Beneath the Paraná Basin, Eastern Paraguay:
Fact or Fiction?
Victor Fernandez Velázquez3
& Celso de Barro Gomes3
di Ingegneria Chimica, dell'Ambiente e delle Materie
Prime. Trieste University. Piazzale Europa, 1. I-34127,
Trieste, Italy. firstname.lastname@example.org
Astrónomico e Geofisico, Universidade de São
Paulo, Rua do Matão 1226, Citade Universitaria.
05508-900 São Paulo, SP, Brazil.
de Geociências, São Paulo University.
Rua do Lago 562, CEP 05508-900, São Paulo,
SP, Brazil. email@example.com
Early Cretaceous to
Tertiary alkaline magmatism in Eastern Paraguay predates
and postdates the tholeiites of the Paraná-Etendeca
system. In South America, platform alkaline and alkaline-carbonatitic
magmatism occur along the main tectonic lineaments.
A similar situation is recognized in Angola and Namibia
in Africa. Sr-Nd systematics reveal heterogeneous mixtures
between a depleted mantle component and EMI, both for
the alkaline rocks and the flood tholeiites of the Paraná-Angola-Namibia
Province. Taken together, the data indicate that the
alkaline-carbonatitic magmatism originated from subcontinental
mantle that is heterogeneous on a small scale. In this
scenario, the Walvis Ridge – Rio Grande Rise and
Vitória – Trindade hotspot tracks might
reflect the accommodation of stresses in the lithosphere
during rifting, rather than continuous magmatism caused
by mantle plumes beneath the moving lithosphere (see
also the South Atlantic
and Africa pages).
magmatic events took place in Eastern Paraguay
subsequent to the Early Cretaceous (Figures
1-3; Comin-Chiaramonti et al., 1999),
i.e. alkaline K-magmatism (at ~ 138
and 128 Ma), tholeiitic flood basalts and
andesite basalts (132-133 Ma, both high- and
low-Ti variants) and Na-alkaline plugs and
lavas (118 and 60-50 Ma). The Sr-Nd isotopic
data indicate that two main mantle components
could have been involved in the genesis of
the Cretaceous to Tertiary magmatism in Eastern
- an extreme, heterogeneous
EMI component, which was prevalent in the
Cretaceous alkaline magmatism, and
- a depleted component,
which became more important in the Cretaceous
and Tertiary sodic magmatism (Comin-Chiaramonti
et al., 1997).
Figure 1: Political
map of South America
Figure 2: Map of Paraguay.
The potassic rocks form a compositional
continuum from moderately to strongly potassic,
i.e. the alkali basalt-trachyte and basanite-peralkaline
phonolite suite. The sodic rocks include mainly
ankaratrites, nephelinites and phonolites.
Two similar but distinct parental magmas emerge
for the potassic suites (pre- and post tholeiitic),
both characterized by strongly fractionated
rare-earth element (REE) and negative “Ta-Nb-Ti
anomalies”. In contrast, small positive
anomalies for Ta-Nb and incompatible element
patterns similar to those of the Tristan da
Cunha magmatism are observed in the Na-alkaline
rocks (Figure 4). Sr-Nd isotope data confirm
the distinction of the potassic rocks, enriched
in radiogenic Sr and low in radiogenic Nd,
from the sodic rocks, close to Bulk Earth
(BE) and transitional to the Paraná
flood tholeiites (Figure 5).
geochemical signatures of the Na-alkaline magmatism
from Paraguay show surprising similarities both
with the Tristan da Cunha and Trindade volcanics
(cf Figures 4 and 5). Crustal contamination
does not appear to have been significant in
the generation of the rocks investigated. The
close association of potassic and sodic suites
in Eastern Paraguay demands that their parental
magmas derived from a small subcontinental mantle
mass, vertically and laterally heterogeneous
in composition, and variously enriched in incompatible
elements. Significant H2O, CO2
and F are also expected in the mantle source
from the occurrence of carbonatites associated
with the K-alkaline complexes.
Crucial to understanding
the genesis of magmas from Eastern Paraguay is the
link with the geodynamic processes which led to the
opening of the South Atlantic. According to Nürnberg
& Müller (1991) sea-floor spreading
in the South-Atlantic at the latitude of Paraná
Basin initiated at ~ 125-127 Ma (chron M4). North
of the Walvis-Rio Grande ridges (latitude < 28°S)
the onset of oceanic crust formation was younger (~
113 Ma; Chang et al., 1988). In general,
the potassic magmatism of central Eastern Paraguay
(the Asunción-Sapucai-Villarrica graben) province
is subcoeval with the flood tholeiites of the Paraná
Basin and, therefore, occurred during the early stages
of rifting, before continental separation. The timing
of the scarce sodic magmatism, however, approaches
that of sea-floor spreading (San Juan Bautista, 118
Ma) or corresponds to an advanced stage of continental
separation (Asunción Province, 60-50 Ma). The
time of the most significant cooling event associated
with the Eastern Paraguay igneous activity occurred
in the Late Cretaceous (80-90 Ma) and overprinted
an older thermal perturbation, probably of Early Cretaceous
age (Hegarty et al., 1996).
Figure 3. Geological
sketch-map showing the distribution of post-Palaeozoic
magmatism in Eastern Paraguay, modified from Comin-Chiaramonti
(1999). 1: Quaternary-Tertiary sedimentary cover;
2: Tertiary sodic alkaline rocks; 3: Late Early Cretaceous
sodic alkaline rocks (San Juan Bautista, SJB); 4:
Early Cretaceous potassic alkaline rocks (post-tholeiites);
5: Early Cretaceous tholeiites of the Paraná
Basin; 6: Early Cretaceous potassic alkaline rocks
(pre-tholeiites); 7: Jurassic Cretaceous sediments
(Misiones Formation); 8: Permian sediments (Independencia
Group); 9: Permo-Carboniferous sedimentas (Coronel
Oviedo Group); 10: Ordovician-Silurian sediments (Caacupé
and Itacurubí Groups); 11: Cambro-Ordovician
platform carbonatic platform (Itacupumí Group);
12: Archean to Neo-Proterozoic crystalline basement:
high- to low-grade metasedimentary rocks, metarhyolites
and granitic intrusions; 13: faults. Inset: Distribution
of the magmatism in the Paraná-Angola-Namibia
Province (South American and African plates, Western
Gondwana, arranged at about 110 Ma; modified from
Comin-Chiaramonti et al., 1997, 1999) and location
of the main alkaline occurrences; sky blue: Early
Cretaceous tholeiitic magmatism.
The alkaline magmatism of Eastern Paraguay
appears to have derived from a lithospheric source,
probably comprising phlogopite-bearing garnet peridotites
(Comin-Chiaramonti et al., 1997). Also, the
alkaline magmatism of Eastern Paraguay and SE-Brazil
(Morbidelli et al., 1995), as well as that
of the Paraná flood tholeiites, was not associated
with important lithospheric extension (Piccirillo
& Melfi, 1988; Peate & Hawkesworth,
1996). Therefore, we support the view that the melting
process was mainly due to heat released by anomalously
hot asthenospheric mantle.
It has been suggested that the Early
Cretaceous alkaline magmatism may be related to a
Mesozoic mantle plume (e.g., Tristan da Cunha;
Milner & LeRoex, 1996). The heat released
from such a plume would have partially melted the
overlying lithospheric mantle without appreciable
contribution of plume components. It has further been
suggested that the peripheral distribution of the
alkaline magmatism with respect to the Paraná
Basin reflects association with the cooler margins
of the plume system, while the subcoeval tholeiites
reflect a hotter regime, corresponding to the inner
plume region. It should be noted that Early Cretaceous
alkaline magmatism is volumetrically minor, relative
to Late Cretaceous magmatism, and is concentrated
in the central Paraná Basin, i.e.
Eastern Paraguay, the Ponta Grossa Arch and the Moçamedes
arch of Angola (Alberti et al., 1999). This
contrasts with the alkaline magmatism of Late Cretaceous
age which is voluminous and mostly concentrated in
the northern Paraná basin (see inset, Figure
3). According to Gibson et al. (1995a), the
Alto Paranaíba alkaline magmatism of Late Cretaceous
age is related to lithospheric mantle source(s) enriched
by small-volume melts subsequent to the Late Proterozoic.
Mantle melting would have occurred at ~ 85 Ma as a
result of heat from a 1000-km-wide “plume head”
presently under Trindade in the Caribbean. This plume
model does not include the most alkaline rocks from
Serra do Mar, Ponta Grossa-Ipanema and Lages, where
several occurrences are subcoeval with those from
Alto Paranaíba (Morbidelli et al.,
1995; Comin-Chiaramonti & Gomes, 1996).
Figure 2. Primordial
mantle (Sun & McDonough, 1989) normalized incompatible
elements of Early Cretaceous to Tertiary magmatic
rocks from Eastern Paraguay. A: pre- and post-tholeiitic
potassic rocks; B: low- and high-Ti Early Cretaceous
tholeiites of the Serra Geral Formation; C: Cretaceous
(Misiones) and Tertiary (Asunción) sodic rocks.
Data sources: Comin-Chiaramonti et al. (1997, 1999
and unpublished data); for Tristan da Cunha: Le Roex
et al. (1990), Le Roex & Lanyon (1998); for Trindade:
Marques et al. (1999).
Even assuming that the Trindade plume
was smaller and cooler than the Tristan da Cunha plume
(which is thought to be ~ 2,000 km wide), it seems
reasonable to expect that an appreciable volume of
tholeiitic basalts, associated with alkaline rocks,
would be generated in its inner and hotter (~ 1380°C?)
portions. However, Late Cretaceous tholeiitic basalts
are virtually absent, lithospheric extension was very
small (beta ~ 1.05-1.1; Chang et al., 1992)
and lithospheric thickness is considerable (> 130
km; James et al., 1993). Therefore, we favour
a hydrous mantle source or sources at about normal
mantle potential temperature (~ 1280°C) to generate
a relatively small fraction of melt (producing ~ 0.6
km for a mechanical boundary layer ~ 150 km thick;
Gallagher & Hawkesworth, 1994), solely
from the lithospheric mantle. As Paraná magmatism
probably migrated east and north (Raposo &
Ernesto, 1995; Renne et al., 1996b),
and not northwest to southeast (Turner et al.,
1994), we suggest that the Late Cretaceous alkaline
magmatism may be related to the same anomalous thermal
regime responsible for the Paraná tholeiites.
The alkaline provinces, which developed from the cooler
margins of that thermal centre, are considered to
be derived from relatively low degrees of melting
of lithospheric mantle. Note that over 20-40 Myr were
required to create, from the lithospheric mantle only,
~ 30-50% of the total melt production with a steady-state
potential temperature lower than 1480°C (Hawkesworth
et al., 1992). We also suggest that the concentration
of Paraná Late Cretaceous alkaline magmatism
along the margin of the São Francisco craton,
Ponta Grossa arch and the Serra do Mar, probably reflects,
at least in part, tectonic control of the basement
(Santero et al., 1988).
5. A. Initial 87Sr/86Sr
(Sri) vs initial 144Nd/143Nd
(Ndi) correlation diagrams; TR, Trindade, TdC,
Tristan da Cunha (Siebel, et al., 2000; Le Roex,
1985, Le Roex et al., 1990). DMM, HIMU, EMI
and EMII are approximations of mantle end-members
taken from Hart et al. (1986, 1992). Data source:
Comin-Chiaramonti et al. (1991, 1996, 1997,
2002), Comin-Chiaramonti & Gomes (1996),
Castorina et al. (1997), Marques et al. (1999).
B. Comparison of Eastern Paraguay with Angola
and Namibia. Data sources: Milner & Le Roex
(1996), Le Roex & Lanyon (1998), Gibson
et al. (1999), Harris et al. (1999), Ewart et
al. (1998), Cooper & Reid (1998), Smithies
& Marsh, 1998, Alberti et al. (1999), Kurzslaukis
et al. (1999). Etendeka: not contamiated samples.
C. Comparison of Eastern Paraguay with Brazilian
("uncontaminated" tholeiites) and
oceanic occurrences. Brazil data sources: Toyoda
et al. (1995), Huang et al. (1995), Walter et
al. (1995), Garda et al. (1995), Marques et
al. (1999), Gibson et al. (1999), Andrade et
al. (1999), Comin-Chiaramonti et al. (2002),
Ruberti et al. (2002). Atlantic Ocean: Walvis
Ridge, Richardson et al. (1982); Rio Grande
Rise, Gamboa & Rabinowitz (1984); Mid Atlantic
Ridge (MAR), Ito et al. (1987), Fontignie &
Schilling (1997); OIB, Halliday et al. (1988;
1995).The Rio Grande Rise basalts (not shown)
plot in the same field as those froom the Walvis
The geochemical provinciality of the
Gondwana flood basalts (both low- and high-Ti types;
Bellieni et al., 1984) is believed to be
melting of an uprising mantle plume (e.g.,
Campbell & Griffiths, 1990; Arndt
et al., 1993) or lithospheric mantle with a
variable asthenospheric component (e.g.,
Piccirillo et al., 1989; Peate &
Hawkesworth, 1996), followed by variable degrees
of crustal contamination (Petrini et al.,
1987; Hawkesworth et al., 1988);
of heterogeneous subcontinental lithospheric mantle,
caused by heat released from an underlying mantle
plume (Gallagher & Hawkesworth, 1994);
mixing of plume-derived
picritic melts with high potassic melts of lamproitic
composition (i.e. high-Ti; Ellam & Cox,
mixing of MORB-like
tholeiitic picrites from a mantle plume with high-
and low-Ti potassic melts derived from subcontinental
lithospheric mantle, followed by crustal contamination
(Gibson et al., 1995b).
The above hypotheses
require an end-member low in incompatible elements and
with a high Sm/Nd ratio, similar in composition to N-MORB
(e.g., Sun & McDonoug, 1989).
Appropriate end members with high incompatible element
contents, variable incompatible element ratios and low
Sm/Nd are required in order to derive the low- and high-Ti
Paraná tholeiites from the southern and northern
provinces, respectively, by mixing processes. Following
Hawkesworth et al. (1992), the major difficulty
is to explain the genesis of the low-Ti tholeiites,
owing to the absence of an end member with low Ti content,
low 143Nd/144Nd, and high 87Sr/86Sr
ratios. Castorina et al. (1994) demonstrated
that this end-member may be represented by the Eastern
Paraguay low-Ti potassic mafic rocks from Asunción-Sapucai-Villarrica
graben (K-ASU rocks; Castorina et al., 1994).
(Nd “Time of separation from a
depleted mantle reservoir”) model ages (Comin-Chiaramonti
et al., 1997; 1999) show that most alkaline rocks,
carbonatites and nephelinites from SE-Brazil and Paraguay,
range in age from 0.5 to 1.1 Ga. This range is virtually
the same as that for the high-Ti Paraná tholeiites.
On the other hand, only K-ASU and carbonatites from
Eastern Paraguay yielded TDM of early Middle
Proterozoic age, similar to most of the low-Ti Paraná
tholeiites, including the Santos-Rio de Janeiro and
Ponta Grossa tholeiitic dykes. These model ages indicate
that two distinct mantle metasomatic events may have
occurred during the Middle and Late Proterozoic as precursors
to the genesis of the tholeiitic and alkaline magmatism
in the Paraná Basin. These metasomatic processes
were chemically distinct, as indicated by the significant
differences in Ti, large-ion lithophile-element (LILE)
and high-field-strength-element (HFSE) concentrations
in the alkaline rocks (e.g., low-Ti, K-ASU
vs. high-Ti in the Alto Paranaíba Igneous Province,
Gibson et al., 1995a) and tholeiites (low-vs. high-Ti
types) of the Paraná Basin. The scarce high-Ti
tholeiites from southern Paraná actually straddle
the boundary between the southern and central regions,
and suggest a lithospheric component of Alto Paranaíba
Igneous Province type related to metasomatic events
of late Middle to early Late Proterozoic times.
note on TDM ages: Given
Sm/Nd and an initial 143Nd/144Nd
ratio it is possible to construct growth curves (143Nd/144Nd
vs. time) for any mantle (or crustal) reservoir. DM
signifies the depleted mantle reservoir. A sample of
old basalt can be tracked back in time given its present-day
143Nd/144Nd and Sm/Nd by constructing
its Nd growth curve. Where this curve intersects the
DM growth curve is time TDM, i.e., the time
at which the basalt sample could have been generated
from the depleted mantle. This may or may not be the
same as the actual age. Back
In summary, the relationships between
Paraná alkaline and tholeiitic magmatism support
a lithospheric mantle origin. Isotopic and incompatible
element data indicate that a significant role in the
genesis of the Paraná tholeiites was played
by an incompatible-element-depleted component. The
Sr and Nd isotopes (0.7052 and 0.5125 respectively)
and other geochemical features of the postulated modern
Tristan da Cunha plume (Weaver et al., 1987;
Le Roex et al., 1990; see
also South Atlantic page) are distinctly different
from N-MORB and a contribution from the latter is
not apparent in the composition of the Paraná
tholeiites (Peate & Hawkesworth, 1996).
Only some Early Cretaceous tholeiitic basalts (“Tafelkop
type”) and alkaline rocks (the “Okenyenya
igneous complex”) from Namibia have Sr-Pb isotopes
similar to those of Tristan da Cunha, Gough and the
Inacessibile islands (Milner & LeRoex,
1996). We support the view that the incompatible-element-depleted
component is represented by the depleted fractions
of a metasomatized, i.e. veined-type, mantle.
Gibson et al. (1995b) have suggested that
the Paraná flood basalts are the result of
a mixing process involving asthenospheric tholeiitic
melts, derived from incompatible-element-depleted
mantle (i.e. Tristan da Cunha plume) and
lithospheric K-magmas and that such tholeiites “may
contain up to 50% mafic potassic lithosphere-derived
melts”. This model implies the production (and
complete mixing) of an unusually large volume of alkaline
magmas (conservatively, > 150,000 km3)
of Early Cretaceous age, which actually are very poorly
represented in the entire Paraná Basin.
A low-velocity zone in the mantle in
northeastern Paraná Magmatic Province was imaged
by VanDecar et al. (1995) and interpreted
as a thermal anomaly corresponding to the “fossil”
Tristan da Cunha plume that moved with the lithospheric
plate. Considering that the lithosphere has a typical
time constant of about 60 Myr for dissipating heat
and consequently attenuating topography (Gallagher
& Brown, 1997), it is quite improbable that
heat from a plume that reached the base of the lithosphere
more than 130 m.yr. ago, could still persist. It would
be more acceptable an association of the low-velocity
zone with a plume related to the alkaline magmatic
provinces nearby, where magmatism younger than 50
m.yr. can be found. However, no geoid anomaly nor
surface expression of the Tristan da Cunha thermal
anomaly is recognized in this region (Molina &
Ussami, 1999), except for the Iporá and
Alto Paranaiba Late Cretaceous alkaline provinces
(Comin-Chiaramonti et al., 1997, and references
therein) further to the north.
Geoid anomalies constitute a source
of information on the thermal state of Earth's interior.
Positive geoid anomalies (> 4 m) extend along the
eastern Brazilian coast (Ussami et al., 1999)
except onshore along the Bahia coast where they are
< 4 m, and between the cities of São Paulo
and Curitiba in the southeast. In the northeastern
area (Borborema Province) anomalies as high as 10
m are calculated. Apatite fission-track age (AFTA;
Harman et al., 1998) results suggest re-heating
of the area (along the Pernambuco lineament) in the
Late Cretaceous (~ 60 Ma). Northward in the Potiguar
basin, at least two episodes of upwelling were identified
in the stratigraphy (Campanian and Oligocene). The
last one possibly occurred at ~ 25 Ma based on the
geoid hight in the area (Ussami et al., 1999).
In the southeast São Francisco
craton, where part of the Alto Paranaíba Igneous
Province and the Serra do Mar Province are located,
the geoid anomaly extends towards de Abrolhos Islands
and Trindade. AFTA data (Gallagher et al.,
1994) indicate Late Cretaceous – Tertiary ages
for the last thermal event in the area, which coincides
with the age of the alkaline magmatism (80-50 Ma).
the light of these facts, the mantle plume/hotspot hypothesis
for the origin of the alkaline lavas must be reviewed,
at least regarding which plumes are most likely to have
been active at the right place and right time when a
specific province is considered. Moreover, the geochemical
signatures of the magmatic rocks are distinct from those
of the commonly advocated plume. For example, this is
the case for Tristan da Cunha and the Paraná
Magmatic Province (Peate, 1997; Marques
et al., 1999) but even for Trindade and the Walvis
Ridge and Rio Grande Rise rocks, the alleged hotspot
traces. Coring at the basement of the western segment
of Rio Grande Rise yielded tholeiitic basalts dated
at ~ 85 Ma, but rocks dredged from the escarpments of
the guyots, towering over the platform of the rise,
indicate the presence of Eocene alkaline basalts (K-Ar
date of ~ 47 Ma; Bryan & Duncan, 1983;
Fodor et al., 1997). In addition, the geochemical
and Sr-Nd-Pb isotope data are different from those of
Walvis Ridge basalts. All the geochemical data (Piccirillo
& Melfi, 1988; Marques et al., 1999)
point to an origin of the Paraná Magmatic Province
tholeiites in melting of heterogeneous lithospheric
mantle reservoirs (Comin-Chiaramonti et al.,
1997). Furthermore, the geochemical and isotope signatures
of Walvis Ridge and Rio Grande basalts may be explained
by detached continental lithospheric mantle left behind
during the continental break-up processes (Comin-Chiaramonti
et al., 2002).
It is important to stress
that the mantle heterogeneity involved in Paraná
magmatism is not confined to the tholeiites, but also
characterises the Early and Late Cretaceous alkaline
magmatism. Even the carbonatites have on the whole Sr-Nd-Pb
isotope characteristics close to those of the related
alkaline rocks and the spatially associated tholeiites
(Comin-Chiaramonti et al., 1997, 1999; Alberti
et al., 1999; Marques et al., 1999), indicating
similar mantle components in their genesis.
Finally, in order
to explain the widespread distribution of South American
Early Cretaceous tholeiitic and alkaline magmatism,
it is not necessary to invoke an active role for an
hypothetical mantle plume head (Ernesto et al.,
2002). We support an “EDGE drive convection”
model (King & Ritsema, 2000; see
also EDGE page), where the rifting processes resulted
in different lithospheric thickness beneath the edge
of cratonic shields, inducing small-scale convection
The simplistic mantle
plume model is unsatisfactory for explaining most continental
flood basalts and recurrent intraplate magmatism and
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the observations better (Ernesto et al., 2002).
In addition to indications from geoid anomalies, the
presence of long-lived thermal anomalies in the mantle
has already been demonstrated by seismic velocity distribution
models based on tomographic techniques using both P-
and S-waves (e.g. van der Hilst et al.,
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