Crustal architecture of the Shangdan suture zone in the early Paleozoic Qinling orogenic belt, China: Record of subduction initiation and backarc basin development

The early Paleozoic Shangdan suture zone in the Qinling orogenic belt (China) marks the main tectonic boundary between the North China and South China blocks. The mafic rocks along this suture zone make up three distinct geochemical groups: (1) MORB-like; (2) boninite-like, and (3) island arc type. The MORB-like basalts display the highest values of TiO2 (0.86 wt.%–1.99 wt.%), LREE depletion and no negative Nb–Ta anomalies. However, their enrichment in LILEs relative to N-MORB indicates subduction influence in their melt source. The boninite-like rocks display lower TiO2 (0.09 wt.%–0.41 wt.%) and FeO* (4.3 wt.%–10.25 wt.%) contents, lower Ti/V ratios and total REEs, and high Cr (303–1495 ppm) and Ni (102–383 ppm) values. These geochemical features collectively indicate that magmas of the boninite-like rocks were derived from partial melting of a refractory mantle source. However, distinctly different ages of the boninite-like rocks imply that not all boninite-like rocks in the Shangdan suture zone formed in association with subduction initiation. The 524 Ma boninite-like doleritic rocks record the subduction initiation magmatism, whereas the younger boninite-like dolerites with more depleted geochemical signatures represent a magmatic stage associated with backarc spreading around 474 Ma. The arc-type lavas display strong negative Nb–Ta and Zr–Hf anomalies, characteristic of intra-oceanic island arc rocks. The combined geological, geochemical and regional tectonic features of the ophiolitic rocks along the Shangdan suture zone in the North Qinling orogenic belt provide a record of subduction initiation, island arc development and backarc rifting in the early Paleozoic, reminiscent of the modern IBM arc-trench system.