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301
The Australandean
Volcanic Zone (South Patagonia),
Rolf KILIAN
Mineralogical and Petrological Institut of the University
Wilhelmstr. 56. 7400 Ttibinaen. W-Germu
of Tubingen.
Abstract
New geochemical
data of volcanics
of the Mt. Burney and Viedma
review on petrology
of the six volcanic centers of the Australandean
49 - 55’S). Volcanic
products of recent activities
field. The volcanics
of the AVZ are acid andesites
volcano of the AVZ shows small geochemic
a special position,
related
ratios. Volcanics
the north,
and
a
Volcanic Zone (AVZ:
of 1988 were observed on the Viedma iceto dacites;
development.
basafts are missing;
every
Volcanics of Cook island occupies
show increasing
K. Rb. Ba contents
of Mt. Burney belong to a low-K
lavas and pumices of Lautaro,
contamination
complete
to volcanics of rift zones. From Mt. Burney in 52’S to Viedma
volcano in 49’ S. the volcanics
%r/*‘Sr
volcano
magma
mixing
as well as K/Rb
andesites
Viedma and Aguilera
are med-K
fused
are discussed
with
crustal
rocks
and
and dacites. More to
dacites. Crustal
for
this SN
development.
Introduction
The volcanism
reports
the beginning
which
in the south Patagonian
are available.
Volcanic
of this century
originated
Swedish expedition
Andes has remained
eruptions
on the Patagonian
almost unknown
by QUENSEL (1910). Ash deposits
in volcanic
centers
of the inaccessible
(1932-34) and examined
and only few
Ice Cap were first reported
in the Argentine
Andes,
were collected
at
pampas,
during
a
by SALMI (1941) and more recently by STERN
(1990). Some rocks of the Lautaro. Aguilera, Mt. Burney and Cook island (fig. 1) have been
geochemically
volcano
analysed
by STERN et al. (1976, 1984) and FUTA
Reclus (Sr57’S)
1988 and MARTINIC
has been described
by South Chilean
1988). In the area of the Patagonian
a volcanic center and poducts of recent eruptions
Tectonic
& STERN (1988). The
geologists
Icefield,
were described
(HARAMBOUR
on the Viedma glacier,
by KILIAN (1990).
setting
The Austral
Volcanic
Zone (AVZ) extends
between
and Viedma volcanoes are situated at the northern
49’ and 55”s. The Lautaro.
Aguilera
end, Reclus, Mt. Burney and Cook Island
are located
more to the south (fig. 1). All these Cenozoic
volcanoes
convergent
plate boundary
plate and the continental
between
the oceanic
Antarctic
are situated
near the
South
x.._____...-_ --
r
302
American
plate. The convergence
rate (3 cm/year.
Minster
et al. 1974). the tectonic
. volcanic activity are lower than to the north of the plate triple junction,
plate is subducted
at a rate of 5 to 10 cm/year.
volcanic gap of 300 km at the continental
and
where the Nazca
South of the plate triple junction.
there is a
margin.
Patagonlan Andes
wmlcmlozekvokUnou
cl
A
----
area ot hvea~noll
cmozokvwkMo
Frwlur.zo~
__lA__
Ageofn-mowmk
_._._..
Ff-
cwslhmU.ye~
South American
Fig. 1: Kanoaoic
volcanoes and plate tectonic structure
of the Patagonian
Andes.
Volcanic activity OQthe Viedma icefield of 1988
An extended
melting,
area was observed
Hot mud flows were formed,
valley net was formed
of eruption
deposits
on the Viedma
icefield
(Patagonia)
caused by the fall of hot pumices and large ash fractions
The eruptions
intensely
at 49’225
periodic
and 73’19’W in an elevation
eruptions
ice
a large area of the glacier. By the hot mud flows a
in the ice surface with a depth of up to 35 m. The probable
was located
of previous
covering
showing
after volcanic euptions.
were mapped;
pumices
center
of 1100 m. Pyroclastic
and lavas were examined.
must have taken place recently between Sept. and Nov. 1988.
.
303
Petrography
Minerals
in volcanics
electronmicroprobe
Viedma,
biotite
(0.5 -
1.5 mm) and
additionally
a second
xonation often occurs.
corroded
Some xenolithes,
phenocrysts
of Mt. Burner
show slightly
K-dacites
often
of volcanics
of Lautaro.
generation
Large plagioclase
of
idiomoph
anorthite
phenocryst
anorthite
xonation
(an 63) of
plagioclase
contamination.
Plagioclase
an 42
in many
contents
of orthoclase.
and indicate
are often rim corroded.
inverse
and
xonation in the range between
consisting
occur in lavas of Viedma and Aguilera
by microscope
(0.5 - 2 mm) and a few
phenocryst
middle xone with the highest
messurements.
biotite and amphibole
Hypersthene
phenocrysts
(1.5 - 4 mm), amphibole
normally shows small anorthit
inverse
show a strongly
our microprobe
were determined
(65). Characteristic
are plagioclase
Plagiocla&
to an 60. A slightly
pyroxene,
and Mt. Burney
messurements
and Aguilera
hypersthene.
cues
of Viedma
and
Plagioclase,
phenocrysts
of lavas
in the range an 45 to 65.
is the most commun mafic mineral with higher Mg/Fe
ratio (mg* 65) than in
of Viedma.
Ceochemical
and petrological
aspects
The volcanics of the AVZ range from 59 to 68 wt.% SiO, and are acid andesites
in the K,O/SiO,
diagramm
(fig. 2). The analyses
of the Viedma.
Lautaro
and dacites
and Aguilera
volcanoes show the highest KsO levels of the AVZ. To the south, the K,O, Rb, Ba. and Zr
levels of rocks from the volcanoes
Reclus and Mt. Burney decrease
latter case to a low-K
series. High-Al
calcalkaline
and correspond
basalts and basic andesites,
types of the SVZ (33 - 46’ S), are missing in the AVZ. Apparently
individual
volcanic
sugesstion
of FUTA
development
primitive
center
of the AVZ in the K,O/SiO,
& STERN
(1988) and STERN
uniform
diagramm
K calcalcaline
the south. Wide ranged calcalcaline
volcanics
of continental
Aguilera volcanoes change to low-K
volcano. This SN development
contamination,
indicated
margin
(fig. 2) confirm
becoming
series are missing. Orogenic
type
in 49’ S of Lautaro,
the
is no local
more
medium-
Viedma
and
series of island arc type in 52’ S from the Mt. Burney
to more continent
by xenolithes
typical rock
plots of each
et al. (1984). that there
of the melts, but marked regional NS change in the chemistry
towards
in the
related series is considered
in the lavas of Viedma and Aguilera
due to crustal
volcanoes.
But
also magma mixing with small amounts of partial fused crustal rocks can not be excluded.
Volcanics
of the Cook island (tab. 1) located more to the south (5s’S) show more relations
to MORB type than to erogenic
and Also,
contents
series (FUTA & STERN
as well as Sr. La concentrations
KsO, Ba, Rb. Cs levels.
1988), with relatively
but low TiO,,
high MgO
HREE, Zr and very low
._-I” ._
.“_
.*_ ..-__._
.
304
3.5
KZO
lccy3
I
l
Reclus
clururrl
2 Mt. Bumey
Fig. 2: K,O/SiOs
diagramm
with distribution
and plots of volcanics
of the Southandean
Volcanic Zone.
References
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Quaternary volcanic centers of the southern Andes. - Earth Planet. Sci. Lett.. 88
(1988): 253-262. Amsterdam (Elsevier).
HARAMBOUR.
S.M. (1988): Sobre et hallazgo del mitico volcan Reclus, ex Mano de1
Diabolo, Hielo Patagonico Sur. Magallanes, Chile. - Rev. Geol. de Chile, 15
(2):173 - 179; Santiago.
KILIAN,
R. (1990): Volcanic eruptions
MARTINIC,
on the Viedma icefield of 1988 (Patagonia).
M. (1988): Actividad volcanica hist6rica
Geol. de Chile 15, 2: 181 - 186; Santiago.
STERN, C.R. (1990): Tephrochronology
-.- (in print)
of Southernmost
en la regi6n de Magallanes.
Patagonia.
- Nat. geograph.
- Rev.
Res. 6:
STERN, C.R., FUTA, K. & MUEHLENBACHS.
K. (1984): Isotope and Trace Element Data
for Orogenic Andesites from Austral Andes. - in: ‘Andean Magmatism Chemical
ed.: Harmon R.S. & Barreiro B.A.: 31 - 48 S.;
and Isotopic Constraints”.
Cheshire, UK (Shiva).
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Subkontinental
Mantle
Lithosphere
below Southern
South America
and
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Santiago, Chile.
STERN,
C.R., SKEWES, M.A. & DURAN, A.M. (1976): Volcanismo Orogenico
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en Chile