PETRO-GEOCHEMICAL INVESTIGATION OF THYOLO MOUNTAIN PERTHITIC SYENITES AND PERTHITE GNEISS

This study investigates the geochemical characteristics of perthitic syenites and perthite gneiss from the Thyolo mountain in southern Malawi to understand petrogenesis and geological history. Field observations, petrographic analysis, geochemical data, and statistical methods were used to characterize these rocks. The key findings indicate that the perthitic syenites are predominantly pink or brownish, dark gray, and whitish with phaneritic textures showing high K2O and Na2O values typical of alkaline nature. Most samples are metaluminous, consistent with rift or post-orogenic magmatism. Granite characterized by high SiO2 values and slightly peraluminous nature, likely representing a late-stage differentiate or separate intrusive phase. Perthite gneisses are dark grey, coarse-grained, dominated by dark mineral grains like hornblende and pyroxene, showing mafic compositions distinct from syenites due to their trace element signatures. The petrogenesis of perthitic syenites suggests derivation from silica-oversaturated alkali-rich magma, indicative of extensional settings possibly related to the East African Rift System (EARS) or post-orogenic events after the Pan-African orogeny—perthitic textures formed during slow cooling typical of deep-seated crustal environments. The granite's high SiO2 values suggest derivation from partial melting of crustal material, potentially representing a differentiated fraction of the syenitic magma. Perthite gneisses likely represent metamorphosed igneous protolith under amphibolite and granulite facies conditions, with deformation textures indicating tectonic forces linked to either Pan-African orogeny or rifting. The alkaline shoshonitic nature of the syenites suggests formation in an extensional or post-orogenic setting linked to Neoproterozoic magmatism or Mesozoic rifting associated with the Chilwa alkaline province (CAP). Gneisses reflect high-grade metamorphism within the Mozambique belt during the Pan African orogeny, with deformation features aligning with EARS-related faulting. These rocks have economic potential: perthitic syenites are suitable for ceramics, glass, and construction applications due to high feldspar content with elevated Ba and Sr, suggesting potential for barite or strontiate deposits. Perthite gneisses are suitable for construction aggregates, road base, or ornamental stone. Granitic rocks are useful in ceramics or dimension stone.