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Plumes Beneath the Paraná Basin, Eastern Paraguay: Fact or Fiction?

Piero Comin-Chiaramonti1, Marcia Ernesto2, Victor Fernandez Velázquez3 & Celso de Barro Gomes3

1Dipartimento di Ingegneria Chimica, dell'Ambiente e delle Materie Prime. Trieste University. Piazzale Europa, 1. I-34127, Trieste, Italy.

2Instituto Astrónomico e Geofisico, Universidade de São Paulo, Rua do Matão 1226, Citade Universitaria. 05508-900 São Paulo, SP, Brazil.

3Instituto de Geociências, São Paulo University. Rua do Lago 562, CEP 05508-900, São Paulo, SP, Brazil.


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).


Distinct 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 Paraguay:

  1. an extreme, heterogeneous EMI component, which was prevalent in the Cretaceous alkaline magmatism, and
  2. 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).

The 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).

Figure 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 Ridge.


The geochemical provinciality of the Gondwana flood basalts (both low- and high-Ti types; Bellieni et al., 1984) is believed to be related to:

  1. variable partial 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);
  2. dehydration melting of heterogeneous subcontinental lithospheric mantle, caused by heat released from an underlying mantle plume (Gallagher & Hawkesworth, 1994);
  3. mixing of plume-derived picritic melts with high potassic melts of lamproitic composition (i.e. high-Ti; Ellam & Cox, 1991), and
  4. 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).

TDM (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.

A 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).

In 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 cells.


The simplistic mantle plume model is unsatisfactory for explaining most continental flood basalts and recurrent intraplate magmatism and alternative thermal sources that do not involve material transfer from the lower mantle to the lithosphere explain 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., 1997 and references therein).


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last updated 29th November, 2004