Home – Home – Jornals – Russian Geology and Geophysics 2003 number 4

A detailed petrological study has been given to high-alumina ferriferous hornfelses characterized by the development of mineral associations (Cld+Bt, Cld+Bt+And, and Crd+Grt+Ms), quite rare for contact metamorphism. The sequence and stability of the observed parageneses were thoroughly analyzed in terms of the Harte-Hudson, Spear-Cheney, and Powell-Holland petrogenetic grids. It has been shown that in the contact aureole of the Ayakhta Massif: (1) the formation of chloritoid atypical of thermal metamorphism and the stability of rare parageneses (Cld+Bt and Cld+And+Bt) are explained by a rare combination of appropriate pressure (3 kbar) with a specific type of rocks enriched in both Al and Fe; (2) the appearance of the Grt+Crd+Ms paragenesis is caused by specific chemical compositions of Mn-enriched garnet and high-Al high-Fe rocks; (3) the development of Crd-And parageneses and staurolite-free associations at the intermediate steps of contact metamorphism is explained by the simultaneous expansion of the Grt+Chl field with participation of Mn-garnets and narrowing, up to complete disappearance, of the St+Bt stability field; (4) the sequence of the observed parageneses and reactions in high-Al Fe-rich hornfelses of the Ayakhta aureole is in agreement with the Spear-Cheney thermodynamic petrogenetic grid.