Structural sequence and geochronology of the Qomo Ri accretionary complex, Central Qiangtang, Tibet: Implications for the Late Triassic subduction of the Paleo-Tethys Ocean

The Late Paleozoic Qomo Ri Accretionary Complex (QRAC) in the Central Qiangtang region of Tibet lies at the southern margin of the Longmu Co–Shuanghu suture. QRAC is tectonically linked with the Nierong and Jitang metamorphic complexes in eastern Tibet and the Yunling and Lancang complexes in western Yunnan province, SW China. This metamorphic complex is mainly composed of Late Paleozoic passive continental marginal strata of the South Qiangtang basin, which was underthrust beneath North Qiangtang in the Late Triassic. At least three stages of deformation, D1, D2 and D3, with two corresponding metamorphic events, M1 and M2, were identified within the QRAC. D1 is characterized by a penetrative foliation, S1, bearing a dextral shear and a corresponding M1 mineral assemblage of phengite, garnet and quartz. This deformation was interpreted to be the result of the northwestward oblique subduction of the Paleo-Tethys Ocean and dated at 211–219 Ma by phengite 40Ar/39Ar ages. D2, a subsequent compression deformation, followed D1 immediately and exhibits flexural folds and pervasive axial crenulation in foliation S2. Deformations from D1 to D2 indicate that an oblique oceanic subduction was immediately followed by a collision between the North and South Qiangtang blocks along the Longmu Co–Shuanghu suture. The D3, recorded by minor folds and divisional axial cleavage (S3), is related to a giant arc structure with an extension of ~ 50 km.