Petrogenetic Aspects of Acid and Basaltic Lavas from the Paran? Plateau (Brazil): Geological, Mineralogical and Petrochemical Relationships     

BELLIENI  G.; COMIN-CHIARAMONTI  P.; MARQUES  L. S.; MELFI  A. J.; NARDY  A. J. R.; PAPATRECHAS  C.; PICCIRILLO  E. M.; ROISENBERG  A.; STOLFA  D. 《Journal of Petrology》1986,27(4):915-944

The acid volcanics (Lower Cretaceous) of the Paran? basin coveran area of about 150000 km² and are represented by dominantrhyodacites and subordinate rhyolites. They may be divided intotwo main types, characterized respectively by relatively lowand relatively high contents of Ti, P, and other incompatibleelements (La, Ce, Zr, etc.), i.e. the Palmas acid volcanics(PAY) and Chapec? acid volcanics (CAV), respectively. PAV arewidespread in the southern Paran? basin and are closely associatedwith basaltic and andesitic rock-types similarly characterizedby low Ti, P, and other incompatible elements. In contrast,CAV are dominant in the northern Paran? basin, where they areclosely associated with basalts containing high Ti, P, and otherincompatible elements. The generation of the Palmas and Chapec? acid melts appearsto be in part consistent with crystal fractionation processes,starting from the associated basic rocks and accompanied bycrustal contamination. However the relative absence of intermediaterock-types (‘silica gap’: 54–56 to 63–65wt. per cent), and the confinement of the acid volcanics towardsthe continental margin suggests that a model involving lowercrustal basic material of significantly different compositionin the northern and southern Paran? basin may be a more plausiblealternative. In this preferred model the basic parent materialmay be represented by mafic granulites of different compositions,or by basalts trapped at the crust-mantle discontinuity andcorresponding in composition to the contrasting low- and high-TiO₂basalts that flooded the Paran? basin in Lower Cretaceous times.The melting of these underplated materials may explain the closegeochemical relationships between fissure acid volcanics andthe closely associated basalt types (e.g., Ethiopia, Paran?).The beginning of the major rifting related to continental break-upshould therefore correspond to the stage when the melting processaffected the lower part of the continental crust. ^*Reprint requests to E. M. Piccirillo  相似文献   

Mantle Heterogeneity and Crustal Contamination in the Genesis of Low-Ti Continental Flood Basalts from the Paran? Plateau (Brazil): Sr-Nd Isotope and Geochemical Evidence     

PETRINI  R.; CIVETTA  L.; PICCIRILLO  E. M.; BELLIENI  G.; COMIN-CHIARAMONTI  P.; MARQUES  L. S.; MELFI  A. J. 《Journal of Petrology》1987,28(4):701-726

Continental flood basalts from the Parana plateau are of LowerCretaceous age and are represented by abundant (c. 45 per centby volume) two-pyroxene tholeiites characterized by relativelylow-TiO₂ (< 2 wt. percent) and incompatible (e.g., P, Ba,Sr, La, Ce, Zr) element contents. Low-Ti basalts are distributedthroughout the Parana Basin and predominate in the southernregions, where they represent over 90 per cent by volume ofthe basic activity. Major and trace elements and Sr-Nd isotope ratios were analysedin 43 low-Ti basalts selected so as to cover the entire Paranabasin. In general, low-Ti basalts with initial ^87Sr86Sr ratios (R₀)lower than O7060 may be divided into two groups: (A) those relativelyenriched in incompatible elements (e.g., average K₂O = O.85and P₂O₅ = 0.27 wt. per cent, and Ba = 346, Sr =289, Rb=16;La =18; Zr=132 p.p.m.) and SiO₂ (average 51.1 wt. per cent);and (B) depleted in incompatible elements (e.g., average K₂O= 0.31, P₂O₅ =0.17 wt. per cent, and Ba=178, Sr= 179, Rb= 11,La = 9, Zr = 93 p.p.m.) and SiO₂ (average 49.7 wt. per cent).Low-Ti basalts of Group A are typical of northern Paran? {Ro= O70550–O70596), but a few are also present in centralParan? (Ro = 070577–0–70591), while those of GroupB are exclusive to central Paran– {Ro = 070463–0–70580) Low-Ti basalts with R₀> O7060 are typical of southern Paran?(R₀ = O7O639 –O71137), but are also present in centralParana (Ro = 070620–070890). These low-Ti basalts havechemical similarity (e.g., Ti, P, Sr) with low-Ti basalts depletedin incompatible elements (Group B) from which, however, theydiffer-in possessing significantly higher concentrations ofSiO₂, K₂O, Rb, and Ba. Such chemical diversity, accompaniedby important Ro variations (070463–071137) suggests thatthe low-Ti basalts from southern and part of central Paranamay result from crustal contamination. On the contrary, low-Ti basalts from northern, and part of central, Parana (GroupA) may be considered virtually uncontaminated. Results indicate that crustal contamination by granitic material(s)may be in the range 7–17 per cent. Such contaminationin central Paran? appears compatible with an assimilation-fractionalcrystallization process (AFC), while in southern Parana, othercontamination processes (e.g., mixing of magmasfrom crustaland mantle sources, assimilation of wall rock while magmas flowthrough dykes, etc.) were probably superimposed on AFC. Thedegree of crustal contamination generally decreases from southernto northern Parana. Sr and Nd isotope ratios suggest that mantle source materialfor low-Ti basalts depleted in incompatible elements (GroupB: southern and part of central Parana) had a lower R₀ value(c. O.7046) and a higher ^l43Nd/¹⁴⁴Nd ratio (Nd + c. 0.51274)than that for low-Ti basalts enriched in incompatible elements(Group A: northern and part of central Parana), namely R₀ c.O.7059 and Nd+ c. 0.51242. These Sr-isotopic differences alsoapply to the northern (incompatible-element rich, R₀ c. O.7053)and southern (incompatible-element poor R₀ c. 0.7046) basaltprovinces of Karoo, suggesting that both Parana and Karoo basaltmagmas, differing by about 70 m.y. in age, probably originatedin a similar batch of subcontinental lithospheric mantle inpredrift times (cf. Cox, 1986). The extension of the Dupal Sr-anomaly (i.e. Rio Grande Rise+ Wai vis Ridge + Gough and Tristan da Cunha islands: Sr = 46=53;Hart, 1984) inside the Brazilian continent (Sr = 46–59)suggests that the lithospheric mantle of the Parana (and Karoo)provinces was possibly also the local source of oceanic volcanismup to advanced stages of the opening of the South Atlantic. ^*Reprint requests to E. M. Piccirillo.  相似文献   

Early Proterozoic Calc-Alkaline and Middle Proterozoic Tholeiitic Dyke Swarms from Central-Eastern Argentina: Petrology, Geochemistry, Sr-Nd Isotopes and Tectonic Implications     

IACUMIN  M.; PICCIRILLO  E. M.; GIRARDI  V. A. V.; TEIXEIRA  W.; BELLIENI  G.; ECHEVESTE  H.; FERNANDEZ  R.; PINESE  J. P. P.; RIBOT  A. 《Journal of Petrology》2001,42(11):2109-2143

The Rio de La Plata craton in Argentina (Azul and Tandil regions)is characterized by Early Proterozoic (2·0 Ga) calc-alkalineand Middle Proterozoic (1·6 Ga) tholeiitic dyke swarmsintruding the crystalline basement involved in the TransamazonianOrogeny (2·2–1·9 Ga). The calc-alkalinedykes have andesitic and rhyolitic compositions and trend east–west,whereas the tholeiitic dykes mainly trend N30°W and arerepresented by basalts with low (0·9–1·7wt %) and high TiO₂ (up to 3·7 wt %). The calc-alkalinedykes have primitive mantle (PM)-normalized trace element patternsenriched in Rb, Ba, K, La, Ce and Nd, and significant negativeNb and Ti anomalies. These dykes are characterized by  相似文献   

The Cameroon Volcanic Line Revisited: Petrogenesis of Continental Basaltic Magmas from Lithospheric and Asthenospheric Mantle Sources   总被引：4，自引：1，他引：3  

MARZOLI  A.; PICCIRILLO  E. M.; RENNE  P. R.; BELLIENI  G.; IACUMIN  M.; NYOBE  J. B.; TONGWA  A. T. 《Journal of Petrology》2000,41(1):87-109

The volcanic activity of Mts Bambouto and Oku (Western Highlands)and of the Ngaoundere Plateau, in the continental sector ofthe Cameroon Volcanic Line, Equatorial West Africa, ranges inage from Oligocene to Recent. It is characterized by basanitic,alkali basaltic and transitional basaltic series. Mineral chemistry,major and trace element bulk-rock compositions, and geochemicalmodelling suggest that the magmatic series evolved mainly atlow pressure (2–4 kbar) through fractional crystallizationof clinopyroxene and olivine ± magnetite, at moderatelyhydrated (H₂O = 0·5–1 wt %) and QFM (quartz–fayalite–magnetite)to QFM + 1 fO₂ conditions. Basalts from Ngaoundere (Mioceneto Quaternary) and from the early activity (31–14 Ma)of the Western Highlands have incompatible trace element andSr–Nd isotopic compositions similar to those of oceanicCameroon Line basalts, pointing to a similar asthenosphericmantle source. By contrast, the late (15–4 Ma) WesternHighlands basanites and alkali basalts have anomalously highconcentrations of Sr, Ba and P, and low concentrations of Zr,which are exclusive features of continental Cameroon basalts.The genesis of these latter magmas is consistent with derivationfrom an incompatible element enriched, amphibole-bearing lithosphericmantle source. Western Highlands basalts show a continuous spectrumfrom high to low Sr–Ba–P compositions, and may resultfrom variable amounts of mixing between melts derived from ananhydrous lherzolite source (asthenospheric component) and meltsfrom an amphibole-bearing peridotite source (lithospheric HSrcomponent). New ⁴⁰Ar/³⁹Ar ages for Mts Oku and Bambouto basalts,combined with previous ⁴⁰Ar/³⁹Ar and K/Ar ages of basaltic andsilicic volcanics, and with volcanic stratigraphy, suggest aNE–SW younging of the peak magmatic activity in the WesternHighlands. This SW younging trend, extending from the Oligocenevolcanism in northern Cameroon (e.g. Mt Oku) to the still activeMt Cameroon, suggests that the African plate is moving abovea deep-seated mantle thermal anomaly. However, the age and locationof the Ngaoundere volcanism does not conform to the NE–SWyounging trend, implying that the continental sector of theCameroon Volcanic Line cannot be easily interpreted as the surfaceexpression of a single hotspot system. KEY WORDS: Cameroon Line basalts;⁴⁰Ar/³⁹Ar geochronology; lithospheric and asthenospheric mantle source; hotspot  相似文献   

Flood Basalt to Rhyolite Suites in the Southern Paran? Plateau (Brazil): Palaeomagnetism, Petrogenesis and Geodynamic Implications     

BELLIENI  G.; BROTZU  P.; COMIN-CHIARAMONTI  P.; ERNESTO  M.; MELFI  A.; PACCA  I. G.; PICCIRILLO  E. M. 《Journal of Petrology》1984,25(3):579-618

The petrology and palaeomagnetism of basic and acidic volcanicrocks of the Paran? plateau (South Brazil) have been investigated.The lower sections of the sequence are largely composed of tholeiiticbasalts and tholeiitic basaltic andesites while the upper portionsare essentially represented by rhyodacites showing intercalationsof basaltic and/or andesitic rock-types. Tholeiitic andesitesprevail at the basic-acidic boundary. Petrography, mineral chemistry and bulk-rock composition (majorand trace elements) data clearly define tholeiitic suites displayingpossible liquid lines of descent related to different degreesof partial melting and crystal fractionation controls. Mass balance calculations, based on phenocrysts and/or microphenocrystspresent in the different rock-types are consistent with thehypothesis that rhyodacitic melts may be derived from basaltsthrough low-pressure crystal fractionation processes accompaniedby crustal contamination. Palaeomagnetic data indicate high rates of magma emission (e.g.700 m in <1 m.y.) and sometimes the contemporaneous eruptionin different areas of basic and acidic lavas. Moreover, thepositions of the palaeomagnetic poles indicate that the volcanicfields of Paran? and Namibia (southern Africa) were joined at120 m.y. B.P. All the data consistently indicate that Paran? volcanism occurredduring the Lower Cretaceous after the formation of up-domingstructures, and that the initial opening of the South AtlanticOcean, at the latitudes of the Paran? basin, occurred not before120 m.y. B.P. * Reprint requests to E. M. Piccirillo.  相似文献