Laterally differential composition and dynamics of a continental collision belt: Evidence from the North Qaidam orogenic system (western China)

Abstract

Revealing the large-scale, along-strike variations of composition and evolution of continental collisional belts remains a challenging task. Here, we report new zircon U-Pb ages, Lu-Hf isotopes and whole-rock major elements of granitoids, combined with a synthesis of published geochemical data of magmatic rocks across the whole North Qaidam collisional orogenic system (western China). Geochemical comparison and mapping reveal a pronounced dichotomy between the eastern and western segments during the ∼460–360 Ma orogeny. We observed that the eastern magmatism was continuous, with the magma source transition from mixed mantle-crust reservoirs to a solely exhumed continental crust at ∼420–410 Ma. A gradual increase in asthenosphere-derived magmatism triggered a magmatic burst at ∼400–380 Ma, followed by the orogen-collapse-related mafic magmatism at ∼380–360 Ma. In contrast, the western segment experienced a two-phase magmatism interrupted by a hiatus at ∼400–380 Ma. The earlier phase originated mainly from the moderately evolved and fractionated felsic magmas with variable older crustal components, while the later switched to asthenosphere-sourced mafic magmatism similar to that in the east. These contrasting features suggest that a relatively flat continental slab in the west gradually attached onto the overlying plate during ∼460–400 Ma, completely shutting down the western magmatism during ∼400–380 Ma. Meanwhile, the oceanic and continental slabs in the east progressively steepened until the slab broke off at ∼400–380 Ma. The whole orogenic root was finally removed together at ∼380–360 Ma. This study shows that slab tearing and differential slab behaviors may be a common feature of convergent and collisional orogens.