Crustal growth from collision to post-collision: implications from Cretaceous back-arc magmatism flare-up in southern Qiangtang of central Tibet

Abstract

Collision zones are primary sites for crustal growth and maturation, where magmatic activities play a crucial role in elucidating the formation and evolution of the crust. Previous studies suggest crustal growth mainly occurs in the continental-arc region during subduction to collisional period, while back-arc area primarily experience stretching and thinning. However, Cretaceous geological records from Bangong-Nujiang suture zone indicates that the original “back-arc region” experienced extensive magmatic activities and crustal thickening during the collision to post-collision period, and the specific mechanism of crustal growth differs from that in the continental-arc region. In this study, intermediate-felsic and high Ba, Sr magmatic rocks from the Cretaceous Bangong-Nujiang suture zone were utilized to constrain the magma sources and mantle metasomatic processes in the back-arc region. Furthermore, the newly identified samples, together with pre-existing mafic rocks and sedimentary evidence, were uesd to constrain the tectonic environment. In contrast to the previously established crustal growth patterns in continental arc regions, the process of crustal growth in the back-arc region can be divided into two stages: (1) Collision Period—during which the overall tectonic setting of the collisional zone was characterized by compression, the slab break-off created a localized extensional environment within the back-arc region. The upwelling asthenosphere through the slab window induced partial melting of the subducting slab, facilitating the continuous underplating and intrusion of mantle-derived magmas into the continental crust; (2) Post-Collisional Period—where the ongoing compression, combined with the underplating of mantle-derived magmas, lead to the continued thickening of the continental crust and lithospheric mantle. Ultimately, local structural weaknesses in the back-arc region resulted in the development of linear fractures oriented perpendicular to the direction of compression. The ascent of mantle-derived magmas through these structural ruptures triggered the partial melting of the enriched lithospheric mantle, which had undergone prior subduction-related metasomatism, thus supplying necessary materials for the growth of the continental crust.