Excelso Ruberti

Mineralogical, petrological and geochemical aspects of alkaline and alkaline-carbonatite associations from Brazil

Abstract 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.

Petrogenesis of coexisting SiO2-undersaturated to SiO2-oversaturated felsic igneous rocks: The alkaline complex of Itatiaia, southeastern Brazil

Abstract 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.

Petrology, geochemistry and Sr–Nd isotopes of the Trindade and Martin Vaz volcanic rocks (Southern Atlantic Ocean)

Abstract Geochemical, mineralogical and Sr–Nd isotopic analyses were performed on the strongly undersaturated volcanic rocks of the Trindade and Martin Vaz islands, located in the South Atlantic Ocean (Brazil). The magmatic activity at Trindade was essentially bimodal, generating various ultrabasic rocks and mesocratic to leucocratic phonolites. The youngest volcanic events are made up exclusively of ultrabasic rocks, whereas the older episodes are characterized by abundant phonolites. Basic rocks (45

Petrography and mineral chemistry of carbonatites and mica-rich rocks from the Araxá complex (Alto Paranaíba Province, Brazil)

The Araxa complex (16 km 2 ) comprises carbonatites forming a central core and a complex network of concentric and radial dykes as well as small veins; additionally, it includes mica-rich rocks, phoscorites and lamprophyres. Fenites also occur and are represented by Proterozoic quartzites and schists of the Araxa Group. The petrographic study of 130 borehole samples indicates that the complex is basically made up by two rock-types, carbonatites and mica-rich rocks, and subordinately by a third unit of hybrid composition. Carbonatites range chemically in composition, the most abundant type being magnesiocarbonatites. Dolomite and calcite correspond to the chief constituents, but other carbonate phases, including the Ce-group RE minerals, are also recognized. Phosphates and oxides are widespread accessories whereas silicate minerals consist of olivine, clinopyroxene, mica and amphibole. Mica-rich rocks are represented by abundant glimmeritic rocks and scarce cumulitic phlogopite-, olivine- and diopside-bearing pyroxenites. Hybrid rocks mainly contain phlogopite and tetraferriphlogopite as cumulus and intercumulus phases, respectively; carbonate minerals may also be found. Chemical data indicate that the carbonatites are strongly enriched in REE and have lower contents of Nb, Zr, V, Cr, Ni and Rb compared to the mica-rich rocks. The higher K, Nb and Zr contents of the latter rocks are believed to be related to metasomatic processes (glimmeritization) of the pyroxenites. Similar REE patterns for carbonatites and mica-rich rocks seem to suggest that they are related to a single

Petrology and geochemistry of the Passa Quatro alkaline complex, southeastern Brazil

Abstract 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.

The Pariquera Açu K-alkaline complex and southern Brazil lithospheric mantle source characteristics

Abstract 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 tholeiitic dyke swarm of the Arraial do Cabo peninsula (SE Brazil): 39Ar/40Ar ages, petrogenesis, and regional significance

Abstract 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.

Geologia e Geocronologia do Maciço Alcalino Máfico-Ultramáfico Ponte Nova (SP-MG)

O macico alcalino mafico-ultramafico Ponte Nova, localizado no setor norte da Provincia Serra do Mar, junto a porcao oriental da Serra da Mantiqueira, na regiao Sudeste do Brasil, encontra-se encaixado em granitoides e gnaisses migmatiticos Pre-Cambrianos. O macico apresenta duas areas distintas de exposicao, estando ambas separadas por rochas do embasamento: uma maior (~5,5 km²), de forma eliptica e com grande variedade de litotipos; outra menor (~1 km²), de forma irregular e petrograficamente menos complexa, e situada a sul da primeira. E predominantemente composto por uma associacao gabroide alcalina, gerada por sucessivos pulsos magmaticos. As rochas mais abundantes sao cumulatos maficos e ultramaficos (e.g., olivina clinopiroxenitos e melagabros com olivina) presentes nos niveis inferiores do macico, ao lado de rochas gabricas e monzogabricas porfiriticas, equigranulares e bandadas nos niveis superiores, todos representativos da evolucao magmatica dos principais pulsos. Um corpo de brecha magmatica e uma suite de diques (lamprofiros a tefrifonolitos) interceptam todas as rochas intrusivas. Analises geocronologicas por K/Ar para diferentes rochas indicam uma idade media de 87,6 Ma (Cretaceo Superior), que e similar as obtidas para as demais ocorrencias do setor norte da referida provincia. A incompatibilidade das idades torna dificil sustentar a hipotese de sua associacao genetica com a pluma mantelica de Trindade

Magmatism and fenitization in the Cretaceous potassium-alkaline-carbonatitic complex of Ipanema São Paulo State, Brazil

The Ipanema alkaline-carbonatitic complex is part of the Meso-Cenozoic alkaline magmatism located within the southeastern part of the Brazilian Platform. Drill-core and field sampling have indicated the occurrence of glimmerites, with subordinate shonkinites (mela-syenites), clinopyroxene-bearing glimmerites, diorites and syenites. The glimmerites are cross-cut by lamprophyric dykes and calciocarbonatites. Fenitization has deeply affected the country rocks, originating dioritic and syenitic rocks. The Ipanema rocks show a distinct potassic affinity. The initial Sr-Nd- isotopic composition of the Ipanema rocks (87Sr/86Sr = 0.70661–0.70754 and 143Nd/144Nd = 0.51169–0.51181) is similar to that of tholeiitic and potassium-rich-alkaline rocks of the Eastern Paraguay. Stable isotope data for the Ipanema calciocarbonatite suggest interaction with fluids at temperatures typical of hydrothermal stages, as hypothesized for other carbonatite complexes from southeastern Brazil. The chemical differences between the lamprophyre, glimmerites, carbonatites, apatitites and magnetitites, and the absence of marked REE enrichment in the evolved lithologies, all indicate that fractional crystallization and accumulus of liquidus phases in a magma reservoir, likely coupled with liquid immiscibility processes, may have played an important role in the genesis of the Ipanema rocks.

The Banhadão Alkaline Complex, Southeastern Brazil: source and evolution of potassic SiO2-undersaturated high-Ca and low-Ca magmatic series

The Cretaceous Banhadão alkaline complex in southeastern Brazil presents two potassic SiO2-undersaturated series. The high-Ca magmatic series consist of initially fractionated olivine (Fo92-91) + diopside (Wo48-43En49-35Ae0-7), as evidenced by the presence of xenocrysts and xenoliths. In that sequence, diopside (Wo47-38En46-37Ae0-8) + phlogopite + apatite + perovskite (Prv>92) crystallized to form the phlogopite melteigite and led to the Ca enrichment of the magma. Diopside (Wo47-41En32-24 Ae3-14) continued to crystallize as an early mafic mineral, followed by nepheline (Ne74.8-70.1Ks26.3-21.2Qz7.6-0.9) and leucite (Lc65-56) and subsequently by melanite and potassic feldspar (Or85-99Ab1-7) 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 (Wo36-29En25-4Ae39-18) + alkali feldspar (Or57-96Ab3-43) + nepheline (Ne76.5-69.0Ks19.9-14.4Qz15.1-7.7) + 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 TiO2 and P2O5 contents and lower SiO2 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 CO2-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.