Petrogenesis of the Zhagadang O-type adakite in Naidong County of Tibet: Constraints from its geochemistry and Sr-Nd-Pb isotopic compositions

Adakite, as a set of intermediate-acid igneous rock assemblages with special geochemical characteristics, has a variety of genetic models and special tectonic significance.The Zedang ophiolite is of great significance to the study of the evolution of the Neo-Tethys Ocean.The trondhjemite outcropped in the Zedang ophiolite was selected to study its petrology, petrochemistry and Sr-Nd-Pb isotopes.The results show that the Zhagadang trondhjemite is a high SiO₂ and Al₂O₃, rich Na₂O(Na₂O/K₂O>2), low MgO, and para-aluminous calc-alkaline series.It is characterized by high Sr and Sr/Y, low Y and Yb.These geochemical characteristics are comparable with those of O-type adakite.The Zhagadang trondhjemite is rich in large ion lithophile elements(e.g., Rb, K and Ba), poor in high field-strength elements(e.g., Nb, Ti and P) and rare earth elements, and exhibits a weak positive Eu anomaly.The Sr-Nd-Pb isotopic compositions indicate that the Zhagadang trondhjemite is characterized by high Sr, Sm, Nd and Pb contents, with I_Sr of 0.704251~0.704329, I_Nd of 0.512776~0.512807, positive ε_Nd(t) values(6.61~7.22) and relatively uniform lead isotopic composition(²⁰⁶Pb/²⁰⁴Pb=18.378~18.495, ²⁰⁷Pb/²⁰⁴Pb=15.542~15.554 and ²⁰⁸Pb/²⁰⁴Pb=38.336~38.406).The study indicates that the Zhagadang trondhjemite bearing mantle characteristics was formed by partial melting of the subducting oceanic slab at a certain depth.The result confirms the existence of the Zedang intra-oceanic.