High-Grade Contact Metamorphism in the Kochumdek River Valley (Podkamennaya Tunguska Basin, East Siberia): Evidence for Magma Flow.

Spurrite-merwinite marbles on the right bank of the Kochumdek River in the Podkamennaya Tunguska basin formed along the top margin of a flood basalt intrusion (Kuzmovka complex) from a marly limestone protolith of the Rhuddanian Lower Kochumdek Subformation, at a pressure of ~200 bars. The contact metamorphic aureole comprises four zones of successively decreasing temperatures marked by the respective mineral assemblages: T ≥ 900 °C (merwinite, spurrite and gehlenite (±rankinite, bredigite); T ≥ 750 °C (spurrite); T ≥ 700 °C (tilleyite, wollastonite, and melilite (Gehl_<50)); and ~500-550 °C (diopside, amphibole, and grossular). Very high temperatures at the contact (T_cont > 2/3 T_melt) result from magma flow along a conduit. The temperature profiles for the Kochumdek metamorphic complex show good fit between measured and geothermometer-derived values at a magma temperature of 1200 °C, an intrusion thickness of ≥ 40 m, a heating time of six months, and a magma flow lifespan within one month. Stagnant magma in a conduit of any thickness cools down and crystallizes rapidly and fails to heat up sediments to the temperatures required for spurrite--merwinite metamorphism (above 790 °C). [ABSTRACT FROM AUTHOR]