Plio–Pleistocene basalts from the Meseta del Lago Buenos Aires,
Argentina: evidence for asthenosphere–lithosphere interactions
during slab window magmatism
Matthew Gorringa,*, Brad Singerb, Jason Gowersa, Suzanne M. Kayc
aDepartment of Earth and Environmental Studies, Montclair State University, Upper Montclair, NJ 07043, USAbDepartment of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI 53706, USA
cDepartment of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, USA
Received 9 July 2001; accepted 9 September 2002
Abstract
Plio–Pleistocene (3.4–0.125 Ma) post-plateau magmatism in the Meseta del Lago Buenos Aires (MLBA; 46.7jS) in
southern Patagonia is linked with the formation of asthenospheric slab windows due to ridge collision along the Andean margin
f 6 Ma ago. MLBA post-plateau lavas are highly alkaline (43–49% SiO2; 5–8% Na2O+K2O), relatively primitive (6–10%
MgO) mafic volcanics that have strong OIB-like geochemical signatures. Their relatively enriched Sr–Nd isotope ratios
(87Sr/86Sr = 0.7041–0.7049; 143Nd/144Nd = 0.51264–0.51279), low 206Pb/204Pb (18.13–18.45), steep REE patterns (La/
Yb = 11–54), and low LILE/LREE and LILE/HFSE ratios (Ba/La < 15, La/Ta < 15, Ba/Ta < 180; Sr/La = 15–22; Th/La < 0.13;
Ce/Pb>15) are distinctive from most other Neogene Patagonian slab window lavas. These data are interpreted to indicate
contamination of OIB-like asthenosphere-derived slab window magmas with an EM1-type component derived from the
Patagonian continental lithospheric mantle (CLM). The EM1-type signature in Patagonian slab window lavas are
geographically associated with the Deseado Massif and indicate important regional differences in lithospheric mantle
chemistry beneath southern Patagonia. We propose that hot, upwelling subslab asthenosphere in slab window tectonic settings
can cause significant thermo-mechanical erosion and thinning of the continental lithospheric mantle and, thus, may be an
important process in slab window magma petrogenesis.
D 2003 Elsevier Science B.V. All rights reserved.
Keywords: Slab windows; Basalts; Patagonia; Asthenosphere; Lithosphere
1. Introduction
Mafic slab window magmatism is produced by the
collision and interaction of mid-ocean ridges with
continental subduction zones and offers a rare oppor-
tunity to investigate the chemistry of the astheno-
spheric and lithospheric mantle beneath continents.
Slab window mafic magmas are thought to be the
product of decompression melting as asthenospheric
mantle upwells through the gap that opens between
the subducting oceanic plates (e.g. Thorkelson, 1996).
Thus, magma sources are thought to dominantly
reflect the chemistry of the asthenospheric mantle
0009-2541/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.
PII: S0009 -2541 (02 )00249 -8
* Corresponding author. Tel.: +1-973-655-5409; fax: +1-973-
655-4072.
E-mail address: [email protected] (M. Gorring).
www.elsevier.com/locate/chemgeo
Chemical Geology 193 (2003) 215–235
beneath the subducting plate (e.g., Thorkelson, 1996).
The strong OIB- and MORB-type chemistry of sev-
eral examples of Neogene mafic slab window lavas,
from British Columbia to the Antarctic Peninsula,
generally support the idea that mantle sources are
dominated by the upwelling subslab asthenosphere
(Johnson and O’Neil, 1984; Thorkelson and Taylor,
1989; Storey et al., 1989; Ramos and Kay, 1992; Hole
et al., 1995; Cole and Basu, 1995; Luhr et al., 1995;
Gorring et al., 1997; Johnston and Thorkelson, 1997;
D’Orazio et al., 2000). However, as with all conti-
nental basalts, the continental lithospheric mantle
(CLM) is a possible reservoir for enriched, OIB-type
mantle components (e.g., EM1 and EM2; Hofmann,
1997), and could potentially play an important role as
a contaminant for asthenosphere-derived slab window
magmas. Thus, the interpretation of mantle sources
for mafic slab window lavas with OIB signatures and
Fig. 1. Tectonic setting of southern South America showing the distribution of Neogene Patagonian plateau lavas (black) from Panza and Nullo
(1994) relative to fracture zones and Chile Ridge segments (Cande and Leslie, 1986; Lothian, 1995). Numbers associated with plateau lava
locations are the published range of 87Sr/86Sr and 206Pb/204Pb ratios, respectively (Hawkesworth et al., 1979; Baker et al., 1981; Stern et al.,
1990; Gorring, 1997; D’Orazio, 2000; Gorring and Singer, 2000; Gorring and Kay, 2001). Ridge collision times are shown in bold numbers
(Cande and Leslie, 1986). Austral Volcanic Zone (AVZ) and southern Southern Volcanic Zone (SSVZ) volcanic centers (black triangles) are
from Stern et al. (1990). Upper Proterozoic– lower Cambrian basement of the Deseado Massif is also shown (line stipple; Pankhurst et al.,
1994). Outline of the Magallanes Basin (heavy line with small ticks) is from Ramos (1989). MFZ=Magallanes Fracture Zone (Klepeis, 1994).
M. Gorring et al. / Chemical Geology 193 (2003) 215–235216
the extent of asthenosphere–lithosphere interactions
is still controversial (e.g., Johnston and Thorkelson,
1997; Gorring and Kay, 2001).
In this paper, we present new geochemical and Sr–
Nd–Pb isotope data on a suite of Plio–Pleistocene
slab window-related lavas from the Meseta del Lago
Buenos Aires (MLBA) in southern Patagonia that
provide an opportunity to further examine this issue.
The MLBA is one of the largest (f 6000 km2) and
northernmost exposures of slab window-related, Late
Miocene to Pleistocene basaltic plateaus that occur
between 46.5jS and 52jS in southern Patagonian
back-arc (Fig. 1). The MLBA is also part of the larger
province of Plio–Pleistocene basaltic magmatism that
occurs in the Andean back-arc as far north as 34jS(e.g., Stern et al., 1990). The MLBA is located in the
northwestern corner of the Santa Cruz province,
Argentina, about 300 km southeast of the Chile Triple
Junction (CTJ) and about 150 km east of the modern
volcanic arc gap between the Southern (SVZ) and
Austral Volcanic Zones (AVZ) (Fig. 1). Based on
available geochemical and radiometric age informa-
tion, Ramos and Kay (1992) and Gorring et al. (1997)
interpreted MLBA magmas to have been generated in
response to the opening of slab windows associated
with collision of a segment of the Chile Ridge with
the Chile Trench at f 6 Ma (Fig. 1). Previous geo-
chemical work on MLBA post-plateau lavas have
shown that these rocks are highly alkaline, nephe-
line-normative basanites and alkali basalts, and have
some of the highest 87Sr/86Sr and lowest 143Nd/144Nd
ratios of all Neogene mafic rocks south of the CTJ
(Hawkesworth et al., 1979; Baker et al., 1981). These
isotope signatures are more enriched than other OIB-
like Neogene slab window lavas from southern Pata-
gonia (e.g., D’Orazio et al., 2000; Gorring and Kay,
2001) and from the Antarctic Peninsula (e.g., Hole et
al., 1995) that are interpreted to be primarily astheno-
sphere-derived (Fig. 1). Therefore, given the slab
window tectonic setting and the preliminary evidence
for unique trace element and isotopic compositions, a
detailed geochemical investigation of the MLBA post-
plateau lavas has primary importance for constraining
petrogenetic models for Late Cenozoic magmatism in
southern Patagonia and for understanding the extent
of asthenosphere–lithosphere interactions in the pro-
duction of basaltic magmas along active continental
margins.
2. Regional tectonic and geologic setting
The Late Cenozoic tectonic history of southern
South America is dominated by near orthogonal
subduction of the Nazca Plate beneath the South
American Plate. In southern Patagonia, convergence
has been punctuated by collision of segments of the
Chile Ridge with the Chile Trench, preceding sub-
duction of the Antarctic Plate (Fig. 1). Plate recon-
structions indicate initial ridge collision near the
southern tip of South America at f 14–15 Ma
(Cande and Leslie, 1986), forming a triple junction
between the South American, Nazca, and Antarctic
Plates. The Chile Triple Junction (CTJ) has since
migrated northwards to its present position (f 46j)by a series of Neogene ridge collisions (Cande and
Leslie, 1986; Forsythe et al., 1986) (Fig. 1). These
ridge–trench collisions are thought to be responsible
for several unique geodynamic, structural, and mag-
matic features of southern Patagonia, such as the
modern volcanic arc gap between the SVZ and the
AVZ (Stern et al., 1984), the eruption of adakitic
magmas in the back-arc (Kay et al., 1993) and in
the AVZ (Stern and Kilian, 1996), the Neogene uplift
of this sector of the Andes, and development of the
Patagonian fold-thrust belt (Ramos, 1989); and the
rapid (15–20 mm) of recent (< 500 years) isostatic
rebound of the Campo de Hielo due to anomalously
low viscosity mantle (< 1�1020 Pa s) and thin litho-
spheric thickness (< 100 km; Ivins and James, 1999).
Another major geologic feature of southern Pata-
gonia is the extensive Late Miocene to Pleistocene
plateau lavas that were erupted over large areas of the
back-arc (Fig. 1). Based on available K–Ar and40Ar/39Ar radiometric age dating, recent investiga-
tions have shown a temporal and spatial link between
ridge–trench collision and slab window formation
(e.g., Ramos and Kay, 1992; Gorring et al., 1997;
D’Orazio et al., 2000) and the eruption of these
plateau lavas, of which the MLBA is one of the major
examples. These slab window lavas are interbedded
with Late Miocene to Pleistocene fluvial gravels and
glacial sediments and were erupted over a sequence of
Jurassic silicic volcanics and Cretaceous to Miocene
mixed marine and continental sediments of variable
thickness (1–4 km). Basement beneath the Magal-
lanes Basin (Fig. 1) in the western and southern
Patagonian back-arc consists of a highly deformed,
M. Gorring et al. / Chemical Geology 193 (2003) 215–235 217
low-grade metasedimentary accretionary complex
which has been tentatively assigned a Devonian–
Carboniferous age by Riccardi and Rolleri (1980)
based on fossil evidence and has a probable meta-
morphic age of 224F 38 based on a Rb–Sr whole
rock date obtained by Herve et al. (1981). Further
east, basement consists of poorly exposed, upper
Proterozoic–lower Cambrian low-grade metamorphic
rocks of the Deseado Massif (Fig. 1) with a K–Ar age
of 549F 20 Ma for amphibolite obtained by Pezzuchi
(1978). Additional details on the regional geology can
be found in Ramos (1989) and Ramos and Kay
(1992).
3. Late Miocene to Pleistocene magmatism in the
MLBA
Available K–Ar and 40Ar/39Ar ages (Sinito, 1980;
Baker et al., 1981; Mercer and Sutter, 1982; Thon-
That et al., 1999) suggest two periods of magmatism:
(1) a voluminous, late Miocene to early Pliocene
tholeiitic main-plateau sequence, and (2) a younger,
less voluminous, highly alkaline, Plio–Pleistocene
post-plateau sequence. MLBA magmatism begins
with the eruption of the older main-plateau unit. These
are voluminous, tabular lava flows with exposed
thicknesses typically of 10–30 m consisting of several
flow units 2–5 m thick each. On the extreme western
side of the MLBA, exposed sections of the main-
plateau unit are up to 300 m thick. This sequence of
lavas flows creates the overall planar, plateau-like
geomorphology of the MLBA. Existing age con-
straints on MLBA main-plateau lavas consist of a
total of four K–Ar and 40Ar/39Ar radiometric ages
that range from 10 to 4.5 Ma (Sinito, 1980; Baker et
al., 1981; Mercer and Sutter, 1982; Thon-That et al.,
1999) with the oldest lavas exposed on the southeast
edge of the plateau. Preliminary data indicate signifi-
cant geochemical differences between this unit and the
younger, post-plateau lavas (Gorring and Singer,
2000); however, this will be more fully explored in
a future manuscript. This paper focuses only on the
geochemistry and geodynamic implications of the
younger, MLBA post-plateau lava sequence.
The MLBA post-plateau unit erupted from more
than 150 individual monogenetic volcanic centers that
formed above the basal main-plateau unit. These
include cinder, spatter, and scoria cones, maars, and
associated flows and pyroclastic debris. Several lava
flows from the larger centers have spilled over the
eastern edge of the MLBA and flowed up to 30 km
down the Rio Deseado and Rio Pinturas drainage
systems. Most of the monogenetic cones rise f 100
–150 m above the surface of the plateau; the largest
centers are up to 400 m above the surface. They have
well-preserved morphology (e.g. scoriaceous crater
rims, flows with pressure ridges, etc.) and, thus, appear
geologically very young. The average thickness of the
post-plateau volcanic pile is not well constrained due to
limited continuous vertical exposure. Baker et al.
(1981) estimated maximum thicknesses of cones and
surface flows to be as much 300 m locally. Based on
field observations, we estimate that the average thick-
ness is probably closer to 100 m, thus totally erupted
volumes are on the order of 600 km3. Existing age
constraints on MLBA post-plateau lavas consist of a
total of 12 K–Ar (whole-rock) and 40Ar/39Ar (step-
heating on basalt matrix and plagioclase separates)
radiometric ages that range from 3.4 to 0.125 Ma, but
most are V 1.8 Ma (Baker et al., 1981; Thon-That et
al., 1999). These ages indicate eruption f 3–5.9 Ma
after ridge collision, and therefore, according to geo-
dynamic models of Gorring et al. (1997), a fully
developed slab window would have existed beneath
the MLBA by this time.
4. Analytical methods
All samples were sawed into slabs, reduced to
0.25–0.5 cm in a hardened steel mortar, and pulver-
ized in an alumina ceramic shatterbox. Major elements
were determined by electron microprobe analysis on
fused glasses using a JEOL JXA-8600 electron
microprobe at both Cornell University and Rutgers
University. Techniques and standards used for microp-
robe analyses of major elements are given in Kay et al.
(1987). Precision and accuracy (2r) are F 1–5% for
elements at >1 wt.% and F 10–20% at < 1 wt.%
abundance levels based on replicate analysis of basal-
tic glass standards. Trace elements were determined by
INAA at Cornell University. Techniques and standards
are given in Kay et al. (1987). INAA precision and
accuracy based on replicate analysis of an internal
basalt standard are 2–5% (2r) for most elements and
M. Gorring et al. / Chemical Geology 193 (2003) 215–235218
F 10% for U, Sr, Nd, and Ni. Pb concentrations were
determined by ICP-MS at Cornell University using a
single collector, Finnigan MAT Element2 instrument
with an external and accuracy of f 5% based on
replicate analyses of USGS basalts standards BIR-1
and BHVO-2.
Sr, Nd, and Pb isotopes were analyzed at Cornell
University on a VG Sector 54 thermal ionization
mass spectrometer. Chemistry and analytical techni-
ques are summarized in White and Duncan (1996).
Pb isotope ratios were corrected for mass fractiona-
tion assuming NBS981 Pb values of 206Pb/204Pb =
16.937, 207Pb/ 204Pb = 15.493, and 208Pb/204Pb =
36.705. Average measured values for NBS981 were206Pb/204Pb = 16.908 F 3 (2r) , 207Pb/204Pb =
15.455F 3, and 208Pb/ 204Pb = 36.595F 10. Sr and
Nd isotope ratios were corrected for mass fractiona-
tion assuming 86Sr/88Sr = 0.1194 and 146Nd/144Nd =
0.7219. Average measured value for NBS987 Sr
standard was 87Sr/86Sr = 0.710235F 34 (2r). Aver-age measured value for the La Jolla Nd standard was143Nd/144Nd = 0.511864F 14 (2r).
5. Petrography
A total of 35 samples of MLBA post-plateau
volcanics were sampled during the austral summers
of 1989–1990 (Kay), 1996–1997 (Singer), and
1998–1999 (Gorring and Singer). The samples are
extremely fresh with only a few showing incipient
alteration (iddingsite, hematite) of olivine phenocrysts
along rims and interior cracks upon microscopic
inspection. MLBA post-plateau lavas are porphyritic,
with 5–20% phenocrysts overwhelming dominated
by euhedral olivine, but clinopyroxene, plagioclase,
and Fe-oxides are also common. One sample (010,
Table 1), classified as mugearite, contained abundant
(30–50%), large (3–5 cm) anorthoclase and horn-
blende megacrysts. Groundmass textures are micro-
crystalline and vary between vitrophyric, intersertal,
and intergranular with plagioclase, clinopyroxene,
olivine, Fe–Ti oxides, and glass as the dominant
phases. Glass content varies from < 10% to as much
as 80%. Flow alignment of groundmass plagioclase
imparts a weak trachytic texture in some samples.
Small (1–2 cm) spinel lherzolite and harzburgite
xenoliths are also common.
6. Geochemical results
6.1. Major and transition metals
The major element and transition metal concen-
trations of MLBA lavas analyzed for this study are
given in Table 1. MLBA post-plateau lavas are a
strongly alkaline series of volcanics (most have 0–
20% normative nepheline; 44–49 wt.% SiO2; 5–8
wt.% Na2O +K2O), with almost all samples plotting
in the alkali basalt, trachybasalt, and basanite fields
on a total alkali–silica classification diagram (Fig. 2;
Table 1). They are considerably more alkaline (higher
in both Na2O and K2O) than other equivalent Neo-
gene southern Patagonian post-plateau slab window
lavas from the central Santa Cruz province (47–50jS;Gorring and Kay, 2001) and from the Pali Aike
Volcanic Field (PAVF, 52jS, D’Orazio et al., 2000)
(Fig. 2). MLBA post-plateau lavas have relatively
high Mg#s (54–69), MgO (6–10 wt.%), and Cr
(100–400 ppm) and Ni (75–225 ppm) (Table 1) that
support the idea that these lavas have suffered only
moderate amounts of crystal fractionation from prim-
itive, mantle-derived basalts. Rapidly decreasing Ni
and Cr with decreasing MgO suggests that olivine and
included Cr-spinel played the dominant role during
crystal fractionation. Two samples (004 and 010)
have anomalously high silica (f 50.5%), low MgO
(f 4 wt.%), and low Cr (f 40 ppm) and Ni (15–35
ppm) and, thus, have suffered more extensive differ-
entiation than the majority of MLBA post-plateau
lavas.
6.2. Incompatible trace elements
Key trace element characteristics of MLBA post-
plateau lavas are shown in Figs. 3–6, and the full
analytical results are listed in Table 1. All samples are
have LREE-enriched patterns (LaN = 75–250) on
chondrite-normalized REE plots (Fig. 3). HREE con-
centrations are roughly the same (YbNf 7–8) for all
samples, and thus, variation in LREE concentrations
produces a large range of La/Yb ratio (11–54). The
buffering of HREEs and the increasing La/Yb fractio-
nation with increasing incompatible trace concentra-
tions strongly suggest a systematic variation in the
degree of partial melting of a garnet-bearing source
during the petrogenesis of MLBA post-plateau mag-
M. Gorring et al. / Chemical Geology 193 (2003) 215–235 219
Table 1
Major and trace element analyses and Sr–Nd–Pb isotope data of post-plateau lavas from the Meseta del Lago Buenos Airesa
Sample AP-01ttb AP-01g AP-02s CV-01ttb CV-01s CV-02s CV-02g AT-01ttb AT-01s
Latitude (S) 46j43.9V 46j43.9V 46j40.0V 46j40.0V 46j35.4V 46j35.4V 46j52.0V 46j52.0VLongitude (W) 70j50.0V 70j50.0V 70j47.0V 70j47.0V 70j40.5V 70j40.5V 70j44.0V 70j44.0VClassification alk bas alk bas alk bas basan basan basan basan basan basan
SiO2 (wt.%) 46.50 47.05 46.73 44.70 45.10 44.03 44.19 46.00 46.29
TiO2 1.97 2.02 2.15 2.72 2.72 2.77 2.85 2.40 2.42
Al2O3 15.50 15.44 15.85 15.60 16.18 14.39 14.51 15.30 15.46
FeO 10.15 10.44 9.95 10.70 10.09 10.16 10.45 9.34 9.19
MnO 0.17 0.24 0.21 0.17 0.14 0.21 0.23 0.17 0.19
MgO 10.25 9.61 9.02 8.51 7.65 9.58 9.57 8.58 8.53
CaO 9.18 9.65 9.71 9.48 9.80 10.92 10.79 7.69 8.09
Na2O 3.02 3.26 3.46 3.90 3.93 3.74 3.91 4.81 4.76
K2O 1.48 1.54 1.60 2.34 2.42 2.32 2.33 2.88 2.74
P2O5 0.49 0.53 0.54 0.98 1.01 1.11 1.12 1.33 1.25
Total 98.7 99.8 99.2 99.1 99.0 99.2 99.9 98.5 98.9
Mg# 67.9 65.9 65.5 62.5 61.4 66.4 65.7 65.8 66.0
ne, � hy 4.0 5.4 6.8 13.0 12.8 15.4 16.1 14.9 14.5
La (ppm) 24 32 30 50 57 66 71 66 79
Ce 64 57 105 127 132 137
Nd 28 30 46 56 57 57
Sm 6.1 5.8 8.6 10.1 10.6 10.2
Eu 1.80 1.70 2.40 2.96 3.05 2.79
Tb 0.80 0.80 0.92 1.14 1.18 1.07
Yb 1.84 1.86 1.77 1.99 2.02 1.80
Lu 0.25 0.26 0.24 0.27 0.28 0.24
Sr 698 694 579 1094 988 1097 1121 1301 1075
Ba 427 399 419 751 729 739 715 731 662
Cs 0.58 0.81 0.48 0.44 0.58 0.57
Rb 32 40 52
U 0.80 0.85 1.48 1.90 1.91 1.62
Th 3.9 3.6 6.6 7.8 7.9 9.0
Pb 7 3.9 4.9 5.4 7.5
Y 25 26 30
Zr 185 283 406
Hf 4.2 4.2 5.1 6.7 6.6 8.4
Nb 42 77 98
Ta 2.51 2.35 4.36 4.76 4.88 5.58
Sc 25 25 17 29 27 15
Cr 320 325 281 181 136 322 313 230 218
Ni 211 165 139 153 134 174 160 198 171
Co 79 49 46 82 42 47 47 58 4087Sr/86Sr 0.704597 0.704393 0.704448143Nd/144Nd 0.512708 0.512649 0.512667206Pb/204Pb 18.233 18.127207Pb/204Pb 15.629 15.592208Pb/204Pb 38.582 38.501
a alk bas = alkali basalt; basan = basanite; hawa = hawaiite; mugea =mugearite. Mg# =Mg/(Mg+ Fe2 +)� 100 assuming Fe2O3/FeO
(wt.%) = 0.15. ne, � hy =wt.% normative nepheline or wt.% normative hypersthene (minus sign).b Analyses published in Thon-That et al. (1999).c Rb, Pb, Y, Zr, Nb by ICP-MS at Cornell University (Gorring and Kay, 2001).
M. Gorring et al. / Chemical Geology 193 (2003) 215–235220
AT-02s AT-03s LC-27 LC-28 001 002 003d 004 005
46j56.0V 46j52.0V 46j49.8V 46j49.4V 46j49.0V 46j48.3V 46j47.8V70j43.0V 70j44.0V 71j03.1V 71j09.7V 71j12.0V 71j08.9V 71j06.3V
basan hawa basan basan hawa hawa hawa hawa alk bas
45.72 45.68 45.57 46.31 51.42 48.49 48.49 50.56 49.28
2.28 2.37 2.33 2.38 1.42 2.33 2.35 1.91 2.00
14.73 14.31 15.06 15.19 16.38 17.23 16.57 18.40 17.32
9.38 9.37 9.87 9.71 8.33 9.42 9.31 8.69 9.57
0.16 0.17 0.22 0.17 0.13 0.14 0.15 0.14 0.14
8.39 10.18 8.37 8.49 8.23 7.28 5.94 3.97 5.99
8.31 8.99 8.12 7.83 8.19 7.79 9.43 8.44 9.54
4.60 4.09 5.40 4.84 3.64 4.12 4.02 4.18 3.46
2.94 2.32 2.68 2.71 1.99 2.47 1.96 2.37 1.47
1.54 1.24 1.58 1.36 0.36 0.76 0.52 0.60 0.41
98.1 98.7 99.2 99.0 100.1 100.0 98.7 99.3 99.2
65.2 69.5 64.0 64.7 67.4 61.8 57.2 49.0 56.7
14.6 12.1 18.3 14.2 0.6 6.8 6.5 3.4 0.9
94 72 99 86 32 40 32 39 27
165 132 178 159 65 80 67 78 57
70 58 75 64 29 39 35 38 29
11.7 9.8 12.1 11.2 6.2 7.4 7.4 7.3 6.1
3.22 2.76 3.41 3.34 1.71 2.12 2.08 2.07 1.82
1.17 1.04 1.32 1.20 0.84 0.94 0.93 0.93 0.88
1.83 1.71 1.85 1.89 2.20 2.17 1.98 2.14 2.18
0.24 0.23 0.27 0.29 0.32 0.31 0.28 0.32 0.32
1213 1030 1479 1343 715 817 780 778 595
711 626 738 653 338 549 407 584 302
0.85 0.47 0.76 0.52 1.81 0.58 1.42 1.03 0.66
2.63 2.26 2.73 1.91 2.27 1.18 1.56 1.51 0.98
11.4 9.0 11.9 10.2 8.7 4.1 5.4 5.8 3.4
8.4 7.0 6.1 4.5
9.2 7.3 9.3 8.8 4.1 5.5 5.0 4.9 4.3
6.32 5.06 6.75 5.99 1.55 3.06 2.21 2.34 1.91
14 18 14 15 23 18 25 17 27
220 386 243 240 409 175 119 38 176
161 226 175 184 176 116 48 15 76
39 44 41 41 40 38 36 27 37
0.704335 0.704875
0.512733 0.512636
18.471 18.453
15.557 15.590
38.429 38.515
(continued on next page)
M. Gorring et al. / Chemical Geology 193 (2003) 215–235 221
mas. The classic, nonmodal batch melting equation of
Shaw (1970) was used to model REE patterns of
MLBA post-plateau lavas. Good agreement between
model melts and actual MLBA REE patterns are
obtained by 1–5% partial melting of LREE-enriched,
garnet-bearing lherzolite mantle source (Fig. 3). This
strongly suggests that MLBA lavas were generated
well within the garnet stability field at depths in
excess of 65–70 km.
The use of an enriched mantle source for the REE
modeling is justified by the strong OIB-like trace
element signatures of MLBA post-plateau lavas that
Table 1 (continued)
Sample 006 007 008 009 010 011 012 013
Latitude (S) 46j49.5V 46j49.7V 46j56.0V 46j58.2V 46j58.7V 46j55.6V 46j58.6V 47j00.4VLongitude (W) 71j06.4V 71j05.0V 71j20.1V 71j18.1V 71j17.2V 71j11.3V 71j05.9V 71j05.6VClassification alk bas alk bas basan hawa mugea basan hawa hawa
SiO2 (wt.%) 47.34 49.66 43.79 48.16 50.68 45.65 49.23 47.82
TiO2 2.52 1.98 2.76 2.25 2.21 2.34 2.27 2.18
Al2O3 15.74 16.69 14.87 16.37 18.07 16.63 17.59 16.67
FeO 9.95 9.91 10.79 9.69 9.93 9.86 8.80 9.50
MnO 0.20 0.15 0.15 0.08 0.12 0.13 0.12 0.16
MgO 8.15 6.96 7.86 7.58 3.94 7.38 5.13 7.66
CaO 10.06 8.96 8.79 6.94 5.16 8.43 8.00 7.56
Na2O 3.89 3.59 4.98 4.58 5.69 4.68 4.55 4.19
K2O 1.66 1.32 2.57 2.28 2.81 2.35 2.16 1.98
P2O5 0.56 0.51 1.52 0.89 0.84 1.01 0.81 0.70
Total 100.1 99.7 98.1 98.8 99.4 98.5 98.6 98.4
Mg# 63.2 59.5 60.4 62.1 45.4 61.1 55.0 62.8
ne, � hy 8.6 0.0 18.6 7.6 8.0 14.0 5.8 6.1
La (ppm) 32 19 88 44 37 63 42 38
Ce 64 41 173 87 78 126 83 79
Nd 30 24 62 38 39 48 38 32
Sm 6.8 5.7 12.2 7.6 8.3 9.4 7.7 7.3
Eu 2.04 1.57 3.53 2.27 2.39 2.71 2.18 2.14
Tb 0.93 0.82 1.34 0.98 0.99 1.15 1.05 1.00
Yb 1.88 1.85 2.10 1.93 1.55 2.04 2.32 2.05
Lu 0.27 0.26 0.28 0.28 0.21 0.29 0.33 0.29
Sr 759 589 1335 878 794 1084 803 794
Ba 390 259 697 476 489 543 476 383
Cs 0.40 0.45 0.58 0.57 0.36 0.58 0.81 0.68
Rb
U 0.86 0.59 2.24 1.36 1.30 1.51 1.16 1.15
Th 3.5 2.2 10.1 4.8 4.2 7.0 5.7 4.5
Pb 8.0 3.3
Y
Zr
Hf 4.8 3.1 8.6 6.2 6.6 7.6 5.7 5.4
Nb
Ta 2.90 0.99 6.08 3.77 3.73 4.46 3.18 3.21
Sc 26 20 19 17 8 21 20 20
Cr 369 192 156 225 40 212 123 176
Ni 130 87 102 162 35 119 65 133
Co 44 40 42 41 30 41 32 4187Sr/86Sr 0.704433 0.704094143Nd/144Nd 0.512676 0.512795206Pb/204Pb 18.208 18.303207Pb/204Pb 15.607 15.574208Pb/204Pb 38.522 38.408
M. Gorring et al. / Chemical Geology 193 (2003) 215–235222
are well displayed on primordial mantle-normalized
patterns (Fig. 4). All MLBA lavas have all the classic
enrichments of LILE, LREE, and HFSE that charac-
terize basalts from intraplate continental and oceanic
settings (e.g. Sun and McDonough, 1989). The strong
intraplate, OIB-like chemical signature of MLBA
lavas is also indicated by their low LILE/LREE and
LREE/HFSE ratios shown in Fig. 5. This plot dem-
onstrates the similarity of Ba/La, La/Ta, and Ba/Ta
ratios of MLBA lavas to a global average of oceanic
alkali basalt (‘‘OIB’’, Sun and McDonough, 1989)
and their low values relative to SSVZ arc lavas (Ba/
014 015 016 017 018 LC-25c LC-26 025 026
47j02.9V 47j02.9V 47j04.7V 47j06.0V 47j06.9V 47j07.7V 47j04.0V 46j49.1V 46j49.1V71j03.3V 71j03.3V 71j01.7V 71j00.5V 70j58.6V 70j52.0V 70j48.0V 70j54.9V 70j54.9Vhawa hawa alk bas hawa hawa hawa hawa hawa alk bas
48.08 49.12 47.84 45.45 47.83 47.20 45.65 48.05 48.87
2.22 2.15 2.48 2.63 2.52 2.31 2.67 2.60 2.06
16.68 16.95 16.24 16.31 17.09 16.10 16.01 16.87 16.48
9.59 9.35 10.38 10.33 9.56 10.05 10.13 9.83 10.19
0.15 0.13 0.17 0.17 0.21 0.16 0.19 0.13 0.11
7.29 7.10 7.14 7.18 5.86 8.01 7.19 5.53 7.36
8.83 7.49 10.04 9.80 9.76 9.42 10.86 9.02 7.98
4.52 4.71 3.64 3.90 3.84 3.87 4.63 4.16 3.27
2.05 2.11 1.19 2.08 2.29 1.97 1.24 1.82 1.60
0.74 0.72 0.44 0.79 0.76 0.66 0.89 0.80 0.57
100.2 99.9 99.5 98.6 99.7 99.8 99.5 98.8 98.5
61.4 61.4 59.0 59.3 56.2 62.6 59.8 54.1 60.2
10.0 7.7 4.2 11.2 7.5 8.6 13.5 5.5 �4.9
44 32 19 47 45 43 50 59 30
86 66 40 92 88 82 92 119 57
38 33 20 41 36 37 45 53 33
7.8 7.2 5.2 8.3 7.7 7.4 8.2 9.4 7.4
2.22 2.10 1.67 2.35 2.25 2.12 2.38 2.60 2.14
0.99 0.94 0.81 1.04 1.07 0.98 0.96 1.10 1.00
2.18 2.10 1.77 1.98 2.00 2.06 1.97 2.05 2.17
0.31 0.29 0.24 0.29 0.30 0.32 0.27 0.30 0.31
818 781 570 868 795 828 966 987 798
487 406 208 500 525 558 581 456 412
0.68 0.51 0.18 0.39 0.65 0.61 0.56 0.70 0.38
33
1.36 0.94 0.46 1.29 1.22 1.30 1.42 1.39 1.04
5.1 3.2 1.6 5.6 5.6 5.4 5.7 5.2 3.6
4.6 4.4 5.0
24
209
5.9 5.2 3.5 5.8 5.5 5.4 5.3 7.4 4.3
50
3.38 2.43 1.51 4.07 3.75 3.65 4.17 3.56 1.62
23 17 24 27 25 24 27 22 22
182 151 222 183 149 236 203 122 241
99 122 73 91 67 134 87 54 169
38 38 44 42 37 44 42 36 44
0.704681 0.704743 0.704106
0.512663 0.512676 0.512623
18.221 18.267
15.604 15.538
38.638 38.384
M. Gorring et al. / Chemical Geology 193 (2003) 215–235 223
Ta>700, La/Ta>35; off-scale in Fig. 5). The strong
OIB-like signature of most MLBA post-plateau lavas
is also supported by Ce/Pb (18–24), Nb*/U (35–60),
and Th/La (0.08–0.13) that fall near or well within the
limits of OIB (Fig. 6).
Despite the overall strong intraplate, OIB-like trace
element signatures of most MLBA lavas, some sam-
Fig. 3. REE plots for representative MLBA post-plateau lavas (fine
solid lines) compared to calculated partial melts (gray field) using a
simple nonmodal batch melting model. Source composition (shown
for reference) and partition coefficients used to calculate partial
melts are given in Gorring and Kay (2001). A constant bulk source
mode of 0.55 olivine, 0.22 orthopyroxene, 0.15 clinopyroxene, 0.06
garnet, and 0.02 Cr-spinel was used. Melt reaction and reaction
coefficients used were as follows: 0.25 opx + 0.61 cpx + 0.20
gar + 0.04 sp! 0.10 ol + liq. Normalization factors are based on
Leedy chondrite (Masuda et al., 1973) and are La (0.378), Ce
(0.978), Nd (0.716), Sm (0.23), Eu (0.0866), Tb (0.0589), Yb
(0.249), Lu (0.0387).
Fig. 2. Alkali– silica classification diagram (Le Maitre, 1989) for
Plio–Pleistocene MLBA post-plateau lavas (filled diamonds: this
study; crosses: Baker et al., 1981). Heavy dark line is boundary
between alkaline and subalkaline rocks of Irvine and Baragar (1971).
Also plotted are data from slab window lavas from Pali Aike
(diagonal rule; Stern et al., 1990; D’Orazio et al., 2000) and Neogene
southern Patagonian slab window lavas (Gorring and Kay, 2001).
Fig. 5. Plot of Ba/Ta versus La/Ta for MLBA post-plateau lavas
(filled diamonds) showing strong OIB signature and low LILE/
HFSE and LREE/HFSE ratios compared to SSVZ mafic arc lavas
(not shown, plot off-scale toward upper right corner). Labeled
samples have anomalously high Ba/Ta and La/Ta ratios (see text)
and are interpreted to reflect minor subduction-related components
in these samples. Also plotted are data from slab window lavas from
Pali Aike (diagonal rule; Stern et al., 1990; D’Orazio et al., 2000)
and the Antarctic Peninsula (fine stipple; Hole, 1988, 1990).
Approximate fields for EM1-, EM2-, and HIMU-type OIB are from
Weaver (1991). Unfilled star is the average OIB composition of Sun
and McDonough (1989).
Fig. 4. Primitive mantle-normalized trace element diagrams for
representative MLBA post-plateau lavas (fine solid lines) compared
to typical OIB (dashed line), N-MORB (heavy solid line), southern
Southern Volcanic Zone (SSVZ) mafic arc lavas (gray field; Hickey
et al., 1986; Lopez-Escobar et al., 1993). OIB, N-MORB, and
normalization factors from Sun and McDonough (1989). Nb* is
equal to 17 times the Ta concentration (e.g. Sun and McDonough,
1989) and is used for plotting purposes only.
M. Gorring et al. / Chemical Geology 193 (2003) 215–235224
ples (001, 003d, 004, 005, 007, 025, 026) have higher
La/Ta (15–20), Ba/Ta (200–250), Th/La (0.15–0.27)
ratios, and lower Ce/Pb (10–15) and Nb*/U (12–30)
ratios (Figs. 5 and 6) that are outside the ranges for
typical OIB and MORB (Hofmann et al., 1986; Sun
and McDonough, 1989; Weaver, 1991). The low Ce/
Pb ratios in these samples are due to excess Pb which
can be clearly seen as positive Pb anomalies on
primitive mantle-normalized multi-element diagrams
(Fig. 4). These geochemical signatures reflect the
influence of minor subduction-related (slab-derived
fluids, sediments) and/or continental crustal compo-
nents in some MLBA lavas.
6.3. Sr, Nd, and Pb isotopic ratios
The Sr, Nd, and Pb isotopic composition of 10
MLBA post-plateau volcanics generally overlap the
isotope compositions of Neogene Patagonian slab
window lavas to south (Gorring and Kay, 2001), and
Fig. 6. Plots of (a) Ce/Pb and (b) Th/La versus Nb*/U for MLBA
post-plateau lavas (filled diamonds) showing that most samples plot
near or within fields for oceanic basalts (OIB+MORB). Labeled
samples are referred to in text and have ratios indicating the influence
of arc and/or crustal AFC components. Fields for OIB+MORB,
altered oceanic crust (diagonal rule), arcs (white box), average
continental crust (CC), and marine sediment (horizontal rule) are
after Klein and Karsten (1995). Curve 1 represents bulk mixing
between a primary MLBA post-plateau lava (1% partial melt of 50%
MORB–50% OIB source) and average S-type Patagonian crust.
Curve 2 is bulk addition (source region contamination) of average
Chile Trench sediments to the estimated asthenospheric mantle
source composition (e.g. 50% MORB–50% OIB source). Tick
labels indicate percentage of crustal component for each curve. See
Table 3 for chemistry of the endmember compositions used.
Fig. 7. Plot of 143Nd/144Nd versus 87Sr/86Sr for Patagonian slab
window lavas (large filled diamonds: this study; small open
diamonds: Hawkesworth et al., 1979) showing their relatively
enriched OIB-like signatures that contrast with the more depleted,
HIMU-like signature of slab window lavas from Pali Aike and the
Antarctic Peninsula (see Fig. 5 for data sources). Also plotted are
fields for other Neogene southern Patagonian slab window lavas
(Gorring and Kay, 2001), Chile Ridge lavas (Klein and Karsten,
1995; Sturm et al., 1999), mafic southern Southern Volcanic Zone
(SSVZ; Hickey et al., 1986; Hickey-Vargas et al., 1989; Lopez-
Escobar et al., 1993), and Austral Volcanic Zone (AVZ; Stern and
Kilian, 1996) arc lavas. MORB field is compiled from literature
(e.g., Hofmann, 1997 and references therein). Mantle components
(EM1, EM2, HIMU, DMM) are from Zindler and Hart (1986).
Curves 1 and 2 are mixing models involving crustal components as
in Fig. 6. Curves 3 and 4 are binary mixing models showing
percentage of EM1-type lithospheric components added to a 5%
partial melt of the estimated asthenospheric mantle source
composition. Tick labels indicate percentage of crustal and EM1-
type components for each curve. See Tables 2 and 3 for chemistry of
the endmember compositions used.
M. Gorring et al. / Chemical Geology 193 (2003) 215–235 225
fall within fields defined bymany continental intraplate
basalts and OIB (Figs. 7–9; Table 1). Sr–Nd isotope
ratios range from 87Sr/86Sr = 0.7041–0.7049 and143Nd/144Nd = 0.51262–0.51279; however, most sam-
ples cluster near 87Sr/86Srf 0.7045 and 143Nd/144Ndf 0.51267 (Fig. 7). Pb isotope ratios range206Pb/204Pb = 18.13–18.47, 207Pb/204Pb = 15.54–
15.63, 208Pb/204Pb = 38.38–38.64 and form a diffuse
array above the Northern Hemisphere Reference Line
(NHRL; Hart, 1984), and thus, are similar to southern
hemisphere EM1- and EM2-type OIB that have pos-
itive DUPAL anomalies (e.g., Dupre and Allegre,
1983; Hart, 1984).
Our data confirm earlier results reported by Haw-
kesworth et al. (1979) on the Sr–Nd isotope compo-
sition of MLBA post-plateau lavas, and extend the
range to the most enriched Sr–Nd isotope ratios and
the most unradiogenic 206Pb/204Pb reported for any
Neogene Patagonian slab window lavas south of the
CTJ. It should also be noted that MLBA post-plateau
magmas, along with other Patagonian slab window
lavas (Gorring and Kay, 2001), are isotopically dis-
tinct from Pali Aike (Stern et al., 1990; D’Orazio et
al., 2000) and Antarctica Peninsula (Hole, 1988,
1990) slab window lavas that have more depleted,
HIMU-type OIB isotopic signatures. The range of
Sr–Nd–Pb isotope compositions of MLBA lavas
suggests mixing between an enriched EM1- and/or
EM2-type components and a more depleted, OIB-like
asthenospheric mantle component.
7. Discussion
The trace element and isotopic characteristics of
MLBA post-plateau lavas are consistent with their
being derived from mantle sources with OIB-like trace
Fig. 8. Plots of (a) 207Pb/204Pb and (b) 208Pb/204Pb versus206Pb/204Pb showing positive Dupal and EM1-type Pb signatures
of MLBA post-plateau lavas. Symbols and data sources as in Figs. 5
and 7. Nazca Plate sediments are from Unruh and Tatsumoto (1976)
and Dasch (1981). Crustal granulite xenoliths from Estancia Lote 17
locality and Jurassic Chon Aike high-Si rhyolite (open diamond
with cross) are unpublished data from Gorring (1997). Average S-
type Patagonian crust and average Chile Trench sediments are from
Kilian and Behrmann (1997). Northern Hemisphere Reference Line
(NHRL) is from Hart (1984).
Fig. 9. Plot of 87Sr/86Sr versus 206Pb/204Pb for MLBA post-plateau
showing the weak EM1-type signatures that contrast with HIMU-
like signatures of Pali Aike and Antarctic Peninsula slab window
lavas. Data sources, symbols, and components as in Figs. 5, 7, and
8. Mixing curves are as in Fig. 7.
M. Gorring et al. / Chemical Geology 193 (2003) 215–235226
element and isotopic characteristics (Figs. 4–8). This
includes both the asthenospheric mantle that upwells
through an opening slab window and enriched mantle
components derived from the Patagonian continental
lithospheric mantle. However, some samples show
some chemical evidence (e.g., low Ce/Pb and Nb*/
U, high Th/La and Ba/Ta), which suggests that sub-
duction-related processes and/or continental crustal
contamination have played a role, albeit minor, in
MLBA magma petrogenesis. In the following section,
we discuss this role of these components and present a
petrogenetic model for MLBA post-plateau lavas.
7.1. Role of subduction-related and continental
crustal components
Several MLBA post-plateau lavas analyzed in this
study (001, 003d, 004, 005, 007, 025, 026) have
anomalously low Ce/Pb and Nb*/U, and high Th/
La, Ba/Ta, La/Ta ratios compared to most of the other
MLBA samples (Figs. 5 and 6). These samples also
have some of the lowest incompatible trace element
concentrations of all MLBA lavas (Table 1) and, thus,
are most susceptible to contamination by a variety of
components derived from either subduction-related
processes (slab-derived fluids/melts, subducted sedi-
ments) or in situ assimilation-fractional crystallization
(AFC) within the Patagonian continental crust, or
both. Resolving these two processes is difficult, if
not impossible, with the present MLBA post-plateau
dataset. Circumstantial evidence for crustal AFC pro-
cesses comes from the fact that these lavas are not
primary mantle-derived magmas. They must have
suffered some crystal fractionation, most likely while
ponded in crustal magma chambers, and thus, could
have assimilated crustal material in the process. Pb
isotope data lends support for crustal AFC in that
these samples have distinctly higher 206Pb/204Pb ratios
(f 18.45) compared to most other MLBA post-pla-
teau lavas (Fig. 8). Pb isotope analyses of Middle
Jurassic Chon Aike rhyolite, crustal xenoliths from the
region (Gorring and Kay, 2001), and estimates of the
average Patagonian crustal composition (Kilian and
Behrmann, 1997) all have relatively high 206Pb/204Pb
(f 18.45–18.70; Fig. 8), and thus, crustal material
could be a viable endmember contaminant during
AFC processes assuming parental MLBA post-plateau
magmas had 206Pb/204Pb < 18.45. The low Ce/Pb ra-
tios could also be explained as continental crust nor-
mally has extremely low Ce/Pb ( < 4) and very high
concentrations of Pb relative to basaltic magmas
(Hofmann et al., 1986). Simple mass balance calcu-
lations indicate that samples with lowest Ce/Pb ratios
(AP-02s, 003d, and 005) could be explained by 10–
20% bulk assimilation of Patagonian crust (see curves
in Figs. 6 and 10).
The HFSE depletion, low Ce/Pb, and Pb isotope
data for MLBA lavas could equally be interpreted as
reflecting mantle source region contamination by
subducted sediment or a slab-derived fluid compo-
nent. Simple mass balance calculations indicate that
samples with lowest Ce/Pb ratios (AP-02s, 003d, and
005) could be explained by V 1% of bulk addition of
Chile Trench sediment (Kilian and Behrmann, 1997)
to the mantle source (see curves in Figs. 6 and 10).
Slab-derived fluids are also thought to be responsible
for HFSE depletion and enrichment in fluid mobile
elements like Ba, Sr, Cs, and Pb relative to LREE in
arc magmas (e.g., Plank and Langmuir, 1993; Wood-
head et al., 1997 and references therein). Thus, some
of the geochemical characteristics of these anomalous
MLBA lavas could be attributed to mantle source
contamination by subduction-related components
(subducted sediments and/or fluids) that were either
Fig. 10. 87Sr/86Sr versus Ce/Pb for MLBA post-plateau lavas
showing influence of EM1-type lithospheric mantle component in
most samples. Labeled samples are those referred to in text as
having evidence for EM2-type crustal AFC components. Data
fields, sources, and symbols as in Figs. 5 and 7. Field for ‘‘pristine’’
southern Patagonian slab window lavas are from Gorring and Kay,
(2001) and represent samples uncontaminated by arc-related and/or
crustal AFC processes. Mixing curves are as in Fig. 6.
M. Gorring et al. / Chemical Geology 193 (2003) 215–235 227
stored in the basal continental lithospheric mantle or
that had contaminated the supraslab asthenosphere, or
both.
Most MLBA post-plateau lavas have trace element
and isotope ratios well within the accepted ranges for
OIB (Figs. 4–8) and, thus, show little evidence for
significant amounts of subduction-related and/or con-
tinental crustal components. Their incompatible trace
element concentrations are generally equal to or much
higher than concentrations in average upper and lower
crust (e.g., Rudnick and Fountain, 1995), therefore,
trace element and isotopic signatures in MLBA post-
plateau lavas are largely buffered against the affects of
small to moderate amounts of crustal contamination
via assimilation-fractional crystallization (AFC) pro-
cesses. Furthermore, the common occurrence of peri-
dotite xenoliths and xenocrysts in these lavas indicates
rapid ascent with little time to interact with the
continental crust. In summary, the geochemical evi-
dence indicates that subduction-related and/or crustal
AFC processes are relatively minor and unlikely to be
responsible for the overall strong OIB-like, trace
element and isotope characteristics of MLBA basaltic
magmas. Instead, the geochemical variability that is
observed mostly reflects the characteristics of the
mantle source(s) and variations in degrees of partial
melting.
7.2. Mantle source signatures of MLBA post-plateau
lavas
The OIB-type chemical and isotopic characteristics
of MLBA post-plateau lavas, without significant crus-
tal and/or arc-related contamination, indicate that they
reflect mantle source signatures. As with all continen-
tal basalts, two mantle source components are possible,
namely the asthenosphere and the continental litho-
spheric mantle (CLM). Given the slab window tectonic
setting of MLBA post-plateau lavas, the asthenosphere
is thought to be the dominant mantle source compo-
nent (Thorkelson, 1996; Gorring et al., 1997). Recent
petrologic studies of slab window lavas further south in
the central Santa Cruz Province (Gorring and Kay,
2001) and in the PAVF (Stern et al., 1990; D’Orazio et
al., 2000) indicate that the southern Patagonian asthe-
nospheric mantle has relatively depleted, OIB-type iso-
tope characteristics (e.g., 87Sr/86Sr = 0.7032–0.7038;143Nd/144Nd = 0.5128–0.5129). In contrast, MLBA
post-plateau lavas generally have more enriched Sr
and Nd isotopic ratios and lower 206Pb/204Pb ratios
than slab window lavas from further south (Figs. 1, 7–
10). Therefore, unless the asthenospheric mantle
beneath southern Patagonia is extremely heterogene-
ous, a CLM component is considered to also have
played an important role in the petrogenesis of MLBA
post-plateau lavas.
The main evidence for an enriched CLM compo-
nent in MLBA post-plateau lavas comes from their
moderately high 87Sr/86Sr and low 143Nd/144Nd and206Pb/204Pb ratios, and strong OIB-like trace element
signatures (e.g. high Ce/Pb, low LILE/LREE and
LILE/HFSE). These chemical signatures have also
been recognized in other Neogene slab window lavas
from the northeastern sector of the Patagonian back-
arc (Gorring and Kay, 2001) and have been similarly
interpreted as having an important enriched CLM
component. Whether the enriched component is
EM1- or EM2-type is difficult to determine exactly.
Arrays on Sr–Nd and Sr–Pb isotope plots are
generally elongated toward an ill-defined area bet-
ween EM1 and EM2 with the samples that have
anomalously low Ce/Pb ratios controlling the appa-
rent elongation toward EM2 (Figs. 7 and 9). How-
ever, the majority of MLBA samples have OIB-like
Ce/Pb (>18), moderate 87Sr/86Sr (0.7040–0.7046)
and relatively low 206Pb/204Pb ratios that are defi-
nitely elongated in the direction of EM1 on Pb–Pb
and Sr–Pb isotope plots (Figs. 8 and 9). As men-
tioned before, this weak EM1-type signature is not
due to crustal AFC or source region contamination
as Patagonian crust and Chile Trench sediments have
higher 206Pb/204Pb ratios compared to all of the
MLBA lavas (see Fig. 8), and mass balance calcu-
lations show that significant crustal additions would
dramatically change certain key trace element ratios,
which is only observed for a few samples (see Figs.
6 and 10). Furthermore, neither source mixing with
Chile Trench sediments nor bulk assimilation with
average Patagonian continental crust can directly
reproduce the Sr–Nd–Pb isotope systematics of M-
LBA lavas (see curves 1 and 2 in Figs. 7 and 9),
unless (1) the endmember mantle source/parental
magmas had higher 87Sr/86Sr, lower 143Nd/144Nd,
and lower 206Pb/204Pb ratios, or (2) the petrogenesis
involves a two-step process of lithospheric EM1-type
contamination followed by crustal AFC processes.
M. Gorring et al. / Chemical Geology 193 (2003) 215–235228
Therefore, we interpret the isotope and trace element
characteristics to reflect the existence of a hetero-
geneous CLM beneath the MLBA that contains
volumetrically minor amounts of an EM2-type com-
ponent that is variably superimposed on a more
prevalent, but weak EM1-type component.
7.3. Origin of the EM-type mantle sources in
Patagonian CLM
The range of Sr–Nd–Pb isotope compositions
suggests that most MLBA post-plateau lavas resulted
from mixing and/or contamination processes requiring
the involvement of EM-type CLM components. The
relatively low 206Pb/204Pb and 143Nd/144Nd ratios of
EM1-type components require time-integrated low U/
Pb and Sm/Nd ratios in the source. Thus, the origin of
EM1-type signatures in continental intraplate basalts
(and OIB) is generally attributed to interaction with an
ancient, silicate melt and/or CO2-rich fluid metasom-
atized CLM or a lower crustal granulite assemblage
(e.g., Zindler and Hart, 1986; Carlson, 1995 and
references therein). Direct involvement of significant
amounts lower crustal contamination seems unlikely
based on arguments given above, thus the EM1
signature is most likely derived from metasomatized
CLM. The high 87Sr/86Sr and low 143Nd/144Nd ratios
of EM2-type components in continental intraplate
basalts is generally attributed to contamination with
upper continental crustal components either through
AFC processes in the crust or to the interaction with
subduction-modified CLM (e.g., Carlson, 1995).
The Patagonian continental lithosphere is relatively
young (V 0.5 Ma, see Section 2), and thus, EM-type
signatures (both EM1 and EM2) are not likely to be
extreme due to the lack of time-integrated growth of
daughter isotopes (Stern et al., 1989, 1990; Zartman et
al., 1991). This is generally supported by the relatively
depleted, but heterogeneous, Sr–Nd isotopic com-
positions (87Sr/86Sr = 0.7027–0.7043; 143Nd/144Nd =
0.51312–0.51279) of peridotite xenoliths from Pali
Aike (Stern et al., 1989, 1999) and from Estancia Lote
17 (Gorring andKay, 2000). These data suggest that the
Patagonian CLM was stabilized from melt-depleted,
MORB-source asthenosphere at some point in the early
Phanerozoic (Stern et al., 1989; Zartman et al., 1991).
Major geotectonic events that have likely affected
the chemical evolution of the southern Patagonian
CLM since stabilization beneath the continent include
the following: (1) the middle Jurassic Chon Aike
rhyolite event associated with Gondwana breakup
(Kay et al., 1989; Pankhurst et al., 1998), (2) exten-
sive Cenozoic plateau basalt magmatism (Ramos and
Kay, 1992), and (3) nearly continuous subduction
since the early Cretaceous (Bruce et al., 1991) and
episodic into the late Paleozoic (Forsythe, 1982;
Herve, 1988). Perhaps continuous metasomatism by
small-percentage melts of MORB-like asthenophere
over 500 Ma, coupled with periodic pulses of OIB-
type melts during Gondwana breakup and Cenozoic
plateau magmatism, could have produced the weak
EM1-type signatures in the Patagonian CLM. Phaner-
ozoic subduction-related processes could potentially
superimposed additional EM2-type heterogeneities
(subducted sediments, H2O-rich fluids) on the CLM,
but this component is difficult to distinguish from the
affects of in situ, crustal AFC.
8. Geodynamic implications
Gorring andKay (2001) proposed a four-component
dynamic slab window model to explain the incompat-
ible element and isotopic variations of Neogene plateau
lavas from central Santa Cruz. The four components
included the following: (1) a relatively depleted, OIB-
type subslab asthenosphere component, (2) a subduc-
tion-related component (e.g., slab-derived fluids and/or
melts) stored in either the basal continental lithosphere
or in the supraslab asthenosphere, (3) an enriched,
EM1-type CLM component, and (4) an upper crustal
component as an AFC-derived contaminant. The
model presented here is similar to, and lends support
for, Gorring and Kay (2001)’s model; however, the
main difference is thatMLBApost-plateau lavas have a
much larger contribution from the EM1-type CLM
component compared to Neoegene slab window lavas
from central Santa Cruz as well as those from the Plio–
Pleistocene Pali Aike field that have stronger astheno-
spheric signatures (Fig. 10).
Our preferred model for the petrogenesis of MLBA
post-plateau lavas and their relationship with regional
tectonic evolution is shown in Fig. 11. The geochem-
ical data presented here and available K–Ar and40Ar/39Ar dates strongly support a slab window origin
for theMLBApost-plateau lavas (e.g., Ramos andKay,
M. Gorring et al. / Chemical Geology 193 (2003) 215–235 229
1992; Gorring et al., 1997). This model involves two
mantle source components that can be explained as an
OIB-like subslab asthenosphere and an enriched CLM
component with ‘young’ EM1-type signatures. Magma
generation most likely occurred near the astheno-
sphere–lithosphere boundary as asthenosphere up-
wells through the slab window and impinged on the
base of the CLM. Based on the HREE depleted patterns
of MLBA lavas, the minimum depth to the astheno-
sphere–lithosphere boundary was at least within the
garnet stability field in excess of 65–70 km. In this
model, upwelling subslab asthenosphere supplies both
heat and magma, and thus, mixing at the base (or
within) the CLM is an in situ, binary process. Binary
mixing of asthenospheric melts with partial melts of
fertile portions of the CLM with EM1-type signatures
produces the range of compositions observed in the
MLBA post-plateau lavas. Mixing curves between
asthenospheric slab window magmas and EM1-type
lithospheric components in Figs. 7 and 9 and mass
balance calculations in Table 2 indicate roughly 10–
40% CLM component in MLBA post-plateau lavas.
This mixing could either have taken place fully within
the lowermost CLM as asthenospheric melts passed
though it, or by preferential melting and mixing of
thermo-mechanically eroded EM1-type ‘plums’ that
were intermingled in a ‘pudding’ of upwelling astheno-
sphere near the base of the CLM. In addition, several
MLBA magmas have acquired weak arc signatures
from further interactions with EM2-type components
derived from subducted sediments and/or slab-derived
fluids stored within the CLM and/or during AFC
processes in crust.
An important regional implication of this study for
Patagonian back-arc magmatism is the observation that
enriched EM1-type components appear geographically
restricted to basalts from the north and northeastern
sectors of the back-arc. Indeed, when one examines the
isotope data for all Neoegene slab window lavas from
46.8jS to 52jS, a systematic regional pattern emerges
with more depleted Sr and more radiogenic Pb isotope
signatures (87Sr/86Sr < 0.7038; 206Pb/204Pb>18.5) in
the south and southwestern part of the back-arc within
the Magallanes Basin, to gradually more enriched
EM1-type signatures (87Sr/86Sr>0.7038; 206Pb/204Pb <
18.5) in the north and northeast within or near the
Deseado Massif (see Fig. 1). We interpret this to reflect
important regional differences in the chemistry of the
Patagonian CLM that may not have been previously
recognized (Table 3).
The results of this study also suggest that astheno-
sphere–lithosphere interactions are an important pet-
rogenetic process for slab window lavas on a global
basis. In our model for MLBA, we envision that
pristine, subslab asthenosphere upwelling through
the slab window will not only partially melt, but also
causes thermo-mechanical erosion and thinning of the
CLM. This is physically analogous to extensional-
induced and plume-related interactions between these
two mantle reservoirs in the petrogenesis of continen-
Fig. 11. Schematic southern Patagonian lithospheric cross-section during the Plio–Pleistocene (1.8–0.1 Ma) opposite where ridge collision
occurred at 6 Ma (see Fig. 1) that shows the geodynamic model for MLBA post-plateau lavas (modified from Gorring et al., 1997; Gorring and
Kay, 2001). Upwelling of OIB-type subslab asthenosphere may have resulted in thermo-mechanical erosion of the basal Patagonian continental
lithosphere. This interaction between asthenosphere and continental lithosphere could explain the isotopic variability and distinctive EM1-type
isotopic signatures of MLBA post-plateau lavas. CC= continental crust; CLM= continental lithospheric mantle.
M. Gorring et al. / Chemical Geology 193 (2003) 215–235230
Table 2
Mixing models showing quantities of EM1-type lithospheric components required to produce trace element and isotopic compositon of MLBA post-plateau lavas
1 2 3 4 5 6 7 8 9 A B C D E
La 26 8.3 151.7 240 66 79 47 43 59 32 42 27 24 15
Nd 25.4 13.8 74.7 207 56 57 41 37 53 62 64 45 38 15
Sr 678 300 2119 1530 1097 1075 868 828 987 29 28 31 29 36
Pb 2.2 0.85 9.73 16 5.4 7.5 4.6 4.4 5 42 70 42 40 2087Sr/86Sr 0.7036 0.7036 0.705 0.705 0.704393 0.704448 0.704681 0.704743 0.704106 29 33 32 39 20143Nd/144Nd 0.51285 0.51285 0.51255 0.51255 0.512649 0.512667 0.512663 0.512676 0.512623 41 35 23 20 28206Pb/204Pb 18.8 18.8 17.8 17.8 18.23 18.127 18.221 18.267 23 32 11 14
Average 37 43 30 32 21
1 and 2 = 1% and 5% partial melt of 50% MORB–50% OIB asthenospheric source mantle, respectively (Gorring and Kay, 2001); 3 = 0.3% partial melt of an enriched lithospheric
mantle component; 4 = average lamproite (Bergman, 1987) with a ‘young’ EM1 isotope signature; 5 to 9 =MLBA post-plateau lavas (CV-02s, AT-01s, 017, LC-25, and 025,
respectively); A= percentage of (1) mixed with (3) to make (5); B = percentage of (1) mixed with (3) to make (6); C = percentage of (2) mixed with (3) to make (7); D = percentage of
(2) mixed with (3) to make (8); E = percentage of (1) mixed with (4) to make (9). All concentrations in 1–9 are in ppm.
M.Gorrin
get
al./Chem
icalGeology193(2003)215–235
231
tal intraplate basalts (e.g. McKenzie and Bickle, 1988;
Ellam and Cox, 1991; Arndt and Christensen, 1992;
Gallagher and Hawkesworth, 1992). Although slab
window tectonic settings are arguably the best places
on Earth to investigate the chemistry of the astheno-
spheric mantle beneath active continental margins,
results of this study emphasizes the need to exercise
caution when interpreting the OIB-like signatures in
slab window lavas as entirely asthenosphere-derived.
9. Conclusions
Abundant (f 600 km3) Plio–Pleistocene post-pla-
teau mafic magmatism in the MLBA region is linked
to ridge collision along the Andean margin f 6 Ma
ago. Most of the lavas are relatively primitive (6–10
wt.% MgO; Ni>80 ppm and Cr>150 ppm), highly
alkaline (5–8 wt.% total alkalis; < 48% SiO2) basan-
ites, hawaiites, and alkali basalts. Their incompatible
trace element characteristics reveal strong intraplate,
OIB-like signatures with little evidence for the influ-
ence of arc and/or crustal components. REE patterns
are all LREE-enriched with high LREE/HREE ratios
(La/Yb = 11–54) and buffered HREE concentrations
at f 6–8� chondrite. These data indicate variable
extents of partial melting (f 1–5%) of a OIB-like,
LREE-enriched mantle source at depths within the
garnet stability field (>65–70 km).
MLBA post-plateau lavas have variably enriched
Sr–Nd isotopes (87Sr/86Sr = 0.7041–0.7049; 143Nd/144Nd = 0.51264–0.51279) and relatively low 206Pb/204Pb ratios (18.13–18.45). Two mantle components
are recognized as major potential sources: a relatively
depleted OIB-like subslab asthenosphere, and an
enriched CLM component with EM1-type signatures.
The isotopic variability of most MLBA lavas can be
considered in terms of mixing of these two compo-
nents in variable proportions. The origin of EM1-type
components in the Patagonian lithosphere is tenta-
tively attributed to continuous basaltic melt metaso-
matism over the last 500 Ma, with enhanced meta-
somatism during the Mesozoic Gondwana breakup
and Cenozoic basaltic plateau magmatism. The EM1-
type signatures of MLBA post-plateau lavas are dis-
tinctive from most other Late Cenozoic Patagonian
slab window lavas from further south that have more
depleted, OIB-type isotope characteristics. This is
interpreted here to reflect an important regional
heterogeneity in the CLM that influences the chem-
istry of southern Patagonian slab window basaltic
magmas. The results of this study indicate that the
interaction of upwelling, hot subslab asthenosphere
with the basal CLM may be an important (and
commonly overlooked) geodynamic process in slab
window tectonic settings that can significantly mod-
ify the asthenospheric chemical signatures of slab
window magmas.
Acknowledgements
The authors would like to give special thanks to
Coco and Petty Nauta at the Estancia Telken for their
gracious hospitality and their invaluable logistical
support in the field. Special recognition also goes out
to Victor Ramos for his expert knowledge of the
regional geologic framework. Assistance in the
laboratory, particularly the efforts of Linda Godfrey
on the TIMS and ICP-MS, Jeremy Delaney (Rutgers)
and John Hunt (Cornell) on the electron microprobes,
and Bob Kay with INAA, are greatly appreciated. The
manuscript was greatly improved by constructive
comments and reviews by C. Hawkesworth and two
anonymous reviewers. This research was generously
supported by grants from Montclair State University
to MLG, NEGSA undergraduate research grant to JG,
Table 3
Chemical compositions of MLBA magmas and crustal components
used to model crustal contamination
1 2 3 4
La 0.481 26 21.5 24.6
Ce 1.337 55.9 47.3 52.9
Nd 1.12 25.4 22.6 28.6
Sr 20.6 678 305 173
Pb 0.05 2.2 13.3 11.1
Th 0.05 5.3 6.9 8.4
U 0.016 1.6 1.9 1.5
Nb 0.65 51.5 9.1 14.387Sr/86Sr 0.7036 0.7036 0.7077 0.72184143Nd/144Nd 0.51285 0.51285 0.51253 0.51232206Pb/204Pb 18.8 18.8 18.65 18.67
1 and 2 = 50% MORB–50% OIB asthenospheric source mantle
and 1% partial melt of this source mantle, respectively (Gorring
and Kay, 2001); 3 and 4 = average of Chile Trench sediments from
ODP Leg 141 and estimate of S-type Patagonian crust, respectively
(Kilian and Behrmann, 1997). All concentrations in ppm.
M. Gorring et al. / Chemical Geology 193 (2003) 215–235232
NSF grant EAR-9219328 to SMK, and Swiss NSF
grant 21-43077.95 to BS. [CA]
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