Emplacement P-T conditions of Pan-African biotite-amphibole granitoids in the Nkambe area, Cameroon

摘要

Biotite- and amphibole-bearing monzogranite, quartz monzonite, and tonalite intrude into the biotite granitoids and biotite-hornblende gneisses of the Pan-African belt in the Nkambe area, northwestern Cameroon. The common mineral assemblage in these rocks is amphibole + biotite + plagioclase + K-feldspar + quartz + zircon + apatite; however, their modal composition varies in each rock-type. Most amphibole grains are relatively homogeneous and locally have slightly Al-rich rims. They are mostly ferro-edenite or ferropargasite, and in a tonalite sample, there is magnesiohornblende with less aluminous and more magnesian composition. The amphibole + biotite + plagioclase + K-feldspar + quartz assemblage was affected by several pressure-sensitive and H_2O-free reactions. The ferro-edenite- and/or ferropargasite-bearing samples show similar pressure-temperature (P-T) ranges of 0.46-0.50 GPa/655-690 ℃ and 0.43-0.69 GPa/675-705 ℃. In contrast, the magnesiohornblende-bearing tonalite records lower temperature conditions of 0.77 GPa/610 ℃, which is attributed to subsolidus re-equilibration during cooling. The P-T estimates for the biotite-amphibole granitoids in this and previous studies suggest that relatively high-pressure granitoids are widespread in the Nkambe area.