Coexisting A1 and A2 granites of Kudaru Complex: implications for genetic and tectonic diversity of A-type granite in the Younger Granite province, north-central Nigeria.

The Nigerian alkaline ring complexes have long been regarded as typical anorogenic A-type granite. However, uncertainty still surrounds many aspects of their petrogenesis and tectonic implication. In this paper, a combined study of whole rock geochemistry, Nd-Pb isotope and zircon U-Pb dating was carried out on the Kudaru ring complex from north-central Nigeria. The complex consists of mildly alkaline fayalite granite porphyry, peralkaline arfvedsonite granite and metaluminous biotite granite, which were emplaced between 176.9 ± 2.5 and 180.55 ± 0.6 Ma. These rocks are characterized by variably high alkalis, HFSEs, Ga/Al ratios, and zircon saturation temperature, typically of A-type granites. Notably, the popular subdivision scheme of A-type granite permits the discrimination of the alkaline-peralkaline granites as typical "A₁-type" granites and the metaluminous granite as "A₂-type". These co-existing A₁ and A₂ granites show distinct isotopic compositions with less negative εNd (t) values and higher ²⁰⁶Pb/²⁰⁴Pb ratios for the former, indicating that the magma which gave rise to the A₁ and A₂ suites was derived from an enriched mantle source but modified to different extent during ascent. Notably, the coexistence of A₁-A₂ granites do not indicate discrete tectonic regimes (intraplate setting for A₁ and post-orogenic setting for A₂), but rather point to a significant role of crustal contamination, as seen also from the differences in Y/Nb ratios. [ABSTRACT FROM AUTHOR]