Early Namaqua low‐pressure metamorphism: deformation and porphyroblast growth in the Zoovoorby staurolite schist, South Africa

Abstract

This paper describes the deformation and metamorphism recorded in the Zoovoorby staurolite schist, a sliver of pelitic supracrustal material in the 1.3–1.0 Ga eastern Namaqua Province, South Africa. The supracrustal Biesjepoort Group, of which the schist is a part, has undergone at least four phases of deformation (D 1 –D 4 ). D 1 and D 2 are preserved in the pelitic schists; staurolite and garnet grew during D 1 , with staurolite growth persisting to the very earliest D 2 crenulation. Andalusite, found in more Mg‐rich schists, grew during D 2 , overprinting both S 1 schistosity and S 0 banding. S 2 has been rotated both with respect to S 1 (preserved as parallel orientated inclusion trails in garnet and staurolite) and with respect to its original orientation (preserved as open D 2 crenulations in staurolite). Staurolite is dissolved against S 2 in zones of progressive shear. The pseudomorphing of staurolite and andalusite by cordierite, and the preservation of relic grains of both minerals in a wide range of garnet–cordierite pelites throughout the eastern Namaqua Province infers that what is preserved fortuitously in the Zoovoorby locality is representative of the early metamorphic history of a much larger terrane. The high thermal gradients needed to attain estimated conditions of 540–550° C and 1.6–2.4 kbar require substantial heat input. Large amounts of foliated (syn‐D 2 ) granite amongst the supracrustal succession are inferred to be the result of delamination of a thickened crust at a destructive plate margin, generating an elevated thermal gradient during D 1 –D 2 times.