Geochemistry and petrogenesis of the mafic dyke swarm of the north eastern part of the Mesoproterozoic Irumide Fold Belt, North eastern Zambia

The north eastern area of Zambia, that lies between Chama and Lundazi towns, is underlain by biotite gneisses of the Mesoproterozoic Irumide Fold Belt. The biotite gneisses are part of the Kampemba Group of the Irumide Fold Belt. A complex of mafic dyke swarm that appears to occupy a North East trending fracture zone occurs mainly between the two towns. The dykes have the same trend with the fracture zone. Individual dykes vary in size from about hundreds of metres long by a few 10s of metres wide, to about 4 kilometre long and about hundred metres wide. This study examined the petrogenesis of the mafic dyke swarm through the application of petrography, whole rock geochemistryas well as isotope geochemistry. Fifteen fresh dyke swarm samples were sent to Actlabs in Canada for whole rock geochemical analysis. Three of the samples were analysed for Rb-Sr and Sm-Nd isotope geochemistry, and age determinations by K-Ar age dating method. The results received on age determination (geochronology) gave the ages, 1522 ± 90 Ma, 1067 ± 55 Ma and 608 ± 35 Ma. Sm and Nd grades ranged from 1.99 – 3.112 ppm and 6.666 – 9.88 ppm respectively. Nd were calculated by the laboratory at -5.23, -8.23 and -9.73. 87Sr/86Sr returned values of 0.710886, 0.714503 and 0.719612.From the petrographic study of the thin sections, the studied dyke rocks were classified as gabbroic. They were composed mainly of plagioclase, clinopyroxene, hornblende with subordinate olivine, quartz, sericite, epidote and opaque phases. The dyke rocks occurred mainly in coarse and medium grained varieties and only two of the twelve samples examined, were fine grained. Diagrams which were generated from whole rock geochemical data and the relative depletion of Nb and Ta to the other incompatible elements revealed that the dyke swarm was emplaced in a back arc continental subduction environment. The geochemical data further revealed that fractional crystallisation affected the magma of both types of rocks identified in the study, tholeiitic and calc-alkalis. Since no two samples came from the same dyke, it was deduced that all the mafic dykes in the study area experienced various levels of fractional crystallisation of possibly olivine, clinopyroxenes, plagioclase feldspars and hornblende