Abstract

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).

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).

T^DM (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 T^DM 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 T^DM 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 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.

Conclusion

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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