EARLY CRETACEOUS SUBDUCTION-RELATED ADAKITE-LIKE ROCKS FROM THE SOUTHERN MARGIN OF THE LHASA TERRANE, TIBET: PRODUCTS OF SLAB MELTING AND SUBSEQUENT MELT–PERIDOTITE INTERACTION?

Conflicting geodynamic models have been proposed for the generation of the widespread Cretaceous igneous rocks in the central and northern Lhasa subterranes. To contribute this issue, we present SHRIMP U-Pb zircon data and geochemical and Sr-Nd-Pb-Hf isotopic data for the Mamen andesites from the southern margin of the Lhasa Terrane, Tibet. The Mamen andesites, emplaced at 136.5 Ma, are sodic (Na₂O/K₂O = 1.2–2.3) and have geochemical characteristics typical of adakites (i.e., high Al₂O₃, high La/Yb ratios and Sr contents, low Y and HREE contents, and positive Eu anomalies), except for high Cr, Ni, and MgO contents. The andesites have initial (⁸⁷Sr/⁸⁶Sr)_t ratios of 0.70413–0.70513, positive eNd(t) values of 3.7–5.8, and (²⁰⁶Pb/²⁰⁴Pb)_t ratios of 18.37–18.51, (²⁰⁷Pb/²⁰⁴Pb)_t ratios of 15.59–15.65, and (²⁰⁸Pb/²⁰⁴Pb)_t ratios of 38.43–38.72. In situ Hf isotopic analyses of zircons that had previously been dated by SHRIMP yielded positive initial e_Hf(t) values ranging from +11.0 to +15.5. A model calculation using trace element and Sr-Nd-Pb isotopic data indicates that several percent of subducted sediment is required to generate the Mamen andesites, which were derived via the partial melting of subducted Neo-Tethyan slab (MORB + sediment + fluid) and subsequently hybridized by peridotite in the mantle wedge. Our data indicate that the Neo-Tethyan ocean floor was subducted northward beneath the Lhasa Terrane during the Early Cretaceous at a high angle. Our results are inconsistent with a tectonic model that advocates the low-angle or flat-slab subduction of Neo-Tethyan ocean floor in generating the widespread Cretaceous magmatism documented in the Lhasa Terrane.