Pressure-Temperature-Time Paths of Granulite Metamorphism and Uplift, Zambesi Belt, N.E. Zimbabwe
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Orogeny
Greenstone belt
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<p>In a geodynamic, geological and geophysical review of global Archean cratons, we find that the survival of Archean cratons depends on the initial conditions of their formation, as well as the tectonic processes to which they were exposed.&#160; In a sense, we must consider both their nature and how they were nurtured.&#160; In a review of existing literature and models, we use stability regime diagrams to understand the factors that contribute to the intrinsic strength of a craton: buoyancy, viscosity, and relative integrated yield strength. We find that cratons formed in the Archean when thermal conditions enhanced extraction of large melt fractions and early cratonization promoted the formation of stable Archean cratonic lithosphere.&#160; In terms of the cratons' nurturing, processes that may have modified and weaken cratonic lithosphere include subduction and slab rollback, rifting, and mantle plumes, as these processes introduced materials and conditions that warmed and metasomatized the lithosphere.&#160; Examining four Archean cratons that are more stable, and four that are categorized as modified or destroyed, we note that continental lithosphere that was cratonized prior to the end of the Archean has more potential to survive deformation during the last 500 My. Although, the survivability of these cratons is highly dependent on their unique positions relative to larger scale tectonic processes, such as subduction.&#160;&#160; We also observe that once an Archean craton begins to undergo even a small amount of modification, it is more likely to continue to be modified, as it loses the preservation advantage that it had upon birth.<br><br></p>
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Since 1969, a major geochronological investigation has been carried out by the South Australian Department of Mines and Energy on rocks of the Gawler Craton. Isotopic dates define three broad groupings corresponding to the Sleafordian Orogeny 2500–2300 Ma, the Kimba Orogeny 1820–1580 Ma, and the Wartakan Event 1580–1400 Ma. The oldest rocks in the craton belong to the Mulgathing Complex (in the north) and the Sleaford Complex (in the south). They consist of strongly folded and metamorphosed paragneisses (probably sediments and volcanics) intruded by several granites during the Sleafordian Orogeny. The Sleafordian Orogeny was followed by a 500 Ma period of magmatic and tectonic quiescence prior to the Kimban Orogeny. During that time sediments of the Hutchison Group were deposited. The Kimban Orogeny is characterized by complex deformation, high‐grade metamorphism and multiple synorogenic granite intrusion. It concluded at ca. 1580 Ma, but was followed by postorogenic acidic magmatism, sedimentation and minor deformation (the Wartakan Event). The spectrum of isotopic dates ends at ca. 1400 Ma, the approximate time of cratonization of the Gawler Craton.
Orogeny
Geochronology
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The original connections of Archean cratons are becoming traceable due to an increasing amount of paleomagnetic data and refined magmatic barcodes. The Uauá block of the northern São Francisco craton may represent a fragment of a major Archean craton. Here, we report new paleomagnetic data from the 2.62 Ga Uauá tholeiitic mafic dyke swarm of the Uauá block in the northern São Francisco craton, Eastern Brazil. Our paleomagnetic results confirm the earlier results for these units, but our interpretation differs. We suggest that the obtained characteristic remanent magnetization for the 2.62 Ga swarm is of primary origin, supported by a provisionally-positive baked contact test. The corresponding paleomagnetic pole (25.2°N, 330.5°E, A95 = 8.1°, N = 20) takes the present northern part of the São Francisco craton to moderate latitudes. Based on the comparison of the paleolatitudes of cratons with high-quality paleomagnetic data and magmatic barcodes, we suggest that the northern part of the São Francisco craton could have been part of the proposed Supervaalbara supercraton during the Archean. Supervaalbara is proposed as including (but not limited to) the part of the São Francisco craton as well as the Superior, Wyoming, Kola + Karelia, Zimbabwe, Kaapvaal, Tanzania, Yilgarn, and Pilbara cratons.
Yilgarn Craton
Greenstone belt
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Radiogenic nuclide
Indian Shield
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Supercontinent
Dharwar Craton
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Southern Superior Province of the Canadian Shield contains cratonic and basinal elements arranged in high- to low-grade metamorphic terranes such that higher grade gneissic cratons are interpreted to represent primary infrastructure to lower grade volcanic-rich (greenstone) basins. Ensimatic accumulation of volcanic components is favoured with derivation of gneissic (granitic) components by ensuing metamorphic differentiation and granitization processes. Such vertically reconstructed basin–craton complexes which are tentatively ascribed to initial Archean mantle convection, are viewed as building units of growing Precambrian shields.Globally, twenty-seven identified Archean cratons belong to three main age groups based on maximum recorded ages as follows: 3.5–3.8 Ga, 2.9–3.1 Ga, and 2.6–2.7 Ga. The three age groups that correspond to major periods of Archean orogeny may represent accretion superevents (after Moorbath).Most cratons as presently exposed display lithologic characteristics of lower superstructure–upper infrastructure of typical basin–craton complexes thereby suggesting a common degree of crustal buoyancy, hence level of erosion. Archean belts of southern Superior Province provide unique opportunity for reconstruction of the typical basin–craton complex.
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Basement
Supercontinent
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The high‐grade crystalline nappes exposed southeast of the São Francisco craton comprise two distinct units of mainly granulite facies rocks that represent a composite section of Neoproterozoic deep continental crust: the Socorro‐Guaxupé nappe above, derived from an arc terrane, and the Três Pontas‐Varginha nappe below. Metamorphism in the Três Pontas‐Varginha nappe is characterized by the exceptional preservation of kyanite granulites (700–750°C, 15 kbar), and followed by limited retrogression. Maximum temperatures around 900–950°C were reached toward the base of the overlying Socorro‐Guaxupé nappe during the intrusion of charnockitic‐mangeritic magmas. Lower‐pressure metamorphism, accompanied by anatexis, prevailed at shallower crustal levels. Our petrological results document an inverted thermal structure with isobaric heating of the top of the high‐pressure granulite nappe. Both granulite nappes were transported more than 200 km eastward above lower nappes involving reworked basement and passive margin units, both metamorphosed to high‐pressure but lower‐temperature conditions. Significant thinning and cooling of the two granulite nappes may have occurred before their emplacement onto the lower nappes. The proposed geodynamic scenario considers that continental subduction took place westward underneath Neoproterozoic oceanic lithosphere. The two granulite units crystallised at ∼ 45 km depths under distinct paleogeotherms within this subduction zone around 630 Ma. The kyanite granulites were rapidely exhumed through the mechanism of low‐angle “forced” extrusion, whereas syncollisional collapse affected the soft, anatectic middle crust of the overlying arc terrane. The final emplacement of the thinned nappe pile onto the cold São Francisco craton and its platform cover, with at most, anchizonal to greenschist‐facies metamorphism, occurred around 600 Ma.
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