Chemistry of biotite as petrogenetic discriminators of plutonic and volcanic granitoids from south-central Côte d’Ivoire, West Africa

Chemical compositions of biotite have been used to understand the petrogenesis of intrusive and extrusive magmatic rocks. Central southern Côte d’Ivoire is home to a wide variety of lithologies, including granodiorites, tonalites, and dacites. In this paper, we compare biotites from intrusive rocks with those from extrusive rocks. We used the composition of the mineral to describe the nature of the granitic magma and estimate the temperature at which the studied rocks were emplaced. Grano- diorite and tonalite are composed of biotite, hornblende, plagioclase, apatite, magnetite, and ilmenite. Dacite contains the minerals biotite, muscovite, quartz, and plagioclase. This work has shown contrasts in composition between biotites. The biotites in the intrusive rocks are muscovite-free. Some of these biotites coexist with amphiboles, while others are not accompanied by other ferromagnesian minerals. Some biotites in extrusive rocks are not accompanied by ferromagnesian minerals, while others coexist with muscovite. Extrusive rock biotites do not contain amphiboles. The major elements of the biotites show that they are located between the annite and siderophyllite poles. However, biotites in granodiorites and tonalites are rich in AlIV, giving them a siderophyllite tendency, while biotites in extrusive rocks, which are low in AlIV, converge towards the annite pole. The biotites in the intrusive rocks studied are rich in Mg (Mg-biotite) and Ti, while the biotites in the extrusive rocks studied appear to be rich in Fe (Fe-biotite) and Mn. Plutons could have formed in an orogenic setting from calc-alkaline magma in a subduction environment. Volcanic rocks are peraluminous. The intrusive rocks derive from I-type granites, while extrusive rocks appear to derive from S-type granites. Based on the incorporation of Ti into biotites, the rocks studied were emplaced in a temperature range estimated at 550-650°C at a pressure of 4-6 kbar.