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 (D1–D4). D1 and D2 are preserved in the pelitic schists; staurolite and garnet grew during D1, with staurolite growth persisting to the very earliest D2 crenulation. Andalusite, found in more Mg‐rich schists, grew during D2, overprinting both S1 schistosity and S0 banding. S2 has been rotated both with respect to S1 (preserved as parallel orientated inclusion trails in garnet and staurolite) and with respect to its original orientation (preserved as open D2 crenulations in staurolite). Staurolite is dissolved against S2 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‐D2) 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 D1–D2 times.