Abstract

Highly magnesian ultrapotassic rocks that erupted ca. 30–40 Ma on either side of the northern Ailao Shan–Red River fault (western Yunnan) have initial 87Sr/86Sr values of 0.7064–0.7094 and ϵNd values of −3.8 to −4.6. Such isotopic compositions are distinct from asthenospheric signatures and most likely probe the lower lithosphere. Sm-Nd model age and trace element data for the Yunnan lavas indicate that at the time of eruption the mantle source was an old (>1 Ga) mica-bearing spinel harzburgite. However, such Proterozoic enriched lithospheric mantle could not have survived the thermo-tectonic processes associated with continental flood volcanism that affected the same part of the western Yangtze craton 250 Ma. Emeishan flood basalt volcanism would have purged the shallow mantle of all its fusible constituents and any enrichments would be younger than 250 Ma. We propose that the lithosphere mantle beneath the western Yangtze craton is exotic and probably represents part of the Tibetan lithosphere extruded to the east 40– 50 Ma. The Indo-Asia collision provides a suitable mechanism and explains the link between the west Yangtze craton and northern Tibet in terms of provenance (Sr-Nd) and the similar two-dimensional seismic velocity structure.

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