Lithosphere Remobilization during Parana CFB Magmatism

C Hawkesworth; M Mantovani; D Peate

doi:10.1093/petrology/Special_Volume.1.205

New major, trace element and Sr-isotope data are presented for samples from four boreholes through the volcanic rocks of the central Parana, and from two detailed road sections in the South Parana. The borehole material constrains the 3-D chemical stratigraphy within the Parana, and demonstrates that magmatism migrated north and evolved from low-Ti (LPT), to high-Ti (HPT) and then to intermediate-Ti (IPT) with time. The road section samples are low-Ti basalts and rhyolites with 87Sr/86Sr = 0·7069–0·7208 as SiO2 increases from 49·2 to 68·4%. The bimodal SiO2 distribution is taken as evidence that the rhyolites are crustal melts, and positive correlations between Rb, SiO2, and 87Sr/86Sr suggest that the rhyolites represent the best estimate of the crustal end member involved in the open system evolution of the lower SiO2 rocks. An inversion technique constrains the isotope and trace element abundances in the parental magmas before assimilation and fractional crystallization (AFC) takes place, and demonstrates that such magmas are similar to subduction related basalts but with elevated 87Sr/86Sr (0·7086). Parental magmas with low 87Sr/86Sr (0·704) are only consistent with the data if the contaminant has significantly higher 87Sr/86Sr (∼0·74), but such AFC models remain very difficult to reconcile with the observed SiO2 gap. Published Pb-isotope data suggests source ages of ∼ 1·8 Ga for the Parana basalts, and so the evidence for deriving continental flood basalts from the mantle lithosphere is briefly reviewed. Mantle xenolith data indicate that the Proterozoic and Phanerozoic sub-continental mantle is significantly more fertile than mantle lithosphere beneath Archaean areas. Moreover, the Mesozoic continental flood basalts of Gondwanaland yield Proterozoic, and not Archaean, source ages. It is concluded that the Parana basalts were derived from mantle lithosphere, but partial melting may have involved less than 10% of a 200 km thick lithosphere.

Lithosphere Remobilization during Parana CFB Magmatism

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