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: gorringm@mail.montclair.edu (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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