Journal of Petrology | Volume 38 | Number 4 | Pages 495-528 | 1997
© Oxford University Press 1997
Potassic and Sodic Igneous Rocks from Eastern Paraguay: their Origin from the Lithospheric Mantle and Genetic Relationships with the Associated Paraná flood tholeiites
1 Dipartimento di Ingegneria Chimica, Dell'Ambiente e Delle Materie Prime, University of TriestePiazzale Europa 1, I-34127 Trieste, Italy
2 Dipartimento di Geofisica Vulcanologia, ‘Federuco II’ University, LargosMarcellino 10, I-80138 Naples, Italy
3 Dipartimento di Scienze Della Terra, University of TriesteVia E Weiss 8, I-34127 Trieste, Italy
4 Instituto de Geocièncias, Universidade de Sãao PauloCP 30627, 01051 São Paulo, SP, Brazil
5 Dipartimento di Scienze Della Terra, ‘La Sapienze’ UniversityPiazzale Aldo Moro 5, I-00185, Rome, Italy
6 Istituto di Mineralogia, Petrografia e Geochimica, University of PalermoVia Archirafi 36, I-90100, Palermo, Italy
Received June 4, 1996; Revised typescript accepted November 7, 1996
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ABSTRACT |
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 TOP ABSTRACT IntroductionGeological SettingClassification and PetrographyPetrochemistryPetrogenesisSignificance of the Asu...Genetic Relationships between...ConclusionsReferences |
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Eastern Paraguay represents the westernmost fringe of Early Cretaceous Parana flood tholeiites (Serra Geral Formation SGF). Besides the SGF, eastern Paraguay has been the site of alkaline magmatism since Mesozoic times: (1) Late Permian-Early Triassic sodic intrusions and lavas: (2) Early Cretaceous potassic igneous rocks and very scarce sodic lavas (3) Late Cretaceous–Oligocene sodic lavas. Two distinct magmatic events are dominant in the Asuncion–Sapucai graben (ASU) eastern Paraguay: (1) wide-spread potassic magmatism and SGF tholeiites (Early Cretaceous) (2) Asuncion sodic magmatism (Late Cretaceous–Oligocene). The potassic rocks form a compositional continuum from moderately to strongly potassic. Two potassic suites are proposed, i.e. basanite to phonolite and alkali basalt to trachyte and their intrusive analogues. The sodic rocks include ankaratites, nephelinites and phonolites. Two similar but distinct parental magmas have been inferred for the potassic suites, both characterized by strongly fractionated REE and negative ‘Ta–Nb–Ti anomalies’. Slight positive Ta and Nb anomalies distinguish the sodic rocks. Sr–Nd isotope data confirm the distinction of the potassic rocks, enriched in radiogenic Sr and low in radiogenic Nd, from the sodic rocks, close to bulk Earth. Crustal contamination does not appear to have been significant in the generation of the investigated rocks, supported by
18O data. The source of potassic rocks is constrained by high LILE, LREE, Th, U and K, relative to a primitive mantle composition, and a garnet peridotite is favoured as a possible mantle source for the investigated rocks. The close association of potassic and sodic rock suites in the ASU demands that their parental magmas derived from a heterogeneous subcontinental mantle, variously enriched in incompatible elements. Significant H2O, CO2 and F are expected in the mantle source(s) for the occurrence of coeval carbonatites. Any genetic hypothesis based on a ‘mantle plume’ system is constrained by strong lithospheric mantle characteristics. This does not preclude that thermal perturbations from the asthenosphere may have trggered magmatic activity in the lithospheric mantle. Model ages indicate that two distinct metasomatic events may have occurred during Late and Early–Middle Proterozoic as precursor to the genesis of tholeiitic and alkaline magmatism in the Parana basin. These metasomatic processes were chemically distinct, indicated by the strong differences in Ti, LILE and HFSE concentrations found in both alkaline provinces (e.g. potassic rocks from ASU vs Alto Paranaiba Igneous Province) and Parana tholeiites (low vs high Ti). Ingeneral, the relationships between the alkaline rocksfrom southeastern Brazil, i.e. AltoParanaiba, Ponta Grossa Arch, Serra do Mar, Lages and the flood basalts of the Parana Basin, support a common origin in the lithospheric mantle. Sr–Nd isotope and other geochemical data indicate that a significant role was played by a mantle component depleted in incompatible elements and with high Sm/Nd ratio. This component (N-MORB type) would be represented by the depleted portions of a mantle which was variously metasomatized during Proterozoic times. The isotopic and geochemical features of the modernTristan da Cunha plume are distinctly different from the component depleted in incompatible elements, and its contribution is not apparent in the compositions of the Parana tholeiites. KEY WORDS: Alkaline magmatism; Paraguay; petrogenesis; Sr–Nd isotopes
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Introduction |
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TOPABSTRACTIntroductionGeological SettingClassification and PetrographyPetrochemistryPetrogenesisSignificance of the Asu...Genetic Relationships between...ConclusionsReferences |
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Potassic magmatism, sensu Foley et al. (1987)
, generally occurs in different tectonic settings. This magmatism is characterized by high concentration of incompatible elements (IE), besides high K, which makes its genesis by partial melting of conventional peridotite mantle sources unlikely (Wyllie, 1987). A ‘veined’ mantle source enriched by various degrees of mantle metasomatism under variable redox conditions has been proposed by Foley, (1988), although the nature of the enriched component(s) has not been uniquely identified, and the role of the tectonic regime and related mantle dynamics at the time of potassic magma genesis remains only partly understood.
For example, mafic potassic rocks from the Alto Paranaíba Province of SE Brazil, widespread in an area of crustal thinning (Santero et al., 1988), have been related to a mantle plume, which made contact with and melted the most fusible parts of lithospheric mantle at the margins of the São Francisco craton during Late Cretaceous times (Gibson et al., 1995b). However, mafic potassic rocks are not exclusive to cratonic or mobile belt settings (e.g. Bergman, 1987; Foley et al., 1987) nor are they confined to regions where the lithosphere is especially thin, e.g. New South Wales (Ewart et al., 1988; Ewart, 1989).
This paper seeks to evaluate new petrological information on the alkaline magmatism of the western margin of the Paraná Basin, where a variety of potassic rocks are associated in time and space with sodic rocks and carbonatites. The investigated rocks occur in eastern Paraguay and span the Mesozoic to Eocene–Oligocene times. Therefore, they are germane to the magmatic and tectonic evolution of the Paraná Basin generally and to the petrogenesis of alkaline rocks from a continental setting in particular. It will be demonstrated that these rocks are closely related to magmatic and tectonic events responsible for the formation of the Paraná tholeiitic flood basalts. Implications and constraints for the origin and evolution of the magmatic activity in and around the Paraná Basin are discussed, also in the light of recent data on the potassic magmatism from Alto Paranaíba Igneous Province (SE Brazil; Gibson et al., 1995b).
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Geological Setting |
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TOPABSTRACTIntroductionGeological SettingClassification and PetrographyPetrochemistryPetrogenesisSignificance of the Asu...Genetic Relationships between...ConclusionsReferences |
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Eastern Paraguay lies along the former western margin of Gondwana, in an intercratonic region which includes the western side of the Paraná Basin of Brazil, bounded by an anticlinal structure established since Early Palaeozoic, the Asunción Arch (Fig. 1), which separates the Paraná Basin (east) from the Gran Chaco Basin (west). The basement formations are largely Precambrian to Early Palaeozoic granitic intrusions and high-grade metasedimentary rocks, considered the northernmost occurrence of the Rio de la Plata craton, and the southernmost tip of the Amazon craton (Fulfaro, 1996
). Eastern Paraguay was subjected to NE–SW-trending crustal extension during Late Mesozoic, probably related to the western Gondwana break-up and the opening of the South Atlantic. NW-SE fault trends, consistent with the dominant orientation of Mesozoic alkaline and tholeiitic dykes, reflect this type of structure. These are crosscut by NE–SW lineaments apparently related to Precambrian–Cambrian basement structures reactivated and enhanced by Early Cretaceous events (Fulfaro, 1996). The resulting structural pattern controlled the development of the associated grabens as a response to NE–SW-directed extension and continued evolving into Upper Tertiary times. Notable for its high concentration of alkaline rocks is the Asunción–Sapucai graben (ASU), 
200 km long and 25–40 km wide, filled with sediments up to 2.5 km thick (Hegarty et al., 1996).
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From the beginning of Mesozoic times, four main magmatic events have occurred in eastern Paraguay. The oldest one (Alto Paraguay Province), is represented by ring complexes of nepheline syenites to alkaline granites and their effusive equivalents, dated at Late Permian–Early Triassic (240–250 Ma; Comin-Chiaramonti & Gomes, 1996
). This magmatism is widespread at the southernmost side of the Guaporé Craton. The second event is represented by the flood tholeiites of the Alto Paraná Formation, i.e. Serra Geral Formation (SGF), of Brazil. 40Ar-39Ar dates from SGF flood tholeiites, obtained from specimens distributed over the entire Paraná Basin, yielded a range of 127–137 Ma (Turner et al., 1994). SGF specimens from the SE margin of the Basin gave well-constrained 40Ar-39Ar dates of 133±1 Ma (Renne et al., 1992), 132±1 Ma and 133±3 Ma (Turner et al., 1994). P. R. Renne (personal communication, 1995) reported a 40Ar-39Ar date of 133±1 Ma from one SGF specimen from Foz do Iguaçu, at the Paraguay state border. The third magmatic event, essentially represented by potassic rocks (137–118 Ma), predated the SGF tholeiites in the Rio Apa and Amambay (NE Paraguay) regions, and postdated the same in the ASU. A biotite 40Ar-39Ar date from the Cerro Charará (i.e. Cerro Santo Tomás, ASU) potassic intrusion in eastern Paraguay gave 126.8±0.3 Ma (Renne et al., 1993). Over 200 alkaline basaltic dykes with dominant NW-SE orientation are associated with alkaline complexes and lava flows in the ASU. Carbonatites occur in the Rio Apa, Amambay and ASU regions (Castorina et al., 1994). Sodic alkaline rocks (120 Ma, K/Ar whole-rock, one sample; J. DeGraff, personal communication, 1995), containing abundant mantle xenoliths, occur as plugs and dykes in the San Juan Bautista region. Finally, the youngest magmatism (70–32 Ma; Comin-Chiaramonti et al., 1991) is represented by ultra-alkaline sodic rocks containing mantle xenoliths (Comin-Chiaramonti et al., 1986), phonolitic plugs and tephritic to phonolitic dykes near Asunción township. The estimated volumes are 500, 20000, 1200 and 100 km3 for the Alto Paraguay Province, SGF, ASU potassic Province and San Juan Bautista Province plus ASU sodic magmatism, respectively. |
Classification and Petrography |
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TOPABSTRACTIntroductionGeological SettingClassification and PetrographyPetrochemistryPetrogenesisSignificance of the Asu...Genetic Relationships between...ConclusionsReferences |
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A total of 527 specimens from the ASU Province, inclusive of dyke rocks, have been investigated in detail, and particulars of field, age, petrography and mineral chemistry have been given by Comin-Chiaramonti & Gomes, (1996)
. Rock nomenclature is after de la Roche, (1986).
The fine-grained nature of most ASU rocks prompted the adoption of a chemical classification based on that of LeMaitre, (1989). The following subdivisions were adopted, on the basis of our chemical database from Paraguay:
- Na2O – 2
K2O for sodic compositions (Na);
- 1<K2O/Na2O
2 for potassic compositions (K);
- Na2O – 2<K2O and K2O/Na2O
1 for compositions transitional to K (tK);
- K2O/Na2O>2 for highly potassic compositions (HK).
- 1<K2O/Na2O
Of the analysed ASU specimens, 18% are Na, 57% K, 18% tK and 7% HK types. Their distribution, illustrated in Fig. 2, shows that the variation of the dyke rocks is consistent with that of the associated intrusive and volcanic rocks and, with the possible exception of the ankaratrites–nephelinites, the ASU compositional variation is virtually continuous. K, HK and tK types generally fall in Barton's, (1979) ‘Roman Region Type’ field and in the Foley et al., (1987) ‘Group III’. Notably, only 7% of the compositions are ‘ultrapotassic’, namely, K2O/Na2O>2, K2O>3 wt %, MgO>3 wt %. The ASU compositions straddle the alkali basalt to trachyte and the basanite–tephrite to phonolite fields of de la Roche's, (1986) diagram (Fig. 2), as well as the corresponding boundary between their TAS analogues (LeBas et al., 1986), i.e. basanite–tephrite to phonolite and trachybasalt to trachyte, respectively. Modal analyses of medium- to coarse-grained potassic intrusives representative of the ASU compositional spectrum, projected into Streckeisen's, (1976) QAPF diagram (Fig. 3), suggest two main suites, i.e. basanite to phonolite (B–P) and alkali basalt to trachyte (AB–T) (see Fig. 2).
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The sodic rocks
These are mainly represented by ankaratrites+ nephelinites (45%), and phonolites (42%).
Ankaratrites and nephelinites contain olivine (Fo82–78), clinopyroxene (Wo47Fs11) and magnetite (ulv. 38%) phenocrysts or microphenocrysts in hypocrystalline groundmass composed of 40–49 vol. % salite (Wo50Fs13), 7–16 vol. % olivine (Fo74–78) and 4–8 vol. % titanomagnetite (ulv. 43%)+nepheline+apatite±alkali feldspar±biotite±carbonate±glass. Mantle lherzolite, harzburgite and dunite xenoliths and xenocrystic debris are common and abundant.
Phonolites are typically microphyric to hypocrystalline with alkali feldspar phenocrysts or microphenocrysts (Or43–83), nepheline (Ne67–79), occasionally altered to cancrinite, acmitic clinopyroxene (acmite up to 63 wt %) and ferroedenite–ferropargasite amphibole. Haüyne, mica, haematite and zircon are typical accessory minerals. Analcime occurs as a nepheline pseudomorph. Ti-andradite and sphene occur in some dykes.
The potassic rocks
B–P suite
Theralites, essexitic gabbros, ijolites and essexites are holocrystalline, seriate, with diopsidic pyroxene (Wo44–51Fs8–17), olivine (Fo75–82 to Fo44–66), mica (Ti-phlogopite to Ti-biotite), Ti-magnetite, alkali feldspar, nepheline (Ne64–80Ks20–36)±leucite±amphibole. Leucite pseudomorphed by analcime and plagioclase are common in K and tK rocks. Variant textures include cumulitic clinopyroxene with alkali feldspar+nepheline+carbonate as intercumulus phases.
Basanites, tephrites and phonotephrites typically show porphyritic textures with phenocrysts of clinopyroxene (Wo40–50Fs10–19), olivine (Fo60–85) and leucite pseudomorphed by sanidine+nepheline, in glassy groundmass containing microlites of clinopyroxene±olivine, Ti-magnetite±ilmenite, Ti-phlogopite–biotite, alkali feldspar (Or15–88), nepheline–analcime (Ne44–59Ks17–26). Phenocrystal plagioclase (up to An74) is present in K and tK rocks. Accessory phases are amphibole (pargasite–kaersutite), apatite±zircon.
Phonolites are characterized by phenocrysts of leucite pseudomorphs, alkali feldspar (Or47–75), clinopyroxene (Wo48–50Fs11–34), Fe-pargasite, nepheline±biotite±sphene±melanite (Ti-andradite up to 68 wt %)±magnetite or haematite. Glassy groundmass contains microlites of alkali feldspar, nepheline, clinopyroxene±Ti-andradite±magnetite or haematite.
AB–T suite
Alkali gabbros, syenogabbros and syenodiorites are usually porphyritic and seriate. They contain clinopyroxene (Wo43–50Fs6–14), olivine (Fo43–82), Ti-biotite, Ti-magnetite±ilmenite, plagioclase (An31–78), alkali feldspar (Or60–84) and interstitial nepheline–analcime (Ne37–82Ks15–23) and alkali feldspar (Or80–97). Accessories are apatite±amphibole±sphene±zircon.
Nepheline syenites and syenites are equi- to subequigranular and seriate. The rock types are characterized by alkali feldspar (Or32–63), clinopyroxene (Wo43–48Fs10–32), nepheline (Ne85) and hastingsite. Common accessories include sphene, apatite±carbonate±zircon.
Alkali basalts, trachybasalts and trachyandesites are porphyritic rocks characterized by phenocrysts and/or microphenocrysts of clinopyroxene (Wo44–49Fs7–15), olivine (Fo65–83), plagioclase (An28–76), magnetite, biotite in glassy groundmass containing microlites of clinopyroxene (Wo46–49Fs13–18), magnetite, ilmenite, biotite, plagioclase (An20–45), alkali feldspar (Or52–65), nepheline–analcime (Ne37–73Ks22–38), amphibole, apatite±sphene±zircon.
Trachyphonolites and trachytes are porphyritic to aphyric. The phenocrysts are alkali feldspar (Or60–65)±clinopyroxene (Wo46–49Fs14–20)±plagioclase (An14–16), pseudomorphed leucite, amphibole and biotite in hypocrystalline to glassy groundmass containing microlites of alkali feldspar, biotite, clinopyroxene±biotite±amphibole±magnetite±Ti-andradite±haematite.
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Petrochemistry |
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TOPABSTRACTIntroductionGeological SettingClassification and PetrographyPetrochemistryPetrogenesisSignificance of the Asu...Genetic Relationships between...ConclusionsReferences |
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Chemical analyses were obtained by X-ray fluorescence (XRF) and by techniques described by Piccirillo & Melfi, (1988)
. Representative specimens from B–P and AB–T suites are given in Table 1. A complete set of bulk-rock analyses has been given by Comin-Chiaramonti & Gomes, (1996). Rare earth elements (REE), Th, U and Ta of selected specimens were determined by inductively coupled mass spectrometry (ICP-MS) (Alaimo & Censi, 1992) (Table 1). Close similarities in major, minor (not shown) and some trace elements (Fig. 4) are obvious on inspection. [REE]CN plots of intrusives, lavas and dykes from B–P and AB–T, respectively, are illustrated in Fig. 5. All specimens are similarly enriched in REE and exhibit steep, subparallel light REE (LREE) trends ([La/Lu]CN=26–161, 17–62 and 11–46 for B–P, AB–T and Na rocks, respectively, which tend to flatten out for heavy REE (HREE) ([Dy/Lu]CN=1.24–1.96, 1.09–2.00 and 0.56–2.05 for B–P, AB–T and Na rocks, respectively), HK dykes excepted. REE profiles with LREE enrichment and flat HREE suggest mantle sources which have been previously depleted by melt extraction and subsequently enriched (e.g. McKenzie & O'Nions, 1995).
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The kamafugitic rocks from Alto Paranaíba Igneous Province (APIP; Gibson et al., 1995b
) from southern Brazil are distinct from the potassic rocks of eastern Paraguay in their higher LREE and MREE contents. APIP kamafugites have [La/Lu]CN ratios (69–173) falling in the upper range of the potassic B–P rocks. Multi-elemental diagrams, normalized to a primordial mantle composition (Fig. 6), show a substantial overlap of B–P and AB–T compositions, negative Nb, Ta, P, Ti and Y spikes, and positive U, K and Sr spikes. Ta, Nb and Ti depletions yielded negative ‘Ta–Nb–Ti anomalies’, interpreted by some (e.g. Pearce, 1983) as characteristic of magmas generated in subduction-related environments. In general, B–P compositions are higher in Rb, K, Zr, Hf, Ti, Y, LREE and MREE than AB–T compositions. By contrast, the rocks of the Na suite yielded La/Nb and La/Ta ratios close to unity, respectively, whereas the Nb/K and Ta/K ratios are >1.0, respectively. It is of interest that the APIP potassic rocks show patterns similar to those of Na rocks from eastern Paraguay and are, therefore, distinct from those of the potassic B–P and AB–T suites. Notably, the variations of incompatible elements of the AB–T compositions mimic those of the SGF high-Ti (and low-Ti) tholeiitic basalts, which approach the lower (Rb to Nd) and higher (Sm to Lu) elemental concentrations of that suite (Fig. 6). Both SGF high- and low-Ti tholeiites and the ASU rocks fall out of the field for non-subduction related compositions in the Th/Zr vs Nb/Zr and Th/Yb vs Ta/Yb diagrams, respectively (Fig. 7). Consistent with the behaviour of Ta and Nb, the investigated Na and APIP rocks fall in the field where rocks are believed to be unrelated to subduction.
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In summary, substantial overlap in bulk-rock chemistry exists between the investigated B–P and AB–T rocks, both characterized by variable K/Na ratio, but K types are dominant. REE and other incompatible elements show similar concentration levels and variation trends in the two suites. The mantle normalized incompatible element patterns of both ASU suites show strong affinities, including negative ‘Ta–Nb–Ti anomalies’, with the Paraná tholeiites. It seems significant that the sodic rocks show a slight positive anomaly for the latter elements (Comin-Chiaramonti et al., 1991
, 1992), similar to that of APIP potassic rocks (Gibson et al., 1995b).
Sr–Nd isotopes
Typical specimens from the ASU K suite yielded initial (128 Ma) 87Sr/86Sr (Sri) and 143Nd/144Nd (Ndi) isotopic ratios within the ranges 0.70612–0.70754 and 0.51154–0.51184, respectively. These are distinct from the values obtained for the Na rocks, i.e. ASU, Sri=0.70362–0.70392 and Ndi=0.51259–0.51277; San Juan Bautista, Sri=0.70435–0.70524 and Ndi=0.51225–0.51242; Alto Paraguay: Sri=0.70350–0.70570 and Ndi=0.51207–0.51223. In the conventional 
tSr vs tNd isotope diagram (Fig. 8) the selected K specimens have higher Sri and lower Ndi values with respect to the ‘Low Nd’ array of Hart et al., (1986), whereas the Na specimens approach bulk-Earth (BE) values, varying from depleted to enriched quadrants. The Amambay potassic rocks (Comin-Chiaramonti et al., 1995) fall on the same general line of the ASU K rocks, but trending to more radiogenic Sr. Likewise, the sodic rocks from Alto Paraguay fall virtually on the same trend of the ASU analogues (San Juan Bautista). The latter approach the generalized field of the Paraná tholeiites from eastern Paraguay, which appear to fall at higher tSr. The radiogenic Sr and Nd are, therefore, crucial in accounting for the source characteristics of the investigated rocks. Model contamination of the melts with crustal material implies unrealistic (up to 50%) contaminant fractions. Likewise, AFC processes do not account for the ASU isotope data, given the poor correlations (not shown) between large ion lithophile elements (LILEs) and Sri and Ndi. Whole-rock 18O data from ASU K rocks (Marzoli, 1991) yielded +5.45 to +5.91%o (V-SMOW), consistent with constituent clinopyroxene (+5.09 to +5.20%o) and biotite (+4.85 to +5.54%o) and expected mantle values. In summary, the data presented support the view that the Na rocks, close to BE, and the associated ASU K rocks, typically high in radiogenic Sr world-wide, represent the range of virtually uncontaminated source magmas from this region.
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Nd-model ages for ASU K rocks (depleted mantle: TDM) range from 1.4 to 2.0 Ga (mean 1.5±0.2 Ga) whereas Na rocks yielded 0.5–0.8 (ASU), 1.0 (Alto Paraguay) and 0.8–0.9 Ga (San Juan Bautista). In general, high-Ti and low-Ti tholeiites from Paraná Basin (H-Ti and L-Ti, respectively) show mean TDM 1.1±0.1 and 1.5±0.2 Ga, respectively. The range of model ages estimated for the potassic rocks implies that the corresponding melts derived from subcontinental mantle sources enriched by ‘metasomatic processes’ (see below) from Middle to Late Proterozoic. These calculations require that Sm/Nd fractionation was not significant during magma genesis.
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Petrogenesis |
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TOPABSTRACTIntroductionGeological SettingClassification and PetrographyPetrochemistryPetrogenesisSignificance of the Asu...Genetic Relationships between...ConclusionsReferences |
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Evolution of the B–P and AB–T suites
Textural, mineralogical and petrochemical evidence points to fractional crystallization as potentially important in the evolution of the ASU suites. Mass balance calculations based on major oxides revealed that the evolution of both B–P and AB–T suites is compatible with fractional crystallization (Table 2). It should be noted that the corresponding Rayleigh's trace element fractionation for the B–P suite yielded observed/calculated ratios within the range 0.8–1.4, Ni and Ba in phonolite excepted. Likewise, trace element modelling for the AB–T suite yielded observed/calculated ratios of 0.8–1.2, except for Ni, Ba, Zr and Nb in the more evolved compositions.
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In an attempt to evaluate the role of fractionation in the B–P and AB–T suites, the variation of Th, Zr, Ni and Cr in the basanite to tephrite and alkali basalt to trachybasalt transitions, respectively, were investigated by means of a model elemental distribution, predicted for open-system fractionation in a periodically replenished magma reservoir (PRF; O'Hara & Mathews, 1981
). Convergence of X and Y values was obtained, particularly for B–P. Similar results were obtained for the proposed tephrite to phonotephrite and trachybasalt to trachyandesite transitions, respectively. The general tendency for X+Y values to approach unity suggests that the PRF model is roughly equivalent to a succession of closed-system fractionation events. The phonotephrite to phonolite and trachyandesite to trachyte model fractionation, respectively, yielded negative values for Cr and Ni. Therefore, the extreme rock compositions are inconsistent with Rayleigh-type fractionation processes, probably reflecting, at least in part, their distinct f(O2) and its influence on partition coefficients.
Mantle source characteristics
The origin of the ASU alkaline magmas is closely related to and probably constrained by the geodynamic processes which promoted the generation of the adjacent and ‘coeval’ SGF tholeiites. The origin and emplacement of the latter basalts occurred before the opening of the South Atlantic and appears to be related to early stages of lithospheric extension, associated with an anomalously hot mantle (Piccirillo & Melfi, 1988; Hawkesworth et al., 1988, 1992; Hergt et al., 1991; Turner & Hawkesworth, 1995). The K/Na and trace element variations and Sr–Nd isotope characteristics of the ASU suites support the view that the lithospheric mantle played an important role in their genesis as well as that of the Paraná flood basalts (Piccirillo et al., 1989; Gibson et al., 1995a; Peate & Hawkesworth, 1996).
The most mafic ASU potassic basanites and alkali basalts are relatively evolved compared with expected primary compositions (e.g. mg-number >0.65, Ni >235 p.p.m.). Possible ASU primary melts (e.g. mg-number 0.74) in equilibrium with Fo90–91 are expected to have fractionated olivine (Ol) and clinopyroxene (Cpx) at or near the mantle source and certainly during the ascent to the surface. Neglecting the effects of polythermal–polybaric fractionation on the chemistry of the fractionates, notional crystal fractions have been calculated to restore the selected ASU parental compositions to possible near-primary melts in equilibrium with their mantle sources (Table 3).
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The strong LREE/HREE ratios of the primary melt compositions of ASU and Alto Paranaíba [(La/Lu)CN = 28–54 and 113, respectively] require melting of peridotite sources with unrealistically high proportions of garnet in the solid residua. Notionally, for 3% batch melting (see below) the garnet proportions in the ASU and Alto Paranaíba peridotite residua are higher than 14 wt %.
Mass balance calculations, starting from different garnet phlogopite peridotites (Erlank et al., 1987; whole-rock and mineral compositions in Table 3), indicate that the ASU and Alto Paranaíba primary melt compositions can be derived from relatively high melting degrees, i.e. 5–8% of the anhydrous garnet peridotite, or 4–11% of phlogopite-bearing peridotites (Table 3). The presence of a relative K enrichment in the ASU potassic rocks (Fig. 6) suggests that a K-bearing phase (e.g. phlogopite) did not represent a residual phase during partial melting. Phlogopite, instead, was probably a residual phase in the mantle source(s) of the ASU sodic rocks and Alto Paranaíba kamafugites. The primary melts, however, have high abundances of strongly IE and high LREE/HREE ratios, which require mantle sources enriched in IE before or subcontemporaneously with melting process (see Menzies & Wass, 1983). Nd model ages approximately indicate that the IE enrichment in the source mantle of the ASU potassic rocks probably occurred during Middle Proterozoic times (mean 1.5 Ga), whereas those relative to the sodic rocks from eastern Paraguay and Alto Paranaíba kamafugites would be related to Middle–Late Proterozoic events (1.0–0.5 Ga).
Trace element concentrations in the hypothetical mantle sources considered were calculated (batch melting) on the basis of the melting degrees obtained (Table 3) and using the partition coefficients of McKenzie & O'Nions, (1991) and Comin-Chiaramonti & Gomes, (1996). The multi-elemental plots for the calculated mantle sources, normalized to the primitive mantle of Sun & McDonough, (1989), highlight a significant enrichment of the most IE, whereas the least IE (mainly from Zr to Lu) would have concentrations similar to or less than those of primordial mantle (Fig. 9). Notably, the patterns of the mantle sources of the ASU potassic rocks are characterized by negative ‘Ta–Nb–Ti’ and positive Ba and Sm spikes. On the other hand, the patterns of the mantle sources of both the ASU nephelinites and Alto Paranaíba kamafugites are distinct in their positive Ta–Nb and Zr, and negative K and Sm spikes. It seems, therefore, that the genesis of the ASU alkaline magmatism is dominated by a lithospheric mantle, characterized by small-scale heterogeneity, as also documented by the occurrence of bleb-like glass in spinel peridotite nodules from the ASU nephelinites (Comin-Chiaramonti et al., 1986, 1991). The substantial differences between the mantle source patterns of the ASU potassic rocks and Alto Paranaíba kamafugites (Fig. 9) indicate also a large-scale regional heterogeneity.
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The metasomatic processes responsible for mantle incompatible element enrichment are believed to be due mainly to fluids or melts related to subduction processes (e.g. Hergt et al., 1991
; Maury et al., 1992) or to volatile-rich small-volume melts from the asthenosphere which would have veined the overlying lithospheric mantle at different depths (e.g. Menzies & Hawkesworth, 1987; Foley, 1992a, b). In general, the high concentrations of the most IE in all the eastern Paraguay alkaline rocks suggest that the effect of crustal contamination on the Sr–Nd isotope system was negligible and that the parental magma composition was dominated by lithospheric mantle melts.
Depletions in high-field-strength elements, HFSE (Fig. 6), are characteristic of magmatic rocks related to subduction processes, suggesting that mantle IE enrichment may have been caused by fluids or melts derived from a downgoing slab. Following Weaver et al., (1987), the ASU potassic rocks with high Ba/Nb and La/Nb ratios could be related to mantle sources contaminated by subducted ocean crust (Ba/Nb=6.5, La/Nb=0.7) with the addition of sediments of variable compositions (Ba/Nb=335–341, La/Nb=5.7–21.3). Assuming that the mixing processes occurred in the source mantle (2.0–1.4 Ga), 4–8% of contaminants are required to obtain the Ba/Nb (26–62) and La/Nb (1.6–3.6) ratios of the ASU potassic rocks. The latter process, however, is not supported by geological evidence of a subduction process which would have involved the Rio de La Plata craton in late Early to early Middle Proterozoic times (Bossi et al., 1993). Alternatively, negative HFSE anomalies may result from lithospheric source mantle whose IE enrichment was caused by small-volume melts which left a titanate phase in their solid residua (e.g. Richardson et al., 1982; Haggerty, 1994). It is notable that the ASU sodic rocks, closely associated in space with the potassic analogues, have slightly positive Nb–Ta anomalies (Fig. 6), which indicate that the IE enrichment of the lithospheric source mantle, as proposed for the genesis of the Alto Paranaíba kamafugites by Gibson et al., (1995b), was produced by low-temperature small-volume asthenospheric melts. This type of enrichment also holds for the sodic magmatism in the Alto Paraguay. The potassic and sodic rocks from eastern Paraguay contain primary carbonates (mean 13C%o=7.04±0.82 VPDB) and are associated with carbonatites whose age ranges from 137 to 128 Ma (Comin-Chiaramonti et al., 1991, 1995). These rocks have Sr and Nd isotope ratios (i.e. Sri=0.70723–0.70784 and Ndi=0.51136–0.51183) similar to those of the ASU potassic rocks. Therefore CO2 may have played a role in the IE enrichment of the lithospheric mantle, as the volatile-rich fluid–melt compositions are dependent on the CO2/H2O ratio (e.g. Dautria et al., 1992; Rudnik et al., 1993).
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Significance of the Asu Province |
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TOPABSTRACTIntroductionGeological SettingClassification and PetrographyPetrochemistryPetrogenesisSignificance of the Asu...Genetic Relationships between...ConclusionsReferences |
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Crucial to ASU magma genesis is the link with the geodynamic processes which led to the opening of the South Atlantic. According to Nürnberg & Müller, (1991)
the sea-floor spreading in the South Atlantic at Paraná Basin latitude was initiated at 125–127 Ma (Chron M4). North of the Walvis–Rio Grande ridges (latitude <28°S) the onset of oceanic crust would be younger (113 Ma; Chang et al., 1988). In general, the potassic magmatism of the ASU Province is subcoeval with the flood tholeiites of the Paraná Basin and, therefore, occurred during the early stages of rifting, before continental separation. The scarce sodic magmatism, instead, approaches the time of sea-floor spreading (San Juan Bautista, 120 Ma) or corresponds to an advanced stage of continental separation (Asunción, 70–32 Ma). The time of the most significant cooling event in the ASU igneous activity occurred in the Late Cretaceous (80–90 Ma) and overprinted an older thermal perturbation, probably of Early Cretaceous age (Hegarty et al., 1996).
As previously outlined, the alkaline magmatism of eastern Paraguay appears to have derived from a lithospheric source, probably formed by phlogopite-bearing garnet peridotites. Also, the alkaline magmatism of eastern Paraguay and SE Brazil (Morbidelli et al., 1995), as well as that of the Paraná flood tholeiites, was not associated with important lithospheric extension (Piccirillo & Melfi, 1988; Hawkesworth et al., 1992; Turner et al., 1994; Turner & Hawkesworth, 1995; Peate & Hawkesworth, 1996). Therefore, we support the view that the melting process was mainly due to heat released by anomalously hot mantle, i.e. asthenospheric.
The IE enrichment of the source mantle for the alkaline magmatism of eastern Paraguay and SE Brazil occurred in Proterozoic times, 2.0–1.4 and 1.0–0.5 Ga. This enrichment would be related to very small degrees of peridotite melting (e.g. GP in Table 3), possibly 0.2% (McKenzie, 1989), to match the Sr–Nd isotope ratios of the ASU K rocks.
The alkaline magmatism of Early Cretaceous age may be related to a Mesozoic mantle plume (e.g. Tristan da Cunha?; White & McKenzie, 1989; Richards et al., 1989; Milner & Le Roex, 1996). The heat released from the plume would have partially melted the overlying lithospheric mantle without appreciable contribution from the Mesozoic plume components. It is then suggested that the peripheral distribution of the alkaline magmatism with respect to the Paraná Basin was associated with the cooler margins of the plume system, whereas the subcoeval SGF tholeiites reflect a higher thermal regime, corresponding to the inner and hotter plume regions. It should be noted that the Early Cretaceous alkaline magmatism is volumetrically minor, relative to the Late Cretaceous analogue, and is concentrated in the central area of the Paraná Basin, i.e. eastern Paraguay, the Ponta Grossa Arch and also the Moçamedes arch of Angola (Piccirillo et al., 1990). This contrasts with the alkaline magmatism of Late Cretaceous age, which is voluminous and mostly concentrated in northern Paraná.
According to Gibson et al., (1995b), the Alto Paranaíba alkaline magmatism of Late Cretaceous age is related to lithospheric mantle source(s) enriched by small-volume melts since Late Proterozoic. Mantle melting would have occurred 85 Ma ago by heat from a 1000 km wide plume ‘head’, at present under Trindade. This plume model does not include most alkaline rocks from Serra do Mar, Ponta Grossa–Ipanema and Lages, where several occurrences are subcoeval with those from Alto Paranaíba (e.g. Barra do Teixeira, Cananeia, Tunas; Buzios, Monte de Trigo, São Sebastião and Vitoria islands, Ponta Nova; Morbidelli et al., 1995).
Even assuming that the Trindade plume was smaller and cooler than the Tristan da Cunha plume (2000 km wide), it seems reasonable to expect that an appreciable volume of tholeiitic basalts, associated with alkaline rocks, would be generated in the inner and hotter (e.g. 1380°C) portions of the Trindade plume. However, Late Cretaceous tholeiitic basalts are virtually absent, lithospheric extension was very small (β1.05–1.1; Chang et al., 1992; Ussami et al., 1994) and lithospheric thickness is considerable (>130 km; James et al., 1993). Therefore, we favour a hydrous mantle source or sources at about normal mantle potential temperature (e.g. 1280°C) to generate a relatively small fraction of melt thickness (0.6 km for a mechanical boundary layer 150 km thick; Gallagher & Hawkesworth, 1994), solely from a lithospheric mantle. As the Paraná magmatism probably migrated toward the east and north (Ernesto et al., 1990; Raposo & Ernesto, 1995; Renne et al., 1996b), and not from NW to SE (Turner et al., 1994), as recently discussed by Renne et al., (1996a), we suggest that the Late Cretaceous alkaline magmatism may be related to the anomalous thermal regime responsible for the Paraná tholeiites. The alkaline provinces, developed from the cooler margins of that thermal peak, are thought of as derivative from relatively low melting degrees of lithospheric mantle. It should be noted that over 20–40 Ma were estimated to be required to achieve, from the lithospheric mantle only, 30–50% of the total melt production with a steady-state potential temperature lower than 1480°C (Hawkesworth et al., 1992). We also suggest that the concentration of the Paraná Late Cretaceous alkaline magmatism along the margin of the São Francisco craton, Ponta Grossa arch and the Serra do Mar probably reflects, at least in part, a tectonic control of the basement (see Santero et al., 1988).
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Genetic Relationships between the Alkaline and Tholeiitic Magmatism of the Paraná Basin |
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TOPABSTRACTIntroductionGeological SettingClassification and PetrographyPetrochemistryPetrogenesisSignificance of the Asu...Genetic Relationships between...ConclusionsReferences |
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The geochemical provinciality of the Gondwana flood basalts (low- and high-Ti types; Bellieni et al., 1984b
; Cox, 1988) is believed to be related to: (1) variable partial melting of an uprising mantle plume (e.g. Campbell & Griffiths, 1990; Arndt et al., 1993) or lithospheric mantle with a variable asthenospheric component (e.g. Piccirillo et al., 1989; Hawkesworth et al., 1992; Peate & Hawkesworth, 1996), followed by variable degrees of crustal contamination (Petrini et al., 1987; Hawkesworth et al., 1988); (2) dehydration melting of heterogeneous subcontinental lithospheric mantle (SCLM), caused by heat released from an underlying mantle plume (Gallagher & Hawkesworth, 1992, 1994); (3) mixing of plume-derived picritic melts with high-potassic melts of lamproitic composition (i.e. high-Ti; Ellam & Cox, 1991); (4) mixing of MORB-like tholeiitic picrites from a mantle plume with high- and low-Ti potassic melts derived from SCLM, followed by crustal contamination (Gibson et al., 1995a).
The above hypotheses require an end-member low in IE and with a high Sm/Nd ratio, similar in composition to N-MORB (e.g. Sun & McDonough, 1989). Appropriate end-members with high IE contents, variable IE ratios and low Sm/Nd are required to derive the low- and high-Ti Paraná tholeiites from southern and northern provinces, respectively, by mixing processes. Following Hawkesworth et al., (1992), the major difficulty is to explain the genesis of the low-Ti tholeiites, owing to the absence of an end-member with low Ti content, low 143Nd/144Nd and high 87Sr/86Sr ratios. Castorina et al., (1994) demonstrated that this end-member may be represented by the ASU low-Ti potassic mafic rocks (K-ASU; Castorina et al., 1994).
Sr–Nd isotopes
Sr and Nd isotope data indicate that the scarce high-Ti tholeiites from southern Paraná may have originated from SCLM without appreciable N-MORB contribution (Fig. 10a). On the other hand, the dominant H-Ti tholeiites from central and northern Paraná plot on the K-ASU (low-Ti)-N-MORB mixing trend, implying 20–25% of the K-ASU lithospheric mantle component. The latter isotopic model, however, is not supported by major and trace element data of the low- and high-Ti tholeiites (Piccirillo & Melfi, 1988).
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The low-Ti tholeiites from central and northern Paraná conform to the K-ASU–N-MORB mixing curve (Fig. 10b), indicating
8% (Paraná tholeiites 3064 and 448) to 35% low-Ti mafic potassic melts derived from SCLM as a possible component. The low-Ti tholeiites from southern and a few occurrences from central Paraná trend to higher Sri, and are broadly compatible with low-pressure crustal contamination (e.g. AFC; DePaolo, 1981), starting from chemically variable parental melts with positive Nd and negative Sr (Petrini et al., 1987; Piccirillo et al., 1989). Peate & Hawkesworth, (1996) considered the southern Paraná low-Ti ‘Gramado-type’ magma (i.e. Sri=0.708–0.710 and Nd-3) as a derivative from a regionally heterogeneous SCLM and interpreted the southern Paraná low-Ti ‘Esmeralda-type’ magma as the result of mixing between a MORB-like asthenospheric melt (e.g. Nd=+4 to +8) and a ‘Gramado-type’ magma. Assuming that the low-Ti K-ASU and high-Ti Alto Paranaíba kamafugites are derivative from a lithospheric mantle, the expected Sri and Ndi for the Paraná tholeiites would be in the mean ranges 0.7070 (low-Ti) and 0.7050 (high-Ti), and 0.51175 (low-Ti) and 0.51225 (high-Ti), respectively. Therefore, we suggest that the high Sr (and low Nd) isotopic compositions of the Paraná ‘Gramado-type’ magma reflect a contribution from crustal contamination. Likewise, the ‘Esmeralda-type’ magma, which reflects a significant N-MORB component (see Paraná tholeiites 3064 and 448, N-MORB; Fig. 10b), was probably contaminated by crustal materials, but to a lesser extent compared with the ‘Gramado-type’ magma.
Ti–Y–Zr relationships
Ti/Y and Ti/Zr ratios (Fig. 11) show that all the high-Ti Paraná tholeiites are consistent with mixing of a component represented by the Paraná samples 3064 plus 448 and 15–20% of a high-Ti APIP component. It should be noted that the alkaline lavas from Gough island and Tristan da Cunha (Weaver et al., 1987) would require 50 and 40% of the latter component, respectively.
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The low-Ti tholeiites from central and southern Paraná do not appear to fit the mixing curve N-MORB (or samples 3064 plus 448)-K-ASU, owing to their lower Ti/Y ratios (Fig. 11). The ‘Esmeralda-type’ and ‘Gramado-type’ magmas require
15–25% and up to 40%, respectively, of a siliceous contaminant, similar to the associated Palmas rhyolites (Bellieni et al., 1986; Hawkesworth et al., 1992), or comparable fractions of a mean crystalline basement composition (Piccirillo & Melfi, 1988). Addition of 4–8% crustal materials (Taylor & McLennan, 1985) to an N-MORB-type source does not account for the low-Ti basalt compositions (see Hergt et al., 1991).
Sr–Nd isotope data show that the high-Ti Paraná tholeiites are not consistent with a mixing curve involving the high-Ti APIP component, a few occurrences from southern Paraná excepted. Notably, the high-Ti Paraná tholeiites do not show evidence of crustal contamination (Petrini et al., 1987; Piccirillo et al., 1989). The Ti/Zr and Ti/Y relationships, instead, indicate that all the high-Ti Paraná tholeiites are compatible with 15–20% of the APIP-type component. These contrasting results are due, at least partly, to different Nd model ages for Alto Paranaíba kamafugites (818 Ma; Gibson et al., 1995a, b) and high-Ti Paraná tholeiites (1101±114 Ma), illustrated in Fig. 12. An age difference of 300 Ma would increase Sri (130 Ma) of Alto Paranaíba kamafugites from 0.7052 to 0.7059, which fits well the mean value for the Paraná high-Ti basalts, i.e. 0.7058 (Cordani et al., 1988). The Low-Ti Paraná tholeiites, straddling the Sr–Nd isotope mixing curve, indicate 8–35% of low-Ti K-ASU component. The other low-Ti basalts show Sr–Nd isotopes, and Ti/Zr–Ti/Y relationships consistent with crustal contamination which probably occurred during their ascent to the surface.
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Metasomatic events
TDM (Nd) model ages (Fig. 12) show that most alkaline rocks, carbonatites and nephelinites from SE Brazil and Paraguay range from 0.5 to 1.1 Ga. This range is virtually the same as that for high-Ti Paraná tholeiites. On the other hand, only K-ASU and carbonatites from eastern Paraguay yielded TDM of early Middle Proterozoic age, as did most of the low-Ti Paraná tholeiites, including Santos–Rio de Janeiro and Ponta Grossa tholeiitic dykes.
These model ages indicate that two distinct mantle metasomatic events may have occurred during Middle and Late Proterozoic as precursor to the genesis of tholeiitic and alkaline magmatism in the Paraná Basin. These metasomatic processes were chemically distinct, as indicated by the strong differences in Ti, LILE and HFSE concentrations found in the alkaline rocks (e.g. low-Ti K-ASU vs high-Ti APIP) and tholeiites (low-Ti vs high-Ti types) of the Paraná Basin. The scarce high-Ti tholeiites from southern Paraná actually straddle the boundary between the southern and central regions, and suggest a lithospheric component of APIP type related to metasomatic events of late Middle to early Late Proterozoic times.
In summary, the relationships between the Paraná alkaline and tholeiitic magmatism support a lithospheric mantle origin. Isotopic and IE data indicate that a significant role in the genesis of the Paraná tholeiites was played by an IE-depleted component. The Sr and Nd isotopes (0.7052 and 0.5125, respectively) and other geochemical features of the modern Tristan da Cunha plume (Weaver et al., 1987; Le Roex et al., 1990) are distinctly different from the N-MORB component, and a contribution from the latter is not apparent in the composition of the Paraná tholeiites (Peate & Hawkesworth, 1996). Only some Early Cretaceous tholeiitic basalts (‘Tafelkop type’) and alkaline rocks (‘Okenyenya igneous complex’) from Namibia have Sr–Pb isotopes similar to those of Tristan da Cunha, Gough island and Inaccessible Island (Milner & Le Roex, 1996). We support the view that the IE-depleted component is represented by the depleted fractions of a metasomatized, i.e. veined-type mantle. Gibson et al., (1995a) have suggested that the Paraná flood basalts are the result of a mixing process involving asthenospheric tholeiitic melts, derived from IE-depleted mantle (i.e. Tristan da Cunha plume), and lithospheric K-magmas and that such tholeiites ‘may contain up to 50% of mafic potassic lithosphere-derived melts’. This model implies the production (and complete mixing) of an unusually very large volume of alkaline magmas (conservatively, >150000 km3) of Early Cretaceous age, which actually are very poorly represented in the entire Paraná Basin.
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Conclusions |
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TOPABSTRACTIntroductionGeological SettingClassification and PetrographyPetrochemistryPetrogenesisSignificance of the Asu...Genetic Relationships between...ConclusionsReferences |
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- Distinct magmatic events are dominant in central-eastern and southwestern Paraguay, i.e. widespread Early Cretaceous potassic magmatism and flood tholeiites (Asunción–Sapucai graben), and sodic magmatism, mainly of Tertiary age (Asunción).
- The potassic rocks form a compositional continuum from moderately to strongly potassic. Two suites are proposed, i.e. basanite to phonolite and alkali basalt to trachyte. The sodic rocks include mainly ankaratrites, nephelinites and phonolites.
- Two similar but distinct parental magmas emerge for the potassic suites, both characterized by strongly fractionated REE and negative ‘Ta–Nb–Ti anomalies’. A slight positive anomaly for Ta and Nb was observed in the sodic rock compositions.
- Model crystal fractionation under a low-pressure volcanic regime shows that substantial evolution of the ASU suites may have developed from distinct parental liquids.
- Sr–Nd isotope data confirmed the distinction of the potassic rocks, enriched in radiogenic Sr and low in radiogenic Nd, from the sodic rocks, close to BE and transitional to the Paraná flood tholeiites. Crustal contamination does not appear to have been significant in the generation of the investigated rocks. 18O data support this conclusion.
- The source of potassic rocks is constrained by high LILE, LREE, Th, U and K, relative to a primitive mantle composition.
- The close association of potassic and sodic rock suites in the Asunción–Sapucai graben demands that their parental magmas derived from a small subcontinental mantle mass, vertically and laterally heterogeneous in composition and variously enriched in incompatible elements. Significant H2O, CO2 and F are also expected in the mantle source from the occurrence of related carbonatites.
- Any hypothesis of mantle plume activity at the margin of the Paraná Basin is constrained by distinct lithospheric mantle characteristics. This does not preclude that thermal perturbations from the asthenosphere may have triggered magmatic activity in the lithospheric mantle in eastern Paraguay.
- It is proposed that an isotopically enriched source, implied by the ASU potassic magmas, derived from a depleted lithospheric mantle pervasively invaded by IE–C–H-rich fluids. These are expected to have promoted crystallization of K-rich phases (e.g. phlogopite) in a pristine peridotite, where they developed a veined network variously enriched in LILE and LREE under various redox conditions. The newly formed veins (‘enriched component’) and peridotite matrix (‘depleted component’) underwent a different isotopic evolution with time, depending on their parent/daughter ratio. This model was extended to the Paraná flood tholeiites and to high- and low-Ti potassic magmatism from southeastern Brazil.
- Isotopically distinct magmas were generated following two ‘enrichment’ events of the subcontinental upper mantle estimated at 2.0–1.4 Ma and 1.0–0.5 Ga, respectively. This would have preserved isotopic heterogeneities over a long period of time, pointing to a non-convective lithospheric mantle beneath different cratons or intercratonic regions.
- The occurrence of potassic magmatism in the Paraná Basin implies appropriate lithospheric sources to generate also the flood tholeiites. Therefore, the hypothesis of an asthenospheric plume origin is not compelling other than as a thermal perturbation and a possible source for the Mesozoic plume melts which contaminated the lithosphere.
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Acknowledgements |
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We are grateful to S. Milner, S. Turner and particularly to A. Ewart for their careful and constructive reviews of an earlier version of the manuscript. We also offer our thanks to R. Alaimo for the use of ICP-MS at CEPA Institution (Palermo). A. C. thanks the Staff of GEOTRACK International at the University of Melbourne for their hospitality and assistance during the preparation of this paper. Financial support from Italian (CNR and MURST) and Brazilian (FAPESP) agencies is gratefully acknowledged.
* Corresponding author. Fax: 0039 40 569 823.
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