Contact metamorphism of Fe- and Al-rich graphitic metapelites in the Transangarian region of the Yenisei Ridge, eastern Siberia, Russia

Prograde evolution or 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-X-H2O conditions of the contact metamorphism the temperatures increase toward the intrusive contact from 430 to 640 degreesC at P = 3.2 +/- 0.3 kbar, X-H2O for the metamorphic fluid decreases toward the intrusive contact from 0.89-0.85 at T = 450 degreesC to 0.49-0.36 at T = 640 degreesC, 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.]. (C) 2001 Elsevier Science B.V. All rights reserved.