Piero Comin-Chiaramonti

Paraná Magmatic Province-Tristan da Cunha plume system: fixed versus mobile plume, petrogenetic considerations and alternative heat sources

Abstract Paleomagnetic reconstructions demonstrate that the Tristan da Cunha (TC) plume, which is usually related to the genesis of the high- and low-Ti flood tholeiites of the Parana Magmatic Province (PMP), was located ∼1000 km south of the Parana Province at the time of the magma eruptions. Assuming plume mobility, and considering the low-velocity zone identified in the northern portion of the PMP as the TC ‘fossil’ plume (∼20° from the present TC position), the plume migrated southward from 133–132 (main volcanic phase) to 80 Ma at a rate of about 40 mm/yr. From 80 Ma to Present the plume remained virtually fixed, leaving a track (Walvis Ridge) compatible with the African plate movement. However, geochemical and Sr–Nd–Pb isotopic data do not support that the tholeiites from Walvis Ridge, Rio Grande Rise and Parana can result from mixing dominated by the TC plume and mid-ocean ridge basalt components. The similarity among the high-Ti basalts from Rio Grande Rise, part of Walvis Ridge (525A) and the Parana Province suggests that delaminated subcontinental lithospheric mantle must be considered in their genesis. Regional thermal anomalies in deep mantle mapped by geoid and seismic tomography data offer an alternative non-plume-related heat source for the generation of intracontinental magmatic provinces.

Continental Flood Volcanism From the Paraná Basin (Brazil)

In the past years study of the continental flood volcanism in the Parana basin had been mainly concentrated in the southern regions, where thick lava sequences are well exposed. Such research essentially concerned basic geology, petrography, radiometric dating and, subordinately, petrochemistry, isotope geochemistry and paleomagnetism (cf. Almeida, 1983; Amaral et al., 1966; Amaral and Crosta, 1983; Asmus and Baisch, 1983; Compston et al., 1968; Cordani et al., 1980; Creer et al., 1965; Ernesto, 1985; Ernesto et al., 1979; Fodor et al., 1985a, 1985b; Fulfaro et al., 1982; Halpern et al., 1974; Leinz et aL, 1966; McDougall and Ruegg, 1966; Mantovani et al., 1985a, 1985b; Marimon et al., 1983; Marques, 1983; Melfi, 1967; Minioli et al., 1971; Pacca and Hiodo, 1976; Pacca and Ernesto, 1982; Ruegg, 1976; Ruegg and Amaral, 1976; Sartori et al., 1975, 1982; Sartori and Gomes, 1980; Sartori and Filho, 1983; Souza, 1983).

Differentiation of partial melts in the mantle: Evidence from the Balmuccia peridotite, Italy

The Balmuccia peridotite shows evidence, in the form of a network of dykes, of partial melting and flow crystallization processes. The partial melting processes probably occurred over a fairly long time interval, and seem to have been related to different “melting pulses”. Resultant liquids were broadly picritic. Melting occurred incongruently according to the scheme cpx+opx+(ol+sp)=Mg-richer ol+Cr-richer sp+L.Partial melts tended at first to accumulate in horizontal layers; then, as the critical melting threshold was exceeded, liquids were able to filter slowly towards lower pressure zones. In doing so liquids fractionated initially in situ, via crystallisation of websteritic dykes of the Cr-diopside suite, and later, in the overlying mantle, via crystallisation of transitional dykes and those of the Al-augite suite. This filter-pressing stage, when flow velocities were very low and discontinuous, probably corresponded to the period of maximum deformability of the peridotite.The type of differentiation testified by the dykes of the Balmuccia peridotite, is characterized by a decrease in SiO2, a rapid enrichment in Al2O3 and a mild increase in FeO, and is substantially in accordance with experimental trends from the fo-an-di-SiO2 system in the spinel-peridotite stability field. A close relationship between type of differentiation, flow velocity and mechanical behaviour of the mantle peridotite is a feature of the proposed model.

Early Cretaceous–Tertiary magmatism in Eastern Paraguay (western Paraná basin): geological, geophysical and geochemical relationships

Abstract Structural characteristics of Eastern Paraguay, at the westernmost side of the Parana basin, show that the tectonics in the region is extensional. This tectonics controlled the emplacement of the alkaline rocks in Early Cretaceous–Tertiary times. Major structures are NW-trending zones of tectonic subsidence, i.e. the ‘Asuncion–Sapucai–Villarrica’ and ‘Amambay’ grabens of the central and northeastern regions of Eastern Paraguay, respectively. Early Cretaceous potassic magmatism is widespread in the ‘gravity lows’, and predates the tholeiitic Early Cretaceous magmatism in the northern areas, whereas in central Paraguay it postdates the tholeiitic flood basalts. Sodic alkaline rocks (late Early Cretaceous–Tertiary) are widespread in cratonized areas of the La Plata Craton. The Early Cretaceous uncontaminated potassic rocks (and tholeiites) have geochemical features of ‘subduction type’ (e.g. Ta–Nb negative anomaly), while the contrary occurs for the closely associated younger sodic magmatism. This implies different mantle sources consistently with Sr–Nd isotopes which are Rb–Nd enriched and depleted for the potassic and sodic rocks, respectively. T DM (Nd) model ages point to two notional distinct metasomatic events occurred in Middle and Late Proterozoic times. A contribution of asthenospheric melts from Tristan da Cunha Plume is not appreciable. We suggest that the source(s) of the recurrent potassic magmatism and interposed tholeiitic event may not be easily accounted for by the Tristan da Cunha plume hypothesis.

Phase relations in the partial melting of the Baldissero spinel-lherzolite (Ivrea-Verbano zone, Western Alps, Italy)

The occurrence of various types of mobilizates in the Baldissero spinel lherzolite is due to partial melting of the same body. The study of the relationships between the peridotite and its mobilizates demonstrates that olivine did not take an active part in the fusion. Estimates of the degree of partial melting vary from 10% for the average composition, to 20% for the most depleted samples. These values refer to an initial pyrolitic composition, and thus are relative, as they can vary depending on the actual primary composition.The calculated composition of the liquid generated by partial melting is quite similar to that of a picritic basalt, and is practically the same irrespective of the 10% and 20% fusion. This fact provides strong evidence that melting took place at a unique invariant point of the natural system, producing a liquid with a remarkably constant composition.Projection of the liquid in the fo-an-di-si diagram is fairly well aligned with the modal compositions of the solid residua, but does not coincide with the minimum of the simplified system. The proposed solution is based on the enlargement of the spinel field (at constant pressure), due to the Cr content in this phase. Therefore, the position of the invariant minimum is not fixed, but rather controlled by the Cr content of the spinel. Is is suggested that, by an increase in the Cr content, spinel might at a given moment become refractory. Thus, saturated or over-saturated magmas are produced depending on the phase relations between olivine, orthopyroxene and clinopyroxene. This would happen in the case of very advanced fusions or in the case of fusion of already depleted peridotites.The relationships between mobilizates of different generations suggest a non adiabatic mantle upwelling.

Carbonatites from Eastern Paraguay and genetic relationships with potassic magmatism: C, O, Sr and Nd isotopes

SummaryGeochemical characteristics were systematically determined for Early Cretaceous samples of carbonatitic rocks from Eastern Paraguay (Rio Apa, Amambay and Central Provinces). The data show that all the occurrences have an enriched isotopic signature and that the carbonatites have negligible or absent crustal signature. A petrogenetic model (parent liquids, fractional crystallization, hydrothermal interactions and weathering) is proposed as a function of incompatible trace element, stable (O-C) and radiogenic (Sr-Nd) isotope variations with the aim to test the significance of carbonatitic complexes as a marker of the metasomatized subcontinental lithospheric mantle. The results indicate that the carbonatites and primary carbonates from eastern Paraguay, and those from the north eastern Paraná Basin (SE Brazil), were affected by metasomatic events distinct in time and composition.ZusammenfassungDie geochemischen Charakteristika von frühkretazischen Karbonatitproben aus Ostparaguay (Rio Alpa, Amambay und Zentrale Provinzen) wurden untersucht. Die Daten belegen, daß alle Vorkommen eine isotopische Anreicherungssignatur zeigen und daß ihnen eine entsprechende Krustensignatur fehlt. Ein Petrologisches Modell (Ausgangsschmelze, fraktionierte Kristallisation, hydrothermale Interaktion und Verwitterung) wird auf Grund der Verteilung der inkompatiblen Spurenelemente, der stabilen (C-O) und radiogenen (Sr-Nd) Isotope vorgeschlagen. Es versucht die Bedeutung der Karbonatitkomplexe als „Markerhorizonte” des metasomatischen subkontinentalen Mantels zu überprüfen. Die Ergebnisse zeigen, daß die Karbonatite und die primären Karbonate in Ostparaguay, und jene aus dem Paraná Becken SüdostBrasiliens durch zeitlich und zusammensetzungsmäßig unterschiedliche metasomatische Prozesse erfaßt wurden.

Low-pressure evolution of basalt sills from Bore-Holes in the Paranà basin, Brazil

Basalt sills (up to 200 metres thick) of tholeiitic and transitional chemistry are widespread in the Paleozoic sedimentary rocks of the Parana basin. They closely correspond in composition to the overlying Lower Cretaceous flood basalts, and are systematically characterized by pigeonite.

The early Proterozoic carbonatite complex of Angico dos Dias, Bahia State, Brazil; geochemical and Sr-Nd isotopic evidence for an enriched mantle origin

Abstract Borehole samples of carbonatites and phlogopite-pyroxenites from the Angico dos Dias (AdD) intrusive alkaline complex, State of Bahia, Brazil, have been investigated in terms of mineralogy, geochemistry and C-O-Sr-Nd isotopes. The AdD complex, of Early Proterozoic age (2 Ga), intrudes the northern side of the Sao Francisco Craton. Mineralogy and petrography indicate that the studied rocks only partially preserved their magmatic textural features owing to their metamorphic re-equilibration (greenschist facies). The REE contents and LREE/HREE ratios of the AdD carbonatites are very high (mean 3979 ± 718 ppm and La/Yb = 215 ± 23, respectively), as for most Precambrian magmatic carbonatites. The AdD carbonatites are also enriched in 18O (δ18O = 11.9 to 15.8‰), possibly due to secondary processes (e.g. metamorphism, alteration) whereas carbon isotopes are in the range of ‘primary carbonatites’ (δ13C = -5.7 to -7.1‰). Most of the initial 87Sr/86Sr and 143Nd/144Nd values of the studied carbonatites were not appreciably modified by secondary processes. Their εtSr and εtNd values (20.0 to 25 and 0.7 to -4.5, respectively) indicate enriched mantle sources very different from the ‘depleted’ ones related to many Precambrian carbonatites from North America (0.6-2.6 Ga) and Africa (0.5-2.0 Ga). The Early Proterozoic Sr-Nd isotopic signatures of the AdD carbonatites are similar to those of the Early Cretaceous carbonatites from the Paraná basin. The latter carbonatites show a great isotopic variability ranging from Bulk Earth to the related potassic magmatism from Asuncion-Sapucai graben in the Eastern Paraguay (K-ASU magmatism: εtSr = 35 to 50 and εtNd = -12 to -20). The very similar isotopic compositions of Precambrian and post-Palaeozoic carbonatites worldwide indicate that the subcontinental mantle variability lasted for long periods of time and indicate a large-scale mantle heterogeneity.

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.

Cretaceous Na‐Alkaline Magmatism from the Misiones Province (Paraguay): Its Relationships with the Paleocene Na‐Alkaline Analog from Asunción and Geodynamic Significance

Alkaline (sodic) volcanic rocks, i.e., ankaratrites‐melanephelinites, basanites‐tephrites, and phonolites, dated at 119 Ma and similar in composition to the 60‐Ma plugs of the Asunción Province, occur in Eastern Paraguay and belong to the Misiones Province. The age relationships confirm that the youngest volcanic events in Eastern Paraguay, at the central westernmost side of the Paraná basin, are represented by alkaline rock types of sodic affinity emplaced in late Early Cretaceous and Paleocene times. This sodic magmatism contrasts with the Early Cretaceous alkaline (potassic) magmatism of the region, and it is associated in space and time with the Paraná basin tholeiites. Geological and geophysical results for Eastern Paraguay indicate transtensional tectonics, with a NE‐SW regional extensional stress field. The geochemistry and Sr‐Nd‐Pb isotope systematics are consistent with a lithospheric mantle source(s) enriched in incompatible elements by metasomatic processes. Nd model ages suggest that these probably occurred during Meso‐ and/or Neoproterozoic times and may be regarded as precursors of both alkaline and tholeiitic magmas in Eastern Paraguay. Potential parents for the alkaline (sodic) liquids have been modeled in terms of small degrees of mantle partial melting. Multielemental plots of calculated mantle sources for these liquids from Asunción and Misiones contrast with the analog mantle sources for the Paraguayan alkaline (potassic) suite, confirming the view that popular geodynamic markers of this type remain implausible indicators of subduction. Our results support the view that the magma genesis and the emplacement of the alkaline magmatism in southeastern Paraguay, and even in northwestern Argentina and Bolivia, is related to and probably driven by reactivation of preexisting lithospheric discontinuities in the various South American blocks, which promoted local decompression melting of previously enriched mantle sources.