40Ar‐39Ar Geochronology and Tectonic Implications of the Blueschist from Northwestern Qiangtang,Northern Tibet,Western China

Blueschist exposed in the northwestern Qiangtang terrane, northern Tibet, western China (∼84°30′ E, 34°24′ N), provides new constraints on the tectonic evolution of Qiangtang as well as northern Tibet. The blueschist represented by lawsonite‐ and glaucophane‐bearing assemblages equilibrated at 375–400 °C and ∼11 kbar. ⁴⁰Ar‐³⁹Ar analysis on mineral separate from one blueschist sample yielded a well‐defined plateau age of ∼242 Ma. Geochemical studies show the blueschist is metamorphosed within‐plate basalts. The high pressure‐low temperature blueschist indicates a Triassic event of lithosphere subduction, and clearly represents an extension of the central Qiangtang metamorphic belt, and defines an in situ suture between eastern and western Qiangtang.