Tectonostratigraphy and provenance of an accretionary complex within the Yarlung-Zangpo suture zone, southern Tibet: Insights into subduction-accretion processes in the Neo-Tethys
Abstract Accretionary complexes record critical information about the history of subduction and accretion along the southern margin of Asia prior to the India–Asia collision. This paper presents detailed field mapping, petrographic and detrital zircon U–Pb data from an accretionary complex within the Yalung–Zangpo suture zone, southern Tibet. From structurally higher to lower levels, the accretionary complex consists of a serpentinite melange, the chert-matrix Tangga melange, the mud-matrix Pomunong melange, and the coherent Rongmawa Formation. The Tangga melange consists of Late Triassic–Early Cretaceous abyssal chert and siliceous shale with blocks of chert and mafic to ultra-mafic rocks. The Tangga melange was accreted beneath ophiolitic rocks during the Aptian. The Pomunong melange consists of a Late Jurassic–Early Cretaceous hemipelagic siliceous shale and chert matrix with blocks from Early Permian seamounts and from Late Cretaceous trench-fill sandstones; it was accreted beneath the Tangga melange after the Aptian but prior to 71 Ma. Structurally beneath these melanges to the south, the uppermost Cretaceous Rongmawa Formation consists of turbiditic sandstone, pelagic chert, and siliceous shale and records a transition in depositional setting from lower abyssal plain to upper trench. Detrital zircons from sandstone blocks of the Pomunong melange and the coherent Rongmawa Formation display similar U–Pb age spectra and are dominated by peaks at 71–231 Ma, 481–693 Ma, and 701–1372 Ma. These age peaks overlap with igneous crystallization ages and detrital zircon ages from sedimentary strata in the Lhasa terrane. Our data indicate that the serpentinite melange, Tangga melange, Pomunong melange and Rongmawa Formation comprise a southward-younging accretionary complex that developed during the northward subduction of Neo-Tethyan oceanic lithosphere beneath the south margin of the Lhasa terrane and that all exposed ophiolitic and accretionary complex assemblages were accreted to the Asian margin prior to the India–Asia continental collision.