Journal of Petrology 45(7) © Oxford University Press 2004; all rights reserved
Structural and P–T Evolution of a Major Cross Fold in the Central Zone of the Limpopo High-Grade Terrain, South Africa
1 DEPARTMENT OF GEOLOGY, RAND AFRIKAANS UNIVERSITY, P.O. BOX 524, AUCKLAND PARK, 2006, JOHANNESBURG, SOUTH AFRICA
2 DEPARTMENT OF PETROLOGY, GEOLOGICAL FACULTY, MOSCOW STATE UNIVERSITY, VOROBIEVY GORY, MOSCOW, 119899 RUSSIA
3 INSTITUTE OF EXPERIMENTAL MINERALOGY, RUSSIAN ACADEMY OF SCIENCES, CHERNOGOLOVKA, MOSCOW DISTRICT, 142432, RUSSIA
4 INSTITUT FÜR GEOLOGIE, MINERALOGIE UND GEOPHYSIK, FAKULTÄT FÜR GEOWISSENSCHAFTEN, RUHR-UNIVERSITÄT-BOCHUM, 44780 BOCHUM, GERMANY
* Corresponding author. Fax: +27-11-4892309. E-mail: ddvr{at}na.rau.ac.za
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 cross folds 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-feldspar (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.
KEY WORDS: deformation; granulites; Limpopo Complex; geothermobarometry; mineral equilibrium; mineral thermodynamics; P–T path
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