Magma mixing recorded by Sr isotopes of plagioclase from dacites of the Quaternary Tengchong volcanic field, SE Tibetan Plateau

摘要

The Tengchong volcanic field east of the Burma arc in SW China comprises numerous Quaternary volcanoes. The volcanic rocks can be grouped into four units, numbered 1-4 from oldest to youngest. Units 1,3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, whereas Unit 2 consists of hornblende dacite. Primary minerals in the dacite include plagioclase, clinopyroxene, amphibole and magnetite, all set in a glassy groundmass. Some of the dacites are moderately to highly weathered and show effects of low temperature alteration, with loss on ignition (LOI) up to 9.6 wt% and positive correlations of LOI with Al_2O_3, but negative correlations with SiO_2, CaO and Na_2O. Fresh dacites (LOI <3 wt%) belong to the calk-alkaline series and have pronounced negative Ti, Nb and Ta anomalies. They have whole-rock ~(87)Sr/~(86)Sr ratios ranging from 0.7090 to 0.7101, eNd values from -10.8 to -11.8, and εHf values from -5.1 to -6.8. Many of the large plagioclase crystals in the fresh dacites have reverse zoning, ranging from An_(55) in the cores to An_(72) in the rims. The smaller phenocrysts have relatively uniform An values between 56 and 50. One plagioclase crystal has an An value of 33.5. Plagioclase in the fresh dacite has relatively high Sr contents and ~(87)Sr/~(86)Sr ratios ranging from 0.7050 to 07125 except for one value of 0.7516, whereas plagioclase in the altered dacite has low Sr contents and highly variable Sr isotopic compositions (0.7039-0.7138), indistinguishable from those of the fresh rocks. Both An values and Sr isotopic compositions of the plagioclase indicate mixing of mantle- and crustal-ly-derived magmas. We therefore propose that mantle-derived basaltic magma caused partial melting of the lower-middle crust and that mixing of the mafic and felsic magmas produced the dacite in staging magma chambers. The dacites contain minerals crystallized from both the mafic and felsic magmas, as well as a few xenocrysts plucked from the country rocks.