Reduced post-collisional magmas for Tengchong volcanoes, southeastern Tibetan Plateau

Abstract

Rapidly cooled and well-preserved magnetite and ilmenite in young volcanic rocks provide good samples for constraining the magmatic redox conditions. We report chemical compositions of magnetite and ilmenite from the youngest Tengchong volcanoes at the southeastern margin of the Tibetan Plateau, to constrain the oxygen fugacity and temperatures for the Tengchong magmas. Oxygen fugacity values (△FMQ) relative to the fayalite-magnetite-quartz (FMQ) buffer are −1.8 ∼ −0.7 (averaging −1.1 ± 0.4) for Dayingshan; −1.3 ∼ −0.7（averaging −1.1 ± 0.3） for Maanshan; and −1.3 for Heikongshan. The temperatures for magnetite-ilmenite pairs of Tengchong lavas range from 789 to 1090 °C. The relatively reducing oxygen fugacity values (△FMQ = −1.8 ∼ −0.7) of the Tengchong magmas are significantly lower than those oxidizing arc magmas (△FMQ = +0.5 ∼ +3.5). The reduced magmas at Tengchong may exemplify other Quaternary post-collisional lavas of the Tibetan Plateau. The crystallization temperatures from mineral-liquid equilibrium are 1160–1192 °C for olivine, 1049–1110 °C for plagioclase, 978–1122 °C for clinopyroxene, 789–1090 °C for Fe-Ti oxides, 743–792 °C for zircon. The high Fe-Ti oxide temperature and Ti-in-zircon temperature indicate warm magmatic conditions (>750 °C) prior to eruptions of Tengchong magmas.