Geochemistry and Sr-Nd isotope characteristics of tonalites in Zêtang,Tibet:New evidence for intra-Tethyan subduction

Tonalites from the island arc rock assemblage in the Zêtang 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. ISr (0.70421―0.70487) is rela-tively low whereas 143Nd/144Nd (0.512896―0.512929) and εNd(t) values (+6.7―+7.3) are high. These fea-tures suggest that Zêtang tonalites were formed by the partial melting of subducted oceanic crust,with involvement of a small amount of oceanic sediments. The identification of Zêtang adakites,derived from slab melting,presents new evidence for the intra-Tethyan subduction and the previous sugges-tion about the existence of intra-oceanic island arc within Tethys.

Tonalites from the island arc rock assemblage in the Zêtang 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 ? Nd(t) values (+6.7–+7.3) are high. These features suggest that Zêtang tonalites were formed by the partial melting of subducted oceanic crust, with involvement of a small amount of oceanic sediments. The identification of Zêtang 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.