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Sistema de Información Científica
Red de Revistas Científicas de América Latina y el Caribe, España y Portugal
doi: 10.5027/andgeoV41n2-a01
Andean Geology
formerly Revista Geológica de Chile
www.andeangeology.cl
Andean Geology 41 (2): 267-292. May, 2014
Geochemistry, U-Pb SHRIMP zircon dating and Hf isotopes of the
Gondwanan magmatism in NW Argentina: petrogenesis and geodynamic
implications
Stella Poma
1
, Eduardo O. Zappettini
2
, Sonia Quenardelle
1
, João O. Santos
3
,
Magdalena Koukharsky
1
, Elena Belousova
4
, Neil McNaughton
3
1
Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires (IGEBA-CONICET), Universidad de Buenos Aires,
Facultad de Ciencias Exactas y Naturales, Departamento de Ciencias Geológicas, Pabellón II-Ciudad Universitaria, Intendente
Güiraldes 2160, C1428 EGA, Argentina.
stella@gl.fcen.uba.ar; sonia@gl.fcen.uba.ar
2
Servicio Geológico Minero Argentino (SEGEMAR), Avda. General Paz 5445, edificio 25, San Martín B1650WAB, Argentina.
eduardo.zappettini@segemar.gov.ar
3
University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia.
orestes.santos@bigpond.com; N.Mcnaughton@curtin.edu.au
4
ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), Macquarie University, Sydney NSW
2109, Australia.
ebelouso@els.mq.edu.au
ABSTRACT.
We have carried out zircon U-Pb SHRIMP dating and Hf isotope determinations as well as geochemical
analyses on three plutonic units of Gondwanan magmatism that crop out in NW Argentina. Two episodes of different
age and genesis have been identified. The older one includes gabbros and diorites (Río Grande Unit) of 267±3 Ma
and granitoids (belonging to the Llullaillaco Unit) of 263±1 Ma (late Permian, Guadalupian); the parent magmas were
generated in an intraplate environment and derived from an enriched mantle but were subsequently contaminated by
crustal components. The younger rocks are granodiorites with arc signature (Chuculaqui Unit) and an age of 247±2 Ma
(middle Triassic-Anisian). Hf isotope signature of the units indicates mantle sources as well as crustal components.
Hf model ages obtained are consistent with the presence of crustal Mesoproterozoic (mainly Ectasian to Calymnian
(T
DM(c)
=1.24 to 1.44 Ga-negative
Hf (T)
) and juvenile Cryogenian sources (T
DM
=0.65 to 0.79 Ga-positive ɛ
Hf (T)
), supporting
the idea of a continuous, mostly Mesoproterozoic, basement under the Central Andes, as an extension of the Arequipa-
Antofalla massif. The tectonic setting and age of the Gondwanan magmatism in NW Argentina allow to differentiate:
a.
Permian intra-plate magmatism developed under similar conditions to the upper section of the Choiyoi magmatism
exposed in the Frontal Cordillera and San Rafael Block, Argentina;
b.
Triassic magmatism belonging to a poorly known
subduction-related magmatic arc segment of mostly NS trend with evidence of porphyry type mineralization in Chile,
allowing to extend this metallotect into Argentina.
Keywords: Gondwanan magmatism, Geochemistry, U-Pb SHRIMP dating, Lu-Hf isotope, NW Argentina.
268
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EOCHEMISTRY
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ONDWANAN
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RGENTINA
...
1. Introduction
A long and discontinuous Permian/Triassic magma-
tic belt occurs along the western margin of Gondwana
in South America, outcropping from central Perú as
far as approximately 39ºS in Argentina. From west to
east it extends approximately 600 km from the Gond-
wana margin into the foreland, reaching the longitude
of the present Sierras Pampeanas and the western
margin of the Río de la Plata craton (Fig. 1, inset).
It comprises epizonal plutonic and volcanic rocks,
the latter including pyroclastic facies. In Argentina
and Chile the assemblage has been named Choiyoi
magmatic province (Kay
et al
., 1989; Llambías
et al
.,
1993; Llambías, 1999), even though originally the term
Choiyoi was used to identify only the volcanic units
(originally Groeber, 1946, 1951, and later Choiyoi
Group;
e.g
., Llambías
et al
., 1993).
In Argentina, the Choiyoi igneous rocks crop out
in the basement of the Neuquén basin and the main
Cordillera of southern Mendoza, in the Cordillera
Frontal of Mendoza and San Juan, San Rafael block
and its southern extension in the La Pampa province.
To the NW the magmatic province extends into Chile
and to the SE into northern Patagonia. The Cordil-
lera Frontal outcrops are the most voluminous and
have been the subject of study over the last 40 years
(
e.g.,
Rolleri and Criado Roque, 1970; Mpodozis
and Kay, 1992; Llambías
et al.,
1993, Breitkreuz
and Zeil, 1994; Lucassen
et al
., 1999). This region
is characterized by felsic rocks with subordinated
basic and mesosilicic rocks. Volcanic sequences
dominate in Argentina and plutonic rocks in Chile,
although important plutons such as the Colangüil
Batholith also occur in Argentina (Llambías and
Sato, 1990, 1995).
Isolated Permian-Triassic plutons have been
recognized in the Puna region of Salta province,
Argentina, suggesting that the magmatic event
reached that latitude (Zappettini and Blasco, 1998;
Page and Zappettini, 1999; Poma
et al
.,
2009).
The aim of this paper is to contribute to the
knowledge of the Gondwanan magmatism through
the presentation and interpretation of chemical and
isotopic data of previously poorly known units that
represent the northernmost outcrops identified in
Argentina. We present new zircon U-Pb SHRIMP
and Hf isotope data and we explore the nature
and characteristics of the magma sources to better
constrain a petrogenetic model and characterize the
crustal components of the Puna region.
2. Geological setting
The basement of the western margin of Gondwana
consists of several terranes amalgamated against
the Río de la Plata and the Amazonas cratons. It
includes the Antofalla-Arequipa, Pampia, Cuyania
and Chilenia terranes that are thought to have
accreted from Late Neoproterozoic to Devonian
times (Ramos
et al
., 2010 and references therein).
Records of subduction and related arc magmatism
RESUMEN. Geoquímica, dataciones U-Pb SHRIMP sobre circón e isótopos de Hf del magmatismo gondwánico
en el NW de Argentina: petrogénesis e implicancias geodinámicas.
Se presentan resultados de la geoquímica,
dataciones U-Pb SHRIMP y de relaciones isotópicas Lu-Hf de tres unidades plutónicas pertenecientes al magmatismo
gondwánico del NW de Argentina. Se identificaron dos episodios de diferente edad y génesis. El más antiguo incluye
gabros y dioritas (Unidad Río Grande) de edad 267±3 Ma y granitoides (pertenecientes a la Unidad Llullaillaco) de
263±1 Ma (Pérmico tardío-Guadalupiense) generados en un ambiente de intraplaca a partir de un manto enriquecido y
subsecuentemente contaminado con componentes corticales. El evento más joven está representado por la intrusión de
granodioritas (Unidad Chuculaqui) de 247±1 Ma (Triásico medio-Anisiense) con características químicas de arco mag-
mático. El comportamiento de los isótopos de Hf de las unidades estudiadas indica la participación de fuentes mantélicas
como así también de componentes corticales en su generación. Las edades modelo Hf obtenidas son consistentes con
la presencia de fuentes corticales del Mesoproterozoico (principalmente Ectasiano a Calimniano (1,24 a 1,44 Ga-ɛ
Hf (T)
negativo) y juveniles del Criogeniano (T
DM
=0,65 to 0,79 Ga-ɛ
Hf (T)
positivo), lo que está de acuerdo con la presencia de
un basamento continuo, de ese rango de edades, bajo los Andes Centrales, como una extensión del macizo de Arequipa-
Antofalla. El ambiente tectónico y la edad del magmatismo gondwánico en el NW de Argentina permiten diferenciar:
a.
Un magmatismo pérmico de intraplaca desarrollado bajo condiciones similares a las del magmatismo carbonífero
de intraplaca descrito para Cordillera Frontal y Bloque de San Rafael;
b.
Un magmatismo triásico perteneciente a un
segmento de arco magmático de orientación predominante NS con evidencias de mineralización tipo pórfiro en Chile,
lo que permite extender este metalotecto en el territorio argentino.
Palabras clave: Magmatismo gondwánico, Geoquímica, Dataciones U-Pb SHRIMP, Isotopía Lu-Hf, NW Argentina.
269
Poma et al. / Andean Geology 41 (2): 267-292, 2014
in the western margin of Gondwana are almost
continuous during the Phanerozoic, although there is
evidence for a time of relative quiescence during the
Devonian and Early Mississippian (390 to 340 Ma)
from northern Perú to southern Chile due to the
development of a passive margin at those times
(Bahlburg
et al
., 2009). In central to southern
Chile this scenario is related to the prior collision
of Chilenia (Willner
et al
., 2009).
In NW Argentina the first records of magma-
tism are related to the Pampean and Famatinian
orogenies (Rapela
et al
., 1998; Pankhurst
et al
.,
1998). The Pampean magmatism is restricted to the
Eastern Cordillera (Cañani, La Quesera and Chañi
units) (Omarini
et al
., 2008) and the Famatinian
magmatism (Bahlburg
et al
., 2009) includes both
the Faja Eruptiva de la Puna Occidental magmatic
arc (Poma
et al
., 2004) and the Faja Eruptiva de la
Puna Oriental back-arc magmatism (Coira, 2008;
Zappettini, 2008).
During the early Carboniferous magmatism was
reinitiated along the Gondwana margin in the cen-
tral and southern Andes (Kay
et al
., 1989; Brown,
1991; Breitkreuz
et al
., 1992; Breitkreuz and Van
Schmus, 1996). In particular, intrusive activity
began around 330 Ma (Lucassen
et al
., 1999 and
references therein). I-type earliest subduction-related
granitoids are recorded in the Eastern Cordillera
FIG. 1. The Gondwanan magmatism in the Socompa region. Inset: distribution of the late Paleozoic-Early Triassic magmatism in South
America. Main tectonic features are also indicated.
270
G
EOCHEMISTRY
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AND
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F
ISOTOPES
OF
THE
G
ONDWANAN
MAGMATISM
IN
NW A
RGENTINA
...
of Perú with U-Pb zircon ages at
ca.
350-325 Ma
(Miskovic
et al
., 2009), between 21° and 22°S in
Chile (Ujina, Rosario, El Colorado and Quebrada
Blanca porphyries, Collahuasi Group, 300 Ma; cf.
Munizaga
et al
., 2008) and between 28° and 29°S
in Argentina (
e.g
., Tabaquito pluton, 326-329 Ma;
Los Guandacolinos Granite, 314 Ma; Cerro Veladero
Granite, 311 Ma; Cerro de las Tunas, 330 Ma; cf.
Alasino
et al
., 2012).
A subduction-related setting has been also ascribed
to Permian magmatism and Triassic units have been
interpreted to be the result of partial rifting and trans-
tension following the suture of the Arequipa-Antofalla
terrane and the Amazonian craton, due to stresses
originated during the Pangea break-up (Kontak
et
al
., 1990; Atherton and Petford, 1990). Although,
other authors have pointed out that an early stage the
magmatism was related to subduction, followed by non
orogenic magmatism related to active rifting (Kay
et
al
., 1989; Mpodozis and Ramos, 1990; Llambías and
Sato, 1990). Kleiman and Japas (2009) and Rocha
Campos
et al
. (2011) consider that the 31°S to 36°S
segment could be regarded as a transitional zone
between different subduction segments after 270 Ma.
The northern segment would be normal in dip angle
and the southern shallower of that. It should be noted
that in northern Chile, there is evidence of arc-related
magmatism of Triassic age with porphyry copper-
related mineralization (243.2±2.1 to 248.7±3.3 Ma
U-Pb SHRIMP; Munizaga
et al
., 2008).
In the Puna region of Argentina, the Gondwanan
magmatism is represented by both plutonic and
volcanic rocks that extend along a discontinuous
NNE-SSW belt from 24° to 26°S. Andean tectonics
have affected the continuity of this belt and Cenozoic
volcanism covers most of the region. Late Paleozoic
outcrops include the León Muerto Granite (25°48’S
/68°24’W), a porphyritic amphibole granite (Page y
Zappettini, 1999) dated at 246±6 Ma (K-Ar whole
rock; Naranjo and Cornejo, 1992); Ojo de Antofalla
Granite (25°26’19”S/67°39’05”W) and a subvolcanic
pluton dated at 235±10 Ma (K-Ar whole rock; Martos,
1981
1
). Northward the Llullaillaco plutonic complex
(Zappettini and Blasco, 1998), herein described as
Llullaillaco Unit, include outcrops between 24°10’S
and 25°S (NE of Aracar volcano, N of Taca-Taca
Range, NW of Agua del Desierto, SW of Pie de
Samenta, NE of Salar Río Grande, East of Salar de
Llullaillaco). It comprises porhyritic rocks, dated at
257±18 Ma (K-Ar whole rock) and a red granite with
a K-Ar (biotite) age of 224±5 Ma. One red granite
that crops out to the west of Incahuasi Salar yielded
266±1 Ma and one microgranitoid was dated at
269±2 Ma (conventional U-Pb on zircon; Page and
Zappettini, 1999). This complex is correlated with
the intrusive bodies known as Plutones Guanaqueros
(282±7 Ma; Gardeweg
et al
., 1993).
Associated volcanic rocks have been identifed to the
north of the Aracar volcano, named as the Laguna de
Aracar Formation (Koukharsky, 1969
2
) that extends
southward along the NW border of the Arizaro Salar.
It comprises acidic volcanic and pyroclastic rocks
dated at 266±28 Ma (K-Ar whole rock; Zappettini
and Blasco, 1998). Equivalent volcanic sequences in
Chile have been dated at 259±8 Ma and 261±9 Ma
(K-Ar whole rock; Ramírez
et al
., 1991).
Other plutonic outcrops of the region have been
assigned to the Gondwanan magmatism as an
outcome of the mapping and geochronological
studies herein presented. They are herein identifed
as Río Grande and Chuculaqui Units and were
originally assigned to the Famatianian magmatic
arc (Zappettini and Blasco, 1998).
We have focused the work on the description of
the Río Grande, Llullaillaco and Chuculaqui Units
(Fig. 1).
The Río Grande Unit form small outcrops to
the East of the Llullaillaco salar and also to the
north of the Río Grande salar where it is intruded
by the Chuculaqui granitoids. These rocks intrude
Ordovician metasediments unit (Zappettini and
Blasco, 1998).
The Llullaillaco Unit constitutes three main groups
of outcrops located to the NE of the Aracar volcano,
to the East of the Llullaillaco salar and to the North of
the Río Grande salar. Outcrops are partially covered
by Cenozoic lava flows and intrude rocks of the Río
Grande Unit (Zappettini and Blasco, 1998).
The Chuculaqui Unit is located to the West in
the center south extreme of the studied region. It
constitutes two main bodies and minor satellital
outcrops partially cover by the Cenozoic volcanites.
1
Martos, D. 1981. Estudio geológico económico del sector sudeste del área de reserva N°5 ‘Antofalla Este’. Facultad de Ciencias Naturales UNT
(Unpublished): 83 p. Tucumán.
2
Koukharsky, M. 1969. Informe preliminar sobre la estratigrafía de la Hoja 6ª Socompa, Provincia de Salta. Instituto Nacional de Geología y Minería
(Unpublished): 22 p. Buenos Aires.
271
Poma et al. / Andean Geology 41 (2): 267-292, 2014
The Chuculaqui rocks intrude red granites asigned to
the Llullaillaco Unit (Zappettini and Blasco, 1998).
3. Petrography
3.1. Río Grande Unit
This unit includes gabbros, diabases and diorites
with hypidiomorphic granular to porphyritic texture,
sometimes exhibiting igneous lamination, as it is
frequently displayed by plagioclase feldspar. The
most abundant minerals are plagioclase, pyroxene
and olivine. Plagioclase crystals are zoned, locally
showing patchy extinction due to complex zoning
patterns and variable Ca-Na compositions, these be-
ing characteristics compatible with mixing-mingling
processes; labradorite-bytownite has been recognized
in the basic varieties, and andesine-oligoclase in the
mesosilicic types. Ortho- and clino-pyroxene are
replaced by fibrous amphibole pseudomorphs with
a few crystals showing preserved cores. Olivine is
present in a few samples showing partial replace-
ment by an orthopyroxene rim. Accessory minerals
include apatite (0.5 to 5.4 mm in length), sphene,
zircon and opaque minerals; some are euhedral but
there is a predominance of rounded shapes like
drops. Locally, plagioclase cumulate textures have
been recognized.
3.2. Llullaillaco Unit
It consists of red granite, with subordinated mi-
crodioritic facies. The main facies is a leucocratic
reddish alkali-feldspar granite. The textures are granular
and microgranular allotriomorphic, with simultane-
ous growth of alkali feldspars and quartz producing
granophyre intergrowth; miarolitic structures are
also observed. These textures indicate rapid cooling
conditions and shallow emplacement. Quartz crystals
usually have rounded borders although some of them
show original bipyramidal habit. Alkali feldspar is
strongly perthitic and plagioclase is scarce or not
visible. Biotite is subordinated (about 1%) and it is
frequently replaced by chlorite and epidote. Accessory
minerals are zircon, apatite needles, fluorite form-
ing mosaic crystals grouped in cavities (upholster
vugs without evidence of lined), and rounded grains
of magnetite. The presence of one alkali perthitic
feldspar is compatible with an hypersolvus granite
(Bowen and Tuttle, 1950; Tuttle and Bowen, 1958).
3.3. Chuculaqui Unit
This unit comprises gray tonalites to granodio-
rites as the main facies, with subordinate granites
and quartz-diorites. Poma
et al
. (2009) described
associated mafic facies in the borders of this plu-
tonic suite, including gabbroid lenses like mafic
microgranular enclaves. These are microdioritic
in composition and the material is hybridized in
various degrees. Field relationships indicate that
during emplacement the silicic melt incorporated
mafic material; additional textural evidence indi-
cates in some localities that the granitic intrusion
was probably coeval with the mafic magmatism.
The rocks are hypidiomorphic, with inequi-
granular medium to coarse grains. In some granitic
rocks monzonitic textures are common showing
poikilitic quartz and K-feldspar enclose euhedral
plagioclase and prismatic amphibole.
The most abundant mineral is zoned oligoclase
(An
26-28
) with light sericitic alteration; alkali feldspar
crystals (orthoclase) are perthitic. In the granodiorite
facies, plagioclase occasionally shows a myrmekitic
intergrowth at grain boundaries with alkali feldspar.
The mafic minerals (10% to 20%) are amphibole
and scarce biotite, both of them partially replaced
by epidote, chlorite and associated opaque miner-
als. Accessory minerals are idiomorphic sphene,
apatite, zircon and opaque minerals.
4. Geochemistry
The studied units cover a wide compositional
range (Fig 2a, Table 1) from gabbros (45% SiO
2
)
to high silica granites (up to 78% SiO
2
). In the
K
2
O
versus
SiO
2
diagram (Fig. 2b) the Río Grande
Unit rocks plot in the medium- to high-K field,
the Llullaillaco Unit is constrained to the high-K
field and Chuqulaqui Unit spans the medium- and
high-K fields.
The Río Grande Unit gabbros and diorites are
metaluminous subalkaline basic to mesosilic rocks
with high values of FeO, CaO and MgO, and vari-
able contents of Na
2
O and Ti
2
O. Zr shows a positive
correlation with SiO
2
. In the MORB-normalized
trace element spider diagrams (Pearce
,
1983) the
rocks (Fig. 3a) display a continuous variation in
HFSE contents, in particular Nb, Ta, Zr, Hf and
Ti depletion. The REE diagram (Fig. 4a) is char-
acterized by a relatively low La/Yb slope and Eu
272
anomalies (0.81 to 1.42) coincident with the pres-
ence of cumular plagioclase as was petrographically
identified.
The red granites of the Llullaillaco Unit have
characteristics of evolved high-silica magma with low
FeO, MgO, CaO contents and are slightly peralumi-
nous with A/CNK ≤1.1 (Table 1). A distinguishing
characteristic is their high Rb, Th and U contents and
low Sr. Zircon contents show a random distribution
versus
SiO
2
. The discrimination diagram of Pearce
(1984) shows (Fig. 5a) the granitoids plot in vol-
canic arc field transitional to the within plate field.
In a spider diagram normalized to MORB (Fig. 3b)
rocks show a consistent pattern characterized by
strong depletion in Ba and Ce, and high Th. The
REE diagram (Fig. 4b) is characterized by having
a sharp negative Eu anomaly related to plagioclase
fractionation and low La/Yb ratio suggesting the
presence of a low-pressure residual mineralogy in
the source.
The Chuculaqui Unit rocks span between 65 and
70% SiO
2
and are metaluminous, plotting in the VAG
field (Fig. 5a). These rocks have characteristically
variable La/Ta ratios even higher than 30 and Ba/Ta
ratios (Fig. 5b) greater than 450. Zr values decrease
with increasing SiO
2
indicating a normal negative
trend. In a MORB-normalized spider diagram
(Fig. 3c) all samples exhibit enriched LILE (Ba)
relative to light REE and both enriched relative to
HFSE Nb, Ta, Zr, and Hf, corresponding to con-
tinental arc-related signatures. The REE diagrams
(Fig. 4c) show small negative anomalies in Eu (0.66
to 0.93) indicative of plagioclase fractionation and
La/Yb ratios <9.
G
EOCHEMISTRY
, U-P
B
SHRIMP
ZIRCON
DATING
AND
H
F
ISOTOPES
OF
THE
G
ONDWANAN
MAGMATISM
IN
NW A
RGENTINA
...
FIG. 2.
a.
TAS plutonic classification by Cox
et al
. (1979);
b.
K
2
O
versus
SiO
2
plot (Peccerillo and Taylor, 1976).
Diamond:
Llullaillaco
Unit,
Circles:
Río Grande Unit,
Triangles:
Chuculaqui Unit.
Poma et al. / Andean Geology 41 (2): 267-292, 2014
273
TABLE 1. GEOCHEMICAL DATA.
Río Grande Unit
Llullaillaco Unit
Chuqulaqui Unit
SA
02/03
SA
06/03
SA
07/03
SA
08/03
SA
09/03
RioG1
RioG2
SA
10/03
M00/72
M643
M634
M30
M644
M665
M664
Chuq
SA
12/03
SA
05/03
SA
01/03
M00/60
M00/63
M00/69
M00/71
M637
SiO
2
49.44
51.70
45.16
45.54
47.60
49.25
50.48
77.98
75.94
77.99
78.00
77.05
73.86
75.96
73.63
68.60
65.88
57.66
67.18
65.67
65.63
70.05
68.38
65.42
TiO
2
1.55
1.05
0.83
1.51
1.63
2.72
2.50
0.12
0.20
0.13
0.10
0.08
0.26
0.16
0.58
0.40
0.61
1.23
0.37
0.42
0.48
0.37
0.48
0.49
Al2O
3
18.46
15.27
21.09
19.35
17.67
15.50
16.10
11.31
12.41
11.64
12.50
12.22
13.38
12.23
12.32
16.09
14.89
16.00
16.33
16.09
16.58
14.01
14.75
15.14
Fe
2
O
3
9.21
8.99
9.94
9.52
10.72
11.82
10.05
1.24
2.06
1.13
0.88
1.60
2.44
1.17
3.11
3.02
4.25
7.22
3.23
3.67
3.67
3.11
3.99
5.08
MnO
0.12
0.17
0.11
0.14
0.16
0.22
0.25
0.01
0.02
0.03
0.02
0.02
0.05
0.05
0.03
0.06
0.07
0.13
0.05
0.06
0.06
0.08
0.09
0.03
MgO
4.38
6.82
6.69
6.55
5.07
4.52
4.22
0.05
0.13
0.07
0.06
0.07
0.36
0.01
0.01
1.34
2.11
3.42
1.09
1.52
1.63
0.93
1.21
1.35
CaO
7.62
9.19
11.86
10.43
9.11
7.24
7.80
0.38
0.39
0.64
0.49
0.41
0.79
0.88
0.58
3.67
2.92
6.00
4.12
3.98
4.41
2.30
3.02
0.86
Na
2
O
3.82
2.94
1.99
2.62
3.24
4.22
5.02
3.55
2.81
3.51
3.38
3.87
3.24
2.69
3.35
4.42
3.89
3.73
4.72
4.39
4.44
2.82
2.84
4.44
K
2
O
1.55
0.96
0.26
0.33
1.11
1.39
0.62
4.73
5.23
4.67
4.75
4.96
5.55
5.23
4.90
1.99
2.79
2.13
1.10
1.87
1.68
3.92
3.92
4.89
P
2
O
5
0.57
0.56
0.10
0.52
0.78
1.04
1.06
0.02
0.05
0.02
0.02
0.06
0.08
0.05
0.22
0.12
0.16
0.47
0.15
0.14
0.15
0.09
0.63
0.23
LOI
2.44
1.57
0.83
2.24
1.53
2.11
1.92
0.46
0.80
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
1.18
1.54
1.28
1.25
0.87
0.89
1.57
0.86
n.d.
A/CNK
0.85
0.68
0.84
0.82
0.77
0.72
0.70
0.97
1.13
0.97
1.08
0.98
1.05
1.05
1.04
1.00
1.01
0.83
0.99
0.98
0.97
1.07
1.02
1.07
La
24.70
34.30
9.87
14.10
35.60
35.00
39.00
28.10
28.90
30.30
15.50
43.70
36.20
20.60
40.60
17.90
33.30
46.00
11.80
18.80
18.50
33.20
34.30
35.00
Ce
54.40
74.50
19.80
32.30
78.90
77.40
91.00
64.70
55.60
74.30
48.80
119.00
106.00
42.00
84.00
34.50
67.90
98.00
24.70
36.60
37.40
61.50
65.60
79.70
Pr
7.14
9.71
2.47
4.61
10.40
9.98
11.20
8.10
5.46
9.28
3.85
11.73
8.05
0.10
0.10
3.72
8.19
12.30
3.06
3.88
4.19
6.06
6.53
7.78
Nd
29.50
38.60
9.85
21.10
42.10
41.20
46.10
33.50
18.60
38.90
14.30
43.40
29.00
15.00
39.00
13.60
29.90
48.30
12.00
15.60
16.60
21.80
24.40
29.00
Sm
6.27
8.39
2.15
4.63
8.88
8.40
9.00
8.12
3.42
10.90
3.03
10.40
5.64
3.10
6.90
2.60
6.01
9.81
2.53
2.91
3.31
3.65
4.31
5.15
Eu
2.01
2.02
0.91
1.70
2.25
2.04
2.32
0.43
0.52
0.51
0.32
0.27
1.03
0.80
1.40
0.69
1.21
2.23
0.74
0.80
0.92
0.94
1.07
1.21
Gd
5.33
7.01
1.79
4.08
7.44
7.30
7.90
8.12
3.52
10.60
2.45
9.30
4.44
0.10
0.10
2.40
4.85
8.02
2.23
2.79
3.09
3.59
4.60
3.70
Tb
0.74
1.07
0.27
0.53
1.08
1.10
1.10
1.53
0.56
2.07
0.50
1.95
0.79
0.70
0.90
0.40
0.81
1.23
0.32
0.38
0.43
0.49
0.65
0.62
Dy
3.98
5.72
1.44
2.64
5.52
5.50
6.00
10.10
3.60
13.00
3.03
11.90
4.53
0.10
0.10
2.20
4.36
6.30
1.68
2.16
2.40
2.84
3.78
3.29
Ho
0.76
1.16
0.28
0.49
1.06
1.10
1.10
2.19
0.79
2.68
0.68
2.48
0.91
0.10
0.10
0.40
0.89
1.22
0.32
0.41
0.47
0.59
0.79
0.61
Er
2.10
3.31
0.77
1.28
3.01
2.80
2.90
6.77
2.69
7.60
2.26
7.53
2.75
0.10
0.10
1.10
2.69
3.58
0.94
1.23
1.38
1.83
2.37
1.78
Tm
0.28
0.48
0.11
0.16
0.42
0.38
0.42
1.05
0.46
1.24
0.43
1.28
0.44
0.10
0.10
0.18
0.41
0.52
0.14
0.19
0.20
0.29
0.37
0.25
Yb
1.73
2.92
0.69
0.88
2.45
2.40
2.50
6.66
3.27
7.67
3.05
7.97
2.91
0.10
0.10
1.20
2.68
3.13
0.91
1.22
1.39
2.01
2.43
1.58
Lu
0.24
0.41
0.10
0.11
0.34
0.33
0.34
0.92
0.55
1.07
0.50
1.16
0.43
0.10
0.10
0.17
0.39
0.46
0.13
0.18
0.20
0.31
0.38
0.24
Sr
730.00
585.00
765.00
804.00
728.00
564.00
666.00
11.00
44.00
17.00
21.00
18.00
181.00
95.00
122.00
419.00
360.00
523.00
530.00
436.00
498.00
138.00
164.00
168.00
Ba
368.00
475.00
175.00
256.00
434.00
141.00
146.00
17.00
349.00
67.00
37.00
205.00
867.00
690.00
908.00
376.00
698.00
586.00
274.00
415.00
755.00
578.00
475.00
1205.00
Cs
9.00
0.70
1.00
1.10
0.90
4.30
1.70
1.00
7.60
1.20
9.70
1.30
5.90
5.00
2.50
1.20
2.20
3.80
0.60
8.00
1.40
9.70
7.90
0.90
U
1.08
1.89
0.34
0.16
1.46
1.30
1.20
3.95
3.24
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
1.60
2.51
2.92
0.76
1.43
1.06
2.47
2.57
n.d.
Th
4.78
9.11
1.49
0.59
6.63
5.10
5.90
24.70
28.20
25.60
50.90
27.50
40.00
11.20
16.30
5.90
14.60
15.20
2.34
6.56
6.14
13.90
14.50
9.82
Hf
2.50
4.20
1.10
0.50
3.30
3.80
3.80
7.60
3.80
7.00
2.40
7.50
6.70
3.00
5.90
2.60
5.70
6.90
3.40
3.30
3.40
3.90
4.50
5.00
Ta
1.90
0.47
0.11
0.05
0.45
0.70
0.80
2.63
2.06
2.56
1.79
2.53
2.02
0.90
1.00
0.70
0.93
1.01
0.25
0.43
0.45
0.92
1.00
1.04
Sc
18.00
27.00
15.00
18.00
21.00
33.00
29.00
3.00
4.00
3.00
2.00
2.00
5.00
3.00
6.00
6.00
10.00
16.00
4.00
8.00
8.00
8.00
11.00
6.00
Cr
40.00
280.00
150.00
60.00
50.00
15.00
15.00
<20
<20
20.00
<20
<20
<20
0.10
7.00
15.00
20.00
40.00
<20
<20
<20
<20
<20
<20
Ni
50.00
80.00
150.00
60.00
270.00
40.00
40.00
<20
<20
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
15.00
<20
30.00
<20
<20
<20
<20
<20
n.d.
Co
30.00
32.00
46.00
36.00
35.00
52.00
34.00
<1
2.00
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
36.00
11.00
19.00
3.00
8.00
7.00
7.00
8.00
n.d.
Rb
87.00
34.00
7.00
11.00
34.00
80.00
33.00
256.00
221.00
235.00
219.00
240.00
261.00
166.00
177.00
50.00
87.00
77.00
33.00
43.00
149.00
149.00
152.00
154.00
Nb
10.90
6.90
1.90
0.60
6.00
11.00
12.00
35.70
9.70
35.00
13.00
42.00
22.00
0.10
0.10
5.00
12.10
15.20
4.00
3.10
5.50
5.50
6.80
16.00
Zr
85.00
163.00
38.00
11.00
113.00
152.00
162.00
169.00
107.00
150.00
50.00
164.00
198.00
74.00
238.00
101.00
203.00
264.00
117.00
119.00
123.00
143.00
161.00
163.00
Y
22.40
33.20
8.30
14.10
30.70
28.00
29.00
65.80
23.50
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
11.00
27.90
37.60
10.10
10.00
12.00
17.50
23.20
n.d.
Latitude-S
24° 52’
31.8”
24° 52’
40.3”
24° 52’
24.8”
24° 53’
18.4”
24° 52’
34.3”
24° 56’
30.6”
24° 56’
30.6”
24° 52’
24.9”
24° 37’
50.3”
24° 57’
05.5”
24° 45’
44.0”
24° 18’
06.9”
24° 57’
07.4”
24° 46’
11.5”
24° 46’
11.4”
24°49’
04.5”
24° 52’
12.8”
24° 52’
16.5”
24° 54’
14.7”
24° 49’
15.5”
24° 49’
15.5”
24° 37’
48.2”
24° 37’
23.8”
24° 55’
20.6”
Longitude-W
68° 05’
55.5”
68° 06’
21.3”
68° 05’
49.3”
68° 06’
26.1”
68° 06’
33.0”
68° 06’
38.9”
68° 06’
38.9”
68° 05’
45.1”
67° 45’
24.5”
68° 06’
48.0”
68° 09’
23.3”
67° 42’
31.5”
68° 06’
50.8”
68° 11’
05.5”
68° 11’
05.5”
68° 06’
01.0”
68° 05’
39.7”
68° 05’
40.0”
68° 05’
02.4”
68° 05’
54.3”
68° 05’
54.3”
67° 45’
41.4”
67° 45’
19.4”
68° 05’
42.0”
274
G
EOCHEMISTRY
, U-P
B
SHRIMP
ZIRCON
DATING
AND
H
F
ISOTOPES
OF
THE
G
ONDWANAN
MAGMATISM
IN
NW A
RGENTINA
...
5. U-Pb and Lu-Hf systematics
Zircons were separated from representative
samples of the Río Grande (RG: 24°56’30.61”S-
68°06’38.88”W), Llullaillaco (SA10-03: 24°52’24.9”S
-68°05’45.10”W) and Chuculaqui (CHUQ:
24°49’4.53”S-68°06’1.04”W) units. A full description
of the samples preparation and analytical methods
is presented in Appendix. Only relevant information
is given in this chapter.
FIG. 3. MORB normalized spider diagrams following Pearce (1983).
a.
Río Grande;
b.
Llullaillaco;
c.
Chuculaqui rock samples.
Symbols same as figure 2.
275
Poma et al. / Andean Geology 41 (2): 267-292, 2014
5.1. U-Pb geochronology
Dating young zircons (Phanerozoic) faces the
problems of low counts of
207
Pb and the difficulty
to detect deviations from slightly older cores and
from subtle amounts of Pb loss. To minimize the
first problem the time of counting
207
Pb was in-
creased from 10 seconds to 20 seconds, and grains
and areas of grains poor in U (<100 ppm) were
avoided. Additionally we have used the TuffZirc
algorithm (Ludwig and Mundil, 2002), which is
largely insensitive to both Pb loss and inheritance
to plot the
206
Pb/
238
U ages corrected using the
207
Pb
counts. Most of the data group reasonably in the
Concordia plots, with few analyses deviating from
the Concordia line. All Concordia ages are within
error with the TuffZirc ages. These ages are presented
as insets in the Concordia plots.
FIG. 4. Leedy chondrite normalized REE concentration patterns (normalizing values from Masuda
et al
., 1973):
a.
Río Grande;
b.
Llullaillaco;
c.
Chuculaqui rock samples. Symbols same as figure 2.
276
G
EOCHEMISTRY
, U-P
B
SHRIMP
ZIRCON
DATING
AND
H
F
ISOTOPES
OF
THE
G
ONDWANAN
MAGMATISM
IN
NW A
RGENTINA
...
5.1.1 Río Grande unit
Rocks of this unit are relatively rich in zircon.
All grains are 100-300 µm prisms terminated in
pyramids (aspect ratio 2:1 to 5:1). The zircon grains
are simple and back-scattered electron images show
no evidence of older inherited cores or of younger
metamorphic rims or zones. They have characteristics
of magmatic grains such as the zoning and the Th/U
ratios averaging 1.6 (Table 2). Examples of dated
zircon are provided in figures 6a to d. Twelve analyses
group at the
206
Pb/
238
U age of 267±3 Ma (MSWD=
0 . 0 1 9 ; 2 σ ) ( F i g . 6 e ) . G r a i n s b . 2 - 4 ( 2 3 2 ± 3 M a ) ,
b.5-3 and b.2-1 may be affected by lead loss and
are not included in the age calculation. Grain b.5-1
is also not used because its age is highly discordant
(-343%). This age is considered the age of crystal-
lization of the gabbro-diorite body.
5.1.2. Llullaillaco Unit
Zircons from sample SA10-03 are short prisms
(aspect ratio 2:1 to 3:1), 80 µm to 200 µm long, clear
and colorless. Zoning is subtle but present in all
grains. Some grains such as b.4-1, g7-1, and g7-3
show recrystallization patches which are brighter
(richer in U) in BSE images (Fig. 7a to d). These areas
are sealing fractures and represent a later event in the
rock. Only one of these patches was analyzed (b.4-1)
and effectively yielded the highest U content of
1,807 ppm and the lowest Th/U ratio of the sample
(0.05), much lower than the Th/U average of the
other zircons (0.68) (cf. Table 2). This patch also
has the youngest age of 258±3 Ma (albeit within
error with the other ages from this sample). The
recrystallization patches may have resulted from
the activity of late-magmatic fluids, which may
FIG. 5.
a.
Rb
versus
(Y+Nb) discrimination diagram for granites (after Pearce
et al
., 1984);
b.
Ba/Ta
versus
La/Ta plot based on Gorring
and Kay (2001). Symbols same as figure 2.
277
Poma et al. / Andean Geology 41 (2): 267-292, 2014
TABLE 2. U-Pb SHRIMP ISOTOPIC DATA OF ZIRCON FROM THE GONDWANAN MAGMATIC UNITS OF WESTERN
PUNA, ARGENTINA.
Isotopic ratios
Ages
U
Th
206
Pb
4f
206
238
U
207
Pb
207
Pb
208
Pb
207
Pb
206
Pb
Disc.
spot
ppm
U
ppm
(%)
206
Pb
206
Pb
235
U
232
Th
206
Pb
238
U
%
Río Grande, diorite, zircon
b.1-1
307
1.89
11.2
0.18
23.574±1.49
0.053±3.61
0.308±3.9
0.013±2.35
316±82
268±4
15
b.1-2
150
1.46
5.4
0.27
24.022±1.72
0.053±5.34
0.305±5.61
0.013±2.66
331±121
263±4
21
b.2-1
997
2.15
34.3
0.06
24.962±1.36
0.051±1.46
0.282±2
0.013±1.5
241±34
253±3
-5
b.2-2
402
1.85
14.8
0.16
23.42±1.32
0.050±1.85
0.296±2.28
0.013±1.55
204±43
270±3
-32
b.2-3
90
1.82
3.3
1.34
23.545±2.11
0.054±12.6
0.314±12.7
0.013±4.11
358±284
268±6
25
b.2-4
783
2.04
25.3
0.36
26.687±1.06
0.052±2.6
0.268±2.81
0.012±1.79
283±59
237±2
16
b.3-1
489
1.82
18.1
0
23.143±1.41
0.051±2.02
0.304±2.46
0.014±1.74
238±47
273±4
-15
b.3-2
126
1.98
4.8
0.45
22.589±1.77
0.052±4.81
0.317±5.12
0.014±2.54
279±110
279±5
0
b.4-2
321
1.74
12.2
0.01
22.585±1.56
0.051±3.46
0.312±3.79
0.014±2.1
243±80
279±4
-15
b.4-3
154
1.15
5.4
-0.36
24.375±1.73
0.054±3.31
0.308±3.73
0.013±2.66
389±74
259±4
33
b.4-4
359
1.9
12.8
0.2
24.133±1.47
0.051±2.48
0.289±2.88
0.013±1.84
224±57
262±4
-17
b.5-1
363
1.86
14.1
0.71
22.336±1.63
0.047±5.12
0.292±5.37
0.014±2.23
64±122
282±4
-343
b.5-2
385
1.83
13.7
0
24.122±1.16
0.054±2.14
0.311±2.43
0.013±1.56
391±48
262±3
33
b.5-3
437
1.87
14.9
-0.15
25.086±1.16
0.054±2.15
0.298±2.45
0.012±1.57
379±48
252±3
34
b.6-1
114
0.85
4.2
0.45
23.521±2.3
0.052±5.85
0.305±6.28
0.014±4.16
286±134
268±6
6
b.8-1
313
2.08
11.2
0.31
24.169±1.5
0.050±2.86
0.283±3.23
0.013±1.9
179±67
261±4
-46
Llullaillaco, red granite, zircon
b.2-1
1,337
0.75
47.7
0.136
24.098±1.37
0.048±2.28
0.274±2.66
0.012±2.01
92±54
262±4
-185
b.3-1
1,650
0.12
58.5
-0.068
24.210±1.34
0.052±1.88
0.297±2.31
0.013±4.12
291±43
261±3
10
b.3-2
732
0.65
26.2
1.298
24.370±1.27
0.053±3.52
0.302±3.74
0.013±4.51
347±80
259±3
25
b.3-3
264
0.84
9.7
2.196
23.961±1.45
0.048±7.42
0.277±7.56
0.012±4.41
103±175
264±4
-156
b.4-1
1,807
0.05
63.5
0
24.451±1.29
0.051±2.07
0.289±2.44
0.014±4.17
253±48
258±3
-2
b.4-2
170
0.82
6.4
0.215
22.678±1.25
0.052±3.37
0.315±3.6
0.013±2.41
275±77
278±3
-1
b.4-3
1,507
0.92
58.3
0.08
22.234±1.08
0.052±1.24
0.320±1.64
0.014±1.44
264±29
284±3
-7
b.5-1
470
0.62
17
0.06
23.769±1.33
0.051±1.93
0.297±2.34
0.013±1.97
248±44
266±3
-7
g.5-1
1,163
0.68
42
0.01
23.782±1.26
0.052±1.44
0.301±1.92
0.013±1.76
282±33
266±3
6
g.5-2
608
0.91
22.8
-0.17
22.863±1.29
0.053±2.66
0.323±2.95
0.014±1.95
349±60
276±3
21
g.6-1
423
0.86
15.3
-0.02
23.676±1.28
0.052±2.04
0.303±2.41
0.014±1.68
288±47
267±3
8
g.7-1
851
0.64
30.3
0.11
24.122±1.31
0.052±2.35
0.295±2.69
0.014±2.07
269±54
262±3
3
g.7-2
1,097
1.03
39.9
0.02
23.639±1.25
0.052±1.26
0.301±1.77
0.014±1.44
266±29
267±3
0
Chuculaqui, granodiorite, zircon
e.1-1
111
0.66
3.8
0
25.176±1.66
0.053±3,52
0.289±3.89
0.013±3.04
317±80
251±4
21
e.1-2
149
0.68
5
0.13
25.692±1.57
0.053±3,04
0.284±3.42
0.012±2.69
324±69
246±4
24
e.2-1
153
0.68
5.3
0
25.039±1.55
0.052±3,09
0.285±3.46
0.012±2.72
275±71
252±4
8
e.4-1
158
0.84
5.1
0
26.683±1.94
0.052±3,37
0.268±3.89
0.012±2.92
276±77
237±5
14
e.4-2
228
0.85
7.5
0.15
26.329±1.47
0.049±2,53
0.257±2.93
0.012±2.13
155±59
240±3
-55
e.5-1
153
0.85
4.9
0
26.893±1.89
0.052±4,32
0.264±4.72
0.011±3.42
263±99
235±4
11
e.6-1
178
1.16
5.8
0.42
26.489±1.7
0.049±5,74
0.257±5.99
0.011±3.04
164±134
239±4
-46
e.9-1
404
1.27
13
0.23
26.690±1.46
0.051±2,88
0.262±3.23
0.012±2
230±66
237±3
-3
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have occurred shortly after the cooling of the granite
(up to 1 or 2 Ma later).
Most of grains (n=10) group at the age of
263±1 Ma
(Upper Middle Permian, Capitanian) but a slightly
older population of three grains is also present at
280±2 Ma (Lower Permian, Artinskian) (Fig. 7e).
The age of 263±1 Ma is considered the age of crys-
tallization of the granitic body that has incorporated
components of a slightly older continental crust,
probably inherited from intra-arc contamination.
5.1.3. Chuculaqui Unit
U-Pb ages have been determined for zircon and
titanite from a granodiorite. The sample has magmatic
titanite, which is dark reddish brown occurring as shards
of 500-600 µm in diameter (Fig. 8a and b). The mineral
is U-rich (average is 631 ppm). The results (Table 2)
are concordant. Common lead (
i.e.,
nonradiogenic)
is up to 3.71% and average 1.98%. The Concordia
age of seven analyses is 247±2 Ma (MSWD=0.76)
(Fig. 8e). This titanite is Th-poor and Th/U ratios are
almost constant and relatively low (average is 0.85).
The zircon population consists of long prisms 100
to 300 µm long of magmatic origin showing zoning
(Fig. 8c and d) and relatively high Th/U ratios (between
0.61 and 1.33). The 13 dated zircons (Table 2) have
the same Concordia age at 246±3 Ma (MSWD=1.7).
The age calculated combining the 13 zircons and
7 titanites gives 246±2 Ma (MSWD=0.086, prob-
ability=0.77) (Fig. 8f).
The age obtained for the titanite is within
uncertainty of the age of the zircons, suggesting that
the minerals were co-magmatic.
5.2. Lu-Hf isotopes
While the zircon U-Pb age for igneous rocks
represents the timing of magma crystallization, Hf
isotopes allow distinguishing juvenile, essentially
mantle-derived crust of a given age, having positive
ε
Hf (t)
, from contemporary crust derived from re-
melting of older crust, characterized by negative ε
Hf (t)
.
Juvenile magmas are defined as those generated
from the depleted mantle or by re-melting of material
recently extracted from it (Belousova
et al
., 2010).
The single-stage Hf model age (T
DM
) value can be
used for zircons with positive ε
Hf (t)
as a proxy for the
maximum age of magma extraction from the depleted
mantle. Otherwise, the two-stage Hf model (T
DM (c)
)
provides a first approximation to the source age of
host magma from which zircon with negative ε
Hf (t)
crystallized. Hf model (T
DM
) based on a depleted mantle
source, is calculated using (
176
Hf/
177
Hf) i=0.279718
at 4.56 Ga and
176
Lu/
177
Hf=0.0384, and producing a
present-day value of
176
Hf/
177
Hf=0.28325 (Griffin
et
al
., 2000, 2004). The T
DM (c)
age in zircon is calculated
from the initial Hf isotopic composition of the zircon,
using an average crustal Lu/Hf ratio (0.015; Griffin
et al
., 2004). The initial Hf composition of zircon
represents the
176
Hf/
177
Hf value calculated at the time
Table 2 continued.
Isotopic ratios
Ages
U
Th
206
Pb
4f
206
238
U
207
Pb
207
Pb
208
Pb
207
Pb
206
Pb
Disc.
spot
ppm
U
ppm
(%)
206
Pb
206
Pb
235
U
232
Th
206
Pb
238
U
%
e.7-1
139
0.64
4.8
0
24.771±1.59
0.051±4.15
0.284±4.44
0.013±2.89
237±96
255±4
-7
e.10-1
213
1.33
7.3
0
25.101±1.55
0.050±2.35
0.275±2.82
0.013±1.98
199±55
252±4
-27
e.11-1
171
0.61
5.7
0.43
25.947±1.69
0.050±5.92
0.263±6.15
0.012±4.47
172±138
244±4
-42
e.12-1
101
0.65
3.4
0
25.370±1.92
0.053±4.49
0.288±4.89
0.012±3.8
329±102
249±5
24
e.13-1
150
1.04
5.1
0.35
25.44±1.72
0.052±6.36
0.279±6.59
0.012±2.94
266±146
249±4
6
Chuculaqui, granodiorite, titanite
d.2-1
461
0.83
14.9
2.77
27.255±1.73
0.050±5.73
0.252±5.99
0.015±3.18
182±134
232±4
-28
d.3-1
1,049
0.87
34.8
0.85
26.142±1.68
0.053±1.74
0.277±2.42
0.007±2.46
308±40
242±4
21
d.3-2
310
0.85
10.4
3.71
26.543±1.79
0.051±8.08
0.265±8.28
0.021±3.19
243±186
238±4
2
d.3-3
391
0.81
13.4
2.4
25.738±1.72
0.051±5
0.273±5.29
0.020±2.58
235±115
246±4
-5
d.3-4
725
0.84
25.2
1.17
25.004±1.68
0.051±2.32
0.279±2.86
0.008±2.66
222±54
253±4
-14
279
Poma et al. / Andean Geology 41 (2): 267-292, 2014
FIG. 6. Examples of dated zircons and concordia diagram for magmatic zircon samples from a Río Grande Unit.
280
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...
FIG. 7. Examples of dated zircons and concordia diagram for magmatic zircon samples from a Llullaillaco Unit granite.
281
Poma et al. / Andean Geology 41 (2): 267-292, 2014
FIG. 8. Examples of dated sphene (
a
and
b
) and zircon (
c
and
d
) samples from a Chuculaqui Unit granodiorite and concordia diagrams
for magmatic sphene (
e
) and for combined magmatic sphene and zircon (
f
).
282
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MAGMATISM
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...
the zircon crystallized, using the U-Pb age previously
obtained in the same spot of the same crystal. Such
model ages indicate the crustal residence time for the
rocks that hosted the zircon.
5.2.1. Río Grande Unit
Eight zircons of the Río Grande unit already
dated by U-Pb were selected for Hf analyses. The
grains were selected according to the degree of
concordance and lower common lead content. All
176
Hf/
177
Hf ratios are similar and the data produced
negative ε
Hf (t)
(from -0.76 to -3.66). The average
Lu-Hf model age of the zircon assuming a crustal
origin (T
DM (c)
in table 3) is about 1.4 Ga (Fig. 9a).
5.2.2. Llullaillaco Unit
Ten measurements for Hf isotopes were under-
taken on zircons from red granite. The ε
Hf(t)
measured
in this fraction varies between +1.55 and -3.64, with
one outlier at -6.26. This range is mostly coincident
with that obtained for Río Grande Formation zircons.
The Hf T
DM(c)
ages obtained range between 1.16 and
1.64 Ga, mostly grouped at 1.24 and 1.36 Ga (Fig. 9b).
5.2.3. Chuculaqui Unit
Hf isotope determinations in the dated Chucula-
qui granodiorite zircons all give positive ε
Hf
values
between +1.92 and +5.66 (Fig. 9c). Because of this
dominantly juvenile input the values of ε
Hf (t)
and
Hf model ages were calculated using the one-stage
depleted mantle model (Table 3). T
DM
ages obtained
range between 0.65 and 0.79 Ga. These data indicate
that the crustal source of the granodiorite has an
important juvenile component of Cryogenian age.
6. Discussion and interpretation
6.1. Magma sources
Late Paleozoic-Early Triassic magmatism timing
in northern Chile shows two main peaks at about
300 Ma and 244 Ma (Munizaga
et al
., 2008), with
porphyry Cu-Mo type mineralization being related
to the younger event. Results obtained in Argentina
also show two main episodes of magmatism, with
different geochemical and Hf isotope signatures. The
younger age obtained (246±2 Ma for Chuculaqui
igneous event) is coincident with the younger age
reported in Chile, but the older ones (267±3 Ma
for the Río Grande, 263±1 Ma for the Llullaillaco
igneous events, as well as the inherited 280±2 Ma
zircon population in the latter unit) are not matched
by rocks of similar ages in Chile.
When comparing the Hf results for both ages’
groups (Fig. 10), the Río Grande and Llullaillaco
events have no equivalent in northern Chile, but they
share a common Hf evolution trend with the 300 Ma
group with more evolved signatures (Cluster 2)
suggesting that both magmatic events were sourced
by crustal protoliths with similar isotopic signature
and Hf model ages. The younger Chuculaqui event
has ε
Hf (t)
and age values essentially coincident with
those of Cluster 3 from Munizaga
et al
. (2008)
-which includes the Characolla granite porphyry
and El Colorado dacite- both sharing sources with
common isotopic imprints.
Hf data obtained from the Río Grande unit sug-
gest the presence of a Mesoproterozoic crust, or
sedimentary material derived from such crust, beneath
the Puna region. The negative ɛ
Hf
also would indicate
that these mafic magmas were derived from mantle
subsequently contaminated by crustal components.
The ε
Hf (t)
variations in the Llullaillaco unit suggest
that the melt is derived from a mixed juvenile-crustal
source. This range is mostly coincident with that ob-
tained for Río Grande unit zircons, as well as Hf T
DM (c)
ages that point to a Mesoproterozoic (Sunsás) crust,
accordingly suggesting a common origin for both
units. The chemistry of high-silica Llullaillaco rocks
indicates crustal participation in their parental melts
and within-plate extensional affinities.
Otherwise, positive ε
Hf (t)
values and T
DM
ages of
about 0.71 Ga in the Chuculaqui unit indicate that the
crustal source of the granodiorite has an important
juvenile component of Cryogenian age.
From a geochemical point of view the Río Grande
unit magmas would appear to have been generated
in a setting with intraplate or no-compressional
affinities, subsequently contaminated by crustal
components as indicted by the Hf isotope data. The
lack of inherited xenocrysts should be noted and may
result from high magmatic temperatures.
The Llullaillaco granites are representative of
a prevailing crustal component without excluding
some juvenile mantelic input at the time of mag-
matism. The dominant crustal source and shallow
emplacement are displayed by their geochemical
and textural features. The REE pattern is represen-
tative of high-silica evolved magmas with LREE
depletion caused by minor accessory minerals and
283
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TABLE 3. Hf ISOTOPIC DATA OF DATED ZIRCONS FROM THE GONDWANAN MAGMATIC UNITS OF WESTERN PUNA, ARGENTINA.
Unit
Analysis No.
176
Hf/
177
Hf
1
σ
e
176
Lu/
177
Hf
1
σ
e
U-Pb age
(Ma)
176
Hf/
177
Hf
initial
epsilon Hf
1
σ
e
T(DM)
(Ga)
T
(DM)c
crustal
Río Grande
09-36B-Rio G-1.1
0.282591
0.000008
0.0013281
0.000004
267.8
0.2825910
-0.56
0.280
0.91
1.29
Río Grande
09-36B-Rio G-2.1
0.282582
0.000010
0.0018528
0.000050
253.2
0.2825820
-1.29
0.343
0.94
1.33
Río Grande
09-36B-Rio G-2.4
0.282563
0.000011
0.0017236
0.000033
237.1
0.2825630
-2.29
0.385
0.96
1.38
Río Grande
09-36B-Rio G-3.1
0.282579
0.000011
0.0010201
0.000018
272.7
0.2825790
-0.82
0.385
0.92
1.31
Río Grande
09-36B-Rio G-3.2
0.282552
0.000012
0.0018275
0.000069
279.2
0.2825520
-1.79
0.420
0.98
1.38
Río Grande
09-36B-Rio G-4.2.5
0.282537
0.000009
0.0015018
0.000027
279.3
0.2825370
-2.25
0.305
0.99
1.41
Río Grande
09-36B-Rio G-4.4
0.282567
0.000010
0.0019259
0.000013
261.7
0.2825670
-1.65
0.350
0.96
1.36
Río Grande
09-36B-Rio G-6.1.4
0.282504
0.000010
0.0004455
0.000016
268.4
0.2825040
-3.46
0.340
1.01
1.47
Llullaillaco
1126B-02.1
0.282443
0.000034
0.0031847
0.000085
262.1
0.2824268
-6.26
1.190
1.17
1.64
Llullaillaco
1126B-03.1
0.28255
0.000021
0.0039609
0.000160
260.9
0.2825300
-2.63
0.735
1.04
1.42
Llullaillaco
1126B-04.1
0.282515
0.000021
0.0023440
0.000067
258.4
0.2825033
-3.64
0.735
1.05
1.47
Llullaillaco
1126B-04.2
0.282576
0.000009
0.0013394
0.000056
278.2
0.2825688
-0.87
0.301
0.93
1.32
Llullaillaco
1126B-04.3
0.28256
0.000019
0.0028311
0.000140
283.6
0.2825445
-1.60
0.665
1.00
1.37
Llullaillaco
1126B-05.1
0.282561
0.000019
0.0023722
0.000091
265.7
0.2825488
-1.86
0.665
0.98
1.37
Llullaillaco
1138G-05.1
0.282659
0.000016
0.0026880
0.000082
265.5
0.2826452
1.55
0.560
0.85
1.16
Llullaillaco
1138G-06.1
0.282623
0.000009
0.0018223
0.000021
266.7
0.2826136
0.46
0.326
0.88
1.23
Llullaillaco
1138G-07.1
0.282633
0.000017
0.0016833
0.000057
261.9
0.2826245
0.73
0.595
0.86
1.21
Llullaillaco
1138G-07.2
0.282611
0.000019
0.0025071
0.000062
267.1
0.2825980
-0.08
0.665
0.91
1.26
Chuculaqui
09-36E-Chu-1.1.3
0.282696
0.000013
0.0007190
0.000004
251.1
0.2826925
2.89
0.455
0.76
1.07
Chuculaqui
09-36E-Chu-1.2
0.282672
0.000008
0.0007845
0.000019
246.2
0.2826683
1.92
0.287
0.79
1.12
Chuculaqui
09-36E-Chu-2.1.5
0.282726
0.000009
0.0009312
0.000014
252.5
0.2827215
3.95
0.319
0.72
1.00
Chuculaqui
09-36E-Chuc-5.1
0.282762
0.000009
0.0013036
0.000048
235.4
0.2827561
4.78
0.305
0.68
0.94
Chuculaqui
09-36E-Chuc-7.1.3
0.282772
0.000010
0.0009495
0.000011
255.1
0.2827673
5.63
0.350
0.66
0.90
Chuculaqui
09-36E-Chuc-9.1
0.282742
0.000009
0.0009610
0.000018
237.1
0.2827376
4.17
0.312
0.70
0.98
Chuculaqui
09-36E-Chuc-10.1
0.28273
0.000009
0.0008607
0.000005
251.8
0.2827258
4.09
0.312
0.71
1.00
Chuculaqui
09-36E-Chuc-12.1
0.282775
0.000011
0.0006523
0.000008
249.2
0.2827719
5.66
0.385
0.65
0.90
Chuculaqui
09-36E-Chuc-13.1
0.282723
0.000009
0.0004224
0.000008
248.5
0.2827210
3.84
0.322
0.71
1.01
The ‘crustal’ model ages (T
DM (C)
) assume that the zircon’s parental magma was produced from a volume of average continental crust (
176
Lu/
177
Hf=0.015; GrifFn
et al
., 2004),
that was originally derived from the depleted mantle.
176
Lu decay is 1.876 x 10-11 yr-1 (Scherer
et al
., 2001).
284
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...
FIG. 9. Diagrams of
176
Hf/
177
Hf (initial)
versus
206
Pb/
238
U ages of zircons from
a.
the Río Grande Unit diorite;
b.
Llullaillaco Unit
granite; and
c.
Chuculaqui Unit granodiorite.
285
Poma et al. / Andean Geology 41 (2): 267-292, 2014
strong plagioclase fractionation, consistent with the
hypersolvus-like features recognized. The nega-
tive ε
Hf (t)
values obtained as well as the presence
of zircon xenocrysts suggest assimilation of older
unexposed igneous material, possibly related to the
early stages of the same magmatic event evidenced
by an older population of zircons of 280±2 Ma with
ages coincident to the oldest zircons dated from the
Río Grande Unit at 279-282 Ma.
The granitoids of the Chuculaqui unit show typi-
cal Cordilleran-type magmatic arc characteristics
considering their geochemistry and their evolutionary
trend. The positive ε
Hf (t)
values for the zircons of this
unit also point to relatively juvenile inputs with little
presence of recycled crust sources.
The data obtained are consistent with the results
presented by Munizaga
et al
. (2008) in Chile, confirm-
ing the observation that the magmas show contribu-
tions from inhomogeneous older crust material as
well as variable magmatic sources, with mixture of
crustal melts and mantle-derived magmas.
6.2. The age of the crust in the Puna region: Iso-
topic constraints
The two stage Depleted Mantle Mesoproterozoic
model ages obtained for the Permian Río Grande
and Llullaillaco units indicate a significant residence
time in the crust for the magma sources and their
emplacement on continental crust. This magmatism
in northwest Argentina thus confirms the presence
of Mesoproterozoic crustal components mainly Ec-
tasian to Calymnian (1.24 to 1.44 Ga-negative ε
Hf (t)
)
supporting the idea of a continuous basement under
the Central Andes, as an extension of the Arequipa-
Antofalla massif (Fig. 11). Coincidentally, Zap
-
pettini and Santos (2011) have reported Ectasian and
Calymnian T
DM (c)
Hf ages (1.36 and 1.48 Ga) with
negative ε
Hf (t)
(between 0 and -22.3) from Cretaceous
syenitic intrusions in Eastern Puna. Preliminary data
obtained by the authors from Ordovician granitoids
from Western Puna also point to Calymnian T
DM (c)
Hf
ages (between 1.42 and 1.62 Ga) and negative ε
Hf (t)
(between 0 and -3.39).
The T
DM (c)
Hf ages of zircons at around 1.4 Ga
obtained by Munizaga
et al
. (2008) for the Perm-
ian magmatism in Chile are also indicative of the
presence of the aforementioned Mesoproterozoic
(Ectasian to Calymnian) crustal source.
Additionally, the 650 to 790 Ma T
DM
zircon Hf
ages for the Chuculaqui Formation as well as similar
data reported by Munizaga
et al
. (2008), with positive
ε
Hf (t)
and T
DM
Hf varying between 604 and 748 Ma
(recalculated from reported data by Munizaga
et
FIG.
10. ε
Hf (T)
in zircons
versus
their respective U-Pb age from the Gondwanan magmatic units of Western Puna, Argentina. Fields of
data from Munizaga
et al
. (2008) are indicated for comparison.
286
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AND
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...
al.,
2008) would point to the existence of juvenile,
essentially mantle-derived Cryogenian crust in the
region, associated with the older Mesoproterozoic
basement crust. Similarly, Zappettini and Santos
(2011) have reported a 153±2 Ma diorite intrusion
in Eastern Puna, with ε
Hf (t)
(+2.5 to +5.0) and T
DM
Hf
varying between 620 and 760 Ma. Moreover, the
presence of enriched mantle in the 700-800 Ma range
(late Rodinia break-up) is already known in Sierras
Pampeanas (Rapela
et al
., 2010) and the data obtained
suggest that such enriched mantle was active beneath
the Puna region. It should be noted that the Chucu-
laqui Unit lacks relict or inherited zircon that would
point to reworking of ancient crust. This excludes the
obtained T
DM
Hf ages to be interpreted as a binary mix
of such an ancient crust and mantle material at the
time of the generation of the granodiorite, although
some Triassic juvenile input is not precluded.
The extension of the Arequipa-Antofalla massif
beneath the Altiplano, as indicated by the reported
Hf data, is consistent with previous Pb-isotopic
signature of plutonic rocks from western Puna that
confirms the presence of Sunsás-San Ignacio age
crust in NW Argentina (cf. Ramos, 2008; Fig. 5 and
references therein).
Loewy
et al
. (2004) have defined two domains
as constituting the Antofalla basement: the northern
one that extends in Chile between Belén and Sierra
Moreno includes juvenile magmatism at 1.5-1.4 Ga,
evidence of metamorphism at 1.2-1.0 Ga and later
FIG. 11. Main basement blocks with ages from basement outcrops and model ages from the Gondwanan igneous rocks (this paper and
Munizaga
et al
., 2008). Other basement ages from Ramos (2008).
287
Poma et al. / Andean Geology 41 (2): 267-292, 2014
magmatism at about 500-400 Ma; and the southern
domain, exposed from Limón Verde in northern Chile
to Antofalla in western Argentina Puna (22°-26°S),
including juvenile material of 700-600 Ma and mag-
matism and metamorphism recorded at 500-400 Ma.
The studied area pertains to the southern domain,
but none of these latter events have been identified,
except for the 700 to 600 Ma juvenile magmatic
episodes that could be correlated with the 710 Ma
T
DM
zircon Hf ages identified in the Chuculaqui Unit.
The 1.24 to 1.36 Ga and 1.0 Ga events would extend
the tectonostratigraphic history of this region at least
to the Mesoproterozoic and, considering the isolated
Statherian T
DM (c)
Hf age (1.64 Ga) with negative ε
Hf (t)
,
up to the late Paleoproterozoic.
The Arequipa-Antofalla massif is interpreted to be
allochthonous to Amazonia (cf. Loewy
et al
., 2004)
and that its accretion to Amazonia would have taken
place during the Grenville-Sunsás Orogeny (1.0-
1.3 Ga) (Chew
et al
., 2007). The recorded evidence of
Early Mesoproterozoic crust in its southern extension
could provide additional evidence for its connection
to the Maz and Río Apa blocks, constituting together
the hypothetical MARA craton, following the model
proposed by Casquet
et al
. (2009, 2010). The 1.24 to
1.36 Ga crustal ages obtained are coincident with the
Andean-type magmatic arc recorded from the Maz
terrane at 1.26 to 1.33 Ga (Casquet
et al
., 2011 and
references therein), as well as to the thermal episode
at 1.3 Ga that affected the Río Apa block (Cordani
et
al
., 2010), pointing to a common history for the three
areas at least during the Mesoproterozoic.
7. Concluding remarks
Gondwanan magmatism developed between
Early Carboniferous and Early Triassic times.
In NW Argentina it comprises two episodes of
different age and genesis: the oldest includes
gabbros and diorites (Río Grande Unit) and
granitoids (belonging to the Llullaillaco Unit) of
late Permian age (Guadalupian) generated in an
intraplate environment, hypothetically from an
enriched mantle subsequently contaminated with
crustal components; the youngest is represented by
granodiorites (Chuculaqui Unit) of middle Triassic
age (Anisian) with Cordilleran-type arc signature.
The results obtained here are comparable to those
presented by Munizaga
et al
. (2008) for northern
Chile (see discussion above) and can be related to
the Pre-Andean cycle as distinguished in Chile by
Charrier
et al
. (2007).
The Choiyoi magmatic province, according to
Llambías and Sato (1995) and Llambías (1999),
developed in a tectonic setting evolving from a
late Carboniferous to Permian subduction-related
magmatic arc through a collisional regime and
subsequent early Triassic post-orogenic granite
magmatism, the latter developed in the Argentine
side of the Frontal Cordillera. Kleiman and Japas
(2009) and Rocha Campos
et al
. (2011) subdivided
the Choiyoi magmatism in a lower section (with ages
between 280 and 265 Ma) and an upper section (with
ages between 265 and 250 Ma). Triassic volcanic
sequences are separately grouped in a synrift phase.
Late Triassic to early Jurassic volcanic sequences
are grouped further south, in the basement of the
Neuquén Basin, into the Precuyano Cycle related
also to a rift setting.
We consider that the upper section of the Choi-
yoi magmatism, with main outcrops in the Frontal
Cordillera and San Rafael Block, reaches the NW
Argentine Puna where it is represented by the Río
Grande and Llullaillaco units. In fact, the tectonic
conditions that originated both the Permian magma-
tism described in this paper and that of the upper
section of the Choiyoi series, as well as their geo-
chemical signatures, are analogous. The ages deter-
mined in the Puna units are slightly older than those
known from further south in the Frontal Cordillera
and San Rafael Block (Kleiman and Japas, 2009),
suggesting that these common tectonic conditions
were originated earlier in the north, with progressive
migration to the south.
Conversely, the Triassic magmatism represented
by the 246 Ma Chuculaqui Unit in the Puna region
cannot be correlated with the upper section of the
Choiyoi magmatism from Frontal Cordillera and
San Rafael Block, the latter being older than 250 Ma
(Rocha Campos
et al
., 2011). Furthermore, the
Triassic magmatism from the Gondwanan Cycle of
similar age (246 Ma and younger), identified in the
San Rafael Block, has been grouped in the above
mentioned synrift phase (Rocha Campos
et al
., 2011)
with geochemical characteristics and tectonic set-
ting emplacement that differ from those identified
for the Chuculaqui unit. Rather, the latter could be
ascribed to a poorly known continental magmatic
arc segment of mostly NS trend. Although during
the Pre-Andean tectonic cycle subduction along the
288
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EOCHEMISTRY
, U-P
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continental margin was presumably interrupted or
considerably diminished (Charrier
et al
., 2007) the
presence of the Chuculaqui Unit and similar rocks
of the same age in northern Chile (Munizaga
et al
.,
2008) are indicative of, at least, some restricted
subduction related magmatism during the Triassic.
Interestingly, it should be highlighted that in Chile
there are porphyry Cu type deposits (La Profunda
and Characolla) genetically related to these rocks
(Munizaga
et al
., 2008). This implies the presence
of an early Triassic metallogenic belt that continues
into Argentina, in the areas where the Chuculaqui
magmatic event has been defined.
Acknowledgments
This research was partially supported by the Servicio
Geológico Minero Argentino (SEGEMAR) and by grants
from the University of Buenos Aires to SP. (UBACYT
X20020100100520) and CONICET to SQ. (PIP453/11).
SHRIMP U-Pb analyses were performed at Curtin Uni-
versity, Perth, Western Australia.
Detailed revisions by C. Casquet (Universidad de Ma-
drid), D. Morata (Universidad de Chile) and R. Pankhurst
(British Geological Survey) greatly improved the original
manuscript.
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Edades U-Pb SHRIMP en circones, determinaciones
isotópicas de Hafnio e implicancias geodinámicas.
In
Congreso Geológico Argentino, No. 18, Actas DVD-
Rom. Neuquén.
Manuscript received: December 11, 2013; revised/accepted: January 16, 2014; available online: January 17, 2014
292
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EOCHEMISTRY
, U-P
B
SHRIMP
ZIRCON
DATING
AND
H
F
ISOTOPES
OF
THE
G
ONDWANAN
MAGMATISM
IN
NW A
RGENTINA
...
Appendix
Description of methods
Representative samples of the three units were analyzed for major elements by inductively coupled
plasma (ICP) and for trace and rare earth elements by ICP Mass Spectrometry (ICP-MS) at Activation Labo-
ratories of Ancaster, Ontario, Canada. Representative data from main group types are presented in table 1.
U-Pb analysis of zircon was carried out at Curtin University of Technology, Perth. Samples RG (dio-
rite from Río Grande Unit), CHUQ (granodiorite from Chuculaqui Unit) and SA10-03 (red granite from
Llullaillaco Unit) have been crushed, milled, sieved, and washed to remove very fine material (clay and silt
sizes). The 60-250 mesh fractions were treated with heavy liquids (to remove light minerals) and magnetic
separator (to concentrate the less magnetic minerals such as zircon). Zircon was handpicked and organized
in an epoxy mount, which was polished and carbon-coated for SEM (Scanning Electron Microscope) study.
Back-scattered images (BSE) were taken using a JEOL6400 SEM at the Centre for Microscopy and Micro-
analyses at University of Western Australia. Images of zircon are critical for identifying internal features
such as core and rims and to help avoiding areas with high common lead content (inclusions, fractures, and
metamict areas). Epoxy mount (UWA 05-85) was gold-coated for SHRIMP analyses.
Sensitive High Mass Resolution Ion MicroProbe (SHRIMP II) U-Pb analyses were performed at Curtin
University, under a Consortium between that university, the Western Australia University, and the Geo-
logical Survey of Western Australia. Data was collected in two sessions using an analytical spot size of
about 20-25 mm. Individual analyses are composed of measurement of nine masses repeated in five scans.
The following masses were analyzed for zircon: (Zr
2
O,
204
Pb, background,
206
Pb,
207
Pb,
208
Pb,
238
U,
248
ThO,
254
UO). The standards D23 and NBS611 were used to identify the position of the peak of the mass
204
Pb,
whereas the calibration of the U-content and the Pb/U ratio were done using the zircon standard BR266
(559 Ma, 903 ppm U). Data were reduced using the SQUID
©
1.03 software (Ludwig, 2001) and the ages
calculated using Isoplot
©
3.0 (Ludwig, 2003). The Phanerozoic ages are mean average
206
Pb/
238
U ages where
the common lead is corrected using the 207Pb content. The uncertainties of individual ages are quoted at
1σ whereas the final ages and those used in the plots are calculated at 2σ level (about 95% confidence).
Hf-isotope analyses reported here were carried out in situ using a New Wave Research LUV213 laser-
ablation microprobe, attached to a Nu Plasma multicollector ICPMS at GEMOC Key Centre, Macquarie
University, Sydney. Most analyses are carried out with a beam diameter of about 40 μm, a 10 Hz repetition
rate, and energies of 0.6-1.3 mJ/pulse. Typical ablation times are 30-120 s, resulting in pits 20-40 μm deep.
The analytical spots of Hf-isotope analyses were located in the same site of the previous U-Pb SHRIMP
analyses. Isobaric interferences of
176
Lu and
176
Yb on
176
Hf were corrected by the Nu Plasma because the mass
bias of the instrument is independent of mass over the mass range considered. Interference of
176L
u on
176
Hf
is corrected by measuring the intensity of the interference-free
175
Lu isotope and using
176
Lu/
175
Lu=0.02669
to calculate the intensity of
176
Lu. Similarly, the interference of
176
Yb on
176
Hf is corrected by measuring
the interference-free
172
Yb isotope and using
176
Yb/
172
Yb to calculate the intensity of
176
Yb. The spiking of
JMC475 Hf standard is used to determine the value of
176
Yb/
172
Yb (0.5865) required to yield the value of
176
Hf/
177
Hf obtained on the pure Hf solution.
The
176
Lu decay constant used to calculate initial
176
Hf/
177
Hf, ε
Hf
values, and model age is 1.983×10-11
(Bizzarro
et al
., 2003). Typical uncertainties on single
176
Lu/
177
Hf analyses are about 1 sigma unit (±0.001-
0.002%) incorporating both spatial variation of Lu/Hf and analytical uncertainties. Chondritic values of
Scherer
et al
. (2001) (1.865x10-11) have been used for the calculation of ε
Hf
values. A model of (
176
Hf/
177
Hf)
i= 0.279718 at 4.56 Ga and
176
Lu/
177
Hf = 0.0384 has been used to calculate model ages (T
DM
) based on a
depleted mantle source, producing a present-day value of
176
Hf/
177
Hf (0.28325) (Griffin
et al
., 2000, 2004).
T
DM
ages, which are calculated using measured
176
Hf/
177
Hf of the zircon, give only the minimum age for the
source material from which the zircon crystallized. We have also calculated a ‘crustal’ model age (T
DM(c)
)
for each zircon which assumes that the parental magma was produced from an average continental crust
(
176
Lu/
177
Hf = 0.015) (Griffin
et al
., 2004) that was originally derived from depleted mantle.
Hf data are given in table 3. ε
Hf
values, also summarized in table 3, were calculated at the
206
Pb/
238
U
age of each grain (T).
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