Thermal History and Fluid Regime during the Formation of the Eldjurta Biotite Granite Massif (Greater Caucasus): Reconstructions Based on Isotope (δ18O, δD) and Geochemical Data

Abstract

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 δ¹⁸O 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 δ¹⁸O 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 Tc values in the lower part of the massif indicate the re-opening of the oxygen isotope system of quartz, with subsequent long-term isotope re-equilibration between minerals. This leads to decrease of the observed Tc 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 δ¹⁸O values in the minerals of the Eldjurta biotite granites can be described in terms of a simple retrograde exchange at the cooling stage.