Browsing by Subject "Laakiobasaltti"

Continental flood basalts (CFBs) in the Karoo large igneous province were formed 189–178 Ma ago during the breakup of the Gondwana supercontinent. The flood basalts are found in the southern part of Africa, in Antarctica, and in the Falkland. The Karoo province can be divided into the North Karoo and South Karoo subprovinces on the basis of the geochemical compositions of the CFBs. The source of the South Karoo lavas has been interpreted to consist of pyroxenitic mantle and depleted upper mantle. The North Karoo lavas have been less well studied. The source of the North Karoo lavas at Luenha River, Mozambique, has been interpreted to be in a primitive part of the African LLSVP (Large Low Shear Velocity Province) in deep mantle. It is also believed that they have surfaced as part of a mantle plume. The results on the mantle source of the North Karoo CFBs have not been conclusive, however. In this study I analyze olivine crystals from the Luenha River picrites and utilize their mineral composition to address the mantle source of the North Karoo lavas. Four of the samples from Luenha River contain fresh olivine. They were analyzed using EPMA. All the analyzed olivine crystals are within Fo77–89. The compositional variation within the samples is relatively small and there is no evidence of multiple olivine populations. The olivine compositions support the proposed primitive peridotitic mantle source of the Luenha picrites. They are very similar to the olivines of the Etendeka CFBs that have been interpreted to be from peridotitic mantle source. The olivines from the Luenha picrites have high Mn/Fe ratios and low Ni, Ca, and Zn concentrations, which indicates a peridotitic source. The portions of pyroxenite and recycled oceanic crust were calculated to be less than 10 weight percent. In two samples, the crystallization temperatures were determined to be 1207–1368°C using an Al-in-olivine thermometer. Combined use of the Al-in olivine and the melt-olivine Mg-Fe equilibrium thermometers enabled the estimation of the temperatures of the primary melts (Fo92). The temperatures of the primary melts were 1300–1472°C, which supports the theory of Luenha picrites being derived from a mantle plume.