Petrography and geochemistry of the Singo granite, Uganda, and implications for its origin

The Singo granite in western central Uganda intrudes metasedimentary rocks that have experienced low-grade regional and contact metamorphism. Rocks of the main body are pink and coarse-grained with a porphyritic texture. Marginal gray medium-grained biotite granite (BG) and several other varieties with intermediate composition occur with limited extent. The Singo granite is generally massive, contains mainly plagioclase, K-feldspar, quartz, biotite, muscovite and opaques. The BG has higher Al2O3, MgO, CaO, Fe2O3, TiO2, Ba, Zr, and V, but lower SiO2, Th, U, and rare earth elements (REE) and alkali totals than the pink porphyritic granite (PPG). Spider diagrams (chondrite-normalized) show negative Eu, Sr, and Nb anomalies, and unfractionated heavy-REE (HREE). The Eu and Sr anomalies and unfractionated HREE suggest the presence of plagioclase and absence of garnet in the source, whereas the Nb anomaly implies a crustal component. Singo granite has both S- and I-type characteristics, and was emplaced in a syn- to post-collision tectonic setting in several magma pulses within relatively short time intervals. The pluton, in general, shows zonation in texture, mineralogy, and geochemistry from the margin to the center. The BG is relatively old and less felsic, whereas the PPG represents a younger and more felsic part of the pluton. There is continuity between the BG and the PPG through the intermediate granite, suggesting a common origin. Field, petrographic, and geochemical characteristics support a magmatic origin from a water-undersaturated, heterogeneous crustal source rock under low pressure conditions. Petrographic and chemical variations were mainly the result of fractional crystallization and source heterogeneity. Late- and post-magmatic stages were dominated by strong hydrothermal activity. (C) 2003 Elsevier Science Ltd. All rights reserved.