Figure 1:
Phanerozoic sedimentary cover and major lineaments of the Parnaíba basin. The boreholes sampled during this study are indicated (eg. 1UN21). As an approximation, the 45°W longitude circle separates the Mosquito Formation, to the west, from the Sardinha Formation, eastwards the Parnaíba basin.
Figure 2:
Corroded (A) and zoned (B) plagioclase xenocrysts in diabases from the Parnaíba basin. Crossed polarized light. Samples: 1UN06 and 1UN32.
Figure 3:
Myrmekite in diabase (A) and basalt (B) from the Parnaíba basin. Crossed polarized light. Samples: 1UN26.
Figure 4:
The total alkalis versus silica (TAS; in wt.%) discrimination and classification diagram (A) and the AFM (A = Na2O + K2O, F = FeO + 0.8998Fe2O3, M = MgO, in wt.%) diagram (B) for the studied samples (LeBas et al. 1986Le Bas M.J, Le Maitre R.W, Streckeisen A., Zanettin B. 1986. A chemical classification of volcanic rocks based on the total alkali-silica diagram. Journal of Petrology, 27:745-750., LeMaitre 2002Le Maitre R.W. (Ed.). 2002. Igneous Rocks. A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. 2nd. ed. Edinburg, Cambridge University Press. 236p.). Curves are from Irvine & Baragar (1971Irvine T.N., & Baragar W.R.A. 1971. A Guide to the Chemical Classification of the Common Volcanic Rocks. Canadian Journal of Earth Sciences, 8(5):523-548.). Data recalculated to 100% on a volatile-free basis. The Fe2O3/FeO ratios for CIPW norm calculations (Middlemost 1989Middlemost E. 1989. Iron oxidation ratios, norms and the classification of volcanic rocks. Chemical Geology, 77:19-26.) are shown in the TAS classification fields.
Figure 5:
Variation diagram for TiO2 for the studied samples showing two different evolutionary trends (low- and high-TiO2 suites) with a boundary at TiO2 = 2.00 wt.%.
Figure 6:
Variation diagram for Ti/Y for the studied samples showing two different evolutionary trends (low- and high-TiO2 suites) with a boundary at Ti/Y = 410.
Figure 7:
La versus Yb diagram showing an AFC curve at F = 0.1 intervals up to F = 0.5. The L-Ti samples are shown. Other parameters used in modelling were: DLa = 0.051; DYb = 0.493; r = 0.3. Upper crust composition from Taylor & McLennan (1981Taylor S.R., & McLennan S.M. 1981. The composition and evolution of the continental crust: rare earth element evidence from sedimentary rocks. Philosophical Transactions of the Royal Society of London, A301:381-399.).
Figure 8:
Y
versus Ti diagram showing an AFC curve at F = 0.1 intervals up to F = 0.7. The H-Ti samples are shown. Other parameters used in modelling are those in
Figure 7.
Figure 9:
Schematic sampled section of borehole 1UN32. MgO (wt.%) and La/Yb ratios are indicated. The approximate in situ contamination zone was estimated by the anomalously values of La/Yb ratios compared to those of the top and the base of the intrusion.
Figure 10:
Chondrite-normalized REE diagram for samples from borehole 1UN32. Normalization factors from Evensen et al. (1978Evensen N.M., Hamilton P.J., O’Nions R.K. 1978. Rare-earth abundances in chondritic meteorites. Geochimica et Cosmochimica Acta, 42(8):1199-1212.).
Figure 11:
Diagram showing the relation of the amount of contamination (or the amount of contaminant fluid) and the La/Yb(c) ratio of fluids to produce the contaminated magmas of borehole 1UN32.
Figure 12:
Photograph of sections of borehole 1UN32 outside (A) and inside (B) the contamination zone. The latter bears a greater volume of veins than the former as depicted by its whitish, more fractured features.
Figure 13:
Bivariate diagrams with AFC curves for F = 0.1 intervals. The L-Ti tholeiites of borehole 1UN21 are shown. Partition coefficients (Kd) from Rollinson (1993Rollinson H.R. 1993. Using Geochemical Data: Evaluation, Presentation, Interpretation. Edinburgh: Longman, 352 p.), with the exception of Ti, Y, and Zr (Johnson 1998Johnson K.T.M. 1998. Experimental determination of partition coefficients for rare earth and high-field-strength elements between clinopyroxene, garnet, and basaltic melt at high pressures. Contributions to Mineralogy and Petrology, 133(1-2):60-68.).
Figure 14:
Chondrite-normalized REE diagram for samples from borehole 1UN26. Normalization factors from Evensen et al. (1978Evensen N.M., Hamilton P.J., O’Nions R.K. 1978. Rare-earth abundances in chondritic meteorites. Geochimica et Cosmochimica Acta, 42(8):1199-1212.).
Figure 15:
Chondrite-normalized REE diagram for samples representing parental compositions in the study area. Normalization factors from Evensen et al. (1978Evensen N.M., Hamilton P.J., O’Nions R.K. 1978. Rare-earth abundances in chondritic meteorites. Geochimica et Cosmochimica Acta, 42(8):1199-1212.).
Figure 16:
Chondrite-normalized multielemental diagram for samples representing H-Ti parental compositions in boreholes 1UN06 and 1UN32. Normalization factors from Thompson (1982Thompson R.N. 1982. Magmatism of the British Tertiary Volcanic Province. Scottish Journal of Geology, 18(1):49-107.).
Figure 17:
F versus La/Ybn diagram with the modal batch partial melting curve. Modal composition in the mantle residue (olivine:orthopyroxene:clinopyroxene:spinel = 68:27:2:3) is from McKenzie & O’Nions (1991McKenzie D., & O’Nions R.K. 1991. Partial melt distributions from inversion of rare earth element concentrations. Journal of Petrology, 32:1021-1091.). The amount (%) of partial melting and their respective La/Ybn ratios are indicated. Crystal-liquid partition coefficients from Arth (1976Arth J.G. 1976. Behaviour of trace elements during magmatic processes - a summary of theoretical models and their applications. Journal of Research of the United States Geological Survey, 4:41-47.) e Schock (1979Schock H.H. 1979. Distribution of rare-earth and other trace elements in magnetites. Chemical Geology, 26:119-133.).
Figure 18:
Chondrite-normalized multielemental diagram for samples representing L-Ti parental compositions in boreholes 1UN19 and 1UN21. Normalization factors from Thompson (1982Thompson R.N. 1982. Magmatism of the British Tertiary Volcanic Province. Scottish Journal of Geology, 18(1):49-107.).
Figure 19:
Chondrite-normalized multielemental diagram for samples representing L-Ti and H-Ti parental compositions in borehole 1UN26. Normalization factors from Thompson (1982Thompson R.N. 1982. Magmatism of the British Tertiary Volcanic Province. Scottish Journal of Geology, 18(1):49-107.).
Figure 20:
Petrogenetic model for the Parnaíba tholeiites in the study area.
Figure 21:
Discriminant diagram of low-TiO2 suites of Paraná-Etendeka Province (Peate 1997Peate D.W. 1997. The Parana-Etendeka province. In: Mahoney J.J., & Coffin M.F. (Eds.). Large igneous provinces: continental, oceanic and planetary flood volcanism. American Geophysical Union, 100, p.217-245. Geophysical Monograph Series.) with samples of low-TiO2 basalts of the Parnaíba Basin studied by other authors (Bellieni et al. 1990Bellieni G., Piccirillo E.M., Cavazzini G., Petrini P., Comin-Chiaramonti P., Nardy A.J.R., Civetta A.J., Melfi A.J., Zantedeschi P. 1990. Low- and high TiO2, Mesozoic tholeiitic magmatism of the Maranhão basin (NE-brazil) - K-Ar age, geochemistry, petrology, isotope characteristics and relationships with Mesozoic low- and high TiO2 flood basalts of the Paraná Basin (SE-Brazil). Neues Jahrbuch Mineralogischer Abhandlungen, 162:1-33., Ernesto et al. 2004Ernesto M., Marques L.S., Bellieni G., Piccirillo E.M., DeMin A., Pacca I.G., Martins G., Macedo J.W.P. 2004. As Rochas Toleíticas Mesozoicas do Nordeste do Brasil: Distribuição Espacial e Temporal dos Sucessivos Episódios Magmáticos. In: Congresso Brasileiro de Geologia, 42., 2004, Araxá. Anais
do XLII Congresso Brasileiro de Geologia. CD. p.783.) and studied in this thesis. The discriminant field for the low-TiO2 basalts of Paraná-Etendeka are shown.
Table 1:
Major and trace elements analyses. n.d. = not detected. Fe2O3t is total iron. L.O.I. is the loss on ignition. Oxides in wt.%. Trace elements in ppm.
Table 2:
Maximum, minimum, average and standard deviation values of CIPW norm of the studied samples. All values are expressed in wt.%. Iron ratios for calculations according to Middlemost (1989Middlemost E. 1989. Iron oxidation ratios, norms and the classification of volcanic rocks. Chemical Geology, 77:19-26.).
Table 3:
Representative compositions of the parental and the most evolved liquids of the low-TiO2 (L-Ti) and high-TiO2 (H-Ti) suites in the study area. n.d. = not detected.
Table 4:
Representative samples of parental magmas, their respective wells, MgO content (wt.%) and (La/Yb)n and (La/Nb)n ratios. High-TiO2 (H-Ti) and low-TiO2 (L-Ti) suites are indicated.
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