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The P–T paths of the exhumation of Precambrian granulite complexes at craton boundaries usually include two stages: subisothermal decompression and a decompression–cooling stage with a more gently sloped P–T path. Our goal is to understand the possible causes of the change in the slope of the P–T exhumation path of the Central Zone (CZ) of the Limpopo granulite complex, South Africa, located between the Kaapvaal and Zimbabwe cratons. For this purpose, rocks (mainly, metapelites) were studied in various structural settings within the Central Zone, i.e., in dome structures, regional cross folds, and in local and regional shear zones. The metapelites are gneisses of similar bulk composition. The rocks contain various amounts of relics of leucosomes composed of quartz–feldspar aggregates with garnet and biotite, and melanocratic domains that are enriched in cordierite and usually mark shear microzones that envelope and/or break garnet porphyroblasts. Study of polymineralic (crystallized melt and fluid) inclusions in the garnet, its zoning with respect to the major (Mg, Fe, and Ca) and some trace (P, Cr, and Sc) elements, fluid inclusions in quartz, as well as phase equilibria modeling (PERPLE_X) showed that the rocks coexisted with granite melts and saline aqueous carbonic fluids ( = 0.74–0.58) at the peak of metamorphism at 800–850°C and 10–11 kbar. Partial melting of the rocks initiated their subisothermal exhumation to 7.5–8 kbar during diapirism of granitic magmas in the Neoarchean (2.65–2.62 Ga). This is reflected in the specific zoning of the garnet grains in terms of the grossular content. A change in the rheology of the rocks as a result of partial removal and crystallization of melt activated the shear zones during further exhumation to 6–5.5 kbar along a decompression–cooling P–T path at 95–100°/kbar, reflecting the slower uplift of the rocks in the middle crust. This process was resumed due to thermal effects and interaction of the rocks with aqueous fluids ( > 0.85) in the Paleoproterozoic (~2.01 Ga). Such a scenario of metamorphic evolution implies that the Limpopo granulite complex in general and its Central Zone in particular resulted from the evolution of an ultrahot orogen, in which vertical tectonic movements associated with diapirism were coupled to horizontal tectonic processes caused by the convergence of continental blocks. [ABSTRACT FROM AUTHOR]

Data on the carbon and oxygen isotopic composition of carbonates from the rocks of the Giyani greenstone belt, Kaapvaal craton, South Africa are presented. This belt is immediately adjacent to the Hout River Shear Zone, along which the Southern Marginal Zone (SMZ) of the Limpopo granulite complex is overthrusted onto the Kaapvaal craton. The δ13C values are –2.1 and –2.2‰ for carbonates from two metapelite samples, –5.1‰ for a meta ultrabasite sample, and –7.1 and –7.7‰ for two metabasite samples, respectively. The δ18О values are 17.1 and 17.5‰ for carbonates from the metapelites, 14.3‰ for carbonates from the metaultrabasite, and 12.9 and 13.0‰ for carbonates from the metabasites. The comparison of the data obtained to the published data on the isotopic composition of carbonates from the rocks of the different greenstone belts indicates both hydrothermal (in the metabasites and the metaultrabasite) and sedimentary (in the metapelites) origins of the carbonate material in the rocks studied. Based on the comparison of these data to δ13C of magnesite that formed in ultrabasic granulites during the interaction of the Limpopo complex with the craton, the graphite and fluid inclusions in leucocratic garnet-bearing granitoids that transported fluids, and the carbon isotopic composition of graphite from metapelites of the SMZ, it is concluded that the carbon source was located outside the SMZ and represented a heterogeneous carbonate-bearing lithologies. It could be carbonate-bearing volcanogenic–sedimentary sequences of the greenstone belts of the Kaapvaal craton, which underwent prograde metamorphism during the interaction with the SMZ granulites. [ABSTRACT FROM AUTHOR]

A clear case study of local-scale, fluid-induced dehydration of the Paleoarchean Sand River biotite–hornblende gneiss from the Central Zone of the Limpopo Complex is presented here. Field and petrographic examination of three adjacent zones—darker Sand River orthogneiss with local occurrence of orthopyroxene and clinopyroxene, a lighter intermediate gneissic zone with more orthopyroxene than the Sand River orthogneiss, and tonalitic veins containing large orthopyroxene-bearing patches—indicates the local transformation of a light grey, fine- to medium-grained, hornblende–biotite gneiss into a greenish brown, medium- to coarse-grained orthopyroxene-bearing dehydration zone. Field evidence indicates that the tonalitic veins were emplaced in discrete ductile shear zones, with development of large orthopyroxene-bearing patches in a sigmoidally transposed foliation bounded by shear planes. Orthopyroxene-forming reaction textures after biotite and amphibole together with the occurrence of microveins of K-feldspar along quartz–plagioclase grain boundaries in the three adjacent zones, and the higher modal abundance of orthopyroxene and K-feldspar with lesser biotite and amphibole from the Sand River orthogneiss to the intermediate gneissic zone to the orthopyroxene-bearing patches, indicate that the three adjacent zones represent progressive stages of the dehydration process. Such K-feldspar microveins along quartz–plagioclase grain boundaries have been proposed as evidence for the presence and passage of a low H2O activity fluid. Further, the occurrence of monazite inclusions in fluorapatite in orthopyroxene-bearing zones suggests dissolution and reprecipitation involving a free fluid phase. Fluid inclusion studies indicate the presence of a fluid with CO2, NaCl and H2O components, with higher salinity of the fluid (up to 29% NaCl) in the orthopyroxene-bearing patches relative to the intermediate gneissic zone. The increase in Cl content in amphibole, biotite and fluorapatite from the Sand River orthogneiss to the orthopyroxene-bearing patches supports the presence of a Cl-rich brine fraction in the fluid responsible for the dehydration process. Further, the increase in An content of plagioclase at the contact with the K-feldspar rims on quartz reflects an increase in potassium activity in the fluid. The whole-rock major, trace and rare-earth element enrichment or depletion patterns of the orthopyroxene-bearing zones relative to the precursor support the dehydration process. The diffuse contact relationship of a granite pegmatite occurring in the vicinity of the dehydration zones, together with fluid inclusion and whole-rock major, trace and rare element characteristics of samples collected along a traverse from the granite pegmatite to the Sand River orthogneiss, suggests a scenario in which the dehydrating fluids derived from an external source utilized lithological contrasts, such as the gneiss–pegmatite boundaries, as fluid conduits. Dehydration of the gneissic wall-rock occurred where permeability was sufficient for fluid penetration. The occurrence of orthopyroxene-bearing tonalitic veins along deformation-transposed foliation planes further attests to a structural control to the channeling of the dehydrating fluids. [ABSTRACT FROM PUBLISHER]