Early Cretaceous tectonomagmatic evolution and basin development of western Bangong–Nujiang suture: A complete history of soft collision

Abstract

Uncertainty surrounding the Early Cretaceous tectonomagmatic evolution and basin development of the Bangong–Nujiang suture zone (BNSZ) limits understanding of the history of Meso-Tethys Ocean and the initial uplift and large-scale metallogenesis of the Tibetan Plateau during the Mesozoic. Here we present new geochronological, geochemical and isotopic data for the Early Cretaceous volcano-intrusive rocks (including quartz-diorites, tholeiitic andesites, and bajaitic latites) around the western segment of the BNSZ (W-BNSZ). We also report an Early Cretaceous deep-water volcano-stratigraphic section newly identified in this region. Based on these new data and information available for the BNSZ, we propose that the W-BNSZ and surrounding regions had already entered a stage of soft-collisional land-locked basin evolution during the Early Cretaceous. The land-locked basin underwent limited uplift related to the initial soft collision during the earliest Early Cretaceous, resulting in the formation of a regional unconformity between Shamuluo Formation shallow-marine sedimentary rocks and underlying ophiolitic mélange. Subsequently, the basin experienced continuous subsidence related to sinking and rollback of the Bangong oceanic slab, leading to the occurrence of deep-water flysch deposits (e.g., the Zhaga Formation). The Bangong oceanic slab was completely detached from the overlying soft-collision zone at ca. 113 Ma, resulting in the strong isostatic uplift of the whole soft-collision zone. Consequently, seawater receded from W-BNSZ and surrounding regions during the late Early Cretaceous–Late Cretaceous. Our study also shows that the western segment of the South Qiangtang terrane experienced long-lived intermediate magmatism (130–104 Ma) under this soft-collision regime. Intermediate magmas of this age originated from partial melting of juvenile lower crust, fractional crystallization of mantle-derived mafic magmas, and partial melting of metasomatized mantle-wedge peridotites, and these processes were driven by the break-off, rollback, and complete detachment of the Bangong oceanic slab, respectively.