Том 32, № 5 (2024)

Based on the geochemical and isotopic (δ¹⁸О, δD) data the thermal and fluid conditions during the formation of the Eldjurta granite massif were reconstructed. Analysis of rocks collected from the core of the Tyrnyauz Superdeep Well (TSW) within the depth range of 1427–3923 m revealed their homogeneous isotopic parameters: the δ₁₈О values of bulk samples, quartz, feldspars, and biotite in 12 samples of biotite granites are 8.50 ± 0.33, 9.55 ± 0.22, 8.40 ± 0.33 and 5.45 ± 0.40‰, respectively. The δD values in the biotite vary from −103.3 to −95.6‰. The closure temperatures of the oxygen isotope system of quartz are 440–980°C. The rock cooling history was reconstructed using a new approach based on the analysis of single quartz grains. This approach can be used for detailed reconstructions of thermal history during formation of intrusive bodies. The definite samples were used to demonstrate that Dodson’s equation is valid for description of the δ₁₈О values of quartz in a granite system. The data obtained suggest that the studied part of the massif was formed in at least two almost simultaneous stages. The lower part of the massif was crystallized first, and the second injection of granite melt arrived immediately after the first portion has been crystallized, but had no yet had time to cool significantly. The Т^с values in the lower part of the massif indicate the reopening of the oxygen isotope system of quartz, with subsequent long-term isotope re-equilibration between minerals. This leads to decrease of the observed Т^с values and the calculated cooling rates, which is related to increasing volume of the intrusive body and cooling within already heated rocks. Estimates of the isotopic parameters of the water component indicate the absence of exotic water fluid (meteoric or buried waters) during cooling of the massif. The variations of the δ¹⁸О values in the minerals of the Eldjurta biotite granites can be described only in terms of a simple retrograde exchange at the cooling stage

The P–T paths of exhumation of Precambrian granulite complexes at the craton boundaries usually include two stages: sub-isothermal decompression and a decompression–cooling stage with a more gentle P–T path. Our goal is to understand the possible causes of the change in the slope of the P–T path of exhumation 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) from various structural positions within the Central Zone, i.e. dome structures, regional crossfolds, local and regional shear-zones, were studied. Metapelites are gneisses of similar bulk composition. Relics of leucosomes composed of quartz-feldspar aggregates with garnet and biotite are variously manifested in rocks, and melanocratic areas enriched in cordierite usually mark micro-shear-zones that envelope and/or break garnet porphyroblasts. Study of polymineral (crystallized melt and fluid) inclusions in garnet, its zoning with respect to the major (Mg, Fe, Ca) and some trace (P, Cr, Sc) elements, fluid inclusions in quartz, as well as phase equilibria modeling (PERPLE_X) showed that rocks coexisted with granite melts and aqueous-carbonic-salt fluids (a_H2O = 0.74–0.58) at the peak of metamorphism at 800–850°C and 10–11 kbar. Partial melting initiated sub-isothermal exhumation of rocks 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 garnet grains in terms of the grossular content. A change in the rheology of rocks as a result of partial removal and crystallization of the melt activated shear-zones during further exhumation to 6–5.5 kbar along the P–T decompression–cooling path of 95–100°/kbar, reflecting a slower uplift of rocks in the middle crust. This process was resumed due to thermal effects and interaction of rocks with aqueous fluids (a_H2O > 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, are the result of the evolution of an ultra-hot orogen, where vertical tectonic movements associated with diapirism were conjugate with horizontal tectonic processes caused by the convergence of continental blocks.