Download Mantle-drip magmatism beneath the Altiplano

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Mantle-drip magmatism beneath the Altiplano-Puna plateau, central
Andes
M.N. Ducea1,2, A.C. Seclaman1.2, K.E. Murray 2, D. Jianu1, and L.M. Schoenbohm3
1
Universitatea Bucuresti, Facultatea de Geologie-Geofizica, Bucharest 010041, Romania
University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA
3
University of Toronto, Department of Geology, Toronto, ON M5S 3B1, Canada
2
INTRODUCTION
The constant recycling of continental lithosphere into the convective mantle is required
by observations of mass and chemical imbalance in overriding plates at convergent margins
(Rudnick, 1995; DeCelles et al., 2009). Lithospheric mantle mass should more than double
in thickness at convergent margins, but this
is not observed seismically (Wernicke et al.,
1996). Similarly, continental crust generated
at orogenic margins needs a complementary
ultramafic (but not peridotitic) lithospheric reservoir (Lee et al., 2006), which is not known to
exist under most continental regions on Earth.
Seismic and other geophysical techniques
(Zandt et al., 2004; Park, 2004) provide observations of significant, tens-of-kilometers-scale
compositional and thermal heterogeneities
in the upper mantle beneath these regions.
These results fuel the hypothesis that parts of
the continental lithosphere at or near orogenic
areas are being convectively removed (i.e.,
dripped) from the overriding plate, a process
also known as delamination or foundering
(Kay and Kay, 1993; Kay et al., 1994; Ducea,
2011). This convective overturn of the upper
mantle should preferentially melt the most fertile, significantly heated and/or greatly decompressed parts of the local mantle. Here we test
the hypothesis of Elkins-Tanton (2007) and
propose that pyroxenites representing the roots
of magmatic arcs are dripping off the overriding plate and provide the major source of these
mantle-drip magmas, using newly acquired
as well as previously published data from the
Altiplano-Puna plateau in the central Andes.
We focus on this region because it is a classic area for which convective removal has been
hypothesized in the past (Kay et al., 1994). We
propose that convective removal can be elsewhere tested using the combination of petrologic tools described here.
BACKGROUND AND HYPOTHESIS
Convective removal is testable through thermal and compositional variations in magmatism
through time (Kay and Kay, 1993). In the classic hypothesis (Kay et al., 1994), a lithospheric
dripping event triggers adiabatic upwelling of
asthenospheric mantle, producing basaltic partial melts. Such syn-drip mantle melts should be
predictably different from local magmatic arc
products erupted prior to dripping, and thereby
identifiable geochemically and/or isotopically
as rocks generated by a source typical of the
convective mantle. However, where regional
geochemical shifts have been observed in areas
suspected of delamination (Manthei et al., 2010;
Gutierrez-Alonso et al., 2011; Putirka and Platt,
2012) those shifts to a more primitive asthenospheric source took place over time scales of
tens of millions of years (15–30 m.y.), which
is interesting because the individual foundering
events occur at ~1 m.y. time scales. Even more
puzzling, areas with evidence for recent dripping, e.g., the southern Sierra Nevada (Zandt
et al., 2004), display predominantly upperplate–derived magmatism (mantle lithosphere
and/or crust) at the time of dripping (Farmer
et al., 2002) and not the expected primitive,
asthenospheric-derived basalts. In addition, this
magmatism is volumetrically insignificant and
GEOLOGY, August 2013; v. 41; no. 8; p. 1–4; Data Repository item 2013255 doi:10.1130/G34509.1
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characterized by relatively low temperatures
(<1350 °C; Ducea and Saleeby, 1998).
More sophisticated models for drip melting predict that the downgoing lithosphere can
undergo wet partial melting or simply melt as
more fusible components, notably pyroxenites,
heat up as the drip descends (Elkins-Tanton,
2007). This is particularly likely if drips are
small enough (~20–50 km diameter) to undergo
relatively fast conductive heating. Indeed, if
amphibole or phlogopite are present, partial
melting of pyroxenites at pressures of 2–4 GPa
due to heating and/or dehydration melting is
predicted in such drips, which need ~1 m.y. to
sink through >50 km of mantle (Fig. 1). Pyroxenites (with or without garnet and/or amphibole
and phlogopite) are common in the lowermost
crust/mantle lithosphere under continents (Herzberg et al., 1983; Wilshire et al., 1988) and form
either as veins of melts frozen in the mantle or
as cumulates/residues of ancient magmatism,
notably during arc magmatism (Girardi et al.,
2012; Ducea and Saleeby, 1998; Jull and Kelemen, 2001). Most pyroxenites are denser than
average peridotite mantle (Hacker and Abers,
2003) and therefore their presence in dense
regions of lithosphere capable of foundering
T (°C)
1100
1300
1500
1
P (GPa)
ABSTRACT
Convective removal of continental lithospheric roots has been postulated to be the primary mechanism of recycling lithospheric mass into the asthenosphere under large plateaux
such as the Altiplano-Puna in the central Andes. Convective instabilities are especially likely
to develop where there is extensive intermediate arc-like magmatism in the upper plate, as
the residual masses complementing these magmatic products are typically denser than the
underlying mantle. Mafic volcanic rocks erupted on the central Andean Altiplano-Puna plateau during the past 25 m.y. contain evidence of this process. Here we use equilibration temperatures, age data, and geochemical constraints––primarily based on transition metals––to
show that the most important source materials by mass for this mantle-derived magmatism
are pyroxenites from the lower parts of the lithosphere, with only minor contributions from
mantle peridotite. Pyroxenites are denser than typical upper mantle whether they are garnet
bearing or not, and are therefore likely to contribute to destabilizing parts of the continental lithosphere. The pattern of melting is consistent with the process of foundering/dripping
of small-scale (<50 km diameter) density anomalies in the lithosphere, where mafic volcanic
fields on the plateau represent the manifestations of individual drips.
2
LAB
PXT
3
PER
F=0.01
F=0.05
F=0.10
Dry peridotite solidus
Dry pyroxenite solidus
Figure 1. Pressure-temperature (P-T ) diagram illustrating the drip-melting hypothesis put forward in this paper. F—fraction of
melt (e.g., 0.1 = 10% melt); PXT—hypothetical downward P-T path for a pyroxenite drip;
PER—adiabatic upwelling path for mantle
peridotite replacing drip; LAB—lithosphereasthenosphere boundary.
Published online XX Month 2013
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DATA SET
We compiled previously published data for
the Altiplano (Davidson and deSilva, 1995;
Knox et al., 1989; Redwood and Rice, 1997)
and the Puna (Kay et al., 1994; Drew et al.,
2009; Risse et al., 2008) and added newly
acquired data from Puna volcanics (see below)
to use in our analysis. Individual volcanic fields
from the Puna for which we present new data
here are Pasto Ventura, Antofagasta–Hombre
Muerto, Arizaro, and Chorillos (see Fig. DR1 in
the GSA Data Repository1).
SAMPLES AND GEOCHEMICAL
RESULTS
Twenty-six (26) new lavas from the central
and southern Puna (sample descriptions and
locations are provided in the Data Repository)
were analyzed for major and trace elements in
this study. In addition, olivine phenocrysts and
surrounding glass or aphanitic groundmass
were analyzed for major element chemistry
by electron microprobe (see the Data Repository for all analytical techniques). Among new
1
GSA Data Repository item 2013255, analytical
techniques, sample location and petrographic description, olivine microprobe data and thermometry, major
and trace element geochemistry of new samples, and
a summary of new and existing geochemical and geochronologic data of mafic rocks from the AltiplanoPuna region, is available online at www.geosociety
.org/pubs/ft2013.htm, or on request from editing@
geosociety.org or Documents Secretary, GSA, P.O. Box
9140, Boulder, CO 80301, USA.
2
70°W
68°W
66°W
64°W
Or
Figure 2. Location map
of volcanic fields analyzed in this study, using
new and previously published data. Or—Oruro
field (Redwood and Rice,
1997); Sa—Sajama (Davidson and deSilva, 1995);
Oll—Ollague (Davidson
and deSilva, 1995); CA,
EA—central and eastern Altiplano (Davidson
and deSilva, 1995); Az—
Arizaro; Ch—Chorillos
(and other) Northern Puna
shoshonites; A-HM—Antofagasta–Hombre Muerto;
PV—Pasto Ventura; Af—
Antofalla. Gray box outlines area for which we
provide a more-detailed
geologic map, with Puna
sample locations, in the
Data Repository (see footnote 1).
18°S
Peru
Sa
A
ALTIPLANO
CA
20°S
22°S
Pacific Ocean
is highly probable, if not required. Pyroxenite
solidi are ~150 °C lower than average peridotite
solidi at upper-mantle pressures (Hirschmann
and Stolper, 1996), and therefore pyroxenitedominated drips would be more likely than
upwelling peridotitic asthenosphere to melt during a foundering event.
A key prediction of this scenario is that progressively hotter melts would form as dripping
progresses. In contrast, if pyroxenites melt due
to adiabatic decompression, their melts have
progressively lower temperatures due to ascent
along a partially molten adiabat, and never
higher temperatures (Fig. 1). As asthenospheric
mantle rises to replace the drip, peridotite partial
melting could also occur at the shallow end of
the adiabat. Qualitatively, this hypothesis predicts that peridotite melts would form late in
the process, and the extent of peridotite melting
would depend on the potential temperature of
the local asthenosphere, presence of water, etc.
Mixing melts of these two unrelated sources are
possible, with pyroxenites dominating the earliest melts. We tested this hypothesis (ElkinsTanton, 2007) on Neogene mantle-derived mafic
rocks from a classic area proposed to have been
subject to dripping in the recent geologic past:
the Altiplano-Puna plateau in the central Andes
(Kay et al., 1994; Garzione et al., 2006) (Fig. 2).
EA
Salar
de Uyuni
Oll
Bolivia
Argentina
Chile
24°S
Az
Ch
PUNA
26°S
Af
A-HM
N
PV
0
200
km
28°S
and previously published data, we use the most
primitive, potentially mantle-derived materials
(MgO > 7%, high Ni and Cr concentrations).
Chemical and isotopic fingerprints in these
mafic volcanics––in addition to the relatively
thin lithosphere and high elevation of the modern Altiplano-Puna (Whitman et al., 1996)––
have been used to suggest large-scale foundering of the sub-Puna root (Kay et al., 1994). Most
of these rocks are basaltic andesites and a few
true basalts that erupted within the back arc of
the central Andes in several distinct fields with
lifetimes of 1–5 m.y.
Zn/Fe ratios for the majority of the selected
rocks are greater than those typical of peridotites, supporting the idea that pyroxenites dominated the sources of these melts. Zn/Fe (Le
Roux et al., 2010) and other first-row transition
elemental ratios (Le Roux et al., 2011) are a
promising tool in separating peridotite versus
pyroxenite melts; peridotite-derived melts have
a ratio less than 12, whereas pyroxenite melts
have a ratio between 13 and 20. Le Roux et al.
(2010) demonstrated that Zn/Fe is not fractionated during peridotite melting or olivine-dominated differentiation, but is highly fractionated
if garnet and/or clinopyroxene are involved in
melting, thus effectively making this ratio a
tracer for pyroxenite versus peridotite source
materials. Zn/Fe ratios are inversely correlated
with temperature for the Altiplano-Puna (Fig.
3A). In the Pasto Ventura field, where we have
100
geochemistry and age data, these ratios decrease
with time suggesting that in that region early
pyroxenite-derived melts were progressively
mixed with peridotite melts. Other first-row
transition elemental ratios, such as Mn/Fe and
Ni/Co, are consistent with Zn/Fe in that they
require derivation of the majority of Puna volcanic rocks discussed here from pyroxenites
(e.g., correlation between Zn/Fe and Mn/Zn;
Fig. 3B). However, there is no obvious correlation between Zn/Fe and MgO (Fig. 3C) or Mg#
within individual volcanic fields, bolstering
the idea that conventional parameters used in
assessing the primitive nature of a basalt cannot
distinguish between peridotite and pyroxenite
melting. Future studies aimed at deciphering
between peridotite versus pyroxenite sources
could incorporate mineral chemistry data on
olivine phenocrysts—specifically, olivine that
crystallizes from magmas of a pyroxenite source
are generally high in Ni, low in Ca and Mn, and
high in Fe/Mn (Herzberg, 2011).
THERMOMETRY
We calculated temperatures for individual
eruptions with the magnesium and silica-activity thermometer, which uses bulk rock chemistry (Lee et al., 2009) and assumes that mafic
melts, whatever their ultramafic source, equilibrated with an olivine-rich mantle. The olivineglass thermometer (Putirka, 2008) was then
used as an independent check of equilibration
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Oruro
Arizaro
Antofagasta
Pasto Ventura
Antofalla
Pyroxenite
10000*Zn/Fe
15
14
13
12
Peridotite
11
A
10
1100
1200
1300
1400
1500
T ( 0C)
16
Oruro
Arizaro
Antofagasta
Pasto Ventura
Antofalla
Pyroxenite
10000*Zn/Fe
15
14
13
12
Peridotite
B
11
10
9
11
13
15
17
19
Mn/Zn
16
10000*Zn/Fe
15
C
14
13
12
11
Oruro
Arizaro
Antofagasta
Pasto Ventura
Antofalla
10 6
7
8
9
10
11
MgO
Figure 3. A: Correlation of Zn/Fe in AltiplanoPuna (central Andes) rocks with temperature.
B: Mn/Zn versus Zn/Fe in Altiplano-Puna
rocks. Both diagrams show mix in transition
metals between pyroxenite and peridotite
source. C: Zn/Fe plotted against MgO showing no correlation within individual fields.
temperatures, because the former thermometer
assumes equilibration of the liquids with an
olivine-rich mantle, which may have taken place
after extraction of these melts from pyroxenite
sources. The olivine-glass thermometer, where
applicable, yielded results within the error of the
silica activity calculations under the assumption
of 1% water in the source (see the Data Repository). The strong correlation between these two
independent tools for measuring temperatures
suggests that the magmas investigated here
were in fact in equilibrium with an olivine-rich
mantle even though they were derived from
pyroxenite sources.
The majority of calculated equilibration
temperatures for the primitive Puna lavas are
between 1200 and 1300 °C, which is within the
range of sub-arc mantle-derived melts. If only
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peridotites were partially melting in the sub-plateau mantle, these temperatures would require
unusually low pressures of melting (~1 GPa,
equivalent to 35 km deep; Fig. 1) given that the
crust thickness is variable (from ~42 to 80 km)
under the plateau (Whitman et al., 1996). If on
the other hand pyroxenite sources were involved
in melting, as the transition metals suggest,
melting would take place within a more realistic depth interval, because pyroxenites melt
15–30 km deeper than peridotites along a given
adiabat. Even with pyroxenites involved in
melting, it is quite clear that the melting only
took place beneath the areas with the thinnest
present crust and that average melting depths
were within 5–10 km of the Moho. The silica
activity barometer of Putirka (2008), which can
be used for pyroxenite sources as well as peridotite sources, suggests melting depths of 2–3
GPa, consistent with the above interpretations.
More importantly, eruption ages inversely correlate with equilibration temperatures (Fig. 4);
temperature increase is as much as 130 °C in the
1 m.y. lifespan of the Pasto Ventura field. This
critical observation is consistent with the dripping scenario described above, in which melts
are sourced from the downgoing lithosphere and
not from the upwelling asthenosphere.
PYROXENITE ORIGIN
Pyroxenites inferred to have sourced the plateau magmas described here were most likely
clinopyroxenites with or without garnet, which
typically complement large arc systems at depth
(Ducea and Saleeby, 1998). Rare earth element
(REE) patterns suggest that garnet may have
been present in the pyroxenites sourcing Chorillos and other northern Puna shoshonites due
to significant enrichment of light REEs over the
heavy ones (e.g., high La/Yb ratio), which correlates with K2O concentration; its presence is
plausible but not required by REE concentrations in the other fields discussed here. The presence of garnet in the deep lithosphere, probably
in pyroxenites, is also implied by the patterns
of trace elemental concentrations of voluminous intermediate volcanic rocks (andesites and
dacites) of similar ages found in the vicinity of
the studied mafic fields (Kay et al., 1994). The
pyroxenite end member has pronounced “arclike” elemental signatures, including the depletion of high field strength elements (not pictured); these characteristics are less pronounced
in the peridotite-dominated melts. We suggest
that previously recognized differences between
arc-like and oceanic island basalt–like reservoir signatures in Puna mafic lavas (Kay et al.,
1994) are indeed related to delamination but
fundamentally reflect the difference in elemental chemistry between pyroxenite and peridotite
sources. Radiogenic isotopic fingerprints clearly
show that multiple sources are involved in the
generation of the magmatic rocks described here
1350
T (°C)
16
Antofagasta
Pasto Ventura
CE Altiplano
1300
1200
0
Age (Ma)
1.7
Figure 4. Age versus temperature relationships for the Antofagasta–Hombre Muerto,
Pasto Ventura, and Central–Eastern (CE)
Altiplano volcanic fields. 2σ error bars are
shown for temperature estimates; age errors are variable, but less than the size of
the symbols in figure.
(Drew et al., 2009) and that the great majority
of these rocks have enriched mantle signatures,
consistent with the model presented here.
CONCLUSIONS
These results indicate that mantle partial
melting was driven by localized instabilities
of small lithospheric roots under the AltiplanoPuna. We propose that they are (1) driven by
local density anomalies produced by the accumulation of arc cumulates/residues, which
themselves are the major source mantle-drip
melts, (2) taking place at the scale of a few tens
of kilometers, and (3) countered by ascent of
peridotite diapirs that experience only limited
partial melting. The combination of pyroxenite/
peridotite melting with the increase in melting
temperature within a volcanic field over short
time scales (~1–5 m.y.) should be found in other
areas experiencing removal of arc roots and we
suggest that it represents a magmatic test of convective removal in arc regions.
ACKNOWLEDGMENTS
This work was funded by U.S. National Science
Foundation Tectonics grant EAR-0910941, Romanian National Sciences CNCSIS grant PN-II-ID-PCE
201/2011, and a research grant from Exxon-Mobil
to the University of Arizona. We thank reviewers
K. Putirka, C. Herzberg, and B. Jicha for significantly
improving the quality of the manuscript via their
thorough and constructive reviews.
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Manuscript received 21 February 2013
Revised manuscript received 2 April 2013
Manuscript accepted 8 April 2013
Printed in USA
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GEOLOGY