Nd-Sr-Pb isotopes of Tengchong Cenozoic volcanic rocks from western Yunnan, China: evidence for an enriched-mantle source

Tengchong Cenozoic volcanic activity in the southeastern Himalayan belt of western Yunnan is considered to be rift-related after the Indo-Eurasia collision. In this study, Nd-Sr-Pb isotopic compositions were analyzed in order to understand the genesis of the Cenozoic volcanic rocks. Five basalts and andesitic basalts are characterized by high Sr-87/(86) Sr ratios (0.7057-0.7081), low epsilon(Nd) values (-1.1 to -5.7), and particularly high Pb-208*/Pb-206* ratios (1.08-1.12). Twenty samples of rocks, which include granite, granodiorite, and amphibolite from the same region, were analyzed to evaluate the role of contamination and assimilation in the volcanic rocks. Granites have high Sr-87/Sr-86 ratios and low epsilon(Nd) values, indicating a crustal origin. Granodiorites have Sr-87/Sr-86 ratios of 0.7069-0.7100 and epsilon(Nd)-values of -2.9 to -7.6 that indicate contribution of a mantle component. Amphibolites that are characterized by high epsilon(Nd)-values of 9.4 to -1.7 may represent remnants of obducted oceanic crust of the Neo-Tethyan basin. The volcanic rocks are distinguished from the granitoids and amphibolites in terms of Pb isotopic compositions. This observation probably implies that crustal contamination played a minor role in the origin of the volcanic rocks. Isotopic evidence for the volcanic rocks indicates that they probably originated from melting of an enriched-mantle source, that is, ascribed to assimilation of subducted oceanic crust and sediments. (C) 2002 Elsevier Science Ltd. All rights reserved.