The Cananeia Alkaline Body in the South Coast of São Paulo State: Mineral Chemistry

Abstract

The Cananeia alkaline occurrence of Late Cretaceous age (Ar-Ar, 83.6 Ma) lies in the Ribeira Valley and it is represented by two small bodies: Morro de São João (1.8 km²) in Cananeia Island, and Morrete (0.4 km²), in the neighboring Comprida Island. It is covered by Quaternary sediments and mostly constituted by intrusive rocks of syenitic composition, assembled into two principal groups: alkali feldspar syenites and quartz-alkali feldspar syenites. Microsyenites of variable texture are subordinate. Regarding the mineralogy, the rocks are very rich in feldspars, mesoperthite being the most abundant phase, and plagioclase is only occasionally found as isolated crystals. Mafic minerals consist dominantly of clinopyroxenes and amphiboles, both belonging to the calcic, calcic-sodic and sodic groups and showing significant chemical variations, such as the increase in the Fe/Mg ratio and Na content, and the decrease in Ca, as a function of the degree of evolution of the rocks. Textural relationships show that clinopyroxene reacts to form amphibole. Biotite is common, mainly in association with amphibole and opaques. Biotite and opaques together are found at the borders of amphibole grains or concentrated along their fractures and cleavage planes. Fayalitic olivine is rarely preserved. Most common accessories include opaques, mainly magnetite with exsolved ilmenite lamellae, apatite, titanite and zircon. Late to post-magmatic alteration processes can explain the replacement textures shown by the principal primary minerals and the exsolution structures found in feldspars and opaques.