GEOCHEMICAL CHARACTERISTICS OF THE UMAT TRACHYTIC PLUG: IMPLICATIONS FOR THEIR ORIGIN

Abstract

The Umat trachytic plug is located in Bokkos area, Plateau State, part of Kurra sheet 189 (NE), North Central Nigeria. This study presents the geochemical results carried out on some samples of the Umat Trachytic Plug. The Umat trachytic hill extruded the Precambrian Basement granite-gneiss complex and in places, the Pan African Granites (Older Granites), Younger Granites Complexes and the Cenozoic volcanoes. A total of 10 rocks samples from the trachytic hill were analyzed for major, trace and rare earth elements using XRF and ICPMS respectively. Major, trace and rare element composition ratios and plots show that they have the same mantle source. They are alkali in nature which have been emplaced within plate volcanism comparable with Ocean Island Basalts as opposed to Mid-Oceanic-Ridge basalts which are tholeiitic. On the TAS classification diagram, the rocks expressed variation in their composition ranging from phonolite to tephriphonolite-trachyandesite. This is consistence with their trace element ratios such as Zr/Y; Ba/Nb, Ba/La, Zr/Nb, Th/Nb and La/Nb which are higher than those of primitive mantle and MORB but similar to those observed in the ocean island basalts. The average Zr/Y, Ba/Nb, Ba/La ratios are in agreement with a derivation related to OIB like mantle source. Similarly, their LREEs are highly incompatible in the mantle-melt system and LREE ratios should be close to those of the mantle source under batch melting condition. On chondrite-normalized REE patterns, the rocks exhibit strong LREE enrichment with (La/Yb) La/Ce ratios varying from 9.29 to 31. These ratios are higher than those of NMORB (0.5), EMORB (1.3) and OIB (12), indicating LREE enrichment which may be interpreted as due to low degrees of partial melting or reflecting an enriched fertile mantle source.