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Deformed A-type granites in northern Malawi, east-central Africa: Pre- or syntectonic?

Abstract:

Metamorphosed and deformed A-type granites ranging in age from 1087 to 1119 Ma occur in strike-slip and reverse-slip shear zones of the Ubendian and Irumide belts of northern Malawi. Peak metamorphic conditions in the granites and their wallrocks reached about 12-13 kbar and 680-740°C. Microstructures reveal that the granites experienced a seemingly progressive deformation from magmatic to solid state. Magmatic deformation is manifested by the coexistence of aligned magmatic potassium feldspar, plagioclase, amphibole, biotite and ilmenite with undeformed quartz and schlieren structures. High-temperature solid-state deformation is characterized by widespread grain-boundary migration, dynamic recrystallization and myrmekitic potassium feldspar replacement. The tectonic foliation is generally subparallel to the magmatic foliation in the granites and also subparallel to the borders of the plutons and to the foliation in the wallrock. Kinematic indicators in both granite and wallrock reveal sinistral movement in the Ubendian-belt shear zones and top-to-the-ESE thrusting in the Irumide-belt shear zones. The overall parallelism of magmatic and tectonic foliations would suggest syntectonic emplacement of the granites with respect to a c. 1100 Ma Irumide orogeny. However, similar deformation patterns of granite-related dykes and some Pan-African pegmatites and geochronologic work suggests that the first deformation of the granites, their related dykes and some of the pegmatites occurred during the Pan-African orogeny. This study implies that a purely structural approach to distinguish syn- and pretectonic granites is rendered difficult when post-emplacement deformation occurred under high-grade metamorphic conditions. Strongly heterogeneous deformation with a pronounced degree of coaxial flattening in the granites allows magmatic structures to be preserved in low-strain zones and to be passively rotated into parallelism with penetrative deformation structures. Our work also implies that there was no Irumide orogen in northern Malawi.