PETROLOGY OF POTASSIUM-POOR METAPELITES OF THE VORONEZH CRYSTALLINE MASSIF WITH REFERENCE TO THE OLIVINE-GEDRITE-ORTHOPYROXENE-GARNET-MAGNETITE ASSEMBLAGE

Abstract

The central part of the Voronezh crystalline massif was determined to contain the following three petrochemical types of metapelites: (1) K-poor magnesian and aluminous cordierite–gedrite–anthophyllite schists, (2) K- and Al-rich muscovite–staurolite schists, and (3) K- and Al-poor Fe-rich grunerite–garnet rocks. A staurolite–sillimanite and a sillimanite–muscovite zone were mapped in the K-rich metapelites. The K-poor magnesian and aluminous metapelites are noted for the relations between the Fe mole fractions of their minerals: Crd22–34 < Opx39.5 < Ath40–43 < Ged46–49 < Grt65–79 . The assemblage of orthopyroxene with gedrite and garnet crystallized in the high-temperature part of the muscovite–sillimanite zone. The grunerite–garnet assemblage is stable in the metapelites of type 3 only within the Fe-rich region and does not overlap with other mineral equilibria. The rare mineral assemblage Ol + Ged + Opx + Grt + Magof the K-poor moderately aluminous quartz-free metapelites was formed during regional metamorphism (T= 639°C, P= 4–5 kbar) at a combination of several factors: a specific bulk-rock chemistry (K-poor, moderately aluminous, and relatively highly magnesian quartz-free lithologies), metamorphic parameters (630°C and 4–5 kbar), and redox conditions (an elevated oxygen fugacity, above the quartz–fayalite–magnetite buffer). The P–Ttrajectory of the prograde metamorphic stage was controlled by the heating of the rocks from 530°C to approximately 630°C at pressures of 4–5 kbar.