The textural features of the granites and invaded rocks of the Singo batholith of Uganda and their petrogenetic significance

Abstract

Summary The batholith invades quartzites, sandstones, sandy shales and shales of the Karagwe-Ankolean System, which were sharply folded before the emplacement of the granite. The structures show that the invaded rocks were not swept aside, but that areas formerly occupied by country rocks are now occupied by granite. The granite is remarkably uniform in character over the greater part of the batholith and is principally a pink or reddish, coarse-grained, porphyritic rock, with large felspar phenocrysts. A finer- and more even-grained marginal variety is often found. The Karagwe-Ankolean sediments, in addition to showing increasing metamorphism of regional type towards the southern part of the area, are also contact-metamorphosed by the granite. Progressive recrystallization of original clastic grains of quartz to form a quartz mosaic, and reconstitution of other constituents into sericite and muscovite, are traceable. Certain of the marginal “granites” contain sericite in place of felspar and are only distinguishable from contact-altered sediments by the absence of bedding. The sericite always exhibits replacement relations towards the quartz and is believed, in part at least, to represent an intermediate stage in the development of felspar. Even in the coarse-grained granites, quartz was always the earliest mineral to form and was replaced successively by two generations of potash felspar as well as by oligoclase. The various felspars also show mutual replacement relations, while the phenocrysts of perthite were formed at a late stage in the development of the granite. It is believed that the granite never existed wholly as a magma or melt, but that it evolved by a series of replacements, effected by differential fixation in time and place of a succession of introductions from below and of materials displaced within the body itself.