-
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 com- posed 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 inclu- sions 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 assimi- lated
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 ele- ment 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
pri- mary 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 indi- vidual 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 cen- ters, however, because both are large ignim- brite
shield volcanoes surmounted by later in- termediate 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 stud- ies at individual
stratovolcanoes in the CVZ,
74" i I I
-- L PERL
7(3 6, I "\. I I
/ ( BOld \' i \
IJarirlacota l i ~
[ aclfic Ocean
Cenozoic- Recent Volcanic Series
- International frontier
0 200 km i ,
~,~12
San Pedro ~"~ j ' ~ , ~ , . -
Capricorn ...,./,' __
t CHILE ~i I :~
) ARGEN FINA
F
/ ' /
2 4
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 strato- volcano, and to quantify
petrogenetic pro- cesses 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 composi- tional 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 posi- tion 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 discus- sions 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 ex- tremely 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
land- scape (Noble et al., 1979; Tosdal et al., 1984).
Volcfin Ollagfie is part of a broad NW-trend- ing belt of late
Cenozoic calc-alkalic and ai- kalic volcanic centers that extends
from south- ern 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_.
68:21'W 21 10'S
2126'S 68:~21 'W
\
Chile
)
N (
t
J / )
7y Sahtr de ( Uhtgt7~" ~ [ Salar
J t ~ ,s~
"x, / / , :~ i
/ ( (
7 J r,)
(
I
i /
i t
Salar de
San Mart{n Cerro
WS::5:SL# ~ LkL=:::;:W :2:kW::: : ~ -- . . . . . !(i ~IT!L~!)) ~
i / ................................ .................... 1" c ~
i
.- i'J~77 7:777 ;7 77777 77 " ' ( ~
N ================================================= "~ t ~ [ : :
)
68 05'W
Explanation
[ ~ colluvitlm arid alluviuna
~ salar deposits
N gtacial moralrle
[] La Celosa series [] post-collapse series I debris ax,
alanchc
deposit
i [ ] Chasca Orkho , series
N altered pyroclaslic deposits
N Vinta 1.oma series
~ older volcanic rocks
ignimbrite
i exposed VCIII-,
hnnarolc
) i ~ ' ~ inlerred trace ,,I { l ~,,,a,~nch ........
:0 2 4 6
6805'W
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 mor- phology of the
lavas and persistent fumarolic emissions from a summit vent. The
lava flows, domes, and pyroclastic deposits of Ollagiie ov- erlie
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 re- lated 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 stratigra- phy and petrographic
features of the lavas at Ollagiie. Volc~in Ollagiie is a complex
compos- ite cone with evidence for a multistage erup- tive 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 earli- est
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 di- vided 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 in- dividual eruptive series are illustrated in
Fig-
~:tllll~ll'OlC
) i
C. Debris Avalanche stage
B. Chasca Orkho stage
San Maiim ~ -
A. Vinta Loma stage
\Vcsl
l \Clli
-
226 T.C FEELEY EI',At..
8
7
c/3
4 < :~ 3
8
7
4 < 3
2
8
7
6
4 < 3
2
8
7
4 < ca: 3
2
La Celosa series
Post-Collapse series
Chasca Orkho series
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 la- vas into the
eruptive series was based principally on vent locations and contact
rela- tionships. 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 la- vas
were assigned to eruptive series on the ba- sis 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 dom- inant 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 vol- ume
(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 collu- vium 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 depos- its 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 la- vas 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) > > py- roxene >
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 sig- nificant amphibole relative to pyroxene (Fig.
6). Microphenocrysts of plagiociase, pyrox- ene, 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 sum- mit 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 con- tains 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 ).
60
q
5o b ~ 40 C 2 ~ 30
i i i i i i i i i i i i i i i i i i
I A C{ '[osz l 5 e r i c 5
{5~i{ Olivine -~ rJ ':!~ Clinopyr{}xenc ~
Orthopyroxcn{' ~ l I'lagioclasc
i : Amphibole i i i Biotit{.' [ ] Oide
I I I I I I I I I I I I I I I I
u
r -
(b
BO
4{)
3()
20
ll)
i i i i i i i I i i i i i i i i i i i
I0~1 C }lhi'pse se "i '/ -r 1. ,
r,
5 ~
li i i i i i I I I I i i I i i m
e-
ca
()0
50
40
30
20
I0
i i i , i i i i i i i h , I i , i i
(-/mSCa ()rkllo s e r i e > ,-~
g m
i i ~ i i i i i i i
'o O o
2~
~0 q2 54 [~6 $8 f3(} (,2 {34 I~h 8 7{3
SiO, wt/./~
Fig. 5. Volume% of phenocrysts versus wt.% SiO2 of rep-
resentative Ollagfic lavas b.~ eruptive series. Data deter- mined
by point counting > 1000 points per thin section. Phenocrysts
are defined as larger than 0.3 mm.
-
228 T.C. FEELEY ET AL.
, A m p h io yx
" ~ Pyx > Amph + Bio
II
S~ale(km)
[~ 2 4 6 S
Fig. 6. Spatial distribution of Ollagiie lavas in which
amphibole and biotite are present in greater proportions than
pyrox- ene 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 south- ern part of the field. Three small-volume, dark- grey,
vesiculated, olivine-phyric basaltic an- desite 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 an- desite. Texturally and
mineralogically this flow resembles the underlying basaltic
andesite flows except that it contains more modal cli- nopyroxene.
The stratigraphic position of the basaltic andesites at Ollagiie is
significant be- cause 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 cov- ers 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 re- sult of
lateral spreading during dome growth. This rift strikes N55 W,
parallel to the align- ment direction of the other exposed vents on
Ollagiie (Fig. 2). In the western part of the dome and coul6e
field, where contact relation- ships 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 tex- tures, 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 in- crease 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, de- fined by subparallel alignment of stubby pla-
gioclase 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 reac-
tion 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 pro-
duced a debris avalance deposit, which was first recognized and
described by Francis and Wells
( 1988 ). The debris avalanche deposit is pres- ently 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 pro- duced 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 se- ries 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 ande- site bombs are
occasionally found on top of the debris avalanche deposit. It is
unclear, how- ever, if these bombs represent a juvenile mag- matic
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 amphithea- ter
(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 av- alanche
deposit. Young morphological fea- tures such as well-developed
lev6es and pres- sure 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 evi- dence is not conclusive.
The presently exposed volume of the sum- mit 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 , be-
cause 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
col- lapse 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 rel- atively young
post-collapse lava flow (Fig. 8 ). Additional late-stage activity
included con- struction of small-volume andesitic phreato- magmatic
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 re- lated 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%; plagio- clase (An6o 40) dominates
the mode as both phenocrysts and as a groundmass phase. In older
post-collapse lava flows low on the west- ern flank, pyroxene is a
more abundant phen- ocrystic phase than amphibole and biotite (
Fig. 6). These flows are overlain by shorter, am- phibole- and
biotite-bearing flows at topo- graphically higher levels on the
volcano. For example, the summit dome (OLA9037; Fig. 5 ) is a
coarse-grained porphyritic to glomeropor- phyritic, 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-col- lapse 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 ) be- cause 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
diffi- cult to be certain what the relative age of the La Celosa
series is from field relationships, be- cause 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 vol- cano. 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, topo-
graphically lower vent flowed down slope and presently partially
encircle this vent with a se- ries of concave upslope pressure
ridges. Lava erupted from the topographically higher vent flowed
down the backside of the older flow to- ward the southwest (Fig. 2
). The lavas erupted from both vents are compositionally and pet-
rographically indistinguishable.
Textures of La Celosa rocks range from por- phyritic 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 be- cause 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 vol- canic 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 in- clusions 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,
or- thopyroxene, and basaltic amphibole in vary- ing proportions
(Fig. 9A). Pyroxene pheno- crysts 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 inter- preted
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 ox- ides in a brown glass matrix (Fig. 9A). Vesic- ulated
mafic inclusions with similar textures have been described from
many other conti- nental 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 inclu-
sions in Ollagiie lavas.
The second type of inclusions are unvesicu- lated 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 usu- ally 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 de- scribed above; they are angular and
defined by crystal boundaries (Fig. 9B). The small gab- broic clots
are distinguished from glomero- crysts because clinopyroxene in the
gabbroic clots commonly forms oikocrysts enclosing plagioclase
(Fig. 9B). The unvesiculated na- ture of these inclusions and the
cumulate tex- tures 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 micro-
phenocrysts of clinopyroxene, orthopyroxene, Fe-Ti oxides, and
sparse brown glass. In addi- tion, 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 activ- ity 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 la- vas 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 phen- ocrysts (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-col- lapse 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 ac- tivity migrated to
the southeast flank where a sequence of olivine basaltic andesites
to am- phibole and biotite-bearing andesites and dac- ites (Chasca
Orkho series) was erupted on top of Vinta Loma flows. The trend
toward erup- tion 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 compo-
sition lavas (c.f. Volc~n Colima; Luhr and Carmichael, 1980). In
this model the tavas represent leaks from the magma chamber dur-
ing progressive degrees of differentiation. The culmination of
magmatism to date at Ollagiie occurred following collapse of the
upper west- ern flank. At this stage, eruptive activity largely
returned to the summit area and modal com- positions of the lavas
became highly variable, although bulk compositions of erupted mag-
mas 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 se- ries 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 sug-
gests 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 pheno- crysts (Fig. 6). Although it is
presently diffi- cult 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 horn- blende. Furthermore, they contend that a sim-
ilar near surface addition of groundwater to the magma system
beneath Volc~in Colima was re- sponsible 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 con- strain 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 illus- trated in Figures 10 and 11 and are given in
Table 1. Compositions are similar to interme- diate 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 com- posed principally of plagioclase (with apatite
inclusions); A1203 and P205 are significantly
elevated and elements not compatible in pla- gioclase 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 de-
crease 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 sym- bols 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 ex-
posed 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, how- ever. Petrographic features
of the vesiculated inclusions such as the large volume of xeno-
crysts indicate large degrees of crustal contam- ination during
differentiation of inclusion- forming magmas. Furthermore,
variations in radiogenic isotopic compositions preclude dif-
ferentiation by closed-system processes alone (Fig. 12).
Rocks of all four eruptive series have indis- tinguishable 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 amphi- bole- and biotite-dominated assemblages (Fig.
5 ). Factors that control the stability of amphi- bole 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,
pres- sure, 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 geochem- istry 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 systemat- ically 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 iso- topic 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 shal- lower (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 crystal- lization (AFC; DePaolo, 1981 )
calculations intended to quantitatively describe the mag- matic
system beneath Ollagtie. It is intended to constrain processes
taking place in the mag- matic 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 con- tamination 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 is- otopically least evolved
inclusion (OLA9027i) and the crustal contaminant. The solid model
curves depicting combined assimilation-frac- tional 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 rela- tive to the
mass of crystals fractionated; De- Paolo, 1981 ). The dashed model
curve (AFC 3 ) shows the effect of simultaneously decreas- ing 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 ele- ment ratios.
In the deep crust we assume that ambient temperatures are high,
allowing large amounts of assimilation relative to crystalli-
zation. 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 la- vas to be more compatible with deep-crustal
differentiation involving large amounts of crustal
assimilation.
In contrast to the data trends for the inclu- sions, 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, pos- sible that these lavas have
undergone little to no crustal contaminat ion during differentia-
tion. 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 sili-
cic lavas present at Ollagtie. This trend is con- sistent with
assimilation of old basement rocks with relatively nonradiogenic Nd
and radi- ogenic St, There is some suggestion that the Sr isotopic
compositions of the andesitic and dacitic lavas trend to more
radiogenic compo- sitions 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 vol-
canic complex upon which Ollagtie is con- structed 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 contam- inant are consistent with a
change from differ- entiation at deep crustal levels to
differentia- tion 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 oc- cur over large distances along
strike in the CVZ. All three volcanoes are classified as composite
volcanoes by de Silva and Francis ( 1991 ) be- cause 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 proba- bly results from
oversteepening of the edifices due to eruption of viscous andesitic
and daci- tic 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 ava- lanche 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 rhyo-
lites) and have a larger compositional range relative to
post-collapse lavas (andesites to ba- saltic 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 impor-
tant 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 un- clear 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
contami- nation. 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
con- tinental crust because Sr isotopic ratios are el- evated
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
crystal- lization calculations without accompanying crustal
assimilation. They also infer crustal as- similation, 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 correla- tion with differentiation (Fig. 14).
Davidson et al. ( 1990b, 1991 b ) explained this feature as a
result of the establishment of "baseline" iso- topic compositions
in parental mafic magmas during contamination in deep-crustal magma
chambers, followed by rise and further differ- entiation 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
sig- nificant 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 be- cause the upper crust is isotopically and chem- ically
similar to magmas previously contami- nated 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 con- tamination 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 sec- tor 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 inclu- sions 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 as-
similation relative to factionation. The re- stricted 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 discus- sions 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 signifi- cant improvements to this manuscript.
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