occidental (Thorpe et al., 1982 ). Ollagiie, San Pedro, and Parinacota are included in this group. Third, volumetrically minor alkali basalt lavas are present in small, isolated fields located mainly to the east of the main arc in Bolivia (Thorpe et al., 1982; Dav- idson et al., 1991a). The age of these lavas is not well constrained although the presence of associated morphologically young maars and tephra cones suggests that they are young. Most of the alkali basalt fields are located within N- S-striking structural depressions and they may be related to rifting (Thorpe el al., 1982 ). ['he late Cenozoic volcanic stratigraphy of the ('VZ indicates that early volcanism was dominantly silicic and that a greater percentage of more marie compositions has been erupted with time.

Volc~in Ollagiie was constructed upon the western edge of the Altiplano about 25 km east of the main axis of Quaternary volcanoes. Al- though few radiometric age data are available for Ollagiie rocks, the young age ( < 1Ma i of

224 Y.C. FEELE~ ET AI_.

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Fig. 2. Simplified geologic map of Volcfin Ollagiie and the surrounding area based on field work and photo interpretation,

the volcano is indicated by the pristine morphology of the lavas and persistent fumarolic emissions from a summit vent. The lava flows, domes, and pyroclastic deposits of Ollagiie overlie the regionally extensive 5.9 to 5.5 Ma old Carcote ignimbrite, which is exposed on its eastern side (Fig. 2; Baker and Francis, 1978 ). Older, extensively glaciated lavas and domes (e.g., Cerro Huanaco, Cerro Chanchajapi- china, Cerro Chijiliapichina; Fig. 2) not related to the main volcanic center are exposed to the east and south of Ollagtie but are not considered in this study.

Geology and eruptive history of Voican Ollagiie

In this section we summarize the stratigraphy and petrographic features of the lavas at Ollagiie. Volc~in Ollagiie is a complex composite cone with evidence for a multistage eruptive history. It has a summit elevation of 5863 m and a maximum edifice height of about 2065 m above the Altiplano. Slow erosion rates due to the~trid climate that has persisted in the CVZ for much of the Late Tertiary (Galli-Oliver, 1967 ) have resulted in excellent preservation

V ( ) L ( N N I C A N D M A G M A T I C E V O L U T I O N O F VOLC,&N ( - )LLAGOE 225

of lhe 80-90 km 3 of volcanic material that constitute the edifice of Ollagiie. However, the lack of erosion has prevented exposure of the oldest rocks at the volcano and thus the earliest eruptive history is inaccessible.

The geology and eruptive history of Volc~in Ollagiie are summarized in Figures 2 and 3.

D. Post-Collapse and ~0 ...... i[ . . . . . . . ~'. La Celosa stage d({l~ ~ re! "[;{i

~ ' ~ '

The lavas and pyroclastic rocks have been divided into four eruptive series. In ascending stratigraphic order they are: (1) the Vinta Loma series; (2) the Chasca Orkho series; (3) the post-collapse series; and (4) the La Celosa series. Compositional and modal data for individual eruptive series are illustrated in Fig-

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B. Chasca Orkho stage

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226 T.C FEELEY EI',At..

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Vinta Loma series

50 52 54 56 58 60 62 64 66 68 70 SiO 2 wt.%

Fig. 4. Histograms of SiO2 contents for lavas from Ollagiie by eruptive series. The data represent approximately 80% of exposed lavas at the volcano.

ures 4 and 5, respectively. Grouping of the lavas into the eruptive series was based principally on vent locations and contact relationships. Due to the difficulty in correlating individual flows and domes where the vents are covered or where stratigraphic position is not obvious from contact relationships, a few lavas were assigned to eruptive series on the basis of flow morphology, mineral mode, and un- published 4Ar/39Ar age determinations (G. W~Srner, pers. commun., 1992). Andesites of relatively uniform composition were the dominant magmas erupted, although significant volumes of dacite vented on the flank of the volcano (Fig. 4).

The Vinta Loma series

The numerous andesitic and dacitic lava flows of the Vinta Loma series are the oldest exposed rocks and they represent the main phase of cone growth at OllagiJe (Fig. 3 ). They are presently exposed at the summit and in the eastern one-half sector of the volcano where they comprise about 60% of exposures by volume (Fig. 2 ). The predominant rock types are medium-grey, blocky to platey, cliff-forming two-pyroxene andesites and dacites. About 80% of the lava flows are andesites (Fig. 4). Flow widths and thicknesses (20-90 m) vary inversely with ground slope; on gentler slopes flows are commonly 2-3 times wider and thicker than on steep slopes. Many flows have well-developed internal flow folds and termi- nal exposures may be autobrecciated. Rare basal exposures reveal oxidized flow breccias up to several meters thick. Primary surface flow features are typically not exposed due to burial by a thin mantle of colluvium. Above about 5000 m elevation the colluvium completely buries some flows giving them a rootless ap- pearance lower on the volcano (Fig. 2 ). Given the slow rates of erosion in the CVZ the colluvium may have accumulated over a consider- able length of time, which is consistent with the

~ 7 V{ )L{ "ANIC AND MAGMATIC EVOLUTION OF VOL( :~N (}LLAG [JE =~

relatively low stratigraphic position of the Vinta Loma series.

Exposed in the wall of a large cirque at the summit of Ollagfie is a 60-m-thick section of strongly altered pyroclastic deposits interbed- ded between Vinta Loma lava flows. Compris- ing the bulk of this unit are small volume, 1--5 m thick, poorly sorted pyroclastic flow deposits alternating with thin, well-sorted pumice Fall deposits. These pyroclastic deposits record in- termittent episodes of small-volume plinian eruptions during the Vinta Loma stage. The lavas and pyroclastic deposits of the Vinta Loma series dip outward and are radially distributed around the present summit region, indicating that they were erupted from a central summit vent.

Mineralogically and petrographically, most Vinta Loma lavas are porphyritic to seriate, sparsely glomeroporphyritic with 22 to 41% phenocrysts of plagioclase (An6o_37) > > pyroxene > Fe-Ti oxides + amphibole (Figs. 5 and 6). Rare quartz, biotite, and olivine are present in some samples and in most cases are probably xenocrystic. In addition, a few of the older flows on the northwest flank contain significant amphibole relative to pyroxene (Fig. 6). Microphenocrysts of plagiociase, pyroxene, and Fe-Ti oxides are abundant in the groundmass of all lavas; amphibole is absent in the groundmass.

The Chasca Orkho series

Subsequent to the main phase of cone growth, eruptive activity shifted from the summit area to locations on the southeast flank of Ollagiie. Here a distinctive lava field overlies Vinta Loma flows (Figs. 2 and 3) and is re- ferred to as the Chasca Orkho eruptive series, after a 300-m-thick dacite dome named Cerro Chasca Orkho. The Chasca Orkho series contains the most mafic and most silicic lavas erupted at Ollagfie (Fig. 4). Vents for the Chasca Orkho flank eruptions strike radial to the summit of Ollagfie (Fig. 2 ).

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228 T.C. FEELEY ET AL.

, A m p h io yx

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II

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[~ 2 4 6 S

Fig. 6. Spatial distribution of Ollagiie lavas in which amphibole and biotite are present in greater proportions than pyroxene and vice versa. Compare with Fig. 2.

The stratigraphic sequence within the Chasca Orkho series is relatively well established by contact relationships, especially in the southern part of the field. Three small-volume, dark- grey, vesiculated, olivine-phyric basaltic andesite lava flows, each about 1 to 2 m thick, are at the base of the sequence. They are the most mafic lavas exposed at Ollagiie. Overlying these flows is a 20-m-thick flow of platy basaltic andesite. Texturally and mineralogically this flow resembles the underlying basaltic andesite flows except that it contains more modal clinopyroxene. The stratigraphic position of the basaltic andesites at Ollagiie is significant because it contradicts the claim of Thorpe et al. (1982) that mafic lavas are never erupted from a CVZ composite volcano after eruption of more silicic lavas (Fig. 4).

Following eruption of the basaltic andesite flows, a sequence of 10 coalescing crystal-rich andesitic to dacitic domes and lavas (coul4es) was erupted. These lavas are morphologically distinct from the Vinta Loma lavas because they are short and steep-sided and commonly

have flow-front scree of autobrecciated lava. Aspect ratios of individual domes and coul6es range up to 0.15. Coul6es were erupted on the mid-slope of Ollagiie and are elongate down- slope. A thin veneer of colluvium usually covers the upper surfaces of the coul6es. Lavas erupted near the base of Ollagiie form domes that are circular to elongate in plan, sometimes with concentric pressure ridges on their upper surfaces. Cerro Chasca Orkho has an 80-m- deep axial rift, which probably formed as a result of lateral spreading during dome growth. This rift strikes N55 W, parallel to the alignment direction of the other exposed vents on Ollagiie (Fig. 2). In the western part of the dome and coul6e field, where contact relationships are relatively well exposed, the lavas pro- gressively increase in SiO2 content upsection from 63.5% to 67% SiO2. The domes and cou- 14es of the Chasca Orkho series do not appear to have flowed very far (if at all) from their vents. They record a stage in the evolution of Ollagiie when eruptive activity shifted from central summit vent eruptions of the Vinta

VOLt'ANIC AND MAGMAT1C EVOLUTION OF V()L(7/~N OLLAG()E 229

Loma series to flank eruptions of more viscous magma.

In hand specimen, Chasca Orkho series rocks range from medium-grey andesites to light-grey dacites. In thin section, textures range from porphyritic-seriate to nearly vitrophyric. Pla- gioclase (An6o_3o), which may have sieve textures, is the most common phenocrystic phase in the andesites and dacites, and is accom- panied by lesser amphibole, biotite, pyroxene, and Fe-Ti oxides (Fig. 5). A few of the more silicic samples contain trace quartz and sphene. A notable difference between Chasca Orkho series rocks and Vinta Loma rocks is the ap- pearance of biotite phenocrysts, and the increase in amphibole relative to pyroxene in rocks with similar SiO2 contents (Figs. 5 and 6 ). The amphibole and biotite phenocrysts are typically quite distinctive in hand specimen, rarely occurring as megacrysts 5 to 10 m m across in radiate splays. One dacite (OLA9021 ) contrasts with the other andesites and dacites because amphibole and biotite are absent (Figs. 5 and 6). Groundmass textures range from hypocrystalline to trachytic, defined by subparallel alignment of stubby plagioclase microlites. The basaltic andesites are porphyritic with 8 to 11% modal phenocrysts of olivine >/ Fe-Ti oxides > clinopyroxene set in a trachytic groundmass of plagioclase (An64), clinopyroxene, and Fe-Ti oxides (Fig. 5 ). Xenocrysts of spongy textured plagioclase (An37) and quartz with clinopyroxene reaction coronae are sparse but conspicuous.

Debris avalanche

Late-stage volcanic activity commenced with sector collapse of the western flank of Ollagiie. Although the sector collapse event did not de- stroy the actual summit, it probably resulted in formation of a collapse-amphitheater about 3 km in diameter on the upper western flank (Figs. 2 and 3). The collapse event also produced a debris avalance deposit, which was first recognized and described by Francis and Wells

( 1988 ). The debris avalanche deposit is presently only preserved west of Ollagiie in the San Martin basin where it extends 16 km from the summit and covers an area of approximately 100 km 2 (Fig. 2). Morphologically it forms a hummocky terrain similar to the deposit produced during the May 18, 1980 eruption of Mount St. Helens, and it was probably formed in a similar manner (Fig. 7; Francis and Wells, 1988). Reconnaissance sampling within the debris avalanche deposit suggests that most of the megablocks are lithologically similar in composition and phenocryst mode to the older amphibole-bearing lavas of the Vinta Loma series on the northwest flank (G. W~Srner pets. commun., 1991 ). The extent of these lavas was, therefore, likely to have been greater than those presently exposed. Polygonally jointed andesite bombs are occasionally found on top of the debris avalanche deposit. It is unclear, however, if these bombs represent a juvenile magmatic component associated with the collapse event.

Post-collapse series

Contemporaneous with and /o r following sector collapse of the upper western flank, eruptive activity continued with extrusion of numerous short flows of grey andesitic lava, which are well preserved on the western flank of the volcano, and with growth of an andesitic dome within the inferred collapse amphitheater (Figs. 2 and 3). All of these extrusions are compositionally very similar (Fig. 4). The lava flows erupted from the western summit area and flowed toward the west over the debris avalanche deposit. Young morphological features such as well-developed lev6es and pressure ridges on upper flow surfaces, and Iow- albedo (Rothery et al., 1986) on satellite im- agery suggest that these lavas are younger lhan Chasca Orkho series lavas, although this evidence is not conclusive.

The presently exposed volume of the summit dome is about 0.35 km 3. At its base are

~ 3 0 F.C. FEELEY El AI .

I i I

Fig. 7. View of the debris avalanche deposit from the western flank of Ollagfie. The deposit forms the hummock) terrain in front of the white salar deposits in the San Martin basin. Note Cerro La Porufiita, a phreatomagmatic tephra cone constructed on top of the deposit, on the right. Peaks in the background are Volcans Chela (right) and Palpana (left).

pi les a n d tongues o f angu la r b locks as large as

l 0 m in d i a m e t e r t ha t r esu l ted f r o m smal l rock a v a l a n c h e s du r ing d o m e g rowt h (Fig. 8; c.f., Swanson et al., 1987 ). T h e d o m e p r o b a b l y fills

a c o l l a p s e - a m p h i t h e a t e r in a m a n n e r analo- gous to the l ava d o m e at M o u n t St. Helens , because wes t -d ipp ing V in t a L o m a lavas and pyr-

oclas t ic depos i t s at the wes te rn s u m m i t are

Fig. 8. View of summit andesite dome from the north. Note the small rock avalanche at the right base of the dome and the active summit fumarole. The arrow points to a young post-collapse lava flow that underlies the dome.

V()LCANIC AND MA(;MATIC EVOLUTION OF VOLCAN OI.LAG()E ?_31

abruptly truncated at the eastern margin of the dome. Conclusive evidence for formation of a collapse-amphitheater and presence of a collapse scar at Volc~in Ollagfie has, however, been obscured by post-collapse eruption of the younger lavas and glacial erosion. The dome is probably the youngest extrusion exposed on Ollagfie because its western edge overlies a relatively young post-collapse lava flow (Fig. 8 ). Additional late-stage activity included construction of small-volume andesitic phreatomagmatic tephra cones (e.g., Cerro La Porui]- ita; Figs. 2 and 7) on the debris avalanche deposit in the San Martin basin, although it is presently unclear how or if these cones are related to the main volcanic center.

Modal compositions of post-collapse series lavas are variable and suggest the possibility of two distinct post-collapse eruptive phases on the western flank (Figs. 5 and 6). In all lavas, crystal contents range from 30 to 42%; plagioclase (An6o 40) dominates the mode as both phenocrysts and as a groundmass phase. In older post-collapse lava flows low on the western flank, pyroxene is a more abundant phenocrystic phase than amphibole and biotite ( Fig. 6). These flows are overlain by shorter, amphibole- and biotite-bearing flows at topographically higher levels on the volcano. For example, the summit dome (OLA9037; Fig. 5 ) is a coarse-grained porphyritic to glomeroporphyritic, amphibole- and biotite-bearing an- desire that contains little pyroxene.

All eruptive activity at Ollagfie predates the last glacial episode ( ~ 11,000 a B.P.; Mercer and Palacios, 1977) because some post-collapse lava flows are incised by glacial valleys and the reformed western flank is mantled in places by a girdle of moraine (Fig. 2). Volc~in Ollagiie is, nonetheless, classified as poten- tially active in a recent compilation of CVZ volcanoes by de Silva and Francis ( 1991 ) because of persistent emission of a 100-m-high fumarolic plume from a vent located within the summit dome (Fig. 8 ).

The La Celosa series

The La Celosa series consists of a dacitic dome and coul6e that erupted on the lower northwest flank of Ollagfie (Fig. 3). It is difficult to be certain what the relative age of the La Celosa series is from field relationships, because Cerro La Celosa is isolated from contact with other units. 4Ar/3'~Ar age determinations (G. W6rner, pers. commun. , 1992) suggest that the La Celosa series lavas are similar in age to the post-collapse lavas.

The La Celosa series lavas retain many of their primary flow features due to the absence of glaciation on the lower flanks of the volcano. Cerro La Celosa is lobate in plan and has a 1.5-km-long, north-striking axial rift on its upper surface. The rift probably formed as a result of lateral spreading during dome growth as also noted at Cerro Chasca Orkho. The cou- 16e is a composite unit of two lavas that erupted from two closely spaced but separate vents (Fig. 2). Lava erupted from the older, topographically lower vent flowed down slope and presently partially encircle this vent with a series of concave upslope pressure ridges. Lava erupted from the topographically higher vent flowed down the backside of the older flow toward the southwest (Fig. 2 ). The lavas erupted from both vents are compositionally and petrographically indistinguishable.

Textures of La Celosa rocks range from porphyritic to vitrophyric. Plagioclase (An6o_ ~o) is the most common phenocrystic phase tol- lowed by amphibole, biotite, and Fe-Ti oxides (Fig. 5 ). The Cerro La Celosa dacite is similar to the more silicic Chasca Orkho dacites because it contains trace, yet conspicuous sphene phenocrysts. Pyroxene is, in general, absent in La Celosa rocks (Fig. 5 ).

Inclusions

A particularly striking feature of the volcanic rocks at Ollagiie is that nearly every lava, excluding the basaltic andesite lavas in the

2 3 2 T.C. FEELE~ ET AI..

Chasca Orkho series, contains ellipsoidal to spheroidal mafic inclusions. Inclusions with vesiculated interiors and nearly vesicle-free, glassy margins are the most common. In most Vinta Loma and post-collapse lavas these inclusions are relatively rare and small ( < < 1.0%; ~< 5 cm). In lavas of the Chasca Orkho series and La Celosa series they commonly make up from 1 to 3% of the rock and may be as large as 15 cm across. Such inclusions are a common feature in many intermediate lavas of the CVZ (Davidson et al., 1990a).

The vesiculated inclusions have micropor- phyritic to porphyritic textures characterized by up to 10% euhedral, paragenetically early phenocrysts ( < 2.0 mm) of clinopyroxene, orthopyroxene, and basaltic amphibole in vary- ing proportions (Fig. 9A). Pyroxene phenocrysts are typically surrounded by a reaction rim of amphibole (Fig. 9A). Xenocrysts of plagioclase, biotite, and quartz (surrounded by cpx coronae) occur in nearly all inclusions. Most of the large plagioclase grains are interpreted as xenocrysts because they have albite- rich cores (mn3o_60) that are riddled with abundant, irregularly shaped glass inclusions, and rims that are euhedral and strongly re- versely zoned (Fig. 9A; An6o_71 ). The ground- masses of the vesiculated inclusions are inter- granular to hyalopilitic and are composed of microlite-sized, acicular laths of plagioclase (Anso-7o) + amphibole _+ orthopyroxene and more equant microphenocrysts of Fe-Ti oxides in a brown glass matrix (Fig. 9A). Vesic- ulated mafic inclusions with similar textures have been described from many other continental magmatic fields and are recognized as blobs of mafic magma quenched in cooler, more silicic magma (e.g., Eichelberger, 1975; Bacon and Metz, 1984; Bacon, 1986; Grove and Donnelly-Nolan, 1986; Davidson et al., 1990a; Feeley and Grunder, 1991 ). This is also our interpretation for the vesiculated inclusions in Ollagiie lavas.

The second type of inclusions are unvesiculated gabbroic clots containing plagioclase

(An35_40) -bclinopyroxene+Fe-Ti oxides _+ orthopyroxene _+ amphibole (Fig. 9B). This type of inclusion has been found in only a few lavas, although they may have been over- looked owing to their small size. They are usually less than 1 cm in diameter, although one, sample OLA9027i (Table 1 ), is 5 cm across. Margins of these inclusions are not spheroidal like margins of the vesiculated inclusions described above; they are angular and defined by crystal boundaries (Fig. 9B). The small gabbroic clots are distinguished from glomero- crysts because clinopyroxene in the gabbroic clots commonly forms oikocrysts enclosing plagioclase (Fig. 9B). The unvesiculated nature of these inclusions and the cumulate textures suggest that they are magmatic cumulate residues. If so, they may preserve evidence of the phases that precipitated from a precursor magma to generate the silicic andesites and dacites (c.f., Grove and Donnelly-Nolan, 1986). OLA9027i is a nearly holocrystalline anorthositic gabbro composed principally of elongate, randomly oriented phenocrysts of plagioclase as large as 2 mm. Many of these plagioclase grains have small, included apatite needles. Interstices between the plagioclase crystals are filled with small, granular microphenocrysts of clinopyroxene, orthopyroxene, Fe-Ti oxides, and sparse brown glass. In addition, a few large oikocrysts of amphibole are present either partially or wholly enclosing small, equant plagioclase grains. This texture, and the chemical composition of OLA9027i (see below) suggest that it is plagioclase accumulative.

Summary and discussion of the geology and eruptive history of Ollagiie

The earliest exposed stage of eruptive activity at Ollagiie (Vinta Loma series) produced two-pyroxene andesitic and dacitic lavas and pyroclastic deposits from a central summit vent. A few amphibole and biotite-bearing lavas appear to have been erupted early in the

VOLCANIC AND MAGMATIC EVOLUTION OF VOLC-i~N OLLAG()E 233

Fig. 9. Photomicrographs of inclusions found in Ollagiie andesitic and dacitic lavas. Width of both photomicrographs is 2.5 ram. Crossed-polarized light. (A) Clinopyroxene phyric andesite inclusion OLA9021i. Euhedral clinopyroxene phenocrysts (center and right) are surrounded by reaction rims of amphibole and are set in a vesiculated glassy groundmass with acicular plagioclase and amphibole, and more equant Fe-Ti oxide microphenocrysts. To the lower right of center is a plagioclase xenocryst with sieve-textured core and euhedral overgrowth. (B) Gabbroic inclusion (center) in post-collapse lava OLA9054. Early grown plagioclase and Fe-Ti oxide phenocrysts are partially and completely surrounded by large clinopyroxene oikocryst.

234 T.C. FEELE~ ET AL,

Vinta Loma series. Through time, eruptive activity migrated to the southeast flank where a sequence of olivine basaltic andesites to amphibole and biotite-bearing andesites and dacites (Chasca Orkho series) was erupted on top of Vinta Loma flows. The trend toward eruption of more silicic compositions with time in the Chasca Orkho series implies that crystal fractionation was operating within a magma chamber to produce the intermediate composition lavas (c.f. Volc~n Colima; Luhr and Carmichael, 1980). In this model the tavas represent leaks from the magma chamber during progressive degrees of differentiation. The culmination of magmatism to date at Ollagiie occurred following collapse of the upper western flank. At this stage, eruptive activity largely returned to the summit area and modal compositions of the lavas became highly variable, although bulk compositions of erupted magmas were relatively restricted (Figs. 4 and 5 ). During all of the stages, andesites were the dominant magmas erupted, although flank vents produced basaltic andesites and a higher proportion of dacite than summit eruptions. Vesiculated inclusions are found in nearly all of the lavas suggesting that a subvolcanic magma chamber, from which the andesitic and dacitic lavas were derived, was repeatedly fluxed from below with parental mafic lavas.

Vents of the Chasca Orkho and La Celosa series are aligned and strike radial to the volcano (Fig. 2). In particular, those from the La Ce- losa series and the southern part of the Chasca Orkho series are aligned with the summit vent and strike N55W. The alignment of the La Celosa, Chasca Orkho, and summit vents suggests that an unexposed NW-striking structure beneath Ollagiie may have served as a princi- pal conduit for magma ascent and surface eruption. Similar linear zones of flank vents at Medicine Lake volcano (Fink and Pollard, 1983), Mount Mazama (Bacon, 1983), and South Sister volcano (Scott, 1987) have also been interpreted to result from magma ascent along fractures radial to the volcanoes. Lavas

erupted from all flank vents are characterized by abundant amphibole and biotite phenocrysts (Fig. 6). Although it is presently difficult to test, the large proportion of amphibole and biotite in these lavas relative to lavas erupted from the summit may also provide evidence for an unexposed lineament beneath Ollagiie (Fig. 6). Fracture or fault controlled, localized addition of groundwater to magmas erupted from shallow reservoirs on the periph- ery of the magmatic system may have been suf- ficient to stabilize hydrous phases relative to pyroxene. Luhr and Carmichael (1980) have argued on the basis ofKuno's (1950) work that near surface addition of water at Hakone Vol- cano was responsible for stabilizing hornblende. Furthermore, they contend that a similar near surface addition of groundwater to the magma system beneath Volc~in Colima was responsible for the relatively hydrous nature of some hornblende andesites. It is important to point out, however, that this model does not account for the presence of the amphibole- and biotite-bearing lavas in the Vinta Loma and post-collapse series. A detailed stable isotopic study is currently in progress and may constrain the origin of the water in OllagiJe tavas.

Geochemistry of the volcanic rocks

Major- and trace-element compositions

Representative major- and trace-element compositions oflavas and inclusions are illustrated in Figures 10 and 11 and are given in Table 1. Compositions are similar to intermediate volcanic rocks from San Pedro, Parina- cota, and other CVZ volcanoes (Fig. 10 ). They define a high-K, calc-alkaline suite (Fig. 10). SiP2 contents of the lavas range from 52 to 67 wt.% with a large compositional gap between 54 and 60%. Compositions of all the inclusions range from 53 to 61 wt.% SiO2 and fill the gap in SiO2 content of the lavas. For the suite as a whole contents of CaP, FeO*, TIP2, MgO, Ni. and Cr decrease, and K20, Na20, and Rb in-

V()LCa.NIC AND MAGMATIC EVOLUTION OF VOLC,g,N OLL&GI~IE 235

0 . 4 0

0 . 3 5

0 . 3 0

0 ~ i::L~ 0.25

0 . 2 0

0 . 1 5

' ' ' I ' , , I ' ' ' I ' ' , I '

o*" d

2 0

1 9

- , - : 1 8

O e - - ~ 1 7 <

1 6

1 5

" ~ " " ~ O L A 9 0 2 7 i

0

O [ ]

0 " D

0

z

0 o r q o

~

~ 3

0 ~ 2

0 50

. . . . ' ' I ' ' F = ' F ' ' m _

X [~a r i n a c o t a

+ Sail Pedro

I ~ ~ i [ : ] + 4 - + X

m K

/ L o w - K b a s a l t i c ~ i andes i te i dac i te a n d ~ s i t ~

5 4 5 8 6 2 6 6 7 0

S i O w t % 2

1 0 , , , , , , , , , , , ~ , , , i , , T -

8

o"!.

o

4 t 3 ~

mE]

2 , 0 T r ' l , , , [ , , ' ~ - , , ~ ! , ~ r

1 , 6

1 . 2

O 0 . 8

0 . 4

@.0

1 o

o~

0

I i

6

o

2

Cumulate inc!uston

[ ~ [ ] zk Magmat ic Inclusions

La Celo sa

Post Col lapse

' - ~ [ ] Chasca Orkho

0 Vinta Loma

; ' ~ I

i i

0 I , , , I , , , I , , = J , L ~ _ J _ ~ ~ 5 0 5 4 5 8 6 2 6 6 7 0

S i O w t . % 2

Fig. 10. MaJor-element oxide var ia t ion versus SiO2. K2O classification boundar ies ( italics ) are from Peccerillo and Taylor (1976) . Nomencla ture for the volcanic rocks is indicated along the bo t tom of the diagram. Circled field indicates the range of inclusion composi t ions . On the K20 versus SiO2 diagram, represents Par inacota lavas from I)avidson et al. (1990b) and X represents San Pedro lavas from O'Cal laghan and Francis (1986) for compar ison with Ollagtie data.

236

TABLE 1

Representative major (wt.%) and trace (ppm) element analyses of Ollagiie rocks

T.C. FEELEY ET AL.

Vinta Loma series

Sample: OLA9014 OLA9023 OLA9031 OLA9032 OLA9056 OLA9058

SiO2 60.9 63.0 58.6 64.4 62.2 ~0,2 TiO2 0.89 0.82 0.95 0.86 0.85 ii.84 AI203 16.9 16.6 16.7 16.7 16.4 i(~.7 FeO* 5.2 5.0 5.7 5.1 4.9 ~. 8 MgO 2.6 2.4 3.5 1.4 2.3 3.2 MnO 0.07 0.07 0.11 0.10 0.09 ~LII CaO 5.1 4.9 6.0 4.0 4.9 L5 Na20 4.0 3.9 3.7 4.1 3.5 ',.8 K20 2.7 3.0 2.5 3.2 3.2 2.8 P205 0.32 0.23 0.25 0.24 0.28 i).22 LOI 1.0 1.1 0.9 0.7 1.2 i~2 Total 99.6 101.0 99.0 100.8 99.9 lo0.?,

Rb 77 91 69 96 95 ,';4 Sr 530 527 547 494 722 >~8 Cr 25 17 43 6 11 B~ Ni 8 5 8 4 6 :,

Chasca Orkho Series

Sample: OLA9015 OLA90 t 6 OLA9020 OLA902 t OLA9026 Ol, A9027

SiO2 67.0 52,7 65.6 63.5 60,1 62 7 TiO2 0.55 1.33 0.57 0.77 0.91 (!.80 AI203 15.4 16.1 15.5 16.2 16.8 i6.3 FeO* 3.3 8.6 3.5 4.6 5.4 47 MgO 1.2 6.6 1.4 2.1 2.9 2.3 MnO 0.05 0.12 0,06 0.06 0.08 007 CaO 3.1 8.5 3.5 4.4 5.2 45 Na20 4.1 3.4 4.1 4.4 3.6 L0 K20 3.8 1.6 3.7 3.2 2.9 ~ i P20~ 0.17 0.24 0.18 0.25 0.23 0 22 LOI 1.2 0.1 1.3 0.7 1.4 i.3 Total 99.7 99.3 99.5 100.0 99.7 q99

Rb 118 37 126 117 89 1~5 Sr 422 702 441 451 485 436 Cr 6 221 10 14 30 , Ni 3 75 25 6 7

crease with increasing SiO2 (Figs. 10 and 11 ). A1203 and P205 contents are roughly constant until about 60% SiO2 and then decrease strongly. Consistent with its petrography, the chemical composition of inclusion OLA9027i indicates that it is a magmatic cumulate composed principally of plagioclase (with apatite inclusions); A1203 and P205 are significantly

elevated and elements not compatible in plagioclase are depleted relative to other samples (Fig. 10).

The compositional variations highlight the importance of crystal-liquid fractionation as a petrological process in the evolution of the Ollagiie rock suite. For example, the strong decrease in A1203 and P205 in samples with

~3 V{)I_Ca, NIC AND MA(iMATIC EVOLUTION OF VO1.CAN OLLAG()E .._ 7

Post-collapse series La Cclosa series

Sample: OLA9037 OLA9047 OLA9054 ()LA31 OLA33

Sit), 62.6 60.5 60.8 64.7 66.4 TiO2 0.79 0.90 0.94 0.67 0.57 AI:O?. 16.6 16.6 16.8 16.2 15.7 FeO* 4.9 5.4 5.4 3.~I 3.3 MgO 2.3 3.1 2.7 1.8 1.2 MnO O. 11 0.09 O. 10 0.06 0.OS CaO 4.6 5.5 5.2 3 " 3.0 NaeO 4.3 3.6 4.1 4.0 4.1 K20 2.9 2.8 2.7 3.6 3.8 P205 0.27 0.22 0.26 0.19 0.1 ~ k()l 1.2 1.4 1.1 0.9 1.3 Tolal 100.8 100.1 100.2 100.1 100.0

Rb 93 89 82 116 123 Sr 506 491 490 460 424 Cr 15 23 30 25 14 Ni 5 6 8 7 4

Inclusions

Sample: OLA901 li OLA9015i OLA9021i OLA9025i OLA9026i OLA14 ()LA32 OLA91)27i

SiO2 59.7 56.2 57.1 59.2 57.1 56.4 53.1 53.1 TiO2 0.90 1.16 0.94 0.97 1.18 1.07 1.42 1.03 AI20:~ 16.5 17.1 16.8 16.8 16.6 17.4 173 19.7 FeO* 5.5 7.5 5.7 5.9 6.6 7.3 8.3 7.8 Mg() 3.0 3.3 3.8 3.5 4.0 3.9 5.2 2.1 MnO 0.09 0.10 0.08 0.08 0.09 0 1 0.11 0.13 CaO 5.3 6.0 7.6 6.1 6.7 6.8 8.7 8.5 Na20 3.3 3.6 3.5 3.9 3.1 3.7 3.5 3.9 K20 2.8 2.3 1.9 2.3 2.0 2.2 1.5 1.0 P2Os 0.24 0.36 {).26 0.27 0.26 0.28 0.27 (}.37 L()I 1.8 1.4 1.9 0.7 1.5 0.5 1.4 1.5 Total 99.2 99.0 9~).6 99.9 99.2 99.7 100.8 99.1

Rb 71 63 43 54 59 55 45 32 Sr 652 610 650 600 520 603 641 813 (7I' 78 7 5 "v 53 79 31 69 7 Ni 9 8 27 13 13 14 15 11

All data collected by standard XRF techniques on dried rock powders using a Rigaku 3070 X-rat spectrometer at the University of Southern California. Precision on major and trace elements is estimated at 1% (one-sigma standard deviation ) except for Cr ( < 3.5%). Analyses of Ni less than 30 ppm are regarded as semiquanlitative. LOI determined by igniting at 900:C a separate aliquot of powder. FeO* is total Fe as Fe 2+. Samples without -90- prefix are from W6rner et al. ( 1992 ).

greater than 60% SiO2 reflects the large amount of plagioclase and apatite in the fractionating assemblage of these compositions. Trends fix MgO, FeO*, Ni, and Cr are slightly concave upward indicating fractionation of pyroxene

and olivine in mafic compositions (Figs. 10 and 11 ). The curved trends for these and other elements such as Rb and A1203 are addition- ally significant because they indicate that the compositional trends of the mafic vesiculated

238 T . C . F E E L E Y E l A 1 .

140

120

IOO E O.

o. 80 J~ rr

60

4 0

2 0

o ~m D

9O0

800

E 700 El.

C~ 600

5 0 0

4 0 0

l

i , , I , i i I , , I J , ~

3 0 0

2 5 0

200 E (D.. Q - 1 5 0

c3 I O 0

50

g

D []

[]

lOO

8o

E 6 o Q.. c~

Z 4 0

20

0 5O

[]

[ ]

54 58 62 66 7 SiO wt.%

2

Fig. 11. Trace-element variation versus SiO2. Data symbols are the same as in Fig. 10. Circled field indicates the range of inclusion compositions.

inclusions cannot be produced by simple two- component mixing of mafic magmas (e.g., Chasca Orkho basaltic andesites) with the exposed andesitic and dacitic lavas at Oltagfie. Magma mixing has been invoked to explain petrographic features and compositional trends of vesiculated inclusions at other localities (Bacon and Metz, 1983; Bacon, 1986; David- son et al., 1990a; Feeley and Grunder, 1991 ). Closed-system crystal fractionation cannot be the only petrologic process operative, however. Petrographic features of the vesiculated inclusions such as the large volume of xenocrysts indicate large degrees of crustal contamination during differentiation of inclusion- forming magmas. Furthermore, variations in radiogenic isotopic compositions preclude differentiation by closed-system processes alone (Fig. 12).

Rocks of all four eruptive series have indistinguishable bulk compositions at similar SiO2 contents (Fig. 10). This is significant because as discussed above, among different eruptive series there is a change in mineralogy from two- pyroxene-dominated assemblages to amphibole- and biotite-dominated assemblages (Fig. 5 ). Factors that control the stability of amphibole in andesites include bulk composition, .[o2, Pn2o, Ptotab and temperature (Gill, 1981 ). Because the change in mineralogy coincides with no change in bulk composition of the rocks, a change in either temperature, pressure, fo2, and (or) fluctuations in the water content of the magmatic system beneath OllagiJe are implied.

Isotopes

Covariation of 875r/86Sr and t43Nd/]44Nd ratios of Ollagiie rocks are illustrated in Figure 12. A more complete data set will be published in a subsequent paper detailing the geochemistry and petrology of the rocks. The range in 875r/86Sr and Ja3Nd/144Nd ratios for Ollagiie rocks is small compared to the overall range from CVZ centers (Fig. 12). In common with

VOL{ "AN1C AND MAGMATIC EVOLUTION OF VOLC&N ()LLAGI~E 239

0.51320

0.51300

0.51280

0.5126{)

"~ 0.51240

~Z ~ 0.31220

0 .5125

0 .5124

0 .5123

, , / / L 5 1 2 2 ~ ~ A v /

7 - - - i / ~ . ~ _ ~ _ _ - N V Z

(0-2S)

~ '~F A x ~ k . g33_42S)

, , , , , , , , i , , , , i , , , , .

I [ T [ ~ i ,\/fiplano Puna ~tQcal/i~ C;llllp]f ~ l

0,706 _ 0 .707 0,708 0.709 0.71 I)

0.51200 _---

0.51180 _._.t 0.7020

c v z (17.5-26%)

0.7060 0.7100 0.7140 0.7180

",~Sr/~',Sr

Fig. 12. Isotope data for select Ollagtie lavas and inclusions compared with island-arc tLlt') data and data from the Andean Northern (NIT), Central (C17), and Southern (SIT) volcanic zones. Diagonall? ruled field in inset sho~s the compositions of rocks of the Altiplano-Puna volcanic complex from de Silva (1987). Symbols are the same as in Fig. 10. Arrows point to assumed composition of bulk Earth (BE). After Davidson et al. { 1990b ).

other CVZ centers, the Ollagiie rocks have higher 87Sr/86Sr and lower 143Nd/144Nd than late Cenozoic volcanic rocks from island arcs and the Northern and Southern Volcanic Zones of the Andes (Fig. 12; Davidson et al., 1991b).

Isotopic composi t ions for inclusions and mafic lavas (basaltic andesites) are systematically correlated, in contrast to composi t ions for the andesitic and dacitic lavas (Fig. 12). Inclusions and mafic lavas together have a large range in isotopic composi t ions whereas isotopic composi t ions of the andesitic and dacitic lavas are more restricted (Fig. 12). Feeley and Davidson ( 1991 ) have explained these trends by differentiation of inclusion-forming magma deeper in the crust where thermal condit ions permit large degrees of assimilation relative to fractionation. The restricted range in isotopic composi t ions of the andesitic and dacitic lavas suggests lower rates of assimilation under shallower (cooler) crustal condit ions (DePaolo , 1981 ; Gans et al., 1989; Feeley and Davidson,

1991). This idea will be discussed further below.

Petrogenesis of magmas at Volcfn Ollagiie: a working model

In this section we describe a simple model to explain the composit ional diversity of the magmas at Ollagi, ie. The model is not meant to be a rigorous description of the petrogenesis of the magmas nor are the values of the variables selected in the assimilat ion-fractional crystallization (AFC; DePaolo, 1981 ) calculations intended to quantitatively describe the magmatic system beneath Ollagtie. It is intended to constrain processes taking place in the magmatic system and to serve as a point of refer- ence for future studies of the rocks.

The magmatic inclusions and the basaltic andesite lavas of the Chasca Orkho series have a large range in Sr isotopic composi t ions over a relatively narrow range in R b / S r (Fig. 13A).

240 T . c , F E E L E Y ET AL.

r.g3

r./3

0 .709

A

0.708 -

(I.707 -

0 .706 -

0 .00

{) x o6

~.: ~ /....x {17 /

~ 1 / ~ 7 / 0 95 / .-" ~"- II{} ;-'--~1 {} " A F C

~ / / . . / / - 7 " / - - " Model Ds,

.'/-.,Z tr~-~ o 95 1 I ~ 2 5 / 2 1.25

[]

l)m, Ma/Mc ~r}

0.3 0.8 {}.3 {}.5 0.3 {}.5

I I I I I 0.05 0 .10 0. t 5 0 .20 0.25

Rb/Sr

{}.3{}

0 .709

/ 0 q 0.708 -- ~i~- ch . . . . o,'u~. ~,-,~,_[~

, c~ . /~ .... F C ' a [] 0

A2ZX g 0 .707 - t -CoU~p,~ st-iie ~ r,3

0 .706 - /

I I I I I 0.00 0.05 0 .10 O. 15 0 .20 0.25 0 .30

Rb/Sr Fig. 13. (A) Sr isotope constraints on bulk mixing and assimilation-fractional crystallization (AFC; DePaolo, 1981 ) models for Ollagiie inclusions and basaltic andesite lavas. The assumed contaminant in all of the models has aTSr/ S6Sr=0.725 and Rb/Sr ~ 0 . 4 (Kntiver and Miller, I981 ). The legend shows the bulk distribution coefficients (Ds, and DRb ) and r values used to calculate the model curves. Tic marks on Bulk Mixing curve indicate the percentage of silicic endmember in the mixture. Tic marks on AFC curves indicate the amount of original magma remaining (F). Symbols are the same as in Fig. 10. See text for discussion. (B). Sr isotope constraints on differentiation models for Ollagtie andesitic and dacitic lavas. See text for discussion.

This trend can be explained by assimilation of a large mass of crustal rocks during crystal fractionation. In Figure 13A, four crustal contamination cases are illustrated. Because of the large number of unconstrained variables in- herent in AFC models, the contaminant used to calculate all of the model curves is an aver- age composition of Paleozoic metamorphic rocks exposed in basement uplifts of the Pam- pean Ranges in northwest Argentina, about 400

km south of Ollagtie (Kniiver and Miller, 1981 ). This contaminant has a 87Sr/86Sr ratio of about 0.725 and a Rb/Sr ratio of about 0.4. The dotted model curve (Bulk Mixing ) in Fig- ure 13A illustrates bulk mixing between the isotopically least evolved inclusion (OLA9027i) and the crustal contaminant. The solid model curves depicting combined assimilation-fractional crystallization (AFC 1 and AFC 2; Fig. 13A) show the effect of decreasing the r value

V( }i (" XNI(7 AND MAGMATIC EVOLUTION OF VOLC-ixN ()[_I_~GI~'E ~4 ]

(the rate of the mass of crust assimilated relative to the mass of crystals fractionated; De- Paolo, 1981 ). The dashed model curve (AFC 3 ) shows the effect of simultaneously decreasing r and increasing the bulk distribution coef- ficient for St.

Model curves labeled Bulk Mixing and AFC 1 were constructed to simulate the effect that differentiation under deep crustal conditions has on isotopic compositions and trace element ratios. In the deep crust we assume that ambient temperatures are high, allowing large amounts of assimilation relative to crystallization. Although a bulk distribution coeffi- cient of 1.25 for Sr is moderate to somewhat high for basaltic andesite to andesite systems, Sr decreases with increasing SiO2 and was therefore compatible during differentiation (Fig, 11 ). The curves AFC 2 and AFC 3 were constructed to simulate the effect of AFC at shallower crustal levels where temperatures are lower so the amount of crust assimilated for a given amount of crystallization (r) is less, and plagioclase constitutes a larger percentage of the crystallizing assemblage. We interpret the data for the magmatic inclusions and mafic lavas to be more compatible with deep-crustal differentiation involving large amounts of crustal assimilation.

In contrast to the data trends for the inclusions, Figure 13B illustrates that when viewed on the scale of individual eruptive series where field evidence indicates that the rocks are com- agmatic, the andesitic and dacitic lavas have little isotopic variability. It is, therefore, possible that these lavas have undergone little to no crustal contaminat ion during differentiation. It is especially difficult to demonstrate AFC trends for Ollagiie andesitic and dacitic lavas on Figure 13B because at any point along the data array of the inclusions, which we inter to be parental magmas to the andesite and dac- ire lavas, it is possible to begin a horizontal fractionation trend through the composit ions of the lavas.

In addition to a major change in the amount of assimilation, there is a change in the type of

crust assimilated. On Figure 12 the steep data array of the inclusions actually extends to Nd isotopic ratios that are lower, and Sr isotopic ratios that are as high as those of the most silicic lavas present at Ollagtie. This trend is consistent with assimilation of old basement rocks with relatively nonradiogenic Nd and radiogenic St, There is some suggestion that the Sr isotopic compositions of the andesitic and dacitic lavas trend to more radiogenic compositions at roughly constant t43Nd/144Nd. This feature indicates that if these rocks have undergone some crustal contamination, Mio- cene ignimbrites of the Al t ip lano-Puna volcanic complex upon which Ollagtie is constructed are a possible contaminant (Fig. 12 ). Ollagfie may then be a case where AFC and fractional crystallization are difficult to tell apart because the upper crust is isotopically similar to previously contaminated parental magmas feeding the upper crustal system. The shift in r and nearly certain change in contaminant are consistent with a change from differentiation at deep crustal levels to differentiation at shallower crustal levels.

Comparison with other CVZ stratovolcanoes

The CVZ contains over 1,100 late Cenozoic volcanic edifices (de Silva and Francis, 1991 ). Until recently, however, only two andesite stratovolcanoes have been studied in suffi- cient detail to permit speculation about magma chamber processes and volcanic evolution: San Pedro (Francis et al., 1974: Thorpe et al., 1982: O'Callaghan and Francis, 1986) and Parina- cota (W6rner et al., 1988: Davidson et al., 1990b). Observations from these volcanoes and Ollagtie indicate that there are regular vol- canological and petrological processes that occur over large distances along strike in the CVZ. All three volcanoes are classified as composite volcanoes by de Silva and Francis ( 1991 ) because they were constructed in two stages sep- arated by collapse of their western flanks with resultant debris avalanches. The formation of large debris avalanche deposits late in the his-

242 T.C. FEELEY ET At..

tory of these and other CVZ volcanoes probably results from oversteepening of the edifices due to eruption of viscous andesitic and dacitic magma, as discussed by Francis and Wells (1988).

Lavas preceding the debris avalanche at San Pedro are predominantly basaltic andesites, whereas those erupted after the debris avalanche are mainly andesites and dacites (O'Callaghan and Francis, 1986). At Parina- cota, lavas erupted prior to the collapse event are generally more silicic (andesites to rhyolites) and have a larger compositional range relative to post-collapse lavas (andesites to basaltic andesites; W6rner et al., 1988). O'Cal- laghan and Francis (1986) found that post- collapse magmatism at San Pedro produced a succession of four eruptive groups. Within each group increasingly more silicic compositions were erupted with time, similar to the pattern for the Chasca Orkho series at Ollagfie. These field observations are consistent with chemical data suggesting that crystal fractionation, with or without crustal contamination, is an important petrologic process in upper crustal magma chambers beneath stratovolcanoes in the CVZ and elsewhere. The significance of amphibole in CVZ andesites and dacites is presently unclear and requires additional work. At Parina- cota, amphibole is an abundant phenocryst phase in pre-collapse lavas, yet it is virtually absent in post-collapse lavas (W6rner et al., 1988 ). Data on the abundances of phenocrysts in San Pedro lavas are not available.

A persistent problem in CVZ magmagenesis is the extent to which the andesitic and dacitic lavas have been affected by crustal contamination. Thorpe et al. (1976), Francis et al. ( 1977 ), and O'Callaghan and Francis ( 1986 ) published rare earth element, Sr isotopic, and major- and trace-element data, respectively, for San Pedro lavas. 87Sr/S6Sr ratios of San Pedro lavas are high (0.7055 to 0.7070) although they are systematically lower than those of Ollagfie lavas (Fig. 14; Francis et al., 1977). Like Ollagiie andesites and dacites they also do not exhibit a clear correlation with any index of

0 . 7 0 9 0

0 . 7 0 8 0

b -

0 . 7 0 7 0

O . 7 0 6 0

h i

0 . 7 0 5 0 ~ , , I , , , I , , , ~ ~_ , , I , . . . . . . .

5 0 5 4 5 8 6 2 6 6 71)

SiO2 w t . %

Fig. 14. Comparison of Sr isotopic compositions of lavas from Parinacota, San Pedro, and Ollagtie. Data from Davidson et al. (1990b) and Francis et al. ( 1977 ).

differentiation (Francis et al., 1977). None- theless, Francis et al. (1977) infer that San Pedro lavas were contaminated by lower continental crust because Sr isotopic ratios are elevated relative to Sr isotopic ratios of lavas from Ecuador (87Sr/86Sr "~ 0.7044), where the crust is 20-30 km thinner than in northern Chile. O'Callaghan and Francis (1986) suc- cessfully duplicated major- and trace-element trends of San Pedro lavas by fractional crystallization calculations without accompanying crustal assimilation. They also infer crustal assimilation, however, mainly on the basis of disequilibrium phenocryst textures. At Pari- nacota, 87Sr/86Sr ratios are very similar to those from San Pedro and also show little correlation with differentiation (Fig. 14). Davidson et al. ( 1990b, 1991 b ) explained this feature as a result of the establishment of "baseline" isotopic compositions in parental mafic magmas during contamination in deep-crustal magma chambers, followed by rise and further differentiation of magmas in shallower crustal magma chambers with or without subsequent contamination. This model is very similar to the one proposed here, except that parental mafic magmas that fed shallow crustal magma chambers at Oltagiie were not isotopically ho- mogeneous. It thus appears that shallow-level magma-chamber processes do not result in significant or systematic changes in radiogenic isotopic compositions. This may result from

v( )L( a~NIC AND MAGMATI(" EVOLUTION OF VOLCAN ()LLAG{~TE 243

closed-system crystal fractionation. An alter- native and more likely explanation is that crustal contamination is difficult to detect because the upper crust is isotopically and chemically similar to magmas previously contaminated deeper in the crust. Future geochemical work on the stable isotopic systematics of Ollagtie lavas will be aimed at documenting in more detail the amount of shallow crustal contamination they have undergone.

Conclusions

Field relations indicate that cone growth at Volc~in Ollagiie evolved during at least four main eruptive stages with an intervening sector collapse event between the second and third stage. During all of the stages, andesitic magma of relatively uniform composition was the dominant eruptive product, although magmas that vented on the flanks included more mafic types and a higher proportion of dacite. Quenched mafic inclusions in nearly all lavas preserve evidence that the magmatic system beneath Ollagiie was repeatedly fluxed from below with parental basaltic andesites and mafic andesites. Petrographic features such as the large proportion of xenocrysts in the inclusions indicate that crustal contamination was an important process in their petrogenesis. Whole-rock geochemical and isotopic trends of the inclusions indicate they are not simple two- component mixtures between a more mafic magma and exposed andesite and dacite lavas, however.

It is possible to explain geochemical trends of the basaltic andesites and mafic andesites by differentiation at deep crustal levels where thermal conditions permit large degrees of assimilation relative to factionation. The restricted range in isotopic compositions of the andesitic and dacitic lavas suggests that these rocks may have undergone smaller amounts of crustal assimilation during differentiation at shallow (cool) crustal conditions. Large amounts of crustal assimilation are possible if the upper crust is chemically and isotopically similar to the andesites and dacites. The pres-

ent data set for Ollagtie lavas does not permit distinction between these two alternatives. Fu- ture geochemical work will concentrate on evaluating in more detail the amount of upper crustal contamination at Ollagtie.

Acknowledgements

Supported by National Science Foundation grant EAR-8915808 to Davidson. We thank the Servicio Geologico de Bolivia (GEOBOL) for arranging field logistics, Peter Holden for as- sistance in the isotope lab, Dave Mayo for help with the XRF analyses, and Anne Loi for com- puter support. Wendy Bohrson assisted during field work. This work benefitted from discussions with Gerhard WiSrner, Shan de Silva, and Mary Reid. Unofficial reviews by Anita Grun- der and Gerhard W6rner and official reviews by Jim Luhr and Bill Rose resulted in significant improvements to this manuscript.

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