Affiliation: | a Department of Earth Sciences, The University, Leeds, England b Department of Geology, Imperial College, London, S.W. 7, England c Department of Geophysical Sciences, University of Chicago, Illinois 60637, U.S.A. d Department of Geology, The University, Leicester, England |
Abstract: | Two sections of the anorthosite ‘complex’ were examined at Messina, South Africa and at Pikwe, Botswana. Thirty XRF whole-rock analyses of samples in stratigraphic order show that alkalies at Messina increase upwards from leuco-gabbros to anorthosites, but no such correlation was found at Pikwe. Electron probe analyses of plagioclases in 33 samples indicate extensive normal zoning and variation (An31-An80) especially at Pikwe. The highest An-contents tend to vary monotonically with stratigraphic position, whereas the lowest values are erratic. Twenty-two amphibole analyses indicate that SiO2, TiO2 and MgO/FeO increase, decrease and increase, respectively, with height at Messina, but 22 analyses from Pikwe show no such trends. Their compositions are similar to those from the Fiskenaesset complex. Aluminous chromites are Fe-rich, also like the Fiskenaesset ones. The Limpopo anorthositic rocks belong to a layered igneous body that was recrystallized by regional metamorphism and subjected to erratic metasomatism. The original Limpopo and Fiskenaesset bodies strongly resemble gabbroic and peridotitic relics in the Peninsular Ranges Batholith in S. California. |