Silicic Layer-Parallel Shear Zones in a Zimbabwean Greenstone Sequence: Horizontal Accretion Preceding Doming

Abstract

Detailed structural metamorphic data from the Trojan nickel mine area in the Shamva-Bindura Greenstone Belt (SBGB) show that two groups of ductile structures associated with separate amphibolite facies metamorphic assemblages can be distinguished. D 1/M 1 structures are related to a network of anastomosing shear zones that accommodated W-directed imbricate stacking of the stratigraphy, while metamorphic conditions in the mine area reached about 500°C at pressures of 3-4 kbar. D 1 shears are locally strongly silicified and preserve fine-grained mylonitic textures. They have been wrongly identified in the past as banded iron stones or stratigraphic chert horizons. Truncations of primary layering associated with such fine-grained mylonitic quartzites are tectonic and not stratigraphic in origin, and the units can not be used as tectonic marker horizons, but instead represent glide planes across which tectonic imbrication of the stratigraphy has occurred. An important silicified shear zone of this nature occurs along the boundary of the Iron Mask and Arcturus Formations. D 2/M 2 structures are related to doming of the Chinamora Batholith, and a contact metamorphic overprint and recrystallisation of M 1 assemblages. M 2 temperatures in the mine area reached 565°C. Such high contact metamorphic temperatures at 3 km from the contact of the batholith can only be explained if the entire Chinamore Batholith was emplaced as a relatively hot (>750°C) intrusive body in an already anomalously hot greenstone sequence. Metamorphic fluids during M 1 and M 2 where CO 2-rich and carbonate alteration was pervasive. D 1/M 1 and D 2/M 2 events may have been separated by as much as 70 Ma. D 1 structures in the Trojan area can be related to a large mantled-gneiss, nappe structure represented by the Madziwa Batholith and a mantle of mafic greenstones to the N of the SBGB. The footwall thrust of this nappe occurs along the N boundary of the Shamvaian sediments in the centre of the SBGB. The imbricate stacking of the stratigraphy in the Trojan area can be interpreted as secondary structures in the footwall of the nappe. Horizontal accretion and stacking of crustal fragments during D 1 was followed by thermal perturbations that resulted in the diapiric rise of domes like the Chinamora Batholith during D 2. After doming the terrain appears to have cooled and stabilized, with further deformation partitioned into narrow strike-slip shear zones.