The amphibolite from the Shuanghu region, Qiangtang Basin, Qinghai-Xizang Plateau:Geochronological and geological implications

A succession of amphibolite crops out along the Qilongwuru valley in the Shuanghu region, Qiangtang Basin, Qinghai-Xizang Plateau. With the aid of SHRIMP zircon U-Pb dating, the amphibolite herein gives a ²⁰⁶Pb/²³⁸U weighted average age of 450.1 ±5.0 Ma, indicating that the amphibolite was formed during the Late Ordovician. Geochemically, the amphibolite is characterized by SiO₂ contents of 45.67%-51.68%, Na₂O contents of 0.98% -2.54%, high Na contents and lower K contents, lower differentiation of LREE and HREE, positive Eu anomaly, relatively enrichment of the elements Rb, Ba and U, and relatively depletion of the elements Nb, Ta and Zr. These geochemical signatures show that the amphibolite has the features of the enriched mid-oceanic ridge basalts (E-MORB), and represents the relicts of the Palaeo-Tethys oceanic crust. The similarity in the ages and geochemical signatures of the amphibolite from the Qilongwuru valley in the Shuanghu region and those of the Tongchang ophiolites and Wanhe ophiolites in the Changning-Menglian suture zone indicates that the Lungmuco-Shuanghu sutute zone and Changning-Menglian suture zone are represented by a unified Proto-(Palaeo-) Tethys ocean.