Miocene to Late Quaternary Patagonian basalts (46–478S

Transcription

Journal of Volcanology and Geothermal Research 149 (2006) 346 – 370
www.elsevier.com/locate/jvolgeores
Miocene to Late Quaternary Patagonian basalts (46–478S):
Geochronometric and geochemical evidence for slab tearing due
to active spreading ridge subduction
Christèle Guivel a,*, Diego Morata b, Ewan Pelleter c,d, Felipe Espinoza b,
René C. Maury c, Yves Lagabrielle e, Mireille Polvé f,g, Hervé Bellon c, Joseph Cotten c,
Mathieu Benoit c, Manuel Suárez h, Rita de la Cruz h
a
b
UMR 6112 bPlanétologie et GéodynamiqueQ, Université de Nantes, 2 rue de la Houssinière, 44322 Nantes, France
Departamento de Geologı́a. Fac. Cs. Fı́sicas y Matemáticas, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile
c
UMR 6538 bDomaines océaniquesQ, UBO-IUEM, place Nicolas-Copernic, 29280 Plouzané, France
d
CRPG-CNRS UPR A2300, BP 20, 54501 Vandoeuvre-les-Nancy, France
e
UMR 5573, Dynamique de la Lithosphère, Place E. Bataillon, case 60, 34095, Montpellier Cedex 5, France
f
LMTG-OMP, 14 Avenue E. Belin, 31400 Toulouse, France
g
IRD-Departamento de Geologia de la Universidad de Chile, Chile
h
Servicio Nacional de Geologı́a y Minerı́a, Avda. Santa Marı́a 0104, Santiago, Chile
Received 18 May 2005; received in revised form 29 August 2005; accepted 14 September 2005
Abstract
Miocene to Quaternary large basaltic plateaus occur in the back-arc domain of the Andean chain in Patagonia. They are thought
to result from the ascent of subslab asthenospheric magmas through slab windows generated from subducted segments of the South
Chile Ridge (SCR). We have investigated three volcanic centres from the Lago General Carrera–Buenos Aires area (46–478S)
located above the inferred position of the slab window corresponding to a segment subducted 6 Ma ago. (1) The Quaternary Rı́o
Murta transitional basalts display major, trace elements, and Sr and Nd isotopic features similar to those of oceanic basalts from the
SCR and from the Chile Triple Junction near Taitao Peninsula (e.g., (87Sr/86Sr)o = 0.70396–0.70346 and qNd = + 5.5 + 3.0). We
consider them as derived from the melting of a Chile Ridge asthenospheric mantle source containing a weak subduction
component. (2) The Plio-Quaternary (b 3.3 Ma) post-plateau basanites from Meseta del Lago Buenos Aires (MLBA), Argentina,
likely derive from small degrees of melting of OIB-type mantle sources involving the subslab asthenosphere and the enriched
subcontinental lithospheric mantle. (3) The main plateau basaltic volcanism in this region is represented by the 12.4–3.3-Ma-old
MLBA basalts and the 8.2–4.4-Ma-old basalts from Meseta Chile Chico (MCC), Chile. Two groups can be distinguished among
these main plateau basalts. The first group includes alkali basalts and trachybasalts displaying typical OIB signatures and thought to
derive from predominantly asthenospheric mantle sources similar to those of the post-plateau MLBA basalts, but through slightly
larger degrees of melting. The second one, although still dominantly alkalic, displays incompatible element signatures intermediate
between those of OIB and arc magmas (e.g., La/Nb N 1 and TiO2 b 2 wt.%). These intermediate basalts differ from their strictly
alkalic equivalents by having lower High Field Strength Element (HFSE) and higher qNd (up to + 5.4). These features are
consistent with their derivation from an enriched mantle source contaminated by ca. 10% rutile-bearing restite of altered oceanic
crust. The petrogenesis of the studied Mio-Pliocene basalts from MLBA and MCC is consistent with contributions of the subslab
* Corresponding author.
E-mail address: [email protected] (C. Guivel).
0377-0273/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2005.09.002
C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370
347
asthenosphere, the South American subcontinental lithospheric mantle and the subducted Pacific oceanic crust to their sources.
However, their chronology of emplacement is not consistent with an ascent through an asthenospheric window opened as a
consequence of the subduction of segment SCR-1, which entered the trench at 6 Ma. Indeed, magmatic activity was already
important between 12 and 8 Ma in MLBA and MCC as well as in southernmost plateaus, i.e., 6 Ma before the subduction of the
SCR-1 segment. We propose a geodynamic model in which OIB and intermediate magmas derived from deep subslab
asthenospheric mantle did uprise through a tear-in-the-slab, which formed when the southernmost segments of the SCR collided
with the Chile Trench around 15 Ma. During their ascent, they interacted with the Patagonian supraslab mantle and, locally, with
slivers of subducted Pacific oceanic crust that contributed to the geochemical signature of the intermediate basalts.
D 2005 Elsevier B.V. All rights reserved.
Keywords: slab window; slab tear; plateau basalts; alkali basalts; ridge subduction; Patagonia
1. Introduction
Neogene and Quaternary magmatic activity in the
Patagonian Andes displays numerous specific features
which can be related to the subduction of the segmented South Chile Ridge (SCR) beneath the South
American plate. During the last 15 Ma, the location of
this ridge subduction (the Chile Triple Junction, CTJ)
migrated northwards as a result of the oblique collision between the Chile ridge and the South American
margin (Herron et al., 1981; Cande and Leslie, 1986;
Cande et al., 1987; Nelson et al., 1994; Bangs and
Cande, 1997; Tebbens and Cande, 1997; Tebbens et
al., 1997). The present location of the CTJ, ca. 50 km
north of the Taitao Peninsula (Fig. 1A), is marked by
near-trench magmatic activity (Forsythe and Nelson,
1985; Forsythe et al., 1986, 1995; Lagabrielle et al.,
1994, 2000; Bourgois et al., 1996; Guivel et al., 1999,
2003) and a corresponding gap in the Andean calcalkaline volcanic belt between the southern part of the
Southern Volcanic Zone (SSVZ, 41815V–468S) and the
Austral Volcanic Zone (AVZ, 49–548S) (Stern et al.,
1990; Ramos and Kay, 1992). East of the Andean
chain, the Patagonian back-arc domain is characterised
by numerous Neogene basaltic plateaus (Mesetas), the
emplacement of which does not seem to be connected
either with back-arc extension or with a topographic
swell or hotspot track (Ramos and Kay, 1992). Numerous authors (Ramos and Kay, 1992; Kay et al.,
1993; Gorring et al., 1997, 2003; D’Orazio et al.,
2000, 2001, 2003; Gorring and Kay, 2001) have proposed that these basaltic magmas were produced by
melting of subslab asthenospheric mantle upwelling
through slab windows generated from subducted
ridge segments (Dickinson and Snyder, 1979; Thorkelson, 1996; Murdie and Russo, 1999). Especially,
Gorring et al. (1997) and Gorring and Kay (2001)
pointed out that the spatial distribution, ages and
chemistries of the Neogene basaltic plateaus of Southern Argentina fit apparently with the locations of
asthenospheric windows which opened successively
when segments of the Chile ridge bounded by large
fracture zones (FZ) were subducted. Fig. 1B shows
that the subduction of these various segments, according to their magnetic anomaly patterns, started at ca.
15–14 Ma (SCR-4, south of Desolación FZ), 14–13
Ma (SCR-3, south of Madre de Dios FZ), 12 Ma
(SCR-2, south of Esmeralda FZ), 6 Ma (SCR-1, between Esmeralda and Tres Montes FZ), 3 Ma (SCR0,
between Tres Montes and Taitao FZ) and finally 0.3
Ma (SCR1, north of Taitao FZ), respectively (Cande
and Leslie, 1986; Forsythe et al., 1986).
In this paper, we test this model using new geochronometric (K–Ar) and geochemical (major, trace element
and Sr and Nd isotopic data) on basalts from the Lago
General Carrera–Buenos Aires area (46–478S) in southern Patagonia. This area is located at the latitude of the
present Chile Triple Junction position (Fig. 1B), along
the Chile–Argentina border, south of Mt. Hudson, the
southernmost active volcano of the SSVZ. As shown in
Fig. 1B, it overlies the SCR-1 slab window present
position inferred from magnetic anomalies (Cande
and Leslie, 1986; Tebbens et al., 1997; Lagabrielle et
al., 2000). Three Miocene to Quaternary basaltic complexes are exposed in this area on both sides of the
Argentina/Chile border (Fig. 1C): Meseta Chile Chico
(Chile) which is capped by a basaltic pile dated back to
8.2–4.4 Ma (Espinoza et al., 2005), Meseta del Lago
Buenos Aires (Argentina) for which available K–Ar
and Ar–Ar ages range from 10.0 to 0.76 Ma (TonThat et al., 1999) and 10.1 Ma to b110 ka (Brown et
al., 2004), and finally Rı́o Murta (Chile) subglacial
basalts, previously considered Holocene (Demant et
al., 1994, 1998; Corgne et al., 2001). We will show
that the timing and geochemistry of most of these
basaltic eruptive events do not fit with the hypothesis
of their derivation from the subslab asthenospheric
mantle from the SCR-1 fragment, and that alternative
models of opening of asthenospheric windows or tearsin-the-slab need to be envisioned.
348
2. Regional geology
Most authors consider that the Miocene–Recent
evolution of the Patagonian Andes has been controlled
by the oblique northward subduction of the Chile
Ridge beneath the Andean continental margin (Fig.
1A). Plate reconstructions by Cande and Leslie
(1986) indicate that initial ridge collision started at
15–14 Ma at ca. 558S, forming a triple junction (the
Chile Triple Junction, CTJ) between South America,
349
Fig. 2. Plot of the ages of the southern Patagonian basalts against latitude. Where reported, errors are in 1r. The latitudes and ages of arrival to the
trench of South Chile Ridge segments SCR-3, SCR-2, SCR-1, SCRO and SCR1 are also shown. Sources of previously published ages: Pali Aike
(D’Orazio et al., 2000), Estancia Glencross (D’Orazio et al., 2001), Condor Cliff (Gorring et al., 1997), Meseta de la Muerte (Gorring et al., 1997),
Meseta Central (Gorring et al., 1997), Meseta Belgrano (Gorring et al., 1997), Northeast region (Gorring et al., 1997), Cerro Pampa (in Kay et al.,
1993), Meseta del Lago Buenos Aires (Ton-That et al., 1999, Brown et al., 2004, and this work), Meseta de Chile Chico (Espinoza et al., 2005),
Murta basalts (this work).
Nazca and Antarctic plates. Then, the CTJ migrated
northwards up to its present position at ca. 468S (Fig.
1B). On the continent, the last major compressive
phase which affected the Patagonian fold and thrust
belt started at ca. 15 Ma (Lagabrielle et al., 2004) and
is generally considered as a consequence of ridge–
trench collision. Then, in the back-arc domain, an
important Neogene magmatic event led to the emplacement of large basaltic plateaus (Mesetas). It started at
ca. 12 Ma, more or less simultaneously with the emplacement of Cerro Pampa adakites which have been
interpreted as partial melts of the young subducted
Pacific oceanic slab (Kay et al., 1993). Gorring et al.
(1997, 2003) and Gorring and Kay (2001) have distinguished two stages of building of the basaltic mesetas. Thick lava flows, generally tholeiitic, were erupted
during the main plateau stage. Then, after a quiescence
period sometimes several million years long, volcanic
activity resumed, emplacing smaller amounts of postplateau basaltic lavas (usually alkali basalts or basanites) richer in incompatible elements than the main
plateau basalts. Post-plateau basalts generally crop out
as strombolian cones, maars and flows filling channels
or paleolandscapes.
The ages of these basaltic plateaus, based on available
K–Ar and Ar–Ar dates, have been plotted against latitude in Fig. 2. The general pattern suggests that magmatic activity started between 12 and 8 Ma all along the
back-arc domain of the Patagonian thrust and fold belt,
from 528S to the present position of the CTJ at 468S.
Unlike Ramos and Kay (1992), Kay et al. (1993), Gorring et al. (1997, 2003) and Gorring and Kay (2001), we
find no evidence for a trend towards younger ages northwards which might be correlated with the chronology of
the subduction of the successive Chile Ridge segments.
Young (Pliocene–Late Quaternary) or relatively young
volcanic activity is evidenced both at ca. 528S (Pali Aike
volcanic field, D’Orazio et al., 2000), 6 508S (Camusú
Aike, D’Orazio et al., 2005), 49856.6V8S (Condor Cliff,
Gorring et al., 1997) and near 47–468S in Meseta del
Lago Buenos Aires (Gorring et al., 2003; Brown et al.,
2004) and Rı́o Murta (Demant et al., 1998).
The studied area (Lago General Carrera–Buenos
Aires, 46–478S and 70–738W) is located within the
Fig. 1. Geological setting of the studied volcanic rocks. (A) Simplified tectonic sketch map of the present-day Chile Triple Junction (CTJ) showing
the location of the studied area and the sense of motion (black arrows) of the Nazca and Antarctic plates with respect to the South American Plate;
numbers are relative velocities in cm/yr (DeMets et al., 1990); (B) simplified map of the CTJ area and location of the fracture zones (FZ) and active
segments of the South Chile Ridge (SCR) indicating the ridge collision ages (grey numbers) and the present-day inferred locations of subducted
active ridges (SCR0, SCR-1, SCR-2) (simplified from Guivel et al., 1999; Lagabrielle et al., 2004). Grey triangles: southernmost volcanoes of the
South Volcanic Zone (SVZ) and northernmost volcanoes of the Austral Volcanic Zone (AVZ); empty triangle: Cerro Pampa adakite; (C) simplified
geological map of the Lago General Carrera–Buenos Aires area in Patagonia (46–478S) showing the location of the studied areas in which Neogene
and Quaternary volcanic rocks are located (modified from Lagabrielle et al., 2004). Local geological sketch maps of these areas are shown in Fig. 3.
350
modern volcanic arc gap, south of the SSVZ and north of
the AVZ. In this zone, the Miocene to Late Quaternary
magmatic rocks investigated were emplaced over and/or
intruded into Palaeozoic to Plio-Quaternary units (Fig.
3). Strongly deformed Palaeozoic low- to medium-grade
metasediments of the Eastern Andean Metamorphic
Fig. 3. Local geological sketch maps of the studied volcanic areas. (A) Rı́o Murta basalts (modified from SERNAGEOMIN 1:1,000,000 unpublished
map); (B) Meseta Chile Chico (MCC) basaltic plateau (simplified from Espinoza et al., 2005); (C) Meseta del Lago Buenos Aires (MLBA) basaltic
plateau (simplified from SEGEMAR 1:750,000 map). The whole rock K–Ar ages and the location of dated samples are indicated.
351
dillera front, in the Cosmeli basin and below the basaltic sequences of the Meseta del Lago Buenos Aires.
They are mostly fluviatile with local marine intercalations and correspond to the Guadal, Galera (in Chile)
and Santa Cruz (in Argentina) Formations (Lagabrielle
et al., 2004). Plio-Quaternary moraines and glacial
deposits are mostly located in the eastern side of the
Lago General Carrera–Buenos Aires. Plutonic rocks are
represented by Meso-Cenozoic subduction-related granitoids of the North Patagonian Batholith (Pankhurst et
al., 1999), and small Late Miocene and Pliocene satellite
plutons (Pankhurst et al., 1999; Suárez and De La Cruz,
2001; Thomson et al., 2001; Morata et al., 2002).
Quaternary volcanic structures belonging to the
SSVZ occur north of the studied area. The Cay,
Maca, Isla Colorada and Hudson volcanoes (Fig.
1B) are the southernmost volcanic centres of the
Complex crop out west and south of the Lago General
Carrera–Buenos Aires (Bell and Suárez, 2000). Middle
to Late Jurassic rhyolitic ignimbrites and lava flows
(with minor andesitic to basaltic intercalated flows) belonging to the large silicic Chon Aike Province (Pankhurst et al., 1998, 2000; Féraud et al., 1999)
unconformably overlie the metamorphic rocks. These
rocks are referred to as the Ibáñez Group in Chile and
the El Quemado Complex in Argentina. Late Jurassic to
Lower Cretaceous marine sedimentary rocks (Coyaique
Group) and subaerial volcanics associated with continental sedimentary rocks (Divisadero Group), which
overlie diachronically the Ibáñez Group (Suárez et al.,
1996), are mostly cropping out in the north of the
studied area and below the basaltic sequences of the
Meseta Chile Chico (Fig. 3B). Cenozoic sedimentary
formations are mainly exposed to the East of the Cor-
Table 1
K/Ar age data. 1: Rio Murta; 2: Meseta Chile Chico, 3: Meseta Lago Buenos Aires
Lab.
number
Sample
6012-8
6023-9
6298-9
6315-2
6316-3
6046-7
6340-5
PG 01a
PG 06a
PG 102
PG 102
PG 102
PG 37a
PG-138a
FE01-36a
CC317-2a
FE01-23a
CC-313a
PG 44
PG 52
PG 51
PG 65
PG 67
PG 69
PG 72
PG 75
PG 105
PG 108
PG 109
PG 113
PG 114
PG 116
PG 119
PG 120
PG 121
PG 130
PG 132
PG 133
PG 134
PG 143
6063-7
6047-8
5962-7
6022-8
6032-9
6373-4
6037-4
6056-8
6301-3
6297-8
6312-7
6287-7
6288-8
6313-8
6302-4
6278-6
6314-1
6279-7
6280-8
6286-6
6296-7
6317-4
Rock type
Basaltic lava flow
Basalt, pillow lava
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic plug
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic plug
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic dyke
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic neck
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Basaltic lava flow
Teschenite
Analytical method is detailed in the text.
a
Data from Espinoza et al. (2005).
Loc
1
1
1
1
1
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
40
K2O
(%)
Ar*
( 10
1.00
0.65
0.33
0.33
0.33
1.42
0.96
0.99
1.57
1.13
1.33
2.78
1.20
1.77
1.23
0.58
1.14
1.74
0.85
1.26
1.28
1.30
0.85
1.10
1.13
0.97
0.86
2.25
1.96
2.38
2.25
1.76
1.60
40
36
Weight
(g)
Age (Ma)
0.971
1.891
1.0069
1.0045
1.0083
1.7992
1.8001
1.0064
1.0033
0.899
1.033
0.945
1.100
0.742
2.505
1.116
2.754
2.557
1.655
1.702
1.346
1.273
0.918
1.442
1.569
1.722
1.356
1.389
1.624
1.936
2.316
1.0074
1.0012
1.0235
1.0009
1.0038
1.0025
1.0077
1.0112
1.0039
1.0077
1.0011
1.0005
1.0052
1.0016
1.0028
1.0016
1.0013
1.0133
1.0385
1.0108
1.0009
1.0045
0.90 F 0.08
0.85 F 0.10
0.27 F 0.39
0.32 F 0.30
0.21 F 0.27
4.63 F 0.13
4.46 F 0.22
4.4 F 0.8
7.6 F 0.4
7.9 F 0.4
8.2 F 0.5
1.08 F 0.04
10.23 F 0.26
3.44 F 0.10
4.98 F 0.15
6.95 F 0.24
4.32 F 0.23
3.91 F 0.11
4.81 F 0.32
6.53 F 0.25
5.80 F 0.19
5.64 F 0.19
5.84 F 0.21
10.84 F 0.28
9.97 F 0.25
10.71 F 0.29
12.18 F 0.34
1.19 F 0.08
3.44 F 0.11
3.32 F 0.10
3.64 F 0.11
3.89 F 0.14
12.36 F 0.33
Ar*
(%)
Ar
( 10
0.291
0.178
0.029
0.034
10.5
8.0
0.7
1.1
0.844
0.694
1.434
1.921
2.123
1.383
0.168
0.468
0.348
0.426
0.966
3.968
1.962
1.979
1.297
1.589
2.143
1.319
2.659
2.414
2.366
1.603
3.854
3.643
3.360
3.389
0.864
2.177
2.549
2.646
2.210
6.396
42.7
19.9
89
58
59
28
26.8
56.6
41.8
37.9
37.1
17.7
39.6
14.1
26.1
33.2
32.0
28.7
50.7
57.4
44.1
42.3
14.5
35.5
39.2
35.8
27.9
48.4
7
3
cm /g)
9
cm3/g)
352
SSVZ, and their location is controlled by the dextral
transcurrent Liquiñe–Ofqui fault zone (López-Escobar
et al., 1995), which originated in response to the
oblique subduction of the Nazca plate beneath South
America (Cembrano et al., 1996). Previous geochemical data on these volcanoes have been published by
López-Escobar et al. (1993), Demant et al. (1994) and
D’Orazio et al. (2003). The geochemical signatures of
the Cay, Maca and Isla Colorada lavas are typically
calc-alkaline, while the Hudson lavas are comparatively
less enriched in large ion lithophile elements (D’Orazio
et al., 2003).
3. Analytical methods
One hundred samples (12 from Rı́o Murta, 22
from Meseta Chile Chico, 62 from Meseta del Lago
Buenos Aires) were selected on the basis of their
petrographic freshness (macroscopic and microscopic), low Loss on Ignition (LOI) values and geological
Table 2
Major and trace element data for Murta basalts
Sample
PG01
Lat. 8S
Long. 8W
wt.%
SiO2
TiO2
Al2O3
Fe2O3*
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
Mg#
ne,- hy
ppm
Rb
Sr
Ba
Th
Sc
V
Cr
Co
Ni
Y
Zr
Nb
La
Ce
Nd
Sm
Eu
Gd
Dy
Er
Yb
46808V21 46812V58 46812V58 46812V23 46812V23 46812V23 46811V26 46808V41 46808V50 46812V25 46812V25 46812V25
72836V52 72848V23 72848V23 72848V21 72848V21 72848V21 72848V10 72837V01 72837V27 72848V17 72848V17 72848V17
PG04a
PG04b
PG05a
PG05b
PG06a
PG07v
PG101
PG102
PG104a
PG104b
PG104c
49.60
2.09
17.70
11.40
8.72
0.19
5.20
8.00
4.34
1.00
0.51
0.32
99.71
51.53
0.65
47.90
1.22
20.00
8.63
6.60
0.13
7.35
9.50
3.25
0.58
0.20
1.26
100.02
66.50
0.70
48.00
1.65
18.55
10.28
7.86
0.16
7.22
9.80
3.70
0.66
0.28
0.11
100.19
62.07
2.95
48.00
1.70
18.30
10.50
8.03
0.17
7.00
9.62
3.60
0.65
0.29
0.28
100.11
60.84
1.73
48.80
1.18
21.30
7.60
5.81
0.12
5.80
10.40
3.57
0.53
0.20
0.49
99.99
64.01
0.98
48.00
1.57
18.25
10.22
7.82
0.16
7.50
9.85
3.50
0.61
0.28
0.20
100.14
63.10
1.75
48.00
1.36
18.00
9.45
7.23
0.15
7.35
9.75
3.40
0.50
0.23
1.64
99.83
64.45
0.15
48.80
1.52
17.80
9.70
48.80
1.50
17.60
9.90
48.00
1.71
18.00
10.52
48.20
1.36
19.10
9.00
47.60
1.25
19.00
9.08
0.16
7.25
10.00
3.40
0.44
0.25
0.73
100.05
63.53
2.28
0.16
7.52
10.40
3.54
0.42
0.25
0.20
99.89
63.90
1.10
0.17
6.95
9.60
3.40
0.68
0.28
0.50
99.81
60.62
0.37
0.14
7.30
10.10
3.30
0.57
0.24
0.27
99.58
65.40
0.46
0.14
8.10
10.00
3.15
0.51
0.23
0.30
99.36
67.52
0.51
15
480
285
2.2
25
221
56
29
33
38
185
11
22.5
52.5
30
6.9
2.25
6.8
6.35
3.4
3.2
9.7
540
88
0.8
20.8
142
53
36
70
19
106
6.5
9
20
12
3.1
1.16
4.2
3.4
2.1
1.84
12
495
110
1.3
26.2
190
60
38
59
26
146
8.8
11.7
29
18
4.4
1.5
4.9
4.65
2.7
2.4
12
485
118
1.3
28.5
210
64
38
49
29.5
154
9
12
29.5
19
4.7
1.57
5.2
5
3
2.6
9
600
90
0.8
18.7
138
48
30
53
19
101
5.7
8.7
19.5
11.5
3
1.14
4
3.3
2
1.85
11.5
485
116
0.9
27.5
200
65
40
62
26
148
8.5
11.2
26
17.5
4.1
1.48
4.8
4.5
2.7
2.4
12
435
100
0.7
27
175
85
41
72
24.5
140
5.8
9.7
25
15.5
3.9
1.35
4.5
4.2
2.5
2.3
12.8
475
98
0.7
32
210
145
37
74
26.8
145
5
10.2
26
15.5
4.1
1.54
4.8
4.7
2.7
2.5
7
480
165
0.9
34
215
165
36
74
27
150
5.1
10.2
25.5
16.8
4.2
1.54
4.7
4.7
2.7
2.6
11.4
480
115
1
30
230
65
38
51
28.5
162
9
12.7
30
19.5
4.7
1.65
5.5
5.15
2.8
2.7
9.7
522
101
0.95
25
180
68
38
69
23
130
7.4
10.3
24
15
3.8
1.4
4.4
4.1
2.3
2.15
9
530
100
0.7
22
165
57
42
84
21
120
6.7
9.3
21.5
13.5
3.5
1.26
3.9
3.65
2
1.92
Analytical method is detailed in the text.
*Total Fe as Fe2O3; LOI: loss on ignition; Mg# = Mg/(Mg + Fe2+) assuming Fe2O3/FeO = 0.15; ne, -hy = wt.% normative nepheline or wt.%
normative hypersthene (minus sign).
position from a set of ca. 150 samples collected
during fieldwork investigations in 1998, 2001 and
2002.
Mineral analyses (available on request to the authors)
were obtained using a five spectrometer Cameca SX-50
electron microprobe (Microsonde Ouest, Brest, France).
Analytical conditions were 10–12 nA, 15 kV, counting
time 6 s. A detailed account of the procedure is given in
Defant et al. (1991). Whole rock 40K–40Ar datings of
Meseta del Lago Buenos Aires and Rı́o Murta basalts
(Table 1) were performed on the 0.5- to 0.15-mm-size
fraction after crushing, sieving and cleaning with distilled water of whole-rock samples. Analyses were carried out at the Laboratoire de Géochronologie,
Université de Bretagne Occidentale (Brest, France).
One aliquot of sample was powdered for K analysis
by atomic absorption after HF chemical attack and
0.5–0.15-mm grains were used for argon isotopic analyses. Argon extraction was performed by the direct
technique under high vacuum (10 5–10 7 hPa) using
induction heating of a molybdenum crucible. The argon
content was measured by isotope dilution and argon
isotopes were analysed in a 1808 stainless steel mass
spectrometer, according the original procedure described by Bellon et al. (1981). Age calculations, following the equation of Mahood and Drake (1982) and
using the Steiger and Jäger’s (1977) recommended constants, are given, with 1r error, in Table 1. The K–Ar
ages on Meseta de Chile Chico basalts discussed in this
paper are taken from Espinoza et al. (2005).
Major and trace-element analyses (Tables 2 and 3)
were conducted on agate-ground powders by inductively coupled plasma–atomic emission spectroscopy (ICPAES) except Rb which was determined with flame
atomic emission spectroscopy, at the Université de
Bretagne Occidentale (Brest, France) and checked
against IWG-GIT standards BE-N, AC-E, PM-S and
WS-E. Relative standard deviation is ca. 1% for SiO2
and 2% for the other major elements except for low
values (b 0.50% oxide) for which the absolute standard
deviation is 0.01%. For trace elements, relative standard
deviation is ca. 5% except for concentrations below six
times the detection limit, for which the absolute standard deviation is about one third of the detection limit.
Detection limits are 2 ppm for Ba, V, Cr, Co, Ni, Zr and
Ce; 1 ppm for Nd, Gd and Er; 0.5 ppm for Rb, Sr, Nb,
La and Sm; 0.3 ppm for Y, Dy and Th; 0.15 ppm for Sc,
Eu and Yb. Specific details for the analytical methods
and sample preparation can be found in Cotten et al.
(1995). The major and trace-element data on Meseta
Chile Chico basalts discussed in this paper are taken
from Espinoza et al. (2005).
353
Sr and Nd isotopic data were measured on a single
batch of HCl 2N leached whole rock powder; 70 mg of
powder were dissolved in a HNO3–HF mixture from
which Sr and Nd were eluted. Nd was run on a double
Re filament and Sr on a W filament with Ta activator.
Measurements were performed on a Finnigan MAT 261
and Thermo Triton T1 mass spectrometers (Université de
Bretagne Occidentale, Brest). Sr and Nd isotopic ratios
were calculated at t = 0 using the 40K–40Ar determined
ages. Nd initial ratios are expressed as qNd. The errors on
87
Sr/86Sr and 143Nd/144Nd ratios are reported in Table 4.
4. Field data and K–Ar geochronometry
4.1. Rı́o Murta basalts (Chile)
These basalts, previously investigated by Demant et
al. (1994, 1998) and Corgne et al. (2001), crop out in
the bottom of the glacial valley of Rı́o Murta, dug into
the North Patagonian Batholith granitoids some 30 km
SSE of Hudson volcano (Fig. 3A). Their total preserved
volume is small (b1 km3, Demant et al., 1998). They
occur either as columnar jointed basaltic flows in the
Rı́o Murta river bed, eroded down to a few metres by
the stream, or as subglacial and sublacustrine volcanics.
These include pillow lavas and lava tubes up to 3 m in
diameter with glassy chilled margins, as well as hyaloclastic breccias associated with varved clays, moraines
and tills. On the basis of field evidence for local subglacial emplacement followed by moderate erosion,
former authors have considered them as Holocene.
However, two out of three K–Ar dates given in Table
1 point out to an emplacement at ca. 0.85–0.9 Ma,
while the third one (b0.5 Ma) obtained on the uppermost part of a flow occupying the main Rı́o Murta
stream bed might be consistent with an Holocene age.
4.2. Meseta Chile Chico (MCC, Chile)
The MCC flood basalt cover (Fig. 3B), up to 900–
1000 m thick, is composed of two sequences (Espinoza
et al., 2005). The basal Lower Basaltic Sequence
(LBS), which unconformably overlies the Mesozoic
Ibáñez Group and the Neocomian Cerro Colorado Formation, is formed by a 500–550-m-thick pile of Eocene
basaltic lava flows (57–40 Ma, Charrier et al., 1979;
Baker et al., 1981; Petford et al., 1996; Espinoza et al.,
2005) crosscut by several basanitic necks and diatrems.
These Eocene basalts can be correlated with the 57–45
Ma Posadas Basalt (Baker et al., 1981; Kay et al., 2002)
and with the 42 F 6 Ma Balmaceda Basalts (Baker et
al., 1981; Demant et al., 1996), which crop out east and
354
Table 3
Selected major and trace element data for Meseta del Lago Buenos Aires volcanic rocks
PG143
PG65
PG67
PG75
PG105
PG113
PG114
PG132
PG133
PG51
PG52
PG69
PG72
PG108
Age
Lat. 8S
Long. 8W
Type
wt.%
SiO2
TiO2
Al2O3
Fe2O3*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
Mg#
ne,-hy
ppm
Rb
Sr
Ba
Th
Sc
V
Cr
Co
Ni
Y
Zr
Nb
La
Ce
Nd
Sm
Eu
Gd
Dy
Er
Yb
12.4
46838V41
71828V08
MP-inter
4.98
47804V
71848V
MP-inter
6.95
47804V
71848V
MP-inter
4.81
47803.602
71849.141
MP-inter
6.53
46843V33
71842V06
MP-inter
5.84
46846V47
71842V07
MP-inter
10.84
46846V33.6
71842V47,9VV
MP-inter
3.32
47809V28
71833’25,6VV
MP-inter
3.64
47809V30
71833V19
MP-inter
3.45
47810V14
71832V22
MP-alk
10.23
47820V15
70848V18
MP-alk
4.32
47804V
71848V
MP-alk
3.91
47804.08
71847.995
MP-alk
5.8
294948 UTM
4820953 UTM
MP-alk
48.50
1.76
16.55
10.30
0.15
6.42
8.45
3.97
1.60
0.43
1.41
99.54
59.23
3.68
47.00
1.48
15.85
11.55
0.17
8.37
9.65
2.80
1.21
0.40
0.75
99.23
62.81
0.74
47.35
1.43
15.70
11.42
0.18
9.40
9.60
3.28
0.61
0.38
0.11
99.24
65.73
1.66
50.00
1.54
17.20
10.66
0.17
5.66
9.10
3.42
0.89
0.29
0.75
99.68
55.30
10.49
47.90
1.64
16.25
10.34
0.16
8.54
8.65
3.70
1.48
0.66
0.28
99.60
65.81
3.78
48.30
1.61
16.70
11.60
0.17
7.15
9.60
3.38
1.03
0.44
0.27
99.72
58.96
1.09
47.15
1.25
15.60
10.36
0.18
10.40
10.00
2.47
1.28
0.41
0.57
99.67
70.06
0.77
55.00
1.40
18.40
8.65
0.19
2.07
5.30
5.85
2.60
0.91
0.26
100.11
35.80
1.13
51.80
1.68
16.50
13.10
0.23
2.63
5.75
4.58
2.33
1.28
0.52
99.36
31.87
10.39
48.80
2.22
16.00
10.80
0.16
7.03
8.20
4.14
1.80
0.68
0.50
99.33
60.27
4.40
48.70
2.44
15.82
12.91
0.18
4.92
8.60
3.56
1.16
0.54
0.39
99.22
47.04
6.71
47.75
2.16
17.60
10.77
0.16
6.28
9.05
3.68
1.41
0.46
0.43
99.75
57.61
3.17
47.80
2.63
17.75
11.92
0.17
4.78
8.00
3.76
1.66
0.53
0.64
99.64
48.31
1.40
47.50
2.06
16.10
11.90
0.17
7.12
9.85
3.54
1.42
0.54
0.23
99.97
58.24
4.61
40.5
685
345
4.95
23
200
167
35
68
23
164
20
28
56
29
6.3
1.86
6.2
4.4
2.2
1.9
27.5
725
310
3.5
29
215
286
43
162
26
157
11.6
24.5
55
34
6.85
1.95
5.7
4.6
2.3
2.18
22
708
287
3.6
29
213
320
45
182
23.7
149
11.4
24
53
30
6.3
1.81
5.45
4.35
2.2
2.12
17.5
444
190
2.5
26
198
150
35
59
25
141
12
14.5
34
18
4.4
1.42
4.3
4.35
2.25
2.12
34.5
870
455
5.2
22
190
220
39
180
25
210
24.5
38.5
75
36.5
7
2.07
5.8
4.7
2.4
2.1
21.5
555
260
2.5
28
225
181
41
95
25
149
15.3
21
45
24
5.6
1.76
5
4.5
2.3
2.16
39.5
700
340
4
30
240
485
45
212
23
124
11
22
47.5
26
5.5
1.64
4.7
4
2
2
46.5
718
965
5.8
10
46
2
11
1
33
294
45
55
109
47
9.2
3.2
7.7
6.2
3
2.76
41
534
600
6.7
12
54
2
20
2
44
500
53
61
124
52
11.7
3.28
9.9
8
4
3.8
31.5
742
470
3.25
19
174
200
38
115
30
240
37
37
74
36
7.5
2.26
6.9
5.5
2.7
2.6
20.5
595
440
2.8
23.5
240
28
42
32
30
205
29
26
53
32
7
2.22
7.2
5.4
2.8
2.3
23.5
688
315
3.05
22
210
42
38
50
24.5
205
31
28.5
60
28
6.1
1.92
5.9
4.6
2.1
1.95
27
682
415
3.35
21
240
12
36
26
29.5
232
36.5
31
68
33
7
2.15
6.8
5.45
2.6
2.32
23.5
682
320
3
26
235
185
43
107
245
180
30
28
57
30.5
6.7
2.07
6
4.8
2.2
1.94
Analytical methods detailed in the text. MP: main plateau; PP: post-plateau; Alk.: alkali; Int.: intermediate. K–Ar ages in Ma.
*Total Fe as Fe2O3; LOI: Loss on ignition; Mg# = Mg/(Mg+Fe2+) assuming Fe2O3/FeO = 0,15; ne,- hy = wt.% normative nepheline or wt.% normative hypersthene (minus sign).
Sample
PG109
PG116
PG119
PG120
PG130
PG134
PG44
PG121
PG41
PG46
PG50
PG123
PG126
PG127
Age
Lat. 8S
Long. 8W
Type
5.64
294962 UTM
4820912 UTM
MP-alk
9.97
47804V29,2VV
71801V21,1VV
MP-alk
10.71
47806V08
70859V59,6VV
MP-alk
12.18
47806V13,6VV
70859V34,5VV
MP-alk
3.44
47809V32
71833V15,3VV
MP-alk
3.89
47810V27
71832V29,4VV
MP-alk
1.08
46852V24
70844V08
PP-alk
1.19
47806V28
46841V24 47803V35 47807V42 47803V05
47803V13 47803V35
70859V16,5VV 70849V48 70846V54 70851V58 71802V55,4VV 71802V50,6 71801V44,4VV
PP-alk
PP-alk
PP-alk
PP-alk
PP-alk
PP-alk
PP-alk
SiO2
TiO2
Al2O3
Fe2O3*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
Mg#
ne,-hy
47.00
2.05
16.00
11.95
0.17
7.45
9.60
3.48
1.35
0.50
0.13
99.68
59.23
4.43
47.10
2.36
16.20
11.95
0.16
7.03
9.55
3.54
1.14
0.44
0.54
100.01
57.82
3.47
47.20
2.38
16.25
12.04
0.16
7.40
9.32
3.80
1.17
0.43
0.59
99.56
58.89
5.02
47.20
2.34
16.10
12.00
0.17
7.40
9.90
3.63
1.10
0.42
0.26
100.00
58.97
4.79
49.00
2.71
17.00
13.20
0.20
3.85
6.85
4.35
2.04
0.86
0.67
99.39
40.47
1.59
48.50
2.31
16.75
10.75
0.16
7.01
7.72
4.16
1.85
0.62
0.06
99.89
60.31
4.50
45.90
2.29
14.50
10.80
0.17
8.10
8.15
5.00
2.90
1.50
0.29
99.60
63.61
16.26
46.85
2.56
17.25
10.75
0.16
5.37
9.70
4.04
2.22
0.74
0.19
99.45
53.79
9.67
43.50
2.64
13.15
12.08
0.18
10.30
10.60
3.50
2.31
1.05
0.05
99.26
66.52
14.71
44.50
2.34
14.35
12.15
0.18
10.14
9.40
3.55
1.90
0.69
0.08
99.28
66.04
11.22
46.00
2.45
14.40
11.50
0.17
8.80
9.42
3.65
2.28
0.86
0.26
99.27
64.07
9.67
47.80
2.03
16.65
10.40
0.15
7.05
7.42
4.65
2.13
0.79
0.51
99.58
61.24
9.02
47.60
2.43
17.45
11.76
0.17
4.54
9.55
3.67
1.27
0.41
1.14
99.99
47.36
2.79
48.10
2.42
16.85
11.35
0.15
5.87
6.70
5.19
2.33
0.83
0.27
99.52
54.65
10.55
Rb
Sr
Ba
Th
Sc
V
Cr
Co
Ni
Y
Zr
Nb
La
Ce
Nd
Sm
Eu
Gd
Dy
Er
Yb
22
685
285
3
25
225
190
42
124
24
185
29
27.5
54
29.5
6.55
2.05
5.6
4.7
2.1
1.88
14.3
600
235
1.75
23
232
205
43
75
23.5
159
24
19
39
22.5
5.5
1.82
5.1
4.3
2.1
1.75
13.8
590
215
1.8
24
235
220
44
86
22.5
152
24
18.4
40
22.5
5.25
1.8
5.2
4.35
2
1.74
12.9
582
220
1.6
25
255
240
48
91
22.5
158
23.5
17.5
38
22
5.2
1.78
5.1
4.2
2
1.69
36
578
480
4.6
20
175
2
27
6
37.5
395
48
47
94
45.5
10
2.92
9
7.15
3.5
3.25
33
761
442
3.7
19
165
175
37
114
29
252
38
36
73
34
7.3
2.25
6.7
5.6
2.7
2.46
46.5
1290
715
9.4
14.5
153
225
42
190
28
420
98
90
153
63
11
3.2
8.25
5.6
2.6
1.8
40
855
560
6.1
25
240
83
32
42
27.5
267
61
47
89
40.5
8.15
2.41
7
5.25
2.4
2.04
37
1020
755
7.4
28
265
345
52
215
27
300
76
67
117
56
9.4
2.91
7.4
5.35
2.5
1.87
33
790
525
5
22.5
225
270
51
215
22.5
225
62
45
85
39.5
7.5
2.23
6.4
4.6
2.2
1.6
43
870
670
6.15
24
240
240
49
170
25
275
67
54
97
46
8.5
2.45
6.8
5.05
2.25
1.8
31.5
810
390
3.3
17
150
140
37
134
25
252
38
32
65
32
6.8
2.14
5.9
4.75
2.2
1.97
15.2
615
250
2.15
25
200
100
42
44
25.5
167
21
18.3
40
22.5
5.15
1.83
5.8
4.8
2.2
2.09
35
885
460
4.8
13.3
130
98
35
87
25
268
60
42
81
38.5
7.9
2.53
6.9
5.15
2
1.81
Sample
355
356
Table 4
Sr and Nd isotopic data
Sample
Loc.
SiO2
PG102
PG01a
PG06a
IBA47a
FE01-36b
PG65
PG75
PG105
PG113
PG114
PG108
PG109
PG116
1
1
1
1
2
3
3
3
3
3
3
3
3
48.80
49.60
48.00
46.75
46.01
47.00
50.00
47.90
48.30
47.15
47.50
47.00
47.10
Age
0.27
0.90
0.85
4.40
4.98
4.81
6.53
5.84
10.84
5.80
5.64
9.97
Mg#
TiO2
La/Nb
(87Sr/86Sr)0
63.90
51.53
63.10
65.00
1.50
2.09
1.57
1.51
1.42
1.48
1.54
1.64
1.61
1.25
2.06
2.05
2.36
2.00
2.05
1.32
0.703532
0.703958
0.703460
0.703590
0.704140
0.704109
0.704260
0.704364
0.704362
0.704215
0.704367
0.704394
0.703996
62.81
55.30
65.81
58.96
70.06
58.24
59.23
57.82
2.03
2.11
1.21
1.57
1.37
2.00
0.93
0.95
0.79
2s
4
3
5
11
10
3
4
6
5
5
7
(143Nd/144Nd)
2r
qNd
0.512920
0.512792
0.512916
0.512900
0.512873
0.512910
0.512751
0.512795
0.512730
0.512821
0.512736
0.512724
0.512799
10
8
7
5.50
3.00
5.42
5.11
4.58
5.31
2.20
3.06
1.79
3.57
1.91
1.68
3.14
10
11
10
9
7
8
7
9
Analytical methods are detailed in the text. International standards (NBS 987, La Jolla) are run regularly on both mass spectrometers. Typical values
are (1) Triton T1 87Sr/86Sr = 0.710250 F 12, 143Nd/144Nd = 0.511850 F 6 and (2) 87Sr/86Sr = 0.710251 F16 143Nd/144Nd = 0.512104 F 6 (JNdi).
Average blanks for this study are 0.2 for Nd and 0.4 ng for Sr.
Loc.: location in Fig. 1 (1 = Rı́o Murta; 2 = Meseta Chile Chico; 3 = Meseta del Lago Buenos Aires).
Ages from Table 1; geochemical values from Tables 2 and 3.
a
Data from Demant et al. (1998).
b
Data from Espinoza et al. (2005).
north of the MCC area, respectively. The rather flat
upper surface of the MCC, which covers ca. 300 km2,
corresponds to the 400-m-thick Upper Basaltic Sequence (UBS) of Miocene–Pliocene tabular basaltic
lava flows and necks which provided K–Ar ages of
8.2, 7.9, 7.6, 4.6, 4.5 and 4.4 Ma (Espinoza et al.,
2005). They are underlain by two rhyolitic flows
dated at 13.1 and 9.8 Ma, respectively. On the basis
of their age range and field features, they can be
considered equivalent to the main plateau basaltic sequence (Gorring et al., 2003) of the Meseta del Lago
Buenos Aires (MLBA) documented below. Very young
cones and associated flows do not occur on the MCC,
which therefore has apparently not undergone any magmatic event equivalent to the post-plateau volcanic
phases of MLBA and other Patagonian plateaus.
4.3. Meseta del Lago Buenos Aires (MLBA, Argentina)
This Meseta (Fig. 3C) is one of the largest (ca. 6000
km2) basaltic plateaus in the Patagonian back-arc domain. Its main plateau sequence (Gorring et al., 1997,
2003) is composed of an up to 300-m-thick pile of
tabular basaltic lava flows overlying the Miocene
molasse sediments of the Rı́o Zeballos Group (Santa
Cruz Formation). Previous K–Ar and 40Ar/39Ar ages of
these main plateau basalts (Sinito, 1980; Baker et al.,
1981; Mercer and Sutter, 1982; Ton-That et al., 1999)
range from 10 to 4.5 Ma and more recently Brown et al.
(2004) obtained three 40Ar/39Ar isochron ages of 10.12,
7.86 and 7.71 Ma on lavas from this sequence. Some of
these flows are interbedded with glacial tills (Ton-That
et al., 1999). The MLBA post-plateau lavas have been
studied in detail by Gorring et al. (2003) and dated by
Brown et al. (2004) and Singer et al. (2004). They
erupted from more than 150 small monogenetic volcanic centres (strombolian and spatter cones and very
well-preserved maars). The flows form a volcanic pile
usually ca. 100 m thick topping the MLBA, and many
of them poured down its eastern slopes. Brown et al.
(2004) published 31 isochron 40Ar/39Ar ages ranging
from 3.3 to less than 0.1 Ma for these post-plateau
MLBA basalts. Different volcanic pulses have been
recognised by these authors at 3.2–3.0 Ma, 2.4 Ma, 1.7
Ma, 1.35 Ma, 1.0 Ma, 750 ka, 430–330 ka, and finally,
b110 ka. Other 40Ar/39Ar and K–Ar ages obtained by
Singer et al. (2004) from lavas interbedded with moraine
deposits range from 1016 F 10 ka to 109 F 3 ka.
We have measured 22 new K–Ar ages (Table 1 and
Fig. 3C) on MLBA basalts: 2 on the post-plateau lavas
(1.19 F 0.08 Ma and 1.08 F 0.04 Ma) and 20 on the
main plateau sequence. These latter ages allow to extend the known range of the main plateau activity from
12.18 F 0.34 Ma (PG 120) to 3.32 F 0.10 Ma (PG 132)
and to identify a quiescence period between ca. 10 and
7 Ma.
The oldest volcanic pile crops out, as noticed by
previous authors, along the southeastern border of the
plateau, and especially along the trail from Estancia La
Vizcaina to Laguna del Sello. Three samples from
tabular lava flows from this cross section gave ages
of 12.18 F 0.34 Ma (PG-120, 1000 m), 10.71 F 0.29
Ma (PG-119, 1050 m) and finally 9.97F0.25 Ma (PG116, 1170 m) from bottom to top, respectively. This
sequence is unconformably overlain by younger lava
flows (sample PG-121, 940 m, 1.19 F 0.08 Ma old) of
the post-plateau sequence (Fig. 3C) which poured out
down its slopes towards the plain near Estancias La
Vizcaina and Casa de Piedra.
A good cross section of the top of the main plateau
sequence is exposed in the southern border of the
Meseta along the Hacienda El Ghio horse-trail near
Rı́o Torrentoso. Four samples of tabular flows from
this cross section provided K–Ar ages ranging from
3.89 F 0.14 Ma (sample PG-134, 980 m) to 3.32 F 0.10
Ma (sample PG-132, 1455 m). A 40-m-thick glacial till
unit is interbedded between this latter sample and two
slightly older flows dated at 3.44 F 0.11 Ma and
3.64 F 0.11 Ma, respectively. These data suggest that
the main plateau stage of the MLGA volcanism ended
at 3.3 Ma, and thus, that there was no significant time
gap separating it from the post-plateau stage which
started at 3.3 Ma according to Brown et al. (2004).
In addition, they provide a rather precise dating for one
of the Pliocene glacial events already documented in
the area by Ton-That et al. (1999).
On the western border of the Meseta, near Estancia
Los Corrales located 38 km south of Los Antiguos
357
along the Paso Roballos road, the lowest lava flows of
the main plateau sequence gave ages of 5.84 F 0.21
Ma (PG 113), 5.80 F 0.19 Ma (PG 108) and
5.64 F 0.19 Ma (PG 109), respectively. These flows
overlie the Rı́o Zeballos Group Miocene molasse
which is crosscut by an older basaltic neck dated at
10.84 F 0.28 Ma (PG 114).
Finally, a few mafic rocks cropping out away from
MLBA also provided K–Ar ages consistent with those
of the main plateau building stage. One of them, a
columnar jointed hypovolcanic intrusion locally referred to as a btescheniteQ body, located north of the
MLBA near Estancia Las Chicas, has been dated back
to 12.40 F 0.33 Ma (sample PG 143) and a basaltic lava
flow 5 km east of Bajo Caracoles, near the SE edge of
the MLBA, at 10.23 F 0.26 Ma (sample PG 52).
5. Petrologic and geochemical data
5.1. Sample classification
A large majority of the studied rocks are petrographically fresh and their Loss On Ignition (LOI) values
range from slightly negative to ca. 1 wt.%. According
to the TAS diagram shown in Fig. 4, they are mostly
basaltic (basalts, basanites and trachybasalts) although a
Fig. 4. Total alkali–silica classification diagram (Le Maitre et al., 1989) for the Miocene to Late Quaternary igneous rocks recalculated to 100 wt. %,
anhydrous basis. The heavy line represents the boundary between alkaline and subalkaline series (Irvine and Baragar, 1971). Open diamonds: Rı́o
Murta basalts. Open triangles: intermediate main plateau lavas from Meseta Chile Chico (MCC). Black triangles: alkaline main plateau lavas from
MCC; Open squares: intermediate main plateau lavas from Meseta del Lago Buenos Aires (MLBA). Black squares: alkaline main plateau lavas
from MLBA. Black diamonds: post-plateau lavas from MLBA. Data on Meseta Chile Chico (MCC) intermediate and primitive lavas are from
Espinoza et al. (2005).
358
few samples plot in the fields of basaltic trachyandesite
and trachyandesite. We have classified MLBA samples
from our data set according to their main plateau or
post-plateau position, based on their field relationships
and K–Ar ages. For the latter, we have postulated that
the transition between the main plateau and post-plateau stages occurred abruptly at 3.3 Ma (Brown et al.,
2004 and our data as discussed above). As discussed
above, no post-plateau activity can be identified in
MCC, where all the Mio-Pliocene lavas crop out either
as a tabular flow pile or as necks and dykes crosscutting
them (Espinoza et al., 2005).
A further discrimination has been operated within
our data set. Indeed, basaltic lavas from both the
MLBA and the MCC display chemical features very
similar to those considered typical of Ocean Island
Basalts (OIB), as already shown by previous authors
(Hawkesworth et al., 1979; Baker et al., 1981; Ramos
and Kay, 1992). However, Stern et al. (1990), Gorring
et al. (1997, 2003), Gorring and Kay (2001) and Espinoza et al. (2005) have shown that some Patagonian
main plateau and post-plateau basalts display weak to
moderate bsubduction-relatedQ geochemical imprints
traduced by relative depletions in High Field Strength
Elements (HFSE) vs. Large Ion Lithophile Elements
(LILE) and Light Rare-Earth Elements (LREE), and
sometimes as well by specific isotopic signatures.
These features are clearly observed in our MLBA and
MCC data set, in which about 30% of the samples
depart from the usual compositional range of OIB by
displaying La/Nb ratios greater than unity (up to 3.7)
and TiO2 contents usually lower than 2 wt.% (Fig. 5A
and B). Their relative depletion in Nb is unlikely to
result from fractionation of Ti-magnetite during differentiation because their Nb contents increase with decreasing Mg numbers (Mg#, Fig. 5C). Consequently,
we have identified by specific symbols in all the geochemical diagrams the MLBA and MCC samples characterised by La/Nb N 1. Stern et al. (1990) termed these
btransitionalQ basalts, but we will rather use this word to
describe the Rı́o Murta basalts which plot in between
the fields of alkalic and subalkalic basalts in most
geochemical diagrams. Therefore, the btransitionalQ
basalts of Stern et al. (1990) will be referred to here
as bintermediateQ samples (i.e., intermediate between
OIB and subduction-related lavas), as opposed to the
other samples, termed bcratonicQ basalts by Stern et al.
(1990) and which display typical OIB characteristics
(including La/Nb b 1). The petrogenesis of the
bintermediateQ lavas will be discussed separately below.
Fig. 5. Selected plots of major (recalculated to 100 wt.%, anhydrous basis) elements, major element parameters, and trace elements (in ppm). (A)
TiO2–Mg#; (B) TiO2–La/Nb; (C) Nb–Mg#; (D) normative nepheline–Mg#. Symbols as in Fig. 4.
5.2. Major elements and petrographic types
Most of the studied basalts are silica-undersaturated
and contain up to 22 wt.% normative nepheline (Fig.
5D), while a minority (including 8 over 12 Rı́o Murta
samples) is hypersthene-normative. In the TAS diagram, the MLBA and MCC samples plot consistently
above Irvine and Baragar’s (1971) limit between alkalic and subalkalic compositions while Murta samples
spread around this limit. The Murta basalts also fit
Middlemost’s (1975) requirements for the definition of
transitional basalts, i.e., they plot in the subalkalic
field in the K2O–SiO2 diagram and in the alkalic
field in the Na2O–SiO2 diagram. They will thus be
termed transitional. All MCC Mio-Pliocene mafic
lavas are alkali basalts, trachybasalts or basaltic trachyandesites (Espinoza et al., 2005; Figs. 4 and 5)
whether or not they display bintermediateQ La/Nb
ratios (N 1). MLBA post-plateau basalts contain usually 8–22 wt.% normative nepheline (Fig. 5D) and plot
in the fields of basanites and tephrites in the TAS
diagram (Fig. 4). MLBA main plateau basalts, intermediate or not, are merely alkali basalts, the CIPW
norms of which contain either small amounts of nepheline (less than 5%) or of hypersthene (Fig. 5D and
Table 3). The TAS diagram of Fig. 4 also shows that
the MLBA-MCC alkali basaltic/basanitic samples displaying La/Nb b 1 and TiO2 N 2 wt.% plot consistently
in the basalt/basanite/trachybasalt fields while the intermediate samples are basalts, trachybasalts or basaltic trachyandesites. The basalts range from primitive
(Mg# N 65, MgO N 8 wt.%) to evolved (Fig. 5A, C, D)
and the patterns of major element variations vs. Mg#
(not shown) suggest the occurrence of fractionation
effects involving olivine, clinopyroxene, plagioclase
and titanomagnetite. Normative nepheline contents
tend to decrease with Mg# (Fig. 5D), a pattern often
observed in alkali basalt series.
In short, the distribution of petrographic types within
the studied sample set is relatively simple: Rı́o Murta
lavas are transitional basalts while all the other ones are
of dominant alkali affinity. Among the latter, two
groups can be distinguished: a genuine alkali one
with typical OIB geochemical signatures (La/Nb b 1
and TiO2 N 2 wt.%), which will be referred to as the
balkali groupQ and a second one (the bintermediate
groupQ) displaying incompatible element signatures intermediate between those of OIB and arc lavas (La/
Nb N 1 and TiO2 b 2 wt.%). The alkali group includes
the post-plateau MLBA lavas, all of which are strongly
silica-undersaturated (basanites and tephrites), together
with alkali basalts and trachybasalts from the main
359
plateau MLBA stage and from MCC. The intermediate
group is only represented in MLBA (main plateau) and
MCC by alkali basalts, trachybasalts, basaltic trachyandesites and trachyandesites. It is noteworthy that main
plateau lavas from both Meseta Chile Chico and Lago
Buenos Aires are silica-undersaturated (alkali basalts
and basanites), while most of other Neogene Patagonian main plateaus are principally made up of tholeiitic
basalts (Gorring and Kay, 2001).
5.3. Mineral chemistry
The Murta basalts are porphyritic, with plagioclase
(An73–48Ab26–50Or1–2), olivine (Fo87–72) and minor augite (Wo46En42Fs12) phenocrysts set in a microcrystalline groundmass of plagioclase (An68Ab31Or1), olivine
(Fo82–73), Ti-magnetite and acicular quenched Ti-rich
augite (Wo50En25Fs25). Megacrysts of plagioclase (up
to 2 cm in size) and clinopyroxene are frequent in some
samples.
The Mio-Pliocene MCC basaltic lava flows, plugs
and dykes (Espinoza et al., 2005) bear low amounts
of olivine (Fo83–65) and minor clinopyroxene (Wo45
En40–46Fs8–15) phenocrysts set into a microlitic to
subdoleritic groundmass containing olivine (Fo75–54),
augite (Wo49–43En44–37Fs13–14), plagioclase (An68–66)
and Ti-magnetite.
The MLBA main plateau basalts contain plagioclase
(An55–70), clinopyroxene (Wo49En42Fs9) and olivine
(Fo86–62) phenocrysts set into a microlitic groundmass
containing plagioclase (An66), olivine (Fo81–60), rare
clinopyroxene, Ti-magnetite and glass. The MLBA
post-plateau basalts are very fresh vesicular aphyric to
moderately porphyritic lava flows, with occasional plagioclase (An66–72), clinopyroxene (Wo52En38Fs10) and
olivine (Fo83–73) phenocrysts. Their glassy to microlitic
groundmass contains plagioclase, olivine (Fo69), clinopyroxene (Wo51En40Fs9), Ti-magnetite and glass.
Corroded quartz xenocrysts, always rimmed by small
clinopyroxene aggregates, are often present in basaltic
lava flows from MCC and MLBA, indicating that they
experienced some extent of crustal contamination.
The compositions of calcic clinopyroxene phenocrysts from MCC and MLBA are typical of those
from alkaline intraplate basalts. They are Ti- and Alrich and plot consistently within the alkalic fields in
Leterrier et al.’s (1982) diagrams (not shown).
5.4. Trace-element features
Compatible element contents of the Murta basalts
are consistently low (Table 2). Those of MLBA lavas
360
(Table 3), as well as those from MCC basalts, range
from concentrations close to those of near-primitive
basaltic magmas (Co = 45–55 ppm, Ni = 200–220 ppm,
Cr = 300–450 ppm) to very low ones. They decrease
rather abruptly with Mg# (diagrams not shown), a
feature consistent with the occurrence of olivine and
clinopyroxene fractionation effects. Incompatible traceelement abundance patterns normalised to the compo-
sition of Primitive Mantle (Sun and McDonough, 1989)
are shown in Fig. 6.
Patterns of selected Murta transitional basalts have
been plotted in Fig. 6a together with those of representative basaltic samples from the Chile Ridge (Klein and
Karsten, 1995) and from Hudson volcano (López-Escobar et al., 1993). Murta basalts display slightly LILEand LREE-enriched patterns (average primitive mantle-
Fig. 6. Primitive mantle normalised (Sun and McDonough, 1989) incompatible multi-element patterns. (A) Rı́o Murta transitional basalts, Chile
Ridge (bold patterns, samples D34-1 and D42-4; Klein and Karsten, 1995) and Hudson volcano basalts (dashed patterns, samples Hud-1 and Hud-3;
López-Escobar et al., 1993) are shown for comparison; (B) intermediate MCC lavas (analyses in Espinoza et al., 2005); (C) intermediate MLBA
main plateau lavas; (D) alkaline MCC lavas (analyses in Espinoza et al., 2005); (E) alkaline main plateau MLBA lavas; (F) alkaline post-plateau
MLBA lavas.
normalised (La/Sm)N = 1.75) and are slightly but significantly depleted in Nb relative to K and La. These
features are very similar to those of Chile Ridge segment 3 sample D42-4 (Klein and Karsten, 1995) displaying subduction-related geochemical affinities.
Sample PG01a is slightly richer in all trace elements
but Sr than other less differentiated Murta samples.
However, its incompatible trace-element pattern is parallel to the others, a feature consistent with olivine and
plagioclase fractionation.
Incompatible trace-element patterns of intermediate
main plateau basalts from MCC and MLBA are shown
is Fig. 6B and C, respectively. Both groups of intermediate basalts share the same trace-element characteristics. They display LILE- and LREE-enriched patterns
(average primitive-mantle normalised (La/Sm)N = 2.72
for MCC basalts and (La/Sm)N=3.00 for MLBA
basalts) and are noticeably but variably depleted in
Nb. This depletion in Nb seems to be attenuated with
increasing differentiation, as Nb contents in intermediate basalts increase when Mg# decreases (Fig. 5C). This
feature implies that the more or less pronounced Nb
depletion of the intermediate basalts is unlikely to be
361
related to differentiation processes (e.g., fractionation of
Ti-magnetite) but is pristine and linked to the nature of
their source. It is worth to note that the range of La/Nb
ratios found in intermediate main plateau basalts (MCC
and MLBA) is far greater (Fig. 5B) than the one (La/
Nb* b 1.3; Nb* = 17 Ta) found by Gorring et al.
(2003) in some post-plateau alkali basalts.
Main plateau alkali basalts from both MCC and
MLBA show trace-element patterns typical of OIB
(Fig. 6D and E). They display levels of LILE and
LREE enrichment similar to those of intermediate
basalts (average (La/Sm)N ratios of 3.09 and 2.90,
respectively) but lack the negative Nb anomalies typical
of the latter.
Post-plateau alkali basalts from MLBA display
very homogeneous trace element compositions. Like
the MLBA main plateau alkali basalts, they show
OIB-like trace element patterns (Fig. 6F) with an
average (La/Sm)N = 3.93, slightly higher than that of
the main plateau lavas. They display a wide range of
LREE concentrations ((La/Yb)N from 6.3 to 37) with
almost constant HREE contents (YbN ranging from
3.25 to 4.26).
Fig. 7. (A) Plot of (87Sr/86Sr)o against qNd for the studied lavas. White diamonds: basalts (b1 Ma) from Rı́o Murta (grey diamond, data from
Demant et al., 1998); white squares: intermediate basalts from MLBA; black squares: alkali basalts from MLBA. (B) Plot of (87Sr/86Sr)o against
qNd for the studied lavas and isotopic ratios from other Patagonian magmatic rocks: fields of the alkaline post-plateau (b1 Ma) basalts from Meseta
Buenos Aires (Gorring et al., 2003) and the main plateau sequences from Patagonian Basaltic Field as defined by Gorring and Kay (2001), the Cerro
Pampa adakites from Kay et al. (1993); Pacific MORB from Peate et al. (1997); black circles: basaltic andesites from the Chile Trench Taitao Ridge
(CTR) (Guivel et al., 2003); and white circles: heterogeneous South Chile Ridge (SCR) basalts (Klein and Karsten, 1995). For geochemical
modelling, two mixing curves have been calculated (ticks every 10%). [A]: mixing model between mantle source similar to that of the alkali basalts
and adakitic melt derived from slightly altered oceanic crust (black star); [B]: mixing model between similar mantle source than in model [A] and
slightly altered mid-ocean ridge basalt (black star); (see Table 5 for model parameters and see text for details).
362
5.5. Sr and Nd isotopic data
The isotopic compositions of the studied Mio-Pliocene magmatic rocks are listed in Table 4 and plotted
in Fig. 7A and B. Published data from Murta basalts
as well as additional isotopic ratios from other Patagonian magmatic rocks are also plotted on this figure.
Initial strontium isotopic ratios (87Sr/86Sr)o are scattered between 0.70346 and 0.70439 and corresponding
(143Nd/144Nd)o between 0.51291 and 0.51272. The
isotopic compositions closest to Mid-Oceanic Ridge
Basalts (MORB) are those from Murta transitional basalts ((87Sr/86Sr)o = 0.70346–0.70353; (143Nd/144Nd)o =
0.512916 0.512920; qNd = + 5.4 + 5.5), with the exception of sample PG-01a ((87Sr/86Sr)o = 0.70396;
(143Nd/144Nd)o = 0.512792; qNd = +3.0) which is more
differentiated as shown by its Mg# = 51.5 (Table 2) and
is also characterised by a strong negative Nb anomaly
(La/Nb = 2, Table 2). The Murta samples plot within
the mantle array. Their signature is clearly different
from those of Cerro Pampa adakites, but rather
similar to that of basaltic andesites from the Chile
Trench Taitao Ridge (Guivel et al., 2003). Main
plateau MLBA alkali basalts plot in the same
field of Fig. 7B that MLBA post-plateau basalts
(Gorring et al., 2003). In this diagram, MLBA
intermediate basalts define a sub-vertical trend rooted in the former field and evolving outside this
field towards higher 143Nd/144Nd at rather constant
87
Sr/86Sr ratios.
6. Discussion
6.1. A subslab asthenospheric origin for the Rı́o Murta
basalts
Rı́o Murta transitional basalts have trace elements
patterns very similar to some Hudson volcano lavas
(López-Escobar et al., 1993) and also to those of
Enriched MORB (E-MORB) from the anomalous
segment 3 of the SCR (Klein and Karsten, 1995)
(Fig. 6A). These E-MORB from segment 3 of the
SCR show trace-element patterns very similar to
those of convergent margin magmas. This feature
has been interpreted as reflecting contamination of
the SCR mantle source by various amounts of either
oceanic sediments and altered oceanic crust or melts/
fluids derived from (Klein and Karsten, 1995). The
Sr/Nd isotopic ratios of Rı́o Murta transitional basalts
overlap those of the very heterogeneous SCR basalts
(Fig. 7B). These features allow us to infer that their
source may be identical to the mantle source of the
SCR basalts. As the SCR-1 ridge segment which
entered the trench 6 Ma ago is thought to be presently located beneath the studied area (Fig. 1B), the
Murta basalts may represent melts from the subslab
SCR-1 asthenospheric mantle passing through the
slab window, as envisioned by previous authors
(Demant et al., 1998; Corgne et al., 2001). Their
chemical similarities with Hudson magmas might
suggest that this window (or its zone of influence)
extended northwards to the edge of the SSVZ, as
already pointed out by D’Orazio et al. (2003). The
specific geochemical features of sample PG-01a, i.e.,
lower Mg#, strong negative Nb anomaly and respectively more (Sr) and less (Nd) radiogenic isotopic
signature compared to the others, might reflect crustal contamination effects of subslab magmas, likely
by the Patagonian Batholith through which they
ascended.
6.2. Origin of the MLBA and MCC main and postplateau alkali basalts and basanites
The petrogenesis of Plio-Pleistocene basalts from
MLBA has been previously studied by Hawkesworth et al. (1979), Baker et al. (1981) and Gorring et al. (2003). These works have demonstrated
the highly alkaline affinity of these basalts (nepheline-normative basanites and alkali basalts) with a
strong OIB-like geochemical signature and relatively
enriched Sr/Nd isotopic ratios (87Sr/86Sr= 0.7041–
0.7049; 143Nd/144Nd = 0.51264–0.51279). These features have been interpreted by Gorring et al. (2003)
as consistent with their derivation from an OIB-like
source involving the deep subslab asthenospheric
mantle, together with a contribution of the enriched
subcontinental lithospheric mantle (predominantly EM
I type). The study of peridotitic xenoliths within Patagonian basalts (Rivalenti et al., 2004) has documented
a lithospheric mantle metasomatised by asthenospherederived alkali basaltic melts. Our new isotopic data on
main plateau MLBA alkali basalts plot within the
previously determined field of Patagonian basalts
(Fig. 7B). Although we basically agree with the interpretations of Gorring et al. (2003), we did not find any
unquestionable evidence for a specific geochemical
imprint of the subslab asthenosphere opposed to that
the supraslab (i.e., mantle wedge) enriched asthenosphere proposed by Stern et al. (1990). Moreover, our
data do not allow us to ascribe the origin of the EM I
signature to either the Patagonian lithospheric mantle
or to an heterogenous subslab or supraslab asthenospheric mantle.
The alkali basalts from MLBA (main plateau) and
the basanites from MLBA (post-plateau; Gorring et al.,
2003) display rather similar Sr and Nd isotopic ratios
(Fig. 7A and B and Table 4), suggesting their derivation
from a single (or similar) type(s) of enriched mantle
source(s). However, the variable slopes of their incompatible trace element patterns (Fig. 6D–F) are consistent
with variable degrees of partial melting of such a source
(Gorring et al., 2003), the lowest ones corresponding to
the post-plateau MLBA basanites which show the highest La/Yb ratios. Luhr et al. (1995) have plotted nearprimitive basalts in a La/Yb vs. Yb diagram to document partial melting degrees of an enriched (lherzolitic)
mantle source with variable contributions of spinel and
garnet. In Fig. 8, where we have used the source
composition proposed by these authors (La = 1.79
ppm and Yb = 0.31 ppm), the position of post-plateau
MLBA basanites is consistent with 1.5–5% melting of a
source in which garnet is slightly more abundant than
spinel. Main plateau MCC and MLBA alkali lavas
would derive from somewhat larger (5–10%) melting
degrees of a less garnet-rich source. However, these
calculations are rather dependent from the assumed
composition of the source. For instance, using the
source composition proposed by Gorring and Kay
(2001), i.e., La = 0.885 ppm and Yb = 0.423 ppm,
leads to obtain very low melting degrees (0.1–2%) for
post-plateau MLBA basanites, and lower ones (2–5%)
for the main plateau lavas (diagram not shown).
363
6.3. Origin of the MLBA and MCC intermediate lavas
Alkali basalts and intermediate lavas from both
MCC and MLBA (main plateau) display roughly similar chemical characteristics and tend to plot along the
same trends in some diagrams. Because of the present
lack of isotopic data on MCC basalts, the following
discussion will be focused on MLBA (main plateau)
intermediate lavas.
The MLBA intermediate lavas erupted synchronously with the MLBA alkali basalts, and both types
have roughly similar compositions in major elements
(with the exception of TiO2) and trace elements (with
the exception of Nb). The intermediate lavas displaying the highest Nd isotopic ratios are also characterised by high Mg# and strong depletions in Nb
and Ti (Fig. 9).
These features, together with the trend they define
with MLBA alkali basalts in Fig. 7, could suggest that
their genesis was controlled by a mixing process between a component related to the alkali basalts (or their
mantle source) and a bcontaminantQ characterised by a
relatively unradiogenic Sr isotopic signature similar to
that of the alkali basalts but with higher Nd isotopic
ratios (above the mantle array) and a selective depletion
in Ti and Nb.
Mature continental crust and oceanic sediments, although depleted in Ti and Nb, have Sr isotopic signatures much more radiogenic than required for the
Fig. 8. Plot of La/Yb against Yb for primitive samples (Mg# N63) of our data set. Symbols as in Fig. 4. Results of non-modal batch melting (Shaw,
1970) of garnet and spinel lherzolite sources (modified from Luhr et al., 1995) are shown. Composition of the source: La = 1.79 ppm and Yb = 0.31
ppm. The mode of the garnet lherzolite is taken as ol/opx/cpx/gt = 60:25:9:6 and that of spinel lherzolite as ol/opx/cpx/sp = 58:30:10:2. Phase
proportions entering the melt are taken as ol/opx/cpx/gt or sp = 10:20:65:5. Partition coefficients for La and Yb were selected from literature values
as 0.0002:0.002:0.069:0.01:0.002 and 0.0015:0.049:0.28:4.1:0.007 for the minerals ol/opx/cpx/gt/sp, respectively.
364
Fig. 9. Plots of Mg#, TiO2, La/Nb and Nb against qNd. Symbols as in Fig. 4.
bcontaminantQ (Andean continental crust and Leg ODP
site 141 Chile Trench sediments, 87Sr/86Sr = 0.715 and
=0.708, respectively; Stern and Kilian, 1996) and can
thus be discarded as potential candidates. Contamination by adakitic magmas may also be envisioned, as
their composition matches the required trace element
and isotopic features, providing the adakitic component
derived from the melting of slightly altered oceanic
crust with a Sr isotopic signature slightly higher than
that of MORB. The addition of up to 10% adakitic melt
(isotopic and trace-element compositions given in Table
5) to the mantle source of MLBA alkali basalts does not
fit the Sr, Nd isotopic trend of the intermediate basalts
(curve A in Fig. 7B). Moreover, the trace-element
patterns of basaltic melts derived from such a blend
are inconsistent with the observed ones (Fig. 10A and
Model A in Table 5), as they display strong positive Sr
anomalies and Yb depletion typical of adakites, which
do not exist in the intermediate basalts.
Thus, we have tested another model (model B)
involving the mixing between up to 10% of a
slightly altered oceanic crust (87Sr/86Sr = 0.70375;
143
Nd/144Nd = 0.5131) and an alkali basalt mantle
source similar to that considered in the former model.
This mixing model (parameters and results given in
Table 5), similar to the former one but for the composition of the contaminant, accounts for the isotopic
signature of intermediate MLBA lavas (curve B in
Fig. 7). In addition, the trace-element features of the
basaltic magma derived from such a mix fits with the
trace elements patterns of the intermediate lavas except
for the lack of negative Nb anomalies (Fig. 10B and
Model B in Table 5). Thus, other processes must be
envisioned to explain these anomalies.
The negative correlation between Nb and Mg#
observed for intermediate lavas (Fig. 5C) suggests
that the negative Nb anomaly is attenuated when
differentiation progresses. This feature implies that
Nb depletion with respect to adjacent incompatible
elements is pristine and linked to the mantle source
of the intermediate basalts. It could correspond to the
bweak subduction componentQ detected in ultramafic
xenoliths of southern Patagonian basalts by Rivalenti
et al. (2004). However, this Nb depletion is unlikely to
represent an overall feature of the asthenospheric
mantle, as it does not occur in the MLBA post-plateau
basanites and main plateau alkali basalts. The origin of
this signature could thus be either lithospheric or
365
Table 5
Results of mixing calculations and compositions of the corresponding end-members
Rb
Ba
Th
Nb
La
Ce
Sr
Nd
Sm
Eu
Gd
Dy
Y
Er
Yb
87
Sr/86Sr
143
Nd/144Nd
qNd
1
2
3
4
5
Mantle source
Adakitic melt
MORB
Model A
Model B
2.20
28.50
0.30
2.90
2.75
5.40
68.50
2.95
0.66
0.21
0.56
0.47
2.40
0.21
0.19
0.704394
0.512724
1.68
–
306.00
4.90
11.73*
26.60
60.90
1,886.00
30.30
4.41
1.16
–
–
–
–
0.72
0.703636
0.513069
8.41
4.44
27.80
0.49
2.30
3.55
10.45
131.00
9.26
3.06
1.10
4.27
4.95
29.30
2.97
2.90
0.703636
0.513069
8.41
–
560.95
7.59
37.12
48.97
101.51
2,232.71
47.58
7.74
2.12
–
–
–
–
1.06
0.703823
0.5129079
5.26
23.95
283.51
3.18
27.87
26.99
54.74
666.91
29.97
6.73
2.08
6.21
5.33
29.49
2.48
2.01
0.704261
0.5128132
3.42
(1) Mantle source composition (PG109/10); (2) adakitic melt (RB5 from Cerro Pampa, Nb* = 17 Ta (Kay et al., 1993); (3) MORB composition
(sample D20-1) from SCR1 (Klein and Karsten, 1995); (4) model A: 10% batch partial melt of 90% of (1) mixed with 10% of (2); (5) model B: 10%
batch partial melt of 90% of (1) mixed with 10% of (3). A constant bulk source mode of 0.58 olivine, 0.275 opx, 0.095 cpx, 0.015 gt and 0.035
spinel was used. Partition coefficients used in calculations are from Gorring and Kay (2001). Isotopic compositions for adakitic melt and MORB are
those of slightly altered oceanic crust (DSDP/ODP sites 417/418, flow 300; Staudigel et al., 1995).
shallow asthenospheric, and related to a blocalQ—i.e.,
not widespread—component derived from slightly altered oceanic crust. Thorkelson (1996) and Thorkelson
and Breitsprecher (2005) have shown that the slab
edges of an asthenospheric window are able to melt
at depth, generating adakitic magmas and leaving
restite fragments which may become long-term residents of the continental lithospheric mantle. However,
if the restite becomes entrained in the asthenosphere, it
may then undergo partial melting. Furthermore, as Nb
concentrations in intermediate basalts are in average
lower than in the alkali basalts (26 vs. 38 ppm), Nb
should be retained in some residual mineral during
these processes. Rutile is the best candidate as it
concentrates only Ti, Nb and Ta and is commonly
observed as a residual phase during partial melting of
oceanic basalts under P–T conditions consistent with
those of hot subduction zones (Ringwood, 1990;
Foley et al., 2000; Schmidt et al., 2004). Amphibole
and/or phlogopite could also be considered, but their
occurrence in the restite should be detectable from the
behaviour of incompatible elements other than Nb and
Ti. Thus, we propose that the origin of the Ti- and
Nb-depleted intermediate MLBA basalts could be
linked to the contribution to their source of rutilebearing restites of partially melted oceanic crust
from the edges of the asthenospheric window, incor-
porated either in the shallow asthenospheric or the
deep lithospheric Patagonian mantle.
6.4. Tectonic setting of MLBA and MCC: successive
opening of ridge-derived asthenospheric windows?
As discussed above, the petrogenetic features of the
studied Mio-Pliocene basalts from MLBA (main plateau) and MCC are consistent with possible contributions of the deep subslab asthenosphere, the South
American subcontinental lithospheric and asthenospheric mantle and the subducted oceanic crust to
their magma sources. In the slab window opening
model previously developed by Ramos and Kay
(1992), Kay et al. (1993), Gorring et al. (1997, 2003)
and Gorring and Kay (2001), melting may occur at the
boundary between the ascending subslab asthenosphere
and the overlying subcontinental lithosphere, with occasional contributions of the downgoing basaltic crust
or of its melting products (Cerro Pampa adakites).
Alternatively, subslab-derived melts may interact with
the subcontinental mantle or with the oceanic crust at
the slab window edges during their ascent towards the
surface.
However, a critical aspect of the slab window model
of Gorring, Kay and co-authors is that they consider
that several windows developed successively beneath
366
post-plateau lavas seems hardly consistent with their
ascent through asthenospheric windows derived from
the successive segments of the SCR. Especially, the
early magmas of MLBA (main plateau stage) and
MCC cannot have ascended through the asthenospheric
window generated by the segment SCR-1 which entered the trench at 6 Ma only and is now thought to be
located beneath the studied area. Indeed, the post-plateau lavas of MLBA (b3.3 Ma) and the Quaternary
basalts of Rı́o Murta may have ascended through this
window (Gorring et al., 1997, 2003; Demant et al.,
1998; Corgne et al., 2001). In short, discrepancies
between the ages of emplacement of Patagonian basaltic plateaus and the age of the subduction of ridge
segments lead us to reconsider the modalities and timing of slab window opening beneath Patagonia.
6.5. Proposition of a new tectonic model involving slab
tearing linked to spreading ridge collision
Fig. 10. Primitive mantle normalised (Sun and McDonough, 1989)
incompatible multi-element patterns of compositions obtained from
batch partial melting (Shaw, 1970) of mantle sources derived from
mixing calculations (Table 5); (A) model A and (B) model B. A
constant bulk source mode of 0.58 olivine, 0.275 opx, 0.095 cpx,
0.0.15 gt and 0.035 spinel was used. Patterns of main plateau MLBA
intermediate samples with Mg# N59 are shown for comparison. See
text for details.
Patagonia during the last 15 Ma, each segment of the
SCR stopping its activity and thus developing its own
slab window after colliding with the Chile trench. Such
a process should result into a younging northward
pattern of plateau building (Fig. 1) that Gorring et al.
(1997) claim to have identified from their age data.
Presently available literature ages, combined with our
own K–Ar results, are plotted against latitude in Fig. 2.
No clear age decrease is observed from South to North,
especially regarding the onset of magmatic activity, and
no connection between the ages and the timing of
arrival of the various SCR segments to the Chile trench
is easily identifiable. Magmatic activity seems to begin
between ca. 12 and 8 Ma for nearly all the dated
volcanic centres including Estancia Glencross (528S),
Mesetas Belgrano, Central and de la Muerte, the Northeast volcanic region and finally MLBA and MCC (46–
478S). Moreover, a phase of relative paucity of volcanic
activity seems to have occurred around 7 Ma (except
possibly in MLBA), followed by a new pulse starting at
ca. 5–4 Ma in many volcanic centres. Thus, the chronology of emplacement of the Patagonian plateau and
As discussed above, emplacement of thick alkali
basalt sequences started at ca. 12 Ma, i.e., after the
tectonic phase that built up the Cordillera of the southern Patagonian Andes. Indeed, the Late Miocene basaltic flows are roughly horizontal and always appear to
post-date the main folding and thrusting event. This
contractional phase ended with a major tectonic event
recognized at the scale of the entire southern Patagonia,
characterised by thrusting of the pre-Cenozoic rocks of
the Cordillera over the frontal Oligocene–Miocene marine to continental molasse (Lagabrielle et al., 2004).
This phase occurred necessarily after 16.3 Ma (youngest known age of the mammal fauna of the continental
molasse) and before 12–10 Ma (the age of the basal
flows of the alkali plateau basalts). It is also recorded
by fission track analysis east of the present-day topographic divide where rapid cooling and denudation
ceased between 12 and 8 Ma (Thomson et al., 2001).
It must be noted that the initiation of the subduction of
the Chile Ridge at 15–14 Ma in southern Patagonia
coincides with this last main contractional phase that
affected the entire Cordillera. A period of very rapid
erosion and peneplanation followed the tectonic uplift,
producing a relatively flat surface on which the alkali
basalts were emplaced.
Considering the ages at which segments SCR1,
SCR0, SCR-1 and SCR-2 entered the trench (0.3, 3, 6
and 12 Ma Fig. 1B), and assuming that the Patagonian
plateau and post-plateau basaltic magmas originated
from a mantle that ascended through a slab window,
it becomes clear that this (or these) window(s) opened
well before the subduction of the corresponding ridge
segments. This has to be taken into account when
attempting to link plateau basalt emplacement to the
opening of such a window. For this reason, we favor a
model based on the process of slab tearing at depth
when collision starts at the trench (van den Beukel,
1990) and leading ultimately to slab breakoff (e.g.,
Davies and von Blanckenburg, 1995; von Blanckenburg and Davies, 1995; Mason et al., 1998). In such a
model, slab tear would start when strong tectonic coupling in the forearc occurs before the subduction of the
ridge axis itself. This situation may occur when a series
of large spreading ridge segments approaches the
trench, that is for the segments SCR-4 to SCR-2, all
of which arrived in less than 3 Ma within the subduction zone (15–14 Ma for SCR-4, 14–13 Ma for SCR-3,
and 12 Ma for SCR-2). Slab tear may then have propagated towards the north into slightly older lithosphere
emplaced at segment SCR-1. The lack of correlation
between latitude and onset of volcanic activity (Fig. 2)
suggests that this propagation was very rapid: indeed,
367
volcanic activity started earlier in MLBA than in Estancia Glencross (528S).
A tentative sketch of our proposed tectono-magmatic
model of Patagonian plateau basalt emplacement is
shown in Fig. 11. As compression occurs in the Cordillera leading to active orogenesis, tension forces are
applied to the descending slab that will break off beneath the continental plate. A slab tearing all along the
active Patagonian margin results ultimately into the
detachment and sinking of the deep part of the subducted plate. OIB-type magmas would be generated
by the partial melting of the subslab asthenospheric
mantle uprising through the tear-in-the-slab, possibly
near its boundary zone with the overlying continental
lithospheric mantle (Coulon et al., 2002). Alternatively,
heat transfer through the tear might have induced partial
melting of the supraslab mantle (Davies and von
Blanckenburg, 1995). Then, the ascending magmas
would interact with the Patagonian lithospheric mantle
and locally with altered Pacific crust from the edges of
the slab tear (intermediate magmas) and ultimately be
emplaced in the back-arc domain between ca. 528 and
468S. Finally, for all segments, after slab tear, the
spreading axis will enter the trench. The final stages of
this evolution will correspond to the opening of btrueQ
(ridge-related) slab windows. The latter are responsible
for the genesis and ascent of the most recent basaltic
magmas, i.e., in the northern and eastern Mesetas (including MLBA post-plateau phase) and Rı́o Murta during a second magmatic pulse starting at ca. 5–4 Ma.
7. Conclusions
Fig. 11. Cartoon showing the main stages of the proposed tectonic
model of slab tearing during ridge collision at the trench.
1. The Quaternary Rı́o Murta transitional basalts display obvious geochemical similarities to the SCR
and CTJ oceanic basalts. We consider them as derived from the melting of a Chile Ridge asthenospheric mantle source containing a weak subduction
component. Their position above the inferred location of the slab window corresponding to the SCR-1
segment subducted 6 Ma ago is consistent with a
slab window opening model previously developed
by Ramos and Kay (1992), Kay et al. (1993), Gorring et al. (1997, 2003) and Gorring and Kay (2001).
2. Two groups may be identified among the main
plateau basalts of MLBA and MCC. The first one
includes alkali basalts and trachybasalts displaying
typical OIB signatures and thought to derive from
the melting of OIB-type mantle sources involving
the deep subslab asthenosphere and the subcontinental mantle, as previously shown by Gorring et al.
(1997, 2003).
368
3. The second group of samples, although dominantly
alkalic, displays incompatible element signatures
intermediate between those of OIB and arc magmas
(e.g., La/Nb N 1 and TiO2 b 2 wt.%). These intermediate basalts differ from their alkalic equivalents by
their HFSE-depleted character and their higher qNd
(up to + 5.4). We ascribe these specific features to
their derivation from an enriched mantle source
contaminated by ca. 10% rutile-bearing restite of
altered oceanic crust, likely derived from the edges
of a slab window or slab tear.
4. The chronology of emplacement of main plateau
basalts from MLBA (12.4–3.3 Ma) and MCC
(8.2–4.4 Ma) is inconsistent with their origin from
an asthenospheric window opened as a consequence
of the subduction of the Chile Ridge segment SCR-1
which entered the trench at 6 Ma. This fact allows us
to question the model developed by Gorring et al.
(1997) and Gorring and Kay (2001), in which the
Neogene basaltic magmas of Southern Argentina
plateaus ascended through asthenospheric windows
which opened successively when segments SCR-4,
SCR-3, SCR-2 and finally SCR-1 of the Chile
ridge were subducted. In our preferred geodynamic
model, OIB and intermediate magmas of MLBA
and MCC, as well as those of other Patagonian
plateaus (Mesetas Belgrano, Central, de la Muerte
and the Northeast volcanic region) originated from
deep asthenospheric mantle uprising through a tearin-the-slab subparallel to the trench, which formed
when the southernmost segments of the SCR collided with the Chile Trench around 15 Ma.
Acknowledgements
This research was supported by the cooperation
program ECOS-Sud ACU01 and was part of the Chilean FONDECYT Project 1000125 and French DyETI
project 2004–2005. We thank Drs. M. D’Orazio and C.
Stern for their pertinent and helpful reviews of the
manuscript. Fieldwork assistance of Leonardo Zuñiga
(Pituso) is acknowledged.
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Miocene to Late Quaternary Patagonian basalts (46–478S

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