On the Sr-Nd-Pb-Hf isotope code of enriched, Dupal-type sub-continental lithospheric mantle underneath south-western China

Cenozoic intra-plate volcanism in Southwest China allows insights into modes of melting and the composition of the sub-continental lithospheric mantle (SCLM). The late Cenozoic Tengchong Volcanic Field (TVF), located in the Yunnan Province Northeast of the Himalayas, comprises a cluster of mafic continental intraplate lavas, represented by 68 volcanic cones and spanning an area of ca. 800 km2. Major and trace element data and Sr-Nd-Pb-Hf isotope ratios of 18 newly sampled rocks from the area are used to identify the composition of their source in the SCLM. Indices of magmatic differentiation are correlated with isotope composition, marking a classic case of assimilation-fractional-crystallization (AFC) during petrogenesis. The most primitive samples, identified on the basis of element and isotope compositions, are interpreted to be directly derived from the metasomatised mantle underneath south-western China. Modelling of a best-fit scenario indicates that the isotope and trace element character of these samples can be reproduced by ca. 1% partial melting of a metasomatised mantle source, composed of depleted MORB mantle infused with 1% addition of a bulk global subducting sediment (GLOSS) component. Average isotope compositions of 87Sr/86Sr = 0.705861, 143Nd/144Nd = 0.512675, 176Hf/177Hf = 0.282962, 206Pb/204Pb = 18.305, 207Pb/204Pb = 15.642 and 208Pb/204Pb = 38.948 are proposed for the present-day enriched part of the SCLM. The origin of the isotopic signatures is ascribed to modifications of parent-daughter ratios in the SCLM by past subduction-related metasomatism, possibly in the form of sediment mélange diapirs. This implied that the mantle source of TVF lavas is not, as previously proposed, related to enriched geochemical reservoirs (EM-1, EM-2) associated with mantle plume activity or to so-called Dupal or Indian asthenospheric mantle. Instead, it is argued that time-integrated parent-daughter fractionation occurred long before India-Asia collision and this produced reservoirs with isotopic compositions that resemble some but not all aspects of the Dupal isotope signature. In analogy, so-called Dupal isotope signatures elsewhere may also be the result of contamination of the mantle by a crustal component, most likely subducted sediment.