Abstract
The occurrence of Jurassic volcanism in the Sokoto State (NW-Nigeria) is documented. Basaltic lavas showing porphyritic seriate texture (An > 50% zoned plagioclase, Mg/(Mg+Fe) = .43 olivine and iron-titanium oxide minerals in a Mg 35Fe 22Ca 43 clinopyroxene, magnetide and glass groundmass) have been found in boreholes carried out along the rivers Gagare and Bunsuru. Mineral chemistry and major element composition are consistent with tholeiitic affinity transitional towards alkali basalts. HYG element abundance indicate a within-plate anorogenic setting. The K/Ar age of 157 ± 5M.a. obtained for the samples suggests a relation with the basaltic activity affecting Nigeria in Jurassic times, contemporaneous with the Younger Granite intrusions. The high initial 87Sr/ 86Sr ratios (ranging from .07597 ± 3 to .70633 ± 3) and the HYG element contents fit the hypothesis that the basaltic lavas evolved by assimilation/crystallization processes starting from E-type MORB compositions. The comparison of the HYGE spectra between basaltic lavas and Younger Granite intrusions suggest that in the genesis of these latter a mantle component played a significant role.
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