Angelo De Min

The Central Atlantic Magmatic Province (CAMP) in Brazil: Petrology, Geochemistry, 40Ar/39Ar Ages, Paleomagnetism and Geodynamic Implications

The CAMP tholeiitic magmatism in Brazil (mean 40 Ar/ 39 Ar age of 199.0±2.4 Ma) occurs on the continental margin to ca. 2,000 km into the South American platform, near the boundary between the ancient terrains of the Amazonia craton and Proterozoic/Brazilian-cycle related mobile belts. Geological evidence indicates that this magmatism was preceded, in Permo-Triassic times, by continental sedimentation, indicating a possible regional uplift. The Brazilian CAMP tholeiites are generally evolved and characterized by a low TiO 2 concentration (less than 2 wt%). The Cassipore dykes, which are usually high in TiO 2 (more than 2wt%) are an exception. The Cassipore low- and high-TiO 2 basalts are characterized by a positive Nb anomaly and Sr-Nd isotopes that are parallel to typical mantle array. Except for one sample, all the other Brazilian CAMP tholeiites that are low in TiO 2 , show Sr-Nd isotopes trending towards crustal components. The latter isotopic characteristics could be related to crustal recycling ancient (Middle-Late Proterozoic) subductions, and/or low-pressure crustal interaction. All the Brazilian CAMP tholeiites show a decoupling between their Sr-Nd isotopic composition and Rb/Sr and Sm/Nd values, suggesting mantle metasomatism, and/or subduction-related crustal interaction before mantle melting. Notably, the chemical data show that tholeiites from specific Brazilian regions are related to mantle sources that reflect compositional mantle heterogeneity, including the lower mantle of the lithospheric thermal boundary layer. In general, paleomagnetic poles for CAMP rocks from South America, Africa and North America match an age of ca. 200 Ma, but also show a distribution pattern trending to younger ages (e.g. 190 Ma), especially for the South American poles relative to the CAMP magmatism of the continental edge. The Brazilian CAMP magmatism cannot be easily explained through plume head (active) models, being instead consistent with mantle geodynamic processes where the unstable buoyancy of the Pangea supercontinent played an essential role to approach isostatic stabilization. Therefore, it is proposed that the Brazilian CAMP magmatism was related to hot upper mantle incubation under thick continental lithosphere, and to edge-driven convection between lithospheric domains with different thickness.

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.

Eastern Paraguay: an overview of the post-Paleozoic magmatism and geodynamic implications

Eastern Paraguay, at the westernmost part of the Paraná-Angola-Etendeka (PAE) system, at the westernmost fringe of the Early Cretaceous flood tholeiites (Serra Geral Formation, SGF), was the site of repeated and different magmatic activity from Mesozoic to Paleocene times. In the early Middle Triassic, sodic alkaline magmatism occurred along the belt represented by the Paraguay River, at the boundaries between Brazil and the Chaco-Pantanal basin. During the Early Cretaceous, potassic alkaline magmatism (where are present also alkaline-carbonatitic complexes) pre- and post-dates SGF. Further alkaline sodic magmatism occurred in Eastern Paraguay during late Early Cretaceous and Paleocene. The latter are characterized by the presence of abundant mantle xenoliths (spinel facies) in the ultramafic rock-types in the Misiones and Asunción provinces. Geological, petrological, mineralogical, geochemical and isotopic results suggest that almost two main mantle components have been involved in the genesis of the magmatism in Eastern Paraguay: an extreme and heterogeneous EM and a HIMU component. The EM-like component appear prevalent in the Early Cretaceous potassic alkaline magmatism, whereas the HIMU was important in the late Early Cretaceous and Paleocene sodic magmatism. Different contributions of mantle component similar to EM and HIMU could also explain the geochemical heterogeneity of the Early Cretaceous flood tholeiites in Eastern Paraguay (i.e. high- and low-Ti types, with high and low contents of incompatible elements, respectively). In the light of these facts, the mantle plume/hotspot hypothesis for the origin of the magmatism in Eastern Paraguay must be reviewed, at least regarding which plumes are most likely to have been active at the right place and right time for a specific province. In fact, in order to explain the widespread distribution of South American (Angolan and Namibian) Early Cretaceous tholeiitic and alkaline magmatism, a hypothetical mantle plume head is not compelling. We support rifting processes which resulted in different lithospheric thickness beneath the edge of cratonic shields, inducing small-scale convection cells. In addition, 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. On the whole, the geophysical evidence and geochemical results, combined with new well controlled ages (mainly40Ar/39 Ar ages) for the magmatic events in Eastern Paraguay, indicate that any model proposed for the evolution of the PAE in terms of HIMU and EM end-members must satisfy the following constraints: (a) HIMU and EMI-II are not restricted to the oceanic environment; (b) end-members are variously associated in space as a function of the various protoliths; (c) mantle regions with HIMU and EMI isotope compositions are capable of generating melts that can lead to the formation of a wide variety of silicate rocks, including melts enriched in CO2 (including alkaline-carbonatitic complexes,i.e. mantle metasomatism); (d) the geochemical features of the sodic alkaline rock-types cluster together, well distinct in comparison with the potassic alkaline from Paraguay, but consistent with the potassic alkaline-carbonatite rocks from Angola and Namibia; (e) the paleomagnetic results in dicate that any mantle plume hypothesis is in disagreement with the fixed and mobile plume models. Finally, it is suggested that a more complete fluidodynamic models must be developed for the plate-tectonic models.RiassuntoIl Paraguay Orientale, situato nella parte più occidentale del sistema Paraná-Angola-Etendeka (Namibia: PAE), è una regione interessata da ripetuta e differente attività magmatica in un arco temporale che si estende dal Mesozoico al Paleocene. Durante il Medio Triassico inferiore, un magmatismo alcalino sodico si estende lungo il corso superiore del Rio Paraguay, al confine tra il Brasile (bacino del Paraná) e il bacino Chaco-Pantanal. Durante il Cretacico Inferiore differenti tipi di magmatismo alcalino potassico (dove sono frequenti i complessi alcalinocarbonatitici) precedono e seguono le colate tholeiitiche della Formazione Serra Geral. Successivamente, durante il tardo Cretacico Inferiore e il Paleocene, si verificano altri due episodi di magmatismo alcalino sodico. Questi ultimi sono caratterizzati dalla presenza di abbondanti xenoliti di mantello (facies a spinello) nei tipi di rocce ultramafiche. In generale, le caratteristiche geologiche, petrologiche, mineralogiche, geochimiche e geochimico-isotopiche suggeriscono che nel Paraguay Orientale almeno due componenti di mantello sono state variamente coinvolte nella genesi del magmatismo dal Cretacico al Paleocene; una componente fortemente arricchita (tipo EM) e una componente «impoverita» (tipo HIMU). In particolare la componente tipo EM è assolutamente prevalente nel magmatismo alcalino potassico, mentre la componente tipo HIMU appare importante per il magmatismo sodico del tardo Cretacico Inferiore e del Paleocene. Contributi differenti dei componenti EM e HIMU sono consistenti anche con la eterogeneità geochimica osservata nelle tholeiiti del Paraguay Orientale (come anche in tutto il bacino del Paraná), cioè varianti ad «alte» e «basse» concentrazioni in Titanio (ed elementi incompatibili). Alla luce di questi fatti deve essere rivista ogni ipotesi dimantle plume ebot spot, relative all’ origine del magmatismo nel Paraguay Orientale, per lo meno in quanto inconsistenti dal punto di vista spazio-temporale. Infatti, per quanto riguarda la diffusione in Sud America (e Angola e Namibia) di magmatismo tholeiitico e alcalino, con caratteristiche geochimiche e petrogenetiche profondamente differenti che chiaramente riflettono eterogeneità di mantello (metasomatismo di mantell), una ipoteticamantle plume è difficile da sostenere. Si suggeriscono pertanto processi distensivi diffusi a differenti profondità litosferiche e attivi in particolar modo ai limiti delle differenti aree cratoniche e che inducono celle convettive a piccola scala. Per di più, la presenza di anomalie termiche di lungo periodo è stata messa in evidenza dai modelli di distribuzione delle onde sismiche, basati su tecniche tomografiche sia delle onde P che delle onde S. Nel complesso, le evidenze geofisiche combinate con i dati geochimici e recenti datazioni radiometriche controllate dal punto di vista degli equilibri isotopici (soprattutto datazioni40Ar/39Ar su rocce e minerali), indicano che nel Paraguay Orientale ogni modello proposto per la evoluzione del PAE, in termini di componenti EM e HIMU, deve soddisfare le seguenti condizioni; (a) i componenti di mantello tipo HIMU e EMI-II non sono ristretti al solo ambiente oceanico; (b) questi componenti risultano variamente associati nello spazio in funzione di differenti protoliti; (c) regioni mantelliche con differenti composizioni isotopiche tipo HIMU and EMI sono in grado di generare fusi che possono portare alla formazione di una grande varietà di rocce silicatiche, inclusi fusi arricchiti in CO2, e quindi complessi alcalino-carbonatitici; (d) le caratteristiche geochimiche del magmatismo sodico, sia Mesozoico che Paleogenico, sono estremamente simili, ma ben differenti rispetto al magmatismo alcalino-potassico del Paraguay, e se mai consistenti col magmatismo potassico dell’Angola e della Namibia; (e) le risultanze paleomagnetiche indicano che ogni modello relativo alla presenza dimantle plume non si accorda né con la presenza di una «mantle plume» fissa, né mobile. Infine si suggerisce che debbano essere sviluppati modelli fluidodinamici che tengano conto di tutte le variabili meccaniche del nostro pianeta per quanto riguarda una migliore definizione della tettonica a placche.

Petrology of potassic alkaline ultramafic and carbonatitic rocks from Planalto da Serra (Mato Grosso State), Brazil

The Planalto da Serra igneous rocks form plugs, necks and dykes of carbonate-rich ultramafic lamprophyres (aillikites and glimmerites with kamafugitic affinity) and carbonatites (alvikites and beforsites). Phlogopite and/or tetraphlogopite, diopside and melanitic garnet are restricted to aillikitic rock-types, whereas pyroclore occurs only in carbonatites. Aillikites and carbonatites are altered to hydrotermalites, having chlorite and serpentine as dominant minerals. Planalto da Serra igneous rock association has kamafugitic affinity (i.e. effusive, ultrapotassic. High LREE/HREE fractionation, incompatible elements data and Sr-Nd isotopes, suggest that the K-ultramafic alkaline and carbonatite rocks originated from a variably metasomatized mantle source enriched in radiogenic Sr. Crustal contamination is negligible or absent. Age values of 600 Ma rule out the geochronological relationship between the investigated intrusions and the Mesozoic alkaline bodies from the Azimuth 125° lineament. The TDM model ages allow to conclude that Planalto da Serra magma is derived from the partial melting of a mantle source metasomatised by K-rich carbonatated melt during the Early to Late Neoproterozoic. On the basis of alkaline magmatism repetitions at 600 Ma and 90–80 Ma we question the subsistence of a stationary mantle plume for so long time.

Magmatism in the Asunción-Sapucai-Villarrica Graben(Eastern Paraguay) Revisited: Petrological, Geophysical,Geochemical, and Geodynamic Inferences

The Asuncion-Sapucai-Villarrica graben (ASV) in Eastern Paraguay at the westernmost part of the Parana Basin was the site of intense magmatic activity in Mesozoic and Tertiary times. Geological, petrological, mineralogical, and geochemical results indicate that the following magmatic events are dominant in the area: (1) tholeiitic basalt and basaltic andesites, flows and sills of low- and high-titanium types; (2) K-alkaline magmatism, where two suites are distinguished, that is, basanite to phonolite and alkali basalt to trachyte and their intrusive analogues; (3) ankaratrite to phonolite with strong Na-alkaline affinity, where mantle xenoliths in ultramafic rocks are high- and low-potassium suites, respectively. The structural and geophysical data show extensional characteristics for ASV. On the whole, the geochemical features imply different mantle sources, consistently with Sr-Nd isotopes that are Rb-Nd enriched and depleted for the potassic and sodic rocks, respectively. Nd model ages suggest that some notional distinct “metasomatic events” may have occurred during Paleoproterozoic to Neoproterozoic times as precursor to the alkaline and tholeiitic magmas. It seems, therefore, that the genesis of the ASV magmatism is dominated by a lithospheric mantle, characterized by small-scale heterogeneity.