Abstract

In the southwest branch of the Diagorou-Darbani greenstone belt, mafic to ultramafic plutonic rocks outcrop in Pogwa area. Field relations show that these rocks are clearly intrusive in flattened pillowed-metabasaltes. They are made up of gabbros and pyroxenites with ultramafic rocks cropping out as panels or scattered lenses. Petrographic characters are suggestive of metamorphic grade varying from greenschist to amphibolite facies. The geochemical characters revealed that most of the gabbroic rocks were generated by fractional crystallization; however, variations in Ni and Cr contents in some samples are attributed to variable degrees of partial melting. The rare earth  elements REE and spider diagrams distinguished two kinds of patterns: 1) most rocks with light rare earth elements LREE enriched patterns, heavy REE HREE depleted patterns, large ion lithophile elements LILE enrichment and negative anomalies in high field strength elements HFSE (Nb, Ta, Zr and  Hf). These features are suggestive of an oceanic arc emplacement, corroborated by Zr-Nb-Y and Cr-Y discrimination diagrams. 2) The other (Ultramafic rocks and metatonalite) with tholeiitic character with REE patterns depleted in LREE and enriched in HREE. In fact, the tonalite rock may represent a highly evolved term of an ultramafic-mafic suite of oceanic crust. The mafic and ultramafic rocks of Pogwa constituted the out cropping root of a paleoproterozoic oceanic-arc in the birimian formations in Liptako, Niger.

Key words: Liptako, Pogwa, palaeaoproterozoic, oceanic arc, root.