Geologic History of the Central Zone of the Limpopo Complex: The West Alldays Area

Abstract

New field, structural, petrologic, and age data show that tectonic styles and metamorphic histories in the West Alldays area of the Central Zone of the Limpopo Complex, southern Africa, can be linked to neo-Archean and Paleoproterozoic high-grade (granulite facies) tectonometamorphic events. The styles comprise (1) a regionally developed high-grade (S2) gneissic fabric that evolved into a regional (D2) system of sheath folds mapped as circular to oval-shaped structures with lineations plunging steeply to the WSW and (2) a system of mainly N-S-trending and north-verging (D3) shear zones characterized by high-grade tectonites ("straight gneisses") with well-developed (S3) gneissic fabrics. In the West Alldays area, the superimposition of D3 onto the regional D2 fold pattern produced the kilometer-scale N-S-trending Baklykraal shear zone, which, before this study, was mapped as the Baklykraal fold and interpreted as part of the regional D2 fold pattern in the Central Zone. The Paleoproterozoic high-temperature D3 shear event is accurately constrained to 2023 ± 11 Ma by a Pb/Pb stepwise leaching garnet date that reflects the syntectonic crystallization of a garnet-cordierite-sillimanite-biotite-quartz paragenesis formed during shearing. This tectonite records a retrograde pressure-temperature (PT) path from 780°C at 5.7 kbar to 600°C at 3.3 kbar. The neo-Archean age of the M2 granulite facies metamorphism, coeval with the tight D2 folding preserved within the Baklykraal shear zone, is evident from several garnet Pb/Pb stepwise leaching experiments that gave dates intermediate between 2000 and 2600 Ma, with large scatter in the data arrays (thus interpreted as mixed ages), and from zircon ages of protoliths of the syn-D2 Singelele-type quartzofeldspathic gneisses. A polymetamorphic garnet-cordierite-orthopyroxene-biotite-quartz paragenesis from a D2 outcrop within the Baklykraal shear zone records two PT paths: a decompression-cooling PT path from ∼850°C at ∼8.5 kbar to ∼675°C at ∼6 kbar at ∼2600 Ma and an isobaric (6 kbar) heating event from ∼675°C to ∼770°C that was immediately followed by a decompression-cooling path that reflects the uplift of the high-grade rocks toward the Earth's surface (to the level of about 8-10 km). Polymetamorphic granulites that resulted from the isobaric heating event introduce new petrologic and geochronologic problems. On the basis of new data, we thus document previously unrecognized N-S-trending high-temperature shear zones and associated polymetamorphic granulites from the Central Zone of the Limpopo Complex. These shear zones developed in the Paleoproterozoic and were superimposed onto older (neo-Archean) regional fold structures. The tectonic history of the Central Zone of the Limpopo Complex is therefore characterized by two high-grade tectonometamorphic events separated by at least 550 m.yr. These data require that existing models for crust formation in the Central Zone, including those that argue for a single granulite facies event linked to an orogeny at ∼2000 Ma, be reconsidered.