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

Citation
U. Ring et al., Deformed A-type granites in northern Malawi, east-central Africa: pre- or syntectonic?, J GEOL SOC, 156, 1999, pp. 695-714
Citations number
82
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
JOURNAL OF THE GEOLOGICAL SOCIETY
ISSN journal
0016-7649 → ACNP
Volume
156
Year of publication
1999
Part
4
Pages
695 - 714
Database
ISI
SICI code
0016-7649(199907)156:<695:DAGINM>2.0.ZU;2-F
Abstract
Metamorphosed and deformed A-type granites ranging in age from 1087 to 1119 Ma occur in strike-slip and reverse-slip sheer zones of the Ubendian and I rumide belts of northern Malawi. Peak metamorphic conditions in the granite s and their wallrocks reached about 12-13 kbar and 680-740 degrees C. Micro structures reveal that the granites experienced a seemingly progressive def ormation from magmatic to solid state. Magmatic deformation is manifested b y the coexistence of aligned magmatic potassium feldspar, plagioclase, amph ibole, biotite and ilmenite with undeformed quartz and schlieren structures . High-temperature solid-state deformation is characterized by widespread g rain-boundary migration, dynamic recrystallization and myrmekitic potassium feldspar replacement. The tectonic foliation is generally subparallel to t he magmatic foliation in the granites and also subparallel to the borders o f the plutons and to the foliation in the wallrock. Kinematic indicators in both granite and wallrock reveal sinistral movement in the Ubendian-belt s hear zones and top-to-the-ESE thrusting in the Irumide-belt shear zones. The overall parallelism of magmatic and tectonic foliations would suggest s yntectonic emplacement of the granites with respect to a c. 1100 Ma Irumide orogeny. However, similar deformation patterns of granite-related dykes an d some Pan-African pegmatites and geochronologic work suggests that the fir st deformation of the granites, their related dykes and some of the pegmati tes occurred during the Pan-African orogeny. This study implies that a pure ly structural approach to distinguish syn- and pretectonic granites is rend ered difficult when post-emplacement deformation occurred under high-grade metamorphic conditions. Strongly heterogeneous deformation with a pronounce d degree of coaxial flattening in the granites allows magmatic structures t o be preserved in low-strain zones and to be passively rotated into paralle lism with penetrative deformation structures. Our work also implies that th ere was no Irumide orogen in northern Malawi.