Petrogenesis and magma residence time of lavas from Tengchong volcanic field (China): Evidence from U series disequilibria and 40Ar/39Ar dating

We present U‐Th disequilibria measurements as well as radiogenic and trace element data for volcanic rocks erupted over the past ∼200,000 years at Tengchong volcano field (TVF), near the collisional belt of India‐Asia, southwestern China. Mineral separates, matrix, and whole rock were analyzed separately by mass spectrometry for 230Th‐238U disequilibria. Whole rocks are characterized by either 230Th enrichment or depletion relative to 238U. Three mineral internal 230Th age isochrons and a CO2 laser 40Ar/39Ar age were determined. A difference of ∼40 ka between the 230Th age and the 40Ar/39Ar age can be interpreted as the residence time of the associated magma. Large variations in Sr, Nd, and Pb isotopic compositions and trace element ratios suggest more than one source for the TVF lavas. High Th contents, Th/U ratios and LILE element abundances, as well as their large variations can be attributed to partial melting of mantle previously metasomatized by subduction‐derived components. Excesses of 238U in some lavas could be the feature of another end‐member, an arc‐type melt, in the source. Fast extraction of the melts may result in the observed chemical disequilibrium.

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