MAGMA MIXING AND PETROGENESIS OF THE 13 NOVEMBER 1985 ERUPTIVE PRODUCTS AT NEVADO-DEL-RUIZ (COLOMBIA)

Magma erupted on November 13, 1985, at Nevado del Ruiz (Colombia) was compositionally diverse, with bulk compositions ranging from andesite (59% SiO2 and 6% MgO) to dacite (65% SiO2 and 3% MgO). The textural heterogeneity of juvenile products is emphasized by clasts of black scoria, gray scoria, black scoria with white pumiceous inclusions, banded scoria, banded pumices and white pumices. Numerous phenocrysts are out of equilibrium in the andesitic component, including magnesian olivine, sodic plagioclase and hypersthene. Plagioclase compositions are bimodal. Such features and textural heterogeneities are interpreted to result from transfers of phenocrysts by a mechanical mixing process, mainly from the felsic component to the mafic one. Three mineralogical associations have been observed: • - dacitic association: andesine (An 35-50), hypersthene (En 68-70), edenite, augite, phlogopite, magnetite and ilmenite. • - andesitic association: labradorite (An 55-65), bronzite (En 75-78), pargasite, augite, phlogopite, magnetite and ilmenite. • - mafic association: magnesian olivine (Fo 79-87) and bronzite (En 78-83). Basic magma was not erupted, but is thought to be responsible for triggering the eruption. It could have been injected into a shallow dacitic reservoir (PH about 1 kbar), destabilized it, and mechanical mixing could have started, in the magma chamber or through a conduit, just before eruption. The basic magma evolved completely to andesite, as suggested by the experiments of Kouchi and Sunagawa in 1983, while some original dacitic magma was preserved. As complete homogenization was not attained, the resulting products are texturally heterogeneous. Such a genesis explains the mineralogical disequilibria. © 1990.