Isotopic characteristics of shoshonitic rocks in eastern Qinghai-Tibet Plateau: Petrogenesis and its tectonic implication

The Cenozoic magmatic rocks of shoshonitic series in the eastern Qinghai-Ti bet Plateau include potassic alkaline plutonic rocks, volcanic rocks, lampr ophyres and acidic porphyries. Analytical results show that these different lithological rocks are extremely similar in Sr, Nd and Pb isotopic composi tions with the range of 0.705 187-0.707 254 for Sr-87/Sr-86, 0.512 305-0.51 2 630 for Nd-143/Nd-144, 18.53-18.97 for Pb-206P/(204)pb, 15.51-15.72 for P b-207/Pb-204 and 38.38-39.24 for Pb-208/Pb-204. They are isotopically simil ar to the EMII end-member. This indicates that mantle metasomatism must hav e taken place in their source region. The formation of these particular roc ks is related to crustal thinning and mantle upwelling in a large-scale str ike-slip and pull-apart fault zone at about 40 Ma in northern and eastern Q inghai-Tibet Plateau.