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J . geol. Soc. London, Vol. 141, 1984, pp. 909-917, 2 figs, 1 table. Printed in Northern Ireland.
ThePatagonianBatholith
S of Tierra del Fuego, Chile:timingand
tectonic implications
M. Herve, M. Suarez & A. Puig
SUMMARY: Reconnaissance field studies on the Patagonian Batholith exposed S of Tierra del
Fuego, together with 49 new K-Ar and Rb-Sr mineral dates, render new insights into two main
aspects of the late Mesozoic geology: the geochronology and composition of the batholith, and
time relations between the emplacement of the plutons and deformation. The envelope of the
batholith in this area is mainly composed of late Jurassic-early Cretaceous sedimentary and
volcanic rocks formed in an island arc-marginal basin system.
Three main plutonicgroups
have been recognized which have yielded mineral dates,
interpreted as crystallization ages, ranging from141to
34Ma.The earliest is agabbroic
assemblage,including
gabbros, diorites and hornblenditesexposedas
relicts within later
granitoids and with dates ranging from 141 to 103 Ma. The Canal Beagle Plutonic Group is
composed of granitoids characterized by a penetrative syn-magmatic foliation and with dates of
113 to 81 Ma. The Seno Ano Nuevo Plutonic Group is formed of granitoids with dates of 60 to
34Maand is mainly exposed in theouter islands, indicating a local oceanwards shift of
plutonism with time.
Lower Cretaceous island arc volcanism was contemporaneous with the intrusion of the
gabbroic assemblage, and, in part at least, of the Canal Beagle Plutonic Group. Folding of the
marginal basin infill, which has been related to the closure of the marginal basin, occurred from
about 100 to 93 Ma ago. The gabbroic assemblage was apparently mainly pre-tectonic, whereas
the Seno AAo Nuevo Plutonic Group was clearly intruded after this deformational event. A
post-tectonic or possibly late-tectonicemplacementforthe
youngest plutons of theCanal
Beagle Plutonic Group and a pre- or syn-tectonic intrusion for the oldest is inferred.
Despitethe
large size of the Mesozoic-Cenozoic
calc-alkaline Patagonian Batholith of southern Chile,
there is only a little reconnaissanceradiometric and
geochemical data available from selected areas (Halpern 1973; Suarez 1977; Stern & Stroup 1982). We
present here the first attempt to subdivide a segment
of thisbatholith
andtorelatethe
timing of the
emplacement of these rocks to tectonism. Three main
plutonic groups based on the petrography, fabric and
on 49 new K-Ar and Rb-Sr mineral dates have been
recognized in the batholithexposed S of Tierra del
Fuego (Fig. 1).
The timerelationsbetweenigneousanddeformational events in the region are difficult to determine.
However,
the
Neocomian
rocks
exposed
in Isla
Navarino (Fig. 1) wereprobablyfoldedsometime
between 100 and 93 Ma ago and, consequently, the
emplacement of the batholith, which has yielded ages
ranging from 141 to 34 Ma, spanned this tectonism.
tholeiitic rocks, known as theTortuga
Complex,
comparable totheupperpart
of an ophiolite and
thought to have been emplaced by sea-floor spreading
between blocks of continentalcrust
(Dalziel et al.
1974; Suarez 1978; Elthon & Ridley 1980). These
rocks form part of the floor of the marginal basin and
are overlain by the Yahgan Formation, composed of
turbidites and pyroclastic rocks, and representing the
infill of this basin. To the S these rocks interfinger with
andesitic and rhyolitic rocks of the Hardy Formation
which represent the products of island-arc volcanism
(Suarez & Pettigrew 1976).
The age of the envelope is poorly known and only in
the YahganFormationhaveafewearlyCretaceous
fossils been found. The youngest age of this unit is
Aptian-Albian (119-98 Ma) (Dott et al. 1977). These
rocks can be correlated with those exposed in South
Georgia Is., now 2000 km to the E (e.g. Dalziel et al.
1975; Sugrez & Pettigrew 1976). An age of 103-100
Ma has been determinedforthe
lower part of a
volcaniclastic unit on SouthGeorgia Is. (Tanner &
Regional geology
Rex 1979) comparable to the proximal facies of the
Therefore,
an age of about
The Patagonian Batholith S of Tierra del Fuego was Yahgan Formation.
intruded into volcanic, plutonic and sedimentary rocks 100 Ma (or somewhat younger) can be given for the
interpreted as having been
formed
in an island youngest strata of the marginal basin infill. No
arc-marginal basin system, related to the subduction of Palaeozoic rocks have been recognized S of Tierra del
the Pacific lithosphere under South America, during Fuego, with the possible exception of the pillow lavas
the late Jurassic-early Cretaceous (Dalziel et al. 1974; and metasedimentary phyllites of Islas Diego Ramirez
Suarez & Pettigrew 1976; Winn 1978; Saunders et al. (lat. 56"15'S, long. 68'45'W approx.), S of the study
1979). The envelope of the batholithincludes basic area (unpublishedobservation). However, further N
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910
M . H e w & et al.
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The Patagonian Batholith of S Chile
the Patagonian
Batholith
is intruded
into
late
Palaeozoic
metasedimentary
rocks belonging to a
subduction complex (Forsythe & Mpodozis 1979).
A highly inhomogeneousstructural
style characterizes the area S of Tierra del Fuego. The turbidites
of the Yahghn Formation exposed in Isla Navarino are
intensely folded with avergencedominantly
away
from the Pacific (Dalziel et al. 1975; Suarez &
Pettigrew 1976). The tight asymmetric folds have
hinge lines trending parallel to the continental margin.
Deformation diminishes greatly
southward
where
rocks of the Yahgan and Hardy Formations are gently
folded (Suarez & Pettigrew 1976; Suarez et al. 1979).
Towards theW,
along
Canal
Beagle,
the highly
deformed marginal basin assemblage presents complex
structures which have been interpreted as the result of
progressive strain (Dalziel 1981).
Mylonitic belts up to 7 km wide are developed in
granitic and gabbroic rocks, and in basic rocks .of the
Tortuga Complex, in the northwestern part of the area
(Fig. 1). Characteristically the granitoids are cross-cut
by a swarm of basic dykestrending parallel tothe
WNW regional strike and emplaced prior to mylonitization.Miocene andRecent volcanic rocks locally
overlie the Mesozoic units in the Isla PacksaddlePeninsula Hardy and Cook Is. areas (Puig et al. 1984).
Radiometric data
49 new mineral dates, mostly determined by the K-Ar
method but also including Rb-Sr determinations, were
obtainedfrom
27 specimens fromthe
Patagonian
Batholith S of Tierra del Fuego (Table 1). These dates
include hornblende, biotiteanda
few hornblendebiotite K-Ar mineral pairs,
and
also K-Ar and
Rb-Sr dates obtained from the same sample. In some
cases,specimens
fromthesamepluton,
butsome
distance apart, havebeen
dated. All the biotitehornblende mineralpairs,except
one, gave concordant ages.
The gabbros are difficult to date by the Rb-Sr or
K-Ar radiometric methods. The very low Rb-Sr values
of the whole rock as well as of the principal minerals
(plagioclase, hornblende and
pyroxene)
precluded
Rb-Sr dating using the available instrument.Hornblende, the most suitable mineral for K-Ar dating
in
theserocks,
was sometimesextremely
difficult to
separate from included pyroxenes. In order to check
the Ar retentivity of plagioclase in the gabbros, some
hornblende-plagioclase pairswereanalysed,but
in
every case the apparent age of plagioclase was much
youngerthan that of thehornblende(e.g.
samples
1-386 and 3-224 (Table 1).
The new dateson specimens of the Patagonian
batholith span the period 141 to 34 Ma (Table 1, Fig.
2), which represents a wider rangein age than the
previous dates of 97 to 81 Ma known for this segment
of the batholith (Halpern & Rex 1972; Halpern 1973).
911
The new radiometric ages correlate wellwith dates
obtained
from
the batholith further between
N
latitudes 50" and 52" S, which span the period 160 to
15 Ma (Halpern 1973). The grouping of dates from 141
to 81 Ma (Fig. 2) corresponds to the mineral ages of
the gabbroic assemblage and the Canal Beagle
Plutonic Group. Aparticularconcentration
of ages
exists from 113 to 81 Ma. A second grouping of ages
from 60 to 34 Ma corresponds to dates obtained from
specimens of the Seno Ario Nuevo Plutonic Group.
These two groups of dates are separated
by about 21 Ma,
during which only a biotite reset age of 72 f 2 Ma has
been determined (Table 1) (HervC 1984).
Previous work
From four samples of the Patagonian Batholith exposed S of Tierra del Fuego, Halpern & Rex (1972)
and Halpern (1973) obtained seven K-Ar dates which
ranged from 97 to 81 Ma (recalculated with the decay
constantsrecommended
by Steiger & Jager 1977).
These datesinclude three concordant mineral pairs and
were interpreted as crystallization ages. Thedated
specimens belong to the Canal Beagle Plutonic Group
as defined in this work. The oldest previously known
age in the area, 118 f 7 Ma (Halpern & Rex 1972),
was obtained from a gabbro boulder included in the
Tekenika Beds (assigned to the Albian-Aptian (11998 Ma) by Dott et al. 1977).
Interpretation of new radiometric dates
When 10 biotite-hornblende mineral pairs of the
same
sample
were dated, concordant ages were
obtained (with one exception). The concordance of
two or more
dates
derived
from
minerals with
different closing temperature, such as hornblende (c.
500°C) and
biotite
(c. 25OoC), is considered as
evidence that the age pair in question corresponds to
the age of emplacement (Evernden & Kistler 1970).
An alternativeexplanation, in which the concordant
ages could be younger than the date of emplacement,
considers intrusion of the plutonic rocks an
in
environment with temperatures above 500 "C and a
delayed but rapid uplift (Krummenacher et al. 1975).
However, this explanation can be discarded for the
area considered here, particularly in
view
of the
low-grade metamorphism of the envelope which
indicates temperatures lower than 500°C. This low
grade of metamorphism also precludes resetting due to
deep and widespread burial (HervC 1984).
It can thus be inferredthat the mineraldates in
theseplutonic rocks generally represent an age near
the time of crystallization of the pluton. The locations
and ages of dated samples are shown in Table 1 and
Figs 1 & 2 and only the dates intepreted as caused by
resetting will be discussed below.
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0 0 0 0 0 0 0 0
m
rm
6
m
W
8
8
8
c?
3
3
m
m
0
3
?
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0
m
N
m
0
W
W
m
3
I'
7
S
>
v)
v)
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3 W W W
m
Wmmm
d
+-H++ m+
o m m m
*
3
m
8
'9
3
2
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m
000
<m
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9?
0
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00
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0
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M . Hervt et al.
914
Volcanlc arc
uplift
7
_ _ _ _ _ _ Closure marginal basin __--deformation
Margtnal basm infill
?
AL NUEVO
AEO
SENO
-----
" 3
40
50
60
-
I
. .
90
100
C a l c u l a t e da g e( M a )
80
70
Number of sample in Table I and Figure
Ages from Halpern (1973) and Halpern
,
.. ~ .
I10
120
Syn-andpretectoniccalc-alkallne
Postectonic calc-alkaline plutonism
I30
140
PlUtOniSm
SenoAiioNuevoPlutonlcGroup
I
8 Rex (1972)
?
island arc volcanlsm
0
CanalBeaglePlutonicGroup
GabbrolcAssemblage
FIG. 2. Histogram of the ages of rocks from thePatagonianBatholith
S of Tierra del Fuego, showing the
distribution of the differentplutonicgroups
and a simplified geotectonic chronology (geologic time scale after
Palmer 1983).
Beagle Plutonic Group. Two samples of tonalite,
comparable to those of the CanalBeagle Plutonic
A comparatively old date of 130 f 3 Ma (Table 1)
Group, collected onthe S coast of Estero Fouque
was obtained on hornblende from a foliated gabbro,
yielded Rb-Sr biotite dates of 53 k S and 40 f 4 Ma.
exposed on the border of a mylonitic zone, 7 km wide,
The significance of these ages is currently uncertain.
E of PuertoFortuna
(Fig. 1). The recrystallized
texture of thehornblende
indicates that this date
represents a reset age, which may correspond to the
The components of the batholith
time of mylonitization.
A K-Ar date of 110 S Ma obtained on plagioclase The Patagonian Batholith S of Tierra del Fuego is a
of a gabbro specimen which yielded a K-Ar horn- complex of separate calc-alkaline plutons(Suarez
blende age of 141 7 Ma (Table l ) , most probably 1977; Suarez et al. 1979) mainly composed of tonalites
represents a reset date. The hornblende date is taken and granodiorites, with lesser amounts of quartz
monzodiorites, diorites and gabbros. The radiometric
as an age near that of crystallization.
Anon-foliated gabbro exposed onthe S coast of data, combined with the petrography,texture
and
SenoLuia,on
Isla Londonderry, has yielded K-Ar fabric of the intrusive rocks, the presence and nature
contact reladates of 103 f 10 Ma on hornblende and 51 f 3 Ma on of the inclusions in themandtheir
plagioclase (Table 1, Fig. 1). The latter may represent tionships with the envelope, allow the subdivision of
this segment of the batholith into three main plutonic
resettingrelated tothe intrusion of rocks fromthe
Seno Afio NuevoPlutonic
Group, exposed in the groups (Fig. 2, Table 1). An earlygabbroic assembvicinity (Fig. l), whilst the 103 f 10 Ma date is lage (141 to 103 Ma) and two granitoid groups named
interpreted as an age near that of crystallization.
here the Canal Beagle Plutonic Group (113 to 81 Ma)
Adiscordant
K-Ar hornblende-biotitepair
was andtheSeno
Ado Nuevo Plutonic Group (60 to
obtained from tonalite from the N coast of Isla Hoste, 34 Ma) .
with a hornblende date of 89 f 3 Ma and a biotite date
of 72 f 2 Ma. The latter either represents a reset age
The gabbroic assemblage (pre-late
orthe result of adelayeduplift,
andtheformer
is
Cretaceous)
interpreted as a crystallization age (HervC 1984).
The Tertiary plutonism of the SenoAno Nuevo
Gabbros and hornblendites, exposed as inclusions of
Plutonic Group may have caused localized loss of Ar up to 30 km2 within younger granitoids, are grouped
in some of the granitoids of the CretaceousCanal
rather arbitrarily here in a single gabbroic assemblage.
Reset dates
+
*
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The Patagonian Batholith of S Chile
Where contactsbetween the gabbroic rocks and the
granitic components of the batholithwereobserved,
the former rocks were always the older. The gabbros
and hornblenditeshave yielded K-Ardates ranging
from 141 to 103 Ma (Table 1, Fig. 2). The youngest
age of emplacement so far recognized for gabbros in
the region is mid-Cretaceous (100-93 Ma). This is
based on the fact that these basic rocks were emplaced
in the Yahgan Formation, for which a minimum age of
about 100 Ma has been assigned. and were intruded by
tonalites which have yielded a K-Ar date of 93 Ma
(our unpublished data). A minimum age forthe
gabbros is also given by tonalites intruded into gabbros
on the NE coast of Isla Londonderry (Fig. l ) , which
have been dated at 109 f 9 and 111 9 Ma (Table 1).
The petrography andtexture of the rocks of the
gabbroic assemblage are extremelyvariable and, in
any outcrop, different rock-types with gradational or
sharp contacts are commonly found. Most of these
rocks generally exhibit cumulate textures and show no
penetrativefoliation. The mineralogy includes combinations of labradorite,hornblende, clinopyroxene,
orthopyroxene, and occasional olivine and biotite.
Mylonitic structures are found locally andgabbros
with polygonized hornblendeand
plagioclase were
occasionally recognized (e.g. E of Puerto Fortuna).
The close spatial and
temporal
relationship of
gabbros with granitoids of different ages and the
presence of abundanthornblende and of occasional
biotitefavoursageneticrelationship
with the calcalkaline plutons. The role played by these basic rocks
in the evolution of the Patagonian
Batholith
is
comparable to that observed in other batholiths such
as the Coastal Batholith of Peru (Cobbing et al. 1981)
and the Sierra Nevada batholith (Bateman 1983).
*
Canal Beagle Plutonic Group (Cretaceous)
The Canal Beagle Plutonic Group constitutes most
of the plutonic rocks exposed along the Canal Beagle
and
granodiorite are
the
area (Fig. 1). Tonalite
dominant rock types with lesser amounts of quartzmonzodiorites and quartz-diorites. The plutons, with
Cretaceous ages, coincide with those which exhibit
a penetrative foliation of syn-magmatic origin and
locally deformedaureoles in theSanta Rosa area.
The widespread
distribution
of dated specimens
gave adequate control for the use of this fabric to map
the distribution of the Canal Beagle Plutonic Group.
However, in doing this, it must be borne in mind that
this fabric was formed during the emplacement of the
plutons and, hence, it does not necessarily represent a
specific magmatic character.
The rockswere
mainly emplaced into marginalbasin assemblages in the structuraldomaincharacterized by strongdeformation Suarez 1978). Inthe
aureoles of some of these plutons, granitic sills show
pinch and swell structures caused either by regional
915
stresses or forceful emplacement. This plutonic group
includes adiapiric
pluton, known as the Castores
Pluton and exposed in the Santa Rosa area, a zone of
only moderate regional deformation.
The
synmagmatic foliation, characteristic of these rocks, will
be referred to here loosely as a ‘granitic foliation’ to
distinguish it from mylonitic structures.This
is a
penetrative fabric usually parallel to the regional trend
and which doesnotgetweakerinwardsfrom
the
pluton contacts. It is defined by the planar orientation
of tabular minerals and elongated basic inclusions.
The latter commonly show pinch and swell structures
parallel tothe foliation. At leastsome of the basic
inclusions may be interpreted as disrupted synplutonic dykes that wereinjected intothe granitic
magma. The ‘granitic foliation’ probably formed prior
to the complete consolidation of the magmas, perhaps
as a responseto diapirism and/or a regional stress-field.
Seno Ano Nuevo Plutonic Group
(Tertiary)
The Seno Ano Nuevo Plutonic group is a plutonic
complex of non-foliatedgranitoids with radiometric
ages ranging from 60 to 34 Ma. These rocks are mainly
exposed in a belt along the Pacific side of the
Patagonian Batholith, indicating an oceanward shift in
time of plutonic activity for that area. However, they
crop out along the inferred location of the late
Jurassic-early Cretaceous island arc (Suarez & Pettigrew 1976). The rock-types range from quartz diorites
to granites, with tonalite and quartz monzodiorites as
the most common. In their petrography, they are
comparable with, but somewhat more K-rich, than the
Canal Beagle Plutonic Group, and differ from it in the
absence of a penetrative ’granitic foliation’. A N-S
trending swarm of basic dykes was intrudedinto
plutons of this group exposed on the southeastern bays
of Peninsula Rous.
TheSeno
Aiio NuevoPlutonic
Groupintruded
plutons of the Canal Beagle Plutonic Group, foliated
gabbros of the gabbroic assemblage, and foliated basic
rocks assigned to the Tortuga Complex. On Isla Cook
apluton of this group cuts across a mylonitic zone
developed in granitoids intruded by a swarm of basic
dykes. Locally theseplutonsincludenon-oriented
xenoliths of granitoids with ‘granitic foliation’ and
mylonitic gabbros assigned to
the
Canal Beagle
Plutonic Group and tothe
gabbroic assemblage,
respectively. The aboveobservationsindicate
that
these rocks were of post-tectonic emplacement.
Relationship between plutonism
and deformation
Mid-Cretaceous tectonism and uplift in the Cordilleran area of the southernmost Patagonian Cordillera
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916
M . Hervt et al.
has been suggested by several authors (Feruglio 1949;
Cecioni 1957, 1960; Katz 1962, 1964, 1973; Scott
1966). This
inference
was mainly based on
the
observation that the first appearance of coarse flysch
strata on the Atlantic side of the Cordillera occurred
in the Cenomanian (c. 91-97 Ma; Scott 1966), suggesting that this was the time of the initial deformation
and tectonic uplift of thearc
and marginal basin
terranes (seeDalziel & Palmer 1979; Dalziel 1981).
This date is in accordance with the approximately
100 Ma age assigned to the youngest strata of the
marginal basin assemblage (Dott et al. 1977: Tanner &
Rex 1979). A new K-Ar date of 93 Ma on biotite from
aninferred post-tectonicgranodioriticdyke
in the
Santa Rosa pluton, may represent a minimum age for
this deformation(our unpublished data).On these
grounds the folding of the Yahgan Formation exposed
in Isla Navarino, which has been related to theclosure
of the marginal basin, can be inferred
to have taken
place at about 100 to 93 Ma, as suggested by previous
authors(Halpern & Rex 1972; Dalziel etal.
1974:
Suarez & Pettigrew 1976; Dalziel 1981).
Most of the batholithicrocks in contact with the
coverrocksbelong
tothe
CanalBeaglePlutonic
Group.However,the
locally observedcontacts are
sharp and parallel to a foliation in the hornfelses and
to the ‘granitic’ foliation in the granitoids. Therefore,
the timing of emplacement of most of the plutons in
relation to tectonism is difficult to determine on the
available field data.
If the mid-Cretaceous dating of tectonism is correct,
of the gabbroic assemblage would have
thenmost
been of pre-tectonic and partly syn-tectonic emplacement, whereas the Seno Ano Nuevo Plutonic Group
clearly was intruded post-tectonically.
The emplacement of theCanal
BeaglePlutonic
Group may have spanned this deformation. The oldest
plutons of this group may have been emplaced before
ornearthe
time of this deformation, whilst the
youngest plutons were apparently intruded after the
main tectonic event, possibly during its waning stages
(Fig. 2). The latter is supported by the observation of
tonalite dykes which cut across boudins in the aureole
of a 87 Ma pluton exposedin the Brazo SW Canal
Beagle and of rocks identical to the tonalite of Bahia
Romanche (with an average date of 93 Ma, Fig. 1,
Table 1) intruded3 km to the N,in Cordillera Darwin,
into a mylonitic granite of 157 f 7 Ma (Rb-Sr whole
rock isochron, Herve et al. 1981). However, in dealing
with this problem, it mustbe rememberedthatthe
region is characterized by different structural domains,
some of which have
more
than
one phase of
deformation, and which may have been deformed at
different times. Therefore, it is notcertainthat
the
deformation in the above Mesozoic rocks exposed in
the W part of the
area
will be of the same
mid-Cretaceous age as inferred for the rocks exposed
farther to the E in Isla Navarino (Fig. 1).
Conclusions
The principal components of the Patagonian Batholith
S of Tiera del Fuego and their radiometric
ages are
presented in Fig. 2, which also shows a simplified
geotectonic chronology of the area.
The importance of basic magmas in the early
evolution of the Patagonian Batholith, as recognized
here, is comparable to that present in other Cordilleranbatholiths(seeCobbing
etal.
1981; Bateman
1983). This basic magmatism occurred while the island
arc volcanism and infilling of the marginal basin by
turbidites was taking place (see Suarez & Pettigrew
1976). Therefore, it is possible that basic plutons may
have formed part at least of the roots of the volcanic
arc. The emplacement of granitic plutons in the area
seems to have started when the island arc wasin a
later stage of evolution (Fig. 2). Furthermore, granitoids belonging to the Canal Beagle Plutonic Group
were mainly intruded at the site of the marginal basin
and possibly in part while it was being infilledwith
turbidites, rather than where the island arc of the time
was located.
The intrusion of the Patagonian Batholith spanned
the mid-Cretaceous tectonism of the area, which has
beenrelated to the closure of the marginal basin at
about 100-93 Ma.The
gabbroic assemblage was
mainly emplaced before this deformation, whereas the
Tertiary Seno Aiio Nuevo Plutonic Group was clearly
intruded after it. Althoughthe
dataonthe
Canal
Beagle
Plutonic
Group
are
less precise,
a
posttectonic, possibly late-tectonic
intrusion
for
the
youngest plutons and possibly a pre- or syn-tectonic
emplacement for the oldest can be inferred.
ACKNOWLEDGEMENTS.
We thank M. Halpern for Rb-Sr
mineral datesand J . Cobbing for fruitful discussions on
batholiths.
This
project was supported by the Chilean
Servicio Nacional deGeologii
y Mineriiand
partly by
CONICYT.
References
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