Abstract
Tonalites from the island arc rock assemblage in the Zetang segment of the Yarlung Zangbo suture zone were analyzed for major, trace elements (including REE) and Sr-Nd isotope. The experimental data demonstrate that the tonalites have the adakite-like characteristics, including high SiO2 (58%–63%), Al2O3 (18.4%−22.4%), Sr (810×10−6−940×10−6), Sr/Y (77–106), low HREE (Y=9×10−6−11×10−6, Yb= 1×10−6−1.3×10−6), with LREE enrichment and faint Eu positive anomaly. I Sr (0.70421−0.70487) is relatively low whereas 143Nd/144Nd (0.512896−0.512929) and ɛ Nd(t) values (+6.7–+7.3) are high. These features suggest that Zetang tonalites were formed by the partial melting of subducted oceanic crust, with involvement of a small amount of oceanic sediments. The identification of Zetang adakites, derived from slab melting, presents new evidence for the intra-Tethyan subduction and the previous suggestion about the existence of intra-oceanic island arc within Tethys.
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