Tectono-metamorphic evolution and strain partitioning along the Mugesse mega shear zone, Ubendian Belt (Northern Malawi)

In this contribution we focus on a portion of the Mugesse mega shear zone (MGSZ) in the Ubendian Belt, for which details on kinematics and strain partitioning remains poorly documented. We present a field-based structural work, a petrological and thermobarometrical study of mylonites and LA-ICP-MS in-situ U-Pb dating on monazite and zircon. The detailed field mapping highlights that the whole study area shows the same sinistral strain pattern with deformation being partitioned within S-C and C-C' structures. Syn-kinematic metamorphic mineral assemblages indicate that the MGSZ mylonites recorded a clockwise P-T evolution with peak temper-ature conditions at upper amphibolite metamorphic facies of 11-15 kbar and 760-790 degrees C. Retrogression and cooling reached P-T conditions of 1-7 kbar and 330-480 degrees C. LA-ICP-MS in-situ U-Pb monazite dating suggests that the main activity of the MGSZ was Pan-African, at ca. 547 & PLUSMN; 5 and 545 & PLUSMN; 4 Ma. Zircon dates at 1985 & PLUSMN; 44 Ma and 1127 & PLUSMN; 29-988 & PLUSMN; 10 Ma are interpreted as inherited ages of potentially detrital origin related to the Ubendian and Irumide orogeny, respectively. A 1146 & PLUSMN; 14 Ma date obtained on zircons from the Lwakwa mylonitic orthogneiss is interpreted as the crystallisation age of the granitic protolith that intruded the Paleo-proterozoic basement. Strain partitioning and strain softening during the growth of the MGSZ was strongly controlled by feedback relationships between deformation and retrograde metamorphism enhanced by fluid ingress and recrystallisation.