Highly Fractionated Granites of the Raumid Massif (S. Pamir): Oxygen Isotope and Geochemical Study

The processes of crystallization differentiation, retrograde isotopic exchange, and autometamorphism are considered with reference to the Eocene granites of the Raumid massif, which consists of eight intrusive phases and serves as an example of a “natural laboratory”. The work is based on oxygen isotope, petrographic, and geochemical study of representative samples from each intrusive phase of the massif. The isotopic and geochemical studies were carried out for all rock-forming minerals (Qz, Pl, Kfs, Bt) and their altered varieties. Based on geochemical features, the Raumid granites correspond both to A-type granites and to highly fractionated I-type granites. Our results show that the rocks of the Raumid massif are not the geochemical analog of the Eocene granitoids from the Qiangtang terrane of the Central Tibet or the Vanj complex, as it previously assumed (Chapman et al., 2018). We estimated that differentiation of felsic melts of the Raumid pluton occurred at T = 750–800°C, and P = 4.5–7.8 kbar and was mainly controlled by Pl crystallization. The melts were intruded into the hypabyssal zone in at least two stages: early (γ1–γ3) and late (γ4–γ8), although it is possible that the rocks of the γ7 and γ8 phases formed an additional separate stage. The closure temperature of the oxygen isotopic system of quartz (T_Qz) ranges from 420 to 610°C. The effect of the multiple intrusions of the melts on the T_Qz and apparent cooling rates is considered. The study of altered and unaltered minerals showed that autometamorphism partially overlapped with the retrograde oxygen isotope exchange in the cooling rock. The modelled δ¹⁸О values during Pl and Kfs alteration describes well the observed isotope data when the crystallization takes place at limited content of water fluid (W/M = 0.3–0.05) which could release during the Raumid’s magmas crystallization.