Geochemistry and petrogenesis of volcanic rocks from Chad basins, Africa.

The boreholes exposed that, during Mesozoic-Cenozoic times, numerous basalts had erupted within Chad basins which are poorly documented. They are mainly olivine tholeiitic basalts, whereas quartz tholefitic basalts and alkaline basalts are very scarce. Major- and trace elements as well as Sr-Nd-Pb isotopic geochemistry studies exhibit that magmas which the basalts in Chad basins derived from were produced by fractional crystallization of olivine and clinopyroxene and had not do suffered from crustal contamination. These basalts art! prominently enriched light rare earth element,,; ( LREE), large-ion lithophile elements (I.ILE) and high field strength elements (HFSE), and have positive Nb, Ta, Zr, Hf anomalies. The late Cretaceous basalts have higher (Sr-87/Sr-86), ratios (0.704167 similar to 0.706564) than the Paleogene basalts (0.7035-45 similar to 0.705380), whereas have lower (Nd-143/(144) Nd)(i) ratios (0.512451 similar to 0.512703) than the hitter (0.512690 similar to 0.512847). The epsilon(Nd)(iota) values of the late Cretaceous basalts have a wide range (- 1.4 similar to 3.5) and evidently lower than those of the Paleogene basalts (2.6 similar to 5.6), but they have a analogical radiogenic Pb ((206)pb/Pb-214= 18.14 similar to 18.96, Pb-207/Pb-204 = 15.56 similar to 15. 62, Pb-208/Pb-204=38.33 similar to 38.98). It is possible that the basalts from Chad basins mainly forrited by mixing of depleted mantle ( DM) and enriched mantle ( EMII) sources and the parental melts were probably generated by high degree partial melting of garnet peridotite (15%). The mantle components with temporal change observed in basalts from Chad basins were probably correlated with the asthenospheric mantle upwelling and lithosplieric Ihinning in Central and Western Africa since Mesozoic.