Abstract:
Prograde evolution of minerals in Fe- and Al-rich graphitic metapelites in the Ayakhtinsk aureole, Siberia, produced the unusual contact metamorphic mineral assemblages: chloritoid+biotite, chloritoid+biotite+andalusite and cordierite+garnet+muscovite. Field-petrologic observations show that: (1) the grade of contact metamorphism ranges from chloritoid to sillimanite–alkali feldspar zone; (2) chloritoid+biotite assemblages have a restricted temperature interval and give way up-grade to garnet+chlorite assemblages; and (3) garnet+chlorite assemblages have a wide temperature interval and give way to cordierite+biotite parageneses with increasing grade. Geothermobarometry and thermodynamic analysis of mineral equilibria give estimates of the P–T–XH2O conditions of the contact metamorphism: the temperatures increase toward the intrusive contact from 430 to 640 °C at P=3.2±0.3 kbar, XH2O for the metamorphic fluid decreases toward the intrusive contact from 0.89–0.85 at T=450 °C to 0.49–0.36 at T=640 °C, assuming ideal and non-ideal mixing, respectively, of H2O–CO2 in the fluid phase. The stability of the rare mineral assemblages chloritoid+biotite and chloritoid+biotite+andalusite within the contact aureole can be explained by the unusual combination of pressure (>3 kbar) and Fe- and Al-rich bulk-rock compositions. The local occurrence of the cordierite+garnet+muscovite mineral assemblage is controlled by Mn in the garnet composition and Fe- and Al-rich bulk-rock composition than P–T conditions. Our data are compatible with the KFMASH grid of Spear and Cheney [Contrib. Mineral. Petrol. 101 (1989) 149.].
