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

Van Reenen , D.D., Perchuk, L.L., Smit, C.A., Varlamov, D.A., Boshoff, R., Huizenga, J.M., and Gerya, T.V. (2004) Structural and P-T evolution of a major cross fold in the central zone of the Limpopo high-grade terrain, South Africa. Journal of Petrology, 45 (7). pp. 1413-1439.

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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 N(Mg) 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.

Item ID: 30677
Item Type: Article (Research - C1)
ISSN: 1460-2415
Keywords: deformation, granulites, Limpopo Complex, geothermobarometry, mineral equilibrium, mineral thermodynamics, P–T path
Funders: National Research Foundation South Africa (NRF), Rand Afrikaans University (RAU), Russian Foundation for Basic Research (RFBR), Russian State Leading Scientific Schools Program (RSLSSP)
Projects and Grants: RFBR grant 02-05-64025, RFBR grant 03-05-64633, RSLSSP project 1645.2003.5
Date Deposited: 30 Sep 2016 03:49
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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