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The Early Cretaceous Barra do Itapirapuā carbonatite, in southern Brazil, belongs to the initial stages of the alkaline carbonatitic magmatism in the Ponta Grossa Arch Province, as related to the opening of the south Atlantic Ocean. The magmatic evolution of the complex comprises four phases of intrusive carbonatite that are composed of medium- to coarse-grained Mg and Fe carbonatites with small volumes of late fine- to very fine-grained Mg carbonatites and show evidence of pervasive hydrothermal events. Drill core samples reveal the existence of silicate rocks of syenitic composition of two different origins. Among the silicate rocks, there are syenites of a likely magmatic origin, as well as fenites derived from the granitic protolith, which constitutes the wall rock of the complex. Fenitization around the Barra do Itapirapuā complex is mainly potassic, and fenites are Ca- and LOI-enriched in comparison with the magmatic syenites. Furthermore, the two types of silicate rocks differ in terms of REE and other trace elements. Associated syenites and carbonatites have similar Sr-Nd isotopic compositions, close to the Bulk Earth at 120 Ma. Mixing curves indicate that the carbonatite-syenite association may be accounted for by assimilation of up to 20% crustal material, whereas the late carbonatite composition is consistent with less than 10% assimilation of crustal rocks. An origin by liquid immiscibility for the carbonatite-syenite association can be considered, followed by magmatic differentiation through fractional crystallization of calcite and dolomite in the carbonatite magma. C-O stable isotopic data indicate further reequilibration under hydrothermal conditions at temperatures between 375 and 80 °C. © 2002 Elsevier Science Ltd. All rights reserved.

The Pb isotope ratios of olivine- and nepheline-normative rocks from the Ponte Nova mafic-ultramafic alkaline massif (SE Brazil) corroborate evidence that the assimilation of crustal components played an important role in this small magma chamber. New whole-rock 206Pb/204Pbi (17.655–18.362) and 208Pb/204Pbi (38.029–38.591) ratios correlate negatively with 143Nd/144Ndi (0.512216–0.512555) and positively with 87Sr/88Sri (0.70432–0.70641). The whole-rock ratios of 207Pb/204Pbi (15.494–15.556) are not clearly correlated with other isotopic pairs. Most of Ponte Nova samples straddle the curve for μ = ∼8.7, but the majority plot to the right of the geochron, indicating U enrichment. The least contaminated samples plot within the Tristan-type isotopic composition and close to the Trindade isotopic field and might have formed through mixing between OIB and EM-I components. Mixing model isotope curves correlating Sr–Nd–Pb isotope ratios, considering the most pristine compositions (central plug) and the main host rocks of this area, indicate low crustal contributions for some intrusions and high contributions for others. More pronounced contributions from local metagranitic host rocks are associated with intrusions showing high amounts of crustal components. The wide interval of calculated Nd depleted-mantle model ages (TDM: 703–1217 Ma) for Ponte Nova rocks indicates that crustal contamination and clinopyroxene accumulation can strongly modify these values versus those from liquid compositions (nearby alkaline dikes). Therefore, rocks affected by crustal contamination and clinopyroxene accumulation should not be considered for periods of enrichment/metasomatism of the mantle source.

The Indaiá-I and Indaiá-II intrusions are hypabyssal, small-sized ultrabasic bodies belonging to the Cretaceous magmatism of the Alto Paranaiba Alkaline Province (southeast-central western Brazil). While Indaiá-I is classified as an archetypal group-I kimberlite, Indaiá-II (its satellite intrusion) presents several petrographic and chemical distinctions: (1) an ultrapotassic composition (similar to kamafugites), (2) lower volumes of olivine macrocrysts, (3) diopside as the main matrix phase (in contrast with the presence of monticellite in Indaiá-I), (4) high amounts of phlogopite, and (5) abundant felsic boudinaged and stretched microenclaves and crustal xenoliths. Disequilibrium features, such as embayment and sieve textures in olivine and clinopyroxene grains, are indicative of open-system processes in Indaiá-II. Mineral reactions observed in Indaiá-II (e.g., diopside formed at the expense of monticellite and olivine; phlogopite nearby crustal enclaves and close to olivine macrocrysts) point to an increase in the silica activity of the kimberlite magma; otherwise partially melted crustal xenoliths present kalsilite, generated by desilification reactions. The high Contamination Index (2.12–2.25) and the large amounts of crustal xenoliths (most of them totally transformed or with evidence of partial melting) indicate a high degree of crustal assimilation in the Indaiá-II intrusion. Calculated melts (after removal of olivine xenocrysts) of Indaiá-II have higher amounts of SiO2, Al2O3, K2O, slightly higher Rb/Sr ratios, lower Ce/Pb and Gd/Lu ratios, higher 87Sr/86Sr, and lower 143Nd/144Nd than those calculated for Indaiá-I. Crustal contamination models were developed considering mixing between the calculated melts of Indaiá-I and partial melts modeled from the granitoid country rocks. Mixing-model curves using major and trace elements and isotopic compositions are consistent with crustal assimilation processes with amounts of crustal contribution of ca. 30%. We conclude that (1) Indaiá-II is representative of a highly contaminated kimberlitic intrusion, (2) this contamination occurred by the assimilation of anatectic melts from the main crustal country rocks of this area, and (3) Indaiá-I and Indaiá-II could have had the same parent melt, but with different degrees of crustal contamination. Our petrological model also indicates that Indaiá-II is a satellite blind pipe linked to the main occurrence of Indaiá-I.

The Catalao II carbonatitic complex is part of the Alto Paranaiba Igneous Province (APIP), central Brazil, close to the Catalao I complex. Drill-hole sampling and detailed mineralogical and geochemical study point out the existence of ultramafic lamprophyres (phlogopite-picrites), calciocarbonatites, ferrocarbonatites, magnetitites, apatitites, phlogopitites and fenites, most of them of cumulitic origin. U–Pb data have constrained the age of Catalao I carbonatitic complex between 78 ± 1 and 81 ± 4 Ma. The initial strontium, neodymium and hafnium isotopic data of Catalao II (87Sr/86Sri = 0.70503–0.70599; eNdi = −6.8 to −4.7; 176Hf/177Hf = 0.28248–0.28249; eHfi = −10.33 to −10.8) are similar to the isotopic composition of the Catalao I complex and fall within the field of APIP kimberlites, kamafugites and phlogopite-picrites, indicating the provenance from an old lithospheric mantle source. Carbon isotopic data for Catalao II carbonatites (δ13C = −6.35 to −5.68 ‰) confirm the mantle origin of the carbon for these rocks. The origin of Catalao II cumulitic rocks is thought to be caused by differential settling of the heavy phases (magnetite, apatite, pyrochlore and sulphides) in a magma chamber repeatedly filled by carbonatitic/ferrocarbonatitic liquids (s.l.). The Sr–Nd isotopic composition of the Catalao II rocks matches those of APIP rocks and is markedly different from the isotopic features of alkaline-carbonatitic complexes in the southernmost Brazil. The differences are also observed in the lithologies and the magmatic affinity of the igneous rocks found in the two areas, thus demonstrating the existence of regional-scale heterogeneity in the mantle sources underneath the Brazilian platform.

The Itatiaia alkaline complex is a Late Cretaceous intrusion (72 Myr) made up of felsic differentiates, with syenitic rocks dominant throughout and with presence of both nepheline- and quartz-rich varieties. Dykes with phonolitic or trachytic composition cross-cut the coarse-grained facies. The rocks are arranged concentrically, with the core of the complex being formed by SiO 2 -oversaturated syenites (with a small outcrop of granites), and are radially displaced by faults related to regional tectonic lineaments. The minerals show gradual but significant changes in composition (salitic and augitic to aegirine-rich pyroxenes, hastingsite and actinolite to richterite and arfvedsonite amphiboles, sodic plagioclase to orthoclase feldspars and so on) and the whole-rock trends are broadly consistent with fractional crystallization processes dominated by alkali feldspar removal. Sr-isotopic data indicate more radiogenic ratios for the SiO 2 -oversaturated rocks (0.7062-0.7067 against 0.7048-0.7054 for the SiO 2 -undersaturated syenites), consistent with small amounts of crustal input. The favored hypothesis for the petrogenesis of the different syenitic groups is the prolonged differentiation starting from differently SiO 2 -undersaturated mafic parental magmas (potassic alkali basalts to ankaratrites, present in the Late Cretaceous dyke swarms of the area), accompanied by variable crustal contamination prior to the final emplacement. The lack of carbonatite as a significant lithotype, the potassic affinity of the Itatiaia complex, and the relatively high Sr-isotopic ratios match the characteristics of the other complexes of the Rio de Janeiro-Sa˜o Paulo states coastline and confirm the ultimate derivation of these differentiated rocks from an enriched lithospheric mantle source.

The Planalto da Serra ultramafic-alkaline rocks are represented by several plugs, lava-flows and N60–80E trending dykes which are mainly represented by glimmerites, harzburgites and carbonatites. These rocks intrude the Cryogenian basal unit of the Northern Paraguay Belt in the Rio dos Cavalos rift (southeastern part of the Amazonian Craton). 40Ar/39Ar dating on the phlogopite from glimmeritic dykes and plugs yielded an age of ca. 600 Ma, which is consistent with Rb/Sr and Sm/Nd determinations on the other rock-types. These age determinations make it difficult to establish a geochronological and genetic relationship between the Planalto da Serra rocks and the Cretaceous bodies of the “Azimuth 125° Lineament”, and it suggests there is a relationship between its emplacement and the beginning of the extensional tectonics responsible for the separation of the Amazonian and Laurentia Cratons. This age also indicates that the onset of the deformation of the Cuiaba Group may be older than 600 Ma.

A general description of Mesozoic and Tertiary (Fortaleza) Brazilian alkaline and alkaline-carbonatite districts is presented with reference to mineralogy, petrology, geochemistry and geochronology. It mainly refers to scientific results obtained during the last decade by an Italo-Brazilian research team. Alkaline occurrences are distributed across Brazilian territory from the southern (Piratini, Rio Grande do Sul State) to the northeastern (Fortaleza, Ceara State) regions and are mainly concentrated along the borders of the Parana Basin generally coinciding with important tectonic lineaments. The most noteworthy characteristics of these alkaline and alkaline-carbonatite suites are: (i) prevalence of intrusive forms; (ii) abundance of cumulate assemblages (minor dunites, frequent clinopyroxenites and members of the ijolite series) and (iii) abundance of evolved rock-types. Many data demonstrate that crystal fractionation was the main process responsible for magma evolution of all Brazilian alkaline rocks. A hypothesis is proposed for the genesis of carbonatite liquids by immiscibility processes. The incidence of REE and trace elements for different major groups of lithotypes, belonging both to carbonatite-bearing and carbonatite-free districts, are documented. Sr and preliminary Nd isotopic data are indicative of a mantle origin for the least evolved magmas of all the studied occurrences. Mantle source material and melting models for the generation of the Brazilian alkaline magma types are also discussed.

The Cretaceous Banhadão alkaline complex in southeastern Brazil presents two potassic SiO 2-undersaturated series. The high-Ca magmatic series consist of initially fractionated olivine (Fo 92-91) + diopside (Wo 48-43En 49-35Ae 0-7), as evidenced by the presence of xenocrysts and xenoliths. In that sequence, diopside (Wo 47-38En 46-37Ae 0-8) + phlogopite + apatite + perovskite (Prv >92) crystallized to form the phlogopite melteigite and led to the Ca enrichment of the magma. Diopside (Wo 47-41En 32-24 Ae 3-14) continued to crystallize as an early mafic mineral, followed by nepheline (Ne 74.8-70.1Ks 26.3-21.2Qz 7.6-0.9) and leucite (Lc 65-56) and subsequently by melanite and potassic feldspar (Or 85-99Ab 1-7) to form melanite ijolites, wollastonite-melanite urtites and melanite-nepheline syenites. Melanite-pseudoleucite-nepheline syenites are interpreted to be a leucite accumulation. Melanite nephelinite dykes are believed to represent some of the magmatic differentiation steps. The low-Ca magmatic series is representative of a typical fractionation of aegirine-augite (Wo 36-29En 25-4Ae 39-18) + alkali feldspar (Or 57-96Ab 3-43) + nepheline (Ne 76.5-69.0Ks 19.9-14.4Qz 15.1-7.7) + titanite from phonolite magma. The evolution of this series from potassic nepheline syenites to sodic sodalite syenites and sodalitolites is attributed to an extensive fractionation of potassic feldspar, which led to an increase of the NaCl activity in the melt during the final stages forming sodalite-rich rocks. Phonolite dykes followed a similar evolutionary process and also registered some crustal assimilation. The mesocratic nepheline syenites showed interactions with phlogopite melteigites, such as compatible trace element enrichments and the presence of diopside xenocrysts, which were interpreted to be due to a mixing/mingling process of phonolite and nephelinite magmas. The geochemical data show higher TiO 2 and P 2O 5 contents and lower SiO 2 contents for the high-Ca series and different LILE evolution trends and REE chondrite-normalized patterns as compared to the low-Ca series. The 87Sr/ 86Sr, 143Nd/ 144Nd, 206Pb/ 204Pb and 208Pb/ 204Pb initial ratios for the high-Ca series (0.70407-0.70526, 0.51242-0.51251, 17.782-19.266 and 38.051-39.521, respectively) were slightly different from those of the low-Ca series (0.70542-0.70583, 0.51232-0.51240, 17.758-17.772 and 38.021-38.061, respectively). For both series, a CO 2-rich potassic metasomatized lithospheric mantle enriched the source with rutile-bearing phlogopite clinopyroxenite veins. Kamafugite-like parental magma is attributed to the high-Ca series with major contributions from the melting of the veins. Potassic nephelinite-like parental magma is assigned to the low-Ca series, where the metasomatized wall-rock played a more significant role in the melting process.

Post-Paleozoic magmatism in Angola and Namibia (SW Africa) is widespread along the continental margin (fl ood tholeiites of the Paraná-Etendeka system), and along transverse lineaments (alkaline and alkaline-carbonatitic complexes; sodic and potassic suites). These different magmatic suites are strictly associated in space and/ or time. Variable melting degrees of a veined lithospheric mantle are proposed for the most “primitive” magmas from geochemical modeling and Sr-Nd isotope systematics. A complex evolution emerges for some ultramafi c rocks (cumulus processes) and for differentiated rock compositions (assimilation and fractional crystallization, AFC, magma mixing), which may also involve anatexis of the crystalline basement and emplacement of S-type granites and rhyolites. Melting of a lithospheric mantle, without an appreciable contribution of the asthenosphere (thermal input excepted), is consistent with regional thermal anomalies in the deep mantle, mapped by gravity of the geoid, seismic tomography, and paleomagnetic analysis. The Walvis Ridge and Rio Grande “hotspot tracks” are interpreted as stress response in the lithosphere during rifting. A plume-related heat source is not favored by our results.

The Jaguarao stratoid dacites (Rio Grande do Sul, Brazil) are limited in areal extent, are comprised of about 3.2 km3 of preserved erupted material, and outcrop only in areas of the region underlain by mylonitic and ultramylonitic rocks. They are S-type volcanic rocks containing cordierite, orthopyroxene, plagioclase, and ilmenite as liquidus phases, and partially melted granite, gneiss, and migmatite enclaves that are very similar to the Precambrian basement rocks. The Jaguarao lavas have distinct geochemical signatures and Sr–Nd isotopes with respect to other volcanic rocks of the region. Available geochronological data for Jaguarao dacites range between 157 ± 5 Ma and 139.6 ± 7.4 Ma. Considering the errors, the younger ages obtained for Jaguarao lavas overlap the 138–128 Ma age of rocks of the Serra Geral Group, and thus indicate that the dacites were erupted prior to the break-up of Gondwana in this region. Petrographic, mineralogical, and petrochemical data, as well as the tectonic context of the Jaguarao lavas, suggest that magma genesis was linked, at least in part, to friction melts. The dacitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartz-feldspathic source terrane, leaving a granulite facies residue in subsurface. These melts were probably generated as a consequence of crustal thinning linked to simple shear extension just prior to Gondwana break-up and rifting of the southern Atlantic Ocean.

The Barra do Itapirapua (BIT) carbonatites in southern Brazil belong to the final stages of the Early Cretaceous alkaline rock – carbonatite magmatism of the Ponta Grossa Arch Province. The BIT complex is a dyke and vein stockwork in which four main carbonatitic phases are recognized, mainly magnesiocarbonatites and ferrocarbonatites. These carbonatites are generally overprinted by pervasive hydrothermal events. The C–O stable isotopic data indicate re-equilibration under hydrothermal conditions at temperatures between 375 and 80°C. Significant amounts of REE fluorocarbonate minerals, relatively Sr- and Th-rich, were deposited. Syntaxy between synchysite-(Ce) and parisite-(Ce) is very common owing to the similarity in structures, with alternating (001) layers of (CeF), (CO3) and (Ca). However, bastnasite-(Ce) occurs as individual crystals, overgrown by the synchysite and parisite polycrystals. Textural and chemical reactions between the REE fluorocarbonates provide insights into the mobility of rare-earth elements during fluid–rock interaction. The BIT complex is considered to be of potential economic interest for production of the rare-earth concentrates.

We present 40Ar/39Ar age determinations and chemical and Sr–Nd isotopic data for the tholeiitic dyke swarm cropping out at Arraial do Cabo peninsula at the eastern end of the Rio de Janeiro coastline. The age determinations indicate that the swarm emplaced approximately 55 Ma ago and thus is similar in age to the K-rich alkaline dykes and plugs also found in the peninsula. The dykes are basalts and basaltic andesites that belong to low Ti tholeiitic series. One dyke group is basaltic in composition, has relatively low Zr/Nb (8.9±1.8), roughly flat mantle-normalized incompatible element patterns, relatively low initial 87Sr/86Sr (0.70434–0.70426), and high eNd(55) (+2.3 to +3.8). These dykes chemically resemble enriched midocean ridge basalts (MORB). A second dyke group is formed by more evolved basaltic andesites with higher Zr/Nb ratios (12.7±2.1), negative Nb anomalies in the mantle-normalized patterns, higher initial 87Sr/86Sr (0.70548–0.70613), and lower eNd(55) (−1.8 to −2.2). Chemical and isotopic data exclude the possibility that the two dyke types are comagmatic or related to each other through crustal contamination processes. The genesis of the Arraial do Cabo tholeiites was likely caused by extension-related melting of a largely incompatible element-depleted mantle, with no trace of the enriched component in the roughly coeval, K-rich mafic alkaline magmas of the Serra do Mar province.

Four of the most important K-alkaline magmatic complexes of southern Brazil are associated with two main NW-trending structures. The Lages occurrence is linked to the uplift of a large crustal block, the Lages Dome, tectonically controlled by old NW-trending fault zones. The Jacupiranga, Juquia and Pariquera Acu (PGAM) occurrences are related to the Ponta Grossa Arch, a NW-trending uplift structure that produced tensional tectonic lineaments with the same direction and influenced the emplacement of both alkaline rocks and dyke swarms associated with Parana flood basalt activity. The Pariquera Acu complex, petrographically and geochemically described here for the first time, comprises coarse- to fine-grained rocks representing two main suites with different degrees of alkalinity: theralite to essexite and syenogabbro to syenodiorite. In both series highly evolved products are lacking. The presence of carbonatites is inferred. In the Jacupiranga complex (ca. 130 Ma), clinopyroxenite (jacupirangite) cumulates with minor dunites are associated with ankaratrites, alkali gabbros and leucocratic rocks such as syenites, nepheline syenites and peralkaline phonolites. Many phases of both calcic and magnesian carbonatite intrusions, to which relevant fenitization processes of jacupirangite are related, characterize the district. The Juquia complex (130–135 Ma) consists of prevalent olivine clinopyroxenite cumulates and nepheline syenites, the latter arranged in subanular concentric patterns together with subordinate ijolites–melteigites–urtites, essexites and syenodiorites. The central part of the intrusion is occupied by a small Mg-carbonatite body. The Lages complex (78–63 Ma) is made up of mafic–ultramafic rock-types (olivine melilitites, olivine nephelinites, basanites) and prevalent leucocratic bodies mainly consisting of peralkaline phonolites. Kimberlitic breccias and carbonatites are also present. Petrography, mineral and bulk-rock chemistry and mass balance calculations show that shallow-level fractional crystallization processes controlled the evolution from mafic to leucocratic rocks in all four complexes. Starting from a mantle source composition as suggested for the eastern Paraguay alkaline (ASU) magmatism, mass balance calculations indicate that primary magmas with high LILE contents and high La/Yb ratios are consistent with a garnet peridotite or phlogopite-bearing garnet peridotite solid residuum and less than 6% melting. The mantle sources are inferred to have been derived by different incompatible element enrichment probably associated with two late Proterozoic metasomatic processes: ASU (1.1 Ga) and Alto Paranaiba Igneous Province (APIP; 0.5 Ga). Mainly on the basis of geochemical evidence (e.g. Nb–Ta anomaly), mantle sources for PGAM and Lages Ol-melilitites and Ol-nephelinites seem to have been affected by an APIP-like event. On the other hand, an ASU-like metasomatic process is suggested for the Lages basanite mantle source, which may be indicative of vertical mantle heterogeneity. Geochemical data also point to a large-scale northward increase in concentration of incompatible elements (e.g. Lages Ol-melilitites and Ol-nephelinites vs APIP kamafugites). An attempt to correlate the chemical composition of silicate rocks and carbonate metasomatism shows that the mantle sources for PGAM and Lages occurrences plot on a mixing curve linking depleted peridotites and the average Ca-carbonatite. This fact supports the view that carbonatitic fluids/melts have been involved in the metasomatic processes responsible for variable incompatible element enrichment of the lithospheric mantle of SE Brazil.

The Passa Quatro alkaline complex is formed by strongly undersaturated, felsic intrusive and subvolcanic rocks (nepheline-syenites and phonolites). The abundance and fractionation of alkali feldspar, together with lesser amounts of femic phases, generates a suite which tends, as a whole, to become peralkaline and nepheline-enriched. Variation diagrams indicate, however, strong scatter of the data, reflecting non-homogeneous distribution of major and accessory phases and, probably, slightly different liquid lines of descent. Judging from the absence of restitic phases of crustal origin and other petrologic features, these rocks are probably products of extreme fractionation from basanitic magmas. On the basis of potassic affinity and initial 87 Sr/ 86 Sr isotopic ratios (0.70505), these rocks closely resemble other alkaline complexes of the Rio de Janeiro littoral belt, confirming provincialism in the Meso-Cenozoic alkaline province of eastern continental Brazil.