Monticellite–Spurrite Symplectites: Evidence for a Regressive Stage of the Kochumdek Trap Contact Aureole (Krasnoyarsk Region).

We study merwinite and products of its retrograde exsolution (monticellite and spurrite) from marbles of a contact metamorphic aureole in the Kochumdek River area (East Siberia). Cooling to 820–880°C (at P ≈ 0.2 kbar and ) has led to pseudomorphic replacement of merwinite by monticellite-spurrite symplectites. Calculations show that most of the components (Ca, Mg, Mn, Fe, and C) are mobile in the reaction of merwinite exsolution while Si is inert. The inert behavior of silicon controls the process of replacement and restricts it to a few local sites. The formation of the monticellite-spurrite symplectites was maintained by the own resources of metacarbonates. Later, limited amounts of the monticellite-spurrite symplectites formed due to fluorine inputs, possibly, from the cooling intrusion. [ABSTRACT FROM AUTHOR]