Structural and P-T evolution of a major cross fold in the Central Zone of the Limpopo high-grade terrain, South Africa

Abstract

The Central Zone (CZ) of the Limpopo Complex of southern Africa is characterized by a complex deformational pattern dominated by two types of fold geometries: large sheath folds and cross folds. The sheath folds are steeply SSW-plunging closed structures whereas the cross folds are north-south-oriented with near-horizontal fold axes. In the area south of Messina this complexly folded terrain grades continuously towards the south into a crustal scale ENE-WSW-trending ductile shear zone with moderate dip towards the WSW. All sheath folds document consistent top-to-the-NE thrust movement of high-grade material. The timing of this shear deformational event (D2) and thus of the gneissic fabric (S2) is constrained (at ∼2·6 Ga) by the syntectonic emplacement throughout the CZ of precursors to quartzo-feldspathic gneisses (Singelele-type gneisses). Cross folds deform the S2 fabric and are characterized by a near-vertical axial planar cleavage (S3). Recent single-phase Pb-Pb dating of garnet from a metapelitic gneiss with an S3 fabric from one of the largest crossfolds in the CZ constrains the timing of the deformational (D3) and metamorphic (M3) event at ∼2·0 Ga Mineral chemistry for metapelites from this cross fold shows a single peak on an NMg histogram for garnet reflecting a single phase of mineral growth. Metapelites from this cross fold also preserve evidence for only one well-developed reaction texture, Grt + Sil + Qtz → Crd. This reaction is accompanied by the simultaneously operating reaction Grt + Fsp + H2O = Bt + Sil + Qtz. Both these divariant reactions belong to the univariant KFMASH equilibrium Crd + Grt + Fsp + H2O - Bt + Sil + Qtz. The progress of the two divariant retrograde reactions leads to the consumption of Grt and Fsp: K-fieldspar (Or94-100) never occurs with both cordierite and garnet. Microprobe profiling coupled with calculated isopleths for Bt, Grt and Crd in divariant equilibria define a decompression-cooling P-T path that reflects a single (M3) high-grade metamorphic event during the evolution of the cross fold. This decompression-cooling P-T path traverses from 780° C, 5·7 kbar to 600° C, 3·3 kbar. © Oxford University Press 2004; all rights reserved.