Paleozoic transpressional tectonics in the Beishan orogenic collage, Northwest China: Evolution of the Hongliuhe suture zone

The Beishan orogenic collage is a southern, central subset of the Central Asian Orogenic Belt, in Northwest China. It is an accretionary orogen that was active during the early Paleozoic, transitioning to convergent collisional tectonics in the late Paleozoic to early Mesozoic. It comprises two main early Paleozoic arcs built through Precambrian pericratonic fragments, the Hanshan and Dundunshan terranes. Detrital zircon geochronology indicates that they share a similar set of Precambrian ages with the Tarim craton. It also establishes that the Hanshan arc developed with magmatism from the Late Cambrian to Early Devonian, then re-initiated with a minor peak of magmatism in the Carboniferous–Permian. Magmatism in the Dundunshan arc follows an identical pattern, yet offset younger by roughly 50 m.y. in all aspects. We interpret this pattern to indicate a staged collision and accretion event, the docking of the arcs from north to south. The event initiated during a regional magmatic hiatus in the Devonian. Known ophiolite ages of genesis and emplacement respectively bracket the main arc-building magmatic period. This supports conclusions that an ophiolite belt in the Beishan does represent a suture. Deformation in the Mazongshan terrane, the central Beishan suture and accretionary complex, is due to episodes of early Paleozoic accretion of arcs, oceanic units, and ophiolite emplacement. To further establish the sequence of tectonism we provide an age and tectonic setting of the Hongliuhe ophiolite, one of the oldest that now lies in the suture zone. It comprises cumulate ultramafic to mafic plutonic rocks formed in the Cambrian (520.3 ± 5.8 Ma, U-Pb, Gabbro). Volcaniclastic and arc-marginal sedimentary rocks overlie exposed ductilely deformed lower crustal ophiolitic plutonic rocks. Petrography, lithostratigraphy, and whole rock and mineral chemistry support a conclusion that the ophiolite developed in an extensional arc-marginal supra-subduction tectonic environment. This illustrates how ophiolites can be created during subduction rollback, not necessarily subduction initiation. The emplacement timing of the ophiolite is constrained by an undeformed Early Devonian granite (413.6 ± 3.5 Ma, U-Pb) that intruded through its tilted, folded sedimentary cover sequence. Paleozoic deformation spanning the Mazongshan terrane can be differentiated into two events: pervasive Silurian–Devonian dextral transpression and Permian–Triassic sinistral transpression. We identif