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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 |
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| Authors: | PETRINI R; CIVETTA L; PICCIRILLO E M; BELLIENI G; COMIN-CHIARAMONTI P; MARQUES L S; MELFI A J |
| Institution: | 1Istituto di Geocronologia e Geochimica Isotopica, Consiglio Nazionale delle Ricerche Pisa, Italy 2Dipartimento di Geofisica e Vulcanologia, University of Naples Italy 3Istituto di Mineralogia e Petrografia, University of Trieste Piazzale Europa 1, 34100 Trieste, Italy 4Istituto di Mineralogia e Petrologia, University of Padova Italy 5Istituto di Mineralogia, Petrografia e Geochimica, University of Palermo Italy 6Instituto Astronomico e Geofisico, University of S{small tilde}o Paulo Brazil |
| Abstract: | 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. |
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