Zircon U-Pb age, geochemistry and geological signifi-cance of high Nb-Ta acid volcanic rocks from Meisu Formation, Bieruo-Zecuo area, Gangdise belt

Meisu Formation in Bieruo-Zecuo area is tectonically located on the south margin of north Gangdise belt. LA-ICP-MS zircon ²⁰⁶Pb/²³⁸U dating shows that rhyolite from Meisu Formation has an age of 39.62±0.77Ma (n=14, MSWD=0.99), corresponding to Late Eocene. Geochemistry shows that the rhyolite belongs to weakly aluminous to peraluminous alkaline series (A/CNK=0.99～1.35) with characteristics of high SiO₂ (72.85%～78.54%), high alkali (Alk=6.36%～8.47%), low CaO (0.33%～1.56%) and low MgO (0.08%～0.58%). The chondrite-normalized REE patterns are characterized by the significant enrichment of LREE (La_N/Yb_N=7.27～16.11) and strong negative Eu anomalies (δEu=0.13～0.26) with total REE content of 122.52×10^-6～208.35×10^-6. The primitive man-tle-normalized trace element spider diagram shows high content of Nb (38.1×10^-6～88.7×10^-6) and Ta (3.36×10^-6～6.35×10^-6), rela-tive enrichment of Rb, K, Th, Ce, Zr and Hf and strong depletion of Ba, Sr, P and Ti. These geochemical characteristics show that the Meisu rhyolite is the product of mixing/contamination by a specific proportion of alkali-rich, Nb-Ta enriched, mantle-derived basaltic magma and crust-derived magma. The authors hold that the Meisu rhyolite was formed in an intraplate extension environ-ment (rift). Geodynamically, the formation of those rocks was associated with the on-plateau intracontinental subduction, strike-slip shear, and crust shortening triggered by northward subduction of Tethys Oceanic crust on the southern side.