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

Abstract

The area of north eastern Zambia between, Chama and Lundazi towns, is underlain by biotite gneisses of the Mesoproterozic Irumide Fold Belt dominated by biotite gneisses of the Kampemba group, of the Irumide Fold Belt. A complex of a mafic dyke swarm that appears to occupy a NE 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 100s of metres long by a few 10s of metres wide, to about 4 km long and about 100 metres wide. This study examined the petrogenesis of the mafic dyke swarm through the application of petrography, whole rock geochemistry as well as isotope geochemistry. Fifteen fresh dyke swarm samples were sent to Actlabs in Canada for whole rock geochemical analysis. Three of the fifteen samples were analysed for Rb-Sr and Sm-Nd isotope geochemistry, as well as 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. eNd 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 and photomicrographs, the studied dyke rocks were classified as gabbroic. They were composed mainly of plagioclase, clinopyroxene, hornblende with subordinate olivine, quartz, sericite, epidote and opaque phase minerals. The dyke rocks occur mainly in course and medium grained varieties as only two of the twelve samples examined, were fine grained. The several diagrams which were generated from whole rock geochemical data and applied in the evaluation of petrogenetic processes 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, tholeiitics and calc-alkalics. 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. These were the main minerals which were identified in petrographic studies of the same samples. Both types of dykes show enrichment of the source magma by addition.