Southern Brazil is part of the Paraná–Etendeka Province (PAEP)  characterized by the emplacement of Early Cretaceous tholeiitic flood basalts and dyke swarms, 129–133 Ma aged [1,2,3,4,5,6], and alkaline and alkaline–carbonatite complexes of Early Cretaceous to Paleogene age [8,9,10,11,12]. The alkaline and alkaline–carbonatitic complexes formed mainly along tectonic structures (Figure 1) active at least since Early Mesozoic, and up to the present day, as indicated also by the distribution of the earthquakes in southern Brazil .
Almost all the carbonatites from southern Brazil are restricted to continental areas. The criteria by which a carbonate-rich rock is considered to be a true carbonatite are difficult to apply with confidence. Although not conclusive, the following characteristics are the strongest clues, especially when they are observed together : (1) field evidences, e.g., subcircular or suboval or ring structures of alkaline complexes with carbonatite body occupying the inner part; (2) association with melilite- or nepheline-bearing rocks; (3) calcite with >1 wt% SrO; (4) presence of perovskite, pyrochlore, F-REE-carbonates, and apatite with high silica content (e.g., >2 wt%); (5) enrichment in Sr, Ba, Nb, U, Th, and rareearth elements (with high LREE vs HREE fractionation) and depletion of other incompatible elements (e.g. K, Rb, Zr, Hf, and Ti) relative to mantle and continental crust ; (6) 87Sr/86Sr initial ratios similar to those of the associated silicate alkaline rocks ; (7) stable isotopes (O–C) in the “primary carbonatitic box” [17,18] or following well-distinct isotopic fractionation trends .
Taking into account the above criteria, this article summarizes field and geological evidences and geochemical characteristics in terms of C–O and Sr–Nd–Pb–Re–Os isotopic data of alkaline–carbonatitic complexes and occurrences in and around the Paraná Basin in southern Brazil, in particular, and in the Paraná–Angola–Namibia (PAN) system in general . In the context of the long-standing controversy about the exact mechanism that produced the South Atlantic igneous province , our results give new important information that allows one to interpret the geodynamic framework of the whole Mesozoic magmatism of the southern Brazilian Platform, in alternative to the mainstream model based on the involvement of a deep mantle plume in magma genesis processes in the region. Notably, noble gases isotopic data suggest that the source(s) are similar to other mantle-derived magmas (e.g., HIMU and MORB) and that the carbon of carbonatites is unlikely to be subduction-related carbon, and supports a picture of C–O fractionation starting from mantle-derived sources, in ametasomatized subcontinental lithospheric mantle (SCLM) based on Sr–Nd–Pb isotope systematic [20,23,24,25].
The structure of this article will follow the same structure as the first part (Speziale et al., this issue) in that the different carbonatitic occurrences are grouped on the basis of petrographic associations  as magmatic carbonatites [27,28], hydrothermal carbonatites, and occurrences with unusual geometric relationship.
- (1)Magmatic carbonatites
- (A)Occurrences associated with rock types of the urtite-ijolite-melteigite series, without the presence of extrusive nephelinites (Brazil, Vale do Ribeira: Anitápolis, Ipanema, Itapirapuã, Jacupiranga, Mato Preto, and Juquiá; Goiás: Caiapó and Morro do Engenho; Paraguay: Cerro Sarambí and Sapucai).
- (B)Occurrences only with olivinites and pyroxenites as ultramafitites (±syenites) as Salitre I and Serra Negra, and with glimmerites as Araxá, Catalão I, Catalão II, and Salitre II.
- (C)Occurrences with intrusive rocks containing melilite, as Tapira and Lages.
- (2)Hydrothermal carbonatites: those produced at temperatures ≤375°C (e.g., Barra do Itapirapuã, Cerro Chiriguelo, Cerro Manomó).
- (3)Occurrences with unusual geometric relationships: a small number of occurrences in the form of small dykes or ocelli in alkaline silicate rocks (e.g., Valle-mí, Cerro Canãda, Cerro E Santa Elena, Itanhaém; cf. Speziale et al., this volume).
2 C–O isotopes
A compilation of the C–O isotopic data for calcite and dolomite-ankerite (δ18O‰ vs V-SMOW and δ13C‰ vs PDB-1) is given in Table 1. The C–O isotope compositions of the studied carbonatites appear to be in general agreement with a magmatic origin from supercrytical fluids with CO2/H2O ≈ 0.2–0.6 molar ratios and temperatures ranging from ∼800°C to 400°C (i.e., from orthomagmatic to pegmatitic environment; cf. ). Primary carbonates of alkaline rock types and associated carbonatites show significant variations in C–O isotope compositions [16,19,20,36,42]. The main trends of δ18O and δ13C fractionation appear to be controlled by the depth of emplacement of the carbonatites. Weathering and groundwater fluids are locally important, as well as meteoric water, which yielded samples strongly enriched in light carbon due to contamination by a biogenic component [16,27,60]. Oxygen and carbon isotopic compositions of individual minerals from selected alkaline–carbonatitic complexes in the Brazilian Platform are reported in Table 2.
Representative C–O isotopic ratios in δ‰ notation for calcite and dolomite/ankerite (sometimes coexisting) in carbonatites from the Brazilian Platform. Analytical methods and uncertainties on the values reported in this table can be found in . The locations of the sampling areas [30,31] are reported in detail following the references relative to the single localities: PARAGUAY: Cerro Sarambí, [32,33]; Cerro Cañada, [20,34,35]; Cerro E Santa Elena, [20,34,35]; Sapucai, [34,36]; Cerro Chiriguelo, [29,36]; Valle-mí, [16,37]. BRAZIL: Anitápolis, ; Ipanema, ; Itapirapuã, ; Jacupiranga, [36,41,42,43,44]; Juquiá, ; Salitre, [46,47]; SerraNegra, ; Araxá, ; Catalão I, [50,51]; Catalão II, ; Tapira, [31,53]; Mato Preto, ; Lages, [36,54]; Barra do Itapirapuã, ; Limeira, [36,42]; Itanhaém, ; Caiapó,  Morro do Engenho, [56,57]; Santo Antònio da Barra, 
|Sample||δ18O‰ Cc||δ13C‰ Cc||δ18O‰Dol/Ank||δ13C‰Dol/Ank||Sample||δ18O‰ Cc||δ13C‰ Cc||δ18O‰Dol/Ank||δ13C‰Dol/Ank||Sample||δ18O‰ Cc||δ13C‰Cc||δ18O‰Dol/Ank||δ13C‰Dol/Ank|
|Cerro Sarambí||Cerro Chiriguelo||Cerro Chiriguelo|
|Cerro E Santa Elena||3,416||16.53||−6.40||Valle-mí|
|Sample||δ18O‰ Cc||δ13C‰ Cc||δ18O‰ Dol/Ank||δ13C‰ Dol/Ank||Sample||δ18O‰ Cc||δ13C‰ Cc||δ18O‰ Dol/Ank||δ13C‰ Dol/Ank||Sample||δ18O‰ Cc||δ13C‰ Cc||δ18O‰ Dol/Ank||δ13C‰ Dol/Ank|
|SAN-8||10.38||−6.82||—||—||LG-20-91||—||—||8.50||−5.95||Barra do Itapirapuã|
|JM 12B||9.38||−4.12||9.51||−3.80||C2A19||8.46||−5.97||—||—||1C (M)||23.80||−5.61||—||—|
|JC 100||7.48||−6.07||7.6||−5.71||S22-4B||10.0||−6.7||13.9||−4.9||Morro do Engenho|
|JC 102||9.38||−4.12||9.51||−3.80||S22-5B||15.4||−5.6||15.6||−5.8||Santo Antônio da Barra|
Carbonatite complexes worldwide display large compositional ranges of oxygen and carbonisotopic ratios in the carbonate phases [17,29,67,68,69,70,71]. In the studied carbonatites, the δ18O values cover a wide interval from about 5 to 25‰ vs V-SMOW notation , but about 50% of the analyses fall into a narrow field between 6 and 10‰. The variation of δ13C is more restricted, i.e., 91% of δ13C values fall in the range between −2‰ and −8‰ vs PDB-1 notation . Ranges of δ18O and δ13C between 6 and 10‰ and between −4 and −8‰, respectively, are considered to reflect primary carbonatites [17,18]. Different emplacement levels (i.e., deep-seated, up to near surface, or near-surface environments ) and superimposed primary and secondary processes are all believed to play an important role in the isotopic variations. These may be related to (1) isotopic compositional differences in the source, (2) fractionation processes during the magmatic evolution, (3) loss of fluids during decompression at the time of emplacement, (4) crustal contamination, and (5) post-magmatic, and deuteric-groundwater processes.
Most of the large variations in heavy oxygen of the carbonatites from the southern Brazilian Platform could be explained by the interaction with hydrothermal fluids, whereas the variations in heavy carbon may be associated with both primary (i.e., isotopic composition of parental magma) and secondary (i.e., hydrothermal re-equilibration) processes [17,68,69]. Determining the extent of primary versus secondary variations of oxygen and carbon isotopic ratios in carbonatites has important implications for evaluating the geochemical characteristics of the source regions, and may be useful for deciphering the nature of the subcontinental mantle [16,37].
In order to address some of these problems, we apply to the whole set of investigated carbonatites a model of isotopic C–O fractionation (see  and references therein). In fact (1), the carbonatitic complexes from the southern Brazilian Platform are among the most intensively studied in the worldwide literature; (2) a large number of C–O isotopic and radiogenic data are available for these occurrences ( and references therein); (3) the current interpretation of the isotopic data from the individual sites is controversial as to the role of primary processes versus crustal contamination.
δ18O‰ (V-SMOW) and δ13C‰ (PDB-1) data for the carbonates in the carbonatites from the southern Brazilian Platform display both typical values of primary carbonatites along with higher δ18O and δ13C values for a number of samples (Figures 2–5). Enrichments in heavy isotopes are interpreted in terms of heterogeneity in the mantle source , or contamination by host rocks , or magmatic – hydrothermal evolution of the carbonatite at a shallow level [16,19,29]. δ18O and δ13C for sedimentary carbonates and groundwater calcite are in the range 20 to 30‰ and 20 to 24‰, and −4 to +5‰ and −6 to −4‰, respectively. It should be noted that marbles from southern Brazil show δ18O and δ13C ranges from 16 to 23‰ and from −4 to +3‰, respectively . There are two main trends of isotopic C–O variations in the carbonatites from southern Brazil: (I) a trend characterized by a positive increase of both δ18O and δ13C (e.g., Anitápolis, Mato Preto, and Jacupiranga, Figure 2a and b; Lages, Figure 3b) and (II) a trend of δ18O increase and δ13C decrease (e.g., Juquiá, Figure 2b).
At the Cerro Chiriguelo carbonatitic complex (Figure 4a), two main linear trends are apparent: the first one defined by carbonatitic samples and primary carbonate of silicate rocks from bore-holes , while the second one represents carbonatitic specimens sampled near to or at the topographic surface, or carbonatitic lava flows. The extrapolations of the two trends intersect at δ18O = 7.5‰ and δ13C = −8.5‰, similar to the values obtained for primary carbonate phases of an ijolite from Cerro Cañada in Central Paraguay, i.e., δ18O = 6.9‰ and δ13C = −8.5‰, respectively.
Additional data for silicates, oxides, and carbonates associated in ocelli from Cerro E Santa Elena and Cerro Cañada ijolitic rocks (Figure 5) or silicate-oxide-carbonate phases in Jacupiranga complex and also apatite-calcite pairs in Jacupiranga and Juquiá carbonatites, respectively [36,61], are all reported in Table 2 along with calculated temperatures of isotopic equilibria.
Measured isotope compositions in δ‰ notation of silicates, oxides, apatite, and calcite in some alkaline and alkaline-carbonatitic complexes from the Brazilian Platform
|Cerro E Santa Elena (ijolite)||δ18O‰ (V-SMOW)||δ13C‰||× 10−6 K2||Ref.||T (°C)|
|Calcitea||6.55||−8.90||Average||822 ± 61|
|Average||642 ± 133|
|Cerro Cañada (ijolite)|
|Jacupiranga (carbonatite)||Average||1,052 ± 180|
|Apatiteb||14.11 ± 1.87||−6.60 ± 0.39||Cc-Ap||1.60||0||5||863|
|Calciteb||12.35 ± 2.34||−6.87 ± 0.81|
Abbreviations: Cpx, clinopyroxene; Ol, olivine; Mt, magnetite; Amph, amphibole; Biot, biotite; Cc, calcite; Ap, apatite. Calculated isotopic temperatures based on fractionation of oxygen isotopes in mineral pairs according to the general equation 1000lnα = A(106T−2) + B, where A and B are coefficients of the equation and T is the absolute temperature (cf. ). References: (1) ; (2) [64,65]; (3) ; (4) ; (5) . The uncertainties are about 0.05‰ (1 σ) for both carbon and oxygen (cf. ).
Concluding, the observed large C–O isotopic variations are mainly due to isotope exchange between carbonates and H2O–CO2 rich fluids, while magmatic processes, i.e., fractional crystallization or liquid immiscibility probably affect the δ18O and δ13C values by no more than 2‰. The isotope exchange model implies that the main isotopic variations occurred at low temperatures, in a hydrothermal environment, e.g., in the range 400–80°C, involving fluids with a CO2/H2O molar ratio ranging from 0.8 to 1, as proposed by , based on their thermodynamic model of C–O isotopic evolution.
3 Sr–Nd isotopes
The carbonatites from the southern Brazilian Platform usually show the same initial isotopic ratios, 87Sr/86Sr (Sri) and 143Nd/144Nd (Ndi), as the associated alkaline rocks, even in the late stages of fluid-rock re-equilibration (i.e., hydrothermal environment), as concluded by [16,19]. Initial ratios relative to Sr–Nd isotopes and model ages of carbonatites are reported in Table 3.
|Sample N||SAN1;14;49;12–78;11–101;16A||SAN 13||119.2||IT 28||HB 005||HB 010||HB 011|
|Early Ca–carbonatite N = 4||Late Mg–Ca–carbonatite||Ca–carbonatite||Ca–Fe–carbonatite||Ca–Fe–carbonatite||Ca–Fe–carbonatite||Ca–carbonatite|
|Sample N||Mato Preto||Juquiá|
|Sample N||Caiapó||Morro do Engenho||Santo Antônio da Barra||Cerro Sarambì||Sapucai|
|CR-09 Ca-Carbonatite||ME-C Ca-Carbonatite||SAB-12Ca-Mg-Carbonatite||Glimmerite-Carbonatite.Whole rock||Carbonate Fraction (dolomite)||Carbonate Fraction (calcite 7%)||PS72 Sap. Whole rock. Phonotephrite||Carbonate Fraction (dolomite 24.3 wt%)|
|Sample N||Araxá||Catalão I||Catalão II|
|AR 893 Phlogopite-bearing Mg-Carbonatite||C1-L 1250 Mg-Ca-Carbonatite||C1CB02 Mg-Ca-Carbonatite||C1C4 Ca-Mg-Carbonatite||C2-AA165907 Ca-Carbonatite||C2A2 Ca-Carbonatite||C2B19 Ca-Carbonatite||C2B18 Phlogopite-bearing Fe-Carbonatite||C2B17 Ca-Carbonatite|
|T1 Ca–carbonatite||T2 Ca–carbonatite||SB05A;SB05B Fe–carbonatite N = 2||SB02 Fe–carbonatite||SB10 Si-phlogopite carbonatite||SB11 phlogopite-bearing carbonatite|
In Brazil (Figure 6a), the Early Cretaceous carbonatites have Sri between 0.70425 and 0.70595 and Ndi between 0.51213 and 0.51280 (mean Sri = 0.70527 ± 0.00034 and mean Ndi = 0.51224 ± 0.00011; cf. ). Late Cretaceous complexes yield the following Sri and Ndi mean values, respectively: Alto Paranaíba Igneous Province (APIP), Sri = 0.70527 ± 0.00036 and Ndi = 0.51224 ± 0.00006 ([76,77,78] and references therein); Taiúva-Cabo Frio and Serra do Mar, Sri = 0.70447 ± 0.00034 and Ndi = 0.51252 ± 0.00008 ; Lages, Sri = 0.70485 ± 0.00053 and Ndi = 0.51218 ± 0.00022 [54,80]. It should be noted that the alkaline–carbonatite magmatism shows trends similar to those of the Cretaceous tholeiites from southern Brazil and from Walvis Ridge and Rio Grande Rise (fields H-Ti, L-Ti, UCA, LCA, and RGR; cf. [81,82]).
In eastern Paraguay (Figure 6b), specimens from the Early Cretaceous K-alkaline rocks (both pre- and post-Early Cretaceous tholeiitic magmatism) and associated carbonatites yield Sri and Ndi within the ranges 0.70612–0.70754 and 0.51154–0.51184, respectively. These are distinct from the values obtained for the Late Early Cretaceous (Misiones, MIS, province) to Paleocene Na-alkaline rocks (Asunción province, ASU), i.e., Sri = 0.70362–70524 and Ndi = 0.51225–0.51277. In Figure 6b, the carbonatites and associated K-alkaline rocks show Sri and Ndi values similar to the low Nd array of  and to the Paraguay array of .
The Sr–Nd isotopic ratios of alkaline and alkaline–carbonatite complexes plotted in Figure 6 appear to follow a well-defined array involving depleted and enriched mantle components. Both the Early and Late Cretaceous Brazilian alkaline–carbonatite complexes vary from close to the HIMU to the enriched quadrant, fitting the fields of the uncontaminated low-Ti and high-Ti tholeiites from the Paraná Basin, that is those unaffected by crustal magma contamination processes [1,16,19,99].
The isotopically Sr-enriched rocks (e.g., eastern Paraguay alkaline–carbonatite complexes) are not easily explained by crustal contamination, since this requires high percentages of crustal components in a mixing process (up to 90%; ). On the contrary, we stress that the oxygen isotope data suggest a primary, mantle-derived origin for silicate phases: whole-rock δ18O data for the potassic rocks yielded +5.45 to +5.91‰ (vs V-SMOW), which are consistent with the values for its mineral constituents, clinopyroxene (+4.85 to +5.20‰) and biotite (+3.96 to +5.54‰), and the expected mantle values (cf. Table 3). On the whole, the Na-rocks, close to the BE, and the K-rocks, typically high in radiogenic Sr worldwide, represent the range of virtually uncontaminated source magmas. The same combination of different isotopic data can be a key to interpret the sources of alkaline and ultra-alkaline magmatism in complex geodynamic settings, such as the Italian ultrapotassic volcanic province [102,103,104,105,106].
In summary, the data support the view that the carbonatites and alkaline rocks from eastern Paraguay, mainly ranging in composition from a depleted component to the BE, may represent the end members of virtually uncontaminated source magmas from subcontinental mantle segments variously affected by “metasomatic s.l.” processes at earlier times .
4 Nd-Model ages
Studies relative to the initial 143Nd/144Nd ratios from Precambrian terrains suggest that the mantle that produced continental crust has evolved with Sm/Nd ratios higher than that of CHUR (chondritic uniform reservoir; cf. [62,107]). For this reason, model ages for the continental crustare usually calculated with reference to the depleted mantle reservoir (DM: 143Nd/144Nd = 0.513151, 147Sm/144Nd = 0.2188) rather than CHUR and they are expressed as TDM model ages.
Although the Nd model ages do not reflect the true ages of the sources, being a function of the Sm/Nd fractionation during the melting and magma differentiation , they may provide a notional estimate of the time when a major metasomatic event may have occurred, as a function of the different geochemical characteristics of the different sectors of Brazil and Paraguay.
Applying the TDM (Nd) model ages on the whole Paraná system (PS) (e.g. [31,109]), we observe that: (1) in the South American Platform, H-Ti flood tholeiites and dykes mainly range between 0.8 and 2.4 Ga; (2) L-Ti flood tholeiites span between 0.8 and 2.7 Ga; (3) the pre-tholeiitic potassic rocks mainly vary from 0.8 to 2.5 Ga; (4) early Cretaceous syn- and post-tholeiitic alkaline and carbonatitic magmatism range from 0.6 and 0.9 Ga; (5) Late Cretaceous alkaline rocks and carbonatites vary between 0.6 and 0.9 Ga [12,31].
If the comparison is made between eastern Paraguay (at the western margin of the south Brazilian Platform) and the eastern end of the Brazilian Platform [31,110], we observe that: (1) TDM of pre-tholeiitic K-alkaline rocks from eastern Paraguay display two main peaks at 1.1 Ga (Valle-mí, Apa Block) and at 1.4 Ga (Amambay region), respectively; (2) the post-tholeiitic K-alkaline complexes and dykes from central Paraguay (ASU) show a mean TDM of 1.7 Ga ; (3) the associated high-Ti tholeiitic basalts have TDM ranging from 0.9 to 1.4 Ga. On the other hand, the low-Ti tholeiites exhibit model ages varying from 0.7 to 2.8 Ga, with TDM increasing from north to south and from west to east; (4) the Na-alkaline rocks display TDM mean values of 0.6 Ga (Na-ASU, Paleocene) and 1.0 Ga (Misiones, late Early Cretaceous, respectively, [34,35]). The youngest model ages are those of the Late Cretaceous alkaline outcrops from the Brazilian Mato Preto carbonatite (Ponta Grossa Arch: 0.58 ± 0.08 ). On the other hand, the Early Cretaceous carbonatites of Barra do Itapirapuã and Jacupiranga (Ponta Grossa Arch) show TDM of 0.7 ± 0.2 Ga.
Thus, the range of model ages in the southern Brazilian Platform (cf. Figure 9 of ) suggests that the corresponding Paleozoic, Cretaceous, and Paleogene magmas are derived from subcontinental lithospheric mantle modified by metasomatic processes (probably involving asthenospheric components) since Neoarchean to Neoproterozoic times [1,20,34,35,110].
In summary, the overlapping of isotopic compositions and/or model ages of different igneous rocks (i.e., high- and low-Ti tholeiites, K- and Na-alkaline rocks and carbonatites) cannot be accidental and suggests sampling of ancient reservoirs formed at the same time from the same subcontinental upper mantle (SCUM). Either in case of heterogeneity induced by recycled crust in the mantle [111,112], or of variably veined material in the SCUM , or both, it is clear that magma genesis involved ancient lithospheric mantle reset at well-defined isotopic ranges.
A Proterozoic lithospheric mantle with veins of amphibole/phlogopite-carbonate-lherzolite and amphibole-lherzolite + CO2 fluid (type III and IV veins of ) may well account for the magmatism of the southern Brazilian Platform (Figure 7). This scenario is compatible with Pb isotope data indicating a mantle source of ca. 1.8 Ga for the Paraná high-Ti tholeiites (see Pb isotopes section) and with the age of formation of much of the crust of southern Brazil (2 Ga; cf. ).
5 Pb isotopes
The available Pb isotopic data for the alkaline–carbonatite complexes and tholeiites from the PS are reported in Table 4 and plotted in Figure 8a and b. The figures show patterns compatible with mixing processes mainly involving HIMU and EMI end members, and subordinately DMM and EMI, as well as crustal s.l. components (e.g., EMII). It should be noted that all the rock types lie to the right of the 132 Ma geochron for the whole Paraná–Etendeka system (, except for the post-tholeiitic K-rocks from eastern Paraguay that plot close to the 132 Ma geochron (Figure 8a).
Representative Pb isotopic ratios (measured and initial ratios) of carbonatites from the southern Brazilian Platform. Data sources as in Table 1
|Cerro E Santa Elena||17.435||15.435||37.796||127||17.200||15.424||37.373|
Carbonatites from southern Brazil and eastern Paraguay plot close to the EMI/DMM-HIMU mixing lines for both Pb–Sr and Pb–Nd , different from Mid-Atlantic Ridge (MAR) basalts and Ocean island basalts (OIB), which define trends between the DMM and HIMU mantle components.
This observation seems to confirm the advantages in using carbonatite over silicate rocks, as indicators of mantle sources, because of their rapid ascent to the surface conditions, and buffering against crustal assimilation due to their high Sr, Nd, and Pb concentrations in the liquids.
The available Pb isotopic data indicate that any model proposed for the evolution of the HIMU and EMI end members must be consistent with the following constraints: (1) HIMU and EMI are not restricted to the oceanic environment; (2) end members are variously associated in space as a function of the various protoliths; (3) mantle regions with HIMU and EMI isotope characteristics can generate a wide variety of silicate melts, including melts enriched in CO2 ; (4) Na-alkaline rock types are systematically grouped together in fields well distinct from the K-alkaline fields in Paraguay; (5) even the Na-alkaline rock types from the Central Rift of the sub-Andean system (Late Cretaceous; cf. ) fit the Triassic to Neogene analogues from eastern Paraguay (cf. ).
Some authors as [11,79,115] postulated that the Early Cretaceous alkaline–carbonatitic and tholeiitic magmatism and the Late Cretaceous alkaline and alkaline–carbonatitic magmatism from Alto Paranaíba-Serra do Mar (southern Brazil) would reflect the variable contributions of the asthenospheric mantle components related to the Tristan da Cunha and Trindade plumes, respectively. On the contrary, other authors [1,16,99,110] suggested that the alkaline and alkaline–carbonatitic magmatism in the examined rock types originated from lithospheric mantle sources without appreciable contribution of plume-derived materials.
On the basis of geochemical and geophysical data, [6,7,118] proposed that the genesis of the tholeiites in the Paraná-Tristan da Cunha system mainly reflects melting of heterogeneous subcontinental mantle reservoirs, and that the geochemical and isotopic signatures of the Walvis Ridge and Rio Grande Rise basalts may be explained by contamination through detached continental lithospheric mantle left behind during the continental break-up processes.
`he Early Cretaceous alkaline magmatism from the southern Brazilian Platform appears to be related to heterogeneous mantle sources spanning from time-integrated HIMU and enriched mantle components (Figure 8). According to , for example, relatively low 206Pb/204Pb and high 207Pb/204Pb compositions could be related to delamination of pyroxenite restites formed by anatexis of the initial basaltic crust in Archean–Proterozoic times. The alkaline magmatism from Brazil and Paraguay mimics, in terms of isotopic compositions, the coeval flood tholeiites. We stress that, in general, the enriched isotopic signatures of the Early Cretaceous alkaline magmatism decreases from West (Paraguay) to East (Brazil and SE continental margins). We observe a similar decreasing trend as a function of the age of the magmatism in Paraguay and Brazil from Early-Late Cretaceous to Paleogene. These results suggest that the magmatism is related to both large- and small-scale heterogeneous mantle sources. It should be noted that, according to , the APIP (Alto Paranaíba Igneous Province) would be the inland surface expression of the “dogleg” track left by the Trindade plume. However, in terms of Sr–Nd–Pb isotopes, the contribution, if any, of the asthenospheric components related to that plume is difficult to account for.
Overall, the data are consistent with a thermally eroded metasomatic SCLM and/or delaminated lithospheric materials stored for long time in the transition zone or deeper mantle in Archean–Proterozoic times . The important role of the Tristan plume claimed by  is not apparent. Therefore, we agree with [6,7] that the hypothesis of asthenospheric plumes for the magmatism from PS is not compelling, while a deep mantle thermal anomaly, corresponding to positive geoid anomaly and low seismic velocity (e.g., [120,121]), may have been its possible heat source based on reconstructions of the position of the South American lithospheric plate in the Cretaceous [7,20,122,123].
6 Some notes on the Re–Os isotopic system
Although carbonatites are not suitable materials for the analysis of Re–Os and platinum group elements ([125,126] and references therein), many carbonatitic complexes from southern Brazil are strictly associated with mafic-ultramafic high-K rocks with kamafugitic and kimberlitic affinity, e.g., in the Rio Verde-Iporá (Goiás), Alto Paranaíba (Minas Gerais), and Lages (Santa Catarina) areas (Late Cretaceous magmatism). Some authors [24,25,71,90,127] provided some Re, Os, and platinoids data for rocks from the APIP (cf. Figure 1 of ), with the aim to develop a petrogenetic model regarding the kamafugitic–kimberlitic magmatism and associated carbonatites. For this purpose, this section presents a brief review for the main APIP rock types, i.e., kimberlites, kamafugites, phlogopite-rich peridotites, glimmerites, and associated carbonatites, in order to compare the Sm–Nd–Pb isotope results with the available Re–Os data.
Representative trace element compositions and isotope ratios are listed in Table 5. 87Sr/86Sr vs 143Nd/144Nd initial ratios are plotted in Figure 9. The APIP rock types straddle the field of the high-Ti tholeiites of the Northern Paraná Basin (Early Cretaceous) and are intermediate between the Kimberlites I and the Kimberlites II of [129,130].
Rb, Sr, Sm, Nd, Re, and Os concentrations and isotopic initial ratios of selected samples from the Alto Paranaíba Igneous Province and one sample from Lages (notional ages 85 and 75 Ma, respectively). Analytical methods and data sources: [20,24,25] and references therein
|Limeira||Tres Ranchos||Pantano||Salitre||Santa Rosa||Verdiana||Canas|
In general, the high concentrations of the most incompatible elements (IEs) in all the APIP alkaline rocks  suggest that the effects of crustal contamination on the Sr–Nd isotopic system were negligible and that the parental magma compositions are more likely produced by mantle-enriched sources affected by variable degrees of metasomatism. The data, plotted in time-integrated ε-notations, are widespread mainly in the enriched quadrant, with εSr and εNd extending in the ranges from −4 to 31 and from −2.5 to −9, respectively. The field relative to the kamafugitic rock types contains all the other lithologies (inset a of Figure 9), i.e., kimberlites, glimmerites, mica peridotites, and carbonatites. The almost constant value of the Sm/Nd ratio in the APIP rocks (147Sm/144Nd = 0.085 ± 0.009; ) suggests that the Nd model age is indicative of the main metasomatic event affecting the lithosphere beneath the Alto Paranaíba region . Model ages TDM (calculated with respect to the depleted mantle; cf. ) for the whole APIP population (65 samples; cf. inset b of Figure 9) average to 1.0 ± 0.1 Ga.
The initial isotopic compositions displayed on 206Pb/204Pb vs 207Pb/204Pb and 206Pb/204Pb vs 208Pb/204Pb diagrams (Figure 10a) define essentially linear arrays that are subparallel to both the Early Cretaceous tholeiites from the Paraná Basin [89,135] and the Northern Hemisphere Reference Line (NHRL; ). A possible explanation for the Pb isotope behavior is that the data represent a secondary isochron, giving an apparent age of 2.5 Ga, suggesting an Archean–Proterozoic mantle source, as already pointed out by  for the tholeiites from the Paraná Basin. Alternatively, the observed trends are due to the mixing of different mantle components. Notably, some kimberlites approach the HIMU mantle component.
Overall, the APIP alkaline rock types and associated carbonatites plot in the field of the Brazilian Late Cretaceous alkaline–carbonatite complexes. The latter contains the fields of all the magmatic rock types from the Paraná Basin (i.e., Early Cretaceous flood tholeiites and alkaline–carbonatite complexes (cf.  and Figure 8). In particular, the APIP extends from EMI and DMM to EMII mantle components, overlapping the field of the Tristan da Cunha volcanics (Figure 10). Considering the diagrams 206Pb/207Pb vs 87Sr/86Sr and 143Nd/144 Nd (initial ratios) of Figure 10b, it should be noted that most of the APIP rocks are in the field of the peridotite xenoliths, except for some kimberlites; the latter show lower radiogenic Nd with respect to the Late Cretaceous kimberlites from Gibeon .
The Re–Os isotope systematics does not allow a clear distinction among the different rock types. Kimberlites have Re and Os between 0.32 and 1.99 ppb, and 0.81 and 1.98 ppb (av. 0.69 ± 0.64 and 1.43 ± 0.40 ppb, respectively). On the other hand, 187Os/188Os initial ratios vary within a small range, i.e., from 0.11 to 0.13 (av. 0.122 ± 0.005). Kamafugites show Re and Os from 0.06 to 0.38 ppb and from 0.12 to 1.76 ppb (av. 0.156 ± 0.140 and 0.672 ± 0.526 ppb, respectively) and have 187Os/188Os initial ratios ranging from 0.11 to 0.15 (av. 0.134 ± 0.013). The glimmerites that present the highest 187Os/188Os (av. 0.295 ± 0.043) and 187Re/188Os (av. 13.98 ± 3.83) ratios are well distinct from the other groups (Figure 10).
Rhenium depletion (TRD) model ages (based on Re–Os isotopes) range between 1.39 and 1.64 Ga, thus setting a minimum Re age of the source region protolith  showing an older event with respect to the APIP Nd model age (1.0 ± 0.1). Notably, these model ages are in the same range of the Paraguay alkaline–carbonatite rocks (1.1–1.7 Ga). The cluster of isotopic data seems to indicate old lithospheric sources.
Arguments in favor of lithospheric sources for the APIP magma types are also postulated by [24,127], by comparing the Re-depletion ages of the kimberlites with Nd-depleted mantle model ages of all their analyzed rock types. The Os isotope data for the APIP rock types are indicative of lithospheric mantle sources for the kimberlites (187Os/188Os ratios 0.11 to 0.13) that were variously Re depleted, probably at Neoarchean to Mesoproterozoic times. These lithospheric sources experienced LILE enrichment by fluid/melt metasomatism at ∼1 Ga, probably during the mobile belt formation along the western border of the São Francisco craton. Kamafugites have radiogenic 187Os/188Os ratios (up to 0.15) suggestive of source veins that appear to have been stabilized in the lithospheric mantle in the Meso- to Neoproterozoic.
Finally, it should be stressed that Carlson et al.  proposed that the Os isotope compositions, coupled with Sr–Nd–Pb isotope systematics, most likely represent the influence of delaminated Brazilian lithospheric mantle mixed into mantle circulation beneath the South Atlantic and are not related to the plume(s) activity commonly attributed to the APIP magmatism (cf. ). As matter of fact, geoid anomaly maps and regional seismic tomography studies are supporting a non-plume-related heat source for the magmatism from southern Brazil and eastern Paraguay [7,110]. The hotspot tracks of Walvis Ridge and Rio Grande Rise, as well as the Victória-Trindade chain, rather than reflecting continuous magmatic activity induced by mantle plumes beneath the moving lithospheric plates might instead reflect the accommodation of stresses in the lithosphere during rifting, similar to the general mechanism proposed by [140,141].
7 Concluding remarks
The isotopic data show that magmatism from the southern Brazilian Platform requires heterogeneous mantle sources, also in terms of radiogenic isotopes, probably related to metasomatic s.l. processes that occurred between Neoarchean and Neoproterozoic times, as also confirmed by Re–Os systematics on mafic-ultramafic rock types associated with the alkaline–carbonatitic complexes.
The areal distribution of magmatism suggests that the time-integrated isotopic enrichment of carbonatites and associated alkaline rocks decreases from west (eastern Paraguay) to east (southern Brazil), concomitantly with the decreasing age of the magmatism (Early to Late Cretaceous). This supports the view that the alkaline–carbonatitic magmatism originated from large- to small-scale heterogeneous subcontinental mantle. Sr–Nd–Pb–Os isotope data yield clear evidences that HIMU and EMI mantle components were important in the genesis of the magmatism in the southern Brazilian Platform. We speculate that a viable mechanism could involve passive rifting, astenosphere upwelling, and consequent melting of a metasomized (fluid-bearing) subcontinental lithospheric mantle.
All the results indicate that asthenospheric components derived from mantle plumes (i.e., Tristan da Cunha and Trindade hot spots; cf. ) did not significantly contribute to the genesis of the alkaline–carbonatitic magmatism, consistent with the conclusions reached by [2,7,90,99] for the petrogenesis of the Paraná flood tholeiites. This confirms that the exact mechanism that produced the South Atlantic igneous province and its relationship with the South Atlantic opening are still an open question (e.g.,  and reference therein).
Regional thermal anomalies in the mantle, mapped by geoid and seismic tomography, are supporting a non-plume-related heat source for the Paraná magmatic province according to [7,110,118]. The hotspot tracks of Walvis Ridge and Rio Grande Rise, as well as the Victória-Trindade chain, might be the manifestation of accumulation of stresses due to small-scale lithospheric convection associated with rifting rather than continuous magmatic activity induced by mantle plumes beneath the moving lithospheric plates.
S. S. acknowledges the support from the Deutsche Forschungsgemeinschaft DFG (FOR 2125). We thank the editor Dr J. Barabach and an anonymous reviewer. F. Stoppa is thanked for his very detailed comments and suggestions, which helped us to improve our manuscript. We also thank O. Gerel, V. A. V. Girardi, and L. Kogarko for their comments.
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