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 1.3-1.0Ga eastern Namaqua Province, South Africa. The supracrustal Biesjepoort Group, of which the schist is a part, has undergone at least four phases of deformation (DI-D,). DI and D2 are preserved in the pelitic schists; staurolite and garnet grew during D,, with staurolite growth persisting to the very earliest D, menulation. Andalusite, found in more Mg-rich schists, grew during D,, overprinting both S, schistosity and S, banding. S, has been rotated both with respect to SI (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 S, in zones of progressive shear. The pseudomorphing of staurolite and andalusite by midierite, 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 1.62.4 kbar require substantial heat input. Large amounts of foliated (syn-D,) 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.

Sequential, syndeformational porphyroblast growth during Hercynian low‐pressure/high‐temperature metamorphism in the Canigou massif, Pyrenees

“…Porphyroblasts from the pelitic rocks in the Canigou massif show many features in common with syndeformational porphyroblasts described from other regional metamorphic terranes (e.g. Bell & Rubenach, 1983;Reinhardt & Rubenach, 1988;Humphreys & Van Bever Donker, 1990;Lang & Dunn, 1990). Bell et al (1986) proposed that porphyroblasts grow in deforming rocks in domains characterized by shortening strain.…”

Polymetamorphism and isobaric cooling of pelitic schists from the Annex sulphide deposit, Gordonia Belt, South Africa

“…D2-D3 deformation in the Areachap Group was accompanied by M 2 low-Plhigh-T metamorphism (Humphreys & Van Bever Donker, 1990). with syn-kinematic And + Sil + Bt + Ms assemblages and Sil + Crd + Grt + Kfs assemblages developed in the north and at Copperton, respectively (Humphreys, 1991).…”

“…At 1300-1285 Ma Areachap Group lava were extruded along the eastern margin of the Gordonia Belt, and were subsequently metamorphosed between 1210 and 1201 Ma (Cornell et al, 1986). This was accompanied by (Namaqua) D1 deformation in the south, in which the Copperton Formation was thrust eastward onto the older Kheis Belt. D2-D3 deformation in the Areachap Group was accompanied by M 2 low-Plhigh-T metamorphism (Humphreys & Van Bever Donker, 1990). with syn-kinematic And + Sil + Bt + Ms assemblages and Sil + Crd + Grt + Kfs assemblages developed in the north and at Copperton, respectively (Humphreys, 1991).…”

“…Since the Gordonia Belt contains supracrustal components that are derived from two main sources (the Areachap Group and sequences deformed in the Orange River orogeny) early P-T histories are markedly different, ranging from steep decompression to the M2 peak (ex-Orange River orogeny rocks) and shallow decompression to the peak (Annex) to 'anticlockwise' andalusitesillimanite facies series metamorphism (northern Areachap Group) (Humphreys & Van Bever Donker, 1990). Furthermore, the pelites in the M3 contact aureole along the edge of the Keimoes Suite batholith preserve a variety of isobaric and decompressional prograde paths at different points along its length (Humphreys, 1991).…”

Empirical ferric iron corrections: necessity, assumptions, and effects on selected geothermobarometers

“…Using the original calibration of Ferry and Spear (1978) and a pressure of 5.0kbar, the actual Fig. 1 and 6; example (B)--assemblage S130 (lowest ferric content) and (C) assemblage S142 (highest ferric content) both from Sevigny and Ghent (1989); and (D) assemblage HHG007 from Humphreys and Van Bever Donker (1990).…”