High Fractionated Granites of the Raumid Massif (S. Pamir): O-Isotope and Geochemical Study

Abstract

The processes of crystallization differentiation, retrograde isotopic exchange, and autometamorphism are considered using the “natural laboratory” – Eocene granites of the Raumid massif, which has eight intrusion phases. The work is based on a comprehensive (oxygen isotope, petrographic, geochemical) study of representative samples of each of the intrusion phases of the massif. Isotopic and geochemical studies of rock-forming minerals (Qz, Pl, Kfs, Bt), as well as their altered varieties, have been carried out. The geochemical features of the Raumid’s granites are correspond to both A-type granites and highly fractionated I-type granites. We show that the rocks of the Raumid massif are not the geochemical analog of the Qiangtang Eocene granitoids of the Central Tibet and the Vanj complex, as previously assumed (Chapman et al., 2018). The differentiation conditions for acidic melts that formed the Raumid pluton (T = 750–800°C, P = 4.5–7.8 kbar with predominant Pl crystallization) were estimated. The intrusion of melts into the hypabyssal zone proceeded at least in two stages: early (γ1–γ3) and late (γ4–γ8), although it is possible that the rocks of the γ7 and γ8 phases belonged to a separate stage. The closing temperature of the oxygen isotopic system of quartz (Тq) varies from 420 to 610°C, which indicates a wide range of cooling rates for these rocks. The effect of the multiphase nature of melt intrusion on the Тq and apparent cooling rates is considered. The study of altered and unaltered minerals makes it possible to establish that autometamorphism partially overlapped with the retrograde oxygen isotope exchange in the cooling rock. The modelling of the δ18О values of feldspars during autometamorphism in a system closed with respect to fluid show that the Pl saussuritization and Kfs kaolinitization proceeded at a limited fluid/mineral ratio (W/M = 0.3–0.05), which could be provided by aqueous fluid separated during the Raumid pluton cooling.