ИСТИНА

During metamorphism and subsequent exhumation, Precambrian high-grade complexes become injected by various granitoid magmas that originated from deeper crustal levels (Morfin et al., 2013, Lithos, 168-169, 200-218). The effect of these magmas on the country rocks becomes apparent through transportation of entrapped granulites, formation of local zones of HT-UHT metamorphism and partial melting, assimilation of granulites, and production of large volumes of exotic fluids that interacted with granulites. We present petrological, fluid inclusion and thermobarometric studies exemplifying these complex effects from rocks associated with high-grade shear zones (Petronella, Matok) in the Southern Marginal Zone (SMZ) of the Limpopo Complex (South Africa). During exhumation onto the Kaapvaal craton during the time period 2.72 – 2.66 Ga, the SMZ has been invaded by trondhjemite-granodiorite melts (Belyanin et al., 2014, Prec. Res., 254, 169-193; Safonov et al., 2014, Prec. Res., 253, 114-145), which are, probably, closely associated with the large diorite-granodiorite-quartz syenite Matok pluton (Laurent et al., 2014, Lithos, 196-197, 131-149). These hot (T ~ 1000ºC) magmas entrapped and heated metapelites at P ~ 7.5-8.5 kbar (23-25 km depth) and dragged them to a depth of 18-20 km (6.3-6.5 kbar) (Safonov et al., 2014, Prec. Res., 253, 114-145). Heating of the country orthopyroxene-cordierite-biotite metagraywakes resulted in localized dehydration melting of the rocks at temperatures up to 950ºC. The peritectic melting reactions produced new generations of garnet, orthopyroxene and sillimanite in the rocks and finally resulted in formation of spectacular K-feldspar-rich garnet-orthopyroxene enclaves. In turn, the heterogeneous assimilation of the trondhjemitic melts by the metagraywake material is also reflected in several distinct garnet generations. Various mineral assemblages from the garnet-bearing trondhjemites, garnet-orthopyroxene enclaves and surrounding rocks allowed tracing the sub-isobaric cooling of the magma-rock system from T ~ 900ºC to ~600ºC at 6.3 - 6.5kbar. Fluid inclusions trapped in garnet and quartz in the trondhjemite show that the magma transported a CO2 fluid coexisting with aqueous-salt fluid bearing NaCl, KCl and CaCl2. Reduction of the CO2 fluid during the assimilation of the sulfide-rich metagraywake material locally produced graphite-bearing varieties of trondhjemites. Similar CO2 and aqueous-chloride fluids were detected in quartz of granodiorite and porphyritic quartz syenite of the Matok complex. The aqueous fluids with water activities 0.5-0.3, being exsolved from the granitoid magmas during sub-isobaric cooling and solidification, provoked formation of various assemblages after cordierite and orthopyroxene in the surrounding rocks. They include late garnet, Na2O-rich gedrite (locally coexisting with anthophyllite), biotite, sillimanite (kyanite), staurolite, sodic plagioclase. Our data show that granitoid melts played a critical role in the exhumation of the Limpopo granulites onto the adjacent granite-greenstone craton, transference of heat in the crust, and transportation of large volumes of external H2O-CO2-salt fluids that subsequently participated in the rehydration of a significant portion of the SMZ (van Reenen et al., 2014, Prec. Res., 253, 63-80).