Abstract
The Tadhak alkaline ring-complex of Permian age provides two whole rock U Pb isochrons giving concordant ages in agreement within relative errors with the Rb Sr isochron age: 235U 207Pb isochron: 271 ± 32Ma(MSWD= 0.3); 238U 206Pb isochron: 254 ± 18Ma(MSWD= 7.8), both on 8 whole-rock samples. The existence of these isochrons indicates that in favorable conditions U (and Pb) can be immobile. This can be due either to the lack of hard oxidizing conditions and/or to the location of U, in very low concentrations, in weathering-resistant minerals. The initial ratios ( 206Pb/ 204Pb = 18.714 ± 70and 207Pb/ 204Pb = 15.589 ± 16), corrected for their Permian age, lie in the range observed for oceanic island basalts or continental alkali basalts and indicate an origin in a similar mantle, without any significant crustal contamination. This was also suggested by the initial 87Sr/ 86Sr ratio of 0.70457 ± 4. Moreover, these Sr and Pb isotopic characteristics belong to the field of the so-called “Dupal” anomaly and indicate that it existed already 270 Ma ago. This study shows the potential interest of isotopic investigations of within-plate alkaline ring-complexes to characterize subcontinental mantle compositions, particularly in the past.
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